Abstract

The archetypal Upper Pennsylvanian Oread Cyclothem contains three minor cyclothems. The middle minor cyclothem consists of transgressive Leavenworth Limestone, maximum-transgressive Heebner Shale, and regressive Plattsmouth Limestone and Heumader Shale units. It was deposited on the southern Kansas shelf in SE Kansas and the northern forebulge of Arkoma foreland basin in NE Oklahoma, within the northern paleoequatorial belt, when large-amplitude glacial eustasy was present. Using 55 measured sections and 100 wells, we established the contrasting stratigraphic architectures of the Heebner Shale and Plattsmouth Limestone units between the shelf and deltaic provinces.

The Heebner in SE Kansas is a thin, organic-rich, and phosphatic shale deposited on an anoxic to dysaerobic starved shelf. It thickens abruptly into deltaic deposits in NE Oklahoma. The deltas were sourced by the Ouachita thrustbelts and prograded across the filled Arkoma Basin during sea-level highstand, because efficient weathering and sediment transport in a humid equatorial climate and an episode of rejuvenated Ouachita thrusting had provided copious sediment supply. Shelf anoxia was probably caused mainly by oceanic upwelling driven by the northeasterly trade wind when the shelf deepened during sea-level highstand. Using the modern upwelling system on the shallow NW African shelf as an analog, strong surface and deep geostrophic alongshore currents of the Heebner upwelling system were circulating clockwise along the southern and southeastern margin of the midcontinent sea. They walled off northward deltaic progradation and diverted it to the west, and caused abrupt juxtaposition between thick deltaic deposits and thin anoxic shale.

Thick phylloidal algal mound facies of Plattsmouth Limestone unit was deposited in the inshore warm water on the Kansas shelf during initial slow sea-level fall and basinward retreat of the upwelling center. It changes abruptly into thin arenaceous grainstone and packstone, calcareous sandstone, and limestone conglomerate onlapping the Heebner deltas in NE Oklahoma. Deltaic progradation ceased due to diminished siliciclastic supply. Strong alongshore currents were still present and prevented siliciclastic contamination of the carbonate platform. The abrupt facies juxtaposition between the shelf and deltaic provinces was caused by the interplay of eustatic, oceanographic, tectonic, and climatic processes, and depositional topography.