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FROM SEDIMENT INTO SEDIMENTARY ROCK Objectives FROM SEDIMENT INTO SEDIMENTARY ROCK Objectives • Identify three types of sediments. • Explain where and how chemical and biogenic sediments form. • Explain three processes that lead to the lithification of sediments. • Explain how features such as ripples, cracks, and fossils tell geologists about the environment in which a rock originated. • Identify plate tectonics that are favorable for the accumulation of sediments. Sediments and Sedimentation • Deposition – The laying down of sediment • Sediment separated into three broad categories – Clastic – Chemical – Biogenic 1 Sediments and Sedimentation Sediments and Sedimentation • Clastic sediment – Sediment formed from fragmented rock and mineral debris – Produced by weathering and erosion – Described by particle shape, angularity, and size • Clastic sediment – Volcaniclastic sediments • Volcanic in origin • Pyroclasts – Distinguished by size » Bombs » Lapilli » Ash 2 Clastic sediment Clastic sediment Clastic sediment Glacial till: poorly sorted Well sorted, well rounded, quartz sand 3 Sediments and Sedimentation • Chemical sediment – Sediment formed by the precipitation of minerals dissolved in lake water, river water, or sea water – Plants and animals alter chemical balance • Limestone – Shallow sea water evaporation causes dissolved salts to precipitate Sediments and Sedimentation • Biogenic sediment – Sediment that is primarily composed of plant or animal remains • Shells, bones, teeth • Wood, roots, leaves – Or, precipitates as a result of biologic processes: foraminifer in the head of a pin Sedimentary Rocks • Lithification – The processes by which loose sediment is transformed into sedimentary rock • Bedding – The layered arrangement of strata in sediment/sedimentary rock • Bedding surface – The top or bottom surface of a rock stratum or bed 4 Lithification Processes Lithification Processes Lithification Processes • Compaction – Reduction of pore space in a sediment as a result of the weight of overlying sediments • Cementation – Substances dissolved in pore water precipitate out and form a matrix in which grains of sediments are joined together 5 Lithification Processes • Recrystallization – The formation of new crystalline mineral grains What kind of sediment? • How do you know? • How do you know? Compare textures of a sedimentary rock with an igneous rock (granite) 6 Types of Sedimentary Rocks • Clastic sedimentary rock – Conglomerate • Has large fragments in a finer grained matrix – Sandstone • Medium grained, where clasts are typically, but not necessarily, dominated by quartz grains – Mudstone • A very fine grained sedimentary rock of the same composition as shale but without fissility – Shale • A very fine grained fissile or laminated sedimentary rock, consisting primarily of clay sized particles Conglomerate Figure 7.6 Quartz sandstone Figure 7.4 7 Breccia Figure 7.7 Shale with plant remains Figure 7.2 Types of Sedimentary Rocks • Chemical sedimentary rocks – Evaporite • Formed by the evaporation of lake water or sea water, followed by lithification of the resulting salt deposit – Banded iron formation • A type of chemical sedimentary rock rich in iron minerals or silica 8 Evaporites Bedrock geology of Lower Michigan http://www.deq.state.mi.us/documents/deq-glm-rcim-geology- 1987_Bedrock_Geology_Map.Pdf 9 Travertine-Mammoth Hot Springs, Yellowstone NP Banded Iron Formation (BIF) Types of Sedimentary Rocks • Biogenic sedimentary rocks – Limestone • A sedimentary rock that consists primarily of the mineral calcite – Peat • Formed from the accumulation and compaction of plant remains – Coal • A combustible rock formed from the lithification of plant-rich sediment 10 Coquina Figure 7.9 Chalk White Cliffs of Dover 11 Depositional Environments • Interpreting environmental clues – Patterns formed by air and water moving over sediments Ripple marks • Preserved and later exposed – Ripple marks – Fossils – Mud cracks Characteristics-fossils Fossiliferous limestone 12 Mud cracks Depositional Environments on Land Depositional Environments on Land • Streams • Lakes – Delta • A sedimentary deposit, commonly triangle shaped, that forms where a stream enters a standing body of water – Glacier – Wind • Eolian sediment – Sediments that are carried and deposited by the wind 13 Depositional Environments on Land Delta Old Lake bed Lacustrine sediments-Lake Michigan Bluffs Depositional environments in and near the ocean • Delta • Estuary – Semi-enclosed body of coastal water, in which fresh water mixes with sea water • Beaches • Shelves • Carbonate platforms and reefs Green beach due to olivine 14 Chesapeake Bay: An Estuary 15 Great Barrier Reef Depositional environments in and near the ocean Carbonate platform in the Bahamas 16 Depositional environments in and near the ocean • Turbidites – A turbulent, gravity driven flow consisting of a mixture of sediment and water, – Conveys sediment from the continental shelf to the deep sea • http://www.youtube.com/watch?v=tfNLI2JW7mg&NR=1 • Seafloor – Rich in nutrients • Calcareous ooze • Siliceous ooze Deep sea sediments deposited by turbidity current (turbidite) How Plate Tectonics Affect Sedimentation • Divergent plate boundaries – Rift valleys • A linear, fault-bounded valley along a divergent plate boundary or spreading center • Convergent plate boundaries – Collisional type – Subduction type • Back-arc basin • Accretionary wedges • Ophiolites (slabs of oceanic lithosphere) 17.
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