LOWER TO MIDDLE (LATE DYERAN–MIDDLE DELAMARAN) CAMBRIAN STRATIGRAPHY AND TRILOBITE BIOSTRATIGRAPHY OF THE MOUNT CLARK AND MOUNT CAP FORMATIONS, EASTERN MACKENZIE MOUNTAINS, NORTHWESTERN CANADA

Earliest Paleozoic strata of the eastern Mackenzie Mountains record deposition in an epicontinental basin bounded by the Canadian Shield to the present-day east and tectonic arches cored by Neoproterozoic strata to the west. In the northeastern Mackenzie Mountains, the Mackenzie Arch separates this semi-enclosed basin from the ocean-facing Selwyn Basin farther to the west. Cambrian formations deposited on the eastern side include the Mount Clark Formation at the base, overlain in turn by the Mount Cap and Saline River formations. Proximal to the arch, the Mount Clark Formation consists of cross-bedded quartz arenites, locally with Skolithos pipe rock, that were deposited as shoreface sands, which pass upwards into offshore heterolithic facies. More distal strata are heterolithic and heavily bioturbated, suggesting that the basin center lay to the east. These rocks are abruptly but conformably overlain by the mudstone-dominated Mount Cap Formation, indicating lower-energy conditions below fair-weather wave base. The Saline River Formation, which lies unconformably upon the Mount Cap Formation, consists of mudstone, gypsum, and dolomite and is interpreted to record shallowing causing restriction of the basin. Trilobite collections represent the Bonnia–Olenellus Zone through the Glossopleura Zone, indicating an age range from late Dyeran to middle Delamaran, i.e. upper Stage 4 to middle Wuliuan. The basal strata of the Mount Cap Formation are older to the east which indicates that the transgression between the Mount Clark and Mount Cap formations is diachronous. The Mount Cap Formation is dominated by outer shelf facies corynexochids broadly similar to faunas described from the southern Rocky Mountains of western Canada and the Great Basin of western USA. Stratigraphic differences in the degree of fossil articulation suggest subtle bathymetric changes through time. At least two corynexochid taxa have a significantly reduced number of thoracic segments and appear to be new species endemic to the basin, suggesting peripatric speciation involving heterochronic developmental changes. This may have been induced by environmental stressors, such as water temperature or oxygenation.