More gaps than shale: stratigraphic incompleteness of marine shale successions using a Toarcian example

Abstract:

Marine shale successions are probably the best archives of earth history. The degree of completeness of a marine shale succession is a critical factor in the interpretation of the geologic record of climatic, oceanic, and biogeochemical processes, in the prediction of timescales of those processes, in the determination of the duration of events, and in the establishment of correlations between successions. The sedimentation rates of marine shale successions are often calculated by dividing the thickness of a succession by the duration of the stratigraphic interval it occupies. Sedimentation rates calculated this way are always much lower than rates measured directly in equivalent modern environments. When we apply modern rates to the deposits left behind by their ancient equivalents, and correct for compaction due to overburden and time, we find that the entire succession can be deposited in a relatively short time. Since we know that the stratigraphic interval occupied by such ancient deposits is much longer, we must conclude that the succession is very incomplete. In this presentation, I will use a few different methods to show that 65 to >80% of the duration of the Toarcian oceanic anoxic event in Yorkshire, U.K., is represented by gaps rather than shale. This means that the Toarcian oceanic anoxic event is not as short as proposed by authors who studied the cyclostratigraphy of the Yorkshire succession, and that it probably represents a much longer-term history of environmental change driven by processes acting on longer time scales.