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2_Sedimentary_Rocks_2 - Quick Review: Sedimentary Rocks...

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Unformatted text preview: Quick Review: Sedimentary Rocks Part I Sedimentary Rocks Sediments are the building blocks of sedimentary rocks Four classes: Four Clastic Clastic Clastic / Detrital – Made from weathered rock fragments Clastic (clasts) (clasts) 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 Which of the agents (water, wind, ice) is responsible for Which weathering, erosion (what’s the difference?), and deposition weathering erosion deposition What parent rock the clasts are derived from In which environment lithification (transformation into In lithification solid rock) took place solid Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks From Sedimentary Particles From to Sedimentary Rocks to Main Processes of Lithification (“rock making”) Main (“rock Compaction Cementation – Minerals grow in pores, “gluing” clasts Cementation together together Clastic Sedimentary Rocks are classified on the basis of Clastic texture and composition texture composition Clast (grain) size Angularity and sphericity Degree of sorting Clast composition Clast Character of cement Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Clastic Particles in Transport With increasing transport and distance, average grain With size decreases size Boulder > cobble > pebble > sand > silt > clay Clasts get rounder or more sperical Sorting becomes better with distance from source Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Common Clastic Sedimentary Rocks Conglomerate & Breccia (difference?) Sandstones Siltstones & Shale Clasts held together by Cement Common cements: Quartz, Calcite, Hematite, Clay minerals Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Biochemical and Organic Rocks Sediments derived from living organisms Sediments Biochemical – Exterior or interior mineral skeletons; mostly marine Mainly limestone, often with shell remains Mainly limestone Also chert (opaline silica, SiO2) Also (opaline Organic – Cells of plants, algae, bacteria and plankton Coal Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Chemical Sedimentary Rocks Precipitated from water solution (no clasts) Evaporites – From evaporated seawater (climate?) seawater Evaporation triggers deposition of chemical precipitates Rock salt, gypsum, limestone, dolostone Precipitated from groundwater (hot springs, caves) Travertine – Calcium carbonate (CaCO3); non-marine Travertine Calcium Reverse of Carbonation weathering!! >>> Nonbiogenic varieties of Chert Flint, jasper, agate, petrified wood Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Carbonation: Carbon dioxide in water (carbonic acid, H2CO3) reacts reacts with carbonate rocks (CaCO3) to produce a very soluble with to product (calcium bicarbonate), which can readily be removed by runoff or percolation removed CaCO3 + H2CO3 >>> Ca (HCO3)2 >>> 2 HCO3- + Ca++ Formation of Chemical Sedimentary Rock: Dissolved calcium Ca++ reacts with bicarbonate (HCO3-) Water evaporates and CO2 is expelled into the air which causes CaCO3 to precipitate which ++ Ca++ + 2 HCO3- >>> 2 CO2 + H2O + CaCO3 CaCO Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks CaCO3 + H2CO3 >>> Ca (HCO3)2 >>> 2 HCO3- + Ca++ With increasing CO2 the reaction is moving towards destruction of CaCO3 >>> detrimental for reefs destruction Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Rocks Part II – Sedimentary Structures Features imparted to sediments at or near deposition Layering Surface features on layers (such as a coating) Surface Arrangement of grains Help decipher conditions at or near time of deposition Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Structures Sedimentary rocks are usually layered or “stratified” Sedimentary Arranged in planar, close-to-horizontal “beds” WHY? Arranged WHY? Bedding plane is one of the chief characteristics of sedimentary rocks sedimentary Bedding is often laterally continuous for long distances Beds are often similar in composition, color and texture Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Structures Bedding reflects changing conditions during deposition: These can be … Changes in energy >> affect grain size Disturbance by organisms Bedding may also reflect non-deposition or erosion Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Structures A series of beds are referred to as strata series strata Formation: Strata recognized on a regional scale Geologic maps display the distribution of formations i.e. Coconino Formation i.e. Note the prominent white band of Coconino Note the prominent white band of Coconino Sandstone clearly visible across Grand Canyon. Sandstone clearly visible across Grand Canyon. Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Structures Water flowing over loose sediment creates bedforms Bedforms linked to flow velocity, direction, sediment size Ripples, cm-scale ridges, and troughs, indicate flow Asymmetric ripples – Unidirectional flow Symmetric ripples – Wave oscillation in two directions Ripples are commonly preserved in sedimentary rocks Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Bedforms Cross beds – Created by ripple and dune migration Cross Created Sediment moves up the gentle side of a ripple or dune Sediment piles up, then slips down the steep face The slip face continually moves downstream Added sediment forms sloping “cross-bedded” layers Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Formation of Cross Beds Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Bedforms Dunes – Similar to ripples except much larger Form from wind-blown sand in desert or beach regions Often preserve large internal cross-laminations Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Booming Sands (7 minutes) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Bedforms Turbidity currents Sediment moves down-slope as a pulse of turbid water As pulse wanes, water loses velocity and grains settle Coarsest material settles first, medium next, then fines Coarsest This process forms graded beds in turbidite deposits This in Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Bed-Surface Markings Occur after deposition while sediment is still soft Mudcracks – Polygonal desiccation features in wet mud Mudcracks Polygonal Indicate alternating wet and dry conditions Indicates deposition in a terrestrial setting Scour marks – Troughs eroded in soft mud by current flow Scour Troughs Fossils – Evidence of past life Fossils Footprints Shell or leaf impressions Shell Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Depositional Environments Locations where sediment accumulates. They reflect … Energy regime Sediment delivery, transport, and depositional conditions transport and depositional Chemical, physical and biological characteristics Environments range from terrestrial to marine Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Terrestrial Depositional Environments Deposited above sea level Glacial – Ice carries and dumps every grain size Creates glacial till; poorly sorted gravel, sand, silt, and clay Creates poorly Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Terrestrial Depositional Environments Mountain streams Carry large clasts during floods >>> coarse conglomerates Carry conglomerates During low flow, these cobbles and boulders are deposited Alluvial fan - Sediments that pile up at a mountain front Alluvial Rapid drop in stream velocity (energy) >> a cone-shaped Rapid wedge with more angular clasts and arkoses (with feldspar) arkoses (with Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Terrestrial Depositional Environments Rivers – Channelized flow transports sediment Sand and gravel fill concave-upward channels Fine sand, silt, and clay are deposited on nearby floodplains Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Terrestrial Depositional Environments Lake – Large ponded bodies of water Gravels and sands trapped near shore (energy!) Well-sorted muds deposited in deeper water (energy!) Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Terrestrial Depositional Environments Sand dunes – Wind-blown piles of well-sorted sand Sand Dunes move according to the prevailing winds (“current”) Result in uniform sandstones with gigantic cross beds Result cross Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Marine Depositional Environments Deltas – Sediments dropped where a river enters the sea Sediment carried by the river is dumped when velocity drops New energy environment: Ocean waves, longshore currents Deltas grow over time, building out into the basin Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Marine Depositional Environments Coastal beaches – Surf zone Sediments are constantly being processed by wave action The result: Well-sorted, well-rounded medium sand The Beach sandstones may preserve oscillation ripples Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Marine Depositional Environments Shallow marine environments – Finer beach sediment Fine silts and muds >> siltstones and mudstones Usually active biotic community Usually >> future fossils/ fossil fuels >> Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Marine Depositional Environments Tropical shallow environments – Carbonates Warm, clear, shallow, normal salinity, marine water Reef communities; skeletons of marine invertebrates Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Marine Depositional Environments Deep marine environments – Far from land sources Fine grain sizes dominate (silts and clays) Skeletons of planktonic organisms make chalk or chert Skeletons chert Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Basins Sediments vary in thickness across Earth’s surface Basins form where tectonic activity creates space Basins Subsidence – Sinking of the land during sedimentation Subsidence May be several tens of km thick in center Crustal flexure and faulting; weight of added sediments Crustal Basins are important locations for natural resources: Coal Petroleum Natural gas Uranium Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Tectonic Activity Creates Sedimentary Basins Rift basins – Divergent (pull-apart) plate boundaries Crust thinning, stretching, and rotational normal faulting Sediment fills the down-dropped basin Passive continental margin basin Crust thinned by previous rifting subsides as it cools Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Foreland Sedimentary Basins Craton side of collisional mountain belt Craton Flexure of the crust from loading creates a downwarp Fills with debris eroded off of the mountains Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Intracontinental Sedimentary Basins Interiors far from margins Result from differential thermal subsidence May be linked to failed crustal rifts Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Basins & Sea-Level Changes Sedimentary deposition is strongly linked to sea level: Sedimentary Changes in sea level are common in geology Changes Depositional belts shift landward or seaward in response Layers of strata record deepening or shallowing upward Transgression – Flooding due to sea-level rise Transgression Flooding Sediment belts shift landward; strata “deepen” upward Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Sedimentary Basins & Sea-Level Changes Regression – Exposure due to sea level fall Regression Exposure Depositional belts shift seaward; strata “shallow upward” Regression tied to erosion; evidence less likely to be Regression preserved preserved Sea-level rise and fall creates a zig-zag pattern Transgressions - Regression Animation Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks Diagenesis Physical, chemical, and biological changes to sediment Physical, Bioturbation Lithification Dissolution Mineral precipitation Temps between burial and metamorphism (~300oC) Integrates changes across the entire sediment history Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks What to remember: Environment of formation of sedimentary Environment particles and sedimentary rocks particles Types of sedimentary rocks and examples Difference between weathering and erosion Difference weathering erosion Main processes of lithification: compaction and Main lithification compaction cementation cementation What are the textural criteria in clastic What sedimentary rocks? sedimentary What do those textures tell us? Essentials of Geology, 3rd edition, by Stephen Marshak Chapter 6: Pages of Earth’s Past: Sedimentary Rocks What to remember: What do sedimentary structures tell us? How do they reflect the (changing) energy of the How environment? environment? There are two types of ripples … How does cross-bedding form? How form? What are turbidite deposits? What turbidite What is an alluvial fan, a delta? How do they What alluvial delta How form? form? Know some mechanisms of basin formation 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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