Stratal hierarchy and sequence stratigraphy—Middle Pennsylvanian, southwestern Kansas, U.S.A.
Published:January 01, 1994
John C. Youle, W. Lynn Watney, Lance L. Lambert, 1994. "Stratal hierarchy and sequence stratigraphy—Middle Pennsylvanian, southwestern Kansas, U.S.A.", Pangea: Paleoclimate, Tectonics, and Sedimentation During Accretion, Zenith, and Breakup of a Supercontinent, George O. Klein
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Sequence-stratigraphic methods are used to interpret the stratal architecture and depositional history of Middle Pennsylvanian cyclothems that onlap the Central Kansas uplift from the Hugoton embayment of the Anadarko basin. Thirteen fourth-order (0.1 to 1.0 m.y.) depositional sequences are recognized between the “V” shale of the Cherokee Group (Desmoinesian) and the base of the “Gray group” (upper Atokan). Depositional sequence boundaries, recognized in core as subaerial exposure surfaces developed on subtidal rocks, were correlated by wireline logs from the lower-shelf to updip depositional limits. They were used to interpret the complex history of sea-level change in the midcontinent.
In lower-shelf settings individual sequences are 3 to 9 m thick, consisting of a basal “hot” black shale (1.25 to 3 m thick) that grades upward into a dense limestone (1.75 to 4.5 m thick). The lower-shelf black shales thin and bifurcate shelfward as they grade into limestones and gray shales. Lower-shelf limestones thicken toward the mid-shelf and grade into bioclastic and phylloid-algal wackestones and pack-stones. Sequences reach maximum thickness in the mid-shelf (35 m), where they consist of multiple shoaling-upward parasequences. Mid-shelf sequences thin landward as successively older parasequences toplap beneath the overlying sequence boundary. These sequences are thus sigmoidal in shape, downlapping onto the lower-shelf and toplapping onto the proximal shelf. Mid-shelf limestones thin shelfward and grade into thin, karsted, subtidal and peritidal carbonates. Farther landward, limestones grade into thin siliciclastics, amalgamated soil profiles, or locally estuarine valley-fill siliclastics.
Changes in the geometries, lithologies, and stacking patterns of “Gray” and lower Cherokee Group sequences record the influence of a third-order relative sealevel cycle (1 to 10 m.y.). This larger-order cycle is treated as a composite sequence and is subdivided into three sequence sets. Lower “Gray group” sequences make up the transgressive sequence set. These sequences are dominated by lower-shelf lithofacies. Younger sequences overstep older ones as they onlap pre-“Gray group” rocks. Sequences in the transgressive sequence set double in thickness from lower-shelf to mid-shelf positions, although intrasequence stratal relationships are indistinct. Upper “Gray group” sequences of the early highstand sequence-set exhibit a wide range of lithofacies, display well-defined internal stratal packages, and thicken 500% from the lower-shelf into the mid-shelf. The late-highstand sequence-set makes up the Lower Cherokee Group. These sequences lack lower-shelf lithofacies, and half of them onlap subjacent sequences.