Carbonate Tidal Flat in Mixed Carbonate-Siliciclastic Clore Formation (Upper Chesterian) in Southern Illinois

Frederick E. Abegg

The upper Chesterian Clore Formation consists primarily of delta-front sandstones of the Tygett Sandstone Member and interbedded offshore mud-rich carbonates and prodelta shales of the Cora and Ford Station Limestone Members.

The basal Ford Station limestone in south-central Illinois contains a carbonate sequence 1.0 m thick marked by (1) laminated pelletal lime mudstones, (2) bird's-eye structures containing internal sediment, (3) vertical burrows, (4) horizontal shrinkage cracks, (5) autoclastic brecciation, (6) root? tubes, and (7) calcispheres. These features indicate tidal-flat deposition. Other examples of peritidal carbonate deposition are unknown in the Chesterian of the Illinois basin. Tidal-flat strata overlie 1.9 m of interbedded shale and lime mudstones containing linguloid brachiopods, pectin bivalves, and ostracods. The lime mudstones and shales are interpreted as shallow-subtidal, restricted-shelf deposits overlying crevasse-splay deposits of the upper Tygett sandstone.

Carbonate tidal-flat deposition in the upper Chesterian of the Illinois basin is an exception to the generally accepted model of nearshore terrigenous and offshore carbonate sedimentation. Delta switching is the most plausible explanation for development of the Clore tidal flat. Following deposition of the deltaic Tygett sandstone, peritidal carbonate deposition occurred in the basal Ford Station limestone when terrigenous sedimentation was deflected westward, as indicated by thin delta-front sandstones in the basal Ford Station limestone in southwestern Illinois. The tidal flat developed locally because south-central Illinois was the region of maximum progradation of the Tygett delta and because upper Tygett crevasse-splay deposits compacted less than adjacent shales.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.