Abstract

The Series 2 and Miaolingian (Lower and Middle) Cambrian succession in the Colville Hills region, Northwest Territories consists (ascending order) of the Mount Clark, Mount Cap and Saline River formations, all of which were deposited in an epicontinental basin, herein named the Colville Basin. The Mount Clark Formation is sandstone-dominated with a shale package near the middle of the unit. The Mount Cap Formation is divided into an informal lower member (shale with dolostone and sandy dolostone beds) and upper member (shale with mixed limestone and dolostone). A high-resolution study of all ten available industry cores used process sedimentology and ichnology to delineate fifteen lithofacies in the Mount Clark and Mount Cap formations: six sandstone dominated; three mudstone dominated; four of heterolithic sandstone and mudstone; and one each of dolostone and glaucony. The facies can be grouped into four facies associations (FAs). FA1 consists of sandstone with well-preserved, wave-formed sedimentary structures, recording deposition in storm-influenced shoreface settings. Bioturbation is minimal to absent, suggesting that wave energy exerted a major stress on burrowing organisms. FA2 consists of sandstone in which bioturbation is extensively developed and trace-fossil diversity is high, including local development of Skolithos “pipe-rock”, but with poor preservation of physical sedimentary structures. Deposition is interpreted to have been in fairweather shoreface settings. FA3 contains varying amounts of mudstone, generally recording deposition in more distal environments. Absence of bioturbation in some successions of FA3 may reflect stresses due to seawater chemistry, possibly low dissolved oxygen contents. FA4 encompasses bioturbated, locally sandy carbonates deposited in relatively shallow water. Three transgressive-regressive (T-R) sequences in the Mount Clark and lower Mount Cap Formation can be traced over an area of approximately 300,000 km². The base of the Cambrian is a regional unconformity and sequence boundary. T-R Sequence 1 is entirely within the Mount Clark Formation. Based on archival trilobite collections, it is of Bonnia-Olenellus Zone age or older, but unlikely to be older than early Cambrian. T-R Sequence 2 spans the Mount Clark-Mount Cap Formation contact and has Bonnia-Olenellus Zone trilobites in its uppermost part, though its base may be older. T-R Sequence 3 is entirely within the Mount Cap Formation and its top corresponds to the boundary between the informal lower and upper members of the formation. The base of the sequence is within the Bonnia-Olenellus Zone, its maximum flooding surface is within the Plagiura-Poliella Zone, and the lowest trilobites of the Glossopleura Zone are present just above its top. The upper member of the Mount Cap Formation could not be subdivided into sequences, but contains a prominent maximum transgressive surface near its base and is capped by an unconformity at the base of the Saline River Formation. The upper member is mainly of Glossopleura Zone age. As noted by previous workers, syndepositional extension during deposition of the Mount Cap Formation was pronounced in the Mackenzie Trough but more subdued beneath the Colville Hills.