Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 243,
part 1
Sedimentology and Ichnology of Paleozoic Estuarine and Shoreface Reservoirs, Morrow Sandstone, Lower Pennsylvanian of Southwest Kansas, USA--page 7 of 14
Open-marine Facies Assemblage
Facies I: Highly Fossiliferous, Planar-crossbedded, Very Coarse Grained to Medium-grained Sandstones and Pebble Conglomerates
Description. Facies I consists of light-greenish-gray, planar-crossbedded to rarely massive, conglomeratic, very coarse grained to medium-grained, calcite- and dolomite-cemented quartzose sandstones and pebble conglomerates (fig. 35). Granules and pebbles are dispersed in the sandstones (fig. 36), concentrated at the base of beds (fig. 37) or forming discrete fining-upward beds. Foresets are normally or inversely graded. Fine-grained sandstone interbeds are common, and, in rare cases, display low-angle cross-stratification, probably hummocky cross-stratification. Clay lenses and lamina, as well as stylolitized clay drapes may occur locally. Bed boundaries are either diffuse or erosional. In the latter case, scours filled with pebbles are typical. Bed amalgamation is common. Facies I is the most abundant facies in both fields. At Nell, it occurs as discrete packages throughout the core, typically at the top of coarsening-upward parasequences. At Kendrick and Gaskill, it makes up the upper part of the succession. In the Gaskill core, the whole facies-I unit fines upward, with thicker and more abundant shale lenses towards the top, which are finally replaced by a thick shale unit (facies O). Crinoid plates, bryozoan fragments, solitary and colonial corals, and articulate brachiopod shells are very abundant (fig. 38), forming lenses or paving the erosive bases of fining-upward beds. In some cases, they form polytypic skeletal accumulations, which are bioclast-supported in the lower part of the bed and matrix-supported in the upper one. In these concentrations, brachiopod shells are concordant, but crinoids, corals, and bryozoans do not display any preferential orientation. These fossil concentrations exhibit a simple internal structure of fining-upward type. Brachiopod shells are concentrated towards the base of the bed, while crinoids, corals, and bryozoans, although more abundant at the base, may be present as out-size fragments in the finer-grained upper parts.
Ichnology. The coarse-grained deposits in facies I are essentially unbioturbated. However, Palaeophycus isp. and Ophiomorpha irregulaire (fig. 39) may occur locally. Degree of bioturbation is low, ranging from 0 to 1. The association consists of dwelling structures of suspension-feeders and represents an example of the Skolithos ichnofacies.
Interpretation. This facies is interpreted as upper-shoreface deposits formed seaward and laterally to the estuary mouth. In some cases, distinction of this facies from the coarse-grained, estuarine-mouth barrier is difficult, as both tend to interfinger towards the open marine-estuary transition. Large-scale foresets are most likely part of trough cross-stratification and therefore indicate migration of subaqueous dunes. The high-angle foresets allow distinction of upper-shoreface deposits from foreshore facies, which are characterized by very low-angle crossbeds. The presence of trough crossbeds and the scarcity of biogenic structures is currently considered as indicative of the upper shoreface (e.g., Pemberton, Van Wagoner et al., 1992; Raychaudhuri and Pemberton, 1992). The local existence of hummocky cross-stratification suggests wave activity during storms. Furthermore, overall features of the skeletal accumulations indicate a sedimentologic origin due to storm action (cf., Kidwell et al., 1986). Similar fossil concentrations have been described from shoreface environments (e.g., Aigner, 1985). The scarce trace fossils are also a feature of the upper-shoreface deposits described by Pemberton, MacEachern et al. (1992), which include occasional Palaeophycus, Ophiomorpha, and escape burrows. The ichnofauna indicates a shifty sandy environment of extremely high energy, with strong physical reworking by currents and waves that keep organic particles in suspension. Facies I of this study is equivalent, at least in part, to facies 5 (crossbedded, fossiliferous sandstone) of Wheeler et al. (1990).
Kansas Geological Survey
Web version November 9,1999
http://www.kgs.ku.edu/Current/1999/buatois/buatois7.html
email:lbrosius@kgs.ku.edu
