Print this pageAbstract: Enigmas in Wisconsin Cambrian and New Depositional Model for Type St. Croixan
The Upper Cambrian of Wisconsin consists of several hundred meters of predominantly sandy sediments, divided into 11 lithostratigraphic units on the basis of grain size, abundance of glauconite, and presence of dolomite. Repetition of lithologic types relative to a succession of disconformities led previous workers to a cyclic-deposition model which postulated a series of onlap sequences as seas repeatedly transgressed onto the Wisconsin dome. Coarse basal sands are covered by finer sands and carbonate rocks. Some enigmatic features are not explicable in terms of the onlap model: lack of shale, rarity and patchiness of fossil assemblages, low species diversity, rip-up conglomerates, exposure indicators, and stromatolites. No true "quiet water," offshore environments are r cognizable, even in strata supposedly representing the transgressive maxima. These units are reinterpreted as products of tidal-flat deposition, and the supposed basal onlap sands tentatively are reinterpreted as subtidal-shelf or tidal-channel deposits. Placement of the coarser sand lithotope seaward of the finer sand lithotope implies that the sequences are progradational, thus reversing the traditional transgressive model.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma