Paleobiogeographic patterns of pectinoid bivalves and the Early Jurassic tectonic evolution of western Canadian terranes

Abstract

Utilizing new data from western Canada, the biogeography of Early Jurassic pectinoid bivalves along the eastern paleo-Pacific margin has been analyzed qualitatively and quantitatively. The studied areas range from the Andean Basin in the southern hemisphere to the Sverdrup Basin of Arctic Canada and include major allochthonous terranes of western Canada. While the Andean Basin exhibits a mixed bivalve fauna of austral, bipolar, and low latitude-East Pacific forms, pectinoid bivalves from the Canadian craton are characterized by a high percentage of boreal and bipolar taxa. The western Canadian allochthonous terranes Wrangellia and Stikinia show a mixed influence of low latitude-East Pacific and boreal/bipolar forms until Pliensbachian times. During the Toarcian/Early Aalenian, taxa typical of low latitudes disappeared. This pattern of a latitudinally differentiated Early Jurassic bivalve fauna, which apparently is climatically controlled, seriously undermines the hypothesis of a uniform West American bivalve province. Based on diversity gradients, similarity coefficients, cluster analyses, and distributional patterns of characteristic taxa, biogeographic data have been used to constrain the latitudinal positions of Wrangellia and Stikinia through time. During the three analyzed time intervals (Hettangian/Sinemurian, Pliensbachian, and Toarcian/Early Aalenian), both terranes were in the northern hemisphere and in the eastern paleo-Pacific. During all of the Early Jurassic, Wrangellia and Stikinia were close together and were moving northward. Paleolatitudes corresponding to those of the Western Canada Sedimentary Basin in Alberta were not reached before Toarcian times. By the end of the Early Jurassic, both terranes were in much the same latitudinal position relative to the craton as they are now. Consistent with biogeographic patterns are Early Jurassic latitudinal displacements of approximately 1300 km.