Abstract

A field and petrographic study of lithologic attributes and stratigraphic relations of the St. Genevieve Formation in its type locality area was made after reconnaissance showed these outcrops to be inadequately correlated by earlier workers. The objectives of this work were to describe in detail individual sections, determine persistent local marker beds, tie together isolated exposures, and redefine the type sections. N. of Ste. Genevieve, Missouri, the type section consists mainly of cross-bedded, biocalcarenitic limestone beds comprising the lower 70 ft of the formation. S. of town, the upper 60-ft section is exposed. These 2 intervals have been correlated by a distinctive marker bed characterized by a unique insoluble residue assemblage of silicified fossils, glauconite, and red chert fragments. About 25 ft of equivalent Ste. Genevieve beds, below a 10-23-ft-thick sandstone member, appears at the top of the type section on the N. and in the lower part of the type section exposed on the S. Within the type locality area a composite section attains a maximum thickness of 122 ft, although the average is perhaps 15 ft less. The most common rock type consists of medium crystalline limestone composed of crinoid stem fragments, ooeids, brachiopod shells, and bryozoan zoaria in a calcite mud matrix which in some beds has recrystallized to a sparry calcite. Sublithographic limestones make up about 10% of the carbonate units. "Argillaceous" limestones and thin shaly beds and partings contain large amounts of fine quartz silt. The sandstone member is a fine-grained, well sorted massive unit cemented by carbonates and hydrated iron oxides. It is almost identical with the overlying Aux Vases Formation in physical character. A zone of algal pellets, mostly of genus Solenopora, is found near the base of the formation above a thin limestone conglomerate. Some question still exists regarding the nature of the St. Louis-Ste. Genevieve contact and the position of the sandstone member in the sequence at several outcrops S. of the town. Alternative interpretations from those presented in previous studies are suggested but not proved.

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