Abstract

Cretaceous strata in the subsurface of Great Slave Plain consist of interbedded shale, siltstone and sandstone of the Albian to Cenomanian Fort St. John Group and sandstones of the Cenomanian Dunvegan Formation. These strata are generally less than 500 m thick but thicken rapidly into the north end of Liard Basin. Within the area, Cretaceous rocks rest unconformably on a variety of lower Paleozoic units. Although the subdivision of the Fort St. John Group into its constituent formations is difficult in the subsurface, radioactive shales and areally extensive maximum flooding surfaces allow detailed internal correlations to be undertaken. This approach to correlation also helps to identify, or infer, the presence of at least two unconformities. A locally developed basal sandstone is present within the Fort St. John Group and can be identified as the Chinkeh Formation. The best development of the Chinkeh Formation occurs in the western part of Great Slave Plain. in a 50 km long. several kilometres wide, paleo-estuarine valley. Gas has been recovered from a sandstone within the Chinkeh Formation at Arrowhead B-41. The two unconformities within the Fort St. John Group truncate downsection eastward, indicating that an eastern uplifted area once existed during periods of the Albian. This uplift was broad and of low amplitude and can be traced into northeast British Columbia. If the uplift has been correctly identified there exists the possibility that sandstone bodies at the margin of the uplift may have formed during periods of uplift and truncation. If present, they are most likely to be found in the subsurface of northeast British Columbia.

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