Abstract

The Silurian of Wisconsin contains biohermal structures at several stratigraphic horizons. These bioherms are essentially cuboidal, domal, or ridgelike masses of porous and cavernous, crudely bedded and poorly jointed, barren to highly fossiliferous dolomites which grade peripherally outward and vertically upward and downward into well-bedded, dense to saccharoidal, relatively unfossiliferous dolomites. The approximately contemporaneous surrounding strata, which commonly are thin bedded, first dip away from the reefy mounds at relatively low angles, except where differential slumping has steepened the initial dip, and then become essentially horizontal and take on the lithological character of the normal stratigraphic sequence of the region.

The complexity of the fauna on some of the bioherms shows clearly that no single phylum of organisms played the dominant role in the construction. On the contrary, it is usually obvious that many different classes of organisms made large and important contributions to the growing calcareous mounds. Chief among the bioherm builders were algae, hydrozooid stromatoporoids, tabulate corals, camerate crinoids, cryptostomatous bryozoans, protrematous and telotrematous brachiopods, pelecypods, gastropods and cephalopods, and trilobites.

Biohermal sites, especially at Wauwatosa, Milwaukee, and in the vicinity of Cedarburg and Grafton, were the main sources of the several large Silurian collections made by local amateur collectors in the closing decades of the nineteenth century; they still remain among the best collecting sites. The interbiohermal strata may or may not be fossiliferous, but if they are their fauna tends to be small and somewhat different from the typical biohermal assemblage.

The bioherms appear to have accumulated in fairly shallow water and to have stood as low mounds a few feet to a few tens of feet above the surrounding bottoms where fine calcareous muds were being deposited.

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