In 1947, in his presidential address, Earl Noble cited 6 masks that hamper explorationists in their search for oil: water, overthrust blocks, multiple unconformities, high-velocity limestones, younger volcanics, and thick deposits of relatively young clastics. Improved technology, courage, and success have removed some of these masks in many areas of worldwide exploration, but these masks continued to hinder the search in other areas.

Relatively thick sections of valley fill and volcanic cover have been the chief deterrents to exploration in the basins of southern Arizona. Meager geologic information, poor maps, and the public land situation have added to the negative attitude of companies and individuals. But new data are changing the picture.

Although Edwin McKee pointed out 25 years ago that Paleozoic seaways covered most of southern Arizona and that petroliferous rocks may be present in the intermontane valleys, only in recent years has field work by the U.S. Geological Survey confirmed the presence of Paleozoic rocks in some of the upfaulted mountain blocks. Oil and gas shows in thick Paleozoic sections in southwestern New Mexico and northern Mexico have added to the attractiveness of southeastern Arizona.

Other Survey geologists have developed information indicating a marine embayment of Pliocene age, extending into southwestern Arizona and southeastern California and covering about 15,000 sq mi. It may be larger. More surprising has been the discovery that salt domes exists in Arizona and that salt deposits may extend nearly 350 mi along the northern edge of the Basin-Range province. North of Kingman the salt is more than 4,100 ft thick. Near Phoenix several wells have proved the existence of a dome underlying a gravity minimum. Salt thickness is well over 3,600 ft. Near Florence, palynologists date a caprock core overlying salt as Pliocene(?).

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