SEDIMENTARY ROCKS. Processes, Environments, Structures and Rocks. Sedimentary Processes and Rocks

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1 SEDIMENTARY ROCKS Processes, Environments, Structures and Rocks Sedimentary Processes and Rocks Origins of Sedimentary Rocks Sediment transport and texture Sedimentary structures Lithification Classifying Sedimentary Rocks Detrital/Clastic sedimentary rocks Chemical/Biogenic sedimentary rocks Interpreting Sedimentary Rocks Depositional environments Sedimentary facies Sedimentary rocks and Plate Tectonics 1

2 Most sediments are deposited in the ocean basins but they are also deposited in other environments such as lakes, streams, desert basins, etc. Sediment is loose, unconsolidated material and is transformed into sedimentary rock through a process known as lithification (cementation and compaction). Sedimentary rocks Preserve evidence of the surface processes responsible for deposition Contain fossils the only record of past life Can be interpreted for depositional environment Can be used to interpret past Plate Tectonic History 2

3 Sedimentary Rocks Mechanical and chemical weathering produce the raw materials for sedimentary rocks. Sedimentary rocks only comprise ~5% of the Earth's crust however, they are much more important than this percentage would imply. Sedimentary rocks occur in the portion of Earth where we live. Important source of resources: energy (coal, petroleum) water (storage of groundwater) minerals (gold, aluminum, lead/zinc,etc) Origins of Sediment Weathering results in two types of products: 1. solid particles (sand grains, clay minerals, etc.) 2. dissolved constituents in water This results in two fundamentally different types of sedimentary rocks: 1. detrital (clastic) sedimentary rocks - formed from transported solid particles (detritus or clasts). 2. chemical sedimentary rocks - formed by the precipitation of dissolved substances by either inorganic or biologic processes river rocks salt flat 3

4 Detrital Sediment Quartz and clay minerals are the dominant constituents of detrital sediment. quartz - very resistant to weathering. clay - product of chemical weathering of silicate minerals - very stable at Earth s surface. Feldspars, micas and other silicate minerals may occur in sediment that has not traveled far. Texture of Detrital Sediment Sedimentary rocks are classified partly on the basis of Texture :particle size and shape. Sizes of particles are related to mode of transport and the environment of deposition: Gravels are found in high energy environments such as mountain streams and some beaches. Sands are found in intermediate energy environments such as beaches and river deposits. Clays are found in quiet environments such as lakes and the deep ocean. 4

5 Transport and Texture Type of transport, e.g., waves, stream flow, wind (capacity, energy) Distance traveled angular or rounded (more or less abrasion) Tendency of medium to sort (well or poorly sorted) Composition of original weathered rock (soft or hard) Changes in the energy of medium in different environments (fast/high energy to slow/low energy) Sedimentary Rock Properties Sedimentary Structures Compaction and Cement (lithification) Texture Composition Rock geometry (Sedimentary Facies) Fossils 5

6 Sedimentary Structures Laminations and Bedding Graded Bedding Cross-Bedding Ripple Marks Mudcracks Soft-sediment deformation Biogenic structures 6

7 Sedimentary structures Laminations and Beds Beds, Outcrop and Formations Death Valley 7

8 Sedimentary Structures Beds Examining Recent Undersea Fan Deposits 8

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10 Sedimentary Structures Graded Bedding Sedimentary Structures Cross-bedding 10

11 Dune Crossbeds Valley of Fire State Park, Nevada Sedimentary Structures Ripple Marks Wind or water currents 11

12 Sedimentary Structures Mudcracks Alternate wetting and drying Mudcracks and Ripple Marks in a Vertical Wall 12

13 Sedimentary Structures Trace fossils and bioturbation LITHIFICATION Diagenesis Compaction Cementation 13

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15 CLASSIFICATION AND IDENTIFICATION Texture (Grain Size, Crystal size) Composition (Mineral Composition) 15

16 Detrital Sedimentary Rocks - Conglomerate and Breccia These rocks consist of dominantly gravel-sized particles. Conglomerate - composed of rounded particles indicating some degree of transport (e.g., mountain stream, beaches). Breccia - composed of angular fragments indicating that they did not travel far from their source (e.g., landslide, alluvial fan). Gravel Bar to Conglomerate 16

17 From Alluvial Fans to Breccia Conglomerate and Sandstone in Outcrop Ancient Stream Deposits, Split Mountain, California 17

18 Detrital Sedimentary Rocks Sandstone Arenite (Quartz Sandstone) is composed of sand-sized quartz particles. Quartz is the dominant mineral in sandstone - due to its durability at the Earth s surface. A wellsorted pure quartz sandstone with rounded grains indicates that it may be the result of a great deal of transport - maybe over several cycles. Arkose is a type of sandstone that contains considerable amounts of feldspar. It probably was deposited close to the source of sediment. Likely source was weathering of granitic rocks. Detrital Sedimentary Rocks Sandstone (cont.) Greywacke is a sandstone composed of sand sized grains of quartz and lithic fragments, usually of volcanic rock, with a matrix of clay. It was probably deposited on an active marine margin as a part of a deep sea fan or turbidite sequence. Likely source of sediment is volcanic mountains. photomicrograph 18

19 Air Photo of Underwater Sand Patterns Gulf of California Sandstone in Outcrop, Domengine Formation, California 19

20 Sandstone in Outcrop, Coldwater Sandstone Formation, Santa Ynez Mountain Range, Santa Barbara, CA Detrital Sedimentary Rocks - Siltstone and Shale Mudstone is a general term for fine-grained sedimentary rocks, usually without fine layering. Siltstone is composed of predominantly silt-sized particles and is often difficult to distinguish from shale. Shale is a sedimentary rock consisting of silt- and clay-sized particles that commonly consists of fine layers of clay minerals and can split into thin layers Shale and siltstone commonly contain fossils of plant remains and other organisms. 20

21 Shale is formed from silt and clay deposited in relatively still waters of lakes and the ocean. Flathead Lake, Montana Russian River entering Pacific Sandstone and Shale 21

22 Silty Shale in Outcrop Ancient Playa Lake Bed Shales Zabriske Point Death Valley National Park California 22

23 Ancient Lake Bed Mudstone and Shale, Bryce Canyon Natl. Park Chemical Sedimentary Rocks derived from material that is carried in solution to lakes and seas. Precipitation from water occurs in two ways: 1. Inorganic processes such as evaporation can produce chemical sediments. (ex. evaporites, dripstone, travertine, etc.) 2. Biologic processes (forming shells) results in chemical sediment. Organisms die and collect on the seafloor (ex. coral, chalk, etc.). 23

24 There is a wide variety of chemical sedimentary rocks. Note that there is a mineral composition that is characteristic for each type of chemical sedimentary rock. Chemical Sedimentary Rocks Inorganic Precipitates and Evaporites Inorganic Limestone composed of calcite precipitated from water without the influence of living organisms, e.g., lithographic limestone, travertine Rock Salt and Rock Gypsum composed of Halite and Gypsum crystalized during evaporation of water in arid conditions. Travertine Rock salt Rock gypsum 24

25 Bedded Limestone, Arkansas 25

26 Rock Salt, an Evaporite Owen s Lake, California Interior of Salt Mine Chemical Sedimentary Rocks Inorganic Precipitates and Evaporites Dolostone similar to calcitic limestone but made of calcium magnesium carbonate Inorganic chert microcrystaline quartz precipitated from supersaturated solutions 26

27 Chemical Sedimentary Rocks Chert Chert is a chemical sedimentary rock composed of microcrystalline silica (SiO2). chert nodules (flint) are found especially in limestones (chalk) - inorganic precipitation of biogenic silica Flint Nodules on a Beach in Southern England dark gray due to inclusions of organic matter Chemical-Biogenic Sedimentary Rocks Biogenic Limestone coquina Coquina - composed of poorly cemented coarse shell fragments Chalk - made up of the shells of microscopic marine organisms (foraminifera, calcareous nannoplankton) Chalk cliffs of Normandy Foraminifera 27

28 Source of Coquina Massive Biogenic Limestone, Carlsbad 28

29 Chemical Sedimentary Rocks Biogenic Chert Layered or bedded cherts are derived from accumulation of silica-rich shells of radiolaria (microscopic sea critters). radiolaria Coal very different from other rocks because it is composed of organic matter. Stages in coal formation: 1. accumulation of plant remains in swamp. 2. material does not decay because of lack of oxygen (reducing environment). 3. as material is buried by successive layers it begins to transform under increased temperature and pressure 29

30 peat soft brown material (recognizable plant matter) lignite soft brown coal bituminous coal common low-grade coal (90% reduction in volume) anthracite rare high grade coal (metamorphic?) Coal in Outcrop Lignite beds in Arkansas Bituminous Coal, Colorado 30

31 Interpreting Sedimentary Rocks Depositional Environments Sedimentary Facies Sedimentary Rocks and Plate Tectonics Depositional Environments Continental Rivers and streams Lakes Deserts Caves Glaciers Transitional Coastal deposits Turbidity Currents Marine Shallow Marine (Shelf) Deep Marine 31

32 Depositional Environments 32

33 Depositional Environments Continental Fluvial (Streams and Rivers) Ancient Stream Deposits, Split Mountain, California 33

34 Continental Environments Desert Sand dunes 34

35 Continental Environment Desert Dunes Dune sandstone Continental Environments - Desert Alluvial fan 35

36 From Alluvial Fans to Breccia Depositional Environments Glacial 36

37 Glacial Till Glacial till consists of rounded to angular fragments that are not sorted by size and not laid down in horizontal beds. South African Tillite, source where Indian Ocean is now 37

38 Glacial Lakes - Varves Transitional Environments Where the land meets the sea Deltas Continental margins Tidal flats 38

39 Transitional Environments - Deltas Colorado River Delta Transitional Environments - Delta Flathead Lake, Montana 39

40 Sediments in Deltas Delta topset and Foreset beds Glacial Lake Deposit 40

41 Barrier Islands Barrier Island 41

42 Depositional Environments Marine Detrital Marine Environments Carbonate Marine Environments Evaporite Environments Marine depositional environments Detrital Marine Environments Continental Shelf Continental Slope Continental rise Deep Ocean 42

43 Detrital Marine Environments Turbidites on Continental Slope, Rise 43

44 Deep Marine Environment Many forms of Planktonic life fill the deep marine sediments. Distribution of deep marine sediments 44

45 Lateral Relationships Sedimentary Rocks Principle of Lateral Continuity Sedimentary rock layers extend outward in all directions until they terminate. Terminations Edge of depositional basin Later erosion of portion of basin Truncation by faults Pinching out Lateral gradation Sedimentary Facies Different adjacent environments produce different rocks and fossil assemblages Marine Transgressions and Regressions Transgression rise in sea level or sinking of land Regression- lowering of sea level or rise of land 45

46 Walther s Law The facies seen in a conformable vertical sequence will also replace one another laterally 46

47 Transgression / Regression 47

48 Reasons for Sea Level Change Amount of water frozen in glaciers Rates of Sea Floor Spreading Rapid spreading, high heat flow causes ridges to rise and displace water onto the continents causing universal transgression Slower spreading, lower heat flow, causes ridges to subside, increased volume of ocean basins causes universal regression. Sedimentary Rocks and Plate Boundaries Convergent-Subduction Greywacke sandstone Turbidite conglomerates Limestone Shale Convergent-Collision/ Transform Continental/ Divergent Continental Arkose sandstone Continental conglomerates Evaporites Fluvial shales Divergent Marine Chert Limestone Shale Passive Margins Quartz sandstone Shale Limestone 48