Throughout the Pennsylvanian Period, I’ve focused a lot on the coal swamps that characterized the time, as well as mountain uplifts. But there were plenty of regions still covered by shallow seas like those that were typical of so much of the earlier Paleozoic Era.
Take Kansas, for example. Many of the Pennsylvanian rocks there are shallow-water limestones similar to those that covered much of the interior United States for much of the previous 200 million years. One difference is that those Kansas rocks do also reflect the alternations in sea level that produced the coal cyclothems further east. Cyclothems are alternating layers of coal and sandy sediment, reflecting high stands and low stands of the sea. In the Lansing Limestone of Kansas, the same cycles are shown by alternations between limestone and shale in the Pennsylvanian strata.
It’s pretty clear that the limestones were formed in very shallow water. A lot of the rock is made up of broken shells, which must have been fragmented by wave action. They were piled up into shoals, just like sand banks at or near the water line or at least above wave base. When sea level rose just a bit, fine-grained deeper-water shaly sediments could wash over those carbonate banks. The skeletal shell fragments in the limestone made for some excellent porosity in the limestone – commonly as much as 15% of the rock, and the tight, fine-grained shale made for a nice impermeable barrier – both necessary conditions for oil and gas accumulations. All that we need is a source rock with time to mature – and we have that too, in the same shales that washed organic material into the sea along with the fine sediment.
|Drawing of shoal environments of Lansing Limestone (from Harbaugh, 1960)|
The Lansing and Kansas City Group of limestones has produced something like a billion barrels of oil over many decades, and that adds up to at least a fifth of all the oil Kansas has produced, and Kansas is one of the leading states in terms of total cumulative production. About 25% of all U.S. oil production comes from rocks of Pennsylvanian age. The Lansing Formation and its equivalents extend into Colorado and Oklahoma too. They were laid down about 210 million years ago, contemporaneous with the coal swamps in Pennsylvania and West Virginia and elsewhere.
—Richard I. Gibson
Petrology of Marine Bank Limestones of Lansing Group (Pennsylvanian), Southeast Kansas, by John W. Harbaugh - Originally published in 1960 as Kansas Geological Survey Bulletin 142, part 5
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