Cenozoic Rocks of the Santa Rita Mountains, Southeast of Tucson, Arizona by HARALD DREWES
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Cenozoic Rocks of the Santa Rita Mountains, Southeast of Tucson, Arizona By HARALD DREWES GEOLOGICAL SURVEY PROFESSIONAL PAPER 746 A systematic descTiption) augmented by 19 mdiometTic age deteTminations) of one of the most complete sequences of Cenozoic Tocks in southeasteTn ATizona UNITED STATES GOVE R NMENT PRINTING OFFICE, WASHINGTON: 1972 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress cata log-card No. 72-600158 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D .C. 20402 - Price $1.50 (paper cover) INTRODUCTION 3 which lie between the Santa Rita Mountains and the some of the intrusives of group 3 are probably as Patagonia Mountains to the southeast (fig. 1). A sociated with the magmatic events of the Helvetian small part of the northeast end of the Santa Rita phase, or late phase, of the Laramide orogeny. They Mountains extends outside the Sahuarita quadrangle, are briefly described in this report, to preserve the however, and is not described here. continuity of the discussion of geologic events, but The geologic units described here are all younger inasmuch as they are ancillary to the major plutonic than the Piman phase (Drewes, 1969), or main masses of the Laramide, they will be described more phase, of the Laramide orogeny. They consist of fully in another report I am preparing on the (table 1) (1) some volcanic and clastic rocks de plutonic rocks. posited between the two local phases of the Lara The geologic investigation of the Santa Rita Moun mide, during Paleocene (?) time, (2) several dike tains is part of a larger program of the U.S. Geo swarms and associated plugs, of Paleocene age, (3) logical Survey to map and interpret the geologic stocks of granitoid rocks and intrusives of quartz history of all the ranges i'oughly between Tucson latite porphyry, also of Paleocene age, (4) quartz and Bisbee (fig. 1). Many geologists are involved veins, volcanics, plugs, and dikes, of Oligocene to in this program, and most of their field studies are Paleocene age, (5) volcanics, laccoliths, and dikes, completed. Cooper (1970) has mapped the Sierrita of late (?) Oligocene age, and (6) assorted gravel Mountains; Finnell (1971)", the Empire Mountains; deposits and soils flanking the mountains. At least Creasey (1967), the Whetstone Mountains; R. B. TABLE 1. - S mnrnal'y of Cenozoic rocks of the Santa Rita Mountains [ Broken lines indicate that the adjacent formations are contemporaneous or that the ir relations are uncertain] Age Rock unit Description Thickness (feet) Youngest gravel Gravel a nd intercalated sand on floor s of m odern wa tercou rses. 0-100± Cobble to pebble gravel and inter calated sand; capped by infa ntile 0-200± Holocene Gravel facies gray soil. Low-level terrace ----- depos its Silt and loam of the Santa Cruz River terrace ; capped by infantile 0-200± Silt facies gray soil. ~ capped by " Late Intermediate-level pediment and Boulder to p ebble gravel, grit. and intercalated sand; 0-400± "c.> terrace deposits weakly developed thin reddish-brown soil. ~ '0; Boulder to pebble gravel and intercalated sand; locally cem ented; 0-50± 0: Middle High-level pediment deposits capped and overlapped by well-developed thick red soil. Piedmont fac ies Cobble to pebble gravel. grit, and inter calated sand a nd silt; com- Pleistocene m only pinkish gray, slightly indurated, and locally well cem ented. 0-2,00O± and Bas in-fill gravel ---- - - Pliocene - - Ri-ver- fac-ies - W ell-sor ted cobble to pebble gravel and sand. locally cem ented. 0-300± Tuff beds V ery Ji g ht g ray rhyolitic tuff and tuffaceous sandstone. 0- 20 Pliocene Cobble to pebble g ravel. which is rich in volcanic clasts, and tuffa- and Gravel of N ogales ceous sand and s ilt, w hich is pale red to brownish g ray, poorly 0-1,000± Miocene sorted. a nd slightly indurated. Rhyolite intrus ives of the northern Rhyolite porphyry of a plug and of two dike swarms; locally a Santa Rita Mountains brown v itrophyre. - - - ------- - Dikes a nd stock of the Rhyodacite porphyry dike s warm and m oderately coarse grained :5~" San Cayetano Mountains light-gray granodiorite. 'QP; - -Dik-es a----nd laccoliths in-- the Rhyodacite v itrophyre intrus ives, medium-gray to light-gray. Late ( ? ) ~ E ~ Grosvenor Hills area ...... ): .... ------- Oglicene - - Rhyodacite sU3~ m ember Rhyodacite Java fl ows, agglom erate, tuff, and welded t uff. 150-2,200 8 E6 Grosvenor ;g~~ Hills Rhyolite 500+ m ember Rhyolite tuff and a little welded tuff and lava. o~ V olcanics "~ 0" Gravel a nd Pale-red s ilt and gravel ,,,ith well-rounded pebbles; includes som e ~ rn 0- 200 s ilt member shale and f resh-water limestone. Quartz vein swarm Mineralized quartz veins of the southern Santa Rita Mountains. Oligocene Rhyolitic volcanics Rhyolitic tufT a nd lava( ?) of W asp Canyon. 0-100 to Paleocene Olivine andesite p lugs V esicular a nd a m ygdaloidal olivine andesite of Deering Spring. A nd esitic dikes Andesite. dacite. a nd diorite dikes, s ills, and some small pipes. Quartz latite porphyry dikes and plugs. light-gray to g rayish- orange-p ink ; phenocrysts; GreatervilIe intrus ives contains stubby bipyramidal quartz associated with base- and noble-metals enrichment ; plugs arc am eboid s haped. Medium-dark-gray m edium-coarse-grained quartz diorite. Paleocene H elvetia ~uartz~ i o rite_ Lig ht-gray m ed ium-coarse-grained stocks of granodiorite to quartz stocks Granodiorite monzonite com position; elliptical shaped. Cottonwood Canyon dike swarm Quartz latite porph yry, fin ely porphyritic to coarsely porphyritic. Li g h t~br own i s h.gray very fine grained g ranodiorite in core of a Plugs of Gringo Microgranodiorite plug. Gulch area -- - - - Dacite porphyry H ornble nde dacite, m edium-dark-gray, locally coarsely porphyritic. V olcanics of Red Mountain Intensely altered rhyolitic a nd andesitic pyroclastic rocks. - ---- --[----- - Rhyolitic to dacitic tuff, w elded tuff, tuffaceous sandstone, and a Paleocene ( ?) Gringo Upper member capping andes ite lava flow. 900 + Gulch . V olcanics Lower m ember Rhyolitic a nd dac itic pyroclastic rocks and som e fl ows; contains intercalated epiclastic rocks. 700 + 4 CENOZOIC ROCKS OF THE SANTA RITA MOUNTAINS, ARIZONA Raup (unpub. data, 1971), the Canelo Hills; Simons and Mesozoic history. The basic data appear on the (1971a, b), the Patagonia Mountains; Hayes, the geologic maps, first open-filed in 1966 and 1968 and Huachuca Mountains, and Raup, the Mustang Moun now published (Drewes, 1971a, c), which will be tains (Hayes and Raup, 1968); and Hayes and referred to frequently. Some collateral reports on Landis (1964), the Mule Mountains. geochemistry of the area and on stratigraphy are ACKNOWLEDGMENTS now or will soon be available (Drewes, 1967, 1968, 1971b; 1972a, b; Hayes and Drewes, 1968; Simons The collateral work produced through a larger and others, 1966). program of the U.S. Geological Survey has provided The oldest known rocks of southeastern Arizona both supporting and restraining data for the inter are the Pinal Schist, which generally includes some pretations of the Cenozoic geology of the Santa Rita gneiss and migmatite. The schist is most extensive Mountains. However, the Cenozoic record is less ly exposed and best studied in the Little Dragoon complete outside the Santa Rita Mountains than Mountains, where Cooper and Silver (1964, p. 11- within them, so the validity of the interpretations 23) described it in considerable detail as a sequence on correlation and environments of deposition has at least 20,000 feet thick of metamorphosed con generally not been widely tested. Nevertheless, some glomerate, sandstone, siltstone, shale, and rhyolite of these interpretations were developed jointly with and basalt lava flows. They further described the my colleagues, whose assistance is gratefully ac major aspect of this sequence as a "cyclic gray knowledged, and will also be applied to geologic wacke-slate lithology," which contains "graded bed situations in adjacent areas. The interpretations ding, the intercalated lava flows, and the abundant here are thus not necessarily the results solely of my volcanic debris." L. T. Silver (oral commun., 1968) efforts; yet the validity of their local application dated a rhyolite flow as 1,715±10 m.y. old using the remains my responsibility. U-Pb isotopic method on a suite of zircon samples. The success of the field investigation was facIl The thickness and lithology of the Pinal Schist were itated by many people. Invaluable assistance in map interpreted by Cooper and Silver (1964) as indic ping and sampling was provided by G. C. Cone, ative of geosynclinal deposits. Some aspects of this Bruce Hansen, C. W. Norton, F. W. Plut, J. R. geosyncline were reviewed by Anderson (1951, Riele, R. A. Rohrbacker, Albert Sutheimer, and W. p.1345). M. Swartz between 1963 and 1968. Discussions in In the Santa Rita Mountains, schist and gneiss the field with R. E. Wallace, M. D. Crittenden, Jr., that have been correlated with the Pinal Schist form J. H. Courtright, and P . H. Pickard helped to clarify inclusions, roof pendants, and remnants of wall some problems and to raise others for which I am rock of large granitoid plutons of Precambrian and particularly grateful. The courtesies of George younger ages. The schist and gneiss are inconse Bradt, Roy Green, Dewey Keith, and George Yakob quential as detrital components of the Cenozoic ian, residents near the Santa Rita Mountains, and clastic deposits of the mountains. of Profs. John Anthony, D. L. Bryant, Evans Mayo, Extensive masses of coarse-grained and porphy and S. R. Titley, at the University of Arizona, and ritic alkali granite, quartz monzonite, or grano of E.