Petrology of the Bell Top Formation
— Russell E. Clemons

The Bell Top Formation was named by Kottlowski (1953). Detailed geologic mapping (1:24,000 scale) in the Sierra de las Uvas began in 1970 (Clemons and Seager, 1973). Subsequent studies (Seager and Hawley, 1973; Seager, Clemons, and Hawley, 1975; Seager, 1975; Seager and Clemons, 1975; and Clemons, in press) and reconnaissance investigations have essentially delineated the extent, thicknesses, ages, and regional relationships of the Bell Top Formation.

 

The Bell Top Formation attains its maximum total thickness of about 1,500 ft in the central Sierra de las Uvas (Fig. 1). It crops out sporadically for 25 mi to the north and northeast, as some members pinch out and others interfinger with the lower Thurman Formation (Kelley and Silver, 1952) in the Rincon Hills, southern Caballo Mountains, and Point of Rocks. Thicknesses also decrease to the south and probably pinch out in the northern West Potrillo Mountains. The Bell Top also wedges out to the west in the Good Sight Mountains. Deposition appears to have been limited abruptly to the east, in the vicinity of Faulkner Canyon and Corralitos Ranch. Thus it was deposited over an area extending 50 mi in a north-south direction and 25 mi wide, which has been named the Good Sight-Cedar Hills volcano-tectonic depression (Seager, 1973).

 

The depression is floored by the Palm Park Formation (Kelley and Silver, 1952) of Eocene age to the east, and the Rubio Peak Formation (Elston, 1957) of probable Eocene age to the west. Exact relationships between these two units are still to be determined. Potassium-argon ages of several members of the Bell Top Formation indicate an early to middle Oligocene age. A K-Ar age of a basal Uvas Basaltic Andesite (Kottlowski, 1953) flow, overlying the Bell Top, is late Oligocene (Fig. 2).