Stratigraphy, depositional environments, and paleoecology of the Upper Silurian (Pridoli) Sneedville Formation in Hancock County, Tennessee

The Upper Silurian Sneedville Formation (Hancock Dolomite of U.S.G.S. usage) exposed in Snake Hollow, near Sneedville, Hancock County, Tennessee, is composed of sedimentary rocks deposited in a shallow marine shelf to tidal flat setting. Of the approximately 20 m exposure at Hatfield Farm (HF) and at Vardy School (VS), the basal (approximately 3 m) unit is a fossiliferous calcareous quartzarenite and quartzose biomicrite. The quartzarenite was deposited during a marine transgression over a previously existing erosion surface on the Rockwood and Clinton formations (Lower Silurian) as a barrier bar complex or intertidal sheet sand. Eleven units of interbedded limestone and dolostone lie above the basal quartz sandstone, representing migrating adjacent lithofacies of subtidal lagoon to supratidal mudflat environ-ments, the latter capping each sequence. Slab and petrographic studies reveal subtidal deposits to have the highest faunal diversity. These deposits are dominated by ostracode-brachiopod-rich biopelmicrite and burrowed micrite. Low intertidal deposits of pelletal-skeletal grain-stone are dominated by a bivalve-ostracode fauna and intertongue with subtidal muds. In high intertidal shoals, reworked pelloids, superficial ooids, rip-up clasts of lithified oointrasparite, micrite intraclasts, and skeletal grains were deposited in graded oointrasparite beds. These are interbedded with supratidal cryptalgal laminated dolostone, evaporite pseudomorphs and mudcracks indicating desiccation.

The biostrome, a central, laterally continuous unit, was formed by a transgression of sea level and migration of lagoonal faunas over supratidal flats. Five genera of stromatoporoids collected from the biostrome, exhibit eight growth forms, including a ramose form, rare in correlative faunas. Species of Stictostroma (S. Pseudoconvictum), Stromatopora, Parallelostroma, Densastroma, and Plexodictyon are collectively characteristic of Pridoli age stromatoporoid assembleges. One genus, Parallelostroma also colonized intertidal oolite shoals within an upper stromatoporoidal dolostone. Here laminar and low hemispherical forms are found, which are more stable in turbulent conditions than ramose, high hemispherical and subspherical coenostea characteristic of the subtidal, non-reefal biostrome. This is in agreement with Laporte's (1967) description of Manlius (Lower Devonian) stromatoporoids of which laminar forms were found in intertidal channels and hemispherical forms in the subtidal facies.

The conodont, stromatoporoid and coral fauna indicates the Sneedville is Pridoli in age, but Siluro-Devonian systemic boundary is not exposed and may fall within a hiatus represented by post-Sneedville erosion. Conodont elements recovered from 19 samples belong to four multielement species and two forms transitional between Ozarkodina remscheidensis remscheidensis and O. r. eosteinhornensis. The association of O. r. eosteinhornensis and O. Confluens with subtidal lagoonal facies, and of O. r. remscheidensis with intertidal and shallow lagoonal, marine facies supports previous findings by Denkler, Harris, and Lierman (1983) in the Tonoloway Formation of Maryland that associations of these taxa are environmentally controlled, and not evolutionarily successive.