ABSTRACT

In the central and west parts of the Ashcroft Basin, outcrops of the Jurassic Ashcroft Formation contain thick sections of mainly dark, carbonaceous shale interbedded with a few thin layers of fine-grained silty, graded sandstone and channelled conglomerate. Current directions and sandstone composition suggest that this Lower to Middle Jurassic shale and sand sequence was deposited west of an eroding, but inactive, island arc, and appear to represent the fine facies of submarine fans and distal turbidites. The apron of coarse clastic debris so often found adjacent to island arcs does not outcrop. A series of thrust faults mapped along the west margin of the basin may have placed impermeable, well-lithified, sedimentary rock of the Cache Creek and Nicola Groups, together with carbonaceous shales of the Ashcroft Formation, on top of coarse clastic debris of the proximal (?) fan and nearshore facies of the Ashcroft Formation. The nearshore facies outcrops in only a few places but is inferred to be extensive in the subsurface. If so, the thrust faults may have created numerous petroleum traps by placing potential source rock and cap rock over potentially good reservoir sand in a region where petroleum exploration has been very limited. This situation may exist along a belt hundreds of kilometres long running from just south of Ashcroft to north of Williams Lake and perhaps beyond. The conditions of sedimentation and thrust faulting may be partly analogous to the Late Cenozoic Ventura Basin and Santa Barbara Channel of California and to the Early Paleozoic South Mayo Trough of Ireland.