LATE CRETACEOUS/EARLY TERTIARY CRUSTAL HEATING IN THE SOUTHERN CANADIAN ATLANTIC REGION: EVIDENCE FROM APATITE FISSION TRACK DATA

Time-temperature modelling of new apatite fission track data from the 1.5 km deep Digby D-1 drillhole, which sampled pre-Carboniferous granitoid basement in Nova Scotia immediately south of the Fundy rift basin, and from Permian and Triassic sandstone samples from Nova Scotia, New Brunswick, and Prince Edward Island, indicate that a Late Cretaceous/Early Tertiary heating event, previously detected in fission track studies in onshore and offshore Nova Scotia, was more widespread and significant than previously though. Time-temperature models suggest that rocks now at or near the surface experienced temperatures of ca. 60°C in Late Cretaceous to Early Tertiary time. Paleo-geothermal gradients determined from samples >1 km apart in the Digby D-1 well were in the range of 15-20°C/km. The models require that significant regional cooling of ca. 40°C has occurred in post-Paleocene time. Although the causes of the heating are unclear, the Cretaceous/Tertiary episode was fundamentally different from the thermal event associated with the break-up unconformity in Late Triassic-Early Jurassic times, which is marked by dikes, large outpours of basaltic lavas (Fundy Group), and well preserved unconformities. The heating was likely due in part to sedimentation resulting from a combination of increased eustatic sea levels between 100 and 35 Ma and possible minor tectonic subsidence. Additional heating may have resulted from increased mean annual surface temperatures in Late Cretaceous-Early Eocene time, an effect that would have propagated to depths greater than the levels of the samples at the time. The Eocene to recent model thermal history is explainable in terms exhumation cooling resulting from decreased eustatic sea levels and propagation to depth of the effects of decreased mean annual surface temperatures. The fission-track models, which are being independently tested with (U-Th)/He, have some bearing on petroleum systems and paleo-landscape development. Potential source rocks were heated to oil-window temperatures in the Cretaceous/Tertiary, and subsequent basin inversion may have led to deposition/redistribution of reservoir sands farther from the coast in offshore basins.