In the Tatsamenie Lake area, intensely folded and regionally metamorphosed Permian, Triassic and older strata are separated from less folded and less metamorphosed Mesozoic sedimentary and volcanic rocks by a pre-Upper Triassic unconformity. Hornblende diorite and quartz-monzonite of Juro-Triassic Age intrude and are in fault contact with the pre-Upper Triassic rocks. These are commonly altered to chlorite, hematite and epidote. The Mesozoic strata are overlain unconformably by flat-lying Upper Tertiary and Pleistocene plateau basalts of the Level Mountain Group.

A major north to northwest trending fault, known as the Ophir Break Zone, extends through the area for over 10 kilometres. This zone is about 3500 metres wide, and is defined by areas of intense fracturing with abundant slickensiding; areas of carbonaceous and siliceous black siltstone and gouge; and linear quartz-iron carbonate-pyrite-fuchsite (listwanites) and quartz-dolomite alteration zones. The listwanites occur in the tuffs. The Ophir Break Zone is bounded on the west by the West Wall fault and on the east by the Ultramafic fault.

The pre-Upper Triassic rocks consist of fine-grained crystal to lapilli tuff, phyllite, limestone, siltstone and intraformational breccia. A sliver of ultramafic rock, consisting of serpentinized amphibolite, occurs adjacent to the West Wall Fault. In thin section, the rock contains antigorite, talc and minor carbonate and magnetite. These rocks are considered to be part of the Stikinia Terrane.