Print this pageAbstract: Depositional Environments and Diagenesis of Madison Limestone (Mississippian), Northern Medicine Bow Mountains, Wyoming
Mississippian strata of southwest 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 underlain conformably by a Mississippian sandstone, and disconformably overlain by red beds which represent a Mississippian terra rossa. Stratigraphic correlation and scanty fossil evidence suggest that the Madison is Osagian, probably correlative with Sando's unnamed member of the Madison Limestone in central Wyoming.
The Madison within the study area consists mainly of pelsparite and oopelsparite. Oolitic intervals have been correlated and are more abundant in the south near Coad Mountain. Subordinate beds of micrite, biopelsparite, and bio-oopelsparite also are laterally persistent. A section at the north end of the study area consists of extensive neomorphic calcite and silicified limestone beds.
Cross-bedding is very common. Cross-bedded units range in geometry from tabular to wedge-shaped, and foreset dips indicate a predominantly south-southwest current direction. Postdepositional deformation of beds by slumping is also common.
A marine depositional model is proposed which consists of a carbonate-sandstone 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 subaerial diagenesis. Copper mineralization, silicification, and massive recrystallization, all at the north end of the study area, are thought to be late in the diagenetic sequence.
AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah