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Sedimentary Rocks — The Archives of Earth History

Sedimentary Rocks — The Archives of Earth History. ?. Disconformity. There are actually 3 types of rocks…. Igneous – cool from liquid (magma or lava) Metamorphic – pre-existing rocks that have been altered by intense temperature or pressure

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Sedimentary Rocks — The Archives of Earth History

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  1. Sedimentary Rocks —The Archives of Earth History ? Disconformity

  2. There are actually 3 types of rocks… • Igneous – cool from liquid (magma or lava) • Metamorphic – pre-existing rocks that have been altered by intense temperature or pressure • Sedimentary – form mainly from deposition of sediments

  3. Historical geology focuses on sedimentary rocks •Why??? • Only rocks that contain fossils • Indicate ancient depositional environments

  4. What is a sediment? • Fragment of pre-existing rock (or animal shell) • Why does water off Galveston look murky, while water off Florida looks clear?

  5. What kind of rocks do we find around Houston? • Not many rocks! • Lots of unlithified sediment • Why do many houses in Houston have foundation problems?

  6. What is a sedimentary rock? • Rock that forms at or near Earth’s surface • 3 types • Clastic • Chemically-precipitated • Biogenic

  7. How do clastic sedimentary rocks form? • Weathering • Transport • Deposition • Lithification

  8. How do other sedimentary rocks form? • Chemical – precipitation of dissolved materials • Biogenic (organic) – accumulations of organic material

  9. Environments of Deposition • At or near surface of Earth • Marine • Continental • Transitional (deltas, barrier islands, beaches)

  10. Marine • Coastal • Shelf • Deep water www.ngdc.noaa.gov/mgg/image/sedthick.jpg

  11. Continental • Fluvial • Meandering • Braided • Desert • Lacustrine • Glacial www.uoregon.edu/~millerm/depenv.html

  12. Continental • Fluvial • Meandering • Braided • Desert • Lacustrine • Glacial www.uoregon.edu/~millerm/depenv.html

  13. Continental • Fluvial • Meandering • Braided • Desert • Lacustrine • Glacial www.uoregon.edu/~millerm/depenv.html

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

  15. Continental • Fluvial • Meandering • Braided • Desert • Lacustrine • Glacial www.mikelevin.com/DLBlissParkTahoe.jpg

  16. Continental • Fluvial • Meandering • Braided • Desert • Lacustrine • Glacial www.peakware.com/encyclopedia/peaks/photos/everest7.htm

  17. Moraines and Till • Moraines and poorly sorted till • Origin of glacial drift

  18. Delta • Form in oceans or lakes (marine and non-marine) www.uoregon.edu/~millerm/depenv.html

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

  20. Where would you find different sedimentary rocks?

  21. Where would you find different sedimentary rocks?

  22. Where would you find different sedimentary rocks? • Grain size is controlled by energy • High energy • River • Beach • Low energy • Lake • Deep ocean Large grains Small grains

  23. Sorting, Rounding • 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 • Grains more rounded with longer transport

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

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

  26. Current Ripple Marks • form in response to water or wind currents flowing in one direction • asymmetric profiles allowing geologists to determine paleocurrent directions http://www3.interscience.wiley.com:8100/legacy/college/levin/0470000201/chap_tutorial/ch03/chapter03-5sedstr.html

  27. Wave-Formed Ripple Marks • As the waves wash back and forth, symmetrical ripples form • Wave-formed ripple marks in shallow seawater

  28. Modern Deposition near Houston • Fluvial • Brazos, Colorado, Trinity, San Jacinto Rivers • Transitional • Deltas, barrier islands • Marine • Gulf of Mexico

  29. Ancient Environments • Important for historical geology • Important for oil companies (need to know where sand was deposited) Why are we looking at modern depositional environments?

  30. Present is the key to the past • Study modern depositional environments to learn about ancient ones • Knowledge of ancient environments helps oil companies and historical geologists

  31. Brazos River • Longest river in Texas – 1450 km • Highest sediment supply of any Texas river • Originates in New Mexico

  32. Where does deposition occur? www.uwsp.edu/geo/courses/geog391/toriv/Diagrams.htm

  33. Point Bars • Sediment deposited within the inside bank of a meander loop • Fining upward sequence (grain size decreases) • Coarsest sediment deposited by highest energy

  34. Brazos River – Point Bar

  35. Brazos River – Cut Bank

  36. Coastal – Galveston Island

  37. Barrier Islands • Formed during sea level rise • Rate of SL rise and rate of sediment deposition approximately equal • Wave-dominated environment

  38. 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 • 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

  39. Barrier Island Complex • Subenvironments of a barrier island complex

  40. Texas Coast • Most Texas beaches relatively fine-grained • Low gradient of rivers like Brazos • Why is this a problem?

  41. Brazos Delta gulf.rice.edu

  42. 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

  43. 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 • Why not deposition by glaciers or along a seashore? • No evidence for either glacial activity or transitional environment

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