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The Dos Cabezas Mountains lie in the northeastern most part of Arizona, in Cochise county. They area medium-sized range of some 150 square miles area, and are almost wholly surrounded by unconsolidated basin-fill material. Most of the range is composed of a number of Precambrian igneous and metamorphic rock masses. Its core is composed of a large, complex, terrain of Cretaceous intrusive volcanic breccias and magmatic aphanites. A large number of Laramide and mid-Tertiary intrusive plutons and dikes cut the range. The Precambrian rocks consist of eight granitoid plutons and three areas of phyllitic and argillitic metamorphosed sediments and volcanics. The metamorphic rocks display a primary greenschist facies dynamothermal metamorphic fabric and a later superimposed biotite-forming hornfelsic thermal metamorphic fabric. The metasediments are mostly phyllites and argillites, but contain over 10,000 feet of metaconglomerate showing marked primary cross-bedding. Many of the metamorphic units are weakly metamorphosed volcanic flows or tuff-contaminated fluvial clastic sediments. These rocks are all classified as Pinal Schist, although some may be equivalent Mazatzal Quartzite. The plutons consist of a pre-Pinal-metamorphism dacite porphyry stock, one quartz monzonite gneiss synkinematic with the Pinal dynamothermal metamorphism, and four gneissic quartz monzonite plutons which appear to post-date the Pinal metamorphism and imply a mild tectonic event at about 1450 million years ago, and two large post-kinematic quartz monzonite stocks which are of circa 1400 million years age. One of these latter stocks displays prominent rapakivi texture; this is considered to be the result of normal magmatic crystallization. The texture is caused by reaction breakdown of hornblende to form biotite among crystals floating in the magma, thereby extracting potassium from the magma and temporarily halting potash feldspar crystallization while allowing plagioclase crystallization. Rb-Sr dating of the plutons reveals that one of the older post-Pinal gneisses is 1470 ± 30 m.y. old, while the rapakivi is 1380 ± 30 m.y. old and the other large stock is 1425 m.y. old and has undergone a marked Sr redistribution at 1000 m.y., ago; this thermal event has biased all the Precambrian K-Ar ages in the northwestern part of the range toward 1000 m.y., also. A large complex assemblage of Cretaceous welded intrusive volcanic breccias underlies 17 square miles of the core of the range. They are largely composed of small angular fragments torn from foundering large fragments of surficial andesite flows, sinking in a fluidized bed. The gas source was a crystallizing magma at depth; entrained quartz and plagioclase crystals from this magma appear in the breccia ground mass. The breccias are cut by a large number of small mafic magmatic instrusives. Several large diabasic and quartz dioritic plutons of Cretaceous or Paleocene age appear in the range and mark Laramide plutonism. K-Ar data from all but the northwestern most part of the Precambrian rocks in the range display a remarkably uniform Paleocene age which reflects a Paleocene thermal metamorphism. Mid-Tertiary plutonism is recorded by several mafic dike sets, including one of "Turkey Track" andesite porphyry, a granodiorite stock, and numerous quartz veins. Basin and Range block faulting is not obvious in the range, but may account for its present high-standing nature, especially along the northern range margin. Dynamothermal metamorphism is recorded strongly in the Pinal Schist, and dynamic tectonism, at circa 1450 million years. Thermal metamorphism is recorded at 1000 million years, circa 55 million years, and circa 35 million years. Plutonism is recorded before Pinal metamorphism, during Pinal metamorphism, then over the 1470-1380 million year interval, in the Cretaceous-Paleocene Laramide interval, and in the mid-Tertiary Oligocene-Miocene interval.

The Swisshelm Quartz Monsonite covers about two square miles on the western slope of the Swisshelm Mountains, Cochise County, Arizona. Field observation and petrographic study indicate that the quartz monsonite was derived by differentiation and late-stage alkali metasomation of probably a quartz dioritic magma rich in alkali and volatile constituents. The high concentration of the volatiles is believed to be of great importance in the development of the different facies and rock types. Four different facies of the Swisshelm Quartz Monsonite have been distinguished as (1) the normal facies, (2) the altered facies, (3) the fine-grained facies, and (4) the contact facies. Also, several aplite dikes, local beryl-bearing pegmatite patches, and numerous quartz veins are present and attributed to late magmatic differentiation. Inclusions of an early and late magmatic facies are sparcely disseminated through the quartz monsonite. The Swisshelm Quartz Monsonite magma has intruded and metamorphosed the Upper Paleozoic sediments of the Mace Group as well as the Lower Cretaceous sediments of the Bisbee Group. The metamorphism is of a contact metasomatic type to which the mineralogical and textural changes in the country rocks have been attributed.

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No description available.

A complex late Quaternary alluvial sequence is exposed in Whitewater Draw arroyo, Cochise County, Arizona. The alluvial history is characterized by: (1) sand and gravel deposition in a through-flowing stream between 15,000-8,000 yr B.P. and 6,700-5,500 yr B.P. and (2) cycles of erosion and clay-and-silt deposition in large wet meadows or cienegas between 8,000-6,700 yr B.P. and 5,500 yr B.P.-historic period. Modern arroyo entrenchment began after A.D. 1885 and was largely completed before 1910. The alluvial sequence of the Douglas basin differs in timing, character, and number of degradational and aggradational events, with the exception of the arroyo cutting and filling episode between 6,700 and 5,500 yr B.P., when compared to the alluvial sequence of the adjacent San Pedro Valley and the generalized alluvial chronology for the West. Megafaunal extinction occurred in the Douglas basin no later than 10,400 yr B.P. as evidenced by the occurrence of articulated camel and mammoth remains in sediments of this age. Mammoth, horse, camel, and dire wolf remains from deposits dating 10,400 to 7,000 yr B.P. are in secondary contexts, redeposited from older units. Archaeological remains of the Cochise Culture occur in nearly all the Upper Quaternary deposits of Whitewater Draw. Artifacts of the Sulphur Spring phase, the earliest phase of the Cochise Culture, are found at four sites in Whitewater Draw and at the Lehner site, where they overlie the Clovis horizon. Ground-stone artifacts are the most common element of the Sulphur Spring artifact assemblage and indicate that the Douglas basin was the site of specialized plant gathering and processing. Flaked-stone artifacts are poorly represented and are primarily unifacially retouched flake tools but also include bifacially flaked projectile points. The Sulphur Spring phase dates from 8,000 to 10,000 yr B.P. and probably to 10,500 yr B.P. Evidence suggests that the Sulphur Spring people may have temporally overlapped with relict populations of Pleistocene megafauna during the onset of the Holocene. The Sulphur Spring and western San Dieguito I complex are considered to be temporal equivalents. The Cazador phase is no longer considered a valid phase of the Cochise Culture. Cazador artifacts at the type site occur in deposits equivalent in age to sediments containing Sulphur Spring remains.

No description available.

No description available.

No description available.

No description available.