Features of depositional environments Desert processes sand dunes

Features of depositional environments

Desert processes: sand dunes

Alluvial fans

Ephemeral lakes

River processes: channels, floodplains

Braided streams are characterized by mid-channel bars. . you can think of a bar as a variety of a current ripple

What channels look like in outcrop Downcutting surface, coarse sediment

Meandering stream: Single channel that Is highly sinuous. The bars in this system are on the inside of curves and are called point bars

Channels are preserved, but so are the bars: inclined layers of sand that comprise the point bar

The floodplain is also preserved: fine-grained muds that are deposited from flooding, along with laterally discontinuous layers of sand that form from breaches of the channel banks (termed levees) that send sand out onto the floodplain

Glacial processes: till, moraines

Lateral moraine: the till pushed to the sides by the glacier; terminal moraine= the material bulldozed to the front Layered muds in glacial lakes

Other evidence of the passage of a glacier: the striated surface of bedrock

DELTAS It may be in a mud puddle, but this little delta has all the features of a “real” delta.

What is different about these deltas are the processes that are dominant on the delta top and front. Tides rework delta top Waves rework delta front

Delta processes: delta top channels; delta front, pro-delta

The sediment plume that forms where the river channel dumps its load. This is why the delta can build up and out: sediment is always being added on the delta front.

BEACHES AND BARRIER ISLANDS A beach is a complex set of environments from the front (the foreshore) to back (lagoon)

Beach processes: wave activity and wind The shoreface The backshore = dune field

Wave-generated ripples are symmetrical in shape The inclined surface of the shoreface leaves behind inclined layers of sand

The backshore dune field Beaches are often separated from land by a lagoon or bay behind them. This is a very low energy setting

TIDAL FLATS - form where there is a large tidal ranges (>3 m) and the coastline is protected bays where wave action is minimal. The intertidal zone: tidal currents produce ripples Mudcracks form from exposure during low tide Fluctuating tidal currents deposit alternating layers of sand mud

Ocean processes There’s vertical exaggeration here, but offshore from the shoreline is the almost-flat continental shelf, and further offshore the rise and slope Where major rivers enter the oceans the shelf is cut by submarine canyons that represent the old river channels formed when sea level was lower in the Pleistocene

The ocean floor is the site of accumulation of fine-grained sediment; both clay and carbonate mud The deep ocean floor is NOT devoid of life!

Ocean floor sediments are relatively rare in the stratigraphic record because ocean floor is destroyed in subduction zones. We DO find a record of deepwater environments that form in basins: turbidites. A turbidity current is like an underwater avalance: sand suspended in water cascades down a slope and comes to rest in deep water muds. The turbidite is the resulting deposit.

Ocean processes; carbonate platform If you have warm marine water and little input of clastic material, you can form limestone The lagoon with reef in the distance. Open ocean to the left, the reef top, or crest.

A reef represents the optimal condition for maximum organic diversity and numbers Fossil reef exposures are common in the geologic record. We can recognize the reef and lagoon environments.