Geology of Fort Grahame E1/2 map-area, British Columbia

Fort Grahamc cast -half map-area (94C E 1/2, bounded by latitudes 56 and 57 and longitudes 124° and 125° in north- central British Columbia, embraces parts of Omineca and Rocky mountains and is bisected diagonally by the north-northwesterly trending· Rocky Mountain Trench. The floor of the Trench in the map-area is now flooded by Williston Lake created by the W. A. C. Bennett dam on Peace River . About three quarters of the map-area is underlain by strata of late Proterozoic age assigned to the lngenika and misinchinka groups. These rocks, dominantly elastic in the lower part but containing at least one important carbonate unit in the upper part, are similar to those of the Upper Protcrozoic Kaza Group and lower part of the Cal'iboo Group in Cariboo fountains. They arc regionally metamorphosed to facics ranging from subgreenschist to amphibolitc. Lower Paleozoic strata in Ornincca .Mountains are relatively thin as they are in Cassiar mountains farther north. They comprise Lower Cambrian elastic and minor carbonate beds, possible Ordovician calcareous shale and siltstone and silty limestone, and very well bedded, in part laminated Silurian (?) and Devonian dolomite. Overlying rocks consist of late Paleozoic slate, bedded chert, minor limestone and a thick sequence of basic volcanics. In northern Rocky mountains a well differentiated and locally highly fossiliferous Cambrian succession, typical of Cambrian sequences farther south in Rocky Mountains , is overlain by a thick unit of Lower Ordovician nodular, silty limestone that forms many of the high peaks in the region. GL'aptolitic siltstone and shale of mid- Ordovician to early Devonian age is locally capped by a persistent middle Devonian carbonate unit. Probable Lower Tertiary conglomerate and finer grained nonmarine elastic rocks outcrop here and there in Rocky Mountain Trench and in the valley occupied by Mesilinka and Omineca rivers. Structures northeast of Rocky i\1ountain Trench in general show a northcasterly directed asymmetry whereas those west of the trench arc commonly weste1°ly directed. The northern Rocky Mountain Trench is therefore, in this area, a zone of structural divergence. Several lead-zinc showings have been explored in the Silurian-Devonian carbonates near Osilinka River and mica has been mined on a small scale in the northern part of Butler Range.