Geology of Mission Ridge, near Lillooet, British Columbia

The purpose of this study is to determine the geometry and timing of movement on internal and bounding structures of the Bridge River terrane in the vicinity of one segment of the 1988 LITHOPROBE southern Cordilleran transect, near Lilloo&, British Columbia (Figure l-12-1). Specifically, the orientation of the major faults (Yalakom, Marshall Creek and others) is addressed in order to assist in interpreting the seismic reflection data collected in 1988. The tectonic history of rock units at the boundary of the Coast plutonic complex and the Interlnontane Belt between 50” and 51’ north latitude is highly enigmatic. An already complex history of Mesozoic accretion and probable Eocene extension is obscured by Tertiary strike-slip faulting, involving northward movement of more outboard allochthonous terranes during and/or after accretion. The Bridge River Group, bounded to the northeast by the Yalakom fault and to the southwest by the low-angle Mission Ridge fault (Figure l-12-2), is a chaotic mClange of ribbcln chert, greenstone, pillow basalt, minor greywacke, lim+ stone olistoliths and narrow serpentinite zones. It ha!; a thickness of 2.5 to 4.5 kilometres in this structural pan’:1 (Figure l-12-3). Layers of grey radiolarian chert, I to 5 centimetres thick, are separated by argillite layers I to IO millimetres thick. Fault-bounded blocks of ctlert have multiple, randomly oriented slickenside surfacers. These blocks are usually in fault contact with similarly deformed blocks of the previously mentioned lithologies. In the southern Shulaps Range, Potter (1986) found chat in depositional contact with limestom:, greenstone and sandstone. Radiolaria from chert found along