Lecture 16 (10042024) Sedimentary Rocks PDF

Chapter 6 Pages of the Earth’s Past: Sedimentary Rocks (….contd) Copyright © 2022 W. W. Norton & Company, Inc. Recap  What are sedimentary rocks  How are they formed – Weathering, erosion, lithification  Types of weathering – physical, chemical  Physical weathering – formation...

Chapter 6 Pages of the Earth’s Past: Sedimentary Rocks (….contd) Copyright © 2022 W. W. Norton & Company, Inc. Recap  What are sedimentary rocks  How are they formed – Weathering, erosion, lithification  Types of weathering – physical, chemical  Physical weathering – formation of joints, frost wedging, root wedging, exfoliation, thermal expansion  Chemical weathering – dissolution, oxidation, hydrolysis  Differential weathering – based on shape, based on rock type  Types of sedimentary rocks – clastic, biochemical, organic, chemical  Clastic sedimentary rocks are classified based on mineral content (quartz, feldspar), rock fragment content and matrix/cement content  Classification of clastic sedimentary rocks based on grain size (ref Table), mineralogy/rock fragments, sorting, angularity/sphericity of grains  Examples of clastic sedimentary rocks – Breccia, Conglomerate, Arkose, Sandstone, Shale and Mudstone  Biochemical sedimentary rocks – Limestone and Chert of organic origin  Organic sedimentary rocks – Coal and Oil shale  Chemical sedimentary rocks – Evaporites like halite and gypsum, Travertine, replacement Chert of inorganic origin. Diagenesis  Diagenesis is a process that occurs when sediment is buried and subjected to increasing pressure and temperature, causing chemical and physical changes.  It describes the sum total of all physical, chemical, and biological changes that occur to a sediment after it is deposited  It includes processes like lithification (compaction and cementation), dissolution, recrystallisation, bacterial action, hydration etc. Sedimentary Structures  Sedimentary structures are features that form when sediments were deposited. They are extremely useful to geologists because they provide strong evidence about conditions in the depositional environment. Bedding and Stratification  Bedding and stratification are prominent features of sedimentary rocks. They include surface features on bedding layers and the arrangement of grains within bedding layers.  Sedimentary rocks are usually layered or stratified in planar, close to horizontal beds. The boundary between two beds is a bedding plane.  Bedding often stands out clearly in sedimentary rocks, appearing as bands or stripes that can be traced laterally across an outcrop. Formation of Bedding  Bedding reflects changing conditions during deposition. These may be changes in transporting medium, changes in the sediment source, etc., which result in changes to sediment composition, grain size, sorting, etc. Stratigraphic Formations  Some rock units are so unique that they can be recognized—and mapped—over large regions; they are termed formations. Formations are named for places where they are best exposed. Geologic maps display the distribution of formations. Ripple Marks and Dunes  Water flowing over loose sediment creates bedforms, which directly reflect flow velocity and grain size. Bedforms in ancient sediments are useful indicators of environmental conditions.  Ripple marks are cm-scale ridges and troughs that develop perpendicular to flow in sandy sediments. Dunes are larger scale (50 cm to over 100 m) versions of ripple marks. Cross Bedding  Cross beds are created by ripple and dune migration. Sand moves up the gentle side and piles up at the crest. Then, it slips down the steep face. The slip face moves down-current and is buried by the next avalanche of sand. The slip faces are preserved as cross beds. Turbidity Currents and Graded Beds  Turbidity currents: form in deep basins that receive periodic pulses of turbid water. As pulse wanes, water loses velocity and grains settle. The coarsest material settles first, medium next, then fine. This process forms graded beds (coarse to fine upward). Mud Cracks and Scour Marks  Mud cracks indicate alternate wet and dry terrestrial conditions. Scours form when debris is dragged along the bottom of a river. The scour is filled in with sediment and can make casts. Class Question 4 (T/F) Ripple marks are ridges and troughs that develop parallel to flow in sandy sediments. A. True B. False Class Question 4 Answer (T/F) Ripple marks are ridges and troughs that develop parallel to flow in sandy sediments. A. True B. False (Correct) Depositional Environments  Depositional environments are locations where sediment accumulates. Different environments vary in energy regime, sediment transport, and depositional processes and chemical, physical, and biological characteristics, all of which conspire to create unique sedimentary rocks. Glacial Environments  In glacial environments, sediments are created, transported, and deposited by the actions of moving glacial ice. Ice carries and dumps every grain size. A common feature of this environment is glacial till, a poorly sorted mixture of all grain sizes, gravel, sand, silt, and clay. Mountain Stream Environments  In mountain stream environments, water carries large clasts during floods. During low-flow conditions, cobbles and boulders are immobile. Coarse conglomerate is a characteristic of this setting. Alluvial Fan Environments  Alluvial fans are cone-shaped wedges of sediments that pile up where a rapid drop in stream velocity occurs at a mountain front. Desert Environments  Sand-dune environments develop where there is an abundance of wind-blown, well- sorted sand. Dunes move according to the prevailing winds and result in uniform sandstones with gigantic cross beds. River Environments  River environments preserve evidence of channelized sediment transport. Sand and gravel fill concave-up channels that often scour into previously deposited floodplain fines. Fine sand, silt, and clay are deposited on nearby floodplains. Lake Environments  Lake environments result from large ponded bodies of freshwater. Gravels and sands are trapped near shore. Well-sorted muds are deposited in deeper water. They are often capped with wetland muds. Deep lake muds may show varves, thin stripes of alternating finer and coarser sediment reflecting seasonal changes in sedimentation. Marine Delta Environments  In a marine delta environment, sediment accumulates where river velocity drops upon entering the sea. Deltas grow over time, building out into the basin. Many sub- environments are present. Shallow-Water Carbonate Environments  Shallow-water carbonate environments develop in tropical, warm, clear, shallow, normal salinity, marine water. Protected lagoons accumulate mud. Wave-tossed reefs are made of coral and reef debris. Deep-Marine Environments  Deep-marine deposits accumulate fines that settle out far from land. Fine silts and clays lithify into shale. Skeletons of planktonic organisms make chalk or chert. Sedimentary Basins: Plate Tectonics  Sedimentary basins are special places that accumulate sediment. Sediments vary in thickness across Earth’s surface from zero to 20+ km in sedimentary basins.  Rift basins form at divergent (pull-apart) plate boundaries.  Passive margins are the edges of continents that are not tectonic-plate boundaries.  Intracontinental basins form in the interior of the craton, far from continental margins or tectonic-plate boundaries.  Foreland basins form on the craton side of collisional mountain belt. Sedimentary Basins: Transgression and Regression  Sediment deposition is strongly linked to sea level. Changes in sea level, common geologically, shift depositional environments relative to one another, and sea-level rise or fall creates a predictable pattern of strata.  A sea-level rise or transgression shifts depositional beds landward.  A sea-level fall or regression shifts environments toward the basin. Transgression and Regression: Narrated Figure Video QUESTIONS