Microfacies of Curdsville through Grier beneath Cincinnati; Characterizing the Utica –Lexington transition in the Subsurface
A series of cores drilled to a depth of ~400 ft from lower Kope Fm. (mid Katian) downward to the High Bridge Group (Sandbian) in the Cincinnati, Ohio area are a link between the Lexington Limestone and the Utica Shale in the Sebree Trough. One 300 foot interval was studied petrographically with 200 thin sections.
Preliminary analysis suggests five microfacies: 1) organic, laminated, brown shale with 0.5–1 cm depositional units separated by single layer dalmanellid pavements; 2) calcareous shale to micrite with fine silt and 2–20 cm units; 3) Stacked dalmanellid pavements with siliciclastic mud and calcisiltite matrix, horizontally oriented, unbroken shells with spar or matrix filled articulated valves, units 0.5 – 20 cm thick; 4) Packstones showing a diversity of partially fragmented, randomly oriented brachiopods, bryozoans, and trilobites, some non-skeletal phosphate particles, infiltration structures and erosive bases; and 5) Grainstones, consisting of comminuted shells of a variety of fossils, with high phosphate content, and erosive bases.
The 30 foot Logana (~260 – 290 ft) consists mostly of facies 1 with facies 2 and 3. The packstones and grainstones are largely restricted to the Curdsville (>290 ft) and the Grier (<260 ft). There is no evidence of long distance transport of any clasts larger than silt sized, but packstones and grainstones appear to have been increasingly reworked in place with phosphate enrichment, reflectling longer exposure on the sea-floor before burial. Hence, petrographic work permits distinguishing beds that record longer term re-working. Further work may permit delineation of small-scale cycles.
AAPG Datapages/Search and Discovery Article #90218 © 2015 Eastern Section Meeting, Indianapolis, Indiana, September 20-22, 2015