ABSTRACT

The eastern half of the study area, the Mackenzie and Anderson River Plains area, is underlain by uniform sequences of carbonates and relatively minor shales, cherts and evaporites, and is marked by widespread, regional disconformities. Middle Ordovician, Upper Silurian, and at least the lower Lower Devonian sediments are completely absent from the area.

The western portion that is, roughly the northern part of Yukon -- was, as a result of more local tectonic instability, characterized by more rapid facies changes, development of troughs, ridges and basins, and more localized disconformities and unconformities.

The Richardson Trough, more or less conforming in outline to the area of the present Richardson Mountains, was a deeper-water, graptolite-shale trough from before earliest Ordovician through to at least early Emsian; was apparently positive between late Emsian and early Givetian; and, finally, was an area of rapid sedimentation throughout the remaining Devonian. South of but in linear continuity with the Richardson Trough, a structure recognized for the first time and termed the Bonnet Plume High may have been emergent throughout the Ordovician, Silurian and the earliest part, at least, of the Devonian. The area west of the present Richardson Mountains, north of the Ogilvie Mountains, and south of British and Barn Mountains, and termed the Yukon Stable Block, was one of platform-carbonate deposition. The Block was more or less diagonally dissected by an essentially NE-SW-trending structure, the Dave Lord High, which was positive from Late Ordovician through to at least mid-Early Devonian times. At times what was probably a subaqueous extension of this High joined with an area of carbonate development west of and beneath the Mackenzie River delta, and appears to have restricted the northern end of the Richardson Trough.

The Neruokpuk Formation of the British Mountains, in the extreme northwestern portion of the region, is considered to be at least Early Ordovician to Late Silurian in age, because of correlation with rocks of the same formation in Alaska, and because of its lithologic similarity to rocks of that age range in the Barn Mountains. These rocks are interpreted as miogeosynclinal equivalents of the more stable platform suites to the south and southeast. Intrusion of two small, granitic masses is believed to have occurred at about Early Devonian time in the Barn Mountains area.

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During late Middle and early Late Devonian times, the sedimentation pattern of the entire region changed, leading to the deposition of black, siliceous shale and localized reefs. This was followed during later Frasnian and Famennian time by strong uplifting in and northwest of the British and Barn Mountains regions, and the deposition of widespread and locally great thicknesses of clastic sediments to the southeast. These are characterized by coarse, greywacke-type sediments in the northwest, grading southeastward to finer sandstones and siltstones, and minor carbonates.