Neoproterozoic strata of the southern Canadian Cordillera and the isotopic evolution of seawater sulfate

Abstract

Neoproterozoic strata of the southern Canadian Cordillera comprise metasedimentary rocks that are impressive in their thickness (6–9 km), areal extent (35,000 km 2 minimum) and predominance of sedimentary rocks of deep-water affinity. The stratigraphy can be subdivided into two broad successions that record the response to rifting and subsequent formation of a passive margin during the breakup of western Laurentia. Syn-rift strata include glaciogenic rocks that likely correlate with earlier Neoproterozoic glaciations elsewhere (e.g. Sturtian). Dating of associated granitic and volcanic rocks in Canada suggests that these units were deposited between 762 Ma and 728 Ma. Post-rift strata are interpreted as the deposits of an elongate terrigenous-sourced turbidite system that formed at the base of the continental slope and shoaled upward through slope into platformal facies. Evidence of a second Neoproterozoic glaciation (Varanger equivalent) is recognized in the post-rift turbidite succession as a regionally mappable condensed interval formed during post-glacial sea-level rise. Deep-water carbonate deposits punctuate the record of dominantly siliciclastic slope sedimentation and hold promise for future C Sr isotope chemostratigraphy. Coeval shallow-water facies are rarely preserved in the southern Canadian Cordillera owing to erosional removal during uplift associated with a younger rift event that preceded Cambro-Ordovician passive margin formation. The Kaza Group (sand-rich basinal turbidites) and the overlying Isaac Formation (mud-rich slope facies) are post-rift strata in the Cariboo Mountains that contain abundant pyrite as disseminated, large (up to 5 cm) crystals and finely crystalline stratabound layers. Isotopic analysis of pyrite separates from these two units demonstrates a broad range in δ 34S values (49 and 53‰, respectively), typical of sulfides formed by the bacterial reduction of seawater sulfate. There is a distinct shift of ca. +8‰ in median δ 34S values and +15‰ in the maximum-minimum values from pyrites in the Kaza Group into the overlying Isaac Formation. The magnitude of the shift, in addition to its stratigraphic position above Sturtian age glacial deposits, suggests that this excursion in the sulfide record overlaps with the large sulfate sulfur excursion observed in late Neoproterozoic to early Cambrian evaporites. We suggest that the Kaza Group and Isaac Formation pyrites are the deep-water, 32S-enriched, complement to the sulfate record. Burial of 32S-enriched pyrite in continental margin strata, such as exemplified by the Windermere Supergroup, likely occurred on a global scale in response to widespread passive margin formation during the breakup of western Laurentia with resultant enrichment of residual seawater sulfate in 34S. Thus open marine sulfate reduction and pyrite burial is interpreted as the driving mechanism by which Neoproterozoic sulfate became enriched in 34S and requires microbial reduction of a substantial volume of the global sulfate reservoir (ca. 25%). The coupled process of sulfate reduction and organic matter oxidation implies that the process of sulfate reduction on this scale would have had a substantial impact on the evolution of the oxygen reservoir in the Neoproterozoic.