Evolution of the Southwestern Midcontinent Basin During the Middle Pennsylvanian: Evidence From Sequence Stratigraphy, Core and XRF in Southeastern Colorado

Abstract

Understanding the interplay of regional tectonic setting, basin geometry, and facies relationships is critical to characterizing the petroleum systems of a basin. This is a challenge for the southwestern Midcontinent Basin due to the lack of outcrops of equivalent lithostratigraphy and facies encountered in the subsurface and the dearth of subsurface studies for this broad region. In order characterize the stratigraphic relationships and basin architecture of this petroleum-rich area, our study focuses on the Atokan and Desmoinesian stages (Atoka, Cherokee, and Marmaton formations) of the Middle Pennsylvanian. We utilize and integrate subsurface data including well logs, core data, X-ray fluorescence data, and formation image logs to support our sequence stratigraphic interpretation and a spatially and temporally complex facies model that encompasses southeastern Colorado to central Kansas. Our results reveal a dynamic character to the southwestern Midcontinent Basin. During the Atokan Stage, the basin edge was characterized by interbedded carbonaceous shale, coal, and limestone with facies suggesting a lagoonal margin periodically dominated by cyclothemic marine flooding events. Trace elements suggest a strongly restricted basin within an overall marine transgression trend. The basin morphology is interpreted as a sedimentary wedge, rapidly thinning to the east towards the basin center. During the subsequent Desmoinesian Stage, this region was characterized by interbedded carbonaceous shale and limestone, dominantly controlled by large-scale glacio-eustastic cyclothems in an open marine setting. Depositional environments range from intertidal platform, tidal flats, and shoals to deep, subtidal platform. In contrast to the Atokan Stage, trace elements suggest a weakly restrictive basin. Carbonate buildups, shoals, and paleosols are possibly coincident with an activated flexural forebulge and sediment baffle within the basin but peripheral the basin center and its condensed stratigraphic section. Our data and analysis support a model of dramatic glacio-eustatic transgression-regression cycles within an overall marine transgression from Atokan through Desmoinesian time. Our observations have implications for purported superestuarine circulation, the degree of Midcontinent basin restriction, and patterns of condensed vs thickened stratigraphic sections, all of which are important to historic and emerging petroleum systems of the region.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015