Transcript Ch 6 ppt
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. • 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 deposited such as beach, desert, stream, lake, marine Beach Environment • 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) Sedimentary rocks • Sedimentary rocks may be – Detrital – chemical, including biochemical – Each rock preserves evidence of how it was formed Investigating 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) Grain Size • 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 Rounding and Sorting • A deposit – of well rounded – and well sorted gravel • Angular, poorly sorted gravel Bedding • Sedimentary rocks generally have bedding or stratification – Individual layers less than 1 cm thick are laminations • common in Sedimentary Structures mudrocks – Beds are thicker than 1 cm • common in rocks with coarser grains Graded Bedding • Some beds show an upward gradual decrease – in grain size, known as 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 • Tabular crossbedding forms by deposition on sand waves • Tabular crossbedding 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 Current 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 Wave-Formed Ripples • As the waves wash back and forth, – symmetrical ripples form • The photo shows waveformed ripple marks – in shallow seawater Mud Cracks • 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, – as along a lakeshore – or a river flood plain – or where mud is exposed at low tide along a seashore Ancient Mud Cracks • Mud cracks in ancient rocks – in Glacier National Park, Montana • Mud cracks typically fill in – with sediment – when they are preserved – as seen here Biogenic Sedimentary Structures • 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 Bioturbation • U-shaped burrows • 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? No Single Structure Is Unique to any one environment • 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 Fossils Are Constituents of Sedimentary Rocks • This variety of limestone, – known as coquina, – is made entirely of shell fragments Depositional Environments Continental environments Transitional environments Marine environments Braided Stream • The deposits of braided streams are mostly – gravel and cross-bedded sand with subordinate mud Braided Stream Deposits • Braided stream deposits consist of – conglomerate – cross-bedded sandstone – but mudstone is rare or absent Meandering Stream • Meandering stream deposits – are mostly fine-grained floodplain – sediments with subordinate sand bodies Desert Environments • 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 Associations in Desert Basin • 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 Dune Cross-Beds • Large-scale crossbeds – in a Permian-aged – wind-blown dune deposit in Arizona Alluvial Fans and Playa Lakes • 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 Glacial Environments – 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 Moraines and Till • Origin of glacial drift Glacial Dropstone • Glacial lake deposits show in Varves – alternating dark and light laminations • Each dark-light couplet is a varve, • Dropstones – representing one year’s accumulation of sediment – liberated – light layers accumulate in summer from – dark in winter icebergs – may also be present – Varves with a dropstone Moraines are made of poorly sorted till Transitional Environments Transitional environments Simple Deltas • The simplest deltas are those in lakes and consist of – topset beds – foreset beds – bottomset beds – 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 • Wavedominated deltas – such as the Nile Delta of Egypt – also have distributary channels – but their seaward margin – is modified by wave action Stream/River-Dominated Deltas • Stream/riverdominated deltas – have long distributary channels – extending far seaward – Mississippi River delta Tide-Dominated Deltas • Tide-Dominated Deltas, – such as the Ganges-Brahmaputra delta – of Bangladesh – have tidal sand bodies – along the direction of tidal flow Barrier Island Complex • Subenvironments of a barrier island complex Tidal Flats • Tidal-flat deposits showing a prograding shoreline – Notice the distinctive cross-beds – that dip in opposite directions – How could this happen? Marine Environments Marine environments Detrital 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 Slope and Rise • 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 Detrital Marine Environments • Shelf, slope and rise environments • The main avenues of sediment transport – across the shelf are submarine canyons Turbidity currents carry sediment to the submarine fans Sand with graded bedding and mud settled from seawater Deep Sea • 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 Carbonate Environments • 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 Limestone 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 Carbonate Shelf • The carbonate shelf is attached to a continent – Examples occur in southern Florida and the Persian Gulf Carbonate Platform • 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 Subenvironments • 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 Evaporite Environments • 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 Evaporites • 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 • Evaporites could form • in an environment similar to this • if the area were in an arid region, – with restricted inflow of normal seawater – into the lagoon – leading to increased salinity and salt depositions Environmental Interpretations and Historical Geology • Present-day gravel deposits – by a swiftly-flowing stream – Most transport and deposition takes place when the stream is higher • Nearby gravel deposit probably less than a few thousand years old Environmental Interpretations and Historical Geology • Conglomerate more than 1 billion years old – shows similar features • 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 Interpretation • 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 Navajo Sandstone Checkerboard Mesa, Zion National Park, Utah – Vertical fractures – intersect cross beds of desert dunes – making the checkerboard pattern Paleogeography • 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