Structure and petrography of the Fraquita Peak area, Santa Cruz County, Arizona Item Type text; Thesis-Reproduction (electronic); maps Authors Reed, Richard Kent, 1942- Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 28/09/2021 17:14:43 Link to Item http://hdl.handle.net/10150/554037 STRUCTURE AND PETROGRAPHY OF THE FRAQUITA PEAK AREA, SANTA CRUZ COUNTY, ARIZONA by Richard K. Reed A Thesis Submitted to the Faculty of the DEPARTMENT OF GEOLOGY In Partial Fulfillment of the Requirements For the Degree of MASTER OF SCIENCE In the Graduate College THE UNIVERSITY OF ARIZONA 19 6 7 STATEMENT BY AUTHOR This thesis has been submitted in partial fulfill­ ment of requirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknow­ ledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the interests of scholarship. In all other instances, however, permission must be obtained from the author. SIGNED: APPROVAL BY THESIS DIRECTOR This thesis has been approved on the date shown below: /ft ' ________ Dr. R. L. DuBois Date Professor of Geology ACKNOWLEDGMENTS The author wishes to thank Dr. R. L. DuBois, who offered many useful suggestions, and who directed the thesis and accompanied the author in the field. Special thanks are also due Dr. J. W. Anthony, who visited the area with the author, and Dr. John Gilbert, for their helpful criticisms and advice. Rolf Erickson, University- of Arizona graduate student, offered valuable instruction in feldspar staining techniques. iii TABLE OF CONTENTS Page LIST OF ILLUSTRATIONS............................. Vi ABSTRACT.......................................... viii INTRODUCTION...................................... 1 Location and Extent of Area..... -1 Topography, Drainage, and Climate Purpose of Investigation........ Method of Investigation......... Previous Work in the Area....... -E-t ONVx -O REGIONAL GEOLOGY........................... 9 DESCRIPTIVE GEOLOGY OF THE FRAQUITA PEAK AREA..... 12 Cretaceous (?) Shale...................... 12 Oro Blanco Conglomerate............... 16 Tertiary (?) Quartz Latite Porphyry........ 24 Rhyolite Dike Rocks........................ 35 Kersantite Dike Rocks...................... 37 POSSIBLE CORRELATIONS............................... 42 Cretaceous Shale........................... 42 Oro Blanco Conglomerate.................... 42 Quartz Latite Porphyry..................... 44 Regional Correlations....................... 45 STRUCTURAL GEOLOGY.................................. 46 Joints...................................... 49 Faults............................... 49 Folds....................................... 49 Stream Directions.......................... 51 Summary and Conclusions................. 51 GEOLOGIC INTERPRETATIONS AND SUMMARY OF THE GEOLOGIC HISTORY.................................. 53 iv V TABLE OF CONTENTS (continued) Page The Oro Blanco Conglomerate................ 53 Summary of the Geologic History........... 58 SUGGESTIONS FOR FURTHER STUDY..................... 61 SELECTED BIBLIOGRAPHY............................. 63 LIST OF ILLUSTRATIONS Figure Page 1. LOCATION MAP OF FRAQUITA PEAK AREA........ 2 2. DETAIL AND EXTENT OF THESIS AREA.......... 3 Plate 1. GEOLOGIC MAP OF FRAQUITA PEAK AREA..in pocket 2. VIEW OF FRAQUITA PEAK AREA................. 11 3* CRETACEOUS SHALE........................... 14 4. CRETACEOUS SHALE........................... 15 5* PHOTOMICROGRAPHS OF CRETACEOUS SHALE......... 1? 6. ORO BLANCO CONGLOMERATE................... 19 7* PHOTOMICROGRAPHS OF ORO BLANCO CONGLOMERATE............................ 21 8. QUARTZ LATITE PORPHYRY..................... 2? 9* PHOTOMICROGRAPHS OF QUARTZ LATITE PORPHYRY................................ 30 10. PHOTOMICROGRAPHS OF QUARTZ LATITE PORPHYRY................................ 31 11. RHYOLITE DIKE............................. 36 12. PHOTOMICROGRAPHS OF RHYOLITE............... 38 13• PHOTOMICROGRAPHS OF KERSANTITE DIKES......... 40 14. POINT DIAGRAM OF SHALE BEDDING............. 4? 15. POINT DIAGRAM OF CONGLOMERATE FOLIATION.... 48 vi vii LIST OF ILLUSTRATIONS (continued) Plate Page 16. ROSE DIAGRAM FOR IGNEOUS JOINT STRIKES.... 50 17. CROSS SECTION in pocketGEOLOGIC ABSTRACT The Fraquita Peak area is located in the Oro Blanco Mountains, about 6 miles south of Arivaca, Pima County, Arizona. The area mapped, covering about 8 square miles, is characterized by Cretaceous (?) sediments and Tertiary (?) intrusive rocks. Five rock types are recognized in the area. From oldest to youngest these are Cretaceous (?) shale, the Cretaceous (?) Oro Blanco Conglomerate, a Tertiary (?) quartz latite intrusive. Tertiary (?) rhyolite dikes, and lamprophyre dikes. The Oro Blanco Conglomerate regionally is a Cretaceous (?) sediment that has been tentatively identi­ fied over wide areas of western Santa Cruz County. Within the thesis area the conglomerate has a strong layering, a result of an alignment of elongate quartz latite fragments, and has the character of an autobrecciated intrusive igneous rock. A very strong northwest structural trend is observed in the area. It is reflected by the alignment in the conglomerate, bedding of the shale, the outcrop pattern of the quartz latite, and joint directions in the viii quartz latite. This is a common direction found to be present in pre-Laramide rocks in southern Arizona. The quartz latite is believed a Tertiary (?) shallow intrusive that was intruded along a northwest- trending pre-Laramide zone of weakness. INTRODUCTION Relatively little is known about the geology of western Santa Cruz County, Arizona. Regional studies (Taylor, 1959) have suggested broad similarities and possible correlations of volcanic sequences throughout Santa Cruz County and into the Tucson Mountains. Detailed local studies are now needed in key areas of Santa Cruz County before further regional generalizations can become meaningful. A detailed study of the igneous and sedi­ mentary rocks of the Fraquita Peak area was begun partially to fill this need. The area mapped meets one crucial and often overlooked requirement: It is not so small and geologically complex that a solution of its problems de­ pends upon, rather than contributes to, a complete understanding of the regional geology. Location and Extent of Area The Fraquita Peak area is located in the Oro Blanco Mountains about five miles south of the town of Arlvaca, Pima County, Arizona (Fig. 1). The area is part of the Coronado National Forest. Approximately eight square miles were studied and mapped, including sections 8, 16, 1?, 20, and 21 of T. 22 S., R. 11 E. Figure 2 shows the detail and extent of the area mapped. 1 .FLAGSTAFF ARIZONA PHOENIX TUCSON o 5 10 Arivaca Jci Pima r.n. Santa Cruz Co- Figure 1. Location Map of Fraqulta Peak Area 3 ' ! \ , Coronado Naftlonal A FoViest Pima Co. _ Santa Cfuz Co. 17 R10E Figure 2. Detail and Extent of Thesis Area 4 The area is accessible year-round, except immediately after heavy rains; and may be reached by driving 37 miles south on U. S. Highway 89 to Arivaca Junction, and then following the Arivaca Road southwest for 22 miles to Arivaca. An improved dirt road leads south from Arivaca to the Tres Bellotas Ranch on the Mexican border, and passes through the area. Other unimproved dirt roads lead to grazing lands and deserted mining prospects in the thesis area, but these roads are passable only to trucks or vehicles with four-wheel drive. Topography, Drainage, and Climate The Oro Blanco Mountains, which include Fraqulta Peak, form with Cobre Ridge a northwest-southeast trending range. The maximum relief of the area is 1,510 feet, with the higher elevations toward the center of the area. Fraqulta Peak is the highest point in the area, with an altitude of 5*370 feet. The lowest point in the area, 3,860 feet, occurs where Fraqulta Wash leaves the mapped area to the north. The many gullies which drain Fraqulta and Yellow Jacket Peaks flow either into Fraqulta Wash which bounds the area to the south, west, and northwest; or into Yellow Jacket Wash to the north and east of the area. Both Fraqulta Wash and Yellow Jacket Wash flow north to join Arivaca Greek near Arivaca. Arivaca Creek is an intermittent stream, except near Arivaca, where a 5 spring yields some year-round surface flow. Arivaca Creek flows westward into the Altar Valley. The streams and gullies of the area often follow contacts and joint directions. The topography of the area is rather rugged. The highest peaks in the area, Praquita and Yellow Jacket Peaks, are composed of quartz latite porphyry. The Oro Blanco Conglomerate, which outcrops in the area, is also quite resistant, and forms hills and steep slopes. The only non-resistant rock in the area is a shale of Creta­ ceous (?) age, which forms valleys. Basic dikes cut across the area and are a little less resistant to weathering than the quartz latite or conglomerate