CONTROL OF RELATIVE SEA LEVEL AND CLIMATE ON COAL CHARACTER IN THE WESTPHALIAN C (ATOKAN) FOUR CORNERS FORMATION, CENTRAL APPALACHIAN BASIN, USA

摘要

Peat successions preserved as coal seams preserve high-resolution records of ancient terrestrial water table (base level) fluctuations that are driven by changes in relative sea level and/or paleoclimatic change. The aim of this study is to establish the roles of relative sea level and 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. Comparison of the characteristics of 10 coals in the Four Corners Formation with accommodation trends identified from analysis of intra-coal elastic strata indicates that their thickness and vitrinite/inertinite ratio correlate readily with the accommodation setting of the depositional sequence in which they occur. Coals that accumulated in relatively high-accommodation fourth-order sequences are high in vitrinite, thin, and overlain by, or intercalated with, marine and lacustrine sediments. The coals have simple internal organization, suggesting that they span (parts of) single high-resolution accommodation cycles. Coals that accumulated in lower-accommodation fourth-order sequences are high in inertinite, thick, and overlain by or intercalated with terrestrial sediments. The coals have composite internal organizations typified by multiple high-resolution accommodation cycles, and characteristic abrupt discontinuities of coal facies, representing periods of depositional hiatus. 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. High-frequency accommodation cycles within the coals reflect water table fluctuations in the original mires and were driven by high-frequency sea level and/or climate change. 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.