Geological and geophysical investigations of an Apollo 9 photo anomaly near Point of Pines, Arizona

Abstract

An infrared photograph of southeastern Arizona, taken during the Apollo 9 multispectral terrain photography experiment in 1969, reveals a ringlike feature, some 3-4 miles (5-6 kin) in diameter, on the Natanes Plateau, 35 miles (56 kin) north of the town of Safford. Because the feature occurs in an area of nearly flat lying Tertiary volcanic rocks, the possibilities of its being a small collapse caldera or an exposed circular intrusive body were considered. Geological and geophysical studies of the area were made to test these hypotheses. The local stratigraphic section consists of approximately 1,500 feet (457 m) of Oligocene and perhaps older volcanic rocks, resting on a moderately irregular basement surface carved from nearly flat lying trending Basin-and-Range faults define a broad horst within which two arcuate cross faults, with 300-600 feet (91-183 m) of displacement, bound a downdropped area. Deep erosion along these faults has created a polygonal network of canyons which constitutes the 'ring' seen on the photograph. A mild arching of the volcanic rocks within the ring is suggested by structure contours on the base of the youngest flows. A sharp 350-gamma positive aeromagnetic anomaly is centered within the ring. In its southwest quadrant the anomaly has an elongate extension that trends northwest along an adjoining Basin-and-Range fault. Associated with both is a subtle gravity low. The geophysical data thus suggest the presence of a small blind silicic pluton, possibly of middle Tertiary or younger age. Although it can be argued that the arcuate faults and mild arching of the volcanic pile are related to this postulated pluton, no evidence of hydrothermal alteration or thermal metamorphism of the country rocks was seen. Thus if a pluton is present and of postvolcanic age, it must have been emplaced as a relatively cool dry body; or alternatively, it is older than the surface volcanic rocks. In either instance, its magnetic expression contrasts with that of the known mineralized Laramide porphyry intrusive bodies of the region.