PROCESSES AND TIMESCALES OF BLACK MUD ACCUMULATION IN SHALLOW EPEIRIC SEAS AND ON THE DEEP OCEAN FLOOR (Invited Presentation)

Laminated carbonaceous mudstones, or black shales, constitute a remarkable facies. Like other mudstones, black shales are commonly interpreted to be the product of slow but continuous settling from suspension of fine mud in quiet environments. The preservation of fragile, thin laminae and labile organic carbon in black shales has been used to suggest that the quiet environments in which the muds accumulated were anoxic. Geochemical paleoredox proxies, such as biomarkers and redox-sensitive elements and their isotopic ratios, also often support the idea that these facies were deposited under anoxic water columns. These proxies further suggest that large volumes of the ocean became anoxic during oceanic anoxic events, when black shales were widespread and even dominated some sedimentary environments.

Sorby, in 1908, was the first to note that laminae in mudstones may be due to currents. In the past decade, interest in the sedimentology of mudstones increased, and it became clear that not all laminated muds were deposited from suspension; many laminated muds reflect transport of sortable silt and of fine mud, aggregated in silt- and sand-sized grains, as low-relief bed waves under the influence of currents. Detailed examination of black shales under the microscope further revealed the presence of erosional structures and cross-lamination indicating environments that were not always quiet but rather often energetic, and also bioturbation that suggests the presence of benthic life and oxygen. This has led many to question whether most black mud accumulated under persistently anoxic water columns.

In this presentation, I will show examples of Jurassic and Cretaceous laminated carbonaceous mudstone facies from shallow-water epeiric seas and from the deep ocean floor. In these examples, black mud accumulated during short episodes of sedimentation and was eroded during short episodes of erosion, both separated by longer periods of nondeposition. I will argue that integration of sedimentological and geochemical observations with the consideration of the range of process-timescales these features represent in a single sedimentary product will solve much of the current divide between sedimentologists and geochemists on the redox environment during accumulation of black muds.