Empire S -Galbraith

Empires-Galbraith-21 THE EMPIRE MOUNTAINS, PIMA COUNTY, ARIZONA F. W. Galbraith University of Arizona INTRODUCTION The Empire Mountains occupy about 30 square miles in southeastern Pima Coun ty (fig. 48) , and consist of two parallel ridges trending north-no rtheast. They are connected with the Santa Rita Mountains to the west by low rolling foothill s and are separated from the Whetstone Mountains to the east by the broad floor of Cienaga Wash valley. The Empire Mountains ( fig. 23) are made up of marine limestone, shale, and quartzite of Cambrian, Devonian, Mississippian, Pennsylvanian, and Permian age, aggreg ating app roximately 5, 700 feet in thickness, and a series of Cretaceous (?) continental clastic deposits possibly 18, 000 feet in thickne ss. The sedimentary rocks are intruded by stock-like bodies of quartz monzonite and granodiorite and by dike s ranging in composition from rhyolite to basalt. The rang e has two structural parts - an underlying block of Cretaceous (?) rocks, and an overthrust block of Paleozoic and Cretaceous (?) rocks which is divided into four segments by northwe ste rly striking tear-faults. The thrust fault is exposed along the we stern edge of the mountains and dip s to the east at alow angle. Within the overthrust block there are at least three separate imbricate thrust sheets. Dome s, anticlines, and overturned folds have been formed in the Paleozoic rocks. STRATIGRAPHIC UNITS Bolsa ( ?) Quartzite A small area of dense, glas sy quartzite is expo sed between the Pantano Hill stock and the Abrigo fo rmation in the northern part of the rang e. Although the ex posure is only a few feet in thicknes s, it is considered to be Bolsa quartzite because it is similar in appearanc e to typical Bolsa quartzite and lies confo rmably below the Abrigo fo rmation. Upper Cambrian Ab rigo Formation The Abrigo formation consists of soft, light gray to blue layers of limestone, alternating with thin beds of green shale, light gray sandstone, and brown to black chert. Beds are generally from one- half to three inche s thick, lenticular, and their exposures have a characteristic mottled appearance. In some places the top of the formation is marked by a 5 to 6 foot bed of pure white quartzite. Maximum exposed thickne ss is 750 feet. Upper Devonian Martin Limestone The Martin limestone is compo sed of finely crystalline limestone, gray and moderately thick-bedded at the base, and thinne r-b edded and le ss resistant to ero l sion toward the top. The top of the formation in the northern part of the rang e is marked by a ho rizon composed largely of Cladopora prolifica. In the southern part of the rang e a zone of soft, shaly beds ha s been interpreted as the top of the Devonian l section. Metamorphism ha s transformed large masses of the lower limestone s into l [ 127 EXPLANATION AlI"',m. �\rC\� / o EI Quartz monzonile. }! Small intrusivu, rhwo1ite 10 basalt. � k�:a Creloceousrock,. }!� l' Ep,'aph, S""", Coooha a" Rain VoUey formations }� �ES?'B. Earp, Colina , and Andrada formatlon_, undlfferentloted � } � """ Hla Lim"'a,,. ..... N Escabrosa Limestone. }� 00 � Mortin }� I!I Limestone. ca A'd,a Lim"'a,,. } A l Contact. } Faull, do ,hid where approXimately located. Thrust fault, dashed where A' opprOlimolely localed. Barbs on side of uwer •. plot Strike and dip of bids. �" ,. , . ".,,,,,,,,[' RJ 000 � fut , I 4000 2000 SCALE4000 6000I FIGURE 23. Generalized geologic map and cross section of the Empire Mountains, Cochise County, Arizona. - Empires-Galbraith- 21 sparkling, sugary to coar sely crystalline marble. The thicknes s ranges from about 300 feet in the southern part to about 100 feet in the northe rn part of the rang e. Lower Mis sissippian Escabrosa Limestone The Escabrosa lime stone is expo sed almost continuously throughout the length of the rang e, but is unusual in that it is not a cliff-former in thi s area. The Esca brosa consists of light gray to blue, fine-grained limestone beds one to two feet thick. Metamorphism ha s chang ed much of the fo rmation to white, coarsely-cry stalline marble; which clos ely resembles the marbleized limestone i:il the Martin, and makes difficult the separation of Mississippian and Devonian limestones whe re the Cladopora prolifica ho rizon is not present. The Escabrosa rang es in thickne ss from 200to 60 0 feet, thickening from south to north. Pennsylvanian Horquilla Formation A disconformity, marked in places by a thin bed of conglomerate separates the Horquilla from the underlying Escabrosa. The Horquilla generally consists of mas sive, blue-gray limestone alternating with soft calcareous shale. Individual beds are from 10 to 100 feet thick. Limestone beds are thicker than shal e beds and stand out on weathered slopes, giving a coarsely banded appearance to the fo rmation as a whole. In the northeastern part of the Empires, however, the lower part of the Horquilla formation consists of alternating limestone and quartzite beds, and shale predominate s in the upper part. The limestone beds are one to three fe et thick, fine grained, dark blue, and contain irregular che rt inclusions. The quartzite beds are light gray or white on fresh surfaces and weather to buff. Metamorphism ha s obliterated the bedding and blue-gray color of the limestone beds and chang ed the shal e to hard, resistant, ho rnfelb-like masses. The Horquilla is nearly 1, 200 feet thick in the central part of the range but thins no rthward to approximately 500 feet. Pennsylvanian and Permian An drada Formation The Earp and Colina fo rmations defined by Gilluly, Cooper, and Williams (1954) , cannot be recognized in the Empire Mountains, and the term Andrada formation has been used by Wilson, E. D. (1951), Bryant (1955), and othe rs fo r the undifferen tiated equivalent beds. In the Empire Mountains the Andrada fo rmation consists mainly of soft shale and marl, with some dense, dark blue-gray lime stone from 10 to 200 feet thick, and, in the upper part, three beds of gypsum from 5 to 50 fe et thick. Many beds of the Andrada fo rmation exhibit a complete change in lithologic character within a few hundred fe et along strike. The fo rmation as a whole is soft, and erosion ha s carved the main no rth- south wash from the se unresistant beds. Locally, metamorphism along intrusive contacts ha s converted part of the An drada formation to a fi ssile, light gray, sericite schi st which resembles the Pre camb rian Pinal schist. Where the fo rmation lies at greater distanc es from intrusive bodie s, the limestone ha s been marbleized, and the shal e has been indurated or si licified to ho rnfels-like masses which closely resemble the metamorpho sed Ho r quilla. The Andrada formation ranges in thickne s s from 300 to 1, 500 feet. It is thicke st 129 in the central part of the range and thins to the no rth. PerIll ian Epitaph, Scherrer, Concha, and "Rainvalley" ForIllations The undifferentiated Epitaph, Scherrer, Concha, and "Rainvalley" forIllations, froIll 1, 250 to 2, 250 feet in thickness, are the IllOSt proIll inent rocks in the EIllp ire Mountains. They fo rIll the cre sts of the highe st ridges and the ir western slopes are usually sheer cliffs that are visibl e for Illiles. Although the se fo rIllations, defined by Gilluly, Cooper, and WilliaIll s (1954), can be readily recognized, they have been Illapped as a unit. The Epitaph consists of a series of thick, Illa ssive, fetid liIlle stone beds 90 to 300 feet thick and forIll s the proIllinent we stward facing cliff s along the highe r parts of the range. The Scherrer fo rIllation is Illade up of two gray, vitreous, orthoquartzite IlleIllbers separated by a Illassive, blue, doloIllitic liIllestone which usually contains irregular Illa sses of silic ious Illaterial. The lower quartzite Ill eIllber rang es in thickne ss frOIll 90 to nearly 800 feet; the Illiddle doloIllitic liIllestone Ill eIllb er is froIll 30 to 150 feet thick; and the upper quartzite Ill eIllber is froIll 30 to 250 feet thick. The thickne sses are greater in the southe rn part of the range, but the great chang e over relatively sho rt di stanc es is believed to be due to unrecognized low angle thrust faulting . The Concha and "Rainvalley" fo rIllations Illake up an unbroken series of liIllestone beds above the Scherrer, and fo rIll the eastern slopes of the highest ridges. The Concha Illaintains a thickne ss of about 500 to 600 feet and con sists of dark gray to blue liIllestone beds which are thick, Illassive, and fetid, and I contain abundant chert nodules in Illany places. The "Rainvalley" consists of light gray doloIllitic liIlle stone which is thinner bedded than the Concha, and a few thin beds of brown to buff quartzite. I Cretaceous (?) Beds [ Cretaceous (?) rocks are divided into two Illaj or units, tho se on the east and tho se on the west sides of the thrust fault. The lack of fo ssils and Illarker beds prevents correlation across the th rust. The strata on the west side of the fault f are furthe r subdivided into a northern and southe rn serie s, separated by a high angle fault. The se strata are considered to be of probable Cretaceous ag e, because of their lithologic siIllilarity to rocks in the Santa Rita Mountains (Schrader, 19 15; I Stoyanow, 1949). On the east side of the EIllpire Mountains a series of congloIllerate, shale, sand I stone, and liIllestone beds lies unconfo rIllably upon a deeply eroded surface cut on Paleozoic rocks.

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