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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.
LINKS:
EARTH AND OCEAN SCIENCE IMAGE COLLECTIONS