THE ROLE OF CLAY MINERAL SURFACE AREA IN ORGANIC CARBON ACCUMULATION IN BLACK SHALES

Recent work on Cretaceous sediments of the Western Interior Seaway (U.S.A.) has suggested that internal surface area in smectite and mixed-layer clays plays an important role in the accumulation, burial, and preservation of organic matter (OM) in marine shales. To assess the importance of this control in organic-rich marine black shales of other ages and settings, total organic carbon (TOC), clay mineral composition, and clay mineral surface area was determined for a suite of Devonian-Mississippian black shales from the central Appalachian Basin (east-central Kentucky). External surface area was determined using the Brunauer-Emmett-Teller (BET) method, whereas the ethylene glycol monoethyl ether (EGME) method was used to determine internal surface area of clay minerals in each sample. Major and trace-element data, along with organic petrographic and Rock Eval data, were available for comparison from a previous study.

TOC contents for the Devonian-Mississippian shales range from less than 1% to more than 21%. X-ray diffraction analysis indicates a predominance of illite and mixed-layer illite-smectite (I-S) clays. Shales containing a higher percentage of mixed-layer I-S clays have slightly higher internal surface areas and TOC contents, but relationships are much weaker than those shown previously for Cretaceous samples. In agreement with previous work, external surface area does not show a significant correlation with organic carbon content. Surface-area measurements for a subset of samples subjected to low-temperature ashing (to remove the organics) suggest no significant OM contribution to internal surface area. These preliminary data suggest that mineral surface area may have been a contributing factor in the preservation of OM in the Late Devonian and Early Mississippian, but it was probably just one factor among many (including preservation of OM under anoxic conditions and productivity).

These results will be compared to those for Pennsylvanian gray and black shales from the Illinois Basin (western Kentucky) and Cretaceous shales from the Western Interior Seaway to evaluate the relative importance of clay mineral type and internal surface area in the accumulation of OM in marine environments.