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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  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 ► 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 (made through life processes)  all preserve evidence of the physical, chemical and biological processes that formed them Sedimentary rocks

5 ► 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

6 ► 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

7 ► Very common minerals in detrital rocks:  quartz, feldspars, and clay minerals ► Only calcite is very common in limestones ► Detrital rock tells ► 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

8 ► 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

9 ► 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 Grain Size

10 ► 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

11 ► 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 Sorting

12 ► 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

13 ► A deposit of well rounded and well sorted gravel Rounding and Sorting ► Angular, poorly sorted gravel

14 ► 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 Sedimentary Structures

15 ► 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

16 ► Some beds show an upward gradual decrease in grain size, known as graded bedding 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

17 ► 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

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

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

20 ► 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 Ripple Marks

21 ► 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

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

23 ► 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  lakeshore  river flood plain  or where mud is exposed at low tide along a seashore

24 ► 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

25 ► 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 Biogenic Sedimentary Structures

26 ► U-shaped burrows Bioturbation ► Vertical burrows

27 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?

28 ► 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

29 ► 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 Geometry of Sedimentary Rocks

30 ► 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 Blanket or Sheet Geometry

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

32 ► 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 Other Geometries

33 ► 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—The Biological Content of Sedimentary Rocks

34 ► coquina is made entirely of shell fragments Fossils Are Constituents of Sedimentary Rocks

35 ► 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 Fossils in Environmental Analyses

36 ► 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

37 ► 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

38 ► 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

39 ► 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

40 Continental environments Transitional environments Marine environments

41 ► 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

42 ► 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 Fluvial

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

44 ► Braided stream deposits consist of:  conglomerate  cross-bedded sandstone  mudstone is rare or absent Braided Stream Deposits

45 ► 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 Fluvial Systems

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

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

48 ► 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

49 ► 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 Associations in Desert Basin

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

51 ► 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 Alluvial Fans and Playa Lakes

52 ► 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

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

54 ► 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 Glacial Varves ► Dropstones liberated from icebergs may also be present

55 ► 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

56 Transitional environments

57 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

58 ► 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

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

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

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

62 ► 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

63 ► Subenvironments of a barrier island complex Barrier Island Complex

64 ► 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

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

66 ► 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 ► 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

69 ► 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

70 ► 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

71 ► 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

72 ► 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

73 ► 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

74 ► 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 Carbonate Environments

75 ► 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 Limestone Environments

76 ► The carbonate shelf is attached to a continent  Examples occur in southern Florida and the Persian Gulf Carbonate Shelf

77 ► 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

78 ► 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

79 ► 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

80 ► 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

81 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

82 ► 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

83 ► 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?

84 ► 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

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

86 ► 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

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

88 ► 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

89 ► Later, vast lakes, river floodplains, and alluvial fans covered much of this area  and the sea had withdrawn from the continent Paleogeography

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