THE MENARD FORMATION IN WESTERN KENTUCKY: A DETAILED RECORD OF HIGH FREQUENCY EUSTATIC FLUCTUATIONS DURING THE EARLY CARBONIFEROUS IN THE SOUTHEASTERN ILLINOIS BASIN

The Lower Carboniferous (Serpukhovian; Chesterian) Menard Formation was deposited on a ramp along the southeastern margin of the Illinois basin. Forty-five feet of fossiliferous Menard strata exposed at a road cut in Hancock County, Kentucky include gray shales interbedded with gray micritic limestones and orange-weathering micritic limestone to dolomicrite. The shales contain mostly fenestrate bryozoans and articulate brachiopods, with various other marine fossils; a few thin intervals of shale contain only pelecypods. The gray limestones contain a diverse marine fauna also dominated by bryozoans and articulate brachiopods. Scattered stromatolitic mounds occur in a narrow zone of laminated, orange-weathering limestone. These lithologies and faunas indicate deposition in aquatic environments ranging from fully marine to brackish, and from subtidal to intertidal.

At least seven levels in the lower 18 feet of this outcrop display one or more subaerial exposure features, including mud cracks (several horizons), fenestral porosity in peloidal limestone, mats of small horizontal rhizomorphs, rubbly-surfaced dolomitic limestone containing small vertical rhizomorphs, microkarsted limestone, and pedoturbated paleosol developed on dolomitic limestone. Although some of these exposure horizons may simply represent the filling of accommodation space, others (identified by rhizomorphs, pedogenesis, and microkarstification) more clearly indicate changes in relative sea level, and suggest minor high-frequency eustatic fluctuations.

Except at the top, the upper 27 feet of the Menard here lacks subaerial exposure features, suggesting a prolonged major highstand during which no indications of minor high-frequency eustasy were left behind. Deep carbonaceous rhizomorphs in shale at the top of the Menard reveal a significant fall in sea level that brought Menard deposition to an end. The overlying fluvial Palestine Sandstone channeled into the top of the Menard during the ensuing lowstand.