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Coral faunas studied from subsurface cores of the Mississippian Madison Group in the Williston basin of North Dakota indicate that Sando's coral zones for outcrops in western North America can be extended into the subsurface of North Dakota. Coral zones II and III are recognized as corresponding roughly to lower and upper Mission Canyon strata, respectively. These data were obtained from 12 wells along the northern border of North Dakota in Divide, Burke, Renville, and Bottineau Counties, and two wells near the center of Williston basin in Dunn and McKenzie Counties.
Coral faunas appear to show a relatively low diversity of 13 species distributed among the following genera: Syringopora, Vesiculophyllum, Sychnoelasma, Amplexizaphrentis, Lophophyllum, Cyathaxonia, Lithostrotion (Siphonodendron), Diphyphyllum, Michelinia, and Stelechophyllum. Vesiculophyllum, Sychnoelasma, and Syringopora are the most abundant genera of the Madison Group in North Dakota.
Corals found in dark argillaceous crinoid-skeletal wackestones representing "deeper" waters are robust, and this may infer a hospitable environment for their growth. However, evidence from the coral and lithologic associations refute the pervading dogma that the occurrence of corals is strictly facies controlled. Abundant smaller corals have been found from buff-colored skeletal wackestones and algal mudstones which alternate with subaqueous anhydrites representing a marginal marine environment. In addition, corals have been found in buff-colored skeletal and peloidal grainstones of adjacent shoals and in brown pisolitic-oolitic packstones-wackestones of possible tidal ponds. These latter deposits may represent allochthonous accumulations, but the amount of time involved in transport f corals would not invalidate their usefulness as biostratigraphic tools.
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