Sedimentary Rocks Chapter 5 Dynamic Earth

Major Concepts Sedimentary rocks form at Earth’s surface by the hydrologic system. Their origin involves the weathering of preexisting rock, transportation of the material away from the original site, deposition of the eroded material in the sea or in some other sedimentary environment, followed by compaction and cementation. Two main types of sedimentary rocks are recognized: (a) clastic rocks and (b) chemically precipitated rocks, including biochemical rocks. Stratification is the most significant sedimentary structure. Other important structures include cross-bedding, graded bedding, ripple marks, and mud cracks.

Major Concepts The major sedimentary systems are (a) fluvial, (b) alluvial-fan, (c) eolian, (d) glacier, (e) delta, (f) shoreline, (g) organic-reef, (h) shallow- marine, (i) submarine fan, and (j) deep-marine. Sedimentary rock layers can be grouped into formations, and formations can be grouped into sequences that are bound by erosion surfaces. These formations and sequences form an important interpretive element in the rock record. Plate tectonics controls sedimentary systems by creating uplifted source areas, shaping depositional basins, and moving continents into different climate zones.

The Nature of Sedimentary Rocks Sedimentary rocks form from fragments of other rocks and by precipitation from water. Weathering of preexisting rock Transportation Deposition Compaction and cementation

The Nature of Sedimentary Rocks Fragments of other rocks and minerals, such as gravel in a river channel, sand on a beach, or mud in the ocean Chemical precipitates, such as salt in a saline lake or gypsum in a shallow sea Organic materials formed by biochemical processes, such as vegetation in a swamp, coral reefs, and calcium carbonate precipitated by in the ocean

The Nature of Sedimentary Rocks The most obvious characteristic of sedimentary rocks is that they occur in distinct layers or strata. You should be able to recognize the major layers or formations in the geologic cross section of the walls of the Grand Canyon of Arizona. Layers that are resistant to weathering and erosion form cliffs, and nonresistant rocks erode into gentle slopes Figure 05.01A: Layers that are resistant to weathering and erosion erode into vertical cliffs. Rocks that weather easily form slopes or terraces.

The Nature of Sedimentary Rocks Figure 05.01A: Layers that are resistant to weathering and erosion erode into vertical cliffs. Rocks that weather easily form slopes or terraces. Figure 05.01B: A cross section of the Grand Canyon graphically illustrates the major sedimentary layers or formations.

Features of Sedimentary Rocks FossilsSand grains from clastic sediment Figure 05.02A: Fossils found in sedimentary rocks include representatives of most types of marine animals. Figure 05.02B: A microscopic view of sand grains in sediment shows the effects of transportation by running water.

Sedimentary Rocks of North America Sedimentary rocks are widespread in the stable platforms and where they are generally less than 2 km thick. Thick accumulations of sediment form on continental margins. Also as deformed layers in the folded mountain belts. Figure 05.03: Sedimentary rocks in North America are widespread in the stable platforms.

Clastic Sedimentary Rocks Primarily grain size Secondarily on textural and compositional variations. Figure 05.04: The classification of common clastic sedimentary rocks is based primarily on grain size.

Clastic Sedimentary Rocks Conglomerate--coarse Figure 05.05A: Conglomerate is a coarse-grained clastic sedimentary rock. Sandstone--medium Figure 05.05B: Sandstone is a clastic sedimentary rock composed of sand- sized particles. Mudstone--fine Figure 05.05C: Shale is a clastic sedimentary rock composed of very fine grains of clay or mud.

Biochemical and Chemical Sedimentary Rocks Limestone Figure 05.05D: Limestone is the most common nonclastic sedimentary rock. It is composed mostly of calcium carbonate. Fossiliferous limestone Figure 05.05E: Limestone may also be made of abundant shell fragments. Gypsum Figure 05.05F: Gypsum precipitates as shallow bodies of water evaporate.

Mostly Biochemical Marine sediments Figure 05.07: Marine sediments form by biochemical precipitation. Carbonate sediments dominate at shallow depths and in warm near-shore waters.

Sedimentary Structures Sedimentary structures provide key information about the conditions under which the sediment was transported or deposited. Stratification, Cross-bedding Graded bedding Ripple marks Mud cracks. Sedimentary structures provide key information about the conditions under which the sediment accumulated.

Sedimentary Structures Stratification Distinct layers expressed by changes in color, texture, and the way they weather and erode. Strata or simply beds. Separated by bedding planes. Occurs on many scales and reflects the changes that occur during the formation of a sedimentary rock. Figure 05.09: Particles of sediment, carried by currents, travel up and over the sand wave and are deposited on the steep down current face. Cross- bedding is formed by the migration of sand waves (ripple marks or dunes).

Sedimentary Structures Cross-bedding Formed by the migration of sand. Particles of sediment move up and over the sand wave and are deposited on the steep down current face to form inclined layers. Figure 05.09: Particles of sediment, carried by currents, travel up and over the sand wave and are deposited on the steep down current face. Cross-bedding is formed by the migration of sand waves (ripple marks or dunes).

Sedimentary Structures Graded bedding Progressive decrease in grain size upward through a bed Commonly produced by turbidity currents on the continental slope. Figure 05.10: Graded bedding is produced by turbidity currents. It occurs in widespread layers, each layer generally less than a meter thick. Slumps off the deep continental slopes commonly produce great thicknesses of graded layers.

Sedimentary Structures Ripple marksMud cracks in dry sediment Figure 05.02C: Ripple marks preserved in sandstone suggest that the sediment was deposited by the current action of wind or water. Figure 05.02D: Mud cracks form where sediment dries while it is temporarily exposed to the air.

Sedimentary Systems and Environments Major steps in the formation of sedimentary rocks Weathering of preexisting rock Transportation Deposition Compaction and cementation Figure 05.13: The major sedimentary systems are represented in this idealized diagram.

Sedimentary Systems and Environments Eolian Glacial Fluvial Alluvial-fan Deltaic Shorelines Organic-reef Shallow-marine Deep-marine. Each of these systems has a specific set of physical, chemical, and biological conditions and therefore develops distinctive rock types and fossil assemblages. Figure 05.13: The major sedimentary systems are represented in this idealized diagram.

Eolian Systems Figure 05.16A: Modern sand dunes of the Little Sahara, Utah. © Doug Lemke/ShutterStock, Inc. Figure 05.16B: Ancient dune deposits in Zion National Park, Utah. © iStockphoto/Thinkstock

Glacial Systems Figure 05.17B: Ancient glacial sediments in central Utah. Figure 05.17A: The margins of a valley glacier in eastern Canada. Courtesy of J.D. Ives

Alluvial-Fan Systems Figure 05.15A: Modern alluvial fans in Death Valley, California. Figure 05.15B: Ancient alluvial- fan deposits in central Utah.

Fluvial Systems Figure 05.14A: Point-bar deposits in a modern river. Figure 05.14B: Ancient stream channel marked by sandstone lens interlayered with fine-grained clay and siltstone. © Vladimir Melnikov/ShutterStock, Inc.

Delta Systems Figure 05.18A: The delta of the Nile River, Egypt, forms where the river empties into the Mediterranean Sea. Figure 05.18B: Ancient deltaic deposits in Cenozoic rocks of the Colorado Plateau. Courtesy of GSFC/JPL, MISR Team/NASA

Shoreline Systems Figure 05.19B: Ancient beach deposits in central Utah form resistant sandstone beds alternating with shale (slopes). Figure 05.19A: A modern beach on Cape Hatteras along the Atlantic Coast of the United States. © Doug Lemke/ShutterStock, Inc.

Lagoon Systems Figure 05.20B: Ancient lagoonal deposits with thick beds of coal in sandstone in eastern Wyoming. Figure 05.20A: A lagoon along the central Atlantic coast of the United States. Courtesy Scientific Visualization Studio, Goddard Space Flight Center/NASA

Tidal-Flat Systems Figure 05.21B: Ancient tidal flat deposits in southern Utah. Figure 05.21A: A modern tidal flat in the Maritime Provinces of Canada. Reproduced with the permission of Natural Resources Canada 2013, courtesy of the National Photo Library

Organic-Reef Systems Figure 05.22A: Australia’s Great Barrier Reef is a coral reef on the eastern shore. © Edward Haylan/ShutterStock, Inc. Figure 05.22B: An ancient reef from the Paleozoic Era in the Guadalupe Mountains of west Texas. © Michael J Thompson/ShutterStock, Inc.

Shallow Marine Systems Figure 05.23B: Ancient shallow-marine sediments in eastern Nevada are made of many layers of limestone. Figure 05.23A: A modern shallow-marine environment in the Bahamas as seen from space. The light ridges are drifts of carbonate sediment. Courtesy of GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team/NASA

Continental Slope Systems Figure 05.24B: Ancient series of folded turbidites in southern France. Figure 05.24A: Submarine fan with its distributary channels sits at the base of the continental slope offshore California. Data from: Michelle Hurst/U.S. Geological Survey

Deep Marine Systems Figure 05.25B: Thin beds of ancient deep-marine chert now exposed in coastal California. Such chert beds are interbedded with fine mudstones. Figure 05.25A: Thin layers of deep marine sediment form on abyssal plains on the flanks of the mid-ocean ridge (right). Courtesy of Chalk Butte, Inc

Stratigraphic Sequences Layers of sedimentary rocks can be grouped into formations. Formations can be grouped into sequences that are bounded by erosion surfaces. These formations and sequences form an important interpretive element in the rock record. Figure 05.26: A sequence of sediments deposited by transgression and regression of a shallow sea is represented in these schematic diagrams. Sand accumulates along the beach, mud is deposited offshore, and calcite is precipitated farther offshore, beyond the mud. As the sea expands over the continent, these shallow-marine environments move inland, producing a vertical sequence of sand, mud, and carbonate sediment. When the sea recedes off the continent, mud is deposited over the carbonate sediment, and sand is deposited over the mud. The net result is a vertical sequence of sedimentary layers: sandstone, shale, limestone, shale, and sandstone

Fig Sequence of sediments deposited by transgression and regression of shallow sea

Sequence Stratigraphy Identifies the global changes of relative sea level. Transgression-regression cycles Major ones large tectonic events that changed the volume of the ocean basins. Shorter cycles are caused by glaciation or regional tectonic events. Figure 05.27: Sequence stratigraphy identifies the global changes of relative sea level.

Sedimentary Systems and Plate Tectonics Profoundly influences the origin of sedimentary rocks. Controls sediment sources, pathways, climate zones, and depositional systems. Each major tectonic setting produces a distinctive sequence of sedimentary rocks. Figure 05.28: Plate tectonics exerts fundamental controls on sedimentary systems as shown in this cross section.

Summary of the Major Concepts Sedimentary rocks form at Earth’s surface by the hydrologic system. Their origin involves the weathering of preexisting rock, transportation of the material away from the original site, deposition of the eroded material in the sea or in some other sedimentary environment, followed by compaction and cementation. Two main types of sedimentary rocks are recognized: (a) clastic rocks and (b) chemically precipitated rocks, including biochemical rocks. Stratification is the most significant sedimentary structure. Other important structures include cross-bedding, graded bedding, ripple marks, and mud cracks.

Summary of the Major Concepts The major sedimentary systems are (a) fluvial, (b) alluvial-fan, (c) eolian, (d) glacier, (e) delta, (f) shoreline, (g) organic-reef, (h) shallow- marine, (i) submarine fan, and (j) deep-marine. Sedimentary rock layers can be grouped into formations, and formations can be grouped into sequences that are bound by erosion surfaces. These formations and sequences form an important interpretive element in the rock record. Plate tectonics controls sedimentary systems by creating uplifted source areas, shaping depositional basins, and moving continents into different climate zones.