CLIMATO-ENVIRONMENTAL DYNAMICS OF THE LATE PENNSYLVANIAN MIDCONTINENT SEAWAY: IMPLICATIONS FROM HIGH-RESOLUTION GEOCHEMICAL AND SEQUENCE STRATIGRAPHIC ANALYSIS OF CORE SHALES

During glacio-eustatic highstands of the Late Pennsylvanian Midcontinent Seaway, widespread anoxia resulted in deposition of laminated organic-rich “core shales,” representing an archive of high-frequency climato-environmental variation. We carried out cm-scale analysis of the Hushpuckney Shale Member of the Swope Formation (Missourian Stage, eastern Kansas), documenting relationships among major components (organic C, authigenic sulfides and phosphate, detrital siliciclastics), and trace-element redox proxies (Mo, U, V, Zn). The 52-cm-thick black shale facies exhibits a distinct first-order cycle in TOC and TE concentrations and SiO₂/Al₂O₃ ratios, with peak values at 17-25 cm. Redox proxies indicate that benthic O₂ levels declined sharply above the base of the black shale (0 cm), euxinia existed from 4 to 34 cm, and fluctuating non-sulfidic anoxic to euxinic conditions existed from 34 cm to the top of the black shale (52 cm). The proportion of macerals of terrestrial origin (i.e., vitrinite and inertinite) was uniformly high (>90%) from 0 to 28 cm, declined precipitously from 28 to 40 cm, and was effectively 0% from 40 to 52 cm. These observations have implications for the sequence stratigraphic interpretation of core shales. Strong covariation among TOC, TE redox proxies, and SiO₂/Al₂O₃ was due to a common forcing mechanism, probably water depth. We interpret the base and top of the black shale facies as marking transgression and regression of the pycnocline across the study area, and the first-order peak in TOC and other variables at 17-25 cm as representing the maximum flooding surface (MFS). If the study unit contains a separate surface of maximum sediment starvation (SMS; representing the maximum rate of transgression), it cannot be distinguished from the MFS. The abrupt shift in dominant maceral types above 28 cm is consistent with this model and suggests major climatic change associated with the early stages of sea-level regression, from warm and humid conditions favoring development of paralic coal swamps (as during the preceding transgression) to cooler and possibly dryer conditions. These climate changes reduced the export of terrestrial organic matter to the Midcontinent Seaway and, at the same time, may have reduced freshwater runoff around its margins, weakening the marine pycnocline and enhancing upwelling-driven productivity.