* --- ./ ''WW .J

sJuaurFpaS jo, uo~~afnurnmv put! sJuawIwo.qAua lauop~sodaa

Glacial till

the sequence of deposition at the site and can provide a three-dimensional picture of the deposit. Similarly, geologists who wish to examine a meandering river's depositional record must either dig one or more pits in the valley floor adjacent to the channel or sample the floor by means of coring.

While some sedimentary features provide highly reliable information about the nature of the environments in which they formed, many others offer ambiguous testimony. Most coal deposits, for example, represent swamps choked with vegetation-but such swamps are typical of both the banks of rivers and the shores of marine lagoons. To determine which environment pertains, geologists must evaluate the beds that lie above or below coal deposits. If sediments containing fossils of marine animals are found, it is likely that a marine lagoon, not a river, lay adjacent to the swamp or marsh in which the coal fonned. Such vertical stacking of sedimentary features often provides the key to recognizing ancient environments.

Nonmarine Environments We will begin our exploration of sedimentary clues to ancient environments by considering depositional set- tings on the surfaces of continents. Then our focus wiU shift to the margins of continents, and finally to the ma- rine realm.

Ancient soils can point to past climatic conditions Soil can be defined as loose sediment that contains or- ganic matter and has accumulated in contact with the atmosphere rather than under water. Soil rests either on sediment of some sort-such as sand or mud--or on rock. It supplies essential nutrients and provides phys- ical support for plants, and it serves as a habitat for many organisms. Soils develop largely through weathering processes and decay of plant material. Topsoil, the up- per zone of many soils, consists primarily of sand and clay mixed with humus. Humus, the organic matter that gives topsoil its dark color, is derived from the decay of plant debris by bacteria and in turn supplies nutrients for other plants; thus it has an important position in the cycling of materials through terrestrial ecosystems.

Soils form in a variety of environments through- out the world-in tropical rain forests, in arid regions, and even on mountaintops-and can often be found buried, albeit chemically altered, within thick se- quences of ancient sediment. The type of soil that forms depends in part on climatic conditions. For ex- ample, in warm climates that are dry part of the year, evaporation of groundwater causes calcium carbonate

CHAPTER 5 . Sedimentary Environrnen

to precipitate as nodular or massive deposits kno as caliche (Figure 5-1). In moist tropical c warm waters percolate through the soil, des humus by oxidizing its components (combining th with oxygen). Silicate minerals in such areas also br down quickly, producing laterites: soils rich in ir oxides, which give them a rusty red color, as wel4 aluminum oxides.

Most soils are destroyed by erosion. In addition, cient, buried soils can be exceedingly difficult to r nize and to interpret, partly because their original ical components are often altered beyond recogm One place where an ancient soil is likely to be fo however, is beneath an unconformify. Ancient soils most likely to be found beneath unconformities that h_ been recognized on the basis of other criteria. Plant r also provide clues to the presence of ancient soils, S ilarly, burrows made by animals such as insects and dents are diagnostic features. Certainly the most u of these excavations are the structures known as '

A

Plant debris Plant

Caliche nodule B

Figure 5- 1. Nodules of caliche in soil. The white nodules of calcium carbonate (A) formed around plant roots, as shown in the drawing (B). (A, Lawrence Hardie, Johns Hopkins University.)

jtoro

Note

Start here again

the current is swiftest. On the inside of the bend, the current is so weak that sediment is deposited rather than eroded, and it accumulates there to form a point bar (Figure 5-16A).

Nonmarine Environments

5- 1 4 A braided stream In Alaska. Bars of t-klivide the flow into numerous winding channels. The d bedlment emerge from the base of a melting glacier

isout of View. (Tom Bean.)

ed streams also form in front of mountain glaciers ence substantial melting in summer, as the are choked with sand and grave1 plowed up

owing ice (Figure 5-14).

ring rivers In gently sloping terrain far from .rivers generally occupy solitary channels that ok and forth like ribbons (Figure 5-15). Unlike stt~ams, these meandering rivers-such as

sippi or Thames-are not choked with sedi- use sediment is supplied to them slowly in

o the rate at which the water flows. Any ir- in the local terrain causes the river's path to muse the water flows least rapidly near the 9 bend and most rapidly near the outside, the

to cut into the outer bank of the bend, where

5- 15 A meandering river. Sand forms point bars e insides of the bends. (Peter Kresan.)

Figure 5- 16 Deposition of sediment by a meandering river. A. At each bend, the river migates outward. The current flows most rapidly on the outside of a bend (long arrow), so the water cuts into the outer bank. On the inner side of the bend, where the current moves more slowly, sand accumulates to form a polnt bar. As the channel migrates outward, the point- bar sands advance over the coarser, cross-bedded sands deposited within the original channel. Pebbles often accumulate at the base of the channel. Muddy floodplain deposits, which form when the rlver floods its banks, migate in their turn over the point-bar sands, As a result of this shifting of depositional environments, coarse sediments at the base grade upward into fine sediments at the top. B. Cyclical meandering- river deposits of Devonian age in the Catskill Formation, south of Harrisburg, Pennsylvania. In each cycle, coarse-grained sandstone (light-colored, thickly bedded deposits), which formed in the river channel, wades upward into fine-gained sediments, which formed in floodplains. (B from F. J. Pettijohn and P. E. Potter, Atlas and Glossay of Primy Sedkmentay Stvwtures, Springer-Verlag Publishing Company, New York, 1964.)

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accumulate and- marsh .plants oeen grow,

The famous Mississippi River delta spills into the Gulf of Mexico in an area that is protected from strong wave action. As a result, the delta projects far out into the sea. Because construction of the delta from river- borne sediment has prevailed decisively over the de- structive forces of the sea, this type of delta is sometimes called a river-dominated delta. The growing portion, or active lobe, of such a delta is the site of the function- ing distributary channels (Figure 5-18). Many previously active lobes can be identified in the delta-plain portion of the Mississippi delta, and these lobes, which have been dated by the carbon 14 method (described in Chap- ter 6), provide a history of deltaic development during

1 . I , - 4 .I ' La '

sand body marine clay

Figure 5- 18 Structure of the active lobe of the modern Mississippi River delta. At this lobe (identified by the pink arrow on the map), sediments are being deposited in the shallow water where the river meets the sea. Here, as shown in the block diagram below, the river channel divides into many distributary channels. Swamp deposits accumulate between the distributaries. Bodies of sand accumulate in front of the distributaries where the river water meets the ocean and slows down. As the active lobe builds seaward, the distributaries extend over these sand bodies. (After H. N. Fisk et al,, 1. Sediment. Petrol. 2476-99,1954.)

CHAPTER 5 Sedimentary Environments

~igure 5-1 9 Lobes of the Mississippi \\

ment. A younger lobe eventually grows on top of abandoned lobe, with each consisting of the typical

of a river-dominated delta is limlted to at any gven time and because older expo easily, a delta can seldom be traced very

'1 I

\ e ~ a t o n Rouge delta. The active lobe is numbered 16. Older lobes that are now inactive are numbered I through 15, with I being the oldest. These older lobes are now subsiding, and some are already partially or entirely submerged. (After T. Elliott, in H. G. Reading, ed., Sediments y Environments and Facies, Blackwell Scientific Publications, Oxford 1978.) w q ~ m - 1 pt tb+lrraki J ~ ! f i

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- 5 hq.. -,- . ,qw -<j -.,. 2 8 100 miles * I , 1!.1,141. .

$h..

iu.4J. : --- G- . the past few thousand years (Figure 5-19). Depos ,q!:y8. - activity (or lobe growth) periolcally slzifted when 4. ; , l L , ,

(. J: L' , caused the river to cut a new channel and to aband~ - . C

I ' 1 :wy:,,% w.2 ) - r ' .' . . - I.

the previously active channel and its distributaries. ,&:.$&7v ; I . The fate of an abandoned delta lobe is the key

Top of cycle miss--, , ,

- - -

" first, the sediments of which it is formed compact u der their own weight, and second, the lobe is part the entire delta structure, which is constantly sinlung a result of the isostatic response of the underlying cn to the weight of the continually growing mass of sec

sands with erosional base; - also delta-Front sands

- , &

ward-coarsening sequence. The result is an accuinu tion of sedimentary cycles that &ffer markedly frc . 'OMPLETE those of meandering rivers, whch, as you will recall, E

CYCLE come finer-grained toward the top.

Some deltaic cycles in the rock record lack tops I: cause sediment was eroded awav before another cvc

.< i' D:&~f@fit.dts and days

the rock record. When preserved, the pol ies in the uppbr parts of deltaic cycles m a g reservoirs for petroleum or natural gas.

Changes in the rate at whlch a delta siiilzs or is s u F plied with sediment can alter the size of the active Iobw Today such changes are causing the active lobe of &e Mississippi delta to shrink rapidly, with alarming con- sequences. The building of levees along the Mississippi River and of dams along its tributaries have reduced &B rate of deposition of sediment on the Mississippi delw In addtion, the removal of groundwater for human use in the region of the delta has increased the rate of s u b ,

sidence of the delta fivefold. Because of these two faw tors, the Louisiana coast is now losing about 100 square

Fipm 5-20 Five deltaic cycles. Cycle 3 is a complete cycle; within it prodelta clays grade upward to delta-plain sands. Each cycle represents an accumulation of sediments resulting from the seaward growth of a deltaic lobe.

d!tMqdne and Open- f4h ' e1b~ i~~~ment s

J, , --.:- , I T , , ? l ,I, 1 1

,lie behindrbiWier LPmds of sand

are weak, and there are few tidal channels or so large lagoons lie behind these Texas barrier islands.

deltas within the lagoon. Other depositional environ- ments are also found along the shores of lagoons. Among them are tidal flats-formed of sand or muddy sand-whose surfaces are alternately exposed and flooded as the tide ebbs and flows. High in the inter- tidal zone, above the barren tidal flats, marshes fringe one or both margins of many lagoons. Here plant de- bris accumulates rapidly and decomposes to form peat or, after long burial, coal.

Fresh water from rivers and streams tends to remain trapped in coastal lagoons for some time, so the waters of lagoons in moist climates are often brackish. The salin- ity of these waters at any given time depends on the rate of freshwater runoff from the land, which varies during the course of the year. Laguna Madre of Texas is typical of lagoons found in warm, arid climates (see Figure 5-21). The ponded waters of this long lagoon are hyper- saline because they receive little fresh water from rivers and suffer a high rate of evaporation. Whether lagoons are brackish or hypersaline, their abnormal and fluctuat- ing salinity excludes many forms of marine life. As a re- sult, the fossil faunas in the ancient sediments of lagoons are not very diverse; those species that are present, how- ever, often occur in large numbers. Usually among them are burrowers, such as segmented worms, that disturb the muddy lagoonal sediments, leaving them mottled and largely devoid of bedding structures.

When a barrier island-lagoon complex receives sed- iment at a sufficiently high rate, it progrades-that is, it migrates seaward-like the active lobe of a delta. Un- like the migration of a delta, however, this progradation takes place along a broad belt of shoreline (Figure 5-22). As the shoreline of a sea migrates seaward, marsh and

v Marsh peat

I Sea level

PROGRADATION

Figure 5-22 The stratigraphic sequence produced when a barrier island-lagoon complex progrades. Sedimems of the lagoon and of the adjacent marshes and tidal flats are superimposed on beach sands of the barrier island.

tidal-flat deposits prograde over sediments of the lagoon and its associated tidal channels. AU of these sediments in turn build out over deposits of the barrier islands and over the tidal deltas and marshes behind them. Thus the horizontal sequence of depositional environments (barrier island, marsh or tidal delta, lagoon, tidal flat, and marsh) comes to be represented by a correspon- ding vertical sequence of sedimentary deposits, in ac- cordance with Walther's law.

Open-shelf deposits include tempestites Seaward of barrier islands, continental shelves display a variety of physical conditions and therefore produce a variety of sedimentary deposits. On open shelves where tides produce strong currents and sand is abun- dant, the currents may pile the sand into large ridges or dunelike structures, 0d shelves where waves have a stronger effect than tidal currents, wave motion tends to flatten the bottom, and the sand spreads out in sheets.

On quieter shelves, mud or muddy sand accumu- lates most of the time, but storms occasionally produce tempestites, which are sandy beds that are usually a few centimeters thick. A storm that pounds a coastline may produce waves that pile up water carrying sand that they have scoured from the shallow seafloor. After the storm passes inland, the piled-up water flows seaward, and as it loses velocity, deposits the suspended sediment on the shelf in the form of a tempestite. The occasional deposition of tempestites on a normally quiet shelfpro-

Figure 5-23 Tempestite beds of Ordovician age in Kentucky. The coarse-grained sediment in the lower part of this photograph was deposited by strong water movements produced by a storm along the shelf of an inland sea. (Frank R. Ettensohn.)

CHAPTER 5 Sedimentary Environ

duces a succession of thin sandy beds in a body grained sediment (Figure 5-23). Individual te are commonly graded, having formed as sand before silt or mud.

Animal burrows are abundant in most shelf) ments, and as we will soon see, particular sils also point to an offshore, open marine e

Fossils serve as indicators of marine environments Ancient sediments deposited within and seaward rier island-lagoon complexes often yield fossils th geologists to recognize particular depositional ments. Figure 5-24 depicts an example in Wales, of central England. Here, in rocks of mid-Silurian~ fossil communities of marine invertebrates are roughly parallel to the ancient shoreline. Adja that shoreline is a narrow zone of fine-graine ments, in which the inarticulate brachiopod Lin especially abundant, but only a few other spec present (Figure 5-24A). Presumably the water was brackish: Lingula, a living fossil genus (see F 3-30B), today tolerates nearshore environm brackish and variable salinity where few other sp are able to live.

Seaward of the zone where Lingula predornina in the Silurian deposits is a more diverse fossil c munity, adapted to the more stable conditions center and seaward margin of a lagoon (Figure 5- Sandy deposits representing a barrier island have been preserved, but we can infer that one was pr because seaward of the lagoonal deposits are sandy, cross-stratified marine deposits in which the most co mon fossil is a type of brachiopod that was adapted such agitated conditions (Figure 5-24C).

In finer-grained sediments deposited fa offshore is a fossil community that includes species, none very common. Many types of chiopods are present, and trilobites are restricte this belt. The high diversity of species reflects the ble conditions of an offshore shelf environment, beyond the influence of river water; the low abun* dance pf species reflects a weak food supply, far from the algae and primitive plants that must have flour- ished in the vicinity of the lagoon and supplied its in- habitants with food. Where muddy sediments occur in this area, planktonic graptolites-frag~le colonial an- imals that settled to the quiet seafloor after death-zute preserved. (Figure 5-240).

Fossils have played a key role in the reconstruction of this set of Silurian environments. In the next section, we will examine deposits that are composed entirely of recognizable fossils and fossil debris. 1

a1 Marine and Open-Shelf Environments

Figure 5-25 A typical barrier reef. The reef grows up to sea level, so the reef flat is exposed at low tide. Waves break across the reef flat, losing energy in the process and leaving a quiet lagoon behind. Sediment accumulates in the back-reef area and in the lagoon, and here and there patch reefs rise, I from the laeoon floor.

CHAPTER 5 Sedimentary hvi

I

, i i

Figme 5-26 Outcrop af6ngMindjana Gorge, northwestern Australia, revealing the internal structure of a hzy'qniah reef. m e reef core consists of unbedded limestone. The ralus is crudelybedded, and ilie~bds:sI6pe away fmm the reef core. The back-reef strata are also crudely bedded, but the beds are approximately horizomal. (P. E. Playford, Geological Survey of Western Australia.)

ine and Open-Shelf Environments

Figure 5-27 A barrier reef. The open sea a on the right, the lagoon on the left. (Ralph and Daphne

thrive. Horseshoe-

Volcano rises from

5-28 Development of a typical coral atoll in the Pacific. A-D. The stages of

CHAPTER 5 Sedimentary Environm

Reefs and carbonate platforms are

warm water than in cold water. (You m that a warm bottle of carbonated sod

.I 1

CO2 + HzO = H&03

rid "e 5-29 A late Paleozoic horseshoe-shaped atoll The cooling of water in contact with the atmo-

lies almost a kilometer (0.6 mile) below the surface of sphere increases the concentration of dissolved carbon the land in Texas. The atoll was discovered when rocks of dioxide in the water, driving this chernicd reaction to the region were drilled for petroleum. The reef appears to have the right and increasing the concentration of carbonii? faced prevailing winds from the south. (After P. T. Stafford, U.S. acid. Similarly, warming the water removes carbon diox- Geological Survey Professional Paper no. 315-A, 1959.) ide from solution, decreasing the concentration of car-

bonic acid. Carbonic acid breaks down calcium carbonate to

porous reef rocks often serve as traps for petroleum, fom ions of calcium and bicarbonate, which remain in drilling in the vicinity of these atolls is often profitable. solution: Figure 5-29 shows the outline of a subsurface atoll of late Paleozoic age that has yielded considerable quan- H2C03 + CaC03 = Ca2+ + 2HC03- tities of petroleum in the state of Texas.

An increase in the carbonic acid concentration 111 Carbonate platforms form in warm seas water causes more calcium carbonate to break down. An organic reef commonly forins part of a carbonate In complementary fashion, a decrease in the carbonic

Figure 5-30 The Bahama banks, which are now separated from FlorMa by the Straits of Florida. Here great thicknesses of sediment have accumulated over 170 million years. (After N. D. Newell and J. K. Rigby, Society of Econonomic Palaeontologists and Mineralogists, Special Paper no. 5, 1957.)

I' Marine and Open-Shelf Environments

onate. Thus, low levels of carbonic acid in warm pleal seas favor the growth of organic reefs and car- rate ~slatforms.

&e United States, but vbecause climates are coder by than they have been during most of Earth's his-

m s border the Antilles, and the phtfoms known .,bide Bahama Bank and Great Bahama Bank lie 1 sast and southeast of Florida (Figure 5-30), he varied sediments ~urrently accumulating on $hama banks resemble those of many ancient car- ephtforms. Here, as on carbonate platforms gen- isediments accumulate rapidly. Since mid-Jurassic &out 170 million years ago, some 10 kilometers I&) [of carbonates have accumulated both on the ~;z& banks and in southern Florida, which was part h e carbonate platform dwing Cretaceous time blier. This heavy buildup of sediments has caused ~an ic crust to subside, so that shallow-water Turas-

ah banks (p. 102). Oolites formed in this manner

Mat of cyanobacteria traps fine-grained carbonate sediment

1 Threadlike 1 I

cyanobacteria 2 -' &ow upward

, through the

/ Fl- 5-3 1 The g 6 W h bf dtkdmtitdit~s. A mat of cyanobacteria traps sediment, and'thb cyanobacteria'gow through it to form another layer. The accumulation of several layers leads to the formation of a stromatolite.

structures. Such mats can grow under water, but they are quickly eaten by gr&g marine animals and dam- aged by burrowers, so they seldom accumulate to form stromatolites or well-layered limestones. Exceptions are large column-shaped stromatolites that grow in st~btidal channels in the Bahamas where tidal currents are very strong. Few animals can survive in these currentiswept areas, so stromatolites flourish there. Stromatolites also flourish in Shark Bay, Western Australia, where the wa- ters are hypersaline and animals are very rare (Figure 5-32). As we s h d see in laterqsghdpt~r-s, veq a@@ent stromatolites f ~ r m e d hexie~flx &@sea before tbe snigin of marine animals that feed; on qan"obactena.

kes ,cross-bedded oohtes in the rock record re- @@tors of shallow seafloors swept by strong

zc'back more than 3 billion vears. '

kil relatively few marine animds can survive in Whus there is little to interfere with the tendency $mats created by cyanobacteria to form layered

Figure 5-32 Living strornatolites. These forms, exposed at low tide, occupy the margin of Shark Bay, a lagoon in Western Australia where hypersaline waters allow few animals to survive. (Paul F. Hoffman.)

Stromatolites are not the only indicators of ex- treme environments near shore. Mudcracks are as telling here as they are on continental interiors. Ridges of sediment that build up along tidal channels in the Bahama banks dry out after occasional flooding by storm-driven seas. As a result, the surface of the sed- iment here is broken by mudcracks resembling those that often form when a mud puddle on land dries up. Mudcracks associated with ancient marine deposits usually represent intertidal or supratidal environ- ments that were alternately wetted by the sea and dried by the sun.

Deep-Sea Environments To reconstruct the distribution of continents and ocean basins for any time interval in the past, geologists must identrfy not only nearshore deposits of the various kinds just described, but also deposits of deep-water envi- ronments beyond continental shelves. Coarse clastic de- posits derived from a continental shelf accumulate along its base, but fine-grained sediments predominate in the middle of a huge ocean, far from sources of clastic sed- iments, and they accumulate very slowly.

Turbidites flow down submarine slopes One of the most remarkable advances in sedimentology took place in the middle of the twentieth century with the recognition that certain sedimentary rocks have

CHAPTER 5 Sedimentary En\riro

been produced by turbidity currents. A turbidity rent is a flow of dense, sediment-charged water ing down a slope under the influence of gravity.

Turbidity currents were first noticed in where flows formed from mu the lake floor. These distance, slowing and dropping their sedirnentio they reached gentler slopes and spread out shore. In the 1930s the Dutch geologist Philip demonstrated in the laboratory that turbidity c can attain great speed, especially when they are laden with sediment and moving down steep slopesiL iment suspended in a turbidity current behaves ,as of the moving fluid, and its presence increases !the< sity of the fluid by as much as a factor of 2.

When the slope beneath a turbidity current to flatten out, the current ping its sediment in the seen again and again: suspension, and then, much later, the lows. The result is a graded bed of poorly sorted sand and granules at its base at the top. Such a graded bed is known as a (Figure 5-33A).

Large turbidity slopes and deposit turbidites along on the abyssal plain. Turbidity c near the edge of the contin sediment to the deep sea but also ero tinental slope and part of the continental rise.

, -- -. .-. I B

Fi- 5-33 Ordovician turbidites in New York State. A. A turbidite bed that grades from coarse at the bottom to fine at the top; the upper surface is irregular because it was disturbed by the succeeding turbidity current and distorted by subsequent compaction. B. The bottom surface of a turbidite bed, showing "sole marks" produced when the sediment forming the bed filled depressions that the turbidity current depositing it scoured in the preexisting sediment surface. Most scour marks taper toward the upper left, indicating that the current flowed in that direction. (Earle F. McBride.)

CHAPTER 5 Sedimentary Envi

Calcareous oozes Deep-sea day Glacial sediments

Siliceous oozes Terrigenous sediments Continental-margin . . sediments

A B

F i g w e 5-35 The global pattern of deep-sea sediments. A. Calcareous oozes are restricted to low latitudes. Most areas of siliceous ooze lie closer to the poles, although some occur close to the equator in areas of the Pacific and Indian oceans. B. Calcareous ooze composed of planktonic foraminifera. (A after T. A. Davies and D. S. Gonline, in J. P. Riley and R. Chester, eds., Chemfcul Oceamgruphy, Academic Press, London, 1976; B, Heather AngeVBiofotos.)

(see Figure 3-16B), and radiolarians, which are single- celled planktonic protozoans related to foraminifera (see Figure 3-19). Recall that the skeletons of both di- atoms and radiolarims consist of a soft form of silica called opal, which tends to recrystallize so that individ- ual skeletons canoot be discerned (see Figure 2-21). In the process, the rack that they form becomes a dense, hard chert (see Figqre 2-20). Before recrystallization, the so& sediment, con~isting largely of diatoms, i s known as

How does vertiqal stacking of distinctive types of strata prodde ,dIues to environments of depsition'? Sometimes one kind of rock alone serves to iden* an ancient environ~ent. Usudy, however, suites of closely associated rock %es are required for this purpose. These deposits dk\pcm-miody organized into cycles in which, in accordance with Walther's law, one kind of sediment tends to lie above another that accumulated in an adjacent environment.

What sedimentany features result from deposition in particular nonmarine environments? ,

Ancient soils resembling those of the modem world are sometimes found beneath unconformities, although

diatomaceous earth-the abrasive component, @ scouring powders.

Thick bodies of diatomaceous sedxaent h in marine areas of strong upwelling. Diatoms ist until late in Mesozoic time, however, an come to an important point: the compos sediments has changed markedly in the logic time as groups of sediment-contribu have waxed and waned within the pelagic

they may be hard to identifj because teration. Some types of ancient soils reflect#& conditions under which they formed. which are'huch less common than,marine d characterized by thin horizontal layers, and an absence of marine fossils. G1 over the surface of the land, often leave a suite of features, including sco surfaces, poorly sorted gravelly se ated lake deposits that exhibit annu basins, erosion of the surroun gravelly alluvial fans. Braided s fans toward the basin center els and sands. Beyond these low lakes and salt flats wh Some arid basins also contain dunes of clean, cro

e the distinctive features of marginal and continental-shelf deposits? a river meets a lake or an ocean, it drops its tary load to form a delta. Deltaic deposition

nonmarine settings do gravelly sediments

g rivers, the kind produced by deltas, and roduced by turbidity currents.

a profile of a barrier island-lagoon complex, the various depositional environments. features typlfy sediments that accumulate in

ported by rigid internal organic frameworks. Coral reefs form parts of many carbonate platforms, although these platforms contain a number of other deposits as well. On continental shelves, storms occasionally produce thin sandy beds known as tempestites.

What are the characteristics of deep-sea sediments? Beyond the edge of the continental shelf, turbidity cur- rents intermittently sweep down continental slopes to the continental rise and abyssal plain, where they spread out, slow down, and deposit graded be& of sediment, known as turbidites. StilI farther frorh continental shelves, only he-grained sediments accumulate. Clay reaches these deep-sea areas very slowly. In some areas the deposition of clay is far surpassed by the accumulation~of minute skeletons of planktonic marine life, which settle to the seafloor to form calcareous or siliceous oozes.

9. uescribe the kind of rock found in a typical organic reef.

10. Where is a lagoon in relation to a banier reef? Where is it in relation to an atoll?

11. Which features of carbonate rocks suggest intertidal or supratidal deposition? Which features suggest subtidal deposition?

12. What types of sediments and sedimentary structures usually reflect deposition in a deep-sea setting?

13. What are the important depositional environments of the basic kinds of sediment (such as mud, well- sorted sand, gravel, evaporites, and various kinds of limestone) and of particular sedimentary structures (such as cross-bedding, graded beds, mudcracks, and ripples)? How are the kinds of sediment and sedimentary structures found within each environment related to processes operating within it? Use the Visual Overview on page 104 for reference.