Mixed Carbonate-Clastic Sedimentation in the Basal Mackenzie Mountains Supergroup (Early Neoproterozoic), Northwestern Canada

The >4 km-thick early Neoproterozoic Mackenzie Mountains supergroup (MMsg; Northwest Territories and Yukon, northern Canada) consists of carbonate, terrigenous clastic and evaporite strata that were deposited in a poorly known, subtly extensional epicratonic setting. The lowest part of the group was until now completely unknown because it lies below the oldest exposed strata in the established outcrop area. Remote exposures of the hitherto unknown basal parts of the MMsg in the Wernecke Mtns.
(Yukon) contain three new formations that are assigned to the revised and formalised Hematite Creek Group (basal MMsg). The oldest formation in the succession, the 250 m-thick Dolores Creek Fm., is a mixed carbonate-clastic formation that consists of brilliant orange-weathering microbial and intraclastic dolostone intercalated with black mudrocks. The carbonate lithofacies indicate a depositional setting above storm wave-base and within the photic zone, but the black mudstones are variably pyritic and petroliferous, and exhibit geochemical patterns indicative of deposition below a redoxcline. This combination of attributes suggests that the earliest phase of the MMsg basin consisted of isolated rift basins characterised by shallow-water anoxia and limited clastic sediment supply. The MMsg is correlated with the Shaler Supergroup (SSG) on Victoria Island and in mainland inliers (Northwest Territories and Nunavut). The basal interval of the SSG (Escape Rapids Fm.), although not known in detail, has been described as consisting of grey mudstones and sparse sandstones (no carbonate rocks) that were deposited in an outer-shelf environment under normal-marine conditions (passsive-margin-like sedimentation). The striking contrast in the basal evolution of the two spatially and temporally related basins carries tectonic information about the paleogeography of northwestern Laurentia during the mid-life of Rodinia and requires further study. The economic potential of the basal MMsg remains unknown.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California