Abstract

Mississippian strata of southeast Wyoming provide new insight into regional Mississippian stratigraphy. The Madison Limestone along the west and northwest flanks of the Medicine Bow Mountains is approximately 10 m thick. The Madison is conformably underlain by a Mississippian sandstone, and disconformably overlain by redbeds which represent a Mississippian terra rossa. Stratigraphic correlation and scanty fossil evidence suggest that the Madison is Osagean, probably correlative with Sando's (1968) unnamed member of the Madison Limestone in central Wyoming.

The Madison within the field area consists mainly of pelsparite and oopelsparite. Oolitic intervals have been correlated and are more abundant to the south near Coad Mountain. Subordinate beds of micrite, biopelsparite, and bio-oopelsparite are also laterally persistent. A section at the north end of the field area consists of extensive neomorphic calcite and silicified limestone beds.

Crossbedding is very common. Crossbedded units range in geometry from tabular to wedge-shaped, and foreset dip directions indicate a predominantly south-southwest current direction. Post depositional deformation of beds by slumping is also common.

A marine depositional model is proposed which consists of a carbonate sand belt composed of individual subparallel bars. This belt, lying only a few kilometers offshore, paralleled the northeast-trending Mississippian shoreline and aggraded south-southwest as a result of longshore drift.

Diagenetic alteration of the Madison Limestone has been minimal. Diagenetic changes seem related to times of deep burial, Laramide deformation, and subaerial exposure. At least two generations of cementation are recognized. Liesegang banding originated prior to second generation cementation. Stylolitization occurred during times of deep burial and Laramide deformation. Similarly, calcite-filled fractures and neomorphic spar patches originated during deformation. Late Cenozoic regional uplift and exhumation have resulted in sub-aerial diagenesis. Copper mineralization, silicification, and massive recrystallization, all at the north end of the field area, are thought to be late in the diagenetic sequence.