Tectonic controls on distribution and stratigraphy of the Cryogenian Rapitan iron formation, northwestern Canada

- Rapitan iron formation accumulated in sediment-starved sub-basins.
- Sediment starvation owing to rotational block faulting at basin margin.
- Iron formation thickness linked to timing and amount of sub-basin subsidence.
- Rapitan Group accumulated ∼711 Ma, later than presumed correlatives.
- Inception of glaciation in northwestern Laurentia was diachronous.

Paleogeographic data indicate that the Neoproterozoic Rapitan Group accumulated in two spatially related, silled basins during the first mid-Neoproterozoic glacial episode (ca. 711 Ma), yet the controls on glaciation, reason for basin development, spatio-temporal relationships among stratigraphic units, and constraints on iron formation deposition and distribution in this region and globally at this time have not been adequately explained. The lack of a coherent understanding of the group's regional stratigraphy has been a significant barrier to deciphering the geologic history of this critically important succession. Based on new stratigraphic data, and incorporating the results of a related geochemical study, the Rapitan Group accumulated in tectonically complex sub-basins of a young rift zone. The spatial and temporal dynamics of these sub-basins controlled the grain size, volume, and distribution of detrital sediment (predominantly turbidites and diamictites) deposited during two glacially influenced episodes with ice cover. During an interglacial episode, the rotated, exposed upper surfaces of newly displaced rift blocks directed drainage away from basin margins, precluding significant clastic sediment delivery, and subaqueous margins of outboard tilted fault blocks acted as sills that isolated local bottom water, allowing the accumulation of an entirely hydrogenous succession (jaspilitic and hematitic iron formation). Two strike-parallel normal faults that strongly controlled basin-margin development are prominent thrust faults in the present-day orogen. Syndepositional activity of strike-normal transfer faults segmented the basins along their length, as reflected in pronounced along-strike variations in the presence/absence, thickness, and composition of the three stratigraphic units (Mt. Berg, Sayunei, and Shezal formations), and in the presence or absence of iron formation (locally >100 m thick; top of Sayunei Formation). Differential subsidence in actively rifting basins was essential for deposition of Rapitan-type iron formation, but glaciation, although coeval with Rapitan Group accumulation, was not a strong control. The timing of the onset of glacially influenced sediment deposition in the closely spatially related Rapitan (∼711 Ma) and Mount Harper groups (∼717 Ma) in northwestern Laurentia differs by ∼6 m.y., arguing for diachronous Sturtian glaciation, both locally and globally.