TECTONOSTRATIGRAPHY AND RADIOGENIC ISOTOPE CHEMOSTRATIGRAPHY OF THE CA. 1 GA BYLOT SUPERGROUP

The ~6 km-thick Bylot Supergroup in the Borden Basin on Baffin Island is the most thoroughly studied stratigraphic succession from a series of previously interconnected late Mesoproterozoic intracratonic basins known as the Bylot Basins. We present new black shale Os, mudrock Nd, and carbonate Sr isotope chemostratigraphic data spanning the Bylot Supergroup. Arctic Bay and lower Victor Bay formations black shale units exhibit highly radiogenic initial ¹⁸⁷Os/¹⁸⁸Os (Os_i; up to 1.5) and low εNd(t) (to -17) compositions, consistent with a principal flux of evolved, continental weathering products to the basin. However, both Os_i and εNd(t) values trend toward more intermediate to juvenile compositions (Os_i ~ 0.7 and εNd(t) ~ 0) just below the carbonate-dominated Angmaat and upper Victor Bay formations, which are tidally-influenced and characterized by ⁸⁷Sr/⁸⁶Sr between 0.7055–0.7065. Together, these data illustrate that the Borden Basin was periodically restricted and that the degree of restriction influenced sedimentation patterns, perhaps due to changes in the geochemical stratification of its basin waters. Organic-rich (up to 20% TOC) Arctic Bay and lower Victor Bay black shales were deposited when the Borden Basin experienced minimal communication with the global ocean and was fed by runoff from the surrounding highly-evolved Archean to Paleoproterozoic granulites of the Rae Province. Conversely, Angmaat and upper Victor Bay formations carbonates were deposited while the Borden Basin was connected to the open ocean in all but shallow tidal environments. Thus, new ⁸⁷Sr/⁸⁶Sr data from this study, calibrated by recent Re-Os geochronology, contribute to the global secular Sr isotopic curve and record the effects of Rodinia’s amalgamation and the Grenville Orogeny on global silicate weathering. Finally, discerning between periods of water mass restriction and open marine conditions is essential to establishing the ecological backdrop to the diverse microfossil assemblages from the Bylot Supergroup.