EVOLUTION OF TERTIARY MARINE SEDIMENTARY BASINS IN BRITISH COLUMBIA

Abstract

ABSTRACT Tertiary marine sediments of terrigenous origin are present on the west coast of Canada. For the Upper Eocene to Pliocene time interval the sediments have a volume which amounts to 10 per cent or less of that which might be expected from the erosional history of the land mass. An attempt is made to explain this apparent deficiency. Regional studies of metamorphic patterns, faults and land forms provide some pointers on continental and oceanic evolution, and also on past relations between the onshore and offshore areas. The coast crystalline complex evidently began to form during the Lower Cretaceous. In the north, major intrusive activity effectively ceased at the beginning of the Upper Eocene, but in the south it continued into the Lower Miocene. A right lateral transcurrent fault system developed within the continent, possibly also at the beginning of the Upper Eocene. Except in the northwest, transcurrent motion appears to have ended by the Middle Miocene. In northwestern Washington, a left lateral transform fault formed during the Oligocene, and strike slip developed a sedimentary pile against the continental mass in the Olympic Peninsula. These observations can be interrelated and further interpreted as follows: Between the Lower Cretaceous and Middle Eocene, the oceanic ridge system lay parallel to the Canadian coast, and crustal spreading generated stresses against the continent which formed the coast crystalline complex. At about Upper Eocene time the ridge moved into an oblique position against the continent, and plutonic rocks ceased to develop in the crystalline belt north of the point of contact. South of the contact, production of new crust by spreading was greatly in excess of available space, and the resulting stresses created both a major re-entrant in the continental mass and also the Olympic transform fault, which was itself finally folded against the continent, together with the sedimentary pile. From the Upper Miocene on, the ridge maintained its over-all position with respect to the land mass, but was faulted by stresses created with further spreading. Subduction can explain why only a small part of Upper Eocene and younger sediments are preserved south of Vancouver Island; it also may be the reason why the older part of the sequence is missing to the north of the island. However, most of the younger beds were probably preserved, but are largely absent in the area today because they were moved toward the north by spreading oceanic crust on the west side of an ancestral Fairweather - Queen Charlotte fault system.