Sedimentary Rocks— The Archives of Earth History Chapter 6 Sedimentary Rocks— The Archives of Earth History

History from Sedimentary Rocks 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? Sedimentary rocks preserve evidence of surface depositional processes contain fossils These things give clues to the depositional environment Depositional environments are specific areas or environments where sediment is deposited

Beach Environment Sand deposition Many ancient sandstones possess features that indicate they were also deposited on beaches

Sedimentary rocks Sedimentary rocks may be Detrital or chemical, including biochemical (made through life processes) all preserve evidence of the physical, chemical and biological processes that formed them

Investigating 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 More careful study of the rocks microscopic examination chemical analyses fossil identification interpretation of vertical and lateral facies relationships compare with present-day sediments Make environmental interpretation

Composition of Detrital Rocks Very common minerals in detrital rocks: quartz, feldspars, and clay minerals Only calcite is very common in limestones 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

Composition of Chemical Sedimentary Rocks 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 Evaporites such as rock salt and rock gypsum indicate arid environments where evaporation rates were high Coal originates in swamps and bogs on land

Grain Size 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 and therefore only accumulate under low-energy conditions such as in lakes and lagoons

Sorting and Rounding Sorting and rounding are two textural features of detrital sedimentary rocks that aid in determining depositional processes Sorting refers to the variation in size of particles making up sediment or sedimentary rocks It results from processes that selectively transport and deposit sediments of particular sizes

Sorting If the size range is not very great, the sediment or rock is well sorted If they have a wide range of sizes, they are poorly sorted For example Wind has a limited ability to transport sediment so dune sand tends to be well sorted Glaciers can carry any sized particles, because of their transport power, so glacier deposits are poorly sorted

Rounding Rounding is the degree to which detrital particles have their sharp corners and edges warn 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 and Sorting A deposit of well rounded and well sorted gravel Angular, poorly sorted gravel

Sedimentary Structures Sedimentary structures are visible features that formed at the time of deposition or shortly thereafter Represent manifestations of the physical and biological processes that operated in depositional environments Structures seen in present-day environments or produced in experiments help provide information about depositional environments of rocks with similar structures

Bedding Sedimentary rocks generally have bedding or stratification Individual layers less than 1 cm thick are laminations common in 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 form when sediment-water mixtures flow along the seafloor As they slow, the largest particles settle out then smaller ones

Cross-Bedding 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 useful for relative dating of deformed sedimentary rocks

Cross-Bedding Tabular cross-bedding forms by deposition on sand waves Typically produce where streams enter large bodies of water (AKA Deltas) Tabular cross-bedding in the Upper Cretaceous

Cross-Bedding Trough cross-bedding formed by migrating dunes Trough cross-beds in the Pliocene Six Mile Creek Formation, Montana

Ripple Marks 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 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

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, lakeshore 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 Also called trace fossils Extensive burrowing by organisms is called bioturbation 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 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

Geometry of Sedimentary Rocks The three-dimensional shape or geometry of a sedimentary rock body may be helpful in environmental analyses but it must be used with caution Nevertheless, it is useful in conjunction with other features

Blanket or Sheet Geometry Some of the most extensive sedimentary rocks in the geologic record result from marine transgressions and regressions These rocks commonly cover hundreds or thousands of square kilometers but are perhaps only a few tens to hundreds of meters thick Their thickness is small compared to their length and widthThus, they are said to have blanket or sheet geometry

Elongate or Shoestring Geometry Some sand deposits have an elongate or shoestring geometry especially those deposited in stream channels or barrier islands

Other Geometries Delta deposits tend to be lens shaped when viewed in cross profile or long profile lobate when observed from above Buried reefs are irregular but many are long and narrow or rather circular

Fossils—The Biological Content of Sedimentary Rocks Fossils are the remains or traces of prehistoric organisms can be used in stratigraphy for relative dating and correlation constituents of rocks, sometimes making up the entire rock 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 Are Constituents of Sedimentary Rocks coquina is made entirely of shell fragments

Fossils in Environmental Analyses Did the organisms in question live where they were buried? Were 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

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

Microfossils Microfossils are particularly useful because many individuals can be recovered from small rock samples In oil-drilling operations, small rock chips called well cuttings are brought to the surface These cuttings rarely contain complete fossils of large organisms, but they might have thousands of microfossils that aid in relative dating and environmental analyses

Trace Fossils In Place Trace fossils, too, may be characteristic of particular environments Trace fossils, of course, are not transported from their original place of origin

Depositional Environments 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 Continental environments Transitional environments Marine environments

Continental 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

Fluvial Fluvial refers to river and stream activity and to their deposits Fluvial deposits accumulate in either of two types of systems braided stream system with multiple broad, shallow channels in which mostly sheets of gravel and cross-bedded sand are deposited mud is nearly absent

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 mudstone is rare or absent

Fluvial Systems The other type of system is a meandering stream 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

Meandering Stream Meandering stream deposits mostly fine-grained floodplain sediments with subordinate sand bodies

Meandering Stream Deposits In meandering stream deposits, mudstone deposited in a floodplain is common sandstones are point bar deposits channel conglomerate is minor

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 alluvial fan, sand dune, 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 stream 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 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

Moraines and Till Origin of glacial drift Moraines and poorly sorted till

Glacial Varves Glacial lake deposits show alternating dark and light laminations Each dark-light couplet is a varve, represents one year’s accumulation of sediment light layers accumulate in summer Dark in winter Dropstones liberated from icebergs may also be present

Transitional Environments 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

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

Marine Deltas 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

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

Wave-Dominated Deltas Wave-dominated deltas such as the Nile Delta of Egyptalso have distributary channelsbut their seaward margin is modified by wave action

Tide-Dominated Deltas such as the Ganges-Brahmaputra delta have tidal sand bodies along the direction of tidal flow

Barrier Islands 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 Island Complex Subenvironments of a barrier island complex

Tidal Flats 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?

Marine Environments 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

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 intertonges with inner-shelf sand Much sediment derived from the continents crosses the continental shelf and is funneled into deeper water through submarine canyons 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 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 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 depositslie 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?

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 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, and alluvial fans covered much of this area and the sea had withdrawn from the continent