ESC102. Sedimentary Rocks. Our keys to the past. Monday, February 11, 13

Transcription

1 ESC102 Sedimentary Rocks Our keys to the past

2 Sedimentary Rocks Sedimentary rocks are rocks that form through the accumulation of sediment and the process of lithification. Lithification occurs after deposition and involves compaction and cementation. Sedimentary rocks divided into two major categories: Detrital or Clastic sedimentary rocks are rocks that are composed of the weathered remains of other rocks. Chemical sedimentary rocks are produced either through life processes (biochemical/bioclastic) or through precipitation of minerals in water environments

3 Clastic Sedimentary Rocks Cement Minerals that fill sediment pores. Fluids with dissolved solids flush through pore system. Dissolved ions slowly crystallize and fill pores. Cementation varies from weak to strong. Common cements: Quartz Calcite Hematite Clay minerals

4 Sedimentary Cover Earth is covered by a thin veneer of sediment. The veneer caps igneous and metamorphic basement. Sediment cover varies in thickness from 0 to 20 km. Thinner (or missing) where ig and meta rocks outcrop. Thicker in sedimentary basins.

5 Sedimentary Rocks Sediments are the building blocks of sedimentary rocks. Four classes: Clastic Made from weathered rock fragments (clasts). Biochemical Cemented shells of organisms. Organic The carbon-rich remains of plants. Chemical Minerals that crystallize directly from water. Clastic Biochemical Organic Chemical

6 Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes. Weathering Generation of detritus via rock disintegration. Erosion Removal of sediment grains from rock. Transportation Dispersal by wind, water, and ice. Deposition Settling out of the transporting fluid. Lithification Transformation into solid rock.

7 Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes. Weathering Generation of detritus via rock disintegration. Erosion Removal of sediment grains from rock. Transportation Dispersal by wind, water, and ice. Deposition Settling out of the transporting fluid. Lithification Transformation into solid rock.

8 Clastic Sedimentary Rocks Lithification Transforms loose sediment into solid rock. Burial More sediment is added onto previous layers. Compaction Overburden weight reduces pore space. Sand 10 to 20% Clay 50 to 80% Cementation Minerals grow in pores, gluing sediments.

9 Clastic Sedimentary Rocks Lithification Transforms loose sediment into solid rock. Burial More sediment is added onto previous layers. Compaction Overburden weight reduces pore space. Sand 10 to 20% Clay 50 to 80% Cementation Minerals grow in pores, gluing sediments.

10 Investigating Sedimentary rocks In this class you will learn how to decode past environment using sedimentary rock. To do this we must pay particular attention to: textures composition fossils (if present) thickness relationships to other rocks Preliminary interpretations in the field - reddish rocks may have been deposited on land - whereas greenish rocks are more typical of marine environments. (caution: exceptions are numerous)

11 Clastic Sedimentary Rocks Mineral Composition: Clastic sedimentary rocks are identified based on mineral composition and texture. Common minerals clasts include quartz, feldspars, and clay minerals Mineral composition can only be used to determine the source rocks and physical/chemical properties of the depositional environment. For example: Quartz sand may have been deposited in a river system on a beach or in sand dunes

12 Clastic Sedimentary Rocks Mineral Composition: Clastic sedimentary rocks are identified based on mineral composition and texture. Common minerals clasts include quartz, feldspars, and clay minerals Mineral composition can only be used to determine the source rocks and physical/chemical properties of the depositional environment. For example: Quartz sand may have been deposited in a river system on a beach or in sand dunes

13 Clastic Sedimentary Rocks Texture refers to the grain size of the individual clasts that form the sedimentary rock With increasing transport, average grain size decreases.

14 Clastic Sedimentary Rocks Sediment Type blocks boulders cobbles pebbles sand silt clay Sediment Description Angular particles greater than 256 mm in size. Rounded particles greater than 256 mm in size. Rounded particles between 64 and 256 mm in size. Rounded particles between 2 and 64 mm in size. Particles between.0625 and 2 mm in size. Particles between 2µm and.0625 mm in size. Particles less than 2µm in size.

15 Clastic Sedimentary Rocks Rounding Indicate degree of transport. Fresh clasts are usually angular and non-spherical. Grain roundness and sphericity increases with transport. Well-rounded Long transport distances. Angular Negligible transport.

16 Clastic Sedimentary Rocks 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 If the size range is not very great, the sediment or rock is well sorted- wind produces very well sorted sediments If they have a wide range of sizes, they are poorly sorted

17 Texture as an environmental indicator Clast size allows us to determine the energy conditions that existed at the time the sediment was deposited. The larger the clast the greater the energy needed to transport that clast. Sand and larger clasts require vigorous currents (fast water= high energy) Silt and clay are transported by very weak currents and and therefore only accumulate under low-energy conditions such as in lakes and lagoons

18 Environmental Analysis Rock A Rock B Based on textural clues answer the following question about each sediment: 1. Was the sediment deposited near or far from its source? Explain your reasoning. 2. Was the sediment deposited in a high or low energy environment? Explain your reasoning. 3. Was the sediment deposited by wind, water, or gravity? Explain your reasoning.

19 Sedimentary Structures 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

20 Sedimentary Structures Sedimentary rocks generally have bedding or stratification (they are deposited in layers) Beds are layers that are thicker than 1 cm common in rocks with coarser grains laminations are layers less than 1 cm thick are common in mudrocks

21 Sedimentary Structures Graded bedding occurs when sediment loaded currents (turbidity currents) experience a relatively quick drop in velocity. what will happen to the sediment as the current slows? Will all sediment sizes behave the same way?

22 Sedimentary Structures Graded bedding occurs when sediment loaded currents (turbidity currents) experience a relatively quick drop in velocity. what will happen to the sediment as the current slows? Will all sediment sizes behave the same way?

23 Sedimentary Structures Graded bedding occurs when sediment loaded currents (turbidity currents) experience a relatively quick drop in velocity. what will happen to the sediment as the current slows? Will all sediment sizes behave the same way?

24 Sedimentary Structures Graded bedding occurs when sediment loaded currents (turbidity currents) experience a relatively quick drop in velocity. what will happen to the sediment as the current slows? Will all sediment sizes behave the same way?

25 Sedimentary Structures 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 Tabular crossbeds: shallow dips such as in this image are indicative of currents enter still bodies of water (Gilbert Delta). Steep foreset beds are indicative of eolian (wind) deposition

26 Sedimentary Structures 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 Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments

27 Sedimentary Structures 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 These herringbone crossbeds have foreset beds that dip in both directions. What may have caused this?

28 Sedimentary Structures Small-scale cross bedding creates ripple marks. These structures 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 symmetrical profile

29 Sedimentary Structures Small-scale cross bedding creates ripple marks. These structures 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 symmetrical profile

30 Sedimentary Structures 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 so in what environments would you expect to find them?

31 Sedimentary Structures 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 so in what environments would you expect to find them?

32 Sedimentary Structures Biogenic sedimentary structures include tracks burrows trails These structures are also called trace fossils Extensive burrowing by organisms is called bioturbation may alter sediments so thoroughly that other structures are disrupted or destroyed

33 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

34 Depositional Environments A depositional environment is anywhere sediment accumulates Three broad areas of deposition include continental transitional marine each of these areas has several specific environments with their own physical, chemical, and biological processes

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