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Geology of the Fraser River Valley between Lillooet and Big Bar Creek Trettin, Hans Peter

Abstract

An area of 550 square miles between Lillooet and Big Bar, B.C. was mapped by the author using the scale of one mile to the inch. In the southern part of the Bowman Range four members are recognized in the Middle (?) and Upper Permian Marble Canyon formation which is partly composed of reefal limestone. This formation forms a northwesterly trending anticlinorium overturned to the northeast. The cherts, argillites, limestones, and volcanic rocks west of the Bowman Range, originally referred to the Permo-Pennsylvanian Cache Creek group are shown to be Permo-Triassic and are here assigned to the Pavilion group, a new group which is made up of two Divisions. Microscopic and stratigraphic evidence is given that the cherts of this group are of radiolarian origin. The Lower Cretaceous Lillooet group here is subdivided into three units. Divisions A and B are shown to form a northwesterly trending anticline. Three members are now recognized in Division A of the Lower Cretaceous Jackass Mountain group. The Lower Cretaceous Spences Bridge group is subdivided into several local and stratigraphic units. Two units previously assigned to the Spences Bridge group are correlated with the Kingsvale group on the basis of new fossil collections. Some volcanic and sedimentary rocks originally referred to the Miocene Kamloops group are here correlated with Miocene to Pleistocene rocks of the Quesnel map-area. West of Lillooet a belt of serpentinite was mapped that has structural and lithological similarities to the Upper Triassic ultrabasic intrusions of the Shulaps Range. Granitic rocks of three ages are recognized and range from early Lower Cretaceous or older to mid-Lower Cretaceous. It had earlier been shown that the Fraser River fault zone consists of several normal faults with relative downward movement to the east. East of these faults the author recognizes another fault with relative downward movement to the west. Lower Cretaceous and early Tertiary rocks thus occupy a graben between Permo-Triassic units to the northeast and to the southwest. This graben probably controlled the deposition of Divisions B and C of the Jackass Mountain group. The faulting may be related to the isostatic rise of adjacent granitic masses. Evidence is given that the latest movement on one of the faults took place in mid-Tertiary time.

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