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Sandstones and siltstones of the Uinta Formation nearPark City, Utah. The Uinta Formation is Eocene in age.(Copyright William E. Ferguson.)

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Ever since the Earth has had an atmosphere and hy-drosphere, sediments have been accumulating on itssurface. The sediments, now formed into sedimentaryrocks, contain features that tell us about the environ-ment in which they were deposited. By interpretingthese bits of evidence in successively higher strata, onecan decipher the geologic history of a part of the Earth.

�THE TECTONIC SETTING

Many factors determine the kind of sedimentary rockthat will be formed in a particular area. Among theseare the nature of the source area; the method of trans-port of sedimentary materials; the physical, chemical,and biologic processes operating in the place of deposi-tion; the climate under which processes of weatheringand erosion take place; and the changes that occur tosediment as it is being converted to solid rock. On agrander scale, the characteristics of an entire assem-blage of rocks are also influenced by the tectonics ofthe region in which deposition takes place. The termtectonics refers to deformation or structural behavior ofa large area of the Earth’s lithosphere over a long pe-riod of time. For example, a region may be tectonicallystable, broadly subsiding, or rising only gently. Otherareas are tectonically active and are experiencing upliftand compressive forces that produce mountain ranges.Where a source area has recently experienced crustalcompression and uplift, an abundance of coarse sedi-ment (forming sandstones and conglomerates) derivedfrom the rugged upland source area will be supplied tothe basin. In the geologic past, such a tectonic settinghas resulted in the accumulation of great “clasticwedges” of sediment that thinned and became fineraway from the former mountainous source area. Inother tectonic settings of the past, the source area hasbeen stable and topographically more subdued, so thatfiner particles and dissolved solids became the mostabundant components being carried by streams.

The tectonic setting influences not only the size ofclastic particles being carried to sites of deposition butalso the thickness of the accumulating deposit. For ex-ample, if a former marine basin of deposition had beenprovided with an ample supply of sediment and was ex-periencing tectonic subsidence (sinking), enormousthicknesses of sediments might accumulate. Over acentury ago, James Hall (1811–1898), the eminent

61

In the high mountains, I have seen shells. They aresometimes embedded in rocks. The rocks must havebeen earthy materials in days of old, and the shellsmust have lived in water. The low places are nowelevated high, and the soft material turned into hardstone.Chu-Hsi, A.D. 1200

O U T L I N E

� THE TECTONIC SETTING� ENVIRONMENTS OF DEPOSITION

The Marine EnvironmentTransitional EnvironmentsContinental Environments

� COLOR OF SEDIMENTARY ROCKSBlack ColorationRed ColorationRed Beds

� TEXTURE OF CLASTIC SEDIMENTARY ROCKSSize and Sorting of Clastic GrainsShape of Clastic GrainsArrangement of Clastic Grains

� INTERPRETING SEDIMENTARY ROCKSInferences from Sedimentary StructuresGeopetal StructuresInterpretation of Sands and SandstonesInterpretation of Carbonate RocksInterpretation of Clays and Shales

� THE SEDIMENTARY ROCK RECORDRock UnitsFaciesThe Pervasive Effects of Sea-Level ChangesCorrelationUnconformities

� DEPICTING THE PASTGeologic Columns and Cross-SectionsGeologic MapsPaleogeographic MapsIsopach MapsLithofacies Maps

C H A P T E R 3

The Sedimentary Archives

American geologist mentioned in Chapter 1, recog-nized that the thick accumulations of shallow-watersedimentary rocks in the Appalachian region impliedthat crustal subsidence had accompanied deposition.His reasoning was quite straightforward. It was easy tovisualize filling a basin that was 40,000 feet deep with40,000 feet of sediment. However, where fossils indi-cated a basin never more than several hundred feetdeep, the only way to get tens of thousands of feet ofsediment into it would be to have subsidence occurringsimultaneously with sedimentation.

In a marine basin of deposition that is stable or sub-siding very slowly, the surface on which sedimentation isoccurring is likely to remain within the zone of wave ac-tivity for a long time. Wave action and currents willwear, sort, and distribute the sediment into broad, blan-

ketlike layers. If the supply of sediment is small, this typeof sedimentation will continue indefinitely. Should thesupply of sediment become too great for currents andwaves to transport, however, the surface of sedimenta-tion would rise above sea level, and deltas would form.

It is also important to consider the tectonic frame-work of entire continents as well as of particular areas.The principal tectonic elements of a continent are cra-tons and orogenic belts (Fig. 3-1). Cratons have twocomponents: large areas of exposed ancient crystallinerocks called shields, as well as surrounding regionscalled platforms, in which these ancient rocks are cov-ered by flat-lying or gently warped layers of sedimen-tary rocks. Cratons have been undisturbed by tectonicevents since Precambrian time (about 540 million yearsago). They comprise the stable interiors of continents.

62 � Chapter 3. The Sedimentary Archives

FIGURE 3-1 The craton andorogenic belts of North America.

Orogenic belts are elongated regions that borderthe craton and have been deformed by compressionalforces since Precambrian time. Today, young orogenicbelts are recognized by their high frequency of earth-quakes and volcanic eruptions, whereas older belts aremarked by severely deformed strata, crustal displace-ments, metamorphic terranes, and huge exposed bod-ies of intrusive igneous rocks.

The history of the Earth provides many examples ofmarginal orogenic belts that were once sites for the ac-cumulation of great thicknesses of sediment. Suchelongate tracts of sedimentation are found today alongmany continental margins. Following long episodes ofdeposition, the sediments along these tracts may be de-formed as a result of an encounter with an oncomingtectonic plate, and a mountain range may form wherethere was once only a sedimentary basin. These eventsand their causes will be examined in Chapter 5.

The tectonic setting of deposition largely deter-mines the nature of sedimentary deposits. Conversely,the kind of tectonic setting often can be inferred from arock’s textural and structural features and from itscolor, composition, and fossils. The tectonic and his-torical significance of some of these characteristics ofsedimentary rocks will be described on the followingpages.

�ENVIRONMENTS OF DEPOSITION

Environment of deposition refers to all the physical,chemical, biologic, and geographic conditions underwhich sediments are deposited. Each environment ofdeposition is characterized by geographic and cli-matic conditions that modify or determine the prop-erties of sediment that is deposited within it. Thus,the type of sediment becomes the key to the environ-ment. Some sediments, such as chemical precipitatesin water bodies, are solely the products of their envi-ronment of deposition. Their component mineralsformed and were deposited at the same place. Othersediments consist of materials formed elsewhere andtransported to the site of deposition. Geologists arekeenly interested in the sediment of today’s deposi-tional environments because the features they find inthe deposits of these modern areas can also be seen inancient sedimentary rocks. Comparing present-daysedimentary deposits to old sedimentary rocks per-mits one to reconstruct conditions in various parts ofthe Earth as they were hundreds of millions of yearsago. We apply, as did James Hutton over 200 yearsago, the principle of uniformitarianism.

The Marine Environment

To facilitate discussion, the marine realm can be di-vided into shallow marine, deep marine, and continen-tal slope environments. The shallow division includes

the oceanic topographic regions called continentalshelves. Continental shelves (Fig. 3-2) are nearly flat,smooth surfaces that fringe the continents in widthsthat range from only a few kilometers to about 300kilometers and depths that range from low tide toabout 200 meters. In a geologic sense, these shallowareas are not part of the oceanic crust but resemble thecontinents in their structure and composition. Theyare, in fact, the submerged edges of the continents andhave a readily apparent continuity with the coastalplains. The outer boundaries of the shelves are definedby a marked increase in slope to greater depths. Thesmoothness of parts of the continental shelves seems tohave been produced in part by the action of waves andcurrents during the last Ice Age. At that time, sea levelwas lowered at times by as much as 140 meters as a re-sult of water being locked in glacial ice. Waves and cur-rents sweeping across the shelves shifted sediment intolow places and generally leveled the surface.

For geologists specializing in the study of sedimen-tary rocks, the continental shelves hold great interest.All of the sediment eroded from the continents andcarried to the sea in streams must ultimately cross theshelves or be deposited on them. Many factors influ-ence the kind of sediment deposited on the shelves, in-cluding the nature of the source rock on adjacent land-masses, the elevation of source areas, the distance fromshore, and the presence of carbonate-secreting organ-isms. Because large grains are heavier than small parti-cles, they tend to be deposited closer to shore. At thesame time, shallow-water currents and wave actionkeep finer particles in suspension and carry them far-ther out to sea. For these reasons, clastic sediment de-posited in the shallow marine environment tends to becoarser than material laid down in the deeper parts ofthe ocean. Sand, silt, and clay are common. Wherethere are few continent-derived sediments and the seasare relatively warm, lime muds of biochemical originmay be the predominant sediment. Coral reefs are alsocharacteristic of warm, shallow seas. They remind us ofthe enormous biologic importance of the shallow ma-rine environment. Over most of this realm, sunlightpenetrates all the way to the sea floor. Algae and otherforms of plant life proliferate. Here one finds the “pas-tures of the sea” on which, directly or indirectly, a mul-titude of swimming and bottom-dwelling animals aredependent (Fig. 3-3).

Those areas of the ocean floor that extend from theseaward edge of the continental shelves down to theocean depths are named the continental slopes. Phys-iographic diagrams of the ocean floor are usually drawnwith a large amount of vertical exaggeration, so that thecontinental slopes appear as steep escarpments. Actu-ally, the inclination of the surface is only 3� to 6�. Fromthe sharply defined upper boundary of the continentalslope, the surface of the ocean floor drops to depths of1400 to 3200 meters. At these depths the slope of the

Environments of Deposition � 63

ocean floor becomes gentler. The less-pronouncedslopes comprise the continental rises (see Fig. 3-2).

Sediment deposited on slopes and rises is mostlyfine sand, silt, and clay. These materials are often trans-ported to sites of deposition by turbidity currents.Water in a turbidity current is denser than surroundingwater because it is laden with suspended sediment. Ittherefore flows down the slope of the ocean floor be-neath the surrounding clear water. Upon reachingmore level areas, the current slows and drops its load of

suspended particles. The deposits, called turbidites,may form submarine fans at the base of the continentalslope (Fig. 3-4). In addition to turbidites, slope-and-rise deposits include fine clay that has slowly settled outof the water column and large masses of material thathave slid or slumped down the slope under the influ-ence of gravity.

In the deep marine environment far from the conti-nents, only very fine clay, volcanic ash, and the calcare-ous or siliceous remains of microscopic organisms set-


FIGURE 3-3 Marine life flourishes in manyareas of the well-lighted continental shelves,for light is required for the growth of marineplants, and plants are the basic components ofthe food chain that supports marine animals.(Courtesy of L. E. Davis, Washington StateUniversity.)

FIGURE 3-2 Diagram illustrating some marine, transitional, and continental environments ofdeposition. �? What features of a shale formed in an ancient lake might be used to distinguish it from a shale formedin an ancient lagoon?

tle to the ocean floor. The exceptions are sporadic oc-currences of coarser sediments that are carried downcontinental slopes into the deeper parts of the ocean byturbidity currents. Coarse sediments may also bedropped into deep water as they are released frommelting icebergs.

Transitional Environments

The shoreline of a continent is the transitional zone be-tween marine and nonmarine environments. Here onefinds deltas and the familiar shoreline accumulations ofsand or gravel that we call beaches. Mud-covered tidalflats that are alternately inundated and drained of waterby tides are also found in the transitional zone.

Deltas are accumulations of sediment formed by theentrance of a stream into quiet water such as the oceanor a lake. The term was proposed about 25 centuriesago by Herodotus, who noted that the Nile Delta hadthe general shape of the Greek letter �. Not all deltas,however, have this shape, for every delta responds dif-ferently to depositional and erosional processes, whichmay act in opposition to one another.

The Mississippi Delta (Fig. 3-5A) is termed a bird-foot delta because its many divergent streams (distrib-utaries) extend seaward to form a pattern that resem-bles the outstretched toes of a bird. In this greatdelta, the sediment supply has historically exceededthe reworking capabilities of ocean waves and cur-rents in the northern Gulf of Mexico. The delta,


FIGURE 3-4 Deep-sea fan built of land-derived sediment emerging from the lower partof a submarine canyon. Such fans occur in association with large rivers, such as theAmazon, Congo, Ganges, and Indus. (Vertical exaggeration 200:1.) �? Of the four majorkinds of sandstones described in this chapter, which is most commonly associated with such deep-sea fan deposits?

FIGURE 3-5 Comparison of the Mississippi (A) and Niger (B) deltas. �? What changes orevents might halt the progradation of these deltas?

therefore, has been able to build seaward or pro-grade. Progradation occurs as lobelike accumula-tions of sand, silt, and clay are deposited at themouths of one or more distributaries. After a lobe hasformed, it will usually sink somewhat as its weightcompresses the underlying material. One or moreyounger lobes may then accumulate over the first,producing cycles of deposits in which fine prodeltaclays are successively overlain by delta front silts andsands. The vertical sequence of sediment for eachsuccessive cycle exhibits an upward progression offine to coarse sediment, and each cycle may becapped by organic-rich marsh deposits.

Largely because dams constructed across tributariesof the Mississippi River have trapped sediment thatmight otherwise have contributed to delta expansion,the rate of growth of the Mississippi Delta has dimin-ished since about 1950. In addition, the delta is sinkingas a result of the tremendous weight of sediment on thecrust and the withdrawal of underground water thatsupplies cities and towns along the margin of the Gulfof Mexico. The unfortunate result is a continuing lossof Louisiana’s coastal wetlands and the rich harvest ofseafood these wetlands provide.

In contrast to the Mississippi Delta, where theamount of sediment brought to the sea by streams ex-ceeds the ability of the marine environment to reworkand transport that sediment, the Niger Delta (Fig. 3-5B) on the west coast of Africa provides an example ofa delta in which marine destructional processes are, to afar greater degree, in balance with the supply of sedi-ment. The action of waves and longshore currentsalong the delta front is capable of reworking newly de-posited sand, silt, and clay almost as fast as it is supplied.Over time, however, sedimentation has somewhat ex-ceeded removal so that there has been concentricgrowth along the entire front of the delta. If sedimentremoval had exceeded sediment accumulation, therewould be no delta at all. The Amazon is a river with nodelta. The sediment load is not great enough to over-whelm the effect of tides, waves, and subsidence nearthe mouth of this largest river on Earth.

Some of the best exposures of ancient deltaic se-quences can be observed in the Allegheny-Cumber-land Plateau. Deltas in this region developed along themargins of shallow seas that covered western Pennsyl-vania and Ohio. An ample supply of sediment was pro-vided by streams flowing from the eroding Ap-palachian highlands.

Deltaic sediments are rich in organic debris and in-clude many alternating and intersecting bodies of per-meable sands and impermeable clays. Because of this,ancient deltas have yielded tremendous volumes ofcoal, oil, and natural gas. Many of the once-prolific oilfields of the Texas and Louisiana Gulf Coast still pumpoil from buried deltaic sedimentary sequences.

In addition to deltas, the transitional zone in-cludes barrier islands, lagoons that lie between thebarrier islands and the mainland, and tidal flats. Bar-rier islands are extensively developed along the At-lantic Coast of the United States (Fig. 3-6) as well asaround Florida and the Gulf of Mexico. Sandy sedi-ments predominate in this high-energy environmentwhere sediment is winnowed and moved about bywaves and longshore currents. Barrier islands areelongate in form and may attain lengths of over 100kilometers. Most are less than a few kilometers inwidth. Coney Island off the coast of New York andPadre Island along the coast of Texas are examplesfamiliar to vacationers. Bivalves, gastropods, echi-noids, and crustaceans are prevalent in the barrier is-land environment.

The lagoons that lie behind barrier islands are pro-tected from strong waves and currents. As a result, la-goonal deposits usually consist of fine-grained sedi-ments. Lagoonal silts and clays tend to be denselyburrowed by mollusks and worms, although speciesaccomplishing this work are relatively few. Limitedspecies diversity results from the water chemistry of


FIGURE 3-6 Barrier island and lagoon on the southshore of Long Island. Waves break along the seaward sideof the barrier island, whereas conditions are relatively calmwithin the lagoon. (Courtesy of G. R. Thompson and J. Turk.)�? Why are sediments coarser on the seaward side of the island?

lagoons. Diversity is adversely affected in arid regionsbecause of the limited tolerance of organisms to highlysaline lagoonal water. Lagoons located in humid re-gions may also have lowered species diversity becauseof the influx of fresh water from streams or less salinewater from connections to the open ocean.

Almost featureless, low-lying plains that are alter-nately inundated and drained by tides constitute tidalflats. These are commonly marshy areas in which fine-grained sediments predominate. Owing to their inter-mittent exposure, tidal flats are harsh environments formany organisms. Nevertheless, hardy species of mol-lusks, crustaceans, worms, and cyanobacteria thrive inmany tidal flat environments.

Continental Environments

Continental environments of deposition include riverfloodplains, alluvial fans, lakes, glaciers, and eolian(wind) environments. The silt, sand, and clay foundalong the banks, bars, and floodplains of streams are fa-miliar to most of us. In general, stream deposits de-velop as elongate bodies that reflect the course of thestream itself. Streams are highly complex systems af-fected by many interacting variables, including thequantity of water moving through the channel, the ve-locity of that moving water, the nature of the sedimentbeing transported, and the shape of the stream channel.A change in any variable will cause a change in the sed-iment being deposited. Thus, sands, silts, and clays

may grade abrubtly into one another in stream de-posits. It is not an easy task to recognize ancient streamdeposits. They tend to have few fossils (and no marinefossils). Traces of root systems in the clays of overbankdeposits may provide evidence of stream deposits.Studies of sand and pebble grain orientation may indi-cate directional stream flow. Also useful is the observa-tion that average particle size of sediment transportedby a stream decreases downstream, whereas the round-ing of grain corners (roundness) and approach to asphere (sphericity) increase.

Stream-transported materials may accumulatequickly when a rapidly flowing river emerges from amountainous area onto a flat plain. The result of theabrupt deposition is an alluvial fan (Fig. 3-7). Exceptfor rare bones of vertebrates, spore and pollen grains, orfragmentary plant remains, ancient alluvial fan depositslack fossils. They are recognized primarily on the basisof their lobate form and wedge-shaped cross-section.

Lakes are landlocked bodies of water. The sedimentsof lakes are termed lacustrine deposits. Somewhatquieter deposition occurs in lakes, which are ideal trapsfor sediment. Silt and clay are common lake sediments,although a variety of sediment is possible, depending onwater depth, climate, and the character of the surround-ing land areas. The chemistry of lake water is influencedby the environment. When traces of that chemistry areretained in ancient lake sediment, those traces mayserve as indicators of former environmental conditions.The playa lakes of arid regions are shallow, temporary


FIGURE 3-7 Coalescing alluvial fans covering part of the floodplain, Gulf of Suez area,Egypt. The stream is unable to transport the huge amount of debris supplied to it and is dryduring part of the year. (Courtesy of D. Bhattacharyya.) �? If the mountains on the left side of thephotograph are composed of granite, what silicate minerals are likely to be abundant in the sediments ofthe alluvial fans?

lakes that periodically become dry as a result of evapo-ration. Thus, evaporites characterize playa lake de-posits. To identify lacustrine sediments, one looks forfreshwater fossils, vertebrate tracks, and polygonalcracks (mud cracks) that may form when lake water lev-els are low and exposed mud dries.

The glacial environment may include a variety ofother environments, including stream, lake, and evenshallow marine. Glaciers have the ability to transport anddeposit huge volumes and large fragments of rock detri-tus. Deposits are characteristically unsorted mixtures ofboulders, gravel, sand, and clay (Fig. 3-8). Where suchmaterials have been reworked by glacial meltwater, how-ever, they become less chaotic and resemble stream de-posits. One may even observe features resembling sedi-ments deposited in deserts, as strong winds pick up fineglacial particles and deposit them in dunes.

In addition to moving ice and flowing water, windcan also erode, transport, and deposit sediment. Wind,however, is much more selective in the particle size itcan transport. Air has only about 1/1000th the densityof water and therefore can erode and transport onlyparticles the size of sand or smaller. Environmentswhere wind is an important agent of sediment trans-port and deposition are called eolian environments(Fig. 3-9). They are characterized by an abundance ofsand and silt, little plant cover, and strong winds.These characteristics typify many desert regions.


FIGURE 3-8 Glacial deposits. (A) Gravels in lateral and terminal moraines of a glacier onBaffin Island, Canada. (B) Till exposed on the flanks of Mt. Ranier, Ranier National Park.(Photo by Steve Sheriff.)

(A) (B)

FIGURE 3-9 Dunes in Eureka Valley, California.(Copyright Lee Rentz/Bruce Coleman, Inc.)

�COLOR OF SEDIMENTARY ROCKS

We have seen that color in igneous rocks can be used toindicate the approximate amount of ferromagnesianminerals present. Color in sedimentary rock can alsoprovide useful clues to identification. For example, va-rieties of chert can be identified as flint if they are grayor black, or as jasper if they are red. Color is also usefulin providing clues to the environment of deposition ofsedimentary rocks. Of the sedimentary coloringagents, carbon and the oxides and hydroxides of ironare the most important.

Black Coloration

Black and dark-gray coloration in sedimentaryrocks—especially shales—usually results from thepresence of compounds containing organic carbonand iron. The occurrence of an amount of carbonsufficient to result in black coloration implies anabundance of organisms in or near the depositionalareas as well as environmental circumstances thatkept the remains of those organisms from being com-pletely destroyed by oxidation or bacterial action.These circumstances are present in many marine,lake, and estuarine environments today. In a typicalsituation, the remains of organisms that lived in ornear the depositional basin settle to the bottom andaccumulate. In the quiet bottom environment, dis-solved oxygen needed by aerobic bacteria to attackand break down organic matter may be lacking.There also may be insufficient oxygen for scavengingbottom dwellers that might feed on the debris. Thus,organic decay is limited to the slow and incompleteactivity of anaerobic bacteria; consequently, incom-pletely decomposed material rich in black carbontends to accumulate. In such an environment, ironcombines with sulfur to form finely divided iron sul-fide (pyrite, FeS2), which further contributes to theblackish coloration. Such environments of deposi-tion are likely to yield toxic solutions of hydrogensulfide (H2S). The lethal solutions rise to poisonother organisms and thus contribute to the process ofaccumulation. Black sediments do not always form inrestricted basins. They may develop in relativelyopen areas, provided the rate of accumulation of or-ganic matter exceeds the ability of the environmentto cause its decomposition.

Red Coloration

Hues of brown, red, and green often occur in sedi-mentary rocks as a result of their iron oxide content.Few, if any, sedimentary rocks are free of iron, and lessthan 0.1 percent of this metal can color a sediment adeep red. The iron pigments are not only ubiquitous

in sediments but also difficult to remove in most nat-ural solutions.

The iron present in sediment often occurs as eitherferrous iron compounds or ferric iron compounds.Ferrous iron oxide (FeO) frequently occurs in oxygen-deficient environments. It is unstable and may slowlyoxidize to form ferric iron oxide (Fe2O3). When oxygenis in short supply, ferric iron may be similarly reducedto ferrous iron. Ferric minerals such as hematite tendto color the rock red, brown, or purple, whereas theferrous compounds impart hues of gray and green. Hy-drous ferric oxide (limonite) is often yellow.

Red Beds

Strata colored in shades of red, brown, or purple byferric iron are designated red beds by geologists. Thecompound Fe203, which occurs as “rust” and the min-eral hematite, provides the color. Oxidizing condi-tions required for the development of ferric com-pounds are more typical of nonmarine than marineenvironments; most red beds are floodplain, alluvialfan, or deltaic deposits. Some, however, are originallyreddish sediment carried into the open sea. Electronmicroscopic studies of red beds forming today in BajaCalifornia indicate that the red coloration developedlong after the sediment was deposited. After burial,the decay of clastic ferromagnesian minerals releasediron that was oxidized by the oxygen in water circulat-ing through the pore spaces. Thus, red coloration maybe imparted in the subsurface and may be independentof climate. The paleoenvironmental interpretations ofred beds should be based to a large degree on the asso-ciated rocks and sedimentary structures. Red beds in-terspersed with evaporite layers indicate warm andarid conditions.

Although red beds are more likely to represent non-marine than marine deposition, occasionally one findsmarine red beds interbedded with fossiliferous marinelimestones. In such cases, the color may be inheritedfrom red soils of nearby continental areas. Lands lo-cated in warm, humid climates often develop such redsoils. When the soil particles arrive at the marine depo-sitional site, they will retain their red coloration, pro-vided there is insufficient organic matter present to re-duce the ferric iron to the relatively soluble ferrousstate. Otherwise, they will be converted to the gray orgreen colors of ferrous compounds.

In summary, sedimentary rocks of red colorationmay be a product of the source materials, may have de-veloped after burial as a result of a lengthy period ofsubsurface alteration, or may be the result of subaerialoxidation. Geologists are suspicious of the last possibil-ity because most modern desert sediments are not redunless composed of sediment from nearby outcrops ofolder red beds.

Color of Sedimentary Rocks � 69

�TEXTURE OF CLASTIC SEDIMENTARYROCKS

The size, shape, and arrangement of mineral and rockgrains in a rock constitute its texture. In addition tothe larger grains themselves, the textural appearanceof a rock is influenced by the materials that hold theparticles together. Matrix is bonding material thatconsists of finer clastic particles (often clay) that weredeposited at the same time as the larger grains and thatfill the spaces between them. Cement, on the otherhand, is a chemical precipitate that crystallizes in thevoids between grains following deposition. Silica(SiO2) and calcium carbonate (CaCO3) are commonnatural cements. Other cements include dolomite(CaMg(CO3)2), siderite (FeCO3), hematite (Fe2O3),limonite (2Fe2O33H2O), and gypsum (CaSO42H2O).

Texture can provide many clues to the history of aparticular rock formation. In carbonate rocks, ex-tremely fine-grained textures often indicate deposi-tion in quiet water. Fine carbonate muds, which arethe source sediment of such rocks, are not likely to set-tle to the bottom in turbulent water. Whole, unbrokenfossil shells confirm the quiet-water interpretation.

Limestones containing the worn and broken frag-ments of fossil shells are likely to be the products of re-working by wave action. They are turbulent shallow-water deposits.

Size and Sorting of Clastic Grains

Geologists universally use a scale of particle sizesknown as the Wentworth Scale to categorize clasticsediments (Table 3-1). After disaggregation of a rockin the laboratory, the particles can be passed through aseries of successively finer sieves, and the weight per-centage of each size range in the rock can be deter-mined. It is obvious that a stronger current of water (orwind) is required to move a large particle than to movea small one. Therefore, the size distribution of grainstells the geologist something about the turbulence andvelocity of currents. It can also be an indicator of themode and extent of transportation. If sand, silt, andclay are supplied by streams to a coastline, the turbu-lent nearshore waters will winnow out the finer parti-cles, so that gradations from sandy nearshore depositsto offshore silty and clayey deposits frequently result(Fig. 3-10). Sandstones formed from such nearshore


TABLE 3-1 Size Range of Sedimentary Particles

Wentworth Scale Fractional Equivalents(mm) (mm) Particle Name

Boulders256128 Cobbles

643216 Pebbles

84

Granules2

Very coarse sand1.0

Coarse sand0.5 1/2

Medium sand0.25 1/4

Fine sand0.125 1/8 Very fine sand0.0625 1/160.0313 1/320.0156 1/64

Silt0.0078 1/1280.0039 1/256

Clay

sands may retain considerable porosity and providevoid space for petroleum accumulations. For this rea-son, petroleum geologists draw maps showing thegrain size of deeply buried ancient beaches andnearshore sandstone formations to determine areas ofcoarser and more permeable clastic rock.

One aspect of a clastic rock’s texture that involvesgrain size is sorting. Sorting is an expression of therange of particle sizes deviating from the average size.Rocks composed of particles that are all about the sameaverage size are said to be well sorted (Fig. 3-11), andthose that include grains with a wide range of sizes aretermed poorly sorted. Sorting provides clues to condi-tions of transportation and deposition. Wind, for ex-ample, winnows the dust particles from sand, produc-ing grains that are all of about the same size. Wind alsosorts the particles that it carries in suspension. Onlyrarely is the velocity of winds sufficient to carry grainslarger than 0.2 millimeters. While carrying grains ofthat size, winds sweep finer particles into the higher re-gions of the atmosphere. When the wind subsides,well-sorted silt-sized particles drop and accumulate. In

general, windblown deposits are better sorted than aredeposits formed in an area of wave action, and wave-washed sediments are better sorted than are stream de-posits. It must be kept in mind, however, that if asource sediment is already well sorted, the resulting de-posit will be similarly well sorted. Provided they arenot pervasively cemented, well-sorted sandstones havegood porosity and permeability. They may, therefore,serve as reservoirs for petroleum and natural gas.

Poor sorting occurs when sediment is rapidly de-posited without being selectively separated into sizesby currents (see Fig. 3-8B). Poorly sorted conglomer-ates and sandstones are deposited at the foot of moun-tains, where stream velocity is suddenly checked. Another example of a poorly sorted conglomerate istillite, a rock deposited by glacial ice containing all par-ticle sizes in a heterogeneous mixture.

Shape of Clastic Grains

The shape of particles in a clastic sedimentary rock canalso be useful in determining its history. Shape can be

Texture of Clastic Sedimentary Rocks � 71

FIGURE 3-10 Idealized gradation of coarser nearshore sediments to finer offshoredeposits.

FIGURE 3-11 Sorting of grains insandstones, as seen under themicroscope, may range from goodsorting (A) to poor sorting (B). (A) Quartz sandstone (light-tangrains) with carbonate (pink) cement. (B) A sandstone known asgraywacke, composed of poorlysorted angular grains of quartz (lighttan), feldspar (green), and rockfragments (orange). The graywackelacks cement; spaces between grainsare filled with a matrix of clay and silt. Width of fields is 1.5 mm.�? ? Which of these sandstones can beconsidered an immature sandstone?

described in terms of rounding of particle edges andsphericity (how closely the grain approaches the shapeof a sphere; Figs. 3-12 and 3-13). A particle becomesrounded by having sharp corners and edges removedby impact with other particles. The relatively heavyimpacts between pebbles and granules being trans-ported by water cause rapid rounding. Lighter impactsoccur between sand grains in water transport; the waterprovides a cushioning effect. The result is far slowerrounding for sand grains. In conjunction with otherevidence, the roundness of a particle can be used toinfer the history of abrasion. It is a reflection of thedistance the particle has traveled, the transportingmedium, and the rigor of transport. It can also be usedas evidence of recycling of older sediments.

Arrangement of Clastic Grains

The third element in our definition of texture is thearrangement of the grains in the clastic rock. Geolo-gists examine the rock to ascertain whether the grainsare the same size and whether they are clustered intozones or heterogeneously mixed. These observationsmay help to determine whether the sediment had beenwinnowed and sorted by currents or had been dumpedrapidly. Such factors as the medium of transport, sur-face of deposition, and direction and velocity of currentcontrol grain orientation. Geologists study grain ori-entation as a means of determining the direction ofprevailing winds millions of years ago, the directiontaken by ancient streams, or the trends of former long-shore currents. In general, sand grains deposited inwater currents acquire a preferred orientation inwhich the long axes of elongate grains are aligned par-allel to the direction of flow. The preferred orienta-tion of sand grains in a sandstone can be statisticallyanalyzed in precisely oriented thin sections of sand-stones (Fig. 3-14). For coarser sediments, studies of the preferred orientation of glacial and stream-

deposited pebbles and cobbles indicate the directionof movement in glaciers and rivers that existed far backinto the Precambrian. There is also a practical reasonfor studying grain orientation, for such informationcan provide clues to the subsurface location and trendof petroleum-bearing sandstone strata.


FIGURE 3-12 Well-rounded grains of quartz viewedunder the microscope. From the St. Peter Formation,near Pacific, Missouri. Width of field is 1.85 mm.

FIGURE 3-13 Shape of sediment particles. (A) An angularparticle (all edges sharp). (B) A rounded grain that has littlesphericity. (C) A well-rounded, highly spherical grain.Roundness refers to the smoothing of edges and corners,whereas sphericity measures the degree of approach of aparticle to a sphere. �? ? Although well-rounded, high-sphericitygrains of quartz are common, feldspar grains are less likely to showgood rounding and sphericity. What attribute of feldspar accountsfor this difference?

FIGURE 3-14 Grain orientation study. One method ofstudying grain orientation is to prepare an oriented thinsection of a rock whose field orientation has beenrecorded. Grain orientations are then measured under amicroscope equipped with a rotating stage. The angle ofthe long axis of each elongated grain from the north lineis determined. From many individual measurements, amean orientation is determined (in this example, aboutN 45�E or S 45�W) and its statistical significance isevaluated. A thin section cut perpendicular to beddingmight reveal the tilt of the grains and might then beused to ascertain that the transportation medium flowednortheastward rather than southwestward.

�INTERPRETING SEDIMENTARYROCKS

Inferences from Sedimentary Structures

Sedimentary structures are those larger features ofsediments that are formed during or shortly after depo-sition and before lithification. Because particular sedi-mentary structures result from specific depositional

processes, the structures are useful to geologists inter-ested in reconstructing ancient environments. For ex-ample, mud cracks indicate drying after deposition.These conditions are common on valley flats, alongthe muddy margins of lakes, and in tidal zones. Mudcracks (Fig. 3-15) develop by shrinkage of mud or clayon drying and are most abundant in the subaerial envi-ronment. Cross-bedding (cross-stratification) is an

Interpreting Sedimentary Rocks � 73

E N R I C H M E N T

You Are the Geologist

Imagine you are a petroleum geologist working for a majoroil company. Your company is informed that four parcels ofcontinental shelf south of Louisiana are being offered forlease. You are informed, however, that the company hassufficient revenue to lease only one of the four parcels. Youare asked to recommend one of the four parcels for lease.You have information that the area is underlain by theGusher Sandstone, a formation which has yielded petro-leum in adjacent area. The formation slopes gently towardthe southeast. A few wells have been drilled into the GusherSandstone. Grain size analyses of Gusher Formation coresfrom these wells yielded the average grain size data on theWentworth Scale (Table 3-1) shown on the accompanyingmap.

1. Based on the size analysis alone, which parcel of con-tinental shelf would you recommend your company lease?Why?

2. What additional information would you hope to havein support of your recommendation?

FIGURE 3-15 Modern and ancient mud cracks. (A) These modern mud cracks formed insoft clay around the margins of an evaporating pond. (B) Mud cracks and wave ripples (causedby wind blowing over a shallow lake) in mudstones of the Oneonta Formation, of Devonianage, near Unadilla, New York. Divisions on the scale are 1.0 cm. (Modern mud cracks courtesy ofL. E. Davis; ancient mud cracks courtesy of W. D. Sevon, Pennsylvania Bureau of Topography &Geological Survey.) �? How might the shape of a mud crack in a cross-section of an ancient mudstone beused to indicate the top of a stratum?

(A) (B)

arrangement of beds or laminations in which one set oflayers is inclined relative to the others (Figs. 3-16 and3-17). The cross-bedding units can be formed by theadvance of a delta (Fig. 3-18) or a dune (see Fig. 3-9). Adepositional environment dominated by currents is in-

ferred from cross-bedding. The currents may be windor water. In either medium, the direction of the incli-nation of the sloping beds is a useful indicator of the di-rection taken by the current. By plotting these direc-tions on maps, geologists have been able to determinethe pattern of prevailing winds at various times in thegeologic past.

Graded bedding consists of repeated beds, eachof which has the coarsest grains at the base and suc-cessively finer grains nearer the top (Fig. 3-19). Al-though graded bedding may form simply as the resultof faster settling of coarser, heavier grains in a sedi-mentary mix, it appears to be particularly character-istic of deposition by the turbidity currents discussedearlier. Turbidity currents are often triggered bysubmarine earthquakes and landslides that occuralong steeply sloping regions of the sea floor. Theforward part of the turbidity current contains coarserdebris than does the tail. As a result, the sediment de-posited at a given place on the sea bottom gradesfrom coarse to fine as the “head” and then the “tail”of the current pass over it.

Ripple marks are commonly seen sedimentary fea-tures that developed along the surfaces of beddingplanes (Fig. 3-20). Symmetric ripple marks are formedby the oscillatory motion of water beneath waves.Asymmetric ripple marks are formed by air or water cur-rents and are useful in indicating the direction ofmovement of currents (Fig. 3-21). For example, ripplemarks form at right angles to current directions; thesteeper side of the asymmetric variety faces the direc-tion in which the medium is flowing. Although someripple marks have been found at great depths on the seafloor, these features occur more frequently in shallow-water areas and in streams.


FIGURE 3-16 Two types of cross-bedding. The upperblock shows tabular cross-bedding, as seen in beachdeposits and dunes; the lower block represents troughcross-bedding, as often formed in river channels. The lineat the base of each set of laminations represents a surface oferosion that truncates older sets below.

FIGURE 3-17 Tabular cross-bedding (cross-stratification) inthe Mountain Lakes Formation, ofProterozoic age, NorthwestTerritories, Canada. (Courtesy of G. Ross.) �? Was the current thatproduced the cross-bedding flowingapproximately from right to left or left toright?

Geopetal Structures

The principle of superposition tells us that in undis-turbed strata, the oldest bed is at the bottom and higherlayers are successively younger. But what if the strataare deformed and overturned in such a way that theoldest beds are found at the top (Fig. 3-22A)? If over-turning is not recognized, the geologic sequence ofevents, the kinds of folds, and features indicating thedirection of sediment transport might be misinter-preted. For this reason, geologists working in areas ofdeformed strata (Fig. 1-1) carefully scrutinize rocks forindications of the original tops and bottoms of beds.Features providing such information are calledgeopetal structures. Among the more commongeopetal structures are symmetric ripple marks (seeFig. 3-21A); included fragments; certain types of cross-bedding; graded bedding (see Fig. 3-19); mud cracks(see Fig. 3-15); bed surface markings, such as foot-prints, trails, and raindrop imprints; scour marks; fos-sils; and various biologically produced structures.

In the case of symmetric ripple marks (Fig. 3-22B),the sharp crests of the ripples normally identify the

tops of beds. They point toward the younger beds. Iffragments of the rippled rock are recycled by erosionand included in overlying strata, the overlying bedmust be younger, and the interpretation is confirmed.

In many kinds of cross-bedding, the cross-beds areconcave upward, forming a small angle with bedsbelow and a large angle with beds resting on their trun-cated upper edges (Fig. 3-22C). Geopetal interpreta-tions based on cross-bedding, however, should be con-firmed by other geopetal structures, as some cross-bedsdo not show the concave upward shape.

Graded bedding, in which grains are progressivelyfiner from the bottom to the top of a bed, is anotheruseful geopetal structure. As described earlier, gradedbeds are formed when fast-moving currents begin toslow, so that large particles are dropped first, followedby progressively finer grains.

Mud cracks are geopetal structures formed whenmud dries, shrinks, and cracks. The cracks narrowdownward, away from the top of the bed. Depositionabove mud cracks would fill them, resulting in a corre-sponding pattern of ridges that identify the bottom ofthe overlying stratum.

As currents flow across beds of sand, they often erodevarious kinds of scour marks. An overlying layer of sedi-ment may later fill these depressed markings, formingpositive-relief casts in the covering bed (Fig. 3-22D).The casts are termed sole markings because they appearon the sole, or bottom, of the younger stratum.

Fossils of bottom-dwelling organisms such as coralsmay also be used to determine way-up, provided theyhave been buried in their natural, upright living posi-tions. Some fossils that have been moved by currentsmay also be useful. For example, the curved shells ofclams washed by currents may come to rest in a convex-upward position, as this is hydrodynamically most sta-ble. Finally, many fossil organisms excavated and livedin burrows, such as the U-shaped burrows shown in


FIGURE 3-18 Cross-bedding in a delta. The succession of inclined foreset beds isdeposited over bottomset beds that were laid down earlier. Topset beds are deposited by thestream above the foreset beds.

FIGURE 3-19 Graded bedding. �? Of the four major kindsof sandstones, which most frequently displays graded bedding?

Figure 3-22E. Such structures are often excellentgeopetal indicators.

Interpretation of Sands and Sandstones

Among clastic sedimentary rocks, sandstones havebeen studied in great detail and provide an extraordi-nary amount of information about conditions in andnear the site of deposition. In particular, the mineralcomposition of sandstone grains can be used to identifysource areas and to interpret what may have occurredprior to deposition. Often, by closely studying thegrains, one can ascertain whether the source materialwas metamorphic, igneous, or sedimentary. The min-

eral content also provides a rough estimate of theamount of transport and erosion of the sand grains.Rigorous weathering and long transport tend to re-duce the less-stable feldspars and ferromagnesianminerals to clay and iron compounds and tend to causerounding and sorting of the remaining quartz grains.Hence, one can assume that a sandstone rich in theseless-durable and angular components underwent rela-tively little transport and other forms of geologicduress. Such sediments are termed immature and aremost frequently deposited close to their source areas.On the other hand, quartz can be used as an indicatorof a sandstone’s maturity; the higher the percentage ofquartz, the greater the maturity.


FIGURE 3-20 Ripple marks. (A) Ripple marks formed in sand along a modern beach. (B)Ancient ripple marks on a bedding surface of the Munising Formation, of Cambrian age,Mosquitoe Harbor, Pictured Rocks National Shoreline, Michigan. (Copyright Robert P. Carr/Bruce Coleman, Inc.) �? What do ripple marks in a marine sandstone indicate about the depth of waterin which the sandstone was deposited?

(A) (B)

FIGURE 3-21 Profiles of ripplemarks. (A) Symmetric ripples. (B) Asymmetric ripples.

In addition to providing an indication of a rock’smaturity, composition is an important factor in theclassification of sandstones into quartz sandstone,arkose, graywacke, and lithic sandstone (sometimestermed subgraywacke; Fig. 3-23).

Quartz sandstones are characterized by a domi-nance of quartz with little or no feldspar, mica, or finematrix. The quartz grains are well sorted and wellrounded (see Fig. 3-12). They are most commonly heldtogether by such cements as calcite and silica. Chemicalcements such as these tend to be more characteristic of

“clean” sandstones such as quartz sandstones and are notas prevalent in “dirtier” rocks containing clay. The pres-ence of a dense, clayey matrix seems to retard the forma-tion of chemical cement, perhaps because fine materialfills pore openings where crystallization might occur.

Calcite cement may develop between the grains asa uniform, finely crystalline filling, or large crystalsmay form, and each may incorporate hundreds ofquartz grains. Silica cement in quartz sandstone com-monly develops as overgrowths on the original grainsurfaces.


FIGURE 3-22 Various kinds of geopetal indicators. �? In what regions of the United Statesmight one encounter overturned strata?

FIGURE 3-23 Four categories of sandstone as seen in thin section under themicroscope. Diameter of field is about 4 mm.

Quartz sandstones reflect deposition in stable, quiet,shallow-water environments, such as the ancient shal-low seas that inundated large parts of low-lying conti-nental regions in the geologic past or some parts of ourmodern continental shelves (Fig. 3-24). These sand-stones, as well as clastic limestones, exhibit sedimentaryfeatures, such as cross-bedding and ripple marks, thatpermit one to infer shallow-water deposition.

Sandstones that are 25 percent or more feldspar (de-rived from erosion of a granitic source area) are calledarkoses (Fig. 3-25). Quartz is the most abundant min-eral, and the angular to subangular grains are bondedtogether by calcareous cement, clay minerals, or ironoxide. The presence of abundant feldspars and iron im-

parts a pinkish-gray color to many arkoses. In general,arkoses are coarse, moderately well-sorted sandstones.They may originate as basal sandstones derived fromthe erosion of a granitic coastal area experiencing anadvance of the sea, or they may accumulate in faulttroughs or low areas adjacent to granite mountains(Fig. 3-26).

Graywackes (from the German term wacken,meaning “waste” or “barren”) are immature sandstonesconsisting of significant quantities of dark, very fine-grained material (Fig. 3-27A). Normally, this fine ma-trix consists of clay, chlorite, micas, and silt. There islittle or no cement, and the sand-sized grains are sepa-rated by the finer matrix particles. Matrix constitutesapproximately 30 percent of the rock, and the remain-ing coarser grains consist of quartz, feldspar, and rockparticles. Graywacke has a dirty, “poured-in” appear-ance. The poor sorting, angularity of grains (Fig. 3-27B), and heterogeneous composition of graywackesindicate an unstable source and depositional area inwhich debris resulting from rapid erosion of highlandsis transported quickly to subsiding basins. Graded bed-ding (see Fig. 3-19), interspersed layers of volcanicrocks, and chert (which may indirectly derive their sil-ica from volcanic ash) further attest to dynamic condi-tions in the area of deposition. The inferred tectonicsetting is dynamic and unstable, with deposition occur-ring offshore of an actively rising mountainous region(Fig. 3-28). Graywackes and associated shales andcherts may contain fossils of deep-water organisms, in-dicating deposition at great depth. Such shallow-watersedimentary structures as cross-bedding and ripplemarks are rarely found.

Quartz sandstone, arkose, and graywacke are ratherdistinct kinds of sandstones. A sandstone that has amore transitional composition and texture is termed alithic sandstone (subgraywacke). In lithic sandstones(see Fig. 3-23), feldspars are relatively scarce, whereas


FIGURE 3-24 Idealized geologic conditions under which quartz sandstone may bedeposited. There is little tectonic movement in this environment. Water depth is shallow,and the basin subsides very slowly. �? What features formed by wave action would you expect to findin the marine sands of this environment?

FIGURE 3-25 Thin section of an arkose, viewedthrough a petrographic microscope. The clear grains aremostly quartz, whereas the grains that show stripes or aplaid pattern are feldspars. The matrix consists of kaoliniteclay and fine particles of mica, quartz, and feldspar.(Courtesy of the U.S. Geological Survey; photo by J. D. Vine.)�? How would you describe the sorting in this sandstone? Is itlikely to have good permeability?

quartz, muscovite, chert, and rock fragments are abun-dant. There is a fine-grained detrital matrix that doesnot exceed 15 percent, and the remaining voids arefilled with mineral cement or clay. Quartz grains aremore rounded and abundant, sorting is better, and thequantity of matrix is lower in lithic sandstones than ingraywackes.

The characteristic environments for lithic sand-stones are deltaic coastal plains (Fig. 3-29), wherelithic sandstones may be deposited in nearshore ma-rine environments or swamps and marshes. Coal bedsand micaceous shales are frequently associated withlithic sandstones.

Interpretation of Carbonate Rocks

Limestones are the most abundant of carbonate sedi-mentary rocks. Although limestone lake deposits do

occur, most limestones originated in the seas. Nearlyalways, the formation of these marine limestones ap-pears to have been either directly or indirectly associ-ated with biologic processes. In some limestones, theimportance of biology is obvious, for the bulk of therock is composed of readily visible shells of mollusksand skeletal remains of corals and other marine organ-isms. In other limestones, skeletal remains are not pres-ent, but nevertheless the calcium carbonate (CaCO3)that forms the bulk of the deposit was precipitated fromsea water because of the life processes of organisms liv-ing in that water. For example, the relatively warm,clear ocean waters of tropical regions are usuallyslightly supersaturated with calcium carbonate. In thiscondition, only a slight increase in temperature, loss ofdissolved carbon dioxide, or influx of supersaturatedwater containing calcium carbonate “seeds” can bringabout the precipitation of tiny crystals of calcium car-bonate. Organisms do not appreciably affect tempera-ture, but through photosynthesis, myriad microscopicmarine plants remove carbon dioxide from the waterand thus may trigger the precipitation of calcium


FIGURE 3-26 Geologic environment in which arkose may be deposited.

FIGURE 3-27 Hand specimen (A) and thin section (B) of graywacke. The poorly sortednature of graywacke and the angularity of its component grains are evident in the thin section asobserved with the petrographic microscope and crossed polarizers. Width of field is 9.0 mm.

(A) (B)

carbonate. Bacterial decay may also enhance precipita-tion of calcium carbonate by generating ammonia andthereby changing the alkalinity of sea water. In eithercase, the precipitate can be considered an indirectproduct of organic processes. Carbonate sedimenta-tion today is most rapid in shallow, clear-water tropi-cal marine areas, such as the Bahama Banks east ofFlorida (Fig. 3-30). The carbonate sediments formingtoday in the Bahama Banks (Fig. 3-31) originate inmore than one way. Some of them are derived fromthe death and dismemberment of calcareous algae,such as Penicillus, an organism that secretes tiny,needlelike crystals of calcium carbonate. The micro-scopic shells of other unicellular organisms also con-tribute to the carbonate buildup. In areas where tidal

currents flow across the banks, oöids accumulate. Asnoted in the previous chapter, oöids are tiny spherescomposed of calcium carbonate that are formed whenparticles roll back and forth on the sea floor and ac-quire concentric rings of carbonate (Fig. 3-32). Someof the sediment results from the precipitation of tinycrystals of lime from sea water that has been chemi-cally altered by the biologic processes of marineplants. Coarser particles result from the abrasion ofthe shells of invertebrates or consist of fecal pelletsproduced by burrowing organisms.

The Bahama Banks are a modern example of a car-bonate platform, defined as a broad, shallow marinestructure composed largely of calcium carbonate thatstands above the adjacent ocean floor on one or more


FIGURE 3-28 Tectonic setting in which graywacke is deposited. Frequently graywackesare transported by masses of water highly charged with suspended sediment. Because of thesuspended matter, the mass is denser than surrounding water and moves along the sloping seafloor or down submarine canyons as a turbidity current. Graywacke sediment characteristicallyaccumulates in deep-sea fans at the base of the continental slope.

FIGURE 3-29 Deltaic environment in which lithic sandstones may be deposited.

sides. The Bahama platform is covered by less than 200meters of water and is bordered by a steep slope intodeep water on its eastern margin. At least 5000 metersof carbonate rock has been deposited on the BahamaBanks platform, indicating the structure has main-tained itself by continuous production of carbonatesediment while simultaneously experiencing long-term subsidence.

The requirement of warm, clear, shallow seas forthe accumulation of modern carbonates seems to applyequally well to ancient deposits. Major sequences ofancient limestones are relatively free of clay and fre-quently contain an abundance of fossils of organismsthat thrived in shallow, warm seas. Ancient carbonaterocks have developed in a variety of tectonic settings.Thick sections of limestones and dolomites haveformed in ancient subsiding basins in western Texas,Alberta, and Michigan. In such areas, optimum condi-tions for carbonate sedimentation resulted in a rate ofaccumulation that approximately equaled subsidence.Thick deposits of limestones have also accumulatedduring the Paleozoic Era on carbonate platforms at theeastern margin of North America. During the geologicpast, sea levels were typically higher than they aretoday and climates generally warmer. Thus, carbonateplatforms were once more abundant and extensive.

One type of carbonate sedimentary rock that contin-ues to perplex geologists is the magnesium-calcium car-bonate rock dolomite. You will recall from Chapter 2that dolomite is a rock composed primarily of the min-eral dolomite, CaMg(CO3)2. Dolomite is believed toform when magnesium that has been concentrated in seawater replaces a portion of the calcium carbonate in pre-viously deposited calcium carbonate sediment. Support-ing this interpretation are fossil shells in dolomite stratathat were originally composed of calcite but that subse-quently have been changed to dolomite. Today,dolomite formation occurs in only a few areas, usuallywhere evaporation of sea water is sufficiently intense toconcentrate magnesium. Yet during parts of the Precam-brian, Paleozoic, and Mesozoic eras, dolomites were ex-tensively developed (Fig. 3-33). To account for these an-cient deposits, it would appear that extensive areas ofevaporative conditions would be required. Recently, ge-ologist David Lumsden discovered a correlation between


FIGURE 3-30 The Bahama Banks. The green areas havewater depths of less than 180 m. Florida lies about 110 kmwest of Bimini.

FIGURE 3-31 Carbonate mud accumulating on the seafloor in the shallow warm waters of the Bahama Bankscarbonate platform. Green algae of the genus Penicillusform the tuftlike growths in the background. These algaeproduce fine, needlelike crystallites of calcium carbonate(aragonite) that contribute to the production of carbonatesediment. Other algae, such as Halimeda, produce similarcalcium carbonate particles. (Courtesy of L. Walters.)

FIGURE 3-32 Thin section view of an oöliticlimestone. The oöids are immersed in a cement of sparry(clear) calcite. The large oöid in the center has a maximumdiameter of 0.74 cm. (Courtesy of G. R. Thompson and J. Turk.)

ancient periods of high dolomite formation and episodesof high sea level. He suggests that during times of eustatic(worldwide) rise in sea level, broad, low-lying tracts ofthe continents were inundated by shallow seas. Whereclimatic conditions were favorable, these shallow seasprovided the ideal evaporative environments needed toenrich sea water with magnesium. As the magnesium re-placed part of the calcium in the calcium carbonate sedi-ment that blanketed the sea floor, calcite was convertedto dolomite. When the sea level subsequently fell, themagnesium enrichment process halted, and dolomiteformation ceased.

Interpretation of Clays and Shales

Shale (Fig. 3-34) is a general term for a very fine-tex-tured, fissile (capable of being split into thin layers)rock composed mainly of silt and clay-size particles ormixtures of the two. In general, the environmental sig-nificance of shales parallels that of the sandstones withwhich they are associated. Frequently, the silt-sizedparticles in shales are similar in composition and shapeto sand grains in the associated sandstone beds. Thesesilty components (Fig. 3-35) can be extracted for studyby disaggregating the shale in water and repeatedlypouring off the muddy liquid, retaining the silt parti-cles as a residue.

In shales associated with quartz sandstones, the siltfraction often consists predominantly of rounded

quartz grains. Such quartz shales result from the re-working of older residual clays by transgressing shal-low seas. Their association with thin, widespread lime-stones and quartz sandstones provides evidence of theirdeposition under stable tectonic conditions.

Feldspathic shales contain silt of at least 10 percentfeldspar and tend to be rich in the clay mineral kaolin-ite. Feldspathic shales are common associates ofarkoses and are presumed to have formed in a similarenvironment. Such shales are representative of the


FIGURE 3-33 Dolomites of Triassic age exposed in theItalian Alps. Dolomite was first recognized as a distinctiverock type by Guy de Dolomieu in 1791. The rock type wasnamed dolomite in his honor by Richard Kirwan in 1794.(Italian Tourist Board.) �? How do dolomites differ fromlimestones?

FIGURE 3-34 Shale. The dark color of this specimenresults from its high content of carbon. Note the finelylayered structure, which is termed fissility. Shale tends tosplit readily along the planes of fissility. (CopyrightRunk/Schoenberger/Grant Heilman.)

FIGURE 3-35 Thin section of shale. As indicated here,silt particles are abundant constituents of shale. The browncolor results from organic matter that is mixed with theclay, which is the major constituent of shale. (Courtesy of G. R. Thompson and J. Turk.)

finer sediment winnowed from coarser detritus and de-posited in quieter locations.

Chloritic shales are usually associated withgraywackes. As implied by their name, flakes of thegreen silicate mineral chlorite are common among thesilt-sized components. The less flaky particles tend tobe angular. Fissility in these rocks is less well developedthan in other shales. The clay and silt particles in chlo-ritic shales are generally derived from mountainous,unstable source areas.

A shale type that is the approximate equivalent of alithic sandstone is designated a micaceous shale. Micaflakes, quartz, and feldspar are all among its silt-sizedcomponents. Micaceous shales are deposited underconditions somewhat less stable than the environmentfor quartz shales. They are particularly characteristic ofancient deltaic deposits.

The clay minerals that occur in shales are complexhydrous aluminosilicates with constituent atomsarranged in silicate sheet structures. Kaolinites, smec-tites, and illites are the three major groups of clayminerals. Kaolinites are the purest and seem to have apreferred occurrence in terrestrial environments.Smectites may contain magnesium, calcium, orsodium or any combination of these three, whereaspotassium is an essential constituent of illites. Illitesare the predominant clay mineral in more ancientshales.

�THE SEDIMENTARY ROCK RECORD

Rock Units

William Smith, the British surveyor mentioned inChapter 1, demonstrated that distinctive bodies ofstrata could be traced over appreciable distances andtherefore could be mapped. He produced an excep-tionally fine geologic map of England and Wales in1815. Smith’s map, the first ever made of such highquality and accuracy, was accompanied by a compre-hensive table of the rock units encountered in the area.Each of these units was given a particular name, such asthe “Clunch Clay,” the “Great Oölyte,” or the “Corn-brash Limestone.” Thus originated the concept of afundamental unit in geology that was lithologically dis-tinctive, that had recognizable contacts with otherunits both above and below, and that could be tracedacross the countryside from exposure to exposure (or inthe subsurface from well to well). Smith referred tosuch a unit as a stratum, but today it is universallyknown as a formation (Fig. 3-36). A formation neednot consist of a single rock type as long as the particularcombination of rock types provides a distinctive aspectto the unit. Thus a formation may be composed en-tirely of beds of shale, or it may be a distinctive se-quence of shale with interbeds of sandstone, evapor-ites, or limestone.

Formations and groupings or subdivisions of forma-tions all constitute rock units. Rock units, also calledlithostratigraphic units, are formally defined as bod-ies of rock identified by their distinctive lithologic andstructural features without regard to time boundaries.They are mappable and are distinctly different from thetime-rock or chronostratigraphic units defined in theprevious chapter. Such features as texture, grain size,clastic or crystalline, color, composition, thickness,type of bedding, nature of organic remains, and ap-pearance of the unit in surface exposures (or in thelithologic record of strata penetrated by wells) are allused to define a rock unit and recognize it in the field.Whereas a chronostratigraphic unit represents a bodyof rock deposited or emplaced during a specific intervalof time, a rock unit such as a formation may or may notbe the same age everywhere it is encountered. Thenearshore sands deposited by a sea slowly advancing(transgressing) across a low coastal plain may form asingle blanket of sand (perhaps later to be named theOriskany Sandstone); however, that sand layer will beolder where the sea began its advance and youngerwhere the advance halted (Fig. 3-37).

NAMING ROCK UNITS Formations are given twonames: first, a geographic name that refers to a localitywhere the formation is well exposed or where it wasfirst described, and second, a rock name if the forma-tion is primarily of one lithologic type. For example,the Kimmswick Limestone was first formally describedin a professional publication in 1904 by Edward O. Ul-rich. Ulrich named the formation for exposures nearthe small town of Kimmswick, Missouri. The Kimm-

The Sedimentary Rock Record � 83

FIGURE 3-36 Formations. The diagram shows threeformations. In practice, these formations would be formallynamed, often after a geographic location near which theyare well exposed. For example, the three formations shownhere might be designated the Cedar City Limestone, BigSprings Sandstone, and Plattsburgh Shale.

swick Limestone is entirely composed of limestone,and therefore the rock name can follow the localityname. When formations are composed of several dif-ferent kinds of rock, the locality name is simply fol-lowed by the term Formation, as in the Toroweap For-mation of the Grand Canyon (Table 3-2).

There are other rock units in addition to forma-tions. Distinctive smaller units within formations maybe split out as members, and formations may be com-bined into larger units called groups because of relatedlithologic attributes (or their position between distinctstratigraphic breaks). For example, in Grand CanyonNational Park, the Whitmore Wash, ThunderSprings, Mooney Falls, and Horseshoe Mesa rock unitsare members of the massive Redwall Limestone. In an-other part of the canyon wall, one finds the Tapeats

Sandstone, Bright Angel Shale, and Muav Limestonecombined to form a larger mappable rock unit knownas the Tonto Group (Table 3-2).

Facies

The aforementioned rock terms provide for direct ob-jective mapping of sedimentary beds as well as bodiesof metamorphic and igneous rocks. If one is to makeinferences about events recorded in rock units, it is use-ful to employ the term facies. A sedimentary faciesrefers to the characteristic aspects of a rock from whichits environment of deposition can be inferred. For ex-ample, a body of rock might consist of a bioclastic lime-stone along one of its lateral margins and micritic lime-stone elsewhere. Geologists might then delineate a


FIGURE 3-37 Diagram showing how the original deposits of a formation may vary inage from place to place.

TABLE 3-2 Rock Units of the Paleozoic Section in Grand Canyon NationalPark, Arizona

System Group Formation Member

Kaibab

PermianToroweap

HermitCoconino

Hermit Shale

Pennsylvanian Supai

Horseshoe Mesa

Mississippian Redwall LimestoneMooney Falls

Thunder Springs

Whitmore Wash

Devonian Temple Butte

Muav Limestone

Cambrian Tonto Bright Angel Shale

Tapeats Sandstone

“bioclastic limestone facies” and interpret it as anearshore part of the rock body, whereas they might in-terpret the micritic limestone facies as a former offshoredeposit. In this case, the distinguishing characteristicsare lithologic (rather than biologic); therefore, the fa-cies can be further designated as a lithofacies. In othercases, the rock unit may be lithologically uniform, butthe fossil assemblages differ and permit recognition ofdifferent biofacies that reflect differences in the envi-ronment. A limestone unit, for example, might containabundant fossils of shallow-water reef corals along itsthinning edge and elsewhere be characterized by re-mains of deep-water sea urchins and snails. Therewould thus be two biofacies, one reflecting deeperwater than the other. These could be designated the“coral” and the “echinoid-gastropod biofacies.”

These examples illustrate that facies are clearly theproducts (sediment, shells of organisms) of particularenvironments of deposition. Today, as we travel acrossswamp, floodplain, and sea, we traverse different envi-ronments of deposition (Fig. 3-38). Each of these envi-ronments of deposition changes laterally into the adja-cent environment of deposition, and each provides itspresent-day facies that likewise change to adjacent syn-chronous facies. Geologists record these facies changeson lithofacies and biofacies maps. Because these mapsare based on time-rock units, they provide a view ofdifferent facies of essentially the same age. If one wereable to make such maps of successively different times,it would become apparent that ancient facies haveshifted their localities as the seas advanced or retreatedor as environmental conditions changed.

Consider for a moment an arm of the sea slowlytransgressing (advancing over) the land. The sedimentdeposited on the sea floor may ideally consist of anearshore sand facies, an offshore mud facies, and a far-offshore carbonate facies. As the shoreline advances in-land, the boundaries of these facies also shift in thesame direction, thereby developing an onlap se-

quence (Fig. 3-39), in which coarser sediments arecovered by finer ones. Should the sea subsequentlybegin a withdrawal (regression), the facies boundarieswill again move in the same direction as the shoreline,creating as they do so an offlap sequence of beds (Fig.3-40). In offlap situations, coarser nearshore sedimenttends to lie above finer sediments. Also, because offlapunits are deposited during marine regressions, recentlydeposited sediment is exposed to erosion, and part ofthe sedimentary sequence is lost. Study of sequentialvertical changes in lithology, such as those representedby offlap and overlap relationships, is one method bywhich geologists recognize ancient advances and re-treats of the seas and chart the positions of formershorelines.

Onlap and offlap patterns of sedimentation wererecognized as early as 1894 by the German geologistJohannes Walther. Walther observed that the succes-sion of facies occurring laterally is also seen in the ver-tical succession of facies. Thus, to find what facies areto be encountered laterally from a given locality, oneneed only examine the vertical sequence of beds at thatlocality. For example, section B in Figure 3-41 shows atypical “fining upward” succession of facies. Point X isin the nearshore silt facies and is overlain by finer shaleand then limestone (hence the expression “fining up-wards”). This same sequence of silt to shale to lime-stone is seen in moving westward to section A. Beneathpoint X is a coarse beach sand, which can be traced lat-erally (eastward) in section C. This relationship, inwhich the vertical succession of facies corresponds tothe lateral succession, has been named Walther’sPrinciple.

If the pattern of sediment spread seaward fromshorelines always graded from nearshore sands toshales and carbonates, as depicted in Figure 3-39, pre-dicting the locations of particular facies in ancientrocks would be comparatively easy. In reality, however,the task is usually more complex. For example,


FIGURE 3-38 Sedimentary facies (lithofacies) developed in the sea adjacent to a landarea. The upper surface of the diagram shows present-day facies, whereas the front face showsthe shifting of facies through time. Notice that bottom-dwelling organisms also differ inenvironments having different bottom sediment and water depth.

nearshore sandy facies are not present at all along somecoastlines. This may occur because little sand is beingbrought to the coast by streams, vigorous wave andcurrent action has carried the sand grains away, or pos-sibly sand grains are trapped in submarine canyons far-ther up the coast. The nature of sedimentation along acoastline is also controlled by the direction of long-shore currents, the location of the mouths of majorstreams that dump their sedimentary load into the sea,the amount of sediment supplied, the presence of bar-riers to dispersal of sediment, and whether the agent

bringing the sediments to the sea is running water,wind, or glacial ice. Any of these factors complicate thestudy of facies, but they also provide fascinating prob-lems for the geologist to solve.

The Pervasive Effects of Sea-Level Changes

Whenever a change in sea level occurs that is world-wide, the change is termed eustatic. Ice accumulatingon the continents during an ice age causes lowering ofsea level because much of the water making up the ice


FIGURE 3-39 Sedimentation during a transgression produces an onlap relationship inwhich finer offshore lithofacies overlie coarser nearshore facies (see inset), nearshorefacies are progressively displaced away from a marine point of reference, and older bedsare protected from erosion by younger beds.

FIGURE 3-40 Sedimentation during a regression produces an offlap relationship inwhich coarser nearshore lithofacies overlie finer offshore lithofacies, as shown in A.The sandy nearshore facies is progressively displaced toward the marine point of reference.Older beds are subjected to erosion as the regression of the sea proceeds. As a result, offlapsequences are less commonly preserved than are onlap sequences.

ultimately comes from the ocean. Conversely, duringwarmer episodes, water from melting ice flows backinto the ocean, causing a rise in sea level and the land-ward advance of shorelines around the world. In addi-tion to eustatic changes associated with ice ages, up-warping of the floor of an ocean basin or thedevelopment of extensive midoceanic ridges can resultin a eustatic change in sea level. Midocean ridges aresubmarine mountain ranges extending for thousands ofkilometers across the floors of the major ocean basins.They are composed of basaltic lavas derived from themantle. Great rifts in the ocean floor widen to admitthe basalt. As will be described in Chapter 5, the newlyformed basaltic ocean floor moves laterally by a processcalled sea-floor spreading, and at a distance may plungeback into the mantle. The basaltic rocks formed alongthe midocean ridges are hot and thermally inflated. Asa result, they displace a considerable volume of seawater and cause a worldwide rise in sea level. When therate of extrusion of the basalts is rapid, there is likely tobe a significant worldwide rise in sea level. Subsequentslower rates of extrusion and spreading would conse-quently result in a lowering of sea level. These changesprofoundly influence the geologic history of continen-tal and shelf areas, for they determine when these areasare inundated, when seas regressed, when there is de-position, and when there is erosion. The alternate ad-vance and retreat of seas associated with events alongmidoceanic ridges permit geologists to recognize dis-tinct packages or sequences of strata having erosionalboundaries that reflect global cycles of sea-level fluc-tuations. The cycles can be plotted as sea level curves,such as the Vail sea level curves developed from seis-mic profiles across continental shelves by P. R. Vail(Fig. 3-42). If the Vail sea level curves detected insediments of the continental shelves truly reflect eu-static changes in sea level, then they are global and

permit worldwide correlation of the package of sedi-ments representing each cycle. Thus, rather thancorrelating individual rock units, one correlates anentire sequence of beds. The term sequence stratig-raphy has been given to this type of correlation. Se-quence stratigraphy developed as a consequence ofimproved methods in oil exploration seismology.The superior technology provides cross-sectionalimages or seismic profiles of strata deep below theEarth’s surface (Fig. 3-43).

Critics of Vail sea level curves contend that someof the cycles were the result of vertical movements ofcoastal regions. Without doubt, changes in the eleva-tion of land areas bordering the ocean can cause ef-fects similar to those resulting from eustatic change.A coastal tract may experience either tectonic uplift ortectonic subsidence. The former is likely to cause aretreat of the sea (offlap) from the rising land area,whereas subsidence might allow the sea to advance. Inorder to determine if a stratigraphic sequence de-posited during the advance or the retreat of the seavalidly indicates a eustatic change in sea level, one mustshow that the sequence can be correlated to similar se-quences on other continents.

Precisely how far the sea will advance or retreatduring a change in sea level is determined by theamount of change in sea level and the topography ofthe land. A low-lying, gently sloping terrain wouldhave a much greater area inundated by a small increasein sea level than would a steeply sloping mountainoustract (Fig. 3-44).

Ultimately, the amount of inundation or regressionalong a coast must be related to the interaction be-tween tectonic movements on the continents and eu-static sea-level changes. If the land area along a conti-nental margin rises at the same time and amount as aeustatic rise in sea level, the tectonic change will cancel


FIGURE 3-41 An illustration of Walther’s Principle, which states that vertical facieschanges correspond to lateral facies changes. (After Brice, J. C., Levin, H. L., and Smith, M.S. 1993. Laboratory Studies in Earth History, 5th ed. Dubuque, IA: William C. Brown.)

out the eustatic one. The effect on shoreline displace-ment will be minimal. In contrast, if the land adjacentto the ocean is subsiding while the sea level rises, agreater marine advance can be expected.

Anyone examining a map depicting one of theworld’s greatest deltas would quickly recognize thattectonic and eustatic changes are not the only causes ofa shift in shorelines. The rapid accumulation of sedi-ment along a coast or at the mouths of rivers will causeland area to be built seaward by progradation.

In the geologic past, there have been repeated ad-vances of seas into low-lying regions of continents.Many appear to be directly related to the Vail cycles justdiscussed. At times these marine transgressions coveredas much as two-thirds of North America. The resultinginland seas are termed epeiric, meaning “a sea over acontinent.” In these epeiric seas were deposited the sed-imentary rock record of much of the Paleozoic andMesozoic eras. The advance and retreat of the epeiricseas were characteristically rather irregular, often inter-rupted by partial regressions, and ultimately followed


FIGURE 3-42 The Vail sea-levelcurve of major cycles of sea-levelchanges. The letters E, M, and Lrefer to Early, Middle, and Late.(After Vail, P. R., et al. 1977.American Association of PetroleumGeologists Memoir 26.) �? How manymajor episodes of rising sea leveloccurred during the Paleozoic Era?

FIGURE 3-43 Seismic profile. The profile depicts tilted(dipping) strata that were beveled by erosion andsubsequently covered by horizontal strata. To produce theprofile, vibrations were generated into the Earth, either bydetonating explosives in shallow drill holes or by hydraulicvibrators. The vibrations reflected off deep layers of rockand returned to the Earth’s surface, where they weredetected by receivers called geophones. Computers thenprocessed the data received from the geophones toconstruct the profile.

by gradual withdrawal back into the major ocean basins.Thus, one can recognize packages of strata separated byregional unconformities. The average rates of advanceand retreat were in no sense catastrophic, for they rarelyexceeded a few inches per century. However, they weresufficient to cause extensive inundation of the conti-nents over the tens of millions of years encompassed bya geologic period.

Correlation

When examining an isolated exposure or rock in a roadcut or the bank of a stream, a geologist is aware that therock may continue laterally beneath the cover of soil andloose sediment and that the same stratum or rock body,or its equivalent, is likely to be found at other localities.The determination of the equivalence of bodies of rockin different localities is called correlation (Fig. 3-45).Correlation of both rock units and choro-stratigraphicunits from locality to locality within and between conti-nents or between bore holes drilled into the ocean flooris an important component of stratigraphy. Stratigra-phy, however, is not confined to correlation alone. It in-cludes all aspects of the study of rocks, including theconditions under which they originated, their mutualrelationships, description, and identification. Becausestratified rocks cover approximately three-fourths of theEarth’s total land area and because strata contain ourmost readily interpreted clues to past events, stratigra-phy forms the essential core of geologic history.

There are three principal kinds of correlation. Thefirst, lithostratigraphic, attempts to correlate rockbodies on the basis of their lithology (composition, tex-ture, color, and so on) and stratigraphic position. Bio-

stratigraphic links units by similarity of fossil content.Chronostratigraphic correlation expresses equiva-lence in age as determined by fossils or radioactive dat-ing. Patterns of transgression and regression (as in Vailcycles), distinctive chemical or isotopic characteristics,logs that reflect a unit’s electrical or magnetic proper-ties, or any combination of these and other attributesmay be useful in correlation.

Because there is more than one meaning for the term,geologists are careful to indicate the kind of correlationused in solving a particular geologic problem. In somecases it is only necessary to trace the occurrence of alithologically distinctive unit. Here, the age of that unit isnot critical, and lithocorrelation will suffice. Other prob-lems can be solved only through the chronocorrelationsof rocks that are of the same age. Such correlations in-volve chronostratigraphic (time-rock) units and are of the utmost importance in geology. They are the basisfor the geologic time scale and are essential in workingout the geologic history of any region.

Lithocorrelation of strata from one locality to an-other may be accomplished in several ways. If thestrata are well exposed at the Earth’s surface, as in aridregions, where soil and plant cover is thin, then it maybe possible to trace distinctive rock units for manykilometers across the countryside by actually walkingalong the exposed strata. In using this straightforwardmethod of correlation, the geologist can sketch thecontacts between units directly on topographic mapsor aerial photographs. The notations can then be usedin the construction of geologic maps. It is also possi-ble to construct a map of the contacts between correl-ative units in the field by using aerial photographs orappropriate surveying instruments.


FIGURE 3-44 A rise or fall in sea level will affect a far greater area along a low coastlinethan along coastlines composed of highlands that rise steeply adjacent to the sea.

In areas where bedrock is covered by dense vegeta-tion and a thick layer of soil, geologists must rely on in-termittent exposures found along the sides of valleys, instream beds, and in road cuts. Correlations are moredifficult to make in these areas but can be facilitated byrecognizing the similarity in position of the bed one istrying to correlate with other units in the total se-quence of strata. A formation may have changed some-what in appearance between two localities, but if it al-ways lies above or below a distinctive stratum ofconsistent appearance, then the correlation of theproblematic formation is confirmed (Fig. 3-46).

A simple illustration of how correlations are used tobuild a composite picture of the rock record is providedin Figure 3-47. A geologist working along the sea cliffs atlocation 1 recognizes a dense oölitic limestome (forma-tion F) at the lip of the cliff. The limestone is underlainby formations E and D. Months later the geologist con-tinues the survey in the canyon at location 2. Because ofits distinctive character, the geologist recognizes theoölitic limestone in the canyon as the same formationseen earlier along the coast and makes this correlation.The formation below F in the canyon is somewhat moreclayey than that at locality 1 but is inferred to be thesame because it occurs right under the oölitic limestone.Working upward toward location 3, the geologist mapsthe sequence of formations from G to K. Questions stillremain, however. What lies below the lowest formationthus far found? Perhaps years later an oil well, such asthat at location 3, might provide the answer. Drilling re-veals that formations C, B, and A lie beneath D. Petro-leum geologists monitoring the drilling of the wellwould add to the correlations by matching all the forma-tions penetrated by the drill to those found earlier inoutcrop. In this way, piece by piece, a network of corre-lations across an entire region is built up.

For correlations of chronostratigraphic units, onecannot depend on similarities in lithology to establishequivalence. Rocks of similar appearance have beenformed repeatedly over the long span of geologic time.Thus, there is the danger of correlating two apparentlysimilar units that were deposited at quite differenttimes. Fortunately, the use of fossils in correlation mayhelp to prevent mismatching. Methods of correlationbased on fossils (biostratigraphic correlation) are fullydescribed in Chapter 4. They are based on the fact thatanimals and plants have undergone change through ge-ologic time, and therefore the fossil remains of life arerecognizably different in rocks of different ages. Con-versely, rocks of the same age but from widely sepa-rated regions can be expected to contain similar assem-blages of fossils.

Unfortunately, there are complications to thesegeneralizations. For two strata to have similar fossils,they would have to have been deposited contempora-neously in rather similar environments. A sandstoneformed on a river floodplain would have quite differentfossils from one formed at the same time in a nearshoremarine environment. How might one go about estab-lishing that the floodplain deposit could be correlatedto the marine deposit? In some cases this might bedone by physically tracing out the beds along a cliff orvalley side. Occasionally, one is able to find fossils thatactually do occur in both deposits. Pollen grains, forexample, could have been wafted by the wind into bothenvironments. Possibly, both deposits occur directlyabove a distinctive, firmly correlated stratum, such as alayer of volcanic ash. Ash beds are particularly goodtime markers because they are deposited over a wide


FIGURE 3-45 Correlation of lower Cambrian rockunits in western Montana. The letters C, B, G, and Aindicate the occurrences of trilobite index fossils Cedaria,Bathyuriscus, Glossopleura, and Albertella. (Modified fromSchmidt et al. 1994. U.S. Geological Survey Bulletin 2045.)�? Do the three lowermost formations in these three columnarsections indicate onlap or offlap?

area during a relatively brief interval of time. Such keybeds are exceptionally useful in establishing the corre-lation of overlying strata. Finally, the geologist may beable to obtain the actual age of the strata using radioac-tive methods, and these values can then be used to es-tablish the correlation.

Unconformities

Interpreting the geologic history of an area would begreatly facilitated if deposition were continuous over

time and there were no erosional losses of sediment.Unfortunately, such an uninterrupted sequence ofstrata is rarely encountered. There are gaps in the geo-logic record where varying thicknesses of strata havebeen lost to erosion or where deposition did not occurfor an interval of geologic time. As mentioned inChapter 1, we call these breaks in stratigraphic conti-nuity unconformities. Whether by erosion or nonde-position, the gap in the geologic record may encom-pass tens or even hundreds of millions of years ofEarth history.


FIGURE 3-46 If the lithology of arock is not sufficiently distinctiveto permit its lithostratigraphiccorrelation from one locality toanother, its position in relation todistinctive rock units above andbelow may aid in correlation. Inthe example shown here, thelimestone unit at locality A can becorrelated with the lowest of thefour limestone units at locality Bbecause of its position between thegray shale and the sandstone units.

FIGURE 3-47 An understanding of the sequence of formations in an area usuallybegins with examination of surface rocks and correlation between isolated exposures.Study of samples from deep wells permits the geologist to expand the known sequence offormations and to verify the areal extent and thickness of both surface and subsurfaceformations.

The four major kinds of unconformities illustratedin Figure 3-48 differ with regard to the orientation ofthe rocks beneath the erosional surface. Of the typesshown, the angular unconformity (Fig. 3-49 and Fig.3-43) provides the most readily apparent evidence of

crustal deformation. James Hutton recognized the sig-nificance of an angular unconformity when he ob-served this feature at Siccar Point on the Scottish coastof the North Sea (see Fig. 1-6).

Examples of unconformities are abundant on everycontinent. Some do not reflect the degree of deforma-tion apparent in the strata at Siccar Point but ratherdocument the simple withdrawal of the sea for a periodof time, followed by another marine transgression.The result may be a disconformity, in which parallelstrata are separated by an erosional surface. The with-drawal and advance of the sea may be caused by fluctu-ations in the volume of ocean water, but more com-monly they are the result of crustal uplift andsubsidence. Nonconformities are surfaces wherestratified rocks rest on older intrusive igneous or meta-morphic rocks (Fig. 3-50). In many nonconformities,crystalline rocks were emplaced deep within the rootsof ancient mountain ranges that subsequently experi-enced repeated episodes of erosion and uplift. Eventu-


FIGURE 3-48 Four types of erosional unconformities.(A) Angular unconformity. (B) Nonconformity. (C)Disconformity. (D) Paraconformity.

FIGURE 3-49 An angular unconformity separatesvertical beds of the Precambrian (Proterozoic)Uncompahgre Sandstone from the overlying, nearlyhorizontal Devonian strata of the Elbert Formation, atBox Canyon Falls, Ouray, Colorado. (Photograph by JohnH. Karachewski, from the Geological Society of America. 1987.Geology 15(5): Cover.)

ally, the igneous and metamorphic core of the moun-tains lay exposed and provided the surface on which theyounger strata were deposited.

Although unconformities represent a loss of geo-logic record, they are nevertheless also useful to geolo-gists. Like lithostratigraphic units, they can be mappedand correlated. They often record episodes of terres-trial conditions that followed the withdrawal of seas.Where regionally extensive unconformities occur, theypermit one to recognize distinct sequences or “pack-ages” of strata of approximately equivalent age.

Depicting the Past

GEOLOGIC COLUMNS AND CROSS-SECTIONS To aid inthe synthesis and interpretation of field observations ofsedimentary and other rocks, geologists prepare a vari-ety of maps, graphs, and charts designed to show rela-tionships of rock bodies to one another, their thick-ness, the manner in which they are deformed, and theirgeneral composition. The most important graphic de-

vices for communicating such information are colum-nar sections, cross-sections, and geologic maps.

Columnar sections, like those depicted in Figure 3-45, are made to show the vertical succession of rockunits at a given location or for a specific region. They areused in correlation and in the construction of cross-sections. Cross-sections show the vertical dimension of aslice through the Earth’s crust. Some cross-sections—namely, the stratigraphic type—emphasize the age orlithologic equivalence of the strata. The vertical mea-surements for such stratigraphic cross-sections are madefrom a horizontal line termed the datum that is drawn atthe top of a definite rock unit or some other marker suchas a particular fossil assemblage. The datum in Figure 3-45 is for the trilobite fossil Glossopleura (G).

Geologists construct stratigraphic cross-sectionswith the datum line horizontal. In the field, however,the marker for the datum line follows the configurationof the strata and may be variously inclined and folded.For this reason, stratigraphic sections do not validlyshow the tilt or position of beds relative to sea level.


FIGURE 3-50 The erosionalsurface of this nonconformity isinclined at about 45� andseparates Precambrian rhyoliticrock from overlying UpperCambrian Bonneterre Dolomite.Lower and Middle Cambrian rocksare missing. Taum Sauk Mountain,southeastern Missouri. (Courtesy ofD. Bhattacharyya.)

FIGURE 3-51 Geologic structural cross-section across Paleozoic rocks in theAppalachian Mountains, southeastern Pennsylvania, extending northwestward from thetown of Salisbury.

FIGURE 3-52 Geologic map of the Grand Canyon of the ColoradoRiver, Arizona. �? Does the outcrop pattern of Paleozoic formations indicate thatthe strata are horizontal or inclined? If you were rafting down this part of theColorado River, what would be the age and type of rocks you would see along the baseof the canyon walls? What is the age of the rocks at the summit of the Shiva Templeon the northwestern part of the map?

95

FIGURE 3-53 Steps in the preparation of a geologic map. (A) A suitable base map is selected. (B) Thelocations of rock exposures of the various formations are then plotted on the base map. Special attention is givento exposures that include contacts between formations; where they can be followed horizontally, they are tracedonto the base map also. Strike (the compass direction of a line formed by the intersection of the surface of a bedand a horizontal plane) and dip (the angle an inclined stratum makes with the horizontal) are measured whereverpossible and added to the data on the base map. After careful field study and synthesis of all the availableinformation, formation boundaries are drawn to best fit the data. (C) On the completed map, color patterns areused to show the areal pattern of rocks beneath the cover of soil. (D) A cross-section is shown along line A-A9.(E) A block diagram illustrates strike and dip. �? What is the oldest rock unit seen at the surface along A-A9? Where isthe youngest unit located?

Stratigraphic cross-sections are most effective in show-ing the way beds correlate and vary in thickness fromexposure to exposure or well to well. To show the waybeds are folded, faulted, or tilted, a structural cross-section can be prepared (Fig. 3-51). In the structuralcross-section, the datum is a level line parallel to sealevel, and the tops and bottoms of rock units are plot-ted according to their true elevations. If the verticaland horizontal scales are similar, the attitude of thebeds will be correctly depicted. Many times, however,it is useful to have a larger vertical than horizontal scaleto emphasize geologic features.

Geologic Maps

Geologic maps show the distribution of rocks of dif-ferent kinds and ages that lie directly beneath the looserock and soil covering most areas of the surface (Fig.3-52). Assume for a moment that all of this loose mate-rial and the vegetation growing on it were miracu-lously removed from your home state, so that bedrockwould be exposed everywhere. Imagine, further, thatthe surfaces of the formations now exposed were eachpainted a different color and photographed verticallyfrom an airplane. Such a photograph would constitutea simple geologic map. In actual practice, a geologicmap is prepared by locating contact lines between for-mations in the field and then plotting these contacts ona base map (Fig. 3-53A and B). Symbols are added tothe colored areas to indicate formations and lithologicregions, mineral deposits, and structures such as folds

FIGURE 3-54 Paleogeographic map of Ohio andadjoining states during an early part of theMississippian Period. The data for this study wereobtained from outcrops and over 40,000 well records. (AfterPepper, J. F., de Witt, W. J., and Demarest, D. F. 1954. U.S.Geological Survey Professional Paper 259.)

FIGURE 3-55 Stages in the construction of apaleogeographic map. (A) Area of occurrence of aparticular time-rock unit. (B) Plot of rock types within thetime-rock unit. (C) Paleogeographic reconstruction.


and faults. Once the geologic map is completed, a geol-ogist can tell a good deal about the geologic history ofan area. The formations depicted represent sequential“pages” in the geologic record. From the simple geo-logic map shown in Figure 3-53C, the geologist is ableto deduce that there was an ancient period of compres-sional folding, that the folds were subsequently faulted,and that an advance of the sea resulted in deposition ofyounger sedimentary layers unconformably above themore ancient folded strata.

Paleogeographic Maps

A map showing the geography of a region or area atsome specific time in the geologic past is termed a pa-leogeographic map. Such maps are really interpreta-tions based on paleontologic and geologic data. Themajority of such maps show the distribution of ancientlands and seas (Fig. 3-54). Paleogeographic maps are,at best, of limited accuracy, because since seas advanceand retreat endlessly through time, the line drawn at thesea’s edge may represent an average of several shorelinepositions. They are nevertheless useful for showinggeneral geographic conditions within regions or conti-nents. To prepare a paleogeographic map, one wouldplot all occurrences of rocks of a given time interval on amap and enclose the area of occurrence in boundarylines (Fig. 3-55A and B). Areas of nonoccurrence maybe places of no deposition or places where deposits onceexisted but were subsequently eroded away. With thehelp of fossils, the nature of the sediments—that is,whether marine or nonmarine—is determined andplotted on the map. The final step is to complete the pa-leogeographic reconstruction (Fig. 3-55C).

Isopach Maps

Isopach maps are prepared by geologists to illustratechanges in the thickness of a formation or chronos-tratigraphic unit. The lines on an isopach map (Fig.

3-56) connect points at which the unit is of the samethickness. On a base map, the geologists plot the thick-ness of units as they are revealed by drilling or in mea-sured surface sections. Isopach lines are then drawn toconform to the data points. Ordinarily, the upper sur-face of the unit being mapped is used as the horizontalplane or datum from which thickness measurementsare made. An isopach map may be very useful in deter-mining the size and shape of a depositional basin, theposition of shorelines, and areas of uplift. Figure 3-57is an isopach map of Upper Ordovician formations inPennsylvania and adjoining states. The map indicates a

FIGURE 3-56 Diagramillustrating the construction of asimple isopach map in an area ofundeformed strata.

FIGURE 3-57 Isopach map of Upper Ordovicianformations in Pennsylvania and adjoining states. (AfterKay, M. 1951. Geological Society of America Memoir No. 48.)



FIGURE 3-58 Diagram illustrating the preparation of a lithofacies map from asubsurface time-rock unit. Well locations are indicated by small circles. Lithostratigraphiccorrelation of rock units between wells is indicated by dashed lines. Because of the few controlpoints, the exact position of lithofacies boundaries on this map is somewhat arbitrary. �? Whyis the ash bed a good datum for the cross-section?

FIGURE 3-59 Lithofacies map ofLower Silurian rocks in theeastern United States. (AfterAmsden, T. W. 1955. Bull Am AssocPetrol Geol 39:60–74.)

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G E O L O G Y O F N A T I O N A L P A R K S A N D M O N U M E N T S

In the year 1540, Hopi Indians di-rected a band of conquistadores to therim of the Grand Canyon of the Col-orado River. One can imagine theirsense of wonder as they gazed at thisstupendous natural spectacle. Awe-some in magnitude and beauty, thegreat chasm was unlike anything West-ern humans had ever witnessed. It re-mains so today, an awesome monu-ment to the erosive force of runningwater and gravitational downslopetransfer of solid rock and weathereddebris.

The historical geologist sees thelayers of sandstone, shale, limestones,and lava flows exposed in the canyonwalls as a great history book that re-veals geologic events and changing lifeover a span of 2 billion years. Rocks ofthe first chapter are sands and mudsthat record the presence of a shallowwater body in the region. Volcanoeserupted nearby, for the rocks are in-terbedded with layers of lava and vol-canic ash. About 1.7 billion years ago,mountain building deformed andmetamorphosed these sediments. Intheir altered state, they comprise theVishnu Schist (Figs. 3-52 and 7-13),seen close up by rafters passingthrough the canyon’s Inner Gorge. TheZoroaster Granite intrudes the VishnuSchist and can also be seen in thedeep clefts of the canyon. An intervalof erosion followed the emplacementof the Zoroaster Granite. Then high-silica melts invaded joints and frac-tures and formed veins and igneous

rock, which were later converted tolight-colored gneisses. The region wasmountainous at this time. Land plantshad not evolved, and there was little toretard the forces of erosion. Eventu-ally, the mountains were reduced tolowlands, and their cores can be seenin the canyon walls.

The next chapter in the GrandCanyon story is written in 3700 metersof sedimentary rocks and lava flowsspread extensively over the VishnuSchist. These are the rocks of theGrand Canyon Supergroup (Fig. 7-14).Following their deposition, the regionwas subjected to tensional forces thatproduced north-south trending fault-block mountains. The higher faultblocks became mountains. Debriseroded from these mountains filled in-tervening low areas where downfaultedblocks existed. The destructionalforces of erosion gradually reduced theentire region to a low-lying terrainrecorded in geologic history by “thegreat unconformity” that separatesPrecambrian from Paleozoic strata.Remnants of the once more extensiveGrand Canyon Supergroup are nestledin remaining downfaulted blocks be-neath the great unconformity.

Ascending the canyon, we reachrocks of the Paleozoic Era. The Paleo-zoic was a time of repeated inundationand regression of shallow seas. Thefirst of the inundations laid down thenearshore Tapeats Sandstone (Fig. 8-10). These sands were followed bythe Bright Angel Shale and Muav

Limestone as the shoreline shiftedeastward. The three formations com-prise the Tonto Group. Their sequentialchange in lithology illustrates the wayin which rock units may transgresstime boundaries (Fig. 8-11).

As if pages in our history book hadbeen ripped out, strata of Ordovicianand Silurian age are not found in theGrand Canyon. They may have beendeposited there, but if so, they havebeen lost to erosion. Thus, an uncon-formity (a gap in the stratigraphicrecord) caps the Cambrian sequence.Above that unconformity, carbonatesof the Devonian Temple Butte Lime-stone were laid down in a shallow sea.Again the sea withdrew, only to returnanother time during the MississippianPeriod. In this Mississippian sea,cherty carbonates of the Redwall Lime-stone were deposited (see accompany-ing photograph). Although freshly bro-ken surfaces of the Redwall Limestoneare gray, its weathered surface isstained red by iron oxide washed downthe face of the precipice from red bedsof the overlying Supai Group and Her-mit Shale. The Redwall forms boldcliffs that front many of the canyon’spromontories. It is richly fossiliferouswith the remains of brachiopods, bry-ozoans, crinoids, and corals.

The withdrawal of the Redwall Seais signaled by the presence of estuar-ian and tidal flat sediments that arepart of the Surprise Canyon Formationnear the top of the Redwall. Abovethese sediments of transitional envi-

Grand Canyon National Park

View from the Nankoweap Indian Site of theMississippian Redwall Limestone, GrandCanyon of the Colorado River, northwesternArizona. The Redwall is actually a bluishgray limestone containing chert nodules. Ittakes its colorful name from a coating ofred iron oxide stain derived from overlyingstrata. (Copyright C. C. Lockwood/EarthScenes.)

ronments, one finds yet another ero-sional unconformity, and above thatsurface lie strata of the Pennsylvanianand early Permian Supai Group. TheBuddha, Zoroaster, and other spectac-ular temples in the park are sculptedin the Supai. Nonmarine beds of theSupai exhibit tracks of amphibians,possibly those of reptiles, as well asimprints of ferns. In the overlying Her-mit Shale, one can see evidence ofnonmarine deposition in the forma-tion’s mud cracks and fossils of in-sects, conifers, and ferns. The flood-plains and marshy tracts on which thesediments of the Hermit Shale weredeposited were soon covered by mi-grating dunes of the Permian Co-conino Sandstone. The nearly whiteCoconino sands are cross-bedded.The frosted, well-sorted, well-rounded grains reflect an origin aswindblown sediment. Reptiles wan-dering across the dunes left their foot-prints in the sand.

Marine limestones and sandstonesof the Toroweap Formation rest on theCoconino. They record the advance ofa sea over the Coconino dune fields.Above the Toroweap are the bold verti-

and other deep gorges in eastern Ari-zona took only about 1.5 million years(a mere moment in geologic time).Thus, although the rocks of the canyonare very old, the great chasm itself isgeologically young.

The Grand Canyon reached its pres-ent depth not only as a result of the ero-sive power of running water but also be-cause of impact and abrasion by cobblesand gravels carried in the rushing cur-rents. Yet if the river alone was the onlymechanism for erosion, the canyonwould have vertical walls. Weatheringand gravitational mass movements oferoded debris downslope broadened thecanyon to its present width of over 17kilometers. The Colorado, like riverseverywhere, acts as a conveyor belt, car-rying its own load as well as the debrissupplied to it by slides, rockfalls, andother gravity-driven movements that wecall mass wasting.

There is much more geology in theGrand Canyon than can be describedin these few pages or that can be en-compassed in a short visit. If you arefortunate enough to go there, stay theday and watch the sun go down overthis magnificent colossus of canyons.

Location map.

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cal cliffs of the Kaibab Limestone. Thisthick and resistant formation forms thesurface of the Kaibab Plateau north ofthe canyon and the Coconino Plateauon the south side. The Kaibab is thefinal rock unit of the Paleozoic in theGrand Canyon. During the Mesozoic,floodplain sands and silts of the Trias-sic Moenkopi Formation and gravels ofthe Shinarump Conglomerate werespread across the region. Near theGrand Canyon, however, all but a fewremnants of these formations wereswept away by erosion.

The region now known as the Col-orado Plateau was tectonically upliftedlate in the Cenozoic. That uplift re-sulted in steeper stream gradients andincreased stream erosive powers. TheColorado River and its tributaries wereable to deepen their channels at veryrapid rates. Fifty years ago, most geolo-gists estimated about 7 million yearswere required to erode the GrandCanyon. Recent measurements of ratesof erosion along similar streams indi-cate that rivers having torrential flowcan erode incredibly rapidly. Geolo-gists involved in these studies now be-lieve that erosion of the Grand Canyon

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Generalized Geologic Column for Grand Canyon National Park. (From McKee, E. D. 1982. The SupaiGroup of the Grand Canyon, U.S. Geological Survey Professional Paper 1173.)

semicircular center of subsidence in southern NewYork and Pennsylvania in which over 2000 feet of sedi-ment accumulated. The isopach pattern further indi-cates a highland source area to the southeast.

Lithofacies Maps

Maps constructed to show areal variations in facies canprovide additional details and validity to paleogeo-graphic interpretations. Such graphic representationsare called lithofacies maps. Figure 3-58 is a hypothet-ical base map of an area subjected to exploratorydrilling by oil companies. The logs for the wells arealso shown. Geologists first correlate the formations.Then, assuming that the unconformity represents onetime plane and the ash bed another, they define thetime-rock unit as “X.” Paleontologic study of the rocksbetween the time planes confirms the validity of thetime-rock unit. Geologists may now prepare the litho-facies map. Time-rock unit X is missing at well number11; this may be the result of its not being depositedthere or, having been deposited, of its being erodedaway. It is logical that the sandy facies was depositedadjacent to a north-south trending shoreline.

Figure 3-59 is a lithofacies map of rocks depositedover 400 million years ago in the eastern United

States. From this map, one can infer the existence of ahighland area that existed at that time along our east-ern seaboard and that supplied the coarse clastics. De-trital sediments from the source area become fine, andthe section thins as one proceeds westward from thesource highlands. Finally, as far west as Indiana, themap indicates that only carbonate precipitates werelaid down. The conglomerates were probably the de-posits of great alluvial fans built out from the ancientmountain system.

The lithofacies maps just described provide aqualitative interpretation of areal changes in rockbodies. Quantitative lithofacies maps can also beconstructed and are frequently used in the study ofsubsurface formations that are known primarily fromwell records. By means of contour lines, such mapsshow the areal distribution of some measurable char-acteristic of the unit being mapped. For example,contours may be drawn on the percentage of onelithologic component (such as clay) compared to thetotal unit or on the ratio of one rock type (such assandstone) to the others within the unit. An isopachmap is ordinarily the base map for any of the quanti-tative maps, since one must know the total strati-graphic thickness of the unit with which individualcomponents are compared.

Sedimentary rocks represent the material record of environ-ments that once existed on the Earth’s surface. For this reason,they are of great importance to the science of historical geol-ogy. All sedimentary rocks are formed by the accumulation andconsolidation of the products of weathering derived from olderrock masses, as well as by chemical precipitation and the accu-mulation of organic debris. Because of their mode of formation,the composition of sedimentary rocks provides informationabout source areas. Rock color can provide clues to the chem-istry of the depositional medium. The materials of sedimentaryrocks are often transported by wind, water, or ice, or they arecarried in solution to be precipitated in a particular environ-ment of deposition. The transporting medium imparts charac-teristics of texture or composition that can be used to recon-struct the depositional history and tectonic setting. Fossils insedimentary rocks are splendid environmental indicators. Theytell us if strata are marine or nonmarine, if the water was deep orshallow, or if the climate was cold or warm.

In the case of clastic rocks, the size, shape, and arrange-ment of grains can provide data about the energy of the trans-porting agent, the distance the grains had traveled, erosionalrecycling, and the degree to which movements of the Earth’scrust had disturbed a basin of deposition. Similar kinds of in-formation are elucidated by such primary sedimentary struc-tures as graded bedding, cross-bedding, and current ripplemarks. Sandstones are particularly useful in paleoenviron-mental studies. Graywackes, arkoses, quartz sandstones, andlithic sandstones accumulate in particular paleogeographic

and tectonic situations. It is the geologist’s task to discoverthe details of those situations by examining the rocks.

Geologists usually divide successions of sedimentaryrocks into rock units that are sufficiently distinctive in color,texture, or composition to be recognized easily and mapped.Such lithostratigraphic units are called formations and arenot necessarily of the same age throughout their areal extent.A chronostratigraphic unit differs from a lithostratigraphicunit in that it is an assemblage of strata deposited within aparticular interval of time. The Cambrian System, for exam-ple, is a chronostratigraphic unit including all the rocks de-posited in the Cambrian Period.

Within any given chronostratigraphic unit, one may findrocks that vary in composition, texture, organic content, orother features from adjacent rocks. These rock bodies of dis-tinctive appearance or aspect are called facies. Facies reflectdeposition in a particular environmental setting. For exam-ple, along a coastline one may find nearshore sand facies thatchange seaward to shale facies and carbonate facies. As shore-lines shift landward or seaward, facies shift accordingly,maintaining their association with a particular local set of en-vironmental conditions.

Shoreline migrations may be the result of such factors asworldwide changes in sea level (eustatic changes), tectonicmovements of continental borderlands, or progradation—the seaward advance of the coastline resulting from rapid de-position of sediment brought to the sea by rivers. Prograda-tion is particularly evident in deltas.

S U M M A R Y


That branch of geology that deals with the origin, composi-tion, sequence, and correlation of stratified rocks is stratigra-phy. Stratigraphic correlation involves determining the equiva-lence of strata in diverse locations by the use of lithology, fossilcontent, radioisotopic age, depositional cycles, isotopic charac-teristics, or any other distinctive physical or chemical charac-teristics of the units to be correlated. Lithostratigraphic corre-lation links units of similar lithology and stratigraphic position,whereas biostratigraphic correlation expresses similarity of fos-sil content and biostratigraphic position. Chronostratigraphiccorrelation links units of corresponding age.

The study of facies is of great importance in the develop-ment of reconstructions of conditions on Earth long ago. Ge-ologists employ various graphic methods to record variationsin facies and other attributes of sedimentary rocks. Thesemethods include the preparation of lithofacies, biofacies, ge-ologic, and isopach maps. If examined in chronologic se-quence, such maps are useful not only in reconstructing an-cient geography but also in providing a picture of the Earth’schanging patterns of ancient lands and seas.

Web Sites � 103

1. What features in a sedimentary rock might indicate itwas deposited in each of the following environments of depo-sition?

a. Shallow marine environmentb. Deep marine, continental rise environmentc. Transitional, deltaic environmentd. Continental, desert environment

2. Why are sandstones and siltstones of desert environmentsrarely black or gray?3. How does matrix in a rock differ from cement? What arethe most common kinds of cements found in sandstones?Which of these is most durable?4. What would be the probable origin of a poorly sortedsandstone composed of angular grains in a 30 percent matrixof mud? What origin might you infer for a well-sorted sand-stone that contains fossils of marine clams, is composed al-most entirely of quartz, and has well-developed ripple marks?

5. What differences in texture and composition serve to dis-tinguish between a mature and an immature sandstone?6. In a columnar section of sedimentary rocks, a limestone isoverlain by a shale, which in turn is overlain by sandstone.What might this coarsening upward sequence indicate withregard to the advance or retreat of a shoreline?7. What conditions in the Bahama Banks carbonate platformresult in the high production of calcium carbonate sediment?8. What features of sedimentary rocks are useful in deter-mining the direction of current of the depositing medium?9. An isopach map shows an accumulation of 10,000 metersof sediments in a Paleozoic marine basin of deposition, yetthe sedimentary rocks contain fossils indicating deposition inwater no deeper than 200 meters. What has occurred in thebasin of deposition?10. In Figure 3-59, note the areas in which Lower Silurianrocks are absent. How do you account for their absence?

Q U E S T I O N S F O R R E V I E W A N D D I S C U S S I O N

Ager, D.V. 1973. The Nature of the Stratigraphic Record. Lon-don: Macmillan.

Boggs, S. Jr. 2001. Principles of Sedimentology and Stratigraphy,3d ed. Englewood Cliffs, New Jersey: Prentice Hall.

Blatt, H., and Tracy, R. J. 1996. Sedimentary Rocks, In:Petrology, 2d ed. New York: W. H. Freeman.

Hsu, K. J. 1989. Physical Principles of Sedimentology: A ReadableTextbook for Beginners and Experts. New York: Springer-Verlag.

Lemon, R. R. 1990. Principles of Stratigraphy. Columbus,OH: Merrill Publishing Co.

Mackenzie, F. T. 1998. Our Changing Planet. 2d ed. New Jer-sey: Prentice-Hall Inc.

McLane, M. 1995. Sedimentology. New York: Oxford Univer-sity Press.

Pettijohn, F. J. and Potter, P. E. 1964. Atlas and Glossary ofPrimary Sedimentary Structures. New York: SpringerVerlag.

Prothero, D. R., and Schwab, F. 1996. Sedimentary Geology.New York: W. H. Freeman.

Scholle, P. A., Bobout, D. G., and Moore, C. H. 1983. Car-bonate Depositional Environments. Tulsa, OK: AmericanAssociation of Petroleum Geologists.

Tucker, M. E. 1981. Sedimentary Petrology. New York: JohnWiley & Sons.

R E A D I N G S

The Earth Through Time Student Companion Web Site(www.wiley.com/college/levin) has online resources to helpyou expand your understanding of the topics in this chapter.Visit the Web Site to access the following:1. Illustrated course notes covering key concepts in each

chapter;2. Online quizzes that provide immediate feedback;

3. Links to chapter-specific topics on the web;4. Science news updates relating to recent developments in

Historical Geology;5. Web inquiry activities for further exploration;6. A glossary of terms;7. A Student Union with links to topics such as study skills,

writing and grammar, and citing electronic information.

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