A STUDY OF THE CHESTERIAN GIRKIN FORMATION (GASPERIAN)

The Early Chester (Gasperian) Girkin Formation is a shallow-water, carbonate-dominated unit in west-central Kentucky. Presence of species of the crinoid Talarocrinus differentiates the Girkin from the lithologically similar Ste. Genevieve Limestone below. Carbonates in the Girkin are in most places fragmental to oolitic calcarenites; dolomitic zones are thin, silty and associated with siliciclastic horizons. Siliciclastic intervals are thin and cyclic, represented mostly by shales and shaley carbonates. Coarser siliciclastics are rare and local. In most places siliciclastics more or less grade upward into carbonates. To the north and west, as the Girkin interval thickens, it is divided into up to seven carbonate and siliciclastic formations. For this study the Girkin is informally divided into three members.

The lower member comprises two coarsening-upward carbonate sequences overlain by paleosols. Thin siliciclastic horizons are present at the base and in the middle of the member at most localities. Upper beds are distinguished by the first occurrence of the coral Caninia veryi (Campophyllum gasperense and Lithodrumus veryi of older literature).

The middle member is composed of predominantly oolitic carbonates; persistent siliciclastic horizons are present at the base, middle and top. The carbonates in this interval reflect coarsening-upward sequences, but local fining-upward sequences are also present. Paleosols are present near the middle and at the top of the member locally. Upper beds of the middle member are locally absent due to pre- and syn-depositional erosion by the overlying Big Clifty Sandstone Member of the Golconda Formation.

The upper member consists of a basal siliciclastic-rich horizon, locally developed above paleosols that grades into upward-coarsening carbonates. Productid brachiopods are distinctive in the member. Locally the member is partially or wholly absent due to pre- and syn-depositional erosion by the overlying Big Clifty Sandstone Member of the Golconda Formation. In some places the member is gradational with siliciclastics of the Big Clifty.

Facies and lithologic changes within the Girkin occur both perpendicular and parallel to basin strike. Changes are related to proximity of siliciclastic source areas, and to syn- and post-depositional structural activity.