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Deltas



The exposed sedimentary rock layers in the Pittsburgh area were deposited in a deltaic system. A delta is a roughly trianglular pile of sediment up to thousands of square kilometers in area which builds up where a river enters an ocean or a lake. The best-known example in the United States is the Mississippi River Delta. 300 million years ago, western Pennsylvania was a lot like Louisiana is now.


Deltas gradually build outward, forming a very flat surface just barely above sea level. When the river reaches this very flat area, it forks into several meandering channels. The top of the river water is very close to the land surface; just a little bit of water added by rain runoff can cause flooding. Delta deposits are characterized by broad expanses of flood-related mud deposits, crossed in places by meandering "ribbons" of sand deposited in the river channels. When these sediments are buried and "cooked", the muds become shales and the sands become sandstones. You can see evidence of these ancient deltas at almost any road cut in the Pittsburgh area; look for thinly bedded shale (the flood - related mud) with occasional layers or lenses of sandstone.


The tremendous weight of sediment in a delta can cause the area to sink. The rocks which make up a continent are more buoyant than those that make up the ocean floor, and so will not sink as deep or as fast under the weight of a delta. An area may sink at a rate equal to the rate of deposition, producing a thick accumulation of similar rocks. If the delta sinks faster or slower than the rate of sediment accumulation, the setting and rock type will change over time. Occassionally, sinking of the land in a particular area or general rise in sea level can cause the sea to spread over the land surface. Limestone layers record these incursions.


A repeating sequence of environments may occur. A cyclothem is a coal-bearing cyclic sequence of sedimentary rocks characteristic of western Pennsylvania. Cyclothems contain limestone and gray-green shale deposited when sea level was high, gradually changing upward through red shale deposited near the mouth of the delta into interbedded sandstone, shale, and coal deposited in the river and its floodplain when sea level was low.


The delta that was in the Pitsburgh area was lifted up during the Pennsylvanian Period as North America slammed into Africa to form Pangea. Western Pennsylvania was never again below sea level.



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EARTH AND OCEAN SCIENCE IMAGE COLLECTIONS