Teasing Out Parasequences in Highly Sediment Starved Devonian Black Shales

Abstract

Careful petrographic examination of centimeter scale stratigraphic stacking patterns in conjunction with elemental analysis of total organic carbon (TOC) create an interesting story for better understanding parasequences and sequence boundaries in distal Devonian black shales. Detailed outcrop, drill core, and thin section petrographic observations were conducted on the Upper Gassaway Member of the Chattanooga Shale (Tennessee), the Bakken Shale (North Dakota), and the Clegg Creek Member of the New Albany Shale (Kentucky) to find commonalities in sedimentary rhythms and their significance for sequence stratigraphic analysis. In thin sections, starvation surfaces are marked by winnowed and reworked silt with calcite cement, and alternate with cm-thick intervals of banded shale. The latter show disrupted silt laminae, benthic fecal pellets, and remains of agglutinated benthic foraminifera and probably accumulated under suboxic conditions. Uncompressed and internally mineralized (pyrite, chalcedony) algal cysts attest to very small rates of net sediment accumulation. The presumed parasequences identified in outcrop and core vary in thickness depending on distance from the Acadian sediment source in the east. Following the cycles westward from the Appalachian into the Illinois Basin, they thin progressively. Whereas in eastern Kentucky cycles in the study interval are on the order of ~20 cm thick, they thin to an average of ~10 cm thick near the crest of the Cincinnati Arch. Further west, near Louisville, Kentucky, the cycles have thinned to less than 5 cm thickness. Powdered samples of the Clegg Creek Member of the New Albany Shale and the Bakken Shale were collected at the centimeter interval for geochemical analysis of TOC. TOC analysis shows a complementary pattern of increasing and decreasing TOC with cycles about 3 – 4 cm thick. The starved-reworked portion of the cycle has the lowest TOC values, and the highest TOC values are found in the intervening banded shale. Comparing these combined petrographic and geochemical analyses between the Bakken, New Albany, and Chattanooga Shale will show whether comparable patterns exist and how depositional conditions may have differed (or not) over large distances (up to 1000 miles) in the Late Devonian inland sea.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015