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Lower Cretaceous rocks newly discovered in the Little Hatchet Mountains in southwestern New Mexico display an exposed thickness ranging from 17,000 to 21,000 feet. The base of the section is covered by valley fill and the top by Tertiary volcanics. The entire exposed section seems to be of Trinity age, and more than 15,000 feet of it is of Glen Rose or upper Trinity age. The section is noteworthy also (1) for the presence of three disconformities that indicate the removal of thicknesses of rock measurable in thousands of feet, (2) for a high proportion of conglomerate, mostly intraformational, (3) for a repetition of zones of similar lithology and fauna, four-fold with respect to some zones, and (4) for a great thickness of basaltic volcanic rocks. The section has been di ided into seven formational units to which local names have been given; these are, in ascending order, the Broken Jug limestone, the Ringbone shale, the Hidalgo volcanics, the Howells Ridge formation, the Corbett sandstone, the Playas Peak formation, and the Skunk Ranch conglomerate.
The more significant fossils thus far identified include Douvilleiceras, Beudanticeras, Exogyra quitmanensis, a large Pecten that is characteristic of Taff's "Quitman bed," and Orbitolina. The formations as a group are equivalent in age to part of the Bisbee group of Arizona, but specific correlation of individual formations with members of the Bisbee group is not possible. Some of the formations in the Little Hatchet Mountains apparently, however, can be correlated with exposures in the neighboring ranges.
With the exception of those of volcanic origin, the rocks are beach and shallow-water deposits, including some fresh-water beds. They were deposited in a rapidly subsiding basin of geosynclinal dimensions whose shore line was generally near and at times within what is now the north half of the Little Hatchet Mountains, the shore line shifting back and forth over a strip perhaps 10-20 miles wide. The center of volcanism from which the volcanic rocks were derived seems to have been near the edge of the basin and somewhat on the landward side.
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