UPPER CROIXAN STRATIGRAPHY, UPPER MISSISSIPPI VALLEY

In the Upper Mississippi Valley Croixan strata above the Franconia formation consist of dolomite and dolomitic siltstone and sandstone overlain by massive nondolomitic sandstone. Previous classifications of this succession have employed both faunal and lithologic criteria for discrimination of formations and members.

The strata can readily be divided on lithologic characters alone, and such divisions are markedly time-transgressive from shoreward to basinward facies. The terminology adopted represents a return to early lithologic classifications in which the St. Lawrence and Jordan formations are recognized as the dolomitic strata and the massive sandstone, respectively. The St. Lawrence is further divided into the Black Earth member (sandy and silty dolomite) and the Lodi member (dolomitic siltstone and fine-grained silty and dolomitic sandstone). Faunal zones are largely independent of the lithologic units.

It is urged that the term Trempealeau, previously employed as a formation name to include all these strata and also as a stage name, be restricted to Trempealeauan Stage.