ORIGIN OF SMALL-SCALE CYCLICITY IN UPPER PENNSYLVANIAN CYCLOTHEMIC CORE SHALES OF THE MIDCONTINENT REGION

Glacio-eustatic fluctuations produced 5-10-m-thick cyclothems in the Midcontinent region during the Pennsylvanian, with interglacial highstands represented by the ~40- to 60-cm-thick core black shale facies. This facies commonly records fine-scale cyclicity in the form of compositional variation at a 5- to 10-cm scale. In this study, we analyzed the geochemistry of the Hushpuckney core shale at ~1-cm intervals in sections from Oklahoma, Kansas, and Iowa in order to better understand the nature and origin of these fine-scale cycles. In each study section, we determined that the small-scale cycles recorded variation in TOC (~5 to 20%), total S, Al, and sometimes P. The cause of such compositional variation may have been fluctuations in upwelling intensity and marine productivity, pycnocline strength and organic matter preservation, clay fluxes, or some combination of these factors. The number of small-cycles varied between study sections, from a low of 5 to a high of 12, with geographically close sections exhibiting a similar number of cycles that appear to be correlatable. If the cycles represent a regular temporal forcing, then some sections accumulated black muds for longer intervals than others—probably due to irregular seafloor bathymetry that allowed anoxic deepwaters to flood some areas of the Midcontinent Shelf sooner than others during eustatic transgressions. If cyclothems average ~400 kyr in duration (Heckel, 1986), then the small-scale cycles may represent the 21-kyr orbital precession cycle and the black shale facies may have accumulated over intervals of 100-200 kyr. On the other hand, if substantial time is missing at the paleosol surfaces capping most cyclothems, then the black shale facies may have accumulated in <<100 kyr, and the small-scale cycles are likely to represent sub-Milankovitch (i.e., millennial) climate fluctuations. Understanding these small-scale cycles better may allow development of (1) a finer correlation framework, and (2) an internal geochronometer for Late Pennsylvanian cyclothems.