Sedimentary Rocks—The Archives of Earth History

Chapter 6

• Are formed at or near the surface at relatively low temperatures.

• Are formed from sediments which include boulders, cobbles, gravels, sands, silts, and clay particles.

• Also included as sediments are particles which are suspended and dissolved in water.

• Sedimentary rocks– preserve evidence of surface depositional processes

– also, many contain fossils– These things give clues to the depositional environment

• Depositional environments are specific areas – or environments where sediment is deposited

Sedimentary Rocks

• Depositional environments are specific areas or environments where sediment is deposited

• How do we know whether sedimentary rocks were deposited on – continents—river floodplains or desert sand dunes?– at the water's edge?– in the sea?

• Sand deposition • Sand-size particles are deposited on a

beach along the Pacific coast of the United States

• After many years and layers of deposition, sand is compacted, and eventually cemented to form sandstone.

• Many ancient sandstones – possess features that indicate they were

also deposited on beaches• Present day despositional environments

are used as models to help decode the rocks of the past. (uniformitarianism)

Beach Environment

• Sedimentary rocks may be– detrital– or chemical, including biochemical– and all preserve evidence – of the physical, chemical and biological processes – that formed them

• Some sedimentary rocks are a natural resource for– phosphorous– liquid petroleum– natural gas

Sedimentary rocks

• Observation and data gathering

– carefully examine • textures• composition• fossils (if present)• thickness• relationships to other rocks

• Preliminary interpretations in the field– For example:

• red rocks may have been deposited on land• whereas greenish rocks are more typical of marine

deposits• (caution: exceptions are numerous)

Investigating Sedimentary Rocks

• Very common minerals in detrital rocks:– quartz, feldspars, and clay minerals

• Detrital rock composition tells – about source rocks, – not transport and deposition

• Quartz sand may have been deposited– in a river system– on a beach or– in sand dunes– Quartz is durable and can withstand water and wind

transport

Detrital Rocks

• Detrital grain size gives some indication – of the energy conditions – during transport and deposition

• High-energy processes – such as swift-flowing streams and waves – are needed to transport gravel

• Conglomerate must have been deposited – in areas where these processes prevail

• Sand transport also requires vigorous currents• Silt and clay are transported

– by weak currents and accumulate – only under low-energy conditions – as in lakes and lagoons

Grain Size

• Composition of chemical sedimentary rocks – is more useful in revealing environmental information

• Limestone is deposited in warm, shallow seas– although a small amount also originates in lakes or caves

• Evaporites such as rock salt and rock gypsum – indicate arid environments where evaporation rates were

high

• Coal originates in swamps and bogs on land

Chemical Sedimentary Rocks

• Sorting and rounding are two textural features – Sorting refers to the variation in size of particles – “well sorted” – particles are similarly sized– “poorly sorted” – particles vary in size within a rock

• Well-sorted material implies transport by water or wind.

• Poorly sorted material implies transport by ice, or debris flow

• Some sediments are not transported; they remain where deposited until lithified

Sorting and Rounding

• Rounding is the degree to which sediments have their sharp corners and edges worn away by abrasion

• Gravel in transport is rounded very quickly as the particles collide with one another

• Sand becomes rounded with considerably more transport

Rounding

• A deposit – of well rounded – and well sorted

gravel

Rounding and Sorting

• Angular, poorly sorted gravel

• Sedimentary structures are – features visible at the scale of an outcrop – that formed at the time of deposition or shortly

thereafter – and are manifestations of the physical and

biological processes that operated in depositional environments

• Structures – seen in present-day environments – help provide information about depositional

environments of rocks with similar structures

Sedimentary Structures

• Sedimentary rocks generally have bedding or stratification

Bedding

– Individual layers less than 1 cm thick are laminations

• common in mudrocks

– Beds are thicker than 1 cm

• common in rocks with coarser grains

• Some beds show an upward gradual decrease – in grain size, known as graded bedding

Graded Bedding

• Graded bedding is common in turbidity current deposits– which form when

sediment-water mixtures flow along the seafloor

– As they slow, – the largest

particles settle out then smaller ones

• Cross-bedding forms when layers come to rest – at an angle to the surface – upon which they accumulate– as on the downwind side of a sand dune

• Cross-beds result from transport – by either water or wind

• The beds are inclined or dip downward – in the direction of the prevailing current

• They indicate ancient current directions, – or paleocurrents

• They are useful for relative dating – of deformed sedimentary rocks

Cross-Bedding

• Tabular cross-bedding forms by deposition on sand waves

Cross-Bedding

• Tabular cross-bedding in the Upper

Cretaceous Two Medicine Formation

in Montana

Cross-Bedding

• Trough cross-bedding formed by migrating dunes

• Trough cross-beds in the Pliocene Six

Mile Creek Formation, Montana

• Small-scale alternating ridges and troughs – known as ripple marks are common – on bedding planes, especially in sandstone

• Current ripple marks – form in response to water or wind currents – flowing in one direction– and have asymmetric profiles allowing geologists – to determine paleocurrent directions

• Wave-formed ripple marks – result from the to-and-fro motion of waves– tend to be symmetrical

• Useful for relative dating of deformed sedimentary rocks

Ripple Marks

• Ripples with an asymmetrical shape

• In the close-up of one ripple, – the internal structure – shows small-scale

cross-bedding• The photo shows

current ripples – that formed in a

small stream channel – with flow from right

to left

Current Ripple Marks

• As the waves wash back and forth, – symmetrical

ripples form

• The photo shows wave-formed ripple marks – in shallow

seawater

Wave-Formed Ripples

• When clay-rich sediments dry, they shrink – and crack into polygonal patterns – bounded by fractures called mud cracks

• Mud cracks require wetting and drying to form,

Mud Cracks

– as along a lakeshore

– or a river flood plain

– or where mud is exposed at low tide along a seashore

• Mud cracks in ancient rocks – in Glacier

National Park, Montana

• Mud cracks typically fill in– with sediment – when they are

preserved– as seen here

Ancient Mud Cracks

• Biogenic sedimentary structures include– tracks– burrows– trails

• called trace fossils• Extensive burrowing by organisms

– is called bioturbation

• It may alter sediments so thoroughly – that other structures are disrupted or destroyed

Biogenic Sedimentary Structures

• U-shaped burrows

Bioturbation

• Vertical burrows

Bioturbation

• Vertical, dark-colored areas in this rock are sediment-filled burrows– Could you use burrows such as these to relatively

date layers in deformed sedimentary rocks?

• Sedimentary structures are important – in environmental analyses– but no single structure is unique to a specific

environment• Example:

– Current ripples are found• in stream channels• in tidal channels• on the sea floor

• Environmental determinations – are usually successful with– associations of a groups of sedimentary structures– taken along with other sedimentary rock properties

No Single Structure Is Unique

• Some Sediments are extensive “sheets” deposited during marine transgressions or regressions.

• Delta deposits tend to be lens shaped – when viewed in cross profile or long profile– but lobate when observed from above

• Buried reefs are irregular – but many are long and narrow – or rather circular

Geometries

• Fossils– are the remains or traces of prehistoric organisms– can be used in stratigraphy for relative dating and

correlation– are constituents of rocks, sometimes making up the

entire rock– and provide evidence of depositional environments

• Many limestones are composed – in part or entirely of shells or shell fragments

• Much of the sediment on the deep-seafloor – consists of microscopic shells of organisms

Fossils—The Biological Content of Sedimentary Rocks

• This variety of limestone, – known as

coquina, – is made entirely

of shell fragments

Fossils Are Constituents of Sedimentary Rocks

• Did the organisms in question live where they were buried?

• Or where their remains or fossils transported there?

• Example:– Fossil dinosaurs usually indicate deposition – in a land environment such as a river floodplain– But if their bones are found in rocks with – clams, corals and sea lilies, – we assume a carcass was washed out to sea

Fossils in Environmental Analyses

• What kind of habitat did the organisms originally occupy?

• Studies of a fossil’s structure – and its living relatives, if any, – help environmental analysis

• For example: clams with heavy, thick shells – typically live in shallow turbulent water– whereas those with thin shells – are found in low-energy environments

• Most corals live in warm, clear, – shallow marine environments where – symbiotic bacteria can carry out photosynthesis

Environmental Analyses

• A depositional environment – is anywhere sediment accumulates– especially a particular area – where a distinctive kind of deposit originates – from physical, chemical, and biological processes

• Three broad areas of deposition include– continental– transitional– marine– each of which has several specific environments

Depositional Environments

Depositional EnvironmentsContinental environments

Transitional environments

Marine environments

• Deposition on continents (on land) might take place in – fluvial systems – rivers and streams– deserts– areas covered by and adjacent to glaciers

• Deposits in each of these environments – possess combinations of features – that allow us to differentiate among them

Continental Environments

• Fluvial refers to river and stream activity – and to their deposits

• Fluvial deposits accumulate in either of two types of systems

• One is a braided stream system– with multiple broad, shallow channels – in which mostly sheets of gravel – and cross-bedded sand are deposited– mud is nearly absent

Fluvial

• The deposits of braided streams are mostly – gravel and cross-bedded sand with subordinate mud

Braided Stream

• Braided stream deposits consist of – conglomerate– cross-bedded

sandstone– but mudstone is rare

or absent

Braided Stream Deposits

• The other type of system is a meandering stream– with winding channels– mostly fine-grained sediments on floodplains– cross-bedded sand bodies with shoestring

geometry– point-bar deposits consisting of a sand body – overlying an erosion surface – that developed on the convex side of a meander

loop

Fluvial Systems

• Meandering stream deposits

Meandering Stream

– are mostly fine-grained floodplain – sediments with subordinate sand bodies

• In meandering stream deposits,– mudstone deposited in a

floodplain is common– sandstones are point bar

deposits– channel conglomerate is

minor

Meandering Stream Deposits

• Desert environments contain an association of features found in – sand dune deposits, – alluvial fan deposits,– and playa lake deposits

• Windblown dunes are typically composed – of well-sorted, well-rounded sand – with cross-beds meters to tens of meters high– land-dwelling plants and animals make up any

fossils

Desert Environments

• A desert basin showing the association – of alluvial fan, – sand dune, – and playa lake deposits

• In the photo, – the light colored area in the

distance– is a playa lake deposit in

Utah

Associations in Desert Basin

• Large-scale cross-beds – in a Permian-aged – wind-blown dune

deposit in Arizona

Dune Cross-Beds

• Alluvial fans form best along the margins of desert basins – where streams and debris flows – discharge from mountains onto a valley floor – They form a triangular (fan-shaped) deposit – of sand and gravel

• The more central part of a desert basin – might be the site of a temporary lake, a playa lake, – in which laminated mud and evaporites accumulate

Alluvial Fans and Playa Lakes

• All sediments deposited in – glacial environments are collectively called drift

• Till is poorly sorted, nonstratified drift – deposited directly by glacial ice– mostly in ridge-like deposits called moraines

• Outwash is sand and gravel deposited – by braided streams issuing from melting glaciers

• The association of these deposits along with – scratched (striated) and polished bedrock – is generally sufficient to conclude – that glaciers were involved

Glacial Environments

• Moraines and poorly sorted till

Moraines and Till

• Origin of glacial drift

• Glacial lake deposits show – alternating dark and light laminations

• Each dark-light couplet is a varve, – representing one year’s accumulation of sediment– light layers accumulate in summer– dark in winter

Glacial Varves

• Dropstones – liberated from

icebergs – may also be

present– Varves with a

dropstone

• Transitional environments include those – with both marine and continental processes

• Example:– Deposition where a river or stream (fluvial system) – enters the sea – yields a body of sediment called a delta – with deposits modified by marine processes,

especially waves and tides• Transitional environments include

– deltas– beaches– barrier islands and lagoons– tidal flats

Transitional Environments

Transitional Environments

Transitional environments

Simple Deltas

– topset beds– foreset beds– bottomset

beds

• The simplest deltas are those in lakes and consist of

– As the delta builds outward it progrades

– and forms a vertical sequence of rocks – that becomes coarser-grained from the bottom to top– The bottomset beds may contain marine (or lake) fossils, – whereas the topset beds contain land fossils

• Marine deltas rarely conform precisely – to this simple threefold division because – they are strongly influenced – by one or more modifying processes

• When fluvial processes prevail – a stream/river-dominated delta results

• Strong wave action – produces a wave dominated delta

• Tidal influences – result in tide-dominated deltas

Marine Deltas

• Stream/river-dominated deltas – have long

distributary channels

– extending far seaward

– Mississippi River delta

Stream/River-Dominated Deltas

• Wave-dominated deltas – such as the Nile

Delta of Egypt– also have

distributary channels

– but their seaward margin

– is modified by wave action

Wave-Dominated Deltas

• Tide-Dominated Deltas, – such as the Ganges-Brahmaputra delta

Tide-Dominated Deltas

– of Ban-gladesh

– have tidal sand bodies

– along the direction of tidal flow

• On broad continental margins – with abundant sand, long barrier islands lie offshore – separated from the mainland by a lagoon

• Barrier islands are common along the Gulf – and Atlantic Coasts of the United States

• Many ancient deposits formed in this environment

• Subenvironments of a barrier island complex: – beach sand grading offshore into finer deposits– dune sands contain shell fragments

• not found in desert dunes– fine-grained lagoon deposits – with marine fossils and bioturbation

Barrier Islands

• Subenvironments of a barrier island complex

Barrier Island Complex

• Tidal flats are present – where part of the shoreline is periodically covered – by seawater at high tide and then exposed at low tide

• Many tidal flats build or prograde seaward – and yield a sequence of rocks grading upward – from sand to mud

• One of their most distinctive features – is sets of cross-beds that dip in opposite directions

Tidal Flats

• Tidal-flat deposits showing a prograding shoreline– Notice the distinctive cross-beds – that dip in opposite directions – How could this happen?

Tidal Flats

• Marine environments include:– continental shelf– continental slope– continental rise– deep-seafloor

• Much of the detritus eroded from continents – is eventually deposited in marine environments

• but sediments derived from chemical – and organic activity are found here as well, such as

• limestone• evaporites• both deposited in shallow marine environments

Marine Environments

Marine Environments

Marine environments

• The gently sloping area adjacent to a continent – is a continental shelf

• It consists of a high-energy inner part that is – periodically stirred up by waves and tidal currents

• Its sediment is mostly sand, – shaped into large cross-bedded dunes

• Bedding planes are commonly marked – by wave-formed ripple marks

• Marine fossils and bioturbation are typical

Detrital Marine Environments

• The low-energy part of the shelf – has mostly mud with marine fossils, – and interfingers with inner-shelf sand

• Much sediment derived from the continents – crosses the continental shelf – and is funneled into deeper water – through submarine canyons

• It eventually comes to rest – on the continental slope and continental rise – as a series of overlapping submarine fans

Slope and Rise

• Once sediment passes the outer margin – of the self, the shelf-slope break, – turbidity currents transport it

• So sand with graded bedding is common

• Also common is mud that settled from seawater

Slope and Rise

• Shelf, slope and rise environments• The main avenues of sediment transport

– across the shelf are submarine canyons

Detrital Marine Environments

Turbidity currents carry sediment to the submarine fans

Sand with graded bedding and mud settled from seawater

• Beyond the continental rise, the seafloor is– nearly completely covered by fine-grained deposits

• no sand and gravel

– or no sediment at all • near mid-ocean ridges

• The main sources of sediment are:– windblown dust from continents or oceanic islands– volcanic ash– shells of microorganisms dwelling – in surface waters of the ocean

Deep Sea

• Types of sediment are:– pelagic clay,

• which covers most of the deeper parts

• of the seafloor

– calcareous (CaCO3) and siliceous (SiO2) oozes

• made up of microscopic shells

Deep Sea

• Carbonate rocks are – limestone, which is composed of calcite– dolostone, which is composed of dolomite

• most dolostone is altered limestone• Limestone is similar to detrital rock in some

ways– Many limestones are made up of

• gravel-sized grains • sand-sized grains• microcrystalline carbonate mud called micrite

– but the grains are all calcite – and are formed in the environment of deposition, – not transported there

Carbonate Environments

• Some limestone form in lakes, – but most limestone by is deposited – in warm shallow seas– on carbonate shelves and– on carbonate platforms rising from oceanic depths

• Deposition occurs where – little detrital sediment, especially mud, is present

• Carbonate barriers form in high-energy areas and may be – reefs – banks of skeletal particles – accumulations of spherical carbonate grains known

as oolites • which make up the grains in oolitic limestone

Limestone Environments

• The carbonate shelf is attached to a continent– Examples

occur in southern Florida and the Persian Gulf

Carbonate Shelf

• Carbonates may be deposited on a platform – rising from oceanic depths

• This example shows a cross-section – of the present-day Great Bahama Bank – in the Atlantic Ocean southeast of Florida

Carbonate Platform

• Reef rock tends to be – structureless– composed of skeletons of corals, mollusks, sponges

and other organisms• Carbonate banks are made up of

– layers with horizontal beds– cross-beds– wave-formed ripple marks

• Lagoons tend to have– micrite– with marine fossils – bioturbation

Carbonate Subenvironments

• Evaporites consist of – rock salt– rock gypsum

• They are found in environments such as– playa lakes– saline lakes– but most of the extensive deposits formed in the

ocean

• Evaporites are not nearly as common – as sandstone, mudrocks and limestone, – but can be abundant locally

Evaporite Environments

• Large evaporite deposits– lie beneath the Mediterranean Seafloor

• more than 2 km thick – in western Canada, Michigan, Ohio, New York, – and several Gulf Coast states

• How some of these deposits originated – is controversial, but geologists agree – that high evaporation rates of seawater – caused minerals to precipitate from solution

• Coastal environments in arid regions – such as the present-day Persian Gulf – meet the requirements

Evaporites

– with restricted inflow of normal seawater – into the lagoon– leading to increased salinity and salt depositions

Evaporites

• Evaporites could form

• in an environment similar to this

• if the area were in an arid region,

• Present-day gravel deposits – by a swiftly-flowing stream– Most transport and

deposition takes place when the stream is higher

Environmental Interpretations and Historical Geology

• Nearby gravel deposit probably less than a few thousand years old

• Conglomerate more than 1 billion years old – shows similar

features

Environmental Interpretations and Historical Geology

• We infer that it too was deposited – by a braided stream in a fluvial system– Why not deposition by glaciers or along a seashore?– Because evidence is lacking for either – glacial activity or transitional environment

• Jurassic-aged Navajo Sandstone – of the Southwestern United states – has all the features of wind-blown sand dunes:

• the sandstone is mostly well-sorted, well-rounded quartz • measuring 0.2 to 0.5 mm in diameter• tracks of land-dwelling animals, • including dinosaurs, are present• cross-beds up to 30 m high have current ripple marks • like those produced on large dunes by wind today• cross-beds dip generally southwest • indicating a northeast prevailing wind

Interpretation

– Vertical fractures

– intersect cross beds of desert dunes

– making the checker-board pattern

Navajo Sandstone

Checkerboard Mesa, Zion National Park, Utah

• Paleogeography deals with – Earth’s geography of the past

• Using interpretations – of depositional environment – such as the ones just discussed

• we can attempt to reconstruct – what Earth’s geography was like – at these locations at various times in the past

• For example, – the Navajo Sandstone shows that a vast desert – was present in what is now the southwest – during the Jurassic Period

Paleogeography

– and from Late Precambrian to Middle Cambrian

– the shoreline migrated inland from east and west

– during a marine transgression

Paleogeography

• Detailed studies of various rocks – in several western states – allow us to determine – with some accuracy – how the area appeared – during the Late Cretaceous

• A broad coastal plain – sloped gently eastward – from a mountainous region – to the sea

Paleogeography

• Later, vast lakes, – river floodplains, alluvial fans – covered much of this area – and the sea had withdrawn

from the continent

• Interpretations the geologic record – we examine later– will be based on similar – amounts of supporting

evidence

Paleogeography