OXYGEN LEVELS AND SUB-MILANKOVITCH SEDIMENTARY CYCLES IN THE HUSHPUCKNEY SHALE (SWOPE FORMATION, KASIMOVIAN, PENNSYLVANIAN)

Black shales, although primarily deposited in deep marine environments, may record changes in climate, sea-level, and fluvial systems on the craton in the form of wind-blown dust, terrestrial organic matter, and sedimentary fabrics. Pennsylvanian black shales from the North American mid-continent accumulated in paleotropical, epicontinental seas. These shales occur in 4th or 5th order transgressive-regressive cycles (also known as cyclothems). Pennsylvanian cyclothems probably reflect cyclic advance and retreat of Southern, and perhaps Northern Hemisphere glaciers driven by Milankovitch orbital cycles. In addition, sub-Milankovitch cyclicity has been hypothesized for some Kasimovian (Late Pennsylvanian) cyclothems.

The Hushpuckney Shale member of the Swope Formation (Kasimovian) is an organic-rich shale consisting of a black shale submember overlain by a gray shale submember. This color change was interpreted as a change from anoxic (black shale) to dysoxic (grey shale) conditions driven by falling sea level. XRF scans of the black shale submember in a Hushpuckney shale core reveal minute laminations, both continuous and disrupted, within this subunit. Disrupted laminae may indicate bioturbation, suggesting that the sediment-water interface was often dysoxic rather than anoxic during deposition of the black shale submember. Likewise, back-filled burrows in the gray shale subunit, suggest that the oxygen availability at the sediment-water interface might have been significantly higher than previous estimates. The high organic content of the Hushpuckney Shale may be due to high rates of organic matter accumulation rather than low amounts of oxygen.

Both the black and the gray shale submembers exhibit higher order stratigraphic cycles, which were possibly controlled by sub-Milankovitch climatic fluctuations. Within the black shale submember, concentrations of silt streaks define approximately 12 sedimentary cycles, similar to the number observed in previous studies. The gray shale submember contains an additional 3 to 5 sedimentary cycles, defined by matrix color and intervals of bioturbation.