The Sedimentary Archives

1. The Sedimentary Archives CHAPTER 3

2. Controls on sedimentary rock features • Tectonic setting • Physical, chemical, and biological processes in the depositional environment • Method of sediment transport • Rocks in source area from which sediment is derived • Climate (and its effect on weathering) • Post-depositional processes of lithification (cementation, compaction)

3. Tectonic Setting Tectonics: • The forces controlling deformation or structural behavior of a large area of the Earth's crust over a long period of time. • Tectonic Settings influence: • Size of clastic particles • Thickness of deposit • Rate of erosion or subsidence

4. Continental Tectonic Regimes • Craton - stable interior of a continent; undisturbed by mountain-building events since the Precambrian • Shields- large areas of exposed crystalline rocks) • Platforms- like shields but covered by flat-lying or gently warped sedimentary rocks • Orogenic belts - elongate regions bordering the craton which have been deformed by compression since the Precambrian

5. Figure 3-1 (p. 62)The craton and orogenic belts of North America.

6. Environments of Deposition • All of the physical, chemical, biologic and geographic conditions under which sediments are deposited. • Sediments formed from the weathering of pre-existing rocks outside the basin, and transported to the environment of deposition Or • Sediments form inside the basin; includes chemical precipitates, most carbonate rocks, and coal.

7. Marine Environments • Continental shelf (shallow) • Continental slope • Continental rise • Abyssal plain (deep)

8. Figure 3-4 (p. 65)Deep-sea fan built of land-derived sediment emerging from the lower part of a submarine canyon. Such fans occur in association with large rivers, such as the Amazon, Congo, Ganges, and Indus. (Vertical exaggeration 200:1.)

9. Transitional Environments Transitional environments = shoreline deposits Examples include: • Deltas • Beaches and Barrier Islands • Lagoons • Tidal flats

10. Deltas Deltas • fan-shaped accumulations of sediment • river flows into a standing body of water, such as a lake or sea • sediments are dropped, forming this progradational feature. Mississippi Delta Niger Delta

11. Barrier Islands • Beaches and Barrier Islands are shoreline deposits exposed to wave energy and dominated by sand with a marine fauna. • Lagoons are bodies of water on the landward side of barrier islands. • Tidal flats are low-lying plains near lagoons. Marshy

12. Barrier Island System • Subenvironments:

13. The Outer Banks Dauphin Island

14. Continental Environments • Continental environments are those environments which are present on the continents. Examples: • Fluvial (River) • Alluvial fans • Lakes (lacustrine) • Glacial • Eolian (wind)

15. Alluvial Fan Fluvial Lacustrine

16. Eolian Glacier

17. Color of Sedimentary Rocks • Clues about depositional environment: • Black and dark gray coloration in sedimentary rocks generally indicates the presence of organic carbon and iron • Reddish coloration in sedimentary rocks indicates the presence of oxidized iron • Green and gray coloration in sedimentary rocks indicates the presence of reduced iron

18. Rock Colors Dolomite Red Siltstone Gray Evaporite

19. Size and Sorting of Clasts • Texture refers to the size, shape, sorting, and arrangement of grains in a sedimentary rock. • Three textural components in clastic rocks: • Clasts • Matrix • Cement

20. Clasts and matrix Clasts Matrix

21. Interpretation of Clastic Sedimentary Rocks • The texture of a sedimentary rock can provide clues to the depositional environment. • Fine-grained= quiet water • Large grains= higher energy (velocity) deposition

22. Grain Size • Sedimentary grains are categorized according to size using the Wentworth Scale. • Wentworth Scale for sedimentary grain size: GRAVEL (>2mm) SAND SILT CLAY (<1/256mm)

23. Sorting • Sorting refers to the distribution of grain sizes in a rock. POOR! GOOD!

24. Sorting • In general, windblown sediments are better sorted than wave-washed sediments. • Well-sorted sands • Have higher porosity and permeability than poorly-sorted sands (if not tightly cemented), • May be good reservoirs for petroleum and natural gas. • Poor sorting is the result of rapid deposition of sediment without sorting by currents. • Examples: • alluvial fan deposits • glacial tillites.

25. Grain Shape • Grain shape is described in terms of rounding of grain edges and sphericity (equal dimensions). • Rounding results from abrasion and grain impact during transport.

26. Figure 3-13 (p. 72)Shape of sediment particles.(A) An angular particle (all edges sharp). (B) A rounded grain that has little sphericity. (C) A well-rounded, highly spherical grain. Roundness refers to the smoothing of edges and corners, whereas sphericity measures the degree of approach of a particle to a sphere.

27. Sedimentary Structures • Sedimentary structures are • visible at the scale of an outcrop (LARGE!) • that formed at the time of deposition or shortly thereafter (before lithification) • Evidence of processes operating

28. 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

29. Graded Bedding • Some beds show an upward gradual decrease • in grain size, known as graded bedding • Graded bedding is common in turbidity current deposits

30. Graded Bedding

31. Cross-Bedding • Cross-bedding forms when layers come to rest at an angle to the surface • Cross-beds result from transport by either water or wind http://www.wwnorton.com/earth/egeo/flash/5_1.swf

32. Cross Beds

33. Ripple Marks • Small-scale alternating ridges and troughs are known as ripple marks and are common in sandstone • Current ripple marks • form in response to water or wind currents • flowing in one direction • and have asymmetric profiles • Wave-formed ripple marks • result from the oscillation of waves • tend to be symmetrical

34. Current Ripple Marks • Ripples with an asymmetrical shape • Internally cross- bedded • Flow upper right to lower left

35. Wave-Formed Ripples • As the waves wash back and forth, symmetrical ripples form • Produced by wave (shallow)

36. Mud Cracks • When clay-rich sediments dry, they shrink and crack into polygonal patterns fractures called mud cracks • Mud cracks require wetting and drying to form,

37. Ancient Mud Cracks

38. Geopetal Structures • Which way is up?? • Sedimentary structures can be used to determine "up direction". • graded beds • cross beds • mudcracks • Flute marks • symmetrical (but not asymmetrical) ripples • stromatolites • burrows • tracks,

39. Interpretating Sands in Clastic Rocks • Quartz-rich (mature; quartz sand) • Feldspar-rich (immature; arkosic sand) • Rock Fragment-rich (immature; lithic sand) • Clay-rich (immature; greywacke)

40. Figure 3-23 (p. 77)Four categories of sandstone as seen in thin section under the microscope. Diameter of field is about 4 mm.

41. Figure 3-24 (p. 78)Idealized geologic conditions under which quartz sandstone may be deposited. There is little tectonic movement in this environment. Water depth is shallow, and the basin subsides very slowly.

42. Figure 3-26 (p. 79)Geologic environment in which arkose may be deposited.

43. Figure 3-28 (p. 80)Tectonic setting in which graywacke is deposited. Frequently graywackes are transported by masses of water highly charged with suspended sediment. Because of the suspended matter, the mass is denser than surrounding water and moves along the sloping sea floor or down submarine canyons as a turbidity current. Graywacke sediment characteristically accumulates in deep-sea fans at the base of the continental slope.

44. Figure 3-29 (p. 80)Deltaic environment in which lithic sandstones may be deposited.

45. Interpretation of Carbonates • Main Processes • Chemical direct precipitates (carbonate mud) • Biochemical: organic contribution (shells, etc.)

46. Characteristics of most marine carbonate environments • Warm water • Tropical climate (30 ° N - 30 ° S of equator) • Shallow water (less than 200 m deep) • Clear water (low to no terrigenous input) • Sunlight required for photosynthesis by algae

47. Some limestones may be the accumulation of shells Shell fragments (coquina) Microscopic Foraminifera (chalk) Fossiliferous limestone

48. Dolomite • CaMg(CO3)2 • Rock and mineral • Original (forming today) is rare • Many older rocks have altered (dolomitized) over time

49. Interpretation of Shales • Shale - very fine-grained rock composed of clay, mud, and silt. Types: • Quartz-rich shales (quartz sandstones) • Feldspar-rich shales (arkoses) • Chlorite-rich shales (greywackes) • Mica-rich shales (greywackes)

50. Unconformities • Unconformities in sequences of strata represent times of nondeposition and/or erosion that encompass long periods of geologic time,perhaps millions or tens of millions of years • The rock record is incomplete!