sedimentary rocks— the archives of earth history sedimentary environments and structures
DESCRIPTION
Chapter 6. Sedimentary Rocks— The Archives of Earth History Sedimentary Environments and Structures. Sedimentary Rocks. formed at or near the surface at relatively low temperatures. from sediments which include boulders, cobbles, gravels, sands, silts, and clay particles. - PowerPoint PPT PresentationTRANSCRIPT
• 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
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
• 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
• 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
• 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
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
• Tidal-flat deposits showing a prograding shoreline– Notice the distinctive cross-beds – that dip in opposite directions – How could this happen?
Tidal Flats
• 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