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The Geology of West-Central Baja California Sur, Mexico By HUGH McLEAN, B.P. HAUSBACK, and J.H. KNAPP U.S. GEOLOGICAL SURVEY BULLETIN 1579 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987 For sale by the Distribution Branch, Text Products Section U.S. Geological Survey 604 South Pickett St. Alexandria, VA 22304 Library of Congress Cataloging-in-Publication Data Mclean, Hugh, 1939- The geology of west-central Baja California Sur, Mexico. (Bulletin; B-1579) Bibliography Supt. of Docs. no.: 119.3:1579 1. Geology-Mexico-Baja California Sur. I. Hausback, B. P. II. Knapp, j. H. Ill. Title. IV. Series: Geological Survey Bulletin; 1579. QE75.B9 no. 1579 557.3s 85-600359 [QE203.B34) [557.2'24) CONTENTS Abstract 1 Introduction 1 Previous work 3 Acknowledgments 3 Description of rock units 3 Bateque Formation 3 Distribution 3 Lithology 3 Thickness and stratigraphic relations 6 San Gregorio Formation 6 Distribution 6 Age 7 Structure 8 Isidro Formation 8 Distribution 8 Composition 9 Age 9 Sedimentology 9 Comondu Formation 9 Distribution and thickness 9 Sedimentology 10 Composition 11 Age 11 Ground water 12 Miocene volcanic rocks 12 Distribution 12 Composition 13 Age 13 Quaternary volcanic rocks 13 Structural and stratigraphic relations 13 Folds 14 Faults 15 Slump blocks 15 Crystalline basement rocks 15 References cited 15 FIGURES 1. Index map of study area 1 2. Location map of arroyos, roads, and communities in the study area 2 3. Diagram of stratigraphic relations and K-Ar ages of Tertiary rocks of Baja California Sur 5 4-21. Photographs showing: 4. Eocene siltstone of the Bateque Formation 6 5. Porcellaneous shale, diatomite, and vitric tuff of the San Gregorio Formation 7 _, 6. San Gregorio Formation at Arroyo La Purisima 7 7. Folded porcellaneous shale and white vitric tuff of the San Gregorio Formation 7 III 8. Folds in the Isidro Formation and basal ComondG Formation 8 9. Bioturbated sandstone and coquina of the Isidro Formati9n 8 10. Basalt flow interbedded with Comondu Formation at Purisima Vieja 8 11. Dipping marine beds of the Isidro Formation grading upward into nonmarine beds of the Comondu Formation 10 12. Basalt flow interbedded with nonmarine strata of the ComondG Formation 10 13. Examples of facies variation in the Comondu Formation 11 14. Tabular beds of the Comondu Formation grading upward into lenticular sandstone and conglomerate 11 15. Outcrop of polymict andesite-tuff breccia in Arroyo San Pedro 11 16. Columnar flow overlying tabular sandstone of the Comondu Formation 12 17. Capping flow overlyin~ Comondo Formation in Arroyo La Purfsima 13 18. Strata of the Comondu Formation 30 km NW of Loreto 14 19. Overview of Arroyo La Purfsima 14 , 20. Slump block of sandstone of the Comondu Formation in fault contact with beds of the Isidro Formation 14 , 21. Fold in rocks of the Isidro Formation in Arroyo La Purisima 14 TABLE 1. Potassium-argon ages of volcanic rocks, Sierra de La Giganta, Baja California Sur, Mexico 4 IV The Geology of West-Central Baja California Sur, Mexico By Hugh Mclean, B.P. Hausback, and j.H. Knapp Abstract INTRODUCTION The western slopes of the Sierra de La Giganta in The geography of Baja California Sur is the central part of Baja California Sur are composed dominated by an assymetrically sloping northwest of Cenozoic sedimentary and volcanic rocks. Pre trending mountain range known as the Sierra de La Cenozoic granitoid rocks underlie the Sierra, but are Giganta. The crest of the range lies relatively close to not exposed at the surface on the western slopes of the the coast of the Gulf of California and has an average range. No evidence of mineralization was observed elevation of about 1,000 m with peaks as high as 1,800 within the stratified sequence. The oldest rocks are m. Steep east-facing slopes drop precipitously to the middle Eocene marine sandstone, siltstone, and minor Gulf coast, whereas the western slopes grade gently diatomite of the Bateque Formation. Late Oligocene down to the Pacific coastal plain (figs. 1 and 2). strata of the San Gregorio Formation (formerly named the Monterey Formation) consist predominantly of diatomite and subordinate phosphatic sandstone, vitric 113°00' tuff, and porcellanite. Late Oligocene K-Ar ages from the tuffs correlate closely with ages determined by diatom and coccolith assemblages. Early and middle Miocene time is represented by the Isidro Formation, a 27°30' shallow marine sequence that contains an abundant and Santa diverse assemblage of mollusk fossils in beds of Rosalia , San Ignacio coquina and massive bioturbated silty sandstone; the 0 fossils indicate an age of early to middle Miocene. The area is dominated, however, by a sequence of nonmarine arc-derived volcaniclastic sandstone, conglomerate, agglomerate, and breccia named the Comondu Formation that thins and becomes progressively younger from east to west. Near the crest of the Sierra, the Comondu sequence was AREA OF THIS REPORT intruded by stocks of hornblende andesite porphyry, locally dated at 19 Ma. On the western coastal plain, 12- to 14-Ma flows are interbedded with the nonmarine sequence. In this region alkaline basalts that range in age from 14 Ma to about 7 Ma directly overlie the La Puri~ima Comondu Formation. Quaternary volcanism is Punta Pequena o represented by cinder cones and flank flows, small Loreto 0 rounded capping flows, and a few small canyon-filling San Jose flows. de Comondu Perennial supplies of fresh water along the western slopes of the Sierra are restricted to wells and springs that flow frqm the nonmarine sedimentary rocks of the Comondu Formation; the largest springs lie close to the center of the Baja peninsula and may be controlled more by fracture systems than by the distribution of individual permeable beds. Water 20 40 KILOMETERS flowing from springs and wells in marine strata are saline. Figure 1. Index map of study area. 1 27°00' To San Ignacio 27°00' San Jose de Comondu 0 5 10 15 20 25 KILOMETERS Figure 2. Location of major arroyos, roads, and communities in the study area. Dashed lines are graded roads. Numbers correspond to K-Ar localities on table 1. 2 This report describes the geology of the lower Consejo de Recursos Minerales of Mexico and the U.S. western slo[)es of the Sierra de La Giganta between Geological Survey that was initiated in 1981. We latitudes 26° and 27° N. Southwest-trending arroyos thank David G. Howell for his persistence in initiating (fig. 2) deeply incise the westward-sloping plateaus, the Baja Project, and we appreciate his support of our creating excellent laterally continuous exposures of efforts. The senior author thanks B.P. Hausback and slightly deformed Cenozoic sedi:rentary and volcanic J.H. Knapp for their help, companionship, and rocks. Approximately 5,500 km were mapped during scientific expertise in the field, with special thanks to three field seasons from 1982 to 1984; the work Brian Hausback for outstanding work in radiometric focused on the regional distribution of Tertiary age dating. We greatly appreciate the valuable sedimentary and volcanic units, on age relations, and paleontological work of John A. Barron, David Bukry, on sedimentology. The geology was mapped at a scale Gerta Keller, Kristin A. McDougall, and Judith Terry of 1:5 0,000 on provisional topographic maps provided Smith. We also acknowledge James G. Smith, John C. by SPP (Secretaria de Programacion y Presupuesto). Dohrenwend, and Brent Turrin for sharing their Maps were used in conjuction with 1:70,000-scale black unpublished K-Ar ages, and also R.W. Kistler for the and white aerial photographs, and Landsat color Rb-Sr analyses. Tracy L. Vallier helped select images. Our work was part of a program to determine chemical analyses and also helped to evaluate the the framework geology and mineral potential within chemical data. Baja California Sur. Geologic mapping in the area prior to 1982 was highly generalized because accurate topographic base maps were not available and access DESCRIPTION OF ROCK UNITS by vehicle was difficult. This report provides newly acquired radiometric Bateque Formation (Eocene) ages for volcanic flows and tuffs that range in age from 21 Ma to as young as 0.7 Ma (table 1). Distribution Paleontological data from marine fauna and flora provide new age assignments for Eocene, Oligocene, The Bateque Formation (fig. 3) of Mina (1957) and Miocene strata. The effects of local structural crops out continuously from the type area near features, as well as geologic history, and geologic Rancho del Bateque southeastward for 70 km to control of ground-water distribution are also Batequi de San Juan. Isolated outcrops in major discussed. We found no outcrops of pre-Cenozoic crys arroyos extend farther southward for another 45 km to talline rocks west of the crest of the Sierra de La the mouth of Arroyo Mezquital. Petrography and Giganta or any evidence of mineralization associated sedimentology of the Bateque Formation for the area with intrusions into the Cenozoic rocks. We consider between Laguna San Ignacio and Arroyo El Cuarenta the metallic mineral potential of the area to be were discussed by Sorensen (1982), but continuous negligible. outcrops which extend southward from El Cuarenta for 25 km have not been studied previously. The reconnaissance geologic map of McLean and others Previous Work (1985) shows the distribution of the Bateque Formation, as well as the geographic points of Previous geologic reports that involve parts of reference, and is a recommended adjunct to the reader the present area contain descriptions of rock types and of this report. stratigraphic relations that we found to be generally accurate.