Tucson, Arizona. The newly introduced “Arizona Sonoran Desert Heritage Act of 2013,” H.R. 1799 by U.S. Representative Raúl M. Grijalva (AZ), would withdraw more than 900,000 acres (~1,400 square miles) of federal lands in the low desert and mountain ranges of western Maricopa County from mineral entry and other uses associated with its current multiple-use status. A new study by the Arizona Geological Survey, “The Sonoran Desert Heritage Proposal: An evaluation of the mineral resource potential of lands proposed for withdrawal from mineral entry,” shows a high potential for substantial economic deposits of sand and gravel, copper, gold, and manganese in parts of the proposed Arizona Sonoran Desert Heritage area (SDH).
In 2011, the Arizona Wilderness Coalition, Sonoran Institute, and The Wilderness Society proposed designating western Arizona public lands as a complex of Wilderness Areas, National Conservation Areas, and Special Management Areas. The result would be to remove these lands from mining and quarrying and ban or limit exploration drilling and geophysical surveying. Lands proposed as part of the SDH are managed by the U.S. Bureau of Land Management (BLM).
Nearly all the lands in the SDH have been mapped by Arizona Geological Survey (AZGS) geologists which provides the basis for an informed evaluation by AZGS Senior Geologist Jon Spencer of the mineral resource potential of the area. Known mineral deposits in the SDH are evinced by the presence of old mines, prospect pits, small mine shafts and adits.
Large desert washes in the SDH are potentially rich sites of high quality aggregate - an essential ingredient in construction of new homes, buildings, roads and other infrastructure. Spencer identified four regions in the SDH with a potential of significant aggregate resources: Gila River below Painted Rock Dam, Gila River near Gillespie Dam, Harquahala Wash south of Saddle Mountain, and Jackrabbit Wash north of the Belmont Mountains.
The potential for encountering precious-metal vein deposits, gold and silver, in the SDH are high in the Harquahala Mountains and Big Horn Mountains. The geology of the two ranges is similar to that of the nearby Vulture and Little Harquahala Mountains, respectively. The Vulture Mountains includes the site of the Vulture gold mine that produced 350,000 ounces of gold and 264,000 ounces of silver with a total estimated value in modern prices of more than $600 million. Saddle Mountain and the Gila Bend Mountains in the southern SDH have a lower potential for substantial gold and silver mineralization.
Arizona hosts one of worlds the great porphyry copper mineral belts. The Big Horn Mountains in the northern SDH straddles the northwestern edge of the this porphyry copper belt and hosts granitic rocks of the same age and composition as those found associated with copper deposits in other parts of the state. Historic mining in the Big Horns produced millions of pounds of copper and lead. Mountain ranges to the south of the Big Horn Mountains, including the Gila Bend and Maricopa Mountains, are less likely candidates for copper mineralization. Young volcanic deposits, however, mask portions of these and other nearby mountain ranges, hampering reliable estimates of the potential for economic mineralization.
At present the U.S. imports manganese, the fourth-most-mined metal and an essential ingredient in steel. Because of its important role in steel production, the National Research Council has labeled it a strategically important metal to U.S. national security. The Aguila manganese mineral district, located in the northern Big Horn Mountains, is included in the proposed SDH, and is one of about two dozen deposits scattered over western Arizona and adjacent areas to the west that make up the largest domestic manganese resource. All the manganese used in the U.S. is currently imported.
Finally, the report notes the inherently speculative nature of mineral-resource estimation, and states the following:
“…changes in technology have made uneconomic deposits economic, and increased the value of previously unattractive commodities. For example, the need for the rare-earth element neodymium for exceptional magnet strength in hard-disk drives, hybrid-car electric motors, and wind-turbine generators has caused a recent surge in exploration activity… Thus, while the estimates of mineral potential may be needed for land-use decisions, they can’t account for unknown future economic conditions or technological advances that affect mineral production processes or commodity value.”
This report is available as a free PDF download at the Arizona Geological Survey Document Repository or as printed copies from the Arizona Experience Store, 416 W. Congress, Tucson, Arizona. Please call 520.770.3500 for pricing.
Citation: Spencer, J.E., 2013, The Sonoran Desert Heritage Proposal: An evaluation of the mineral resource potential of lands proposed for withdrawal from mineral entry: Arizona Geological Survey Open File Report, OFR-13-03, 30 p.
For more information please contact:
Arizona Geological Survey
416 W. Congress, Ste 100
Tucson, AZ 85701
Tucson. Each year 600,000 people visit the variegated badlands and fallen forests of petrified wood at Petrified Forest National Park. A new geologic map and accompanying 18-page report, Geologic Map of Petrified Forest National Park, Arizona, display the distribution and nature of fossil-bearing rocks and sediments within the pre-expansion Petrified Forest National Park boundaries. (In 2004, the U.S. Congress expanded the park, but less than 1/3rd of the expanded lands are controlled by the park.)
The 1:50,000-scale map, where 1 inch ~ 4160 feet or 0.8 miles, is a joint effort of research teams from the U.S. National Park Service (Jeff Martz and William Parker) and Northern Arizona University (Lisa Skinner, Jason Raucci, Paul Umhoefer and Ron Blakey). The report includes 22 photographs of major rock units, a stratigraphic column, and cross-sections showing how the landscape has evolved over the past 65 million years.
Petrified Forest National Park (PEFO) is located in the Little Colorado River Valley, 25 miles east of Holbrook, Arizona. Widespread exposures of sedimentary rocks – shales, siltstones, and sandstones – of the 225 to 207 million years ago (Ma) Chinle Formation dominate the park landscape.
Rocks of the Chinle Formation represent deposits of river systems originating in what is now western Texas and fed by tributary streams from highlands to the south and north of PEFO. The Chinle Formation preserves a suite of lowland terrestrial environments that includes river channels, floodplains, swamps, and small lakes operating in a strongly seasonal subtropical climate.
Locally, rocks of the Miocene-age Bidahochi Formation (8 to 4 Ma) cap the Petrified Forest and Owl Rock Members of the Chinle Formation. Sediments of the Bidahochi Formation were deposited in ancient Lake Hopi, which persisted from about 16 Ma to 4 Ma. To the north, basaltic rocks of the Hopi Buttes volcanic field, emplaced between 8.4 – 4.5 Ma, create maar volcanoes, diatremes and lava flows.
Erosion over the past several million years has scraped off most of the Bidahochi rocks, limiting outcrops to the northern reaches of the Park.
Fossil Organisms of Petrified National Park. Two-hundred and fifteen million years ago, dinosaurs tracked the swampy lowlands and forests of what is now PEFO. As noted in the Martz et al. report, “sandstones of the Sonsela Member are the primary source of petrified wood within PEFO. Wood traditionally assigned to the conifer genus Araucaryoxylon is most commonly found in the Jasper Forest bed, Rainbow Forest Bed, and Black Forest Bed , and the conifer genera Schilderia and Woodworthia are also known from the Black Forest Bed …”. The spectacular “forests” found throughout the park, as well as the spectacular multi-colored trees at Jasper Forest, Crystal Forest, and Rainbow Forest lie within the Jasper Forest and Rainbow Forest Beds.
Among the many vertebrate fossils found in PEFO are bony fish and sharks, therapsids, archosauromorphs and pseudosuchians (crocodylian-line archosaurs) and dinosauromorphs. Invertebrate macrofossils include locally abundant bivalves, gastropods, freshwater crustaceans and trace fossils of a wide variety of insects and other small arthropods.
This illustration, Figure 6 in the report, shows the Newspaper Rock Bed of the Chinle Formation within Petrified Forest National Park.
The map and report are available as free, downloadable PDFs from the Arizona Geological Survey’s Document Repository. Printed copies can be purchased from the Arizona Experience Store, 416 W. Congress, Tucson, AZ 85701; email: email@example.com, phone: 520.770.3500.
Petrified Forest National Park covers ~146 square miles in Navajo and Apache Counties. Park headquarters is about 25 miles east of Holbrook, Arizona. Petrified wood logs and dinosaur fossils weathering out of brightly colored shale and variegated, fine-grained sandstones, evinces a subtropical environment of slow-moving, meandering rivers and streams making the Park one of the premiere sites in the world for understanding life during the Triassic Period.
Activities for park visitors include sightseeing, hiking, photography and backpacking. Average elevation is just over 5,000 feet and temperatures vary seasonally from summer highs of 100 °F to below freezing in winter. Park landscape ranges from flat-topped mesas rising from a broad plain to badlands topography.
Martz, J.W., Parker, W.G., Skinner, L. and Raucci, J.J., Umhoefer,P. and Blakey, R.C., 2012, Geologic Map of Petrified Forest National Park, Arizona. Arizona Geological Survey Contributed Map CR-12-A, 1 map sheet, 1:50,000 map scale, 18 p.
Arizona Geological Survey
416 W. Congress, Ste 100
Tucson, AZ 85701