ARCHITECTURE AND STRATIGRAPHIC IMPLICATIONS OF A CARBONATE-FILLED CHANNEL COMPLEX: PEERLESS PARK MEMBER (KEOKUK FORMATION, MISSISSIPPIAN), ST. LOUIS, MO

The Peerless Park Member (Keokuk Formation, Osagean, Mississippian), present locally in east-central Missouri and adjacent Illinois, has represented a very thin, stratigraphically enigmatic unit because it includes lithologies and fauna distinct from under- and overlying strata. New laterally continuous outcrops exposed in the St. Louis area reveal that the Peerless Park includes a carbonate-filled channel complex. The complex is confined within a channel of 100 m apparent width that is under- and overlain by cherty crinoid-bryozoan-skeletal packstones and grainstones with little evidence for currents. Laterally, the valley includes two subchannels, separated by a subtle high. The deeper channel truncates up to 5 m of strata. The maximum thickness of the channel fill is 6 m, but an internal truncation surface is present within the succession. The lower part of the channel fill includes spectacular lateral accretion foresets up to 5 m thick in non-cherty quartz silty-foraminifer-peloid-crinoid-bioclastic grainstone. It is locally conglomeratic at the base. The truncation surface truncates up to 1.5 m of the lower part of the channel fill and is overlain by (inaccessible) cherty packstone to grainstone (?; based on weathering character) that is up to 3 m thick. On the channel flanks, correlative strata are < 1 m thick, non-cherty crinoid-foraminiferal-bioclastic grainstone. Field observations revealed no conclusive evidence for subaerial exposure in underlying strata in these areas. Given the data presently available, we currently are entertaining two alternate hypotheses for the origin of these strata: 1) they represent a channel cut by tidal processes, not associated with subaerial exposure; and 2) they represent an incised valley complex (associated with subaerial exposure) that was filled during the subsequent relative rise in sea level. In either case, however, the facies offsets are interpreted to represent a pronounced and abrupt relative fall in sea level in mid-Keokuk time.