The eastern part of the northern Interior Plains is underlain by rocks of Cenozoic, Mesozoic, and Paleozoic ages. The region is bounded on the east by the Coppermine arch, composed of lower Paleozoic and Precambrian rocks. The plains region is a northwest-dipping homocline, interrupted in its western part by the Kugaluk arch, a north-trending pre-Cretaceous uplift.

Mesozoic rocks of the Interior Plains consist of Cretaceous sandstone, mudstone, and shale with a composite thickness of about 3,000 ft along Anderson and Horton Rivers. The Lower Cretaceous units are correlated with similar rocks on Banks Island. On the mainland, these are disconformably overlain by varicolored clastic rocks of Late Cretaceous and early Tertiary ages.

Westward, in the region of the Mackenzie delta, the Tertiary Reindeer Formation consists of a northward-thickening sequence of poorly consolidated to unconsolidated cherty gravel, cross-bedded sandstone, coal, and ash beds. Its maximum outcrop thickness is about 4,000 ft. In the nearby B. A.-Shell-I.O.E. Reindeer D-27 well the Reindeer Formation is 3,970 ft thick and underlies 790 ft of Quarternary and Holocene sediments. Microfaunal studies show that the Reindeer Formation overlies 2,200 ft of Late Cretaceous clastic rocks which may be partly equivalent to the Moose Channel Formation that crops out on the west side of the delta adjacent to the Richardson Mountains. The Upper Cretaceous rocks in the Reindeer well unconformably overlie 5,690 ft of Lower Cretaceous sandstone and mudston which can be correlated with similar units in the eastern Richardson Mountains.

Offshore seismic profiles obtained during the 1969 Arcticquest survey indicate the presence of a thick sequence of sedimentary rocks, the lower part of which has been deformed into broad domal structures. These lower rocks are unconformably overlain by nearly flat-lying younger rocks. This unconformity may be the same one that separates the Lower and Upper Cretaceous rocks in the Reindeer well. Analyses of the profiles indicate that these younger rocks may have been intruded by diapiric structures.

If the Canadian Government's Hudson 70 program in the Beaufort Sea is successful additional information on the geology of the Beaufort Sea basin will be available.

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