Depositional Environments


Simplistically, the earth's surface can be divided into two broad divisions on the basis of whether there is net erosion or deposition:

(1) Sourcelands - areas which are sources of sediment, usually meaning mountainous regions, but including any place erosion and sediment removal takes place, and

(2) Depositional Basins - areas which receive eroded sediment.

Depositional basins are subdivided a number of ways.

One is by the predominant sediment type found in the basin; thus, Siliciclastic Dominated Systems and Carbonate Dominated Systems. A general observation we can make is that sediment type and supply changes from the sourceland to the depositional basin such that depositional environments evolve in systematic and predictable sequences from sourceland to depositional basin. In general, depositional environments and thus rock types are predictable from sourceland to depositional basin. Each exists in a specific location for specific reasons.
 For example, large siliciclastic particles dominate near the sourceland in alluvial fans and braided rivers, and fine siliciclastic particles dominate at the end of the sequence in ocean basins far away from the sourceland. An ideal downstream sequence of environments is illustrated on the Depositional Systems model below.

Schematic of Main Depositional Environments

 


Depositional Environments

Another, more useful method, is to categorise depositional environments according to their location and by the erosional/depositional processes which distinguish them. A depositional environment is a portion of the earth's surface characterized by a unique combination of physical, chemical, and biological processes. These processes control how sediment is transported and deposited, what chemical modification it undergoes, and what kind of organisms live in and affect the sediment. The processes result in the characteristic and distinctive sedimentary deposits typical of each environment by which we recognize and identify them. Several dozen depositional environments have been identified. The more common ones are listed in the table below.

There are three major environments of deposition: Terrestial/Continental, Transitional and Marine.

 

Classification of Environments by Location and Depositional Processes
Terrestrial/Continental
Transitional
Marine
Siliciclastic

Talus Cones

Fluvial/Alluvial (inc. Alluvial Plain)

Lacustrine (Lakes)

Arid

Sabhka

Paludal (Swamps, Marshes)

Glacial

The Coast

River-Dominated Coast

Wave-Dominated Coast

  • Beach
    • Foreshore
    • Backshore
  • Coastal Sand Dunes
  • Beach Ridges and Cheniers
  • Lagoon
  • Washover Fans

 

Tidal-Dominated Coast

Marshes/Swamps

 

Continental Shelf

Continental Slope

Continental Rise

Deep Marine (Bathyal, Abyssal, Basinal)

  • Pelagic Organic Deposition

Carbonate

 

 

Shelf

Basin



Carbonate Systems

Carbonate dominated systems occur usually in tectonically stable regions where the supply of siliciclastic sediment is minimal or non-existent.
Carbonate systems develop where siliciclastic sourcelands are low and/or very distant, the water is shallow, and climates are tropical to subtropical. In the warm, clear, shallow water calcareous algae flourish and generate micrite, while invertebrate animal skeletons accumulate as sedimentary particles (bioclasts). The oolitic, pelletal, and intraclastic allochems are also produced locally, depending on conditions.

Carbonate systems are, of course, not going to have terrestrial environments (unless we include supertidal flats). Transitional and marine environments dominate. Carbonate environments are often modeled as "belts" running parallel to the coastline. The full model has 9 environmental belts, but here it is simplified to five: tidal flat, lagoon, reef, shelf, and basin (see Carbonate Depositional Environments).

after: Lynn S. Fichter, James Madison University, USA.

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