Depositional History of Upper Triassic Carbonate Platforms on Wrangellia Terrane, Western British Columbia, Canada

Andre Desrochers

Upper Triassic (Karnian-Norian) limestones of the Quatsino Formation, Vancouver Island, and the Kunga Formation, Queen Charlotte Island, were deposited within a complex rifted island arc system, which was accreted to the western margin of North America as Wrangellia during the mid-Cretaceous.

Carbonate sedimentation began after a transgressive event submerged the subaerially exposed Karmutsen volcanics. In both Vancouver and Queen Charlotte Islands, the lower part of the carbonate sequence records the widespread development of a shallow, subtidal platform composed mainly of two facies: open-platform deposits (bio-turbated fossiliferous wackestones) and platform sand-shoal deposits (oolitic-bioclastic grainstones). This early depositional stage evolved gradually into a second stage characterized by the fragmentation of the platform and development of a carbonate bank/basin topography controlled by extensional tectonics within Wrangellia. The interior facies of these carbonate banks consist of cyclic subtidal lagoonal and tidal-flat limestones, each cycle commonly capped by vadose diagenetic layer. The bank margin facies comprises exclusively high-energy, oolitic sand-shoal deposits. The adjacent basin facies include (1) pelagic/hemipelagic limestones, (2) platform-derived and slope-derived carbonate breccia, and (3) proximal to distal carbonate turbidites.

In both island systems, carbonate sedimentation ended abruptly when the isolated carbonate banks were drowned and covered by basinal clastics in response to rapid subsidence probably caused by tectonic activity within Wrangellia.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.