PORCUPINE CREEK – A PERMIAN-TRIASSIC BOUNDARY SECTION FROM THE ENIGMATIC CACHE CREEK TERRANE, BRITISH COLUMBIA

The Cache Creek Terrane is one of the more mysterious exotic terranes of the Canadian Cordillera. It is interpreted to be allochthonous to North America, with a largely Tethyan fauna suggesting that it originated at low latitude far to the west of the contemporary margin of Pangea. As a result of its distal open ocean location at the time of deposition, it may be representative of conditions in the broader Panthalassic Ocean, far from any major continental influence, making it a valuable resource to study the oceanographic dynamics of the end-Permian mass extinction and Early Triassic recovery. However, the dissected nature of the terrane means that few continuous sections have been reported.

Here we present preliminary biostratigraphic and trace element geochemical results from an approximately 100 m measured section at Porcupine Creek in the Marble Range of southern British Columbia, spanning the Middle Permian to early Middle Triassic interval. Massively bedded, partially recrystallized limestone containing sporadic bioclasts including Yabeina and Neoschwagerina is present in the Middle Permian. The carbonate unit becomes increasingly devoid of bioclasts and is interbedded with argillaceous intervals, up to approximately the Permian-Triassic boundary where Hindeodus praeparvus and H. parvus have been recovered. The carbonate is terminated by an apparently rapid deepening that may have been tectonically controlled.

The Early Triassic is present as a succession of foliated green argillite, with highly variable chert content that may reflect cyclical variations in radiolarian productivity in the open Panthalassic Ocean on Milankovitch time scales. These argillites are interspersed with fissile black shales, which likely record episodic anoxia. Carbonate, in the form of early diagenetic concretions, is mostly absent in the Early Triassic at Porcupine Creek, but begins to reappear around the base of the Middle Triassic suggesting shallowing above the carbonate compensation depth. Olenekian (Smithian) bioclastic carbonate with microbialites has been described in a fault bounded unit in the same region. Subsequent Middle Triassic strata are highly deformed, and likely unsuitable for stratigraphic or geochemical analysis.