Sedimentary Rocks— The Archives of Earth History Sedimentary Environments and Structures

Sedimentary Rocks—The Archives of Earth History

Sedimentary Environmentsand

Structures

Chapter 6

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

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

• OR 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: continent, marine, transitional

– environments where sediment is depositedsuch as beach, desert, stream, lake, marine

Sedimentary Rocks

• 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– chemical,

including biochemical

– Each rock preserves evidence of how it was formed

Sedimentary rocks

• Observation and data gathering

– carefully examine • textures• composition• fossils (if present)• Thickness of rock bed• 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

• Detrital grain size– energy conditions – transport and deposition

• High-energy processes – streams and waves

• transport gravel– Rock type: Conglomerate – Sand transport also requires vigorous currents

• 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

• A deposit – of well rounded – and well sorted

gravel

Rounding and Sorting

• Angular, poorly sorted gravel

• 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

Sedimentary Structures

• 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

• 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

• 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 to any one environment

• This variety of limestone, – known as

coquina, – is made entirely

of shell fragments

Fossils Are Constituents of Sedimentary Rocks

Depositional EnvironmentsContinental environments

Transitional environments

Marine environments

• 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

• Meandering stream deposits

Meandering Stream

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

• 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

– Glacial deposits and 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 – Indicates that glaciers were involved

Glacial Environments

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 Dropstone in Varves

• Dropstones – liberated

from icebergs

– may also be present

– Varves with a dropstone

Moraines are made of poorly sorted till

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

• 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

• Stream/river-dominated deltas – have long

distributary channels

– extending far seaward

– Mississippi River delta

Stream/River-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

• Subenvironments of a barrier island complex

Barrier Island Complex

• 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

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