THE NATURE AND ORIGIN OF THE DEVILS HOLLOW MEMBER, LEXINGTON LIMESTONE, CENTRAL KENTUCKY

The Devils Hollow Member (late Middle Ordovician) of the Lexington Limestone outcrops in a northwest-striking belt that extends from Frankfort to southeast of Lexington, KY. It is an unusual member of the Lexington Limestone in that it contains subtropical carbonate constituents compared to mostly cool-water carbonates in the rest of the Lexington Limestone. Three bodies of Devils Hollow are found separated from each other along strike and are composed of three different facies: a light-colored, cross-bedded, porous calcarenite to calcirudite (coquinite); a fissile calcisiltite to calcilutite that often has mudcracks, laminae, and/or bird's eye structures; and a massive, argillaceous, fine-grained calcarenite or calcisiltite that usually contains ostracods. The former is found in all three bodies, whereas the latter two facies occur only in the northwesternmost body near Frankfort. Sedimentary structures indicate that the coquinite is a carbonate beach-barrier deposit and the other two facies represent lagoonal deposits that accumulated in back-barrier regions. From this, it can be concluded that the unit represents an overall regressive beach sequence, in which the lagoonal deposits now lie above the beach facies. Regional structural and stratigraphic relationships suggest that the barrier sequence accreted on top of a thick Tanglewood shoal sequence that probably developed on a penecontemporaneously uplifted block bound by two northwest-trending structural lineaments with basement precursors. Like some other Lexington members on the Tanglewood buildup in central Kentucky, it appears that the Devils Hollow Member is indirectly related to synsedimentary structural reactivation by growth faulting related to Taconian far-field processes. Presence of a local bentonite in the Devils Hollow indicates that the member is largely isochronous and can be used as a chronostratigraphic horizon to help analyze complex structural and stratigraphic relationships within the Lexington Limestone. Finally, the porous, light-colored calcarenites and calcirudites in the member are very similar to the approximately coeval petroleum reservoir rocks in the subsurface "Granville facies" to the south and may provide a model for reservoir development.