Depositional environment: Places where sediments are deposited. The physical characteristics and geographical location determines the type of sediment that will normally be deposited. In broad strokes, we classify depositional environments as:
- Continental: Deposited on land or in fresh water.
- Fluvial: stream or river. Deposits.
- Alluvial: Deposits of flash floods and intermittent streams in mountainous environments, such as alluvial fans. Deposits.
- Glacial: Deposited by glaciers. Deposits.
- Eolian: Deposited by wind (in deserts). Deposits.
- Lacustrine: Lake deposits. Dominated by finely laminated clastic sediments. Deposits.
- Paludal. Swamp deposits. Plant-choked, periodically inundated environments. Deposits.
- Transitional: Deposited in an environment showing influence of both fresh water or air and marine water.
- Deltaic: Deposits at the mouths of large rivers. Map view. Deposits.
- Esturine: Deposits in valleys drowned by rising sea level. Map view, deposits.
- Lagoonal: Deposits in the waters separating barrier islands from the shore. Map view. Deposits.
- Beach: Deposits in shallowest marine water influenced by waves. Deposits.
- Marine: Only influenced by sea water.
- Shallow marine clastic: Regions near the mouths of rivers are usually clastic dominated because the critters that secrete CaCO3 tend to have trouble living in muddy water. Deposits.
- Carbonate shelf: Regions with clear water shallow enough to be penetrated by sunlight are often dominated by the skeletons of marine organisms. Map view. Ancient reef.
- Continental slope: Dominated by the deposition of submarine landslides. (See deposits at right.)
- Deep marine: Very thin sediments formed by the slow accumulation of skeletons and clasts dropped into the ocean by wind.
- Sedimentary facies: A characteristic association of sedimentary rock features linked to a distinct sedimentary environment. In a given location, one sedimentary facies tends to grade laterally into others. For example, beach rocks from a barrier island may grade into lagoonal rocks which can grade into delta deposits.
Note pronunciation: People have faces, rocks have facies.
- A swift mountain stream might keep even gravel sized objects in suspension, so its deposits would only consist of pebbles and boulders
- A large river might be so slow that it couldn't even keep sand in suspension, so its deposits would contain sand sized grains and larger.
- A body of almost still water, like a lake, migth have so little energy that even clay comes out of suspension and settles to the bottom.
Thus, the smaller the dominant clasts, the lower the energy of the depositional environment.
Sedimentary structures: The second key to the identification of sedimentary facies and environments is small scale physical features they display. We will cover these in more detail when we examine each environment, but here are some major ones.
- Stratification - layering / bedding
-individual layers are called strata (sing. stratum) or beds
beds are lain down parallel, with younger beds on top of older ones (superposition): Ancient. Recent
- Cross bedding:- beds (parallel to each other) alligned at an angle to the surface upon which they accumulated
-result of deposition of dunes or ripples (can be subaqueous or subaerial.):
- Graded bedding: Mixture of grain sizes carried by asubmarine landslide. As current slows down it drops the largest particles first and smallest particles last, resulting in a stratum with the largest clasts on the bottom. ancient, recent. Additional information.
- Surface features:
- Ripples: Formed by movement of a current over loose sand to clay sized sediment. Ripples formed by waves have a symmetrical cross section. Those formed by a stream current are asymmetrical. Ancient. Recent
- Rain drop prints Ancient, recent.
- Mud cracks Ancient. Recent
- Tool marks: Formed when the current carries an object across the sediment. Ancient. Recent
- Sole marks: Formed on the undersurface of a bed that conforms acts as a mold for the surface on which it is deposited. Ancient.
And now, here's a test of your insight. What's going on in this image:
Key concepts and vocabulary:
- Depositional environments:
- Shallow marine clastic
- Carbonate shelf
- Continental slope
- Deep marine
- Sedimentary facies
- Energy of depositional environment
- Sedimentary structures:
- Cross bedding
- Graded bedding
- Ripple marks
- Rain drop prints
- Mud cracks
- Tool marks
- Sole marks