Energy in Canada's North

Thermal histories of Cambrian strata and basement rocks from the Great Slave Lake region, Northwest Territories

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(Student abstract)


T.F. Pinto (Presenting)
University of Calgary
E. Enkelmann
University of Calgary
V. Terlaky
Northwest Territories Geological Survey

The southwest region of the Northwest Territories (NWT) comprises Phanerozoic sedimentary rocks that cover the Archean-Proterozoic basement. This region is marked by anomalous high heat flow and a geothermal gradient that increases from west to east. The origin of the high heat flow is mostly unknown, and several models have been suggested. We aim to evaluate the origin, source, and longevity of the high heat flow to better understand the potential for geothermal energy production. Towards this aim we use low-temperature thermochronology and thermal modeling to assess the burial and erosion history of the region. The sensitivity window of the (U-Th-Sm)/He system is 55–75°C for apatite and 160–200°C for zircon, thus recording changes in the upper crust. During this first phase of this project, we analyzed basement and the overlying Cambrian strata collected from outcrops and boreholes along the northwestern edge of Great Slave Lake. From the fifteen samples collected, five yielded apatite and nine yielded zircon that are suitable for (U-Th-Sm)/He dating. In general, the (U-Th-Sm)/He single-grain data scatter widely and preclude the reporting of mean sample ages. Apatite single-grain dates range from 16 Ma to 1262 Ma with Cretaceous ages (74 – 143 Ma) being the most common. Zircon single-grain dates range from 59 Ma to 1857 Ma, with Mesoproterozoic ages (1044 – 1579 Ma) being the most common. Due to the thermal sensitivity of the (U-Th-Sm)/He system, detrital data can be considered reset when the sediment was heated to temperatures higher than the partial retention zone (PRZ) after deposition. Most of the Cambrian strata yielded apatite (U-Th-Sm)/He dates that are younger than deposition age and are interpreted to be thermally reset. Two samples yielded single-grain dates that predate and postdate deposition, suggesting partial resetting of the apatite system. The zircon system requires burial and heating to higher temperatures (>200ºC) to be fully reset. Our zircon results show that three samples yielded reset single-grain dates, one sample is partially reset, and five samples are not reset. The partial and total reset samples suggest post-depositional heating to >55°C for apatite and >160°C for zircon. The five sedimentary samples with zircon (U-Th-Sm)/He data that are not reset (older than the depositional age) places a limit to the maximum amount of heating to <160ºC. Preliminary thermal modeling of samples that are thermally reset present reheating and cooling after the Cambrian. Most samples record multiple phases of cooling. In total we identified cooling phases during Triassic, Upper Jurassic, Lower Cretaceous and Eocene.

In the second phase of this project, apatite fission track data will be acquired on the same samples to better constraint the thermal history of the samples. The same geo- and thermochronological approach will be applied to outcrop and borehole samples from the western portion of the sedimentary basin in the southwestern corner of the Northwest Territories, including the Liard Basin and the deformation front of the Canadian Cordillera.