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 High-energy processes energy conditions transport and deposition High-energy processes streams and waves transport gravel Rock type: Conglomerate 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 Angular, poorly sorted gravel 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 mudrocks Sedimentary Structures 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 cross-bedding forms by deposition on sand waves 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

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 wave-formed 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 cross-beds 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 Till is poorly sorted, nonstratified drift 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 in Varves 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 Dropstones liberated from icebergs may also be present Varves with a dropstone

Moraines are made of poorly sorted till

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 such as the Nile Delta of Egypt also have distributary channels but their seaward margin is modified by wave action

Stream/River-Dominated Deltas have long distributary channels extending far seaward Mississippi River delta

Tide-Dominated Deltas such as the Ganges-Brahmaputra delta of Ban-gladesh 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 checker-board pattern

Paleogeography Paleogeography deals with Using interpretations 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 A broad coastal plain 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, Interpretations the geologic record 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