About This Item
Share This Item
Computer-generated crossplots of the gamma-ray log versus depth in the Pinedale area, northern Green River basin, Wyoming, exhibit an abrupt shift in API gamma units over the structurally influenced depth range of 7,100-8,500 ft (2,100-2,600 m). Higher API units are recorded for all lower Tertiary rocks than for underlying Upper Cretaceous rocks. The shift, which is not obvious from viewing analog prints, coincides with the Cretaceous-Tertiary boundary as located in this area by previous workers using different methods.
Evaluation of gamma-ray spectral logs from five wells, combined with petrographic and x-ray diffraction analysis of core samples from the Pinedale area, shows that this shift is caused by a higher potassium content in Tertiary rocks than in Cretaceous rocks, due to the presence of arkosic sandstones. Within the lowermost 600 ft (180 m) of the Tertiary, the potassium feldspar content of the sandstones gradually decreases with depth down to the Tertiary unconformity and then abruptly declines, essentially to zero, in the Cretaceous.
Crossplots of gamma-ray intensity versus depth from other wells in the northern Green River basin show a diminution of this shift to the west and southwest of the Pinedale area. This relation suggests that the Precambrian crystalline core of the Wind River Range, which was upthrust and eroded during the Laramide orogeny, provided the source of the arkosic Tertiary sandstones.
In many Rocky Mountain basins, identification of the Cretaceous-Tertiary boundary is problematical due, in part, to the similar lithology and depositional environments of the nonmarine rocks as well as to the absence of outcrops and reliable biostratigraphic data. The approach described may be applicable to other areas.
End_of_Article - Last_Page 946------------