COMPARISON OF MIDDLE MISSISSIPPIAN OSAGIAN SHELF AND SHELF MARGIN CHERT DEPOSITS IN CENTRAL KANSAS
At least four Osagian depositional sequences are evident on the shelf margin and at least two have been recognized on the shelf. The lower sequence on the shelf is dominated by echinoderm-rich facies containing abundant other diverse fauna and the upper sequence is dominated by sponge-spicule facies with an abundance of evaporites and subaerial exposure events. In general, compared to shelf margin locations, silicification is less abundant and variably replaces facies in both sequences.
Several lines of evidence indicate that most silica dissolution, redistribution, and silicification of facies occurred early and were completed prior to significant burial in the shelf settings. The major subaerial exposure and erosion event in the late Mississippian-early Pennsylvanian exerted less influence on silica diagenesis as compared to the shelf margin.
Depositional sequences along the shelf margin from bottom to top comprise argillaceous dolomite mudstone, cherty dolomite mudstone, nodular to bedded chert, massive autoclastic chert, autoclastic chert with clay infill, and bioclastic wacke-grainstone.
On the shelf margin silica diagenesis is interpreted to be both early and late. Although much silicification was early, the late Mississippian-early Pennsylvanian exposure event coupled with extensive fracturing of chert related to structural activity led to pronounced meteoric diagenesis including brecciation and dissolution. Erosion and transportation of the chert led to localized chert conglomerates.
The variation in chert abundance and diagenetic history between shelf and shelf margin areas results in important differences in chert character. Chert on the shelf tends to be tight (poor reservoir) whereas on the shelf margin chert is more porous and forms important reservoir facies.