Sedimentary_Rocks_1

Sedimentary_Rocks_1 - Pages of Earth’s Past: Sedimentary...

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Unformatted text preview: Pages of Earth’s Past: Sedimentary Rocks Part I Sedimentary Cover Earth is covered by a thin ‘veneer’ of sediment The veneer caps igneous and metamorphic “basement” The Sediment cover varies in thickness from 0 to 20 km Thicker in structures like basins Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Rocks Sediments are the building blocks of sedimentary rocks Four classes: Four Clastic Clastic Clastic – Made from weathered rock fragments (clasts) Clastic Made Biochemical – Cemented shells of organisms Biochemical Cemented Organic – The carbon-rich remains of plants Organic The Chemical – Minerals that crystallize directly from water Chemical Biochemical Biochemical Essentials of Geology, 3rd edition, by Stephen Marshak Organic Organic Chemical Chemical Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Clastic sedimentary rocks reflect several processes Weathering – Generation of detritus via rock disintegration Erosion – Removal and transport of sediment grains from Erosion rock, via wind, water, and ice rock, Deposition – Settling out of the transporting fluid Lithification – Transformation into solid rock Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Lithification – Transforms loose sediment into solid rock Compaction – Overburden weight reduces pore space Sand – 10 to 20% Clay – 50 to 80% Cementation – Minerals grow in pores, “gluing” sediments Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Classified on the basis of texture and composition (yay!) Classified texture composition Clast (grain) size Angularity and sphericity Degree of sorting Clast composition Clast Character of cement E ssentials of Geology, variablesMarshak These 3 edition, by Stephen produce rd a diversity of clastic rocks. Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Clast (grain) size – The average diameter of clasts Range from very coarse to very fine Boulder > cobble > pebble > sand > silt > clay With increasing transport, average grain size decreases With WHY? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Clast composition May be individual minerals or rock fragments Mineral identities provide clues about … The source of the sediment The environment of deposition Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Angularity and sphericity – Indicate degree of transport Angularity Indicate Fresh detritus is usually angular and non-spherical Fresh Grain roundness and sphericity increases with transport Grain Well-rounded – Long transport distances Angular – Negligible transport Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Sorting – Uniformity of grain size and grain composition Well-sorted – Uniform grain sizes and composition Poorly sorted – Wide variety of grain sizes and composition Sorting becomes better with distance from source Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Time For Another Food Analogy Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks 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 Clay Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Breccia & Conglomerate Both rocks below have clast sizes that are larger than 2 mm (gravel size). The rock on the left has rounded clasts and is called conglomerate. The rock on the right has angular clasts and is classified as breccia. Angularity indicates a lack of transport processing Deposited relatively close to source Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Conglomerate Coarse, gravel-sized rounded clasts Coarse, Indicates water transport Clasts bang together forcefully in flowing water Collisons round angular corners and edges of clasts Conglomerates are deposited at a distance from the source Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sandstone Clastic rock made of sand-sized particles Forms in many depositional settings Quartz: far, the dominant mineral in sandstones – WHY? Quartz: WHY Sandstone varieties: Sandstone Arkose – Contains abundant feldspar (left) Quartz sandstone – Almost pure quartz (right) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Sedimentary Rocks Fine clastics - Composed of silt and clay Fine Silt-sized sediments are lithified to form siltstone Silt-sized Clay-sized particles form shale Clay-sized shale Fine clastics are deposited in quieter waters Fine Floodplains, lagoons, mudflats, deltas, deep-water basins Floodplains, Organic-rich shales are the source of petroleum Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Biochemical and Organic Rocks These are sediments derived from living organisms These Biochemical – Hard exterior or interior mineral skeletons Organic – Cells of plants, algae, bacteria and plankton Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Biochemical Rocks Biochemical limestone – CaCO3 skeletal (shell) remains Biochemical limestone Warm, tropical, shallow, clear, O2-rich, marine water -rich, Diverse organisms (plankton, corals, clams, snails, etc.) Many textural varieties: Reefs fossils Shell debris Lime mud (micrite) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Biochemical Rocks Biochemical Chert – Rock made of cryptocrystalline quartz Chert Rock Formed from opaline silica (SiO2) skeletons Diatoms (plants) Radiolarians (animals) Opaline silica added to bottom sediments dissolves Opaline Silica pore fluids solidify to form chert nodules or beds Silica Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Organic Rocks Made from organic carbon Coal – Altered remains of fossil vegetation Coal Accumulates in lush tropical wetland settings Accumulates Requires deposition in the absence of oxygen Oil shale – Shale with heat-altered organic matter Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Chemical Sedimentary Rocks Comprised of minerals precipitated from water solution Evaporites – Created from evaporated seawater Created Evaporation triggers deposition of chemical precipitates Examples include halite (rock salt) and gypsum Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Chemical Sedimentary Rocks Travertine – Calcium carbonate (CaCO3) precipitated Travertine Calcium from groundwater where it reaches the surface Dissolved calcium (Ca2+) reacts with bicarbonate (HCO3-) CO2 expelled into the air causes CaCO3 to precipitate Thermal (hot) springs Caves Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Chemical Sedimentary Rocks Dolostone – Limestone altered by Mg-rich fluids Dolostone CaCO3 altered to dolomite CaMg(CO3)2 by Mg2+-rich water by Dolostone looks like limestone, except … It has a sugary texture and a pervasive porosity Usually darker than limestone; doesn’t fizz easily Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Chemical Sedimentary Rocks Replacement chert – Nonbiogenic in origin Replacement Many varieties: Many Flint – Black or gray from organic matter Jasper – Red or yellow from Fe-oxides Petrified wood – Wood grain preserved by silica Agate – Concentrically layered rings Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks ...
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This note was uploaded on 02/25/2012 for the course EARTH 2 taught by Professor Thomas during the Summer '11 term at UCSB.

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