UPLIFT OF REDSTONE ARCH (MACKENZIE MOUNTAINS, NWT) DURING DEPOSITION OF CAMBRIAN SERIES 2: LOCAL EXPRESSION OF A REGIONAL TECTONIC EVENT?

Redstone Arch was an elongate, northwest-trending (present-day frame of reference) region of tectonic uplift that exerted a major control on early Paleozoic depositional patterns in an area now coincident with part of the Backbone Ranges of the Mackenzie Mountains, Northwest Territories. Compilation of newly collected and archival stratigraphic data from western Wrigley Lake map areas (NTS 95M) points to a previously unappreciated uplift event along Redstone Arch during deposition of Cambrian Series 2 (trilobite-bearing Lower Cambrian). In this area, the sandstone-dominated Terreneuvian (sub-trilobite basal Cambrian) upper member of the Backbone Ranges Formation is overlain sharply by a succession of variegated shale, siltstone, dolostone, limestone, and sandstone assigned to the Sekwi Formation (Cambrian Series 2). The upper member of Backbone Ranges Formation varies in thickness from tens to hundreds of metres and locally is absent beneath the Sekwi Formation. In this region, the base of the Sekwi Formation is an onlap surface and probably an erosional unconformity. Sekwi Formation in this region has yielded only trilobites typical of the Bonnia-Olenellus Zone, whereas to the west it also includes a thick lower succession that yields trilobites of the underlying Fallotaspis Zone and Nevadella Zone. We suggest that uplift-related exposure of Redstone Arch was part of a regional tectonic event that led to development of karst within Sekwi Formation at the boundary between the Nevadella Zone and Bonnia-Olenellus Zone, to the widespread deposition of a sandstone marker tongue at the same level, and to the shedding of archaeocyathan-bearing olistoliths into the early Selwyn Basin. A return to relative tectonic quiescence during deposition of Bonnia-Olenellus Zone coincided with a transgression that at least partially inundated the arch. Regional comparisons suggest that a similar transition from tectonically active to more quiescent conditions is recorded within the Bonnia-Olenellus Zone westward in Yukon and eastward beneath the northern mainland plains.