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Algal banks are widely distributed, are important in late Paleozoic carbonate provinces, and commonly are good reservoirs for accumulation of hydrocarbons. The term "bank" as used in this report indicates an unusually thick sequence of carbonate mudstone built by in-place organisms in an environment of low turbulence. The bank stood slightly above the surrounding sea floor.
The Winterset Limestone bank (Dennis Formation, Kansas City Group, Pennsylvanian) is a complex, biogenic accumulation associated with a variety of rock types. The dominant bank-forming organism is a red alga, Archaeolithophyllum, which flourished in a quiet-water environment. Environmental factors reflected in lithologic-biota characteristics are light penetration, turbidity, turbulence, influx of terrigenous clastics, and changes in sea level. The westward thickening of Winterset Limestone in the subsurface is attributed to a more extensive development of the algal-bank complex.
The algal bank influenced biota distribution and sedimentation by providing a shallow platform on which other environments could develop. Later, during deposition of the upper Winterset unit, the algal bank acted as a barrier against influx of terrigenous clastic sediment from the south and restricted circulation of water to the north; this may have caused an increase in salinity.
Limestones of the Winterset comprise seven major rock types: skeletal mudstone, algal-bryozoan boundstone, skeletal calcarenite, oolitic calcarenite, laminated mudstone, algal-fusulinid mudstone, and gastropod-fusulinid mudstone.
The skeletal mudstone is interpreted as an off-bank accumulation in the deepest water area or basin. The algal-bryozoan boundstone records a quiet-water environment or marginal shallow shelf. The skeletal calcarenite represents a shoal environment intermediate between that of the algal-bryozoan boundstone and oolitic calcarenite. The latter indicates areas of agitation above a shallow-water bank. The laminated mudstone was deposited in an intertidal to supratidal environment. The algal-fusulinid mudstone represents an environment intermediate between the upper algal-bryozoan boundstone and the gastropod-fusulinid mudstone, which accumulated in slightly deeper water than the algal-bryozoan boundstone. The lack of diverse marine organisms suggests that environmental factors other than d pth were of primary importance for this facies.
Other than lithification, dolomitization was the major diagenetic process in the Winterset Limestone. Dolomitization was selective, replacing (1) drusy calcite mosaic in the algae, (2) carbonate matrix adjacent to algae, and (3) fossils which originally were composed of aragonite. Concentration of the magnesium ions was prior to deposition of the upper Winterset unit. The subsequent dolomitization seems to have been controlled, at least in part, by algal-bank development that caused a migration of magnesium-rich waters downward and laterally through the porous ooid sands into the algal bank. Penecontemporaneous dolomite formed northward along the southern part of the supratidal environment, whereas secondary dolomitization occurred farther south.
Dolomitization may result from either concentration of magnesium from coralline algae, by the "seepage refluxion" model, by the Dorag model, or a combination of the three.
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