Description

This geologic map of the Twin Buttes 7.5' quadrangle, located south of Tucson in southern Arizona, includes both bedrock and surficial mapping. The map shows the 2120 level in the Twin Buttes mine, along with geology mapped by Anaconda geologists that are now buried beneath rock heaps around the mine. Access to the now inactive Twin Buttes mine was not obtained during the current mapping effort. The most prominent feature of Paleozoic and Mesozoic units in the Twin Buttes area is the northwest-trending alignment of lithologic contacts. In outcrops on Twin Buttes and hills to the west, layering in marbles and rare relict bedding in clastic units are sub parallel to this trend except for local irregularities. A northwest-trending fault, parallel to the trend of lithologic units, is intruded by Ruby Star Granodiorite east of Twin Buttes (UTM 494300N, 3531000E). In the Twin Buttes level 2120 map, a short segment of a northwest-trending fault separates Horquilla Limestone from Proterozoic granite. A third significant, northwest-trending fault is inferred within the porphyry intrusion in the Twin Buttes mine, south of the Twin Buttes fault. The stratigraphically highest unit intruded by the porphyry southwest of the fault is the Concha Limestone, but on the northeast side of the intrusion, Earp and possible Horquilla formation are intruded. These relations require that the intrusion is inserted along a fault. In all three cases, the northwest-trending faults have younger rocks to the southwest and older rocks to the northeast. One 1:24,000 scale map sheet with cross-section.