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Trace Metal Variability in the Lower Bakken Formation — Implications for Late Devonian Global Ocean Redox

Abstract

Whereas the Devonian Period is thought to mark a major step increase in global ocean-atmosphere oxygenation—likely linked to the diversification of vascular plants and large predatory fish—many intracratonic basins in North America record extensive anoxia and deposition of organic-rich units. The Late Devonian-Early Mississippian Bakken Formation in the Williston Basin hosts two organic-rich black shale units, which are prolific source rocks for conventional and unconventional hydrocarbon exploration. Although previous studies have addressed stratigraphic and physical characteristics of the Bakken Shale, the details of basin redox evolution that controlled source rock development have not been described.

To better understand both the local and global redox conditions that led to black shale formation, here we use the differential behavior of molybdenum (Mo), vanadium (V), uranium (U), and chromium (Cr) in organic-rich units from the Lower Bakken Formation. Data from multiple cores across the basin show that Mo, U, and Cr enrichments typical of modern euxinic (anoxic+H2S) environments are observed throughout the Lower Bakken. This suggests that: 1) locally euxinic conditions prevailed throughout deposition of the LBF, 2) the oceanic reservoir of these metals was large enough to allow for modern-style enrichments, and 3) the Williston Basin was sufficiently connected to the open ocean to allow renewal of these metals despite local euxinic drawdown. In contrast, however, V is not substantially enriched above crustal levels until the latter part of the Lower Bakken. This lack of enrichment in a largely open-marine basin, suggests a global reservoir drawdown effect because V is expected to be enriched in sediments under euxinic conditions. Because suboxic (low H2S) environments are a net source for V and a sink for Mo, mass-balance modelling suggests that the Late Devonian oceans were perhaps extensively suboxic with a comparatively limited spatial extent of euxinia. These results have important implications for understanding the global ocean redox landscape across the critical Devonian-Mississippian transition.