Anoxia controlled by relative sea-level changes: An example from the Mississippian Barnett Shale Formation

Abstract

A continuous core drilled in the Barnett Shale Formation (Fort Worth Basin, Texas, United States), a remarkable example of Paleozoic organic-rich sediments deposited in a narrow inland seaway, was analysed by means of sedimentological, geochemical and biostratigraphic data. As the influence of sea-level fluctuations on benthic anoxia in ancient seas is still poorly understood, our work aims to shed light on the control exerted by relative sea-level variations on the paleoceanographic processes that drove fine-grained sediment deposition, organic matter dispersal and preservation during the sedimentation of the Barnett Shale Formation. Using spectral gamma-ray log a large second-order sequence modulated by a total of thirteen third-order sequences was inferred. Third-order relative sea-level cycles modulated the level of restriction of the basin through time, alternating pulses of bottom-water oxygenation and relatively more efficient watermass circulation with times of sluggish bottom-water and stagnation, which controlled the facies stacking pattern and organic matter preservation. In particular, relatively increased stagnation occurred during lowstand phases, while relatively reduced restriction was principally associated with transgressive and highstand systems tracts with more efficient bottom current circulation. On a larger scale, second-order eustatic cycles played a crucial role in determining the paleoceanographic and depositional processes responsible for anoxia. During the early phase of sea-level rise, the pre-existing topography was flooded establishing the conditions for a shallow strongly restricted anoxic basin. During the second-order regression the basin experienced increased isolation with bottom-water stagnation and deep-water renewal of ~6000years. Anoxia terminated abruptly, corresponding to the eustatic sea-level rise associated with the beginning of a new second-order sequence. Therefore, the Barnett Shale Formation represents a significant example of the complex interplay between sea-level-controlled basin physiography and local paleoceanographic conditions.