Tidal and deltaic controls on carbonate platforms: Glen Dean Formation (Upper Mississippian) of Illinois Basin, U.S.A.

Abstract

About 1200 vertically oriented carbonate, sandstone and shale samples were collected from twenty-three stratigraphic sections of the Glen Dean Formation in the Illinois Basin. Detailed petrographic investigation using the microfacies approach has led to the recognition of sixteen distinct carbonate microfacies. Their vertical succession and reciprocal relationships in the different sections and their correlation coefficients led to the recognition of an ideal shallowing-upward sequence in turn converted into a horizontal depositional model. The model is a shallow open-marine subtidal carbonate platform consisting of the juxtaposition of two broad banks. The following environments from offshore to onshore were recognized: lower slope, upper slope, outer bank with three sub-bank environments (foraminiferal, crinoidal, and oncolitic), inter bank, inner bank with three sub-bank environments (bryozoan, oncolitic, and oolitic), lagoon. The carbonates display a complex diagenetic sequence with several pathways which includes the following environments: stagnant and active marine phreatic, freshwater vadose, undersaturated and saturated freshwater phreatic, mixing freshwater-marine phreatic, and burial. The petrographic analysis of the siliciclastic rocks led to the recognition of five microfacies: siltstone, quartz arenite, quartz arenite with argillaceous matrix, quartz wacke and shale. A depositional model was developed which consisted of a deltaic complex periodically prograding over the eastern shelf of the Illinois Basin. Consequently, three distinct environments were juxtaposed: delta system, carbonate platform and basin. At times sands and muds cut across the carbonate platform by means of channel systems and were deposited in the basin by gravity flows. The clay mineral assemblage of the shales from the deltaic fringe consists of discrete illite, kaolinite or chlorite, and mixed layer illite/smectite. Kaolinite and chlorite display a north-south zonation due to their physical segregation in the depositional environment with kaolinite in more shoreward and chlorite in more marine environment. The illite/smectite composition ranges from 50 to 85% illite and from 15 to 50% smectite. Its mixed layered structure is short range ordered and/or random. These samples mainly reflect the effects of provenance. The clay mineral assemblage of the shales from the central part of the basin contains long range ordered illite/smectite with the composition of 90–95% illite and 5–10% smectite. These samples mainly reflect the effects of thermal diagenesis due to burial.