Poster / Soc 397a / 2015 Poverty in Tucson Field Workshop

Poster / Soc 397a / 2017 Poverty in Tucson Field Workshop

Poster / Soc 397a / 2017 Poverty in Tucson Field Workshop

Poster / Soc 397a / 2017 Poverty in Tucson Field Workshop

Poster / Soc 397a / 2015 Poverty in Tucson Field Workshop

Poster / Soc 397a / 2015 Poverty in Tucson Field Workshop

Poster / Soc 397a / 2015 Poverty in Tucson Field Workshop

The Cienega Gap area, located about 27 miles southeast of Tucson, Arizona, has long been noted for its structural complexity. Discordances between sedimentary rocks of Paleozoic, Mesozoic, and Cenozoic age and between these rocks and their granitic basement have led previous workers to propose large -scale northward thrusting in an effort to explain the complex structures observed. An alternative hypothesis invoking southward gliding has been considered by several authors but has been assigned a subordinate role in explaining the deformation. The present study was undertaken in an effort to evaluate the relative suitability of dominantly northward versus dominantly southward movement. In the course of this study nine separate localities were examined and mapped in detail in an effort to evaluate movement direction. Evidence of displacement on low -angle faults or glide surfaces was found in each of the areas examined, and definite evidence of movement direction was recognized in five of these. Large-scale recumbent folding, previously unrecognized in the Colossal Cave and Agua Verde Wash areas, was found to be closely related to local uplift. The asymmetry of the folds, plus a very few observed offsets marginal to the uplifts, were the only criteria found for determining the direction of movement. Evidence of northward, southward, and nearly westward movement was found in the course of the study. Areas in the northern Empire Mountains south of Cienega Gap gave evidence of west – northwestward and northward movement. Areas in the southern Rincon Mountains north of Cienega Gap were found to have undergone dominantly southward movement. Involvement of the Pantano Formation in several of the localities suggests that deformation occurred at least as late as early Oligocene time and probably after middle Miocene time. Cienega Gap was therefore the focus of movement for material which was moving laterally away from areas undergoing uplift in Tertiary time.

Golden Gate Mountain appears as a spur projecting westward from the Tucson Mountain range. It is made up of the capping Cat Mountain Rhyolite, the slope - forming Amole Formation, and a variety of intrusions of differing compositions. The emplacement of the andesitic portion of the intrusions occurred during, and probably lasted long after, the deposition of Amole Formation. The hot magma fluidized the wet sediments. Part of the fluidized materials formed pipes and dikes of tuffisites and part was brought up into the basin and contributed to the sedimentation of Amole Formation. During upper Amole time the intrusion of andesite increased in intensity. Part of the basin rapidly subsided and thick deltaic sediments and graywacke were formed. The development of a hinge line accompanied this subsidence. The hinge line controlled the occurrence of fluidization which undercut the Amole beds. The beds slumped into the fluidized parts. The process culminated in forming a large orifice through which the Cat Mountain Rhyolite welled up. The orifice is reflected in the sedimentary beds by the development of a funnel- shaped structure in the central part of which the capping of Cat Mountain Rhyolite is located. The bordering brecciated Amole beds represent the associated slump effects.

The Atlas Mine is located on the northwest flank of the Silver Bell mountains; Silver Bell mining district, Pima County, Arizona. The deposit is high grade (?) sine-copper mineralization in an altered sedimentary sequence. Rocks in the area include Precambrian (?) alaskite; Permian (?) limestone, quartzite and siltstone; Tertiary (?) monzonite, quartz monzonite, quarts latite porphyry and dacite porphyry; and Quaternary alluvium. The limestone has been largely metamorphosed to a mass of tactite, siltstone has been locally metamorphosed to hornfels, and the quartzite has been silicified, locally shattered and altered. Mineralization is related to NE and E-W trending fault systems, and similarly trending intrusive dikes. Predominate ore minerals are sphalerite and chalcopyrite, associated with pyrite, specular hematite and “high temperature" silicates. Copper mineralization is related to the silicified sediments. Zinc mineralization is present in silicates but is more predominate in areas of recrystallized calcite and extensive garnetization, suggesting incomplete replacement of the original sediments by the silicates. Further exploration and development should be undertaken in areas of favorable structural control, and adjacent to favored intrusives.