Assessment of bi-metal paleo-redox proxies in the Bakken Formation black shales, Williston Basin, North Dakota, USA

Abstract

The influence of paleoredox conditions and water-mass restriction on trace metal (TM) accumulation in the lower (LBS) and upper black shales (UBS) of the Devonian–Mississippian (D–M) Bakken Formation was assessed utilizing V/Cr, V/(V + Ni), Ni/Co, U/Th, and enrichment factors (EF) for Mo vs. U. These assessments were compared to previously published estimates based on degree of pyritization (DOPT), sulfur‑iron and C–S–Fe relationships, and sedimentological data. Bi–metal ratios suggest >80 % of samples were deposited under suboxic to anoxic/euxinic conditions, whereas DOPT results suggest >60 % of samples were deposited under dysoxic (0.42–0.75 DOPT) bottom–water conditions. DOPT results are in good agreement with C–S–Fe and total S vs. Fe assessments of paleoredox conditions and sedimentological evidence. Good agreement is also seen between Mo/TOC and Mo EF/U EF that both suggest the Bakken shales were deposited under a relatively unrestricted water mass conditions resulting in consistent renewal of TMs into the basin. With TM resupply outpacing drawdown, the basin could support enhanced primary productivity as well as high concentrations of TMs. Trace metal concentrations for the Bakken Fm. show considerable range for Co (0–10,324 ppm), Mo (0–2018 ppm), Ni (0–1574 ppm), U (0–1604 ppm), and V (0–3194 ppm), and bi–metal ratios for the Bakken Fm. are up to 5× greater than those reported for other D–M black shale formations. Bivariate and principal component analysis indicate Mo, Ni, U, and V accumulation was more closely associated with the organic fraction than with reducing conditions, thus accumulation of organic matter during deposition and generation, migration, and biodegradation of hydrocarbons during diagenesis may have influenced the accumulation and distribution of these TMs. Furthermore, changes in biogeochemical processes during the D–M transition may have fundamentally altered marine chemistry, with the evolution of vascular land plants and enhanced terrestrial weathering leading to an increased influx of nutrients into marine waters resulting in the Annulata, Dasberg, and Hangenberg black shale events. Factors controlling TM accumulation during time of deposition (e.g., TM availability, bottom-water redox conditions, adsorption onto organic matter) and during diagenesis and catagenesis (e.g., alteration and break down of organic matter, movement of fluid hydrocarbons or other basinal fluids) likely contribute to the lack of agreement between redox proxies, and subsequently, the lack of applicability of bi–metal ratios in assessing bottom–water conditions for the Bakken shales. Therefore, it is suggested that these bi-metal redox proxies are not an effective means to describe bottom-water conditions during the deposition of the Bakken shales.