Module 9B: Sedimentary Environments

• Sedimentary environments - sites of erosion, non-deposition or deposition that are physically, chemically and biologically distinct from adjacent terrain
• Sedimentary environments are defined by:

  Physical - geology, geomorphology climate, temperature, depth

  Chemical - Salinity, oxygen content

  Biological - fauna and flora
• Sedimentary environments can be generally classified into: terrestrial, transitional, or marine environments.
• Both alluvial and fluvial environments are formed by flowing water as a erosional agent

  
• Alluvial environments - are areas where river processes operate to transport the sediments from upland areas to flatlands. Sediments are deposited because of the drop in energy from a higher elevation to depositing it on a plain.

  
• For a typical alluvial environment, the river is the transporting agent and brings in varying sizes of sediments.

  The stronger the river current, the larger the sediments that can be deposited in the alluvial fan. The sediments deposited are poorly-sorted because it can bring in fine and coarse particles.

  Sandstones and conglomerates are usually formed in alluvial environments.
• Alluvial fan - fan-shaped accumulation of sediments (sediments are called alluvium)
• Fluvial Environment (river environment) - This type of environment is also governed by the same processes as the alluvial environment. In river systems, sedimentary processes such as erosion, transportation, and deposition operate.
• The basic parts of a river channel are the following: (1) headwaters/source, (2) transfer zone (body / channel), and (3) depositional zone (mouth).

  
• headwaters - is the area where water is replenished in the uplands due to high precipitation, and where sediments are eroded from.
• transfer zone - is the section where materials are eroded and transported and eventually will be deposited in the depositional zone of the river system
• The three channel types are: Meandering, Braided, and Straight
• Meandering channel - A single channel with heavily winding bends called meanders. These river systems are often found in low gradient areas where the channel can laterally migrate thus becoming more sinuous
• Braided channel - A single channel with low sinuosity but with multiple bars within a channel.
• Straight Channel - A single channel with very low sinuosity, usually found during the early stages of a river system development, or when a river system is structurally controlled by a fault or resistant bedrock.
• Anastomosing channel - Multiple channels that branch out and join each other at different points of the river system. The channels may either be straight, braided, or meandering.
• Meanders - The formation of meanders is due to deposition in the inner curve of the bend (point bar), and erosion in the outside curve (cut bank).

  
• oxbow lake - When a meanders overextend laterally, a meander may be cutoff along the neck.

  Meander scar - dried up version of the above.
• Lag deposits - Sediments found in the channel bottom and are characterized by gravels and sand.

  Point bars - typically show cross bedding
• Braided channels - Experience a highly irregular flow regime, ranging from minimum discharge of water which only brings in little sediments to maximum discharge which causes flooding in the channel bringing in abundant sediment supply
• Bars - An identifying feature of braided channels which are usually composed of coarse-grained sediments

  Braided channels typically exhibit trough cross bedded conglomerates and sandstones.
• Terraces - Flat, elevated areas at the sides of channels, formed either by deposition or by cutting down on bedrock.

  Yazoo Tributary - A tributary that runs along the main channel for a considerable distance, before connecting into the main channel. This tributary is usually located within the back basin of the main channel’s floodplain.

  Natural Levees - These are accumulation of sediments along the banks of large rivers, formed by multiple flooding events. They form natural barriers that may prevent stream water from reaching the back basin, except for larger floods.

  
• Lacustrine environment (lakes) - are usually low energy environments. As such, common sediments and rocks are laminated muds and mudstones.
• Varves - are lacustrine deposits that show alternating light- and dark-colored bands of fine sediments, which correspond to seasonal changes within the lake.
• Aeolian environments - are shaped by wind erosion and deposition. Wind erosion may be in the form of deflation, where of finer sediments are blown away, and abrasion, which shape features by sandblasting.
• Desert pavements - resulted from deflation which form lag deposits
• Ventifacts - are angular facets that form at the windward side of boulders.

  
• Yardangs - are formed from the erosion of softer bedrock, and are usually aligned along the prevailing wind direction.

  
• Tafoni - is a general term for relatively small, cavernous features such as shelters and hollows caused by chemical weathering and abrasion.

  
• Sand Dunes - The most common deposition features in aeolian environments. They are formed when the following requirements are satisfied: a ready supply of sand, a steady wind, and some kind of obstacle such as vegetation, rocks, or fences, to trap some of the sand.

  
• Stoss side - is the shallower and windward slope, where erosion occurs.

  Once the sand reaches the highest point of the dune (crest), it is deposited by slipping down along the - lee side.

  
• Glacial Environments - Glaciers incorporate the material it erodes to the ice sheet. The erosional process can be through: plucking which is lifting pieces of bedrock out of place and abrasion which is grinding and scraping of a rock surface.

  Plucking can carry larger sediments up to boulder sizes while abrasion carries finer grained sediments.
• Drift - the sediments in a glacial environment
• Glacial till - a common example of drift which is unsorted sediment deposited directly by the ice. It is also deposited on the ice through abrasion
• Moraines - are accumulation of glacial sediments (till) found at the peripheries of glaciers
• Stratified drift - refers to relatively well-sorted sediments laid down by glacial meltwater
• Deltas - found where a river meets the sea or lake and the processes that operate are similar to that of alluvial environments.

  In this transitional environment, the abrupt loss of energy is due to the opposing forces of the river and the sea
• There are different types of delta based on the dominant current, either river, wave or tide.

  
• River-dominated deltas - These types of delta has a large sediment volume because rivers have a high capacity and are competent transporting agents. When the sediments settle down in the delta, they are distributed widely, hence the delta appears to be elongated and lobate.

  This type of delta often encounters delta lobe switching because of channel migration of the river system. This delta may also form what is known as a bird foot delta.
• Tide-dominated deltas - occur in response to lower energy as compared to the river-dominated and wave-dominated delta. These types of deltas form multiple mouth bars and often form tidal flats at the back of the coast. The sediments deposited are affected by tidal flow hence it forms bars that are parallel to the tidal flow