GIS-enabled structure maps of subsurface Phanerozoic strata, north-western Northwest Territories, Canada
Authors: MacLean B C 
Published: December 2012

New geological maps in GIS shapefile format, accompanied by explanatory text and cross-sections, provide a regional synthesis of the subsurface Phanerozoic geology of the northwest mainland of the Northwest Territories. These maps were derived from the
interpretation of an extensive grid of reflection seismic data, shot over a period of forty-three years, supported by borehole, surface mapping, and regional potential field data. Sixteen structure and thickness maps were constructed in reflection time units and converted to metric depth using
regional velocity fields calculated from well control. Data tables accompanying each map layer provide descriptions and references for each map item. The maps and sections document a five-phase geological history: 1) initial extension and subsidence during the Cambrian; 2) marginal subsidence
lasting, with two interruptions, from the latest Cambrian through to the late-middle Devonian; 3) foreland subsidence related to the Ellesmerian Orogeny that lasted until at least the lower Carboniferous; 4) a hiatus in the rock record that extends to the mid-Lower Cretaceous and 5) regional
subsidence and foreland basin development, triggered by development of the Cordillera during the Columbian and Laramide orogenies. Tectonic effects of these orogenies are observed throughout the section. The lowermost mapped horizon, the Top of the Proterozoic, sets the pattern for the mapset. It
is exposed to the east on the flank of the Canadian Shield and, to the southwest in the Mackenzie Mountains. The Top Proterozoic horizon dips gently westward from where it is exposed on the Canadian Shield and remains relatively shallow across the Colville High until it plunges westward under the
foreland basin of the Mackenzie Mountains, northward under the Anderson and Beaufort Basins and southward under Great Bear Basin. Normal faults offsetting this surface and others are almost entirely due to a Cambrian (Phase 1) period of extension while the reverse faults are products of either the
Ellesmerian or Laramide orogenies.
Maps and cross-sections of Cambrian surfaces and thickness reveal a history of local uplift and rifting that produced a central graben system within a much larger Cambrian basin. Similar maps and sections of the Cambro-Ordovician through mid-Devonian (Phase 2) section document regional westward
subsidence and the multiple phases of evolution of the Keele Tectonic zone. The present-day McConnell and 'St. Charles' ranges are seen as Laramide inversions of very large Cambrian normal faults. Relatively thin strata across the large Colville High attest to its overall stability. What strata
were deposited there was later affected by pre-Cretaceous and later erosion that cut progressively deeper towards the east. As a consequence, maps of Devonian and younger strata are largely confined to the foreland basins, Great Bear Basin and Anderson Basin. The mid-Cretaceous Unconformity surface
and the Lower Cretaceous thickness map hint at a NW-SE trend to the Keele Arch, somewhat different than the N-S trend assumed for earlier manifestations of the arch. The maps present a number of unanswered questions and hints at their solution. For instance, a study of thickness variations and
depositional and erosional edges may enable a better understanding of the Keele Tectonic Zone. Laramide structures, when presented at a regional scale, reveal remarkable patterns that have not been fully investigated. One such pattern is the complimentary nature of the transverse movements across
Whirlpool and Gambill fault zones that contributed to the formation of the Discovery Range and the Franklin Mountains.