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 • 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 formDetrital Sedimentary Rocks • Chemical sediments – Minerals precipitated from water due to chemical reactions (ex. calcite, gypsum, halite). These sediments formChemical 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: CoarseMediumFine Sediment: Boulder | Cobble | Pebble | Sand | Silt | Clay Size (mm): 256 64 2 0.062 0.0039 Rock Name: Conglomerate Sandstone Siltstone Shale Conglomerate Sandstone 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: • Laminar flow – The simplest type of flow. Streamlines do not cross, typically seen in slow moving waters • 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 • Braided streams are characterized by • High sediment load • Variable discharge • High gradient • Relatively young in age
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
