Sulfur geochemical constraints on mesoproterozoic restricted marine deposition: lower Belt Supergroup, northwestern United States

Abstract

Despite nearly a century of study by a diverse group of Precambrian workers, competing interpretations have persisted that favor both marine and nonmarine depositional settings for the Mesoproterozoic Belt Supergroup of the northwestern United States. Our data for sedimentary pyrite formation in unmetamorphosed organic-rich shales of the Newland Formation of the lower Belt Supergroup, western Montana, argue that at least a portion of the Belt basin, during its early history, was characterized by restricted marine conditions with only episodic incursions of seawater. This model is supported by ratios of organic carbon (C org) to pyrite sulfur that, once corrected for selective diagenetic and thermal loss of C org and the inferred high reactivity of the C org reservoir in the absence of vascular land-plant remains, suggest degrees of sulfate limitation consistent with a restricted marine setting. The isotopically heavy disseminated pyrite that dominates the black shales shows a broad range of δ 34S values of bacterial origin (δ 34S ranges from −8.7 to +36.3‰; mean = +7.6‰, n = 41). The pyrite data overlap with the δ 34S range of coeval (barite) sulfate within the basin and are similar to independent estimates for the sulfate of Mesoproterozoic seawater. These relationships have allowed us to delineate a record of bacteriogenic pyrite that formed in a modified marine reservoir where the supply of sulfate was limited. A bacterial mechanism is further suggested by the morphological character of this early-formed pyrite, including the ubiquitous framboids. After comparing our results with those from present-day localities, such as the Black Sea and sites of lacustrine deposition, we conclude that the abundant 34S-enriched bacteriogenic disseminated pyrite observed in black shales of the Newland Formation must record a style of sulfate limitation possible only within an isolated marine basin receiving an intermittent flux of seawater. Although the entire Belt basin likely remained a restricted marine setting throughout deposition of the lower Belt Supergroup, sulfur isotope data from the Newland Formation at two localities separated by more than 150 km in the eastern Belt basin reveal stratigraphic trends in the strength of the marine connection. Specifically, the connection evolved such that inputs of seawater sulfate increased progressively in frequency and possibly strength during deposition of the Newland Formation. Overall, the hypothesized restricted marine setting, when viewed in light of independent tectonic, sedimentologic and geochronologic evidence, suggests syndepositional and likely episodic continental rifting, which helps constrain the Proterozoic paleotectonic history of western Laurentia.