AN INVESTIGATION OF PARASEQUENCES IN THE CAMP RUN MEMBER OF THE UPPER DEVONIAN NEW ALBANY SHALE

The shales representing the Camp Run Member of the Upper Devonian New Albany Shale reflect the interplay between sea level variations, terrigenous supply, and surface productivity of the Devonian inland sea. Sedimentary features of these shales reflect their depositional history as well as the chemistry of surface sediments and bottom waters. Ideally, parasequences are characterized by black to gray cycles of decimeter thickness, but may also consist entirely of black shale. Successive parasequences show sharp basal contacts with some erosion of previously deposited sediment.

X-ray diffraction data show that the organic matter (OM) rich black beds have higher concentration of FeS2 (pyrite and marcasite), and that the OM poor gray beds contain higher concentrations of FeCO3 (siderite). The contrasting diagenetic mineralogy between black (pyrite) and gray portions (siderite) of parasequence cycles reflects the co-flocculation of abundant labile organic matter with clays (favoring sulfate reducers and pyrite formation) in black portions, and early consumption of labile organic matter in gray beds (leaving recalcitrant OM and favoring fermenting bacteria).

Whereas in a “classical” parasequence the quartz/clay ratio would be expected to increase upwards, the highest quartz/clay ratios occur in the basal portion of parasequences and then ratios drop to a lower level and stay uniform for the rest of the parasequence. The basal increase in quartz silt probably reflects clay removal by winnowing, and the uniform quartz/clay ratio for the remainder of the parasequence is most likely due to the very distal nature of these deposits.

Better OM preservation in the base of parasequences may reflect the combined effects of deeper water (less oxygen) and formation of organo-clay aggregates that enhance the preservation of OM due to intimate spatial association of labile organic matter and clays. This latter hypothesis is currently being tested in flume experiments with clays and marine snow proxies. In the upper portion of parasequences shallower water (more oxygen) and bioturbation enhance the potential for microbial degradation of labile OM.

Ongoing research is focused at further detailing these parasequences and to develop an integrated depositional model that combines sedimentology, geochemistry, and mineralogy.