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Sedimentary Rocks Sedimentary Rocks: 5% of the crust, but 75% of the surface area. PROCESSES Transportation: Rounding - increases with length of transportation history. Sorting: increases with length of transportation history (weaker minerals broken down). Deposition: any process that lays down material. Environment of Deposition: location in which deposition occurs. Use the Principle of Uniformitarianism to study ancient environments. Preservation: deposition and burial in a basin. Reworking degrades preservation. Fig. 06.01 Transportation: Rounding increases with length of transportation history. Boulders have been abraided Transportation Sorting: increases with length of transportation history (weaker minerals broken down). Fig. 06.04 Deposition: any process that lays down material. Environment of Deposition: location in which deposition occurs. Use the Principle of Uniformitarianism to study ancient environments. Types of Sediment Clastic or Detrital Deposited grains of rocks and minerals derived intact from source, transported by gravity, water, wind, ice. Organic Shells, shell fragments, bones, reefs, etc. May be transported or precipitated in situ. Inorganic Chemical Chemical precipitates due to precipitation of dissolved constituents. Transport possible, but rare (salt, limestone, gypsum). Clastic Sediments Grains: main or large “bits” of sediment Matrix: fine interstitial particles. Grain and matrix composition depends upon: Source material – what type of rock is being weathered and eroded? The chemical and physical properties of the minerals – stronger ones are at the bottom of Bowens Reaction Series (e.g., Quartz). Degree of weathering and erosion processes to transport – secondary dependence upon climate. Clastic Sediments Texture: Depends on transport history. Size (see Table 6.1): Clastic Sediment Textures Roundness/Angularity: Transport by wind or water – rounding occurs. Transport by ice or gravity – angular. Degree of Sorting: Selection/separation of grains is on the basis of size, shape, specific gravity. Poorly sorted: fast deposition, high energy. Well sorted: slow deposition, less chaotic. Poorly Sorted Clastic Sediment Textures Maturity: Chemical and physical weathering gradually breaks down rock to quartz and clay minerals. Maturity is a measure of this process. Depends on: Time, Climate, Transport History, Depositional Environment. Immature: poorly sorted, clays + mineral fragments (olivine, pyroxene, amphibole, etc.), rock fragments. Grains are angular and vary in size. Mature: well sorted, no clays, mostly made up of rounded quartz grains. Lithification: Compaction and Cementation. 1) Compaction: reduction of pore space due to weight of overlying sediments. Porosity = percentage of voids: Loose sand: 30-50% Lithified sand: 10-20% Siltstone: <5% Granite: 0%! Permeability: ability of rock to allow water to pass. Requires that pores are interconnected. Reorientation of grains reduces permeability/porosity. Lithification 2) Cementation: precipitation from pore water (CaCO3, SiO2, Fe2O3, etc.). Crystalline Limestone F.O.V. = 3 mm Fig. 06.09 Conglomerate: coarse (> 2 mm), rounded clasts (gravel) + finer groundmass. Breccia: coarse (> 2 mm), angular clasts (gravel) + finer groundmass. Fig. 06.10a Sandstone: medium (1/16-2 mm), 90% quartz + matrix. Siltstone: fine (1/256 – 1/16 mm), fine-grained sandstone, feels gritty. Arkose: quartz, feldspar, matrix. Area of deposition close to area of erosion - feldspar weathers relatively easy. Greywacke: >15 % clay matrix, poorly sorted, immature, formed by turbidity currents. Earthquake triggers slumping of sediments down continental slopes. Can form V-shaped valleys in continental slopes. Shale: mostly clay, fissile (splits because of preferred orientation of clays. Feels smooth. Quiet deposition. Shale Organic Sediments Derived due to organic activity. Bioclastic limestone: fossils + calcite/dolomite. Fine-grained Coralline Mud Coarse-grained Bioclastic Limestone Fig. 06.16 Corals precipitate CaCO3 to form limestone in a reef. Water depth about 25 feet. Warm and clear water. A living coral-algal reef sheds bioclastic sediment into the fore-reef and back-reef environments. The fore reef consists of coarse, angular fragments of reef. Coralline algae are the major contributors of carbonate sand and mud in the back-reef environment. Beaches and dunes = bioclastic sand. The sediments in each area can lithify to form highly varied limestones. Coralline algae on the seafloor off the Bahamas can produce large quantities of carbonate mud. Organic Sediments Coquina: rock composed of shell fragments only. Organic Sediments Chalk: made up of calcareous fine grained planktonic micro-organisms. Organic Sediments Diatomite/Chert: Siliceous fine grained planktonic organisms (diatoms). Bedded Cherts Chert Nodules Organic Sediments Coal: modified plant material. Coal Bed Table 06.02 Inorganic/Chemical Sediments Formed by chemical precipitation from solution. Precipitation sequence from seawater: Calcite (CaCO3) Anhydrite (CaSO4) Gypsum (CaSO4.H2O) Halite (NaCl) Sylvite (KCl) Salt (and mud) deposited on the floor of a dried up desert lake, Utah. Inorganic/Chemical Sediments Oolitic Limestone: Ooliths/ooids and Pisoliths Limestone is precipitated directly from seawater. Sedimentary Structures Give evidence of depositional environments. Sedimentary rocks are deposited originally in horizontal beds. Later deformation causes the beds to be inclined. Which was the original way up? Structures can give that information. Bedding – horizontal sheets differing in composition. Sedimentary structures can give idea of paleocurrents. Horizontal Bedding: Beds get younger upward. Deposited in water. Box 06.02.f1 Layered sedimentary rocks on Mars. Candor Chasma, Mars Development of Cross Bedding Fig. 06.27 Cross-bedded Sandstone. Sedimentary Structures Graded Bedding (turbidity current) – beds grade from coarse grained at the bottom to fine grained at the top. Sedimentary Structures Mud Cracks – drying on top of bed. Sedimentary Structures Ripple Marks – wave action, form on top of beds. Fig. 06.32c Ripple Marks Fig. 06.32d Ripple Marks Fossils – traces of plants or animals preserved in sedimentary rocks. Formations Extremely thick body of rock with characteristics that distinguish it from adjacent rock units and is large enough to map. Formations are based on rock type. Can be a single thick bed, several thin beds of the same rock type, or an alternating sequence of two rock types. Main criterion for distinguishing and naming a formation is some distinguishable characteristic that makes it a recognizable unit. This could be rock-type or sedimentary structure. Name: Geographic location followed by rock-type Chattanooga Shale. Fig. 06.35 Formations Source Area Where the sediments were eroded from. Source Area Locating the source area. Deposit thicker to SW Grains coarser to SW Cross-bedding shows paleocurrents came mostly from the SW Depositional Environments Continental: glaciers, streams (fluvial), lakes (lacustrine), springs and groundwater, wind. Transitional: deltas and beach deposits. Marine: shallow (continental shelf) and deep (abyssal plains) deposits. Depositional Environments Continental Deposits Glacial: glacial sediment = till - unsorted mix of unweathered clasts in a clay matrix. Alluvial Fan: coarse, arkosic sandstones and conglomerates, marked by cross-bedding and lensshaped channel deposits. Form where a river emerges on to a valley floor from a mountain chain. Continental Deposits River Channel: lenses of conglomerate or sandstone (arkosic or sand-size rock fragments). Typically cross-bedded with ripple marks. Continental Deposits Lake: Thin-bedded shales, possibly with fish fossils. May contain mud-cracks and interbedded evaporates, if it periodically dried up. Floodplains: thin-bedded shales with mud-cracks & fossil footprints. Hematite may color the floodplain deposits red. Transitional Deposits Delta: formed when river flows into a body of standing water (lake or sea). Mostly thick sequences of siltstone and shale, marked by low-angle cross-bedding, cut by coarse-grained channel deposits. Can contain peat/coal beds as well as marine fossils. Transitional Deposits Beach, Barrier Island, Dune: a Barrier Island is an elongate sand bar built by wave action. All are comprised of well-sorted quartz sandstones with rounded grains. Beach and Barrier Island: low angle cross-bedding and marine fossils. Dune: high-angle and low-angle cross-bedding and occasional fossil footprints. All 3 environments can also contain carbonate sand in tropical areas producing cross-bedded clastic limestone. Dune Deposits Cross-bedded Sandstone. Barrier Island Beach Marine Deposits Lagoon: semi-enclosed body of water between a barrier island and the mainland. Fine grained dark shale cut by tidal channels of coarse sand containing marine fossils. Limestones may also form in lagoons adjacent to reefs. Marine Deposits Shallow Marine Shelves: grain size decreases offshore. Widespread sandstones, siltstones, shales. Sandstone & siltstone contains ripple marks, low-angle cross-beds, & marine fossils. If tidal flats near shore are alternately covered & exposed, mud-cracked marine shales form. Reefs: Massive limestone in core of reef, with steep beds of limestone breccia forming seaward, horizontal beds of sand-sized and finer-grained limestones form landward. All are full of fossil fragments (coral, shells, etc.). Deep Marine Deposits: shale = quiet deposition; greywacke sandstones (with graded bedding and current ripple marks) deposited by turbidity currents. General Aspects of Sedimentary Rocks Composition of grains/clasts: tells about source. Age of grains: tells about age of source and transportation history (e.g., zircon). Texture/Maturity: tells about transportation history. Sedimentary structures: tell about depositional environment. Fossils: tell about depositional environment and age of deposit. Plate Tectonics Convergent/Transform Plate Boundaries: Rapid uplift = rapid erosion and sedimentation. Coarsegrained clastic sedimentary rocks. Look at the composition of large clasts in conglomerates/ breccias to see the record of rocks already eroded. Erosion exposes rocks from depth (igneous intrusions and metamorphic rocks). Marine sedimentary rocks found at the top of mountains! Fig. 06.41 Plate Tectonics Divergent Plate Boundary: Rifting during formation of a new ocean basin. Rift valley forms and fills with thick wedges of gravel and coarse sand along faulted margins. Lake deposits & associated evaporates in the bottom of the rift. 0047 0048 0049 0050 0051 0052