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

Despite nearly a century of study by a diverse group of Precambrian workers , competing interpretations have persisted that favor both marine and nonma rine depositional settings for the Mesoproterozoic Belt Supergroup of the n orthwestern United States. Our data for sedimentary pyrite formation in unm etamorphosed organic-rich shales of the Newland Formation of the lower Belt Supergroup, western Montana, argue that at least a portion of the Belt bas in, during its early history, was characterized by restricted marine condit ions 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 reacti vity of the C reservoir in the absence of vascular land-plant remains, sugg est degrees of sulfate limitation consistent with a restricted marine setti ng. The isotopically heavy disseminated pyrite that dominates the black sha les shows a broad range of delta(34)S values of bacterial origin (delta(34) S ranges from -8.7 to +36.3 parts per thousand; mean = +7.64 parts per thou sand, n = 41). The pyrite data overlap with the delta(34)S range of coeval (barite) sulfate within the basin and are similar to independent estimates for the sulfate of Mesoproterozoic seawater. These relationships have allow ed us to delineate a record of bacteriogenic pyrite that formed in a modifi ed marine reservoir where the supply of sulfate was limited. A bacterial me chanism is further suggested by the morphological character of this early-f ormed pyrite, including the ubiquitous framboids. After comparing our resul ts with those from present-day localities, such as the Black Sea and sites of lacustrine deposition, we conclude that the abundant S-34-enriched bacte riogenic disseminated pyrite observed in black shales of the Newland Format ion must record a style of sulfate limitation possible only within an isola ted marine basin receiving an intermittent flux of seawater. Although the e ntire Belt basin likely remained a restricted marine setting throughout dep osition of the lower Belt Supergroup, sulfur isotope data from the Newland Formation at two localities separated by more than 150 km in the eastern Be lt basin reveal stratigraphic trends in the strength of the marine connecti on. Specifically, the connection evolved such that inputs of seawater sulfa te increased progressively in frequency and possibly strength during deposi tion of the Newland Formation. Overall, the hypothesized restricted marine setting, when viewed in light of independent tectonic, sedimentologic and g eochronologic evidence, suggests syndepositional and likely episodic contin ental rifting, which helps constrain the Proterozoic paleotectonic history of western Laurentia. Copyright (C) 2000 Elsevier Science Ltd.