Abstract

A hierarchy of sedimentary cycles has been recognized in Middle and Upper Pennsylvanian strata in the northern Mid-Continent, U.S.A., and a genetic and sequence stratigraphic classification has been used to characterize them. The basic building block in this succession is the cyclothem-scale depositional sequence, which is bounded by subaerial unconformities on the shelf. Each of these sequences includes several lithogenetic units: a basal flooding unit, a condensed section, a late highstand carbonate or siliciclastic unit, and a paleosol. The thin condensed sections are generally radioactive, conodont, and ammonoid-rich marine black shales that can be readily correlated over large distances in the surface and subsurface using gamma ray logs and conodont and ammonoid biostratigraphy. The subaerial exposure surfaces and paleosols that mark sequence boundaries are difficult to recognize with well log data alone, but the radioactive condensed sections, which in many cases closely overlie the sequence boundaries, can be used to correlate the sequences. Because of these limitations, genetic stratigraphic units, bounded by these correlatable condensed sections, have been used in this study as proxies for depositional sequences. These genetic stratigraphic units are practical for regional subsurface mapping, and can be linked to their corresponding depositional sequences in areas where cores are available. The boundaries of a genetic stratigraphic unit and the associated depositional sequence virtually coincide on the shelf, but diverge in shelf margin and basinal settings, due to significant sedimentation during the early stages of relative sea-level rise.

Sets of these genetic stratigraphic units that exhibit uniform stacking patterns (termed genetic sets) are developed along the shelf margin of the Anadarko and Arkoma basins. Each genetic set is approximately 330 ft (100 m) thick and contains from five to seven primary genetic stratigraphic units. Internally, each set is characterized at the base by an abrupt backstepping of up to 50 mi (80 km) relative to the top of the underlying set, followed by more gradual "lateral accretion," as progressively younger units step basinward. The backstepping marking the bases of individual genetic sets is apparently associated with major readjustments of the shelf, which clearly exerted a significant control on the development of sequences and genetic units. Recognition and characterization of these sets of genetic stratigraphic units in the interior regions of the craton present challenges due to lack of distinctive stacking patterns. Depth profiles of the thorium-to-uranium (Th:U) ratios derived from spectral gamma logs are used in these interior regions to extend this longer term cyclicity originally established along the shelf margins.

A two-dimensional (2-D) computer model was used as a means to investigate the authors'understanding of the processes (including eustasy, subsidence, sedimentation rates, and shelf configuration) that may have controlled the observed stratal hierarchy. Similarities in the patterns of observed and modeled third- and fourth-order cyclicity suggest a high degree of deterministic cause and effect between these processes and the observed stratigraphy, suggesting that stratal patterns in areas away from known control may be predictable via modeling. Increasingly advanced simulation models may be useful in synthesizing observations and interpretations with a goal of identifying areas prone to subtle stratigraphic traps.