17 Undergraduate Research II (Posters)

Thursday, 2 April 2009
1:00 PM - 5:00 PM

STRATIGRAPHY, PALEOENVIRONMENTS AND CORRELATION OF THE LOWER PART (C2-C3 BOUNDARY) OF AN OUTCROP OF UPPER ORDOVICIAN ROCKS NEAR FREDERICKTOWN, KENTUCKY

Paper number: 17-3
Program: Theme Sessions
Time: 1:00 PM – 5:00 PM

Authors

Abstract

The lower part of an Ordovician road cut exposure on US Hwy 150 in Nelson Co., KY 2 km NW of Fredricktown is mapped as uppermost Grant Lake Limestone and the Rowland Member of the Drakes Formation, but has also been identified as the interval spanning the C2-C3 boundary. Its shallow facies makes it difficult to accurately correlate to deeper type Cincinnatian sections.

In preparation for refined correlation, we divided the 17 m of described section into 5 units (U1-U5). Current work includes cm-scale descriptions of these units with thin section petrography to detect potential lithologic markers or meter-scale cyclicity, and detailed whole faunal survey.

The lower units shallow upward. U1 is the Grant Lake Limestone which includes siltstone and wavy-bedded packstones. It contains brachiopods including Vinlandostrophia and the bivalve Modiolopsis. U2 is the base of the Rowland Member characterized by greenish muddy siltstones and thin layers of wackestone with a limited fauna of bryozoans, including a small hemispherical form resembling Stigmatella irregularis which could suggest correlation with the type Cincinnatian Miamitown Shale.

Transgressive deposits begin at 10 m with U3, a cross-bedded 0.5 m-thick encrinite grainstone. This designates the basal unit of the C3 sequence and is a regional marker bed. U4 is dominated by vuggy (>20 cm) wackestones that thicken upward, interbedded with minor shaley siltstones. Rare fossils include stromatoporoids and gastropods. This represents a restricted marine environment with low sediment influx possibly due to transgression.

U5, an interval of wavy-bedded packstone with a few 10-15 cm grainstones starts at 13.6 m. It contains the brachiopods Rafinesquina and Vinlandostrophia, and a variety of bryozoans including large hemispherical colonies. In U5, transgression has led to a return to open marine conditions.

Meter-scale cyclicity is seen in most units: U1 has alternating mudrock and wavy-bedded packstones; U2 contains alternating mudrock and wackestones; U4 is a single cycle with a mudrock and wackestone; and U5 shows wavy bedded packstones and grainstones. These cycles could form a framework for higher-resolution regional correlations