Sedimentary Rocks— The Archives of Earth History Sedimentary Environments and Structures

1. Chapter 6 Sedimentary Rocks—The Archives of Earth History Sedimentary EnvironmentsandStructures

2. 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 depositedsuch as beach, desert, stream, lake, marine

3. 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)

4. Sedimentary rocks • Sedimentary rocks may be • Detrital • chemical, including biochemical • Each rock preserves evidence of how it was formed

5. 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)

6. 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

7. Rounding and Sorting • A deposit • of well rounded • and well sorted gravel • Angular, poorly sorted gravel

8. Bedding • Sedimentary rocks generally have bedding orstratification • 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

9. Graded Bedding • Graded bedding is common in turbidity current deposits • which form when sediment-water mixtures flow along the seafloor • Some beds show an upward gradual decrease • in grain size, known as gradedbedding • As they slow, • the largest particles settle out then smaller ones

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

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

12. 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

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

14. 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

15. 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

16. 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

17. Bioturbation • U-shaped burrows • Vertical burrows

18. 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?

19. 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

20. Fossils Are Constituents of Sedimentary Rocks • This variety of limestone, • known as coquina, • is made entirely of shell fragments

21. Depositional Environments Transitional environments Continental environments Marine environments

22. Braided Stream • The deposits of braided streams are mostly • gravel and cross-bedded sand with subordinate mud

23. Braided Stream Deposits • Braided stream deposits consist of • conglomerate • cross-bedded sandstone • but mudstone is rare or absent

24. Meandering Stream • Meandering stream deposits • are mostly fine-grained floodplain • sediments with subordinate sand bodies

25. 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

26. 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

27. Dune Cross-Beds • Large-scale cross-beds • in a Permian-aged • wind-blown dune deposit in Arizona

28. 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

29. 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

30. Moraines and Till • Origin of glacial drift

31. 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

32. Moraines are made of poorly sorted till

33. Transitional Environments Transitional environments

34. 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

36. Wave-Dominated Deltas • Wave-dominated deltas • such as the Nile Delta of Egypt • also have distributary channels • but their seaward margin • is modified by wave action

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

38. Tide-Dominated Deltas • Tide-Dominated Deltas, • such as the Ganges-Brahmaputra delta • of Ban-gladesh • have tidal sand bodies • along the direction of tidal flow

40. Barrier Island Complex • Subenvironments of a barrier islandcomplex

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

42. Marine Environments Marine environments

43. 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

44. 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

45. 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

46. 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

48. 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

49. 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

50. 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