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The bioherms present in the Silurian of eastern Iowa are confined to the Niagaran Series and are largely restricted to the Gower Dolomite. Those described here are solely Goweran.
Four of the bioherms were chosen for detailed study from the hundreds present, as being typical of the Iowa structure and individually offering exposures of elements which no single bioherm displayed. Typically, they are oval mounds of undisclosed heights some exceeding 180 feet. They range from a few tens of feet to at least a quarter mile in diameter.
The bioherm consists of two parts: a central unstratified core, and peripheral, well stratified flank beds which abut against the core on the one hand and dip radially away from it, merging with the flat-lying inter-reef facies on the other. The core and flanks are petrographically nearly identical but structurally and paleontologically dissimilar.
The core is the most abundantly fossiliferous of the facies, both in variety of species and in numbers present. The coelenterates are the most abundant group, with the pelmatozoans and brachiopods of secondary numerical importance. The flanks have fewer fossils, with pelmatozoans and brachiopods being the most abundant. The presence of stratification in the flanks is the most obvious contrast between them and the core, however.
Both the flanks and core are characteristically porous, the result of solutional activity and attendant fracturing. The initial solution activity at least is a function of the amount and size of the fauna present. For this reason the core is by far the more porous and the more highly fractured.
In some locations collapse breccia and clay fillings form extensive deposits, particularly in the core. Solution activity and the resultant brecciated nature of the core have made this part of the bioherm much less resistant to stream erosion.
The development of the flanks is not entirely dependent upon core debris. Much, if not most, of the flank material is autochthonous.
Although generally similar in architecture and composition, the bioherms display textural, structural, and faunal variations. These variations are ultimately dependent on faunal differences. The most obvious explanation for faunal differences is that the bioherms are a polyphylitic conglomeration unified only by their mound-like out line. Faunal differences also may be explained as the result of growth-stage variations or significant age differences.
The general lack of coarse debris in the bioherm indicates that they must have develped under much less violent energy conditions than those of Illinois. It is concluded that the Iowa structures were formed in a shallow, lagoonal environment near the western shore of the Niagaran sea. The protection thus afforded reduced turbulence.
The bioherms included in this study represent but a small fraction of those present. Certainly they exist in the hundreds within the outcrop belt of the Gower in eastern Iowa.
Iowa Geological Survey
Iowa City, Iowa
v, 52 pages, 13 figures, 1 table, 1 appendix
Bioherms; Stratigraphic geology; Silurian Geologic Period
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Hinman, Eugene E.. A Biohermal Facies in the Silurian of Eastern Iowa. Iowa City, Iowa: Iowa Geological Survey, 1968. (Report of Investigations, 6)