ABSTRACT: Petrology and Stratigraphy of the Middle Run Formation (Precambrian) in Kentucky

David C. Harris, Kentucky Geological Survey, University of Kentucky, Lexington, KY; [email protected]

Recent drilling, and discovery of an old core, have provided new data on the origin and distribution of the Precambrian Middle Run Formation in Kentucky. Middle Run clastic rocks have been penetrated in three wells drilled in Hart and Larue Counties, Kentucky. The KII 1 Brooks well in Hart County penetrated almost 1,800 ft of Middle Run Formation. Middle Run core from a well drilled in Jessamine County in 1966 was recently donated to the Kentucky Geological Survey. Interpretation of these new data has refined our understanding of the provenance and depositional environment of the Middle Run, and has confirmed previous interpretations of the areal distribution of this unit.

Well-sample description integrated with digital spectral gamma-ray, photoelectric (Pe), and formation-density data from the Brooks well have revealed an upward increase in the lithic content of sandstones in the Middle Run interval. More quartz-rich sublitharenites occur near the bottom of the well, and have minor porosity. Lithic components consist of felsic and less common mafic volcanic rock fragments. The change in composition is apparent on the Pe and density logs, where values for both increase up section. The spectral gamma-ray data indicate that gamma response is influenced by the presence of K-feldspars in the sandstones. Shales are minor, and vary little in the thorium or uranium component of the gamma-ray response. The observed change in sandstone composition corresponds to a sequence boundary observed on seismic data near the Brooks well. Interpretation of Middle Run core from the Texaco 1 Sherrer well in Jessamine County supports a subaerial alluvial-fan depositional environment. The upward increase in lithic content observed in the Brooks well reflects increasing contribution from volcanic source areas, probably from within the East Continent Rift Basin.

Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada