High-resolution geochemistry and sequence stratigraphy of the Hushpuckney Shale (Swope Formation, eastern Kansas): implications for climato-environmental dynamics of the Late Pennsylvanian Midcontinent Seaway

Abstract

The Hushpuckney Shale Member of the Swope Formation (Missourian Stage, eastern Kansas) is the core shale of a Kansas-type cyclothem, formed during the late transgressive to early regressive phases of a Late Pennsylvanian glacio-eustatic cycle. Coeval high-frequency climato-environmental dynamics of the Midcontinent Seaway are preserved in the KGS Orville Edmonds No. 1A study core as centimeter-scale variation in major components (organic carbon, authigenic sulfides and phosphate, detrital siliciclastics), organic macerals, trace-element redox proxies (Mo, U, V, Zn), and ichnofabric features. Benthic O 2 levels declined sharply from the base of the black shale (0 cm), went sulfidic at ∼4 cm, and reached a redox minimum at ∼21 cm; above this level, redox potential gradually rose, fluctuating between sulfidic and nonsulfidic conditions from ∼35 cm to the black shale/gray shale contact at 52 cm. Onset of dysoxic conditions at that contact allowed establishment of a benthic community of soft-bodied organisms comprised of deep-tiered tracemakers tolerant of low-O 2 conditions ( Trichichnus), and shallow-tiered tracemakers favoring “soupground” ( Helminthopsis) or “firmground” substrates (the Zoophycos– Phycosiphon– Schaubcylindrichnus– Planolites association). Most geochemical records exhibit a “low-order” cycle spanning the full 52 cm thickness of the black shale submember, reflecting a dominant glacio-eustatic control. The base and top of the black shale submember record lateral migration of the pycnocline across the Midcontinent Shelf during the transgressive and regressive phases, respectively. The maximum flooding surface (MFS) is at ∼17–23 cm, an interval containing the euxinic peak and characterized by high concentrations of illite (representing a cratonic siliciclastic flux) and terrestrial organic macerals, the product of transient increases in humidity, weathering rates, and the export of coal-swamp vegetation associated with the interglacial highstand of the Swope cyclothem. Although a transgressive “surface of maximum starvation” (SMS) cannot be confidently identified, a phosphate-rich “regressive condensation surface” at ∼28–34 cm records pycnoclinal weakening due to increased aridity associated with renewed southern hemisphere icesheet growth; a correlative shift in dominance from terrestrial- to marine-derived organic macerals reflects increased upwelling of nutrient-rich deepwaters and enhanced primary productivity. Penecontemporaneous paleogeographic and -climatic factors (e.g., semirestricted circulation, monsoonal precipitation) predisposed the Midcontinent Seaway toward sensitivity to high-frequency climato-environmental fluctuations, which are preserved as 2- to 7-cm-thick cycles in the study core. The 52-cm-thick black shale submember contains ∼12 such “high-order” cycles, which have an estimated average duration of ∼2 to 9 kyr, implying a sub-Milankovitch-band (i.e., millennial scale) climatic control. The results of this study are relevant to the sequence stratigraphic interpretation of Kansas-type cyclothems, the dynamics of Gondwanan Ice Age glacio-eustatic cycles, and the development of boundary conditions for Late Pennsylvanian paleoclimate models.