Microstratigraphic resolution of Devonian black shales: A combined paleoecological, ichnological, and geochemical approach

Fine grained mudstones interpreted to record conditions of reduced bottom water oxygen are common in the geologic record and preserve a range of sedimentological and biological features associated with varying oxygen levels. Fifteen localities from central and western New York, exposing Middle and Upper Devonian aged mudstones were sampled to investigate the scale of resolution in relative bottom water oxygen levels and the corresponding biological signal preserved as both body and trace fossils. Because variation in burrow size and relative amount of bioturbation are physiologically controlled and directly correlative with bottom water oxygen levels, relative oxygen levels can be inferred from the ichnological signal in the rock record. Within these units, variability in bottom water oxygen levels is recognized on a sub-cm scale. The highest resolution in oxygen variability is recognized using a combined trace and body fossil approach. The trace fossil signal in these units is significantly different from post-Paleozoic strata and therefore requires a refined approach to recognize fluctuating bottom water oxygen conditions as outlined in this study. Notably, body fossils are common in dysaerobic strata in these units and are used to provide increased resolution through this zone.; Using a combined trace and body fossil approach, relative oxygen levels are defined on a bed by bed scale across the paleobasin. Variation in size of individuals, distribution of life habits, and taxonomic distribution in predictable order are recognized in association with what are interpreted to be variable oxygen levels defined by the ichnological signal. Preliminary data from redox proxies including DOP, TOC, and trace metals including Mo and Mn, supports that the biological signal more accurately reflects small scale fluctuations in relative oxygen levels.