slide1 n.
Skip this Video
Loading SlideShow in 5 Seconds..
Sedimentary Rocks— The Archives of Earth History PowerPoint Presentation
Download Presentation
Sedimentary Rocks— The Archives of Earth History

Sedimentary Rocks— The Archives of Earth History

194 Vues Download Presentation
Télécharger la présentation

Sedimentary Rocks— The Archives of Earth History

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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

  2. 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 • also, many contain fossils • These things give clues to the depositional environment • Depositional environments are specific areas • or environments where sediment is deposited

  3. Beach Environment • 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

  4. Sedimentary rocks • 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

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

  6. Investigating Sedimentary Rocks • 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)

  7. 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

  8. 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

  9. 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

  10. 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 accumulate • only under low-energy conditions • as in lakes and lagoons

  11. 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

  12. 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 • 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

  13. 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

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

  15. Sedimentary Structures • 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

  16. 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

  17. 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 graded bedding • As they slow, • the largest particles settle out • then smaller ones

  18. 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 • They are useful for relative dating • of deformed sedimentary rocks

  19. Cross-Bedding • Tabular cross-bedding forms by deposition on sand waves • 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. 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 • 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

  22. 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

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

  24. 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

  25. 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

  26. 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

  27. Bioturbation • U-shaped burrows • 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. 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

  30. 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 • 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

  31. Blanket or Sheet Geometry • 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

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

  33. Other Geometries • 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

  34. 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 • 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

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

  36. Fossils in Environmental Analyses • 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

  37. 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

  38. 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

  39. 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

  40. 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

  41. Depositional Environments Transitional environments Continental environments Marine environments

  42. 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

  43. Fluvial • 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

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

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

  46. Fluvial Systems • 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

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

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

  49. 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

  50. 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