Named as a formation in the Artillery Mountains manganese area for exposure on Cobwebb Hill in upper Chapin Wash, Mohave Co, AZ, Plateau sedimentary province. No type locality given. Crops out along most of Artillery Mountains side of the manganese basin, especially along Chapin and Burro Washes, but is nowhere exposed along Rawhide Mountains side. In its largest exposures along upper Chapin Wash, it tends to form buttes. Is a hypersthene?-augite-olivine basalt. Most of rock is gray to brown, fine grained to aphanitic, and contains only a few vesicles, but where relatively thick the upper part is black, mostly aphanitic, and highly vesicular. Thickness varies from 0-250+ ft. Conformably overlies Chapin Wash formation (new), but overlaps onto the Artillery formation (new) along Burro Wash, where Chapin Wash formation thins out. Unconformably underlies Sandtrap conglomerate (new). Structure map. Geologic maps. Because of its conformity with underlying Chapin Wash formation and its unconformity with overlying Sandtrap conglomerate, Cobwebb is thought to belong to same geologic epoch as Chapin Wash formation. It is, therefore, tentatively assigned an early Pliocene age.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Has an apparent age of 13 +/-2.1 using K-Ar whole rock method. Sample collected southeast of Parker, Yuma Co, AZ in the Basin-and-Range province.

Source: GNU records (USGS DDS-6; Denver GNULEX).