Carbonate-filled channel complexes on carbonate ramps: an example from the Peerless Park Member [Keokuk Limestone, Visean, Lower Carboniferous (Mississippian)], St. Louis, MO, USA

Abstract

The Peerless Park Member [Keokuk Limestone, Lower Carboniferous (Visean, Mississippian)], exposed in the St. Louis, MO, area includes a carbonate-filled channel complex with lithology and faunal elements distinct from under- and overlying strata. The complex is confined within a channel of 100-m apparent width that is under- and overlain by cherty, glauconitic crinoid–bryozoan–skeletal wackestone, packstone and grainstone with abundant hardgrounds and little evidence for wave or current energy. Laterally, the valley includes two subchannels separated by a subtle high. The deeper subchannel truncates up to 5 m of strata. The maximum thickness of the channel fill and associated deposits is 6 m. The channel fill includes a matrix-supported conglomerate locally at its base, non-cherty quartz silty–foraminifer–peloid–crinoid–bryozoan–skeletal grainstone with spectacular lateral accretion foresets up to 5 m thick, and cherty skeletal packstone. A capping foraminifer–skeletal grainstone is erosionally overlain by a 10-cm thick quartz siltstone that grades up into glauconitic crinoid–bryozoan–skeletal wackestone and packstone. On the channel flanks, correlative strata of the Peerless Park Member are less than 1 m thick, and include a lower non-cherty crinoid–foraminiferal–skeletal grainstone, a middle cherty packstone and an upper skeletal grainstone. The upper grainstone is in turn erosionally overlain by the thin quartz siltstone that abruptly grades into the crinoid–bryozoan–skeletal wackestone and packstone of the upper part of the Keokuk Limestone. The strata of the Peerless Park Member are interpreted to be the result of a relative fall of sea level that places shallow water, high-energy inner ramp grainstone immediately above deeper water, outer ramp packstone and grainstone. There is no evidence for gradual downward or basinward migration of facies belts. Instead, shallow water facies appear to have abruptly jumped downdip. Likewise, no field or petrographic evidence for subaerial exposure has been recognized, so the relative fall in sea level may not have subaerially exposed the area. This occurrence of an isolated, downdip carbonate grainstone filled channel complex not accompanied by progradational, forced regressive deposits appears to be unique in carbonate ramp depositional systems and the channel includes fill with character different from many previously documented ramp carbonate-filled channels.