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 

IV 

1. Potassium-argon ages of volcanic rocks, Sierra de La Giganta, Baja California Sur, 

Mexico 4 
The Geology of West-Central Baja 

California Sur, Mexico 

By Hugh Mclean, B.P. Hausback, and j.H. Knapp 

Abstract 

The western slopes of the Sierra de La Giganta in 

the central part of Baja California Sur are composed 

of Cenozoic sedimentary and volcanic rocks. PreCenozoic granitoid rocks underlie the Sierra, but are 

not exposed at the surface on the western slopes of the 

range. No evidence of mineralization was observed 

within the stratified sequence. The oldest rocks are 

middle Eocene marine sandstone, siltstone, and minor 

diatomite of the Bateque Formation. Late Oligocene 

strata of the San Gregorio Formation (formerly named 

the Monterey Formation) consist predominantly of 

diatomite and subordinate phosphatic sandstone, vitric 

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 

shallow marine sequence that contains an abundant and 

diverse assemblage of mollusk fossils in beds of 

coquina and massive bioturbated silty sandstone; the 

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 

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 

Comondu Formation. Quaternary volcanism is 

represented by cinder cones and flank flows, small 

rounded capping flows, and a few small canyon-filling 

flows. 

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 

flowing from springs and wells in marine strata are 

saline. 

INTRODUCTION 

The geography of Baja California Sur is 

dominated by an assymetrically sloping northwesttrending mountain range known as the Sierra de La 

Giganta. The crest of the range lies relatively close to 

the coast of the Gulf of California and has an average 

elevation of about 1,000 m with peaks as high as 1,800 

m. Steep east-facing slopes drop precipitously to the 

Gulf coast, whereas the western slopes grade gently 

down to the Pacific coastal plain (figs. 1 and 2). 

27°30' 

113°00' 

San Ignacio 0 

Santa 

Rosalia , 

AREA OF THIS REPORT 

La Puri~ima Punta Pequena o 

0 

San Jose de Comondu 

20 40 KILOMETERS 

Figure 1. Index map of study area. 

Loreto 

1 
2 

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. 
This report describes the geology of the lower 

western slo[)es of the Sierra de La Giganta between 

latitudes 26° and 27° N. Southwest-trending arroyos 

(fig. 2) deeply incise the westward-sloping plateaus, 

creating excellent laterally continuous exposures of 

slightly deformed Cenozoic sedi:rentary and volcanic 

rocks. Approximately 5,500 km were mapped during 

three field seasons from 1982 to 1984; the work 

focused on the regional distribution of Tertiary 

sedimentary and volcanic units, on age relations, and 

on sedimentology. The geology was mapped at a scale 

of 1:5 0,000 on provisional topographic maps provided 

by SPP (Secretaria de Programacion y Presupuesto). 

Maps were used in conjuction with 1:70,000-scale black 

and white aerial photographs, and Landsat color 

images. Our work was part of a program to determine 

the framework geology and mineral potential within 

Baja California Sur. Geologic mapping in the area 

prior to 1982 was highly generalized because accurate 

topographic base maps were not available and access 

by vehicle was difficult. 

This report provides newly acquired radiometric 

ages for volcanic flows and tuffs that range in age 

from 21 Ma to as young as 0.7 Ma (table 1). 

Paleontological data from marine fauna and flora 

provide new age assignments for Eocene, Oligocene, 

and Miocene strata. The effects of local structural 

features, as well as geologic history, and geologic 

control of ground-water distribution are also 

discussed. We found no outcrops of pre-Cenozoic crystalline rocks west of the crest of the Sierra de La 

Giganta or any evidence of mineralization associated 

with intrusions into the Cenozoic rocks. We consider 

the metallic mineral potential of the area to be 

negligible. 

Previous Work 

Previous geologic reports that involve parts of 

the present area contain descriptions of rock types and 

stratigraphic relations that we found to be generally 

accurate. Many of the early workers traveled with 

pack animals and their mapping was restricted by the 

sparse distribution of water. Reports that describe 

parts of the geology of our study area were published 

as early as 1882 by Willian M. Gabb; followed later by 

Darton (1921), Heim (1922), Beal (1948), Mina (1957), 

McFall (1968), Chavez (1978), Gastil and others (1979), 

and Sorensen (1982). Our field observations confirm 

many of the ideas presented by the previous workers. 

Our geologic mapping connects the area to the north 

and east mapped by Sawlan and Smith (1984), and the 

area to the south mapped by Hausback (1984). 

Geologic maps that cover much of the area described 

in this report have been recently published by McLean 

and Hausback (1984), and by McLean and others (1985), 

and are essential adjuncts for readers of this report; 

many of the geographic names used in this report are 

located on the two maps referred to above. 

Acknowledgments 

Mapping and sampling were done under the 

auspices of a cooperative agreement between the 

Consejo de Recursos Minerales of Mexico and the U.S. 

Geological Survey that was initiated in 1981. We 

thank David G. Howell for his persistence in initiating 

the Baja Project, and we appreciate his support of our 

efforts. The senior author thanks B.P. Hausback and 

J.H. Knapp for their help, companionship, and 

scientific expertise in the field, with special thanks to 

Brian Hausback for outstanding work in radiometric 

age dating. We greatly appreciate the valuable 

paleontological work of John A. Barron, David Bukry, 

Gerta Keller, Kristin A. McDougall, and Judith Terry 

Smith. We also acknowledge James G. Smith, John C. 

Dohrenwend, and Brent Turrin for sharing their 

unpublished K-Ar ages, and also R.W. Kistler for the 

Rb-Sr analyses. Tracy L. Vallier helped select 

chemical analyses and also helped to evaluate the 

chemical data. 

DESCRIPTION OF ROCK UNITS 

Bateque Formation (Eocene) 

Distribution 

The Bateque Formation (fig. 3) of Mina (1957) 

crops out continuously from the type area near 

Rancho del Bateque southeastward for 70 km to 

Batequi de San Juan. Isolated outcrops in major 

arroyos extend farther southward for another 45 km to 

the mouth of Arroyo Mezquital. Petrography and 

sedimentology of the Bateque Formation for the area 

between Laguna San Ignacio and Arroyo El Cuarenta 

were discussed by Sorensen (1982), but continuous 

outcrops which extend southward from El Cuarenta for 

25 km have not been studied previously. The 

reconnaissance geologic map of McLean and others 

(1985) shows the distribution of the Bateque 

Formation, as well as the geographic points of 

reference, and is a recommended adjunct to the reader 

of this report. 

Lithology 

Fine-grained sandstone and coarse-grained 

siltstone compose most of the Bateque Formation; 

mudstone is rare. Color is mainly yellowish brown to 

grayish brown, and induration ranges from moderate to 

friable. The formation is easily eroded where not 

capped by basalt. The petrographic study done by 

Sorensen (1982) indicates that the sandstones are 

te.xturally immature, moderately to poorly sorted, and 

range from arkose to lithic arkose; the average 

composition is Q12F65L23, where feldspar (F) is 

mainly plagioclase and lithic fragments (L) are mainly 

silicic and intermediate-composition volcanic rocks. 

Most grains are angular, indicating that the sediments 

were only minimally exposed to stream- and wavedominated environments enroute to the site of 

deposition, although some of the angularity might be 

due to secondary porosity. A granitic source terrane 

for the sandstone is inferred mainly from the presence 

of grains of plagioclase which lack oscillatory zoning, 

but display intergrowth twinning and are similar to 

plagioclase in granitic rocks of the peninsular 

3 
Table I.--Potassium-argon ages of volcanic rocks, Sierra de La Giganta, 

Baja California Sur, ~xi co 

[* K-Ar age determinati?n by J.G. Smi~h; **, K-Ar age determination_~0 John Dohrenwend a~~ Brent Turrin. 

Ages calculated us1ng the followmg constants:\~ = 0.581 x 10 yr, ic 6 = 4.962 x 10 yr] 

Map Sample No. Rock Material Percentage 40Ar rcr~~entage Age in million Stratigraphic and 

No. locality type dated potassium X 10-ll years before location comments 

(W.R., ±standard mol/g present, ±2 

whole rock) deviation standard deviations 

83BSJZ41 

1* 26°51'08"N. Basalt W.R. 0.129 0.1827 7.0 8.1±0.4 Capping flow, 

112°59'55 w. ±0.0025 Mesa El Yeso 

83BSJ244 

2* 26°43'40"N. Basalt W.R. 0.146 0.2655 24.0 10.5±0.4 Capping flow, 

112°59'15"W. ±0.0026 Mesa Las Gallinas 

384-6-1 

3** 26°48'05"N. Basalt W.R. 4.07 6.08 16.3 8.6±0. 7 Canyon-filling flow, 

112°46'07"W. ±0.064 Arroyo Patrocinio 

384-6-2 

4** 26°44'52 "N. Basalt W.R. 2.82 3.61 45.8 7 .4±0.3 Small capping flow, 

112°45'40"W. ±0.054 Mesa El Bule 

384-6-3 

5** 26°45'00"N. Basalt W.R. 0.052 0.2.33 11.4 8.8±1.2 Canyon-filling flow, 

112°45'55"W. ±0.009 Arroyo San Pedro 

384-7-1 

6** 2.6°51'20"N. Basalt W.R. 1.728 3.975 78.7 13.2±0.6 Interbedded flow, 

112°36'58"W. ±0.039 Arroyo San Pedro 

384-7-2. 

7** 26°52'35"N. Andesite W.R. 1.360 3.265 74.4 13.8±0.6 Monomict 

112°34'18"W. ±0.032 breccia clast, 

Arroyo San Pedro 

384-8-1 

8** 26°49'21"N. Basalt W.R. 1.57 2.855 36.8 10.4±0.5 Capping flow, 112°39'16"W. ±0.036 Arroyo San Pedro 

384-13-3 

26°31'02"N. Tuff Glass 4.48 1.17 73.5 21. 9±0.8 Tuff of San Gregorio ll2°30'00"W. ±0.866 Formation, Arroyo 

San Raymundo 

384-15-1 

10 26°34'22"N. Basalt P1ag. 1.158 2.61 71.3 12.9±0. 7 Interbedded flow, 112°24'51"W. Arroyo San Martin 

384-11-3b 

11 ** 26°22'45"N. Pillow W.R. 0.250 4.658 50.7 10. 7±1.14 Pillow flow, 112°36'12"W. basalt ±0.013 Arroyo San Raymundo 

12* z6°22'18"N. Data incomplete Canyon-filling flow, ll2°30'58"W. Arroyo Cadeje 

101983-4 

13** 26°25'00"N. Basalt W.R. 1.410 0.188 94.4 0.7±0.1 Small capping flow, 112°29'00"W. Arroyo Cadeje, west side 

101983-3 

14** 26°25'00"N. Basalt W.R. 1.638 0.292 88.7 0. 9±0.08 Small capping flow, 11Z0 27'56"W. Arroyo Cadeje, east side 

383-11-1 Transitional 

15 26°18'41 "N. alkalic W.R. 1.267 Interbedded flow, 112°10'11"W. basalt ±0.052 3.189 57.4 14.5±1.2 Arroyo San Gregorio 

383-10-1B 

16 26°29'08"N. Alkalic W.R. 1.803 3.492 56.9 11.1±0.8 Canyon-filling flow, 112°07'40"W. basalt ±0.041 Arroyo Guajademi 

383-10-1A Calc-alkalic 

17 26°29'08"N. basalt W.R 0.948 2.052 55.5 12.4±0.4 Basal capping flow, 112°07'40"W. ±0.013 Arroyo San Gregorio 

383-9-1 

18 26°15'32"N. Alkalic W.R. 2.585 3.747 56.2 8.3±0.3 Capping flow between • 

112°05'58"W. basalt ±0.042 Paso Hondo and La Purisima 

383-8-1 

19 26°16'36"N. Alkalic W.R. 2.176 0.3270 4.6 0.8±0.1 Cinder cone flank flow, 

112°02'18"W. basalt ±0.092 west of La Purisima 

383-10-3 Alkalic 

20 z6°27'57"N. basalt W.R. 2.13 2.2.34 11.0 0.6±0.06 Canyon-fill flow, 

112°13'18"W. ±0.09 Arroyo Agua Verde 

383-17-4 

21 26°09'56"N. Basalt W.R. 1.658 4.253 63.8 14.7±0.7 Capping flow south of 

112°01'02"W. ±0.038 Arroyo La Purlsima 

4 
K-Ar AGE UNIT AGE 

(Ma) 

Cinder cones and flows Pleistocene 

Pliocene 

5 

10 

Capping lavas 

Miocene 

15 

Comondu Fm. 

20 Isidro Fm. 

25 San Gregorio Fm. 

Oligocene 

27 

~~~~--1 

Bateque Fm. Eocene 

Figure 3. Generalized stratigraphic relations and K-Ar ages of Tertiary rocks of Baja California Sur. Dated 

samples shown by an x. 

5 
batholith. Microcline (also a granitic component) is 

present in small percentages. 

Diatomite is found in isolated outcrops in Arroyo 

San Raymundo, about 2 km north and 200 m south of 

La BaHena (a small ranch). Here, grayish-white 

laminated diatomite and siliceous shale overlie 

yellowish-brown fine-grained arkosic sandstone. 

Diatomite samples studied by J.A. Barron indicate a 

late middle Eocene age; this diatomite represents a 

new facies of the Ba.teque Formation. Eocene 

diatomite also crops out in the south bank of Arroyo 

Patrocinio and along coastal bluffs a few kilometers 

north of the mouth of Arroyo Patrocinio. 

The diatom assemblage from Arroyo San 

Raymundo strongly resembles the flora of the 

Kreyenhagen Formation exposed near Coalinga, 

California (McLean and others, 1985). The Ba.teque 

Formation in Arroyo San Raymundo lies north of 

latitude 26°15' N. and is the southernmost outcrop of 

Eocene diatomite known along the west coast of North 

America. The outcrop latitude corresponds closely to 

the southern limit of modern diatoms along the Pacific 

coast. The presence of late middle Eocene diatomite 

in central Baja California Sur may provide a geologic 

constraint for postulated post-Late Cretaceous 

northward movement of the Peninsular terrane that is 

suggested by the paleomagnetic data of Hagstrum and 

others (1985). 

Previous work by Mina (1957) and Sorensen (1982) 

indicates that the age of the Bateque Formation based 

on the foraminiferal assemblage ranges from early 

Eocene at the base, to middle Eocene near the top. 

Mixed shallow- and deep-water faunas indicate middleto upper-bathyal water depths. Shallow-water 

megafossils also suggest downslope transport of 

sediment (Sorensen, 1982). 

The geologic map of Mina (1957) shows flatlying, yellowish-brown, fine-grained sandstone and 

Discocyclinid-bearing siltstone exposed on the south 

side of the mouth of Arroyo Mezquital, as the 

Tepetate Formation of Eocene age (fig. 4). We suggest 

that these beds should be mapped as the Bateque 

Formation, because the nearest outcrop of the 

Tepetate Formation is more than 200 km to the south 

(Knappe, 1974). 

Thickness and stratigraphic relations 

The total exposed thickness of the Bateque 

Formation is probably between 500 and 800 m. Most 

dip angles are less than 5° and nowhere exceed 10°. In 

the northernmost part of the the San Isidro 1:250,000 

quadrangle (north of Mesa El Yeso), the Bateque 

Formation is overlain by basalt-rich gravels of late 

Tertiary or Quaternary age. But at the mouth of 

Arroyo Patrocinio the Eocene strata are overlain by 

the Isidro Formation consisting of 5 to 10 m of lightgray coquina and calcareous shallow-marine sandstone 

of early or middle Miocene age, which is in turn 

capped by laterally extensive basalt that yields K-Ar 

ages of about 8.1 Ma (table 1, No. 1). At the bottom 

of Arroyo San Raymundo, the Bateque Formation is 

locally overlain by siliceous and phosphatic shale, 

diatomite, and tuff of the late Oligocene San Gregorio 

6 

Figure 4. Eocene siltstone of the Bateque Formation 

overlain by fossiliferous beds of the Isidro Formation 

and capped by alluvial gravel. Outcrop shown is 

located 2 km upstream from mouth of Arroyo 

Mezquital. 

Formation. Most outcrops of the Bateque Formation, 

however, are overlain by the Isidro Formation, 

indicating that the San Gregorio was extensively 

eroded prior to deposition of the shallow-marine 

Miocene beds of the Isidro Formation. The tectonic 

significance of these stratigraphic relations is 

discussed in a later section. 

San Gregorio Formation (Oligocene) 

Distribution 

The San Gregorio Formation of Beal (1948) is 

characterized by small outcrops along the banks and 

stream bottoms of the largest arroyos between Arroyo 

San Raymundo in the north and Arroyo La Pur1sima in 

the south (figs. 5, 6, and 7). A thickness of 72 m was 

measured in the bottom of Arroyo La Pur1sima near 

the base of El Pilon, just upstream from the town of 

La Purisima. Laminated, white-weathering diatomite, 

porcellaneous shale with brown fish scales, and dark 

gray phosphatic shale are interbedded with pelletoidal 

phosphatic sandstone and white vitric tuff. Phosphatic 

rocks weather white, but are dark gray on fresh 

surfaces and release a fetid petroleum-like odor when 

broken. X-ray analyses of siliceous porcellanite 

indicates that the silica phase is opal-CT (cristobalitetridymite), rather than quartz. These rocks were first 

named "Monterey-beds" by Darton (1921), and then 

Monterey Formation by Heim (1922) based on the 
Tuff 

Figure 5. Porcellaneous shale, diatomite, and vitric 

tuff of the San Gregorio Formation. East bank of 

Arroyo La Pudsima 16 km downstream from La 

Purfsima. 

superficial resemblance to the Miocene Monterey 

Formation of California. Strata correlated with the 

San Gregorio Formation also crop out about 300 km 

south of La Purfsima at San Hilario and along the west 

side of the Bay of La Paz at San Juan de la Costa 

(Hausback, 1984). 

Diatoms and nannofossils both indicate a late 

Oligocene age for the San Gregorio Formation. K-Ar 

ages from tuff interbeds range in age from 27 to 22 Ma 

Figure 6. San Gregorio Formation on east bank of 

Arroyo La Pur!sima between towns of La Pudsima and 

San Isidro. 

Figure 7. Folded porcellaneous shale and white vitric 

tuff of the San Gregorio Formation unconformably 

overlain by light-colored shallow-marine sandstone of 

the Isidro Formation and incised by a channel filled 

with nonmarine sandstone and conglomerate of the 

Comondu Formation. East bank of Arroyo San 

Raymundo about 16 km upstream from the main road 

to San Jose de Gracia (table 1, No. 9). 

and correspond to the ages of the fossils (McLean and 

others, 1984). The K-Ar age for the youngest tuff 

indicates that the formation may extend into early 

Miocene time (table 1, No. 9). 

Diatom assemblages identified by J.A. Barron 

contain Rocella gelida (Mann) Bukry, indicative of a 

late Oligocene age. The diatom Rocella gemma 

reported in the San Gregorio Formation by Hanna 

(1930) is synonymous with R. gelida. Benthic diatoms 

and resting spores increase upward through the section 

exposed at the type locality in Arroyo San Gregorio, 

indicating possible shoaling of the section as deposition 

proceeded. The concept of late Oligocene shoaling is 

supported by the inferred shallow water depths of the 

overlying Isidro Formation, whose age is probably early 

Miocene at its base. 

Rare molds of planktic foraminifers were found 

(probably Globigerina bulloides, or G. ouachitaensis); 

although they are not age diagnostic, they may be 

indicative of either a late Eocene or Oligocene age. 

However, the assemblage suggests upper bathyal water 

depths of 200-1,500 m. The residue of most samples 

processed for extraction of forams contains a few 

gastropods, ostracods, glauconite, and phosphate 

nodules (G. Keller and K.A. McDougall, written 

commun., 1983). 

Coccoliths from the type locality of the San 

Gregorio Formation on the east bank of Arroyo San 

Gregorio are sparse and moderately etched, but the 

assemblage contains seven genera, including 

Discoaster. These coccoliths indicate a late Oligocene 

age and open marine conditions (D. Bukry, written 

commun., 1982). 

The distribution of late Oligocene strata in Baja 

California Sur provides some insight into the 

paleogeography of the time. Open marine conditions 

are indicated by the rocks in the La Purisima and San 

Hilario areas (fig. 2), whereas shallow marine and 

nonmarine conditions are indicated by strata exposed 

7 
Figure 8. Gentle folds in light-colored Isidro 

Formation and basal part of the Comondu Formation, 

unconformably overlain by younger fluvial gravels of 

the upper part of the Comondu Formation. Location is 

8 km downstream from La Purisima; viewed toward 

the southeast. 

along the east side of the southern Sierra de La 

Giganta at San Juan de la Costa (lat 24°20' N.) 

(Hausback, 1984). Subaerial ash-flow tuffs at Punta 

San Telmo (lat 25°15' N.) and on the west side of the 

Bahia Concepcion peninsula yielded K-Ar ages of 28 

Ma (McFall, 1968; Hausback, 1984). The distribution 

of these rocks indicates westward deepening of the 

late Oligocene basin between 25° N. and 26°30' N. 

latitude, and that a shoreline may have trended 

northwest nearly coincident with the present crest of 

the Sierra de La Giganta. 

Structure 

The structure of the San Gregorio Formation is 

locally disharmonic with respect to overlying and 

underlying formations. Attitudes vary from gentle 

open folds to locally complex small-scale slump folds 

with steep near-vertical dips (figs. 5 and 6). The 

irregular distribution of the complex folding and the 

absence of folding in underlying strata suggests a 

nontectonic origin. The deformation may be related to 

compaction resulting from silica-phase changes during 

diagenesis of diatomaceous sediment (Issacs, 1981). 

Isidro Formation (Miocene) 

Distribution 

The Isidro Formation of Heim (1922), informally 

named the "Yellow beds" by Darton (1921), is easily 

distinguished from other formations in the area by its 

light-yellowish-gray (almost white) color, and by 

massive, commonly bioturbated sandstone and white 

coquina containing abundant shallow-marine 

mollusks. From the type locality in Arroyo La 

Purfsima (figs. 8 and 9), the Isidro Formation extends 

8 

Bu 

Figure 9. Heavily bioturbated sandstone and coquina 

of the Isidro Formation. West bank of Arroyo La 

Purlsima at road crossing approximately 6 km 

northeast of La Purlsima. 

to the northwest and southeast. To the north, the 

formation apparently interfingers with the slightly 

younger San Ignacio Formation of Mina (1957) and 

Smith (1984). To the east, yellowish Isidro beds 

interfinger with brownish and grayish nonmarine 

siltstone and sandstone of the Comondu Formation, a 

predominantly nonmarine volcanic-derived sequence 

widely exposed throughout the Sierra de La Giganta. 

Marine beds of the Isidro Formation interfingering 

with nonmarine beds of the Comondu Formation are 

well-exposed at Pur1sima Vieja in Arroyo San Gregorio 

(fig. 10). Outcrops of the Isidro are also reported far 

to the southeast in the San Juan de la Costa area and 

Figure 10. Basalt flow interbedded with Comondu 

Formation at Purisima Vieja, Arroyo San Gregorio. KAr age of flow is 14.5±1.2 Ma (table 1, No. 15). Lightcolored marine beds at base of section belong to the 

Isidro Formation. 
near Punta San Telmo on the Gulf coast (Beal, 1948; 

Escandon 1978; Hausback 1984). 

In the northern part of the study area, between 

Arroyo Patrocinio and Batequi de San Juan, the Isidro 

disconformably overlies the Eocene Bateque Formation 

and is capped by late Miocene basalt flows that yield 

K-Ar ages of 8 and 10 l\Ia (table 1, Nos. 1 and 2). 

Composition 

Typical lithologies of the Isidro Formation 

include calcareous sandstone and siltstone, lightgrayish-green porcellaneous shale, white coquina, and 

yellowish pebble conglomerate. Because sandstone 

beds in the Isidro Formation are in part derived from 

older strata, the composition varies from arkosic to 

volcanic lithic. North of Arroyo San Raymundo, the 

sandstone composition reflects reworking of the 

underlying Bateque Formation, but the Isidro 

Formation sandstone is readily distinguished 

petrographically by rounded bone fragments and 

phosphatic grains that are recycled from sandstone of 

the San Gregorio Formation. Reworking of Eocene and 

Oligocene sedimentary detritus into the Miocene 

sandstones indicates uplift and erosion of pre-Miocene 

beds in earliest Miocene time. Also in Isidro time, 

active volcanism to the east shed volcaniclastic debris 

westward that was locally incorporated into beds of 

the Isidro Formation. The diversity in sandstone 

composition reflects a lithologically diverse 

provenance. 

Several species of Pecten and Turritella are the 

most abundant molluscan fossils in the Isidro 

Formation, in addition to a large assortment of less 

numerous bivalves and gastropods. Other fossils 

include echinoids (sand dollars), small shark teeth, rare 

large shark teeth ( Charcharodon m egalodon, related to 

the great white shark), teeth and dense tusk-like bones 

of Desmostylus (a marine mammal that resembles the 

modern sea cow), and a variety of barnacles. Much 

data on the molluscan paleontology of the Isidro 

Formation have been published over the past 60 

years. Most workers suggest that the fossil 

assemblage indicates an early to early middle Miocene 

age (Hertlein, 1925; Hertlein and Jordan, 1927; Smith, 

1984). For an excellent summary of paleontologic 

reports prior to 1927, refer to Hertlein and Jordan 

(1927' p. 613-615). 

Radiometric ages of tuff beds in the underlying 

San Gregorio Formation and of the overlying basalt 

flows indicate an early Miocene age for the Isidro 

Formation in the La Purfsima-Cadeje area. The K-Ar 

data are consistent with the paleontologic age 

assignments. For example, the youngest tuff in the 

San Gregorio Formation is about 22 Ma, whereas the 

oldest of the overlying basalt flows are about 14 Ma 

(table 1, No. 15). In the Patrocinio area, the oldest 

capping basalts are 8 to 10 Ma. The marine fossil 

assemblage in the San Ignacio Formation (north of the 

study area along the Arroyo San Ignacio) appears to be 

slightly younger than the assemblage in the type area 

of the Isidro Formation (Smith, 1984). 

Sedimentology 

The assortment of lithofacies and assemblage of 

fossils in the Isidro Formation are consistent with an 

open estuarine environment, an environment that 

might have closely resembled that of the present 

Laguna San Ignacio. The diverse molluscan fauna of 

the Isidro Formation indicates a tropical shallow-water 

environment, and extensive bioturbation in sandstone 

and siltstone beds attests to a highly active bottomdwelling infauna (fig. 9). Beds locally contain upright 

burrow casts that are as large as 3 em wide and 9 em 

long. Many of the molluscan species have tropical or 

Caribbean affinities, and some have north-Pacific 

affinities (Smith, 1984). 

Coquina beds vary widely in fossil content and 

each bed tends to contain a dominant genus. Some 

beds consist mainly of Pecten shells that in turn 

overlie a bed of mainly Turritella, which in turn 

overlies a bed that contains mainly oyster shells. 

Unbroken shells in beds of coquina indicate subdued 

wave and current action, in certain areas. A second 

type of coquina consists of comminuted fossil hash in 

laminated planar beds, an assemblage that may reflect 

a beach environment. 

Comondu Formation (Miocene) 

Distribution and Thickness 

The Comondu Formation, named by Heim (1922) 

for exposures of sedimentary rock in the walls of 

Arroyo Comondu near the town of San Jose de 

Comondu, is the most widely exposed Neogene 

formation in Baja California Sur and the Sierra de La 

Giganta. Comondu strata in the study area consist of 

diverse litho facies, including fluvial sandstone and 

conglomerate, lacustrine mudstone, sedimentary 

breccia (volcaniclastic debris flows), volcanic breccia, 

and rare ash-flow tuffs. The lithofacies interfinger 

and each may be locally predominant, with the 

exception of the mudstone and tuffs. North of Mulege 

and south of San Ignaeio, rocks that are coeval with 

the Comondu Formation consist of andesitic volcanic 

breccia and lava flows (Sawlan and Smith, 1984). In 

the western part of the study area, the formation 

interfingers locally with marine beds of the Isidro 

Formation (fig. 11). In two localities, Comondu 

sandstone and mudstone is interbedded with flows of 

basalt (4 km south of Paso Hondo in Arroyo San 

Gregorio, and in Arroyo San Martin 20 km north of the 

confluence with Arroyo Cadeje, figs. 10 and 12; table 

1, Nos. 10 and 15). Much of the Comondu Formation is 

overlain by basalt flows that tend to increase in 

number and thickness eastward toward the crest of the 

Sierra de La Giganta. The thickness of the Comondu 

Formation increases from a feather-edge in the west 

to at least 1,800 m along the crest of the Sierra. 

McFall (1968) reported a total thickness for his 

Comondu Group of more than 13,000 feet ( 4,000 m) in 

9 
Older terrace 

gravel 

Figure 11. Gently dipping marine beds of the Isidro 

Formation grading upward into brown nonmarine beds 

of the Comondu Formation and unconformably overlain 

by younger flat-lying fluvial sandstone and 

conglomerate of the Comondu Formation. West bank 

of Arroyo La Purisima 5 km downstream from town of 

La Purfsima. 

the Bahia Concepcion area. His geologic cross 

sections, however, show only about 8,000 feet (2,500 

m), and if allowances are made for interfingering 

stratigraphic relations, total thickness would decrease 

to approximately 2,000 m. Bed thickness, overall 

coarseness, and the ratio of conglomerate and breccia 

to sandstone increase eastward toward the core of the 

volcanic arc that produced the voluminous 

volcanogenic debris that composes the Comondu 

Formation. The volcanic core of the Comondu is a 

zone of coarse-grained andesi tic breccia, flows, and 

coeval hypabyssal intrusives that trend approximately 

north-south along the crest and eastern margin of the 

Sierra de La Giganta (Hausback, 1984). 

Previous workers do not agree on whether the 

volcanic rocks (mainly capping and canyon-fill flows of 

andesite and basalt) are part of the Comondu 

Formation or are part of separate formations. 

Different aspects of the nomenclature problem 

associated with the Comondu Formation and the 

volcanic flows are discussed by Hausback (1984), and 

Sawlan and Smith (1984). We prefer to follow the 

nomenclature suggested by Hausback (1984), that the 

name Comondu be restricted to "the Miocene arcderived silicic and intermediate-composition 

volcaniclastics that make up the bulk of the Sierra de 

La Giganta and excludes the late middle and late 

Miocene and younger capping flows related to post-arc 

rifting" (Hausback, 1984, p. 226). However, the 

lithologic complexity of coeval rocks in the Bahia 

Concepcion area supports the nomenclature used by 

McFall (1968) in the Bahia Concepcion area. McFall 

elevated the name Comondu to group status and 

subdivided the group into several formations. 

Reconnaissance photogeology of the north part of the 

study area indicates that sedimentary strata of the 

Comondu are interbedded with monomict breccia that 

10 

omondu 

Fm . 

Figure 12. Basalt flow about 3 m thick, interbedded 

with nonmarine strata of the Comondu Formation; the 

flow has a K-Ar age of 12.9±0.7 Ma (table 1, No. 10). 

West bank of Arroyo San Martln 21 km upstream from 

its confluence with Arroyo Cadeje. 

Sawlan and Smith (1984, p. 239) would probably include 

in their informal unit called the andesite of Sierra 

Santa Lucia. 

Sedimentology 

Sedimentary facies of the Comondu Formation 

show rapid lateral and stratigraphic variation; lenticular sandstone and conglomerate often grade into 

evenly bedded tabular sandstone and conglomerate 

(figs. 13 and 14). Lenticular sandstone and 

conglomerate dominate the exposures of Arroyo La 

Purisima near the town of La Pur1sima. In the same 

arroyo, however, about 8 to 10 km upstream from the 

community of San Isidro, the Comondu is mainly 

composed of tabular-bedded sandstone and 

conglomerate with laterally persistent wedge-shaped 

beds; these beds probably represent deposition by 

streams that flowed from an alluvial fan onto an 

alluvial plain with an estuarine embayment. Massive 

beds composed of poorly sorted matrix-supported 

conglomerate and andesitic breccia are well-exposed 

in the upper part of Arroyo San Pedro in the northern 

part of the area (fig. 15). The thickness and large 

clast sizes reflect proximity to active volcanic centers 

probably located east of the Sierra crest near the 

latitude of Mulege. 

Flow directions determined from cross 

stratification and channel orientations suggest 

southwest paleoflow along a system of coalescing 

alluvial fans. Environments of deposition include 

braided and meandering streams, as well as massive 

debris flows, or lahars. Breccia of both sedimentary 

and volcanic origin is also present in the Comondu 

section. 
Figure 13. Example of facies variation in the 

Comondu Formation. Volcanic boulder conglomerate 

overlying tabular sandstone and conglomerate stringer 

illustrates variation in sedimentary facies within the 

Comondu Formation; hammer for scale at bottom of 

photograph. Outcrop shown is in Arroyo San Martin, 

11.5 km upstream from its confluence with Arroyo 

Cadeje. 

Composition 

Sandstone in the Comondu Formation in the 

study area is mainly volcaniclastic and is composed of 

framework grains of distinctly zoned and partially 

resorbed plagioclase, andesitic and vitrophyric lithic 

,fragments, clinopyroxene (mostly augite), and both 

Figure 14. Tabular beds of the Comondu Formation 

grading upward into lenticular sandstone and 

conglomerate; interpreted as stream deposits. Section 

shown is about 16 m thick and is located in Arroyo 

Cadeje approximately 10 km upstream from the 

confluence with Arroyo San Martin. 

Figure 15. Outcrop of polymict andesite-tuff breccia 

located in Arroyo San Pedro near locality 6 shown on 

figure 2. Volcanic breccia in the Comondu Formation 

becomes predominant in highlands of the Sierra de La 

Giganta. 

green hornblende and oxyhornblende. Also present are 

small percentages of quartz and phosphatic grains and 

shell fragments derived from reworked underlying 

units. The grains are typically subrounded and 

unaltered, cemented by varying proportions of clay 

minerals and zeolites. 

One facies of Comondu sandstone exposed in 

arroyos Cadeje, San Martin, and San Raymundo has a 

distinctive bluish gray color that was first noted by 

Beal (1948, p. 7 4). The petrography of the sandstone 

indicates no unusual framework minerals; we suspect 

that either a clay mineral or mixture of clay and 

zeolite minerals in the matrix creates the bluish 

color. The bluish beds lie between brownish sandstone 

and conglomerate sequences, which results in three 

recognizable lithofacies of Comondu strata in the 

Cadeje area. 

The age of t he Comondu Formation was 

determined from K -Ar ages obtained from volcanic 

rocks that underlie, overlie, and are interbedded with 

nonmarine sedimentary rocks. Hornblende andesite 

porphyry intrudes the Comondu outside of the study 

area (between Loreto and San Javier), providing a 

control for minimum ages. Newly reported K-Ar ages 

are listed in table 1. These and previously reported 

ages of Gastil and others (1979) and Sawlan and Smith 

(1984) provide fresh insight to the areal and 

stratigraphic-age variation of the Comondu 

Formation. The picture that emerges from the 

radiometric data is that the Comondu is as old as 20 

Ma and no younger than 8 Ma, and that between 26° 

and 27° N. latitude the formation is older along the 

summit of the Sierra de La Giganta than along the 

Pacific coastal plain, and older in the south than in the 

11 
north. The long span of time represented by the 

Comondu is consistent with observed lithologic and 

stratigraphic complexities, such as interfingering with 

early and middle Miocene marine strata, and 

intraformational angular unconformities. 

Two basalt flows that are interbedded with 

sedimentary rocks of the Comondu Formation yield 

ages of about 13 and 14 Ma (table 1, Nos. 10 and 15). 

The younger flow is located on the west side of Arroyo 

San Martin at latitude 26°35' N. and lies stratigraphically in the upper one-third of the exposed 

Comondu Formation. The older flow is exposed on the 

west bank of Arroyo San Gregorio at Pur!sima Vieja 

(abandoned settlement and palm oasis located 4 km 

south of the town of Paso Hondo); the flow was dated 

previously at 13.2 + 1.9 Ma by Gastil and others 

(1979). The Pur1sima Vieja flow is stratigraphically 

positioned in the lower one-third of the exposed 

Comondu Formation and lies about 20 m above a 

fossiliferous sandstone tongue of the Isidro Formation. 

The section that includes sandstone, 

conglomerate, and volcanic-breccia members of the 

Comondu Formation and forms the escarpment 18 km 

west of Loreto has a minimum age of 20 Ma 

established by a K-Ar age assigned to a hornblende 

andesite porphyry stock that intrudes the section and 

forms the prominent peak of Cerro de las Parras. 

Gastil and others (1979, p. 847) report that the section 

west of Loreto at the base of the grade to San Javier 

contains hornblende andesite breccia of 17 to 23 Ma 

but give no analytical data. By comparison, the 

Comondu section exposed in Arroyo San Pedro near lat 

27° N. yields K-Ar ages (from the base of the exposed 

section) of approximately 13 Ma (table 1, Nos. 6 and 

7). Along the Pacific coastal plain, Comondu strata 

are overlain by capping flows of alkalic basalt that 

range in ag~ from 8 to 14 Ma (table 1, Nos. 1,2, 18, and 

21). The ages of the capping basalts suggest that most 

of the Comondu had been deposited by late Miocene 

time. Ages of lavas interbedded with Comondu strata 

range from 13 to 14.5 Ma and record a time dominated 

by fluvial sedimentation punctuated by sporadic calcalkalic and alkalic volcanism (table 1, Nos. 6, 7, 10, 

and 15). 

In summary, the Comondu Formation exposed 

along the western slopes of the Sierra de La Giganta 

was formed mainly by sedimentary processes 

associated with erosion of a calc-alkaline volcanic 

massif with breccia derived from local vents, and by 

local alkalic volcanic flows. Ages of andesite breccia 

and flows of alkalic basalt overlap from about 14 to 12 

Ma. Most of the Comondu sequence was deposited 

before the outpouring of mesa-capping flows between 

12 or 13 Ma and 7 or 8 Ma. Radiometric ages of flows 

that overlie the Comondu Formation on the Pacific 

coast plain are several million years younger than ages 

of rocks that intrude the formation along the crest of 

the Sierra de La Giganta. 

Ground Water 

Most of the fresh water springs, seeps, and 

shallow hand-dug wells flow from the Comondu 

Formation. Traverses along the major arroyos in the 

region indicate that strata of the Isidro and San 

12 

mondu 

Fm. 

Figure 16. Columnar flow overlying tabular sandstone 

facies of the Comondu Formation. East wall of Arroyo 

La Pur1sima 11 km upstream from San Isidro. 

Gregorio Formations are relatively impermeable to 

ground-water flow, and that existing springs and wells 

are often located close to the basal contact of the 

Comondu with the underlying formations; examples of 

this phenomenon are located, from north to south, at 

Rancho Patrocinio, San Jose de Gracia, Arroyos San 

Raymundo, San Martin, Cadeje, Mezqui tal, and San 

Gregorio. 

The largest fresh water springs are located in the 

upper parts of Arroyo San Martfn and Arroyo La 

Purfsima. Both springs lie close to the geographic axis 

of the Baja peninsula and flow southwest toward the 

Pacific Ocean, supplying irrigation water for 

downstream agriculture. 

Miocene Volcanic Rocks 

Distribution 

Lava flows cap much of the highland area of the 

Sierra de La Giganta, as well as several of the broad 

mesas in the coastal area, including Mesa El Yeso, 

Mesa Las Salinas, Mesa Las Gallinas, and Mesa La 

Cordillera (see McLean and others, 1985, for 

geographic locations). Most flows overlie sedimentary 

rocks of the Comondu Formation (figs. 16 and 17) but 

the coastal mesa-capping basalts also overlie the Isidro 

and Bateque Formations. In the vicinity of Punta 

Pequefia, lava flows extend to the shoreline and pillow 

lava is well exposed in the sea cliffs at the fishing 

village of San Juanico. 

In the highlands of the Sierra de La Giganta, the 

capping flows reach thicknesses of as much as 400 m. 

Sections of flow rocks tend to thicken toward the crest 

of the Sierra, but most flows do not extend eastward 

as far as the crest. A series of flows appears to have 

flowed southwest from a source north of Arroyo San 

Raymundo. South of Arroyo San Martin, another series 

flowed southwestward from near the crest of the 

Sierra. Capping flows are absent in the highlands 

between Arroyos San Raymundo and San Martfn; the 
G 

Isidro 

Fm. 

Figure 17. Erosional remnant of a capping flow 

overlying Comondu Formation on El Pilon in Arroyo La 

Pur1sima. Beds of the San Gregorio Formation crop 

out along the stream bottom behind palm trees. 

distribution of flows shows clearly on color Landsat 

photos of the region. 

The steplike distribution of basalt flows along 

many of the major arroyos gives the impression that 

the flows are interbedded with the sedimentary rocks 

of the Comondu Formation. However, close field 

inspection, confirmed by radiometric ages, indicates 

that the topographically highest flows are older than 

the canyon-filling flows at lower elevations. Eruption 

of the younger flows appears to have occurred after 

the present arroyo system began to form because the 

arroyos control the distribution of the flows. 

Flows with roughly circular outcrop patterns 

overlie the Comondu Formation in the northern part of 

Mesa El Bule. These flows are similar to flows of 

Pleistocene age near Cadeje that form Cerro La 

Piedra Rodada (table 1, Nos. 13 and 14), but the Mesa 

El Bule flows are several million years older with a KAr age of 7.4 Ma (table 1, No. 4). 

Composition 

Chemical analyses indicate that most of the 

middle and late Miocene flows are alkalic basalts 

(Demant, 1975; Gastil and others, 1979; Hausback, 

1984; and Sawlan and Smith, 1984). Many of the 

coastal mesa-capping flows are low-potassium tholeiitic 

basalt (Sawlan and Smith, 1984). Flows in the study 

area tend to be fine-grained, holocrystalline, and 

slightly porphyritic. Phenocrysts (without respect to 

age or chemistry) include zoned calcic plagioclase, 

pigeonite, augite, olivine, and opaque oxides. 

Accessory minerals include phlogopi te, oxyhornblende, 

and bronzite (orthopyroxene). Secondary minerals 

commonly include iddingsite that replaces olivine, 

zeolites that fill vesicles, and calcite that locally 

replaces groundmass glass. Some flows have trachytic 

textures that are highlighted by flow-oriented 

plagioclase microlites, and some flows have felted 

textures with randomly oriented microlites. 

The oldest mesa-capping flows that we have 

dated are about 14 Ma. A slightly weathered flow 

located on the plateau just to the south of San Isidro 

and Arroyo La Pur!sima yields a K -Ar age of 14.7 Ma 

(table 1, No. 21). This age is supported by another KAr age (14 Ma) perhaps from the same flow located 

approximately 3 km farther west (Sawlan and Smith, 

1984). A basal flow from the capping sequence 

exposed on the east wall of Arroyo Guajademi, located 

about 11 km upstream from San Juan de Guajademi, 

yields an age of 12.4 Ma (table 1, No. 1 7) and a 

topographically lower canyon-filling basalt yields a 

slightly younger age of 11.1 Ma (table 1, No. 16). Four 

basalt flows that cap the coastal mesas between 

Arroyo Patrocinio and Arroyo San Raymundo yield 

ages that average about 8 Ma (table 1, Nos. 1, 2, 3, and 

4). 

The distribution of K-Ar ages and the fact that 

most flows cap Comondu sections indicates that most 

of the Comondu sedimentary apron was deposited prior 

to about 14 Ma, and that during the period between 14 

and 12 Ma volcanism was contemporaneous with the 

latest phases of nonmarine (Comondu) sedimentation 

(fig. 3). 

Quaternary Volcanic Rocks 

Volcanic rocks of Quate2nary age consist of local 

flows (usually less than 5 km ) and cinder cones. The 

flows form topographic highs on mesas and also occur 

as canyon-filling units. Two prominent cinder cones 

overlie mesa-capping basalts about 12 km northeast of 

La Pur1sima (fig. 17). A flank eruption from the cones 

yields a K-Ar age of 0.86 Ma (table 1, No. 19). 

A canyon-filling flow in Arroyo Agua Verde, 

about 13 km north of Paso Hondo, yields a K-Ar age 

0.60 Ma (table 1, No. 20). Other flows downstream 

may be coeval. K-Ar ages from two roughly circular 

flows on the east and west sides of Arroyo Cadeje, 

located about 6 km north of the town of Cadeje, yield 

ages of 0.96 Ma and 0.76 Ma (table 1, Nos. 13 and 14); 

the latter overlies fluvial gravels that unconformably 

overlie lenticular fluvial sandstone and conglomerate 

of the Comondu Formation. 

The Quaternary flows, which are shown on the 

geologic map of McLean and others (1985), are located 

between Cadeje and Arroyo Agua Verde and are 

aligned along a trend (N. 80° E.) that extends for a 

distance of more than 30 km and is oriented almost 

perpendicular to the regional trend of the Sierra de La 

Giganta. 

STRUCTURAL AND STRATIGRAPHIC RELATIONS 

The structure of the western Sierra de La 

Giganta is characterized by flat-lying, gently dipping 

beds, and distinctive formation boundaries (figs. 18 and 

19). But on a local scale this simple picture is 

complicated by small folds, faults, and gradational 

contacts (figs. 7, 11, 20, and 21). 

13 
Figure 18. Strata of the Comondu Formation 

constituting the eastern escarpment of the Sierra de 

La Giganta about 30 km northwest of Loreto. 

Elevation of left peak is 1,690 m and rises 1,600 m 

above plain in foreground. 

Folds 

A prominent anticline in the Bateque Formation, 

with gentle flank dips and a northwest-trending axis, 

lies along the lava-capped coastal mesas. The axis is 

represented stratigraphically by a paraconformity 

between the Bateque Formation and the overlying 

Isidro Formation. The fold axis is also shown on the 

geologic map of Beal (1948). The absence of San 

Gregorio strata along the crest and the presence of 

clasts of ripped-up siliceous shale in basal beds of the 

Isidro Formation indicate that uplift and erosion 

Figure 19. Overview of Arroyo La Purisima looking 

southwest toward El Pilon, the basalt-capped butte on 

the horizon. Road in foreground goes south to San 

Jose de Comondu. 

14 

Figure 20. Slump block of brown sandstone of the 

Comondu Formation in fault contact with light-colored 

beds of the Isidro Formation; see figure 19 for 

location. View is toward the north. 

occurred in early Miocene time. The axes of smallerscale open folds in the Isidro Formation also trend 

northwest. Fold axes are well exposed in the banks of 

Arroyos San Gregorio and La Purisima. Flank dips die 

out in overlying nonmarine beds of the basal part of 

the Comondu Formation (figs. 8, 11, and 21). The 

relations between these folds and the transition from 

marine to nonmarine deposition indicates that marine 

regression was synchronous with folding and regional 

uplift. 

Figure 21. Fold in .Isidro Formation overturned to the 

west; located on north bank of sharp bend in Arroyo La 

Pur1sima approximately 5 km downstream from La 

Pur!sima; folded beds assigned to the Pur1sima Nueva 

Formation by Heim (1922, see photograph in plate 

XXII). In this report the beds are assigned to the Isidro 

Formation. Flat-lying beds of the Isidro Formation 

overlie folded beds, indicating that deformation was 

synchronous with deposition of the Isidro Formation. 
Faults 

Normal faults as much as several kilometers long 

that displace capping flows are clearly visible on air 

photos. Most faults trend northwest and are most 

visible in the highland area close to the crest of the 

Sierra de La Giganta. Displacements estimated from 

air photos and topographic maps range from 20 m to 

about 100 m. The number of faults and fractures 

(observed on air photos and during field 

reconnaissance) increases dramatically east of the 

crest of the Sierra and may be related to tectonism 

associated with the opening of the Gulf of California 

in Pliocene and Pleistocene time. 

Slump Blocks 

Blocks of Comondu strata several hundred 

meters long have slumped down along the banks of 

some of the largest arroyos. Examples of slumping are 

well exposed along the east and west banks of Arroyo 

La Purisima about 2 km upstream from the town of 

San Isidro, where brownish rocks of the Comondu 

Formation have slumped downward toward the arroyo 

and now rest discordantly on yellowish beds of the 

Isidro Formation (fig. 20). 

CRYSTALLINE BASEMENT ROCKS 

No outcrops of granitic rock or pieces of granitic 

float were observed west of the crest of the Sierra de 

La Giganta. However, diorite and gneissic xenoliths 

were found in a canyon-filling basalt flow in Arroyo La 

Pur1sima, located about 6 km upstream from the town 

of San Isidro. The xenoliths are as much as 10 em in 

diameter and are composed of either a two-pyroxene 

diorite or diorite granulite. Essential minerals include 

calcic plagioclase, aegerine(?)-augite, hypersthene, 

and opaque oxides. Measurements of Sr isotopes (by 

R. w. KistW of ~he u.s. Geological Survey) indicate a 

range of Sr/ Sr values from 0.70372 to 0.70384; 

radiometric ages have not been determined for the 

xenoliths. A granodiorite outcrop on the west side of 

the Bahia Concepcion peninsula has ~ biot~te K-Ar age 

of 78.4 Ma (McFall, 1968) and a 8 Sr/8 Sr value of 

0. 70503 (R. W. Kistler, written commun. 1984). The 

xenoliths in the basalt flow north of San Isidro and the 

granitoid outcrops described above indicate that most 

of the Sierra de La Giganta is underlain by crystalline 

basement rock. Gravity and magnetic models by 

Coperude (1977) indicate that granitic basement 

extends westward to a point well beyond the Pacific 

coastline. 

Between lat 24°30' and 28°00' N., Mesozoic 

granitoid rocks are mostly covered by a veneer of 

Tertiary sedimentary and volcanic rocks, but on the 

east side of the Sierra de La Giganta, granitic rocks 

are locally exposed in several areas, including Arroyo 

Las Palmas near the town of Santa Rosalia (Wilson, 

1948), on the west side of the Bahfa Concepcion 

peninsula (McFall, 1968), in the low hills northwest of 

Loreto (Gabb, 1869; Chavez, 1978), and on several of 

the offshore islands between Loreto and La Paz (Mina, 

1957). The granitic rocks between Santa Rosalia and 

Loreto may connect the peninsular batholith to the 

north with the granitoid rocks exposed on several 

islands north of La Paz and the Sierra La Laguna south 

of La Paz. 

REFERENCES CITED 

Beal, C.H., 1948, Reconnaissance of the geology and 

oil possibilities of Baja California, Mexico: 

Geological Society of America Memoir 31, 138 p. 

~vez Valois, V .M., 1978, Estudio geolbgico general del 

area l.Dreto-caoond-6-Bahia Concepci6n, Estado de Baja 

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