THE UPS AND DOWNS OF THE CANADIAN CRATON: WHAT ARE WE MISSING IN THE PHANEROZOIC HISTORY OF THE HUDSON BAY INTRACRATONIC BASIN?

The Canadian Shield is an iconic example of a cratonic area characterized by an old, thick, cold and stiff lithosphere. As with most continental interiors, the Canadian craton is considered stable, and most geological models infer a slow and more or less continuous exhumation punctuated by minor sedimentary and ice sheet loading events. However, several lines of evidence (presence of Phanerozic outliers, sedimentary xenoliths in kimberlite pipes, sparse organic maturation data) imply that younger sedimentary units have been deposited and subsequently eroded away from vast areas. Quantification of the thickness, age and geographical distribution of the missing geological record is not an easy task, but has major implications for the geological history and hydrocarbon prospectivity of the Hudson Bay basin, the largest intracratonic basin of North America. The preserved sedimentary succession of the Hudson Bay basin is up to 2500 m thick, Upper Ordovician to Upper Devonian in age and was deposited under shallow marine conditions.

New apatite fission track (AFT) results from the Hudson Bay area indicate that AFT ages are younger than the age of the host rocks indicating that samples experienced significant annealing and were subjected to temperature > 60°C during the Phanerozoic. This study applied an inverse modeling strategy taking into account the subsidence history for basement samples from the bottom of hydrocarbon wells and the fixed geometrical relationship for the samples from a 3.6 km vertical profile in the LaRonde mine.

Thermal histories from AFT data record cycles of heating and cooling that are coherent with the sedimentary record preserved in the Hudson Bay basin, but also indicate temporal and geographic variations in the timing and degree of Phanerozoic heating episodes. The maximum temperature experienced during the Paleozoic is relatively well constrained but the respective effects of higher surface temperatures, changes in the paleo thermal gradient and changes in depth due to burial remain difficult to assess as well as the timing of maximum heating. The fact that some samples located at the present-day edge of the basin experienced temperatures of the same order of magnitude or even higher than samples from the central part of the basin is particularly noteworthy.