Relative Controls of Sea-Level and Climate on Coal Seam Composition and Thickness in the Westphalian C (Pennsylvanian/Upper Carboniferous) Four Corners Formation (Breathitt Group), Central Appalachian Basin, USA
Jerrett, Rhodri; Hodgson, David; Flint, Stephen S.; Davies, Roy
Peats preserved as coal seams retain records of ancient terrestrial water table (i.e. base level) fluctuations that are driven by changes in relative sea level and/or climatic change. These fluctuations are revealed by changes in the vertical petrological composition of the coal seams. The aim of this study is to establish the roles of relative sea level versus climate in controlling water table fluctuations within coal seams from the fluvio-deltaic Westphalian C (Bolsovian, Atokan) Four Corners Formation of the central Appalachian Basin, USA. This is achieved through comparison of (1) the thickness, (2) the bulk petrological composition and (3) within-seam vertical changes in the petrological composition of 10 coals in from the Four Corners Formation, with the record of accommodation change revealed by the intra-coal clastic strata. The results indicate that the coals' thickness and composition correlate readily with the accommodation setting of the fourth-order (c. 140-210 ky) depositional sequences in which they occur. Coals that accumulated in high-accommodation fourth-order sequences are thin, high in vitrinite, and overlain by, or intercalated with, marine and lacustrine sediments. The coals' microstratigraphy is simple, suggesting that the coals span (parts of) single high-frequency (fifth-order or greater) sequences. Coals that accumulated in low-accommodation fourth-order sequences are thick, high in inertinite, and overlain by or intercalated with terrestrial sediments. These coals have complex internal organisations typified by high-resolution compositional cyclicity and abrupt discontinuities of coal facies, representing periods of depositional hiatus. These coals likely span multiple high-frequency (fifth-order or greater) sequences. The correlations between the composition and thickness of the coals and the magnitude of associated marine transgression imply that relative sea level change was the principal control governing bulk accommodation in these mires landward of the paleoshoreline. It is not possible, however, to distinguish relative sea-level from climate as the control on water-table (base-level) change at higher orders of cyclicity. The results of this study provide considerable insight into the manner in which coal composition and stratigraphy at a single locality vary over time in response to changing accommodation, and provide a means to predict coal-seam quality in an established sequence stratigraphic framework.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013