Abstract

The results of Apatite Fission Track (AFT) analysis for Cretaceous, Paleozoic and Precambrian borehole samples along two major transects through the Western Canada Sedimentary Basin (WCSB) are reviewed here. The northern transect crosses the Peace River Arch (PRA) and extends approximately 700 km from northeast British Columbia to the Alberta-Saskatchewan border northeast of Fort McMurray; the southern transect through central Alberta extends from the edge of the disturbed belt near Rocky Mountain House to the Alberta-Saskatchewan border north of Cold Lake. Sample AFT age and length data show a wide range in the degree of annealing due to the complicated thermal history of the basin and require interpretation through use of a quantitative model. A temperature-dependent fission track annealing model and a controlled random search technique were used to determine sample thermal histories from measured AFT parameters. Many of the Cretaceous samples along the PRA transect appear to have been deposited with effectively zero fission track age, implying a contemporaneous volcanic source for the apatite or rapid exhumation of the source area. Fully annealed samples yield an estimate of 60 Ma for the time of maximum temperature and inferred maximum burial. Maximum AFT paleotemperatures and maximum paleoburial depths (estimated from coal moisture data) were used to estimate paleogeothermal gradients increase systematically from approximately 20 degrees C/km near the deformation front in the southwest to as high as 60 degrees C/km in the up-dip northeastern portion of the WCSB near the cratonic edge of the basin. Present geothermal gradients show a similar overall pattern but with a narrower range of values ( approximately 30 degrees -45 degrees C/km). For the PRA transect, paleogeothermal gradients are elevated in the oil sands region to the east (35 degrees -60 degrees C/km) and in the deep basin to the west (35 degrees -40 degrees C/km). The temporal and spatial variations in geothermal gradient are probably best explained by thermal disturbances caused by regional paleofluid flow across the WCSB near the end of the Laramide Orogeny.

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