GRAVITY AND MAGNETIC STUDY OF PETRIFIED FOREST NATIONAL PARK, ARIZONA

A detailed gravity survey was conducted within and surrounding Petrified Forest National Park in northwestern Arizona. Three hundred and twenty-three new gravity stations were collected and merged with existing data to create new complete Bouguer and residual gravity anomaly maps. These maps were interpreted in conjunction with existing aeromagnetic data to determine the subsurface structures within the Park. Residual gravity and magnetic anomaly maps constructed using wavelength filtering (10-90 km band-pass) were found to adequately represent density and magnetic susceptibility changes in the near surface (less than 10 km in depth) that may be correlated to subsurface features with the Park. The residual gravity and magnetic anomaly maps and a two-dimensional gravity model were interpreted to mainly reflect thickness and density differences within the Paleozoic sedimentary rocks found in the Holbrook Basin. Gravity minima in the Painted Desert region are interpreted to reflect thick (0.5 km) deposits of Permian Evaporites (Halite and Gypsum) and deeper depths (1.3 km) to the Precambrian granites and metamorphic rocks. A smaller amplitude gravity minimum along the Park's southern boundary is also interpreted to be caused by thick Permian Evaporites. A gravity maximum in the center of the Park is thought to be caused by the thinnest layer (0.5 km) of Paleozoic sedimentary rocks within the Park. Gravity and magnetic data did not provide evidence of new subsurface igneous intrusions. A possible northeast-trending Precambrian fault was indicated on the complete Bouguer gravity and residual magnetic anomaly map.