Facies, Depositional, and Diagenetic Environments of Golconda Group (Mississippian) in Southwestern Illinois and Southeastern Missouri

Clayton D. Harris, George H. Fraunfelter

In southwestern Illinois and southeastern Missouri, the Golconda Group (Late Mississippian) is composed of interbedded calcareous and noncalcareous shales, bioclastic and oolitic limestones, and quartz siltstones and sandstones, in order of decreasing abundance. These were deposited on a shallow-marine shelf under the influence of a fluvial-deltaic complex originating from the north.

The primary factors responsible for the abundance and lateral variability of these lithologies were environmental energy level and terrigenous sediment supply. These factors contributed to the formation of 12 distinct Golconda lithofacies (two siliciclastic and ten carbonate facies). They formed in a variety of shallow shelf environments.

Localized bioclastic and oolitic accumulation, resulting in shoal formation, and storm surges were the predominant depositional processes for limestones on this shelf. Environmental energy appears to have increased toward the southwest with a corresponding decrease in water depth. This was the result of an area of emergence toward the southwest, from which minor fine-grained, siliciclastic sediment may have been supplied to the basin.

Based on petrographic observations, the diagenetic sequence of the Golconda was as follows: eogenesis, marine phreatic and freshwater phreatic succeeded by a fresh/saline water mixing zone; mesogenesis, shallow to deep subsurface zone; and telogenesis, freshwater vadose to phreatic surficial zone. Exposure to each of these zones resulted in a variety of diagenetic products, the most significant of which were freshwater cements, dolomite, and sutured grains.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.