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Sedimentary Rocks— The Archives of Earth History Chapter 6.

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1 Sedimentary Rocks— The Archives of Earth History Chapter 6

2 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 –also, many contain fossils –These things give clues to the depositional environment Depositional environments are specific areas –or environments where sediment is deposited History from Sedimentary Rocks

3 Sand deposition –on a beach along the Pacific coast –of the United States Many ancient sandstones –possess features –that indicate they were –also deposited on beaches Beach Environment

4 Sedimentary rocks may be –detrital –or chemical, including biochemical –and all preserve evidence –of the physical, chemical and biological processes –that formed them Some sedimentary rocks are or contain resources –phosphorous –liquid petroleum –natural gas Sedimentary rocks

5 For instance, phosphorous –from phosphorous-rich sedimentary rocks –is used in metallurgy preserved foods ceramics matches chemical fertilizers animal-feed supplements Phosphorous

6 Observation and data gathering –visit rock exposures (outcrops) –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

7 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 Investigating Sedimentary Rocks

8 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 Detrital Rocks

9 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 Composition of Chemical Sedimentary Rocks

10 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 accumulate –only under low-energy conditions –as in lakes and lagoons Grain Size

11 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 and Rounding

12 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 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 Sorting

13 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

14 A deposit –of well rounded –and well sorted gravel Rounding and Sorting Angular, poorly sorted gravel

15 Sedimentary structures are –features visible at the scale of an outcrop –that formed at the time of deposition or shortly thereafter –and are 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 Sedimentary Structures

16 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

17 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

18 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 They are useful for relative dating –of deformed sedimentary rocks Cross-Bedding

19 Tabular cross- bedding forms by deposition on sand waves Cross-Bedding Tabular cross- bedding in the Upper Cretaceous Two Medicine Formation in Montana

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

21 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 –and 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 Ripple Marks

22 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

23 As the waves wash back and forth, –symmetrical ripples form The photo shows wave- formed ripple marks –in shallow seawater Wave-Formed Ripples

24 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

25 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

26 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

27 U-shaped burrows Bioturbation Vertical burrows

28 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?

29 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

30 The three-dimensional shape or geometry –of a sedimentary rock body –may be helpful in environmental analyses –but it must be used with caution –because the same geometry may be found –in more than one environment –can be modified by sediment compaction –during lithification –and by erosion and deformation Nevertheless, it is useful in conjunction –with other features Geometry of Sedimentary Rocks

31 Some of the most extensive sedimentary rocks –in the geologic record result from –marine transgressions and regressions The 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 width Thus, they are said to have –blanket or sheet geometry Blanket or Sheet Geometry

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

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

34 Fossils –are the remains or traces of prehistoric organisms –can be used in stratigraphy for relative dating and correlation –are constituents of rocks, sometimes making up the entire rock –and 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—The Biological Content of Sedimentary Rocks

35 This variety of limestone, –known as coquina, –is made entirely of shell fragments Fossils Are Constituents of Sedimentary Rocks

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

37 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 Environmental Analyses

38 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 Microfossils

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

40 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

41 Continental environments Transitional environments Marine environments

42 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 Continental Environments

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

44 The deposits of braided streams are mostly –gravel and cross-bedded sand with subordinate mud Braided Stream

45 Braided stream deposits consist of –conglomerate –cross-bedded sandstone –but mudstone is rare or absent Braided Stream Deposits

46 The other type of system is a meandering stream –with 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 Fluvial Systems

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

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

49 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

50 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

51 Large-scale cross- beds –in a Permian-aged –wind-blown dune deposit in Arizona Dune Cross-Beds

52 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

53 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 Glacial Environments

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

55 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 Varves Dropstones –liberated from icebergs –may also be present –Varves with a dropstone

56 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

57 Transitional environments

58 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

59 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 Marine Deltas

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

61 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

62 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

63 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 Islands

64 Subenvironments of a barrier island complex Barrier Island Complex

65 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

66 Tidal-flat deposits showing a prograding shoreline –Notice the distinctive cross-beds –that dip in opposite directions –How could this happen? Tidal Flats

67 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

68 Marine environments

69 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

70 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

71 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

72 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

73 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

74 Types of sediment are: –pelagic clay, which covers most of the deeper parts of the seafloor –calcareous (CaCO 3 ) and siliceous (SiO 2 ) oozes made up of microscopic shells Deep Sea

75 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

76 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

77 The carbonate shelf is attached to a continent –Examples occur in southern Florida and the Persian Gulf Carbonate Shelf

78 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

79 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

80 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

81 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

82 –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,

83 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

84 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

85 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

86 –Vertical fractures –intersect cross beds of desert dunes –making the checker- board pattern Navajo Sandstone Checkerboard Mesa, Zion National Park, Utah

87 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

88 –and from Late Precambrian to Middle Cambrian –the shoreline migrated inland from east and west –during a marine transgression Paleogeography

89 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

90 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

91 Summary The physical and biological features –of sedimentary rocks reveal something about –the depositional processes that form them Environmental analysis –of sedimentary rocks uses –mainly sedimentary structures and fossils –but also textures, rock body geometry –and even composition Geologists recognize –three primary depositional areas –continental, transitional, and marine –each with several specific environments

92 Summary Fluvial systems might be braided or meandering –Braided streams deposit mostly sand and gravel, –whereas deposits of meandering streams are mostly mud and subordinate sand bodies with shoestring geometry An association of alluvial fan, sand dune, –and playa lake deposits –is typical of desert depositional environments Glacial deposits consist mostly of till –in moraines and outwash

93 Summary The simplest deltas, those in lakes, –consist of a three-part sequence of rocks –grading from finest at the base, –upward to coarser-grained rocks Marine deltas dominated by –fluvial processes, waves, or tides –are much larger and more complex A barrier island system includes beach, –dune, and lagoon subenvironments, –each characterized a unique association –of rocks, sedimentary structures, and fossils

94 Summary Inner shelf deposits are mostly sand, –whereas those of the outer shelf are mostly mud; –both have marine fossils and bioturbation Much of the sediment from land –crosses the shelves and is deposited –on the continental slope and rise as submarine fans Either pelagic clay or oozes –derived from the shells of –microscopic floating organisms cover –most of the deep seafloor

95 Summary Most limestone originates in shallow, –warm seas where little detrital mud is present Carbonate rocks (just as detrital rocks) –may possess cross-beds, ripple marks, –mud cracks, and fossils –that provide information –about depositional processes Evaporites form in several environments, –but the most extensive ones were deposited –in marine environments In all cases, though, they formed –in arid regions with high evaporation rates

96 Summary With information from sedimentary rocks, –as well as other rocks, –geologists determine the past distribution –of Earth's surface features


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