Geopic of the week: Ancient delta deposits

Deltaic deposits edited

The above picture shows rocks that were deposited in a delta.  A delta is a place where a river or stream empties into a standing body of water, typically the ocean or a lake.  As it enters the larger body of water, the stream loses velocity and starts dumping the sediment it’s carrying.  Coarse sediment settles out first and finer sediment is carried further from the shore before being deposited. 

As the delta develops, it builds a lobe of sediment out into the ocean.  The stream advances into the sea atop its own lengthening delta, and new sediment is carried ever further seaward.  Eventually, today’s coarse sediment is being deposited on yesterdays fine sediment.  Coarse-grained rock at the top, and fine-grained rock at the bottom, is characteristic of delta deposits.

The picture above shows two lobes of deltaic sediment, each with the characteristic coarsening upward architecture.  The coarsest, top-most bed of each lobe juts out, creating  a waterfall.  This picture is of the Atoka Formation and was taken near Lake Fort Smith, Crawford County, Arkansas.

 

Below is a land satellite image of recent Mississippi River deltas in the Atchafalaya Bay.

AtachfalaDelta

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