Abstract

An integrated sedimentologic analysis of outcrops, cores, and geophysical well logs has delineated lithofacies and determined their stratigraphic relationships in siliciclastic cyclothems of the upper portion of the Cherokee Group in eastern Kansas and northeastern Oklahoma. Coals that represent coastal swamp settings maintained by rising sea levels mark the initiation of transgression in nonmarine interfluve settings (transgressive systems tract = TST). Gray shales represent marine shelf margin environments (shelf margin systems tract = SMST) and prodeltaic, progradational muds (regressive systems tract = RST). Deltaic complexes prograded across the margins of the Middle Pennsylvanian epeiric sea during times of regression and eustatic lowstand in response to increased amounts of siliciclastics supplied as fluvial valleys were incised (RST). Black, phosphatic clay shales represent marine maxima that can be laterally traced in outcrops, in cores, and with geophysical logs across the entire Mid-Continent (highstand systems tract = HST). Sandstones consist of both shoestring-shaped and thin sheet-like units within thicker siliciclastic sedimentary lobes. Shoestring-shaped sandstones formed as (1) incised fluvial valleys that were filled during sea-level rises and, therefore, represent TST in fluvial valleys, and (2) delta-distributary channel sands that were deposited as parts of deltaic progradational complexes (RST). Sheet-like sandstone bodies represent the reworked deltaic lobes that were reworked during marine transgressions and, therefore, represent a marine portion of the TST. Seat rocks beneath coals represent paleosols that mark emergent portions of shoreline complexes and represent maximum regression (lowstand systems tract = LST) within traditionally defined cyclothems. These emergent units constitute sequence boundaries. These cyclothems, which record glacio-eustatic sea-level changes that resulted from global climatic changes, are "allocyclothems," or allocyclic sequences based on sequence stratigraphic concepts.

All or parts of four sequences occur within the interval studied and can be traced across the study area along the outcrop belt and in the subsurface. These sequences range in thickness from about 2.5 to 20 m and represent time spans that approximate 100,000 years each. Sequence 1 consists of the Verdigris Formation, which is underlain by an exposure surface represented by a paleosol beneath the Croweburg Coal. Sequence 2 overlies a paleosol and consists of the Bevier Coal and a thin marine shale overlain by a regressive interval of shale and sandstone. Sequence 3 overlies a paleosol and consists of the Iron Post Coal in northern Oklahoma and equivalent strata northward in Kansas, including the Breezy Hill Limestone and a widespread shale that contains thin sandstone lenses capped by a paleosol underlying the Mulky Coal. Sequence 4 consists of the Mulky Coal, the overlying Excello Shale, and the Blackjack Creek Limestone members of the Fort Scott Formation. Because the base of the widespread, easily recognized Excello Shale is considered to be the base of the Marmaton Group, the Cherokee-Marmaton contact is at the base of a maximum flooding surface (MFS); according to the traditional (Exxon) sequence stratigraphy concepts, this surface is a parasequence boundary contained within a sequence. The base of the Oakley black shale of the Verdigris Formation and that of the Excello black shale would be considered sequence boundaries, according to "genetic stratigraphy" concepts. Although genetic sequences can more easily be traced across much of the Mid-Continent, traditional sequences offer the possibility of a higher resolution of stratigraphic correlations.

The extent of each sea-level change and concomitant climatic changes were major controlling factors that determined the distributions of lithofacies associations. Along the eastern margin of the seaway, deltaic lobes prograded southwestward in response to minor sea-level drops and concomitant fluvial valley incision. As individual delta lobes shifted and were abandoned, waves and currents winnowed their upper portions, leaving thin sheet-like lenses of sand. These reworked sands and overlying marine muds constitute autocyclothems that are relatively local in extent in eastern Kansas and northern Oklahoma, and are embedded within the regressive portions of allocyclic sequences.