INTERPRETING THE CENOZOIC EVOLUTION OF THE TOFINO AND QUEEN CHARLOTTE BASINS, BRITISH COLUMBIA, CANADA USING OFFSHORE WELL DATA, MICROPALEONTOLOGY, STRONTIUM ISOTOPES, AND REGIONAL GEOPHYSICS.

New and significant advancements in understanding the complex evolution of the Cenozoic Tofino and Queen Charlotte basins, which lie mostly offshore British Columbia, have been achieved using multiple data sets including: well data, micropaleontology, strontium isotopes, and regional geophysics. The work exploited the cuttings from several offshore exploration wells, typically to about 4000m depth, drilled in the 1960s, specifically over 2,145 drill cutting samples from 6 offshore wells and 1,035 coastal surface samples all within the Tofino Basin and over 3,550 drill cutting samples from 8 offshore wells in the Queen Charlotte Basin. Subsets of samples were analyzed for microfaunas, strontium ages, lithology, mineralogy, and related data. The resulting high resolution Cenozoic chronostratigraphy is constrained by strontium isotope ages (36.9–1.3 Ma) from 59 analyses of Late Eocene to Pleistocene foraminifers together with revised biostratigraphies (foraminifers and ichthyoliths) from offshore wells and outcrop samples, specimen thermal alteration values, and existing well log and offshore multichannel seismic and magnetic data. These data establish the different regional geologic, tectonic and deformation events during the Cenozoic between and within these major basins. For example, six regions, multiple sub-basins, and different thrust fault styles are recognized as the Crescent terrane and other sediments were subsequently deformed and fragmented following accretion in the Tofino Basin. Results, especially for the Ucluelet and Carmanah sub-basins, indicate periods of deformation during Late Eocene, Late Oligocene, Middle–Late Miocene, and post middle Pliocene, whereas the Early Oligocene and Early Miocene had periods of relatively slow and less disturbed deposition. In the southern Queen Charlotte Basin, the data show extension and sediment transport resulted in thick Miocene basin infill with heating, deformation or slumping, and mobilization of fluids/gasses upwards since the Early Miocene and support syn-rift and post-rift depositional and deformational phases. Comparisons between the different geological processes and events in the Tofino and Queen Charlotte basins reveal details of the complex evolution of these Cenozoic basins, plate margins, and ridge junctions.