Sedimentary Rocks A sedimentary rock is formed by
Sedimentary Rocks • A sedimentary rock is formed by the accumulation and compaction of sediments (rock pieces, minerals, animal parts, or chemical precipitates)
• Sediments are derived from weathering of rocks – Chemical weathering – dissolution or decomposition of minerals grains due to chemical reactions
Bowen’s Reaction Series: The last minerals to crystallize are the most resistant to weathering. Susceptible to weathering Resistant to weathering
– Mechanical weathering – Breakage of rock due to cracking, scratching, crushing, freezing, thawing, and abrasion
• There are 3 different types of sediments 1) Detrital (clastic) sediments – Pieces of eroded rock fragments and minerals grains. Sediments of this type form Detrital Sedimentary Rocks
• There are 3 different types of sediments – Detrital (clastic) sediments – Pieces of eroded rock fragments and minerals grains. Sediments of this type form Detrital Sedimentary Rocks – Chemical sediments – Minerals precipitated from water due to chemical reactions (ex. calcite, gypsum, halite). These sediments form Chemical Sedimentary Rocks
• There are 3 different types of sediments – Detrital (clastic) sediments – Pieces of eroded rock fragments and minerals grains. Sediments of this type form Detrital Sedimentary Rocks – Chemical sediments – Minerals precipitated from water due to chemical reactions (ex. calcite, gypsum, halite). These sediments form Chemical Sedimentary Rocks – Biochemical sediments – Mineral remains of organisms or minerals precipitated due to biological processes (ex. shells, plant fragments, carbon). Sediments of this type form Biochemical Sedimentary Rocks
• Detrital sedimentary rocks are classified according to their grain characteristics: – Size of the grains: refers to the range of sizes of the individual sediment pieces found within the rock. Can be divided into three categories: • coarse-grained – Boulder, cobble, pebble, gravel • medium-grained – Sand • fine-grained – Silt, clay
Classification of Clastic Sedimentary Rocks based on grain sizes Category: Sediment: Coarse Medium Boulder | Cobble | Pebble | Sand Size (mm): 256 Rock Name: Conglomerate 64 2 Fine | 0. 062 Sandstone Silt | Clay 0. 0039 Siltstone Shale
• A depositional environment is any place where sedimentary rocks form • Environments can be characterized as high, medium or low energy
• The larger the pieces being moved, the higher the energy of the environment – Mountain river, alluvial fan = high energy – Swamp, deep ocean = low energy • The more the pieces are being moved about, the higher the energy of the environment – Swash zone of beach = high energy – Floodplain, playa lake = low energy
Stream Processes
• The water in all streams, large or small, can flow in two ways: a) Laminar flow – The simplest type of flow. Streamlines do not cross, typically seen in slow moving waters b) Turbulent flow – Complex flow pattern in which streamlines cross and form eddies. Typically seen in fast moving streams
• Streams transport a variety of sediment as either suspended load or bed load
• Most sand grains within a stream are transported by the process of saltation. This is the intermittent jumping of particles, and the smaller the particle, the higher it jumps and the farther it travels
• Experimental work has shown that there is a relationship between the velocity of a stream and the size of particles it can erode, transport, and deposit. This is called Hjulstrom’s curve
• The term stream valley encompasses both the actual stream channel and its floodplain
• The two main types of stream channel patterns are braided and meandering 1) Braided streams are characterized by • • High sediment load Variable discharge High gradient Relatively young in age
2) Meandering streams are characterized by: • • Low Sediment Load Low Gradient Meanders change shape Old, mature stream
• Over time meanders change shape because on the cut bank side sediment is being eroded and along the point bar side deposition is occurring
• As streams flood natural levees are built up. Sometimes the levees build up so high that the floodplain level is below the stream level
• The longitudinal profile of a stream is generally always concave upward, with the steepest section being near the streams head, and leveling out near the mouth
• A delta is a fan shaped body of sediment that gets deposited at the mouth of a stream entering an ocean or lake. As the stream enters a standing body of water, deposition of the coarsest sediment occurs first
• Over time deltas build forward, and occasionally the main channel will shift to distributaries with shorter routes. This has occurred several times along the Mississippi delta
• Alluvial fans are similar to deltas in that they are fan shaped and formed from the sudden deposition of material, but alluvial fans are located at mountain fronts
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