Depositional Environments
Landscapes form and constantly change due to weathering and sedimentation. The area where sediment accumulates and is later buried by other sediment is known as its depositional environment. There are many large-scale, or regional, environments of deposition, as well as hundreds of smaller subenvironments within these regions. For example, rivers are regional depositional environments. Some span distances of hundreds of miles and contain a large number of sub-environments, such as channels, backswamps, floodplains, abandoned channels, and sand bars. These depositional sub-environments can also be thought of as depositional landforms, that is, land-forms produced by deposition rather than erosion.
Depositional environments are often separated into three general types, or settings: terrestrial (on land), marginal marine (coastal), and marine (open ocean). Examples of each of these three regional depositional settings are as follows: terrestrial-alluvial fans, glacial valleys, lakes; marginal marine-beaches, deltas, estuaries, tidal mud and sand flats; marine-coral reefs, abyssal plains, and continental slope.
During deposition of sediments, physical structures form that are indicative of the conditions that created them. These are known as sedimentary structures. They may provide information about water depth, current speed, environmental setting (for example, marine versus fresh water) or a variety of other factors. Among the more common of these are: bedding planes, beds, channels, cross-beds, ripples, and mud cracks.
Bedding planes are the surfaces separating layers of sediment, or beds, in an outcrop of sediment or rock. The beds represent episodes of sedimentation, while the bedding planes usually represent interruptions in sedimentation, either erosion or simply a lack of deposition. Beds and bedding planes are the most common sedimentary structures.
Rivers flow in elongated depressions called channels. When river deposits are preserved in the sediment record (for example as part of a delta system), channels also are preserved. These channels appear in rock outcrops as narrow to broad, v- or u-shaped, "bellies" or depressions at the base of otherwise flat beds. Preserved channels are sometimes called cut-outs, because they "cut-out" part of the underlying bed.
Submerged bars along a coast or in a river form when water currents or waves transport large volumes of sand or gravel along the bottom. Similarly, wind currents form dunes from sand on a beach or a desert. While these depositional surface features, or bedforms, build up in size, they also migrate in the direction of water or wind flow. This is known as bar or dune migration. Suspended load or bedload material moves up the shallowly inclined, upwind or upcurrent (stoss) side and falls over the crest of the bedform to the steep, downwind or downcurrent (lee) side. If the bedform is cut perpendicular to its long axis (from the stoss to the lee side) one would observe inclined beds of sediment, called cross-beds, which are the preserved leeward faces of the bedform. In an outcrop, these cross-beds can often be seen stacked one atop another; some may be oriented in opposing directions, indicating a change in current or wind direction.
When a current or wave passes over sand or silt in shallow water, it forms ripples on the bottom. Ripples are actually just smaller scale versions of dunes or bars. Rows of ripples form perpendicular to the flow direction of the water. When formed by a current, these ripples are asymmetrical in cross-section and move downstream by erosion of sediment from the stoss side of the ripple, and deposition on the lee side. Wave-formed ripples on the ocean floor have a more symmetrical profile, because waves move sediments back and forth, not just in one direction. In an outcrop, ripples appear as very small cross-beds, known as cross-laminations, or simply as undulating bedding planes.
When water is trapped in a muddy pool that slowly dries up, the slow sedimentation of the clay particles forms a mud layer on the bottom of the pool. As the last of the water evaporates, the moist clay begins to dry up and crack, producing mud cracks as well as variably shaped mud chips known as mud crack polygons. Interpreting the character of any of the sedimentary structures discussed above (for example, ripples) would primarily provide information concerning the nature of the medium of transport. Mud cracks, preserved on the surface of a bed, give some idea of the nature of the depositional environment, specifically that it experienced alternating periods of wet and dry.
All clastic and organic sediments suffer one of two fates. Either they accumulate in a depositional environment, then get buried and lithified (turned to rock by compaction and cementation) to produce sedimentary rock, or they are reexposed by erosion after burial, but before lithification, and go through one or more new cycles of weathering-erosion-transport-deposition -burial.
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