Shallow-Water Marine Siltstones in Paleozoic Shelf Deposits of Kentucky
Frank R. Ettensohn, Jack C. Pashin, Gary W. Jacobs
Across much of the Mid-Continent United States, shallow-water marine deposits are dominated by carbonates. Where clastics do occur, they commonly represent marginal marine environments such as tidal flats and deltas. However, fully marine shelf sands and silts also are present, although the processes involved in their deposition are not as well understood. Two such sandstones are present in the Paleozoic exposures of central and eastern Kentucky: the Ordovician Garrard Siltstone and the Devonian-Mississippian Berea siltstone. Although large parts of the Berea are attributed to deltaic deposition, silts and sands on either side of the main delta lobes are clearly not deltaic. Both the Garrard and Berea are parts of net regressive sequences accompanying major orogenies, bot extend far onto the shelf away from marginal marine situations, and both are dominated by siltstones with abundant flow rolls and wave ripples.
The Berea is a coarsening-upward sequence of quartzose siltstone and sandstone with minor amounts of mudstone. The Garrard shows no vertical grain-size gradient and consists of calcareous, quartzose siltstones and sandstones interbedded with fine to coarse-grained limestones and minor amounts of mudstone. Sedimentary structures in both units can be described in terms of "ideal" hummocky sequences, but structures reflecting other process responses may be superimposed on them.
Hummocky sequences in the Berea and Garrard apparently reflect deposition by storm wave processes, but secondary processes related to gravity, tides, and calcareous, benthic communities may alter the ideal sequence considerably. Although both the Garrard and Berea are storm wave deposits, major differences between the two are related to structural control, nature of the source area, and apparent depth.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.