Using leaf margin analysis to estimate the mid-Cretaceous (Albian) paleolatitude of the Baja BC block

Abstract

The “Baja BC hypothesis”, which postulates that western Washington State, British Columbia and southern Alaska originated at the latitude of Mexico, has pitted paleomagnetic results against long-held interpretations about the tectonic evolution of western North America. In this paper we develop a new paleobotanical method for estimating paleolatitude and apply it to this problem. We start by showing that the modern MAT field for North America is well correlated with latitude, demonstrating the feasibility of using MAT to estimate paleolatitude. A compilation of MAT and floral data from 84 modern sites in Central and North America is used to establish a new prediction relationship, MAT = 1.32 + 28.99 P, where P is the proportion of smooth-margined species within a floral sample at a site. Our analysis also includes a more complete estimate of the uncertainties associated with estimating MAT from a measurement of P. Using modern data, we show that MAT and P can be used to estimate latitude as well. We then apply this approach to resolve the paleolatitude of Baja BC. Eleven floral sites from stable North America are used to establish the latitudinal MAT profile for North America during the Albian and Cenomanian. A floral site from the Winthrop Formation, a mid-Cretaceous (110–100 Ma) fluvial unit in the Methow basin of northern Washington State, is linked to the Baja BC block and predates its proposed northward offset. Forty-three morphospecies of dicotyledonous angiosperm leaves from the Winthrop Formation give P = 0.76, which is equivalent to a MAT of 23.4 °C, indicating a subtropical to tropical climate. We use the North American MAT profile to estimate a paleolatitude of 38.4°N for the Winthrop flora, indicating ∼ 2200 km of northward offset relative to stable North America.