Detecting detrital carbonate in shale successions - Relevance for evaluation of depositional setting and sequence stratigraphic interpretation

Abstract

The Portwood Member of the New Albany Formation is a dolomitic black shale succession that was deposited in the western portion of the Appalachian Basin. Correlative of the Tully Limestone interval found to the northeast in the Appalachian Basin and slightly more than 5 m thick, it nonetheless consists of three erosion-bounded stratal packages that record Late Givetian eustatic sea-level variations. Detailed petrographic study shows that its carbonate interbeds, though readily interpreted as transgressive in nature, are largely detrital, and represent lowstand deposits eroded from the Cincinnati Arch in the west. Petrographical inspection showed abundant silt-sized detrital carbonate within the black shales, as well as a range of other key features, such as mud-dominated composite particles and agglutinated benthic foraminifera, that shed a new and much differentiated light on depositional setting and history of sediment accumulation. Whereas earlier recognized internal erosion surfaces still anchor the sequence stratigraphic framework, reinterpreting carbonate beds as lowstand deposits, and identifying condensed intervals and maximum flooding surfaces via distinct glauconite maxima, supports a reassessment of system tracts that is in agreement with petrographic constraints. Thin section based microfacies analysis identified multiple sedimentary settings, ranging from comparatively shallow (10's of meters, suboxic to dysoxic, tempestites) to very shallow (meters, dysoxic to oxic, wave and current action), and enabled subdivision of the Portwood into a series of upwards shallowing parasequences. This study demonstrates that petrographic due diligence is essential to correctly identify the origin of shale hosted carbonate intervals, and as an added benefit yields a sophisticated understanding of depositional conditions and long-term depositional trends that rivals what is possible for sandstone or carbonate successions.