FINE-SCALE TEMPORAL VARIATIONS IN MUDROCKS: RE-OS AND CORROBORATING GEOCHEMICAL TOOLS (Invited Presentation)

Mudrocks record minute variations in detrital input, biota and chemistry of the water column, and environmental conditions at the site of deposition, with a critical caveat – in fine-grained rocks it can be difficult to detect gaps in sediment accumulation and to determine the duration of those gaps. Re-Os geochemistry, in conjunction with other geochemical tools, serves as both an environmental proxy and geochronometer.

While precision of individual Re-Os ages for organic-rich sedimentary rocks (ORS) is, at best, 0.5% of the age, Bayesian refinement of multiple ages in a stratigraphic sequence greatly improves the resolution. Typically, the nominal ages of shales are in the correct stratigraphic order, but with overlapping uncertainties. The known sequence permits reduction of the uncertainties using Bayesian statistics. Examples will be shown in which key stratigraphic ages are defined in this way, and another in which an offset in sequential ages is demonstrably caused by faulting.

Initial ¹⁸⁷Os/¹⁸⁸Os ratios (Os_i) in ORS reflect the Os isotopic composition of seawater, which, in turn, is controlled by the relative contributions of primitive (mantle or cosmogenic) and evolved (continental) Os sources. Variations in Os_i reflect marked changes in rates of either mid-ocean ridge volcanism, oxidative weathering of continental crust, or both – or rarely, a sharp decline caused by a bolide impact. Examples will be shown in which temporally controlled Os_i document such tectonic or climatic swings. These interpretations are bolstered by other geochemical data, such as carbon isotope stratigraphy and variations in other redox-sensitive elements.

Finally Re-Os geochronology can ascertain and/or verify regional correlations where biostratigraphic controls are lacking or imprecise. For example, our group has acquired Re-Os and accompanying trace metal and C isotopic data for sections of Late Jurassic to Early Cretaceous ORS from the Barents to the North Sea, providing correlations and identifying missing sections through this interval of key hydrocarbon source rocks. Stratigraphic trends in Os_i, other isotopic ratios, and trace metal concentrations corroborate paleoenvironmental changes on a regional scale, and proposed correlations across the Arctic continental margin.