Chapter 8: Sedimentary Rocks PDF

Chapter 8: From Sediment to Sedimentary Rock Chapter cover photo Weathering and erosion produces solid debris (sand and clay) which will form clastic sediment and dissolved ions from which chemical sediment can form  sediment = rego...

Chapter 8: From Sediment to Sedimentary Rock Chapter cover photo Weathering and erosion produces solid debris (sand and clay) which will form clastic sediment and dissolved ions from which chemical sediment can form  sediment = regolith that has been transported streams move it downslope until the rivers flow out on to flatter plains decrease in slope = less energy = some of the sediment will be deposited in the channel-floodplain system, or will continue to a lake or the sea it will accumulate in these areas as flat layers of sediment  the layering is caused by changes in the flow velocity causing changes in the grain size of the sediment being deposited at lower velocities finer grained material is deposited. if the velocity changes abruptly a sharp contact develops between the coarser and finer layer  the layering is called bedding or stratification a layer is called a bed or a stratum sharp contacts between the beds show that abrupt changes in velocity  Varves  an example in flow velocity changes there is a gradual change in each of these layers from the coarser silts (lighter material) deposited in the summer to the darker clays deposited in the winter. each summer layer rests sharply on the previous winter layer- termed VARVES Sediments which are not clastic can be divided into: 1) biochemical and 2) chemical Clastic Sediments Clastic Sediment Magnified quartz sand Clastic Sediment Magnified quartz sand As the sediment moves it collides with the bed and other sediment grains This action slowly wears the edges and corners of the grain down, making it more smooth and rounded the further it travels grain size distribution  waves on beaches are even better than streams at sorting sand the constant back and forth action of the waves washes all the fine material away the average energy of the waves controls the size of the material which will form the beach  glaciers cannot sort sediment like streams can - deposits are always poorly sorted streams and currents in lakes and the ocean are much better at sorting sediments well sorted and rounded poorly sorted and angular cobbles on a beach cobbles in a glacial deposit Sand: roundness also has to do with mineral hardness Sediment Biogenic Sediment Biogenic sediment Sediment that is composed primarily of plant and animal remains  E.g., Shells, bones, teeth, Figure 8.4(a) – Biogenic sediment wood, roots, leaves Or, sediment that precipitates as a result of biologic processes  Sedimentary Rock Types of Sedimentary Rocks Biogenic sedimentary rocks Limestone A sedimentary rock that consists primarily of the mineral calcite Most abundant biogenic sedimentary rock Coal A combustible rock formed from the lithification of plant-rich sediment  biochemical sediments are produced by organisms most are made of calcite and are formed from shells this type of sediment dominates in a lot of tropical areas  reefs are good examples of these sediments, but shell fragments can form the sand on beaches and comprise the dominant sediment in large offshore areas or entire islands- such as Bermuda. carbonate shell debris dominates in areas where there is little supply of clastic sediment. Biochemical Sedimentary Systems  Limestone is by far the most important type of biochemical sedimentary rock it is commonly formed out of either whole or fragmented shells.  Biogenic Sediment Diatoms, a group of algae with outer casing made of amorphous silica that accumulates on the ocean floor Scanning Electron Microscope image by Berezovska Coal is a biochemical sedimentary rock composed of the altered remains of land plants Copyright: (c) Missisya | Dreamstime.com Sediment Chemical Sediment Sediment formed by inorganic precipitation of minerals dissolved in lake water, river water, or ocean water Plants and animals alter the chemical balance: Limestone deposits Shallow sea water evaporation causes dissolved salts to precipitate. Chemical Sedimentary Rocks These rocks are formed by the evaporation of saline water, either the ocean or a salt lake  Gypsum Halite Chert Sedimentary Rock Clastic sedimentary rock Conglomerate has large fragments in a finer-grained matrix. Sandstone is medium-grained, where clasts are typically, but not necessarily, dominated by quartz grains. Mudstone is a very fine-grained sedimentary rock of the same composition as shale but without fissility. Shale is a very fine-grained fissile or laminated sedimentary rock, consisting primarily of clay-sized particles. Sedimentary Rock Types of Sedimentary Rocks Chemical sedimentary rocks Result from the lithification of chemical sediment E.g., Banded iron formation A type of chemical sedimentary rock rich in iron minerals or silica Atmospheric change recorded in rock What a Geologist Sees, page 213 Banded Iron Formation, estimated age of 2.5 billion years, formed when dissolved iron in seawater precipitated as chemical sediment Sedimentary Rock Rock Beds Lithification The processes by which loose sediment is transformed into sedimentary rock Bedding Figure 8.10 – Bedding The layered arrangement of strata in sediment/sedimentary rock Bedding surface The top or bottom surface of a rock stratum or bed Sedimentary Rock Lithification In order for sediment to be lithified (turned into rock), individual particles must be bound together, undergoing three processes: 1. Compaction 2. Cementation 3. And/or Recrystallization How lithification occurs How lithification occurs Figure 8.12 – How lithification occurs How lithification occurs Recrystallization Sedimentary Rocks Clues to a Bed’s Origin Photographer: David K. Lynch  Sometimes layers of sediment are deposited inclined to the horizontal – we call this cross-bedding or cross-stratification produced by ripples or dunes moving along the bed the avalanching of sand down the backs of these features produces the inclined layering Large-scale cross-bedding caused by the avalanching of sand down the back of wind-blown sand dunes. Sedimentary Rock Interpreting Environmental Clues Ripples forming in a shallow water Exposed ripple marks on environment sandstone bedding surfaces Sedimentary rocks can preserve all the features found in sediment Sedimentary Rock Interpreting Environmental Clues Gull footprints imprinted on a sandbar Fossilized foot-prints preserved in sandstone Figure 8.14(e) – Ancient and modern Figure 8.14(f) – Ancient and modern features (footprints) features (footprints) Delicate raindrop imprints formed by rain that fell on a mud flat about a billion years ago Sedimentary Rock Interpreting Environmental Clues Mud cracks formed on a modern Mud cracks preserved on exposed river bed shale Figure 8.14(c) – Ancient and modern Figure 8.14(d) – Ancient and modern features (mud cracks) features (mud cracks) Interpreting Environmental Clues Fossil fern found in Greenland from the Triassic Period Fossil Worm burrows (Arthrophycus) Credit SCIENCE STOCK PHOTOGRAPHY / SCIENCE PHOTO LIBRARY Sedimentary Rock Sedimentary Facies Figure 8.16 – Sedimentary Facies