Lab 5: Sedimentary Rocks - Depositional Environments


A sedimentary environment (or environment of deposition) is a place where a given type of sediment accumulates. Through reading the previous sections, you have already encountered many of the following examples:




A place on the surface of the land.




This is one of the most familiar depositional environments. Any river or stream will have a channel, where coarser sediment, gravel and sand mainly, is deposited by the faster moving water. Away from the channel is a flat terrain, forest covered in many areas, where the river or stream floods periodically, depositing finer mud (silt and clay). Low areas along a river system that consistently have wetland vegetation and standing, stagnant water, are called swamps.


Alluvial Fan


Along mountain fronts, along the base of a mountain range where the ground gets flatter in an adjacent valley, there is an abrupt change in slope. Water coming down streams in the mountains is flowing fast, and is able to move even boulder size grains. But, when the water flow slows down as it reaches the edge of a valley floor, the coarser grains (larger grain sizes) will abruptly be deposited, making an alluvial fan. It is fan-shaped, like the fold-out hand fans used by many to fan themselves when a bit warm. Often alluvial fans are found in arid environments, where rainfall is scarce, but when it rains, there is enough water flow to produce flash floods which move the sediment along.


Sand Dunes


Great deserts like the Sahara Desert in Africa contain dramatically large dune fields, and these are the places where we see the full complement of dune shapes and sizes. Sand dunes can form locally, however, where the wind moves against a mountain range. Dust (silt and clay) blown across the landscape can pile up in such a place, in a sense trapped by the rising slope of the mountain range. We use the term eolian to describe wind blown deposits like this.


Playa Lakes and Salt Flats


A playa lake forms in a low spot on the land surface, anywhere stream drainage takes water into the low area, but there is no stream draining it out. Often playa lakes dry up during drought, only to be full of water again during the next rainy period. Salt flats form in such environments, when there isn't much rainfall. During summer months, water evaporates to become concentrated enough to precipitate salt.




Natural lakes form as a result of crustal movements, forming a low area, or as a result of movement along faults, or as a result of the landscape-changing effects of the Ice Age. "Lakes" that have been made by people should really be called reservoirs.


Transitional (Coastal)


Between the continental and marine realms is the zone along the beach, where there are dramatically different environments adjacent to one another. Giant waves can be breaking on the beach on one side of coastal sand dune system, while water in a quiet lagoon sits nearby.




Wave action is the steadiest, and sand is well-sorted. There is usually a gentle ramp, as you go up the beach slope to higher ground. Beach sediment is often sand-size, but it can be made of rock fragments derived from local rock types, as with the black beaches of Hawaii, made of volcanic material weathered across the islands. In places where rocky outcrops front the beach, the sediment can be gravelly.




At the mouth of a river, where it flows into the ocean, there is a drop in steady flow of the current. While water is moving down the river channel, it has enough movement to keep clay-size material up in the water column, and there is enough current to be steadily rolling sand and silt along the bottom. But, where the river meets the ocean, there is a drop in stream flow velocity as you go offshore from the beach. Silt and clay particles settle out to form the body of a delta, with sandier deposits lining channels that spread water flow across the delta.


Barrier Island and Lagoon


The Texas coast is the best place to see these features. Barrier islands are long linear piles of sand formed as wave action has gradually moved sand down the beach to accumulate. Barrier islands owe their formation to a slowdown in the rate of sea level rise in the past few thousand years. Behind the barrier island, which is often covered with sand dunes along the crest, is a wide coastal lagoon, where mud accumulates. Lagoons are shallow, and in places like south Texas, evaporation causes higher salinity of the water, making it a special place, only suitable as a home for organisms who are able to take the wide range of water conditions here.


Estuaries (Bays)


Where rivers meet the ocean, there is often an embayment of the ocean, where the coastline sweeps inland up a river valley. River valleys are lower ground than the adjacent countryside, so the ocean is bound to go up into the river valley a bit. But, because of rapid sea level rise during the recent part of the Ice Age, the size of estuaries can be surprising. Chesapeake Bay, on the eastern coast of the United States, is one such estuary, where the shoreline intricately snakes up and around all of the tributaries of the river valley. Mud is dominant in estuaries, because, except for storms, there is usually not too much wave action or wind-generated currents. Just about every river that flows into an ocean has a bay system associated with it.




Landward of estuaries and landward of beaches, there is often a low-lying, mud-dominated area of marsh, where tidal action brings nutrients in and moves water to and fro. Marshes are chock-full of organic material, and dominated by grasses. They are important spawning grounds for shrimp, crabs, fish, etc.




Away from the beach tract itself, in open water over the continental shelf and beyond, out into the deep ocean, there is a wide variety of depositional environments. Reefs form in places where reef-building organisms, such as corals, are happy to live. For the most part, the conditions needed are clear and warm water that is shallow enough so that sunlight penetrates (You don't find classic reefs in deep water). Reefs are found along continents, where river flow into the area is not too great (if it were, the water would be too muddy for the organisms to live), and way out, around volcanic islands. We have already spoken of the turbidite deposits flanking the continental shelves, and you read about the nature of ooze deposits on the seafloor in deep water areas.