ABSTRACT

The structural geology of British Mountains, Yukon, between Crow and Spring rivers is characterized by a series of high-angle (about 30°) thrust faults that are parallel to each other and to the mountain front, trend in a northwesterly direction, and dip to the southwest. The thrusts have been traced in the field over a distance of 10 mi. They are difficult to detect on the available high-altitude aerial photographs but, once located in the field, the ridges (which are their most common topographic expression) can be traced on the photographs.

Two stratigraphic units are involved in the faulting. Unfossiliferous quartzites, argillites, and limestones assigned to the Neruokpuk Formation, believed to range in age from late Proterozoic to early Devonian, form the hanging wall of the faults. Where the thrusts can be detected, the foot-wall is a limestone or a calcareous chert-pebble conglomerate of the Triassic Shublik Formation, which can be identified by the common presence of the pelecypod Monotis sp. The Triassic beds can be seen to lie unconformably on Neruokpuk strata. At most only 50 feet of the lowest part of the Shublik Formation is preserved in the footwall of the faults.

The amount of movement along the thrusts is unknown but it is believed not to be great, meaning that it is measurable in hundreds of feet rather than in miles. To the northwest and southeast of the area between Crow and Spring rivers, the Shublik Formation is absent and the thrusts are not traceable but seem to disappear in the Neruokpuk Formation. Thrusting is believed to have taken place during the Cretaceous and to represent a phase of the Laramide Revolution.