Volcanic Rocks of the El Modeno Area Orange County California

GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-L Volcanic Rocks of the El Modeno Area Orange County California

By ROBERT F. YERKES

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-L

Description of extrusive pyroclastic and flow rocks of El Modeno volcanics of middle to late Miocene age

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1957 UNITED STATES DEPARTMENT OF THE INTERIOR Fred A. Seaton, Secretary

GEOLOGICAL SURVEY Thomas B. Nolan, Director

For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - Price 31.00 (paper cover) CONTENTS

Page Descriptive geology—Continued Page Abstract______313 Igneous rocks—Continued Introduction- ______313 Associated igneous rocks—Continued Previous investigations- ______314 Tuffaceous material in La Vida member of Localities from which samples were obtained ______314 Puente formation,______322 Descriptive geology----_---_---____----______316 Summary of petrography. ______323 Igneous rocks ______316 Sedimentary rocks, Cenozoic system. ______324 Tuffs of the Topanga formation ______316 Silverado formation______324 El Modeno volcanics______316 Santiago formation______325 Basalt flow member..______316 Vaqueros and Sespe formations, undifferentiated- 325 Palagonite tuff and tuff breccia member.. _ 319 Topanga formation______326 General description. ______319 Puente formation.______326 Palagonite tuff breccias______319 Quaternary terrace deposits ______326 Bedded palagonite tuff______319 Stratigraphic position and correlation of the volcanic Andesite flow and flow breccia member_ ___ 320 rocks______326 General description______320 Structure-______327 Flow breccias ______320 Folds.-______327 Lava flows______320 Faults______327 Hydrothermal alteration...______321 Disconformities in the volcanic sequence.______.. 327 Limy volcanic breccia______321 Mode of deposition of the volcanic rocks.______327 Undifferentiated volcanic rocks.______321 Subsurface data ______328 Andesite dikes--__-____-______--_- 321 Distinction between extrusive and intrusive rocks___ 328 Associated igneous rocks.______322 Distribution of the volcanic rocks______328 Breccia- ______322 References cited.______331 Basalt dikes______322 Index______333

ILLUSTRATIONS

[Plates 46 and 47 in pocket] PLATE 46. Geologic map of El Modeno area. 47. Structure sections, El Modeno area. Page FIGURE 55. Index map of southern California showing location of areas underlain by El Modeno and Glendora volcanics___ 314 56. Index map of eastern Los Angeles basin showing wells drilled into Miocene volcanic rocks.______315 57. Pillow structures in basalt flow member, Panorama Heights______317 58. Photomicrograph, basalt flow member______--_-______317 59. Tuff breccia in palagonite tuff and tuff breccia member, Panorama Heights ______319 60. Bedded palagonite tuff in palagonite tuff and tuff breccia member, Panorama Heights__.______319 61. Photomicrograph, bedded palagonite tuff, in palagonite tuff and tuff breccia member__-______320 62. Lava block with radial and concentric cooling cracks, in andesite flow and flow breccia member, east-northeast of El Modeno______320 63. Photomicrograph, vesicular andesite in andesite flow and flow breccia member.______321 64. Andesite dike cutting palagonite tuff and tuff breccia member, east-northeast of El Modeno ______-_-_--____ 322 65. Photomicrograph, porphyritic augite andesite in andesite flow and flow breccia member______322 66. Photomicrograph, tuff in La Vida member of Puente formation.______323 IV CONTENTS TABLES

Page TABLE 1. Petrography of tuffs of Topanga formation.______-_____-____--__-______-_-___-----___--_------317 2. Petrography of basalt flow member______-______------318 3. Petrography of tuffaceous sediments, La Vida member of Puente formation______--_------_- 323 4. Log of wells drilled into volcanic rocks.______329 A SHORTER CONTRIBUTION TO GENERAL GEOLOGY

VOLCANIC ROCKS OF THE EL MODENO AREA, ORANGE COUNTY, CALIFORNIA

By ROBERT F. YERKES

ABSTRACT Miocene age and are tentatively correlated with the The El Modeno volcanics form a series of extrusive pyroclastic Glendora volcanics (fig. 55). and flow rocks which crop out near El Modeno, Orange County, Calif. The volcanics have been divided into three members. The The detailed mapping of the volcanic rocks was basal unit is a basalt flow, which is overlain successively by a undertaken as part of the geologic mapping of the palagonite tuff and tuff breccia member and an andesite flow and west flank of the Santa Ana Mountains (Schoell- flow breccia member. The volcanics are products of both sub- hamer and others, 1954). In order to show details of aerial and submarine extrusion of materials of intermediate com­ the volcanic sequence, the geologic map which accom­ position. They are middle Miocene to early late Miocene in age and are tentatively correlated with the Glendora volcanics panies this report (pi. 46) has been prepared at a scale 25 miles to the northwest. The maximum outcrop thickness of 1 to 12,000. As shown on its index map, the geology of the volcanics is 850 feet. of adjacent areas was mapped by D. M. Kinney and The extrusive volcanic rocks may be distinguished from the J. G. Vedder. John S. Shelton of Pomona College middle Miocene intrusive rocks in this region by their vesicular loaned a collection of thin sections of rocks from well texture and characteristic alteration to crumbly, earthy masses. cores from which some of the data of table 4 was taken. The intrusive rocks are commonly coarser grained, hard and dense, commonly contain biotite, have an ophitic texture, and Cores from wells drilled for oil show that volcanic alter to chlorite-epidote-feldspar rocks. rocks similar to those of the El Modeno volcanics underlie the oil-bearing sedimentary rocks of the Puente INTRODUCTION formation of late Miocene age beneath a large part of The El Modeno volcanics are extrusive, largely pyro­ the eastern Los Angeles basin (fig. 56). In addition, clastic rocks, in part marine, that crop out in an eight- they show that the lowest part of the Puente formation square-mile area east of El Modeno, Orange County, and the pre-Puente sedimentary rocks are cut by in­ California, on the west flank of the Santa Ana Moun­ trusive rocks of similar composition. A petrographic tains. study of the El Modeno volcanics has been included in The volcanic rocks rest conformably upon the strata this investigation, partly to facilitate distinction be­ of the Topanga formation, which include at least two tween the extrusive and intrusive igneous rocks en­ thin tuff beds (tui on the geologic map, pi. 46). The countered in wells. El Modeno volcanics are made up of three members. The terminology of the pyroclastic rocks, including The lowest member is the basalt flow member (Terrify, the term palagonite, is the one proposed by Wentworth which rests upon sandstone beds of the Topanga forma­ and Williams (1932, p. 45-53). Chlorophaeite, a tion (middle Miocene) and includes at the top a thin widely used but ambiguous term, refers herein to a stratum of marine siltstone not differentiated on the green to brown alteration product of ferromagnesian map. This member has a maximum thickness of 200 minerals and glass in basaltic rocks. The composition feet. Overlying the basalt flow member is the palag­ of the feldspar has been determined in some rocks by onite tuff and tuff breccia member (Temt), which extinction-angle methods but in most cases by immer­ ranges from 125 to 450 feet in thickness. This tuffaceous sion of fragments in oils of known refractive index. member is overlain by the andesite flow and flow Special studies were made with the universal stage as breccia member (Tema , which has an average thick­ needed. The relative volumes of the constituents— ness of 200 feet. Locally overlying the andesite flow phenocrysts, groundmass, cavity fillings, and clastic and flow breccia member is the La Vida member of the grains—were obtained by the point-counter method of Puente formation (Tplv). Chayes (1949), except where otherwise noted, and are The El Modeno volcanics are of middle to late given as volume percentages of the whole rock. 313 314 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

118°30' _____118°00' •

OUTLINE MAP I- WITH LOCATION OF INDEX MAP 50 0 150 Miles

120° 118° 116° 118°15' 118°00' 117°45'

FIGUEE 55. Index map of southern California showing location of areas underlain by El Modeno and Glendora volcanics.

PREVIOUS INVESTIGATIONS LOCALITIES FROM WHICH SAMPLES WERE OBTAINED English (1926) included the volcanic rocks of the El In this report rock samples that have been studied in Modeno area under the designation intrusive and ex­ detail are numbered in the order in which they are trusive rocks of Tertiary age. Larsen (1948, p. 108) described. The following list is a key to their corre­ briefly described the El Modeno volcanics as a series of sponding field numbers, the symbols by which the con­ andesitic lava flows, clastic beds, and intrusives, which taining formation members are represented on plates he mapped as a unit of Tertiary age. 46 and 47, and the precise descriptions of the localities No detailed study of the volcanic rocks has been published. from which the samples were obtained. VOLCANIC EOCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 315

R. 10 W. I17'52'30» R. 9 W.

R II W us' "R. 10 W. "" H7-52'30" R9W

FIGURE 56.—Index map of eastern Los Angeles basin showing wells that were drilled into Miocene volcanic rocks. 316 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY Locality descriptions the formation. These tuffs afford evidence of the earliest volcanic activity during Tertiary time. The Field no. No. ia this Rock unit, Description of localities (measurements in report plates 46, 47 feet) tuffs are massive and well sorted, and many specimens are crowded with 0.5- to 2-millimeter opaque white 5b—— __ — 1 2300 E. and 225 S. of northwest glass bubbles. Fragments average 0.05 millimeter in corner projected sec. 36, T. 4 S., R. 9 W., Elev. 800. maximum diameter, are angular, and include clear, 6e_ ____ 2 <«!-___ _ Highway cut, 2250 N. and 200 colorless glass, plagioclase, micas, iron ore minerals, W. of southeast corner sec. 26, T. 4 S., R. 9 W., Elev. 600. and, rarely, detrital quartz. The matrix is composed 8e______. 3 tUi __ _. Road cut, 2050 W. and 325 N. of fragmental and partly altered glass. Sparse fish of easterly corner Irvine Block 15, Elev. '730. scales and molds of small mollusks indicate marine 19a2____ 4 1450 W. and 925 S. of easterly corner Irvine Block 16, Elev. deposition. 590. Glass fragments make up as much as 58 percent of la___-_-_ 5 Temb. - 2325 N. and 1725 W. of southeast the rocks but average about 40 percent. The index of corner sec. 15. T. 4 S., R. 9 W., Elev. 470. refraction of the glass in four samples ranges from 6 Temb.. 2925 S. and 75 W. of northeast 1.494 to 1.504, with a mean of 1.498, all ±0.002. corner projected sec. 35, T. 4 S., R. 9 W., Elev. 630. These values suggest that silica is present in amounts 6c.-.____ 7 Temb-- 3775 E. and 1540 S. of northeast greater than 70 percent (George, 1924, p. 365). Plagi­ corner sec. 13, T. 4 S., R. 9 W., Elev. 1110. oclase crystals and fragments make up no more than 6d.__.._. 8 Temb.. 2050 E. and 575 S. of northwest 5 percent of the rock, and the composition of the corner sec. 13, T. 4 S., R. 9 W., Elev. 990. plagioclase ranges between An3o and An50 ; the average 7a_.__ _ 9 Temb. 1600 S. and 425 W. of northeast value for the rock is about An35. The incongruity corner projected sec. 35, T. 4 S., R. 9 W., Elev. 610. between the intermediate composition of the plagi­ 7e ___ __ 10 Temb-- Road cut, 1200 E. and 2000 N. oclase and the silica-rich glass fragments suggests a of BM 277, near southeast corner sec. 34, T. 4 S., R. 9 composite source for the material. No massive vol­ W., Elev. 415. canic rocks of silicic composition are known from out­ 4e.___.__ 11 Temt... 2250 N. and 2150 W. of southeast corner sec. 26, T. 4 S., R. 9 crops or well cores in this region. W., Elev. 460. Data pertaining to four samples of the tuffs are 2c2__.___ 12 Tema _ _ Highway cut, 2075 N. and 400 W. of southeast corner sec. shown in table 1. Samples 1 and 2 are from the tuff 26, T. 4 S., R. 9 W., Elev. 590. bed that crops out in Section 25 near the center of lb2-_ _ 13 Tdi__._ 1200 N. and 875 W. of southeast corner sec. 23, T. 4 S., R. 9 the map, 150 feet below the top of the Topanga for­ W., Elev. 600. mation, and samples 3 and 4 are from the bed 325 feet 6b..__._. 14 tu.2 _ _ Road, 900 S. and 3550 E. of northeast corner sec. 13, T. 4 above the base of the formation, which crops out on S., R. 9 W., Elev. 1105. both sides of Peters Canyon. llb.-__. 15 tu2 _ -- 1025 S. and 2300 E. of southwest corner sec. 26, T. 4 S., R. 9 W., Elev. 540. EL MODENO VOLCANICS 13e_._ _ 16 tU2...-- 2230 N. and 1300 W. of southeast BASALT FLOW MEMBER corner sec. 26, T. 4 S., R. 9 W., Elev. 660. 14L__..__ 17 1025 N. and 250 W. of southeast A vesicular, porphyritic olivine basalt (Temb) rests comer sec. 26, T. 4 S., R. 9 W., conformably on sandstone of the Topanga formation. Elev. 800. 31d-_..._ 18 tu2 _ __ The basalt is light gray to olive green or brown. A west from easterly corner of fresh specimen has not been obtained. The rock Irvine subdiv. Blk. 68, Black Star Canyon quadrangle: Elev. characteristically weathers to spheroidal masses. Com­ 940. monly the rock is somewhat fresher and darker near 31e. _ _ iq 1950 southwest and 1000 north­ west from easterly corner the surfaces of spheroids or joints and is most deeply Irvine subdiv. Blk. 68, Black altered in the centers of the joint blocks. This Star Canyon quadrangle; Elev. 900. anomalous characteristic is ascribed by Fuller (1938) to alteration due to entrapment of volatile materials. Where the volatile materials escaped, as near joints DESCRIPTIVE GEOLOGY and fissures, alteration is less pronounced. IGNEOUS BOCKS The texture of the rock is everywhere porphyritic TUFFS OF THE TOPANGA FORMATION and subophitic. Variation occurs mainly in the size of Two or more thin beds of white vitric tuff 5 to 10 plagioclase phenocrysts and preservation of olivine. feet thick (tu^ are interbedded in sandstone of the The principal minerals are plagioclase, olivine, chloro- Topanga formation; one is about 150 feet below the phaeite, and magnetite. Plagioclase phenocrysts av­ top, and another is about 325 feet above the base of erage 0.5 millimeter in maximum diameter, but some VOLCANIC ROCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 317

Table 1.—Petrography of tuffs of the Topanga formation

Sample 1 Sample 2 Sample 3 Sample 4

Glass fragments 58 20 30 60 Do __ 1. 494 1. 504 1.447 1. 499 Plagioclase _ _ 1 5 1 4 Do__-______1. 557 1. 545 1.554 1. 548 Do ._ 53 35 + 47 42+ Micas _ 4 10 2 ? Ore minerals ______do____ 2 20 1 1 Matrix ______do.___ 35 45 60 30 Other minerals 2 ______--____do_-_- 6 5

1 Accurate to ±0.002. 2 Includes detrital quartz, tourmaline, augite, and hornblende. are as large as 8 millimeters. The plagioclase microlites alteration of the rock. The vesicles are commonly of the groimdmass average 0.05 millimeter in maximum filled or lined with chlorophaeite or zeolites, or, rarely, diameter. Olivine crystals are well preserved only in with calcite. rocks whose groundmass is made up of grains whose The basalt flow member is generally between 10 and average diameter is less than 0.03 millimeter. The 200 feet thick, but it is locally absent. In the only unaltered olivine phenocrysts are stubby anhedral unfaulted sequence (structure section C-C', pi. 47) it grains. is 200 feet thick. The upper surface of the flow shows The rock is everywhere massive and apparently no sign of penecontemporaneous erosion. Associated consists of a single extensive flow; good exposures show with the basalt at its upper contact is a limy siltstone pillow structures with siltstone seams (fig. 57). Flow bed that averages 4 feet in thickness, which weathers structure is rare. Only sample 5, from a weathered white or grayish white. The siltstone contains fish outcrop in Cerro Villa Heights north of Santiago Creek scales of probable middle Miocene or early late Miocene (northwest corner of map area), shows flow structure age (determination by W. Thomas Kothwell). On of trachytic type. Vesicles are usually abundant and Burruel Ridge north of Santiago Creek, a bed of light- not oriented. They range from 0.01 to 16 millimeters grayish-white claystoiie 20 inches thick locally occupies arid average about 1 millimeter in maximum diameter. a similar stratigraphir position. The claystone bed The ready access to permeating fluids afforded by this vesicularity probably accounts in part for the deep

FIGUEE 58.—Photomicrograph, basalt flow member, sample 9. Shows large (0.45- millimeter) pseudomorph of chlorophaeite and magnetite after olivine, with smaller FIGURE 57.—Basalt flow member, Panorama Heights, showing vesicular basalt in pseudomorphs in lower half of view. Groundmass is composed of plagioclase pillows with siltstone seams. Pencil in center of photograph is five inches in microlites, granular augite, olivine, magnetite, and chlorophaeite. Plane polarized length. light. 397756—57—2 318 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY crops out for a very short distance and rests on a rem­ The average composition of the plagioclase—close nant of the basalt flow member (near north end of to An55, the intermediate silica percentage of the structure section A-A', pi. 47). The claystone con­ essentially unaltered glass—(52 percent), and the tains an abundant fauna of silicified Foraminifera that moderate amount of olivine indicate that the rock is are considered to be middle Miocene in age (see p. 326). probably an olivine basalt, though it may be a basaltic Petrography.—Samples 5 to 10 of the basalt flow member andesite. have been studied microscopically. These differ mainly in the Chlorophaeite replaces olivine and pyroxene indis­ amount and size of plagioclase phenocrysts; the groundmass is criminately, as well as most of the glass formerly almost the same in all the samples, as shown in table 2. Each specimen contains large amounts of chlorophaeite, which is present in the groundmass, and fills or lines all cavities. commonly present as pseudomorphs after olivine or pyroxene It is nearly opaque and has a dark olive-green to yellow- (fig. 58). Very little fresh olivine or pyroxene is found. It is brown color. It commonly occurs as isotropic or nearly estimated that olivine originally made up between 5 and 15 per­ isotropic structureless or spherulitic fibrous patches. cent of the rock, while pyroxene formed from 5 to 10 percent. The weakly birefringent fibers have parallel extinction Wherever determined, olivine was optically negative, with 2V close to 85°. Pyroxene is of uniform character, apparently of a and positive elongation and an index of refraction that magnesian or diopsidic variety with positive elongation and is variable but close to 1.530. The weak birefringence range in Z to c from 37° to 40°. is attributed to incipient formation of chlorite, which

TABLE 2.—Petrography of the basalt flow member [The term chlorophaeite is used as defined on p. 313.]

Composition No. of Megascopic texture Groundmass texture Sample Phenocrysts (sizes are maximum diameter) Groundmass

Holocrystalline: Indeterminate, Plagioclase 20.2 percent, maximum 10 mm, Plagioclase microlites, in a basis Porphyritic: badly altered. average 3 mm, albite and Carlsbad of chlorophaeite and clay min­ Trachytic. twins, An55-62; chlorophaeite 24.2 per­ erals, 48.8 percent: magnetite, Subophitic. cent, in fan-shaped aggregates of radiating well dispersed 0.01-mm grains, needles filling cavities and as pseudo­ 5 percent. morphs after olivine and pyroxene. Holocrystalline (?): Plagioclase 25.7 percent, fresh albite- Magnetite 11 percent in rod- Porphyritic: twinned crystals up to 4 mm, average shaped grains 0.005 mm by Subophitic. .67 mm, Anss-ss; chlorophaeite 33.3 per­ 0.05 mm: plagioclase micro­ cent, in aggregates of spherulitic fibers lites 15 percent, An25-eo; chloro­ with weak birefringence, parallel ex­ phaeite basis 15 percent. tinction, positive elongation, isotropic when fresh, index of refraction close to 1.520, about one-fourth in pseudomorphs after olivine and pyroxene, the rest filling cavities averaging 0.5 mm in maximum diameter. Holocrystalline: Plagioclase 42.7 percent, Anso-RsJ augite 4.7 Magnetite 6 percent; calcite 1.8 Porphyritic: percent; olivine 0.4 percent; chlorophaeite percent; apatite 1.2 percent; Subophitic. 33.2 percent; tridymite(?) trace, with chlorophaeite and clay min­ alpha =1.480, biaxial positive, 2F=70°. erals 10 percent. Holocrystalline: Generally inter- Plagioclase 28 percent, averages 0.25 mm, Plagioclase 23 percent, as micro­ Porphyritic: granular; av­ An55_6o; chlorophaeite 30 percent as lites averaging 0.03 mm, Subophitic. erage grain pseudomorphs after olivine and pyroxene; An33-6s; unresolvable basis of size 0.01 to less devitrified glass 5 percent, filling chlorophaeite, clay minerals 0.05 mm; cavities averaging 0.3 mm, index of and magnetite, 14 percent. commonly refraction =1.580, pale olive-buff, clear, near 0.03 mm. isotropic. Holocrystalline: Plagioclase 23 percent, as fresh albite twins, Plagioclase 22 percent, as micro­ Porphyritic: average 0.75 mm, An47_5b ; chlorophaeite, lites averaging 0.03 mm, Subophitic. 34 percent, as pseudomorphs after olivine An3g-6o; olivine 7 percent, and pyroxene. grains averaging 0.03 mm, 2F=90°; pyroxene 9 percent, in grains averaging .01 mm, Z to c=31°; magnetite 5 percent. 10 Holocrystalline: Plagioclase 41 percent, in large pro­ Plagioclase 11 percent in micro­ Coarsely porphy- gressively zoned albite-twinned euhedrons lites averaging 0.007 mm by ritic. averaging 0.5 mm by 2.1 mm, An^-eiJ 0.05 mm, An57-65; magnetite 1 chlorophaeite 37 percent, in pale straw- percent; chlorophaeite 10 per colored fibrous pseudomorphs after olivine cent, in finely divided fibers; and pyroxene, parallel extinction, positive tridymite present in small 0.03 elongation, index of refraction just above mm crystal. that of Canada balsam, birefringence about .015. VOLCANIC ROCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 319 also seems to cause a slight increase in the index of they are the product of reworking. The matrix of this refraction. rock has not been examined microscopical!}7". The rare appearance of tridymite as vesicle lining* is attributed to deuteric processes. Shelton (oral BEDDED PALACONITE TUFF communication) reports similar occurrences in the Interbedded with the fragmental rocks are much Glendora volcanics. Other minor secondary minerals thicker and more extensive sequences of thinly bedded, are calcite and zeolites. well-sorted, and completely palagonitized tuffs (fig. 60).

PALAGONITE TUFF AND TUFF BRECCIA MEMBER These are most prominent in the central and southern parts of the mapped area. Beds in these sequences GENERAL DESCRIPTION range from Ke to 16 inches in thickness but average Resting conformably upon the thin limy siltstone % inch. Fragments of volcanic rocks are common and bed associated with the basalt flow member are pyro- are of granule size or smaller. Locally, small-scale clastic rocks (Temt) averaging 200 to 300 feet in thick­ cross bedding is seen. Some medium to coarse sub- ness. The thinnest unfaulted sequence measures 125 rounded grains of quartz are present and calcite feet and the thickest more than 450 feet. The color cement is common. of this rock, either fresh or weathered, is light grayish Petrography.—Sample 1 1 was selected as representa­ buff or tan, in some occurrences with a greenish cast. tive of the bedded tuff. The rocks in some exposures show lateral and vertical This sample is composed of medium, fairly well sorted grains; alternations of unsorted fragmental breccias with well- its texture is uniformly clastic (fig. 61). Clasts have sharp, sorted and bedded tuffs. angular boundaries and maximum diameter of 0.3 millimeter; some glass shards show the effects of flattening. The matrix is

PALACONITE TUFF BRECCIAS minutely vesicular palagonite; its sparse vesicles have a mean diameter of 0.05 millimeter. Angular blocks of light-gray vesicular augite andesite The estimated composition is 35 percent plagioclase frag­ are abundant in the tuff breccias. They are as large as ments (Anso-eo): albite and carlsbad twins sharply defined, in­ tensely zoned in both oscillatory and progressive fashion, with 8 to 10 inches in maximum diameter, but average 1 to 2 single crystals ranging from An70 at the cores to An30 at the rims; inches. These blocks, together with lapilli of similar 1 percent augite and hypersthene(?); 5 percent rounded to sub- composition, make up about 20 percent of the rock. angular adventitious quartz grains; 4 percent partly digested The tuffaceous matrix of the breccias is in general glassy basalt in fragments 0.3 millimeter in mean diameter; 55 vesicular, and the cavities are commonly filled or lined percent matrix: somewhat vesicular, rarely spherulitic and with palagonite and clay. The haphazard, unsys­ fibrous light-amber to deep brownish-black palagonite and some patches of calcite, the whole moderately charged with finely tematic arrangement of the fragments in these deposits, disseminated clay or dust particles. Some of the palagonite as shown in figure 59, precludes the possibility that shows incipient crystallization by fairly definite extinction, but most of it is changed very little between crossed nicols, retaining its amber color throughout. The resolvable anisotropic detail

FIACRE 59.—Palagonite tuff and tuft breccia member, Panorama Heights. Palago­ nite tuff breccia with blocks and lapilli of vesicular augite andesite. An irregular contact between two such deposits appears in lower left corner. Handle of pick is FIGURE 60.—Palagonite tuff and tuff breccia member, Panorama Heights. Bedded about twelve inches in length. palagonite tuff. Heavy bed near center is eleven inches in thickness. 320 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

blocks (fig. 62). Microvesicles in the tuff aceous matrix are common.

LAVA FLOWS The lavas are commonly hard, fresh, and light gray. In many places they are jointed and fractured. The rock is a vesicular porphyritic calcic andesite with the vesicles drawn out parallel to the lower contacts. Some of the vesicles reach a length of several inches. Chilling and baking of the lower contacts are nowhere pro­ nounced. At one locality in the north-central part of the mapped area, just south of Santiago Creek, a red­ dish scoriaceous lava is overlain by a dense, blocky flow with prominent joints, which is overlain in turn by a second scoriaceous flow. These three flows are trun­ cated at a low angle by a younger dense flow. The scoriaceous flows are commonly intensely altered and are reddish or yellow, whereas the denser rocks are gray and fresher. A few exposures are made up chiefly of ellipsoidal pillows with thin seams of baked siltstone or altered glass and tuff separating the pillows. The FIGURE 61.—Photomicrograph, bedded palagonite tuff and tuff breccia member. andesite of the lavas is not distinguishable from that of Adventitious quartz, plagioclase, augite, and altered olivine in a matrix of pala- the blocks in the flow breccias. gonitized tuff with calcite and clay minerals. Note angularity of grains. Crossed nicols. Petrography.—Sample 12 is representative of the lavas. takes the form of chloritelike plates with apparently higher This rock is light gray, hard, fresh-appearing and moderately indices of refraction. The mean index of refraction of the palago­ vesicular (fig. 63). The vesicles are as large as 1 centimeter and nite is 1.47. average 2 millimeters in maximum diameter. The texture is ANDESITE FLOW AND FLOW BRECCIA MEMBER porphyritic with phenocrysts that are as large as 0.38 millimeter and average 0.2 millimeter in maximum diameter. The texture GENEKAL DESCRIPTION of the groundmass is intergranular. Composition is 37.6 percent A series of randomly interbedded light- to dark-gray plagioclase, including many progressively zoned and albite- and reddish vesicular calcic andesite flow breccias, twinned crystals, in part andesine and labradorite phenocrysts lavas, and minor tuff breccias (Tema) rests conformably on the tuffaceous rocks and caps almost all the hills in the volcanic area. The lower contact of this unit is slightly irregular and rough but is essentially parallel to the bedding in the underlying tuffs. This series of flows and flow breccias is the most resistant rock in the sequence, as well as the youngest. Its thickness is commonly about 200 feet.

FLOW BRECCIAS The andesite flow breccias are composed of coarsely vesicular angular blocks of light-gray porphyritic calcic andesite in a matrix of greatly altered fine-grained ande­ site (fig. 65.) The blocks are as large as 6 feet in diameter, but average 3 to 4 inches. They are charac­ teristically quite fresh but some blocks present a vari­ colored appearance due to partial alteration. Blocks make up about 80 percent of the rock. At places altered glass or tuff is included in the matrix. Gravity sorting or other evidence of stratification was not detected. Radial and concentric cooling cracks are FIGURE 62.—Andesite flow and flow breccia member, gravel pit approximately one mile east-northeast of El Modeno. Large vesicular andesite block with radial and commonly present but rarely pronounced in the larger concentric cooling cracks in flow breccia. Pick handle about 12 inches in length. VOLCANIC BOCKS, EL MODENO ABEA, OBANGE COUNTY, CALIF. 321

specular hematite, and limonite are present in small amounts. The feldspar commonly remains quite fresh, whereas the tuffaceous matrix shows pronounced palagonitization and random addition of calcite.

LIMY VOLCANIC BPECCIA Interbedded in the andesite flow and flow breccia member in the west central part of the mapped area is a 6- to 10-foot bed of deep red-brown limy volcanic breccia (bx). The rock is composed of vesicular and porphyritic augite andesite, in clasts of granule to block size, closely packed in a matrix of calcite grains whose maximum diameter is 0.1 millimeter. A few plagioclase crystals occur as clastic grains that average 0.3 millimeter in maximum diameter; the patchy, glassy matrix has an index of refraction of 1.555±0.002.

UNDIFFERENTIATED VOLCANIC ROCKS About 750 feet of undifferentiated volcanic rocks (Temu) crop out in a small area near the east margin of the map, just south of Santiago Creek. These are very poorly exposed but are known to include both the basalt Figure 63.—Photomicrograph, vesicular andesite from andesite flow and flow breccia flow and the andesite flow and flow breccia members member, sample 12. Augite, plagioclase, and vesicles lined with devitrified glass. of the El Modeno volcanics. The palagonite tuff and The glassy groundmass is solidly charged with magnetite grains. Crossed nicols. tuff breccia member also is believed to be present. The and in part andesine microlites; 36.3 percent augite; 16.3 percent undifferentiated volcanic sequence was probably con­ chlorophaeite as alteration product of ferromagnesian minerals tinuous with the differentiated volcanic sequence im­ that may have included some olivine; 9.7 percent magnetite in mediately to the west, but these sequences are now well disseminated grains and rod-shaped aggregates; 0.1 percent separated by faulting (see structure section A-A', apatite(?), as submicroscopic inclusions in plagioclase crystals. Tridymite and zeolites are commonly present in small amounts; pi. 47). secondary calcite is common. A zeolite from sample 14 was ANPESITE DIKES determined as a variety of phillipsite similar to wellsite. It had the following properties: Angle /3=126^°; indistinct cleavage Steep or vertical, vesicular and porphyritic andesite parallel to (010); twinning present with composition plane paral­ dikes (TdC) intrude both the palagonite tuff and andesite lel (010) and perpendicular to b; optic plane and Z( = b) per­ flow members as well as sandstone of the Topanga pendicular to (010); X to c=52°, Y to c=38°; optically positive; formation (fig. 64). Of the 16 andesite dikes mapped, 2V ranges from 26° to 50°, with a mean of 42°; a = 1.4971, twelve intrude the palagonite tuff member. The dikes 0=1.4978, 7=1-5021, all ±0.0004; birefringence 0.0050. The mineral is colorless and has strong negative relief in canada are generally less than 10 feet thick and commonly less balsam. than 5 feet thick. Some of the dikes follow fault zones, The composition of the unzoned plagioclase phenocrysts ranges especially those forming the contact between the from An42 to An63, and the mean composition of the microlites is palagonite tuff and andesite flow members. The dike probably close to An 45. The phenocryst zones range from An70 rocks are commonly deeply altered and have a strong to An35. The classification of the rock as a calcic andesite is thus somewhat arbitrary and is based primarily on the mean compo­ clayey odor. The rock at places shows a decrease in sition of the feldspar of several samples. Plagioclase constitutes vesicularity and grain size toward the contacts, but it as much as 60 percent of other samples. does not show notable chilling or development of glass. At some places the vesicles are elongated parallel to the HYDROTHERMAL ALTERATION walls of the dikes, but no prominent flow structure has In the central and northern parts of the area, near been noted. The cavities are commonly filled or lined the mouth of Santiago Creek, the rocks of the andesite with zeolites and calcite. The dikes are all composed of flow and flow breccia member are more deeply altered porphyritic augite andesite identical with that described than in the south. The ferromagnesian minerals are under the andesite flow and flow breccia member. The altered to chlorophaeite and chlorite, which gives the similarities in composition and field relations suggest rock an overall deep-reddish to yellow or greenish color. that the dikes are nearly equivalent in age with the Calcite and zeolites fill many cavities. Tridymite, andesite flow and flow breccia member. 322 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

(Eocene) formations. One of these is in Weir Canyon in the east-central part of the mapped area, and the other is in SEtf Sec. 23, T. 4 S., R. 9 W. These dikes are 2 to 3 feet thick and have indistinct chilled margins. The two dikes are petrographically similar, and both are rather finely porphyritic with phenocrysts up to 0.7 millimeter in maximum diameter. The dikes are younger than the Topanga formation; they may be contemporaneous with the El Modeno volcanics, or they may be younger. Petrography.—Sample 13 is representative of the dike that cuts sandstone of the Topanga formation. This dike rock is dense, black, rionvesicular, and hyalocrystalline, with a finely porphyritic texture. The groundmass texture is intersertal to intergranular. Its composition is 36 percent unaltered plagioclase (Ans5-7o),in prominently twinned and pro­ gressively zoned crystals; 24 percent olivine, in phenocrysts up to 0.73 millimeter in maximum diameter, but 0.15 millimeter in mean diameter, commonly altered to aiitigorite; 10 percent rather poorly preserved augite; 14.5 percent magnetite, in well- Fir.rRE 64.—Small vesicular augite andesite dike cutting palagonitc tuff and tuff disseminated grains; 13.5 percent aritigorite and chlorophaeite, breccia member. Contact is at lower left. Locality is on north slope of small hill south of gravel pit, one mile east-northeast of El Modeno. Head of pick is pseudomorphous after olivine and pyroxene and as alteration about seven inches in length. products of the glassy groundmass; and 1 to 2 percent deep-olive to light-brown basaltic glass. ASSOCIATED IONEOUS ROCKS Several minor occurrences of igneous rocks are asso­ TUFFACEOUS MATERIAL IN LA VIDA MEMBER OF PUENTE FORMATION ciated with the El Modeno volcanics but are not In the El Modeno area the base of the La Vida considered a part of the main volcanic sequence. These member of the Puente formation is characterized by include a volcanic breccia (Temx) that rests on sand­ a thin bed of light-yellow to buff or tan, somewhat stone beds of the Topanga formation, two widely sandy tuff or volcanic sandstone (Tu2). The grains separated basalt dikes (Tdi), and also a tuff in the La in this rock are between 0.5 and 1 millimeter in maxi­ Vida member of the Puente formation, which overlies mum diameter. The tuff consists mainly of colorless the El Modeno volcamcs. BRECCIA Two poorly exposed outcrops of faulted and deformed breccia were mapped at the northeastern extremity of the central outcrop area, just south of Santiago Creek. The base of this breccia is apparently conformable upon sandstone of the Topanga formation. Where weath­ ered, the rock is brownish gray. It has a rubbly appearance, owing to the presence of sharply angular lithic fragments ranging from 2 inches to 10 inches in diameter. The clasts are chiefly vesicular gray ande­ site similar to that of the andesite flow and flow breccia member of the El Modeno volcanics, but they also include tan coarse-grained sandstone. The matrix is a coarse-grained sandstone with some lithic granules, typical of the sandstone of the Topanga formation. The upper contact is not exposed. The thickness of the breccia is not accurately determinable but may be as much as 200 feet. The relation of this rock unit to the El Modeno volcanics is not clear.

BASALT DIKES TWO dense black basalt dikes (TdJ CUt the Sedi- FloUKE ^.-Photomicrograph andesite flow and flow breccia member sample 12. v ' . Porphyritic augite andesite, illustrating oscillatory twinning and selective altera- mentary rocks of the Topanga (Miocene) and Santiago tion of plagioclase phenocrysts. crossed nicois. VOLCANIC ROCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 323 glass and plagioclase fragments with some quartz and A similar tuff occurs elsewhere on the contact be­ lithic fragments in a matrix of strongly altered glass, tween sedimentary rocks of the Topanga and Puente usually an isotropic claylike material. The colorless formations. On Burruel Ridge north of Santiago glass commonly has an index of refraction between Creek this tuff is apparently interbedded in the basal 1.477 and 1.523 ±0.002, indicating hydration; the part of the La Vida member of the Puente formation, average index of refraction is approximately 1.492. and it is therefore mapped as a part of that member. The average composition of the plagioclase is close to The volcanic constituents of the tuff were probably An50. Table 3 summarizes the petrography of six derived from the El Modeno rocks and deposited samples from this unit. locally as an older unit of the La Vida member.

TABLE 3.—Petrography of tuffaceous sediments in La Vida member of Puente formation

Sample 14 Sample 15 Sample 16 Sample 17 Sample 18 Sample 19

Matrix _ _ tvpe Clav '______Clav i or Clav 1-—-. Clav.1 sericite. 55 ______45___-_-___ 65_____ . 40 - 83______60. 1. 535±____ 1. 450± ___ 1.450±-___ 1.555±____ 1. 550±____ 1.550±. 10___._____ 5. ______few 35______5______10. 1. 523______1. 477______(?) - - 1. 490__ __ 1.500—.. _ 1. 520. 5 15______10______25______5_ — ------10. 1. 557______1. 558— ___ 1. 558 ______1. 557___ .__ 1. 556-_____ 1. 555. volcanic quartz, cal- 20 percent siltstone. rock frag­ cite, mus- quartz, ments. covite. musco- vite.

1 The clay matrix of these rocks is semicrystalline with average grain size less than 0.004 millimeter in maximum diameter. It is isotropic or nearly so, with refractive indices commonly less than that of Canada balsam. Halloysite is apparently the only mineral which satisfies the optical characteristics of this clay matrix. 2 Accurate to ±0.002.

The alteration of the groundmass glass to "palago- lieved to be pyroclastic in origin, perhaps deposited in nite" and then to a clay (halloysite?) is illustrated in a water, the alteration of the matrix has commonly been thin section of sample 14 (fig. 66) and is attributed to so complete that the original nature of the rock is not hydrothermal processes (Ross and Hendricks, 1945, clear. The substantial percentage of detrital quartz p. 67, 71). Although most of these samples are be- and foreign lithic fragments in some occurrences shows that the tuff may grade laterally into tuffaceous siltstone or sandstone.

SUMMARY OF PETROGRAPHY Although the El Modeno volcanics include olivine basalts (or basaltic andesites), they are dominantly andesitic in composition. The basalt, the earliest rock in the sequence, is only slightly more ferromagnesian in composition than later extrusive rocks. Mineralogically, these rocks are marked by the com­ plete absence of the late reaction products biotite and hornblende. Hypersthene is also apparently absent in the crystal­ line rocks, though possibly present in small amounts in pyroclastic rocks. Fresh monoclinic pyroxene is usually present, at least as groundmass grains. It is generally colorless and without pleochroism, with positive elongation and an optic angle ranging from 50° to 60°. The extinction angle, Z to c, ranges from 37° to 40°—rather low for augite. The mineral appears to be ar iron-poor diopsidic augite. Alteration of the augite phenocrysts FIGURE 68.—Photomicrograph, tuff in La Vida member of Puente formation. Color­ as well as the olivine and glass to chlorophaeite is less glass bubbles and fragments in a matrix of palagonitized glass and mont- morillonite. Plane polarized light. commonly complete. This type of alteration has been 324 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY ascribed to the deuteric activity of entrapped volatile These data may indicate only minor changes in the materials (Peacock and Fuller, 1928). Substantiating general course of differentiation. Tuffs are poor guides this interpretation is the deeply altered condition of the to the character of their parent magmas, as sorting by volcanic rocks penetrated by wells drilled for oil in this wind may segregate the more siliceous material because part of the Los Angeles basin. Even in occurrences of its lower specific gravity. Also, in the mapped area where the volcanic works are rather definitely un- there is little evidence concerning the position of the weathered they present the same complete state of vent or vents. Subsurface data show that the volcanics alteration. thicken westward, suggesting that the vents were some Olivine is commonly present in most of the crystalline distance west of the El Modeno area. rocks. In the basaltic rocks it originally made up per­ An opaque mineral tentatively identified as magne­ haps 5 to 15 percent of the volume, and in the andesitic tite is always present. It occurs in the zoned plagio­ rocks 5 percent or less. It generally occurs as ovoid clase phenocrysts as inclusions and in olivine grains as grains in the groundmass, although larger pseudo- minute specks. In the groundmass of every sample it morphs in nearly every sample have its characteristic occurs as dendritic clusters or rod-shaped aggregates, as orthorhombic shape. The optical properties are: well as disseminated grains. a= 1.660±0.002, 7= 1.702±0.002; (negative) 2V=84°, Apatite occurs occasionally as minute inclusions in about 80 percent forsterite and 20 percent fayalite in plagioclase phenocrysts. composition. The former phenocrysts of olivine are SEDIMENTARY BOCKS OF THE CENOZOIC SYSTEM nearly always altered to antigorite and chlorophaeite with some magnetite. In some thin sections this altera­ SILVERADO FORMATION tion is incomplete and a crystal consists of a fresh core The strata of Paleocene age in the Santa Ana Moun­ and altered rim. tains were named the Silverado formation by Woodring Zeolites, which are attributed to hydrothermal solu­ and Popenoe (1945). The westernmost outcrop of this tions, occur widespread in all the volcanic rocks. formation in the mountains appears on the east margin Plagioclase is the most abundant and the least altered of the mapped area, on the limbs and nose of a large mineral in every sample and ranges in composition from westward-plunging anticline that makes up the core of An30 to An65. The average composition of the plagio- the northern Santa Ana Mountains. The Silverado clase fragments of the Topanga formation tuffs is An35 ; formation is unconformable on the Upper Cretaceous it is An50 in the tuffs of the La Vida member of the sedimentary rocks. About 500 feet of Upper Creta­ Puente formation. In both the basaltic and andesitic ceous strata are missing beneath this discordance on crystalline rocks the plagioclase phenocrysts range in the crest of the mountains, about six miles east of the composition from An42 to An62. The average composi­ mapped area (Schoellhamer and others, 1954). tion in the basalt is An55; in the andesite it is Ari45. In In this area the Silverado formation consists of five the lavas of the andesite flow and flow breccia member lithologic units recognized by Vedder (1950). In many of the plagioclase phenocrysts show prominent stratigraphic order, from bottom to top, these are: progressive zoning. The groundmass feldspars in the (1) a basal conglomerate bed; (2) a lower arkosic sand­ crystalline rocks have either the same or more sodic stone unit; (3) the Claymont clay bed; (4) a second composition than the rims of the phenocrysts. arkosic sandstone unit; and (5) a marine sandstone bed. Glass is abundant only in the tuffs interbedded near The basal conglomerate bed averages from 20 to 40 the top of the Topanga formation and at the base of the feet in thickness and consists of subrounded pebbles, La Vida member of the Puente formation. Several cobbles, and boulders derived from the metasedimentary samples of each of these tuff beds have been examined and crystalline rocks of the basement complex to the in an attempt to detect the course of differentiation of east. The matrix of the conglomerate is a red to buff the source magmas. The values in the following sum­ coarse-grained micaceous and arkosic sandstone. At mary were derived from the data of tables 1,2, and 3, most exposures in the northern Santa Ana Mountains and from the text descriptions. a thin bed of reddish sandy clay overlies the basal con­ glomerate and tends to stain the conglomerate out­ Average per- Average cent SiOz of percent An crops a reddish color. glass of plagio- Rock Unit clase The lower arkosic sandstone unit is poorly sorted and Tuffs of the Topanga formation. _ 70 plus__ 35. cemented, and is predominantly massive or coarsely Basalt flow member______52______55. cross-bedded; it averages 100 feet in thickness in this Andesite flow member______none____ 45. Andesite flow member, limy brec- 57______None. area. The color of the sandstone is gray to buff; the cia bed. angular grains of quartz and feldspar are medium- Tuffs of the La Vida member__ _ 70 plus__ 50. sized to coarse. Some beds up to 4 feet thick have the VOLCANIC ROCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 325

appearance of a rotten mica schist as they are com­ Buff to gray-brown fine- to medium-grained con­ posed almost entirely of biotite flakes. Thin beds of cretionary sandstone, containing a molluscan fauna of gray or brown sandy clay and siltstone are locally inter- middle to late Eocene age, overlies the basal conglom­ bedded in the sandstone. At the top of this sandstone erate unit. This concretionary sandstone unit has unit is a thick sequence of beds characterized by the an average thickness of 125 feet. alteration of biotite to the clay mineral anauxite. The upper unit of the Santiago formation is conglom­ A yellowish brown to reddish or black pisolitic clay eratic sandstone of possible nonmarine origin that of commercial importance, the Claymont clay bed, averages 450 to 500 feet in thickness. This sandstone occurs about 120 to 150 feet above the base of the for­ is buff to brown, poorly cemented *, and massive, mation. The clay is a resistant bed, 2 feet in average coarse-grained, poorly sorted, and of arkosic composi­ thickness, which has an almost continuous outcrop. It tion. Stringers of volcanic and quartzite pebbles form, is hard, massive, and brittle, with a conchoidal frac­ six-inch to one-foot discontinuous lenses in the sand­ ture. Quartz is usually abundant in the lower part of stone. Large fragments of silicified logs are common the bed as angular grains. The clay commonly con­ and serve to distinguish this unit. tains abundant 3- to 8-millimeter pisolitic aggregates of quartz and clay, particularly in the upper part of VAQUEROS AND SESPE FORMATIONS, UNDIFFERENTIATED the bed. The thickest member of the Silverado formation is a Nonfossiliferous sedimentary rocks of the Sespe coarse-grained arkosic sandstone unit, which rests on formation of late Eocene to early Miocene age rest with the Claymont clay bed and contains much biotite and apparent conformity on strata of the Santiago forma­ anauxite. Locally, this member contains a hard white tion and grade upward and laterally into the fossiliferous quartz sandstone bed about 20 feet thick, as well as marine strata of the Vaqueros formation. thinner beds of varicolored sandy clays, siltstones, Interstratified beds of variable maroon and white or carbonaceous shales, and lignite. A meager fresh-water buff conglomeratic sandstone are characteristic of the fauna occurs locally in the unit. The sandstone mem­ lower part of the Sespe formation; occasional thin bands ber is about 700 feet thick in the westernmost outcrops of greenish-gray to red clay and mudstone appear higher of the formation. in the section. Well-rounded pebbles and cobbles of The marine member of the Silverado formation is varicolored volcanic rocks and gray quartzite, averag­ about 300 feet in thickness, and consists of soft, gra}^, ing about 3 inches in maximum diameter, are character­ well-bedded fine- and medium-grained sandstone. A istic of the conglomeratic portions of the sequence. poorly preserved fauna of Paleocene age includes Tur- The matrix of these rocks is composed of angular ritella pachecoensis and Glycymeris cf. major (Vedder, quartz, weathered feldspar, and biotite in a semi- 1950, p. 33). consolidated clay cement.

SANTIAGO FORMATION In the Santa Ana Mountains it has not been possible The sedimentary rocks of Eocene age in the Santa to differentiate the Sespe formation described above Ana Mountains were named the Santiago formation by from the overlying marine strata of the Vaqueros for­ Woodring and Popenoe (1945). Strata of the Santiago mation. Mollusks characteristic of the Vaqueros forma­ formation rest with apparent conformity on rocks of tion are present locally in maroon and green beds that the Silverado formation, although fossil evidence indi­ are typical of the Sespe formation. cates a disconformity or a hiatus between the two The Vaqueros formation of early Miocene age (Loel formations (Vedder, 1950, p. 35). The approximate and Corey, 1932) overlies the Sespe formation and also thickness of the Santiago formation in this area is 700 intertongues with it. The Vaqueros formation con­ feet. sists of medium- to coarse-grained gray to buff sand­ The basal unit of the formation is a pebble-cobble stone and conglomeratic sandstone with local greenish- conglomerate that interfingers irregularly with massive gray sandy siltstone. Strata of the Vaqueros formation buff sandstone. The conglomerate is made up chiefly contain an abundant molluscan fauna which in­ of well-rounded pebbles and cobbles of gray quartzite cludes Turritella inezana santana, Rapana cf. vaquero- and reddish rhyolite, with less common clasts of light- sensis, Olivella santana, "Terebra" santana, smdAnadara colored plutonic rocks, set in a matrix of yellowish- (Larkinia) santana, all of early Miocene age. brown coarse-grained sandstone. Locally this basal In the mapped area the combined thickness of these unit grades laterally into a massive, occasionally cross- formations averages about 1,400 feet; elsewhere in the bedded, coarse-grained arkosic and micaceous marine Santa Ana Mountains the thickness is as great as 3,000 sandstone. feet. 326 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

TOPANGA FORMATION Bulimina uvigerinqformis, Eponides rosajormis, Valvu- The middle Miocene rocks of the Santa Ana Moun­ lineria grandis, Uvigerina subperegrina, Buliminella tains were assigned to the Topanga formation by curta, Bolivina vaughani, and Epistominella capitanesis— English (1926, p. 24). The strata of the Topanga all referred to the lower part of the Mohnian stage of formation rest with apparent conformity on beds of the Klainpell (1938, p. 121-131). Vaqueros formation. The thickness of the Topanga The average thickness of the Puente formation in formation in this area is about 1,100 feet. this area is about 1,800 feet. In the type area in the Buff-colored massive and thickly bedded medium- to Puente Hills, to the northwest of El Modeno, the coarse-grained gritty sandstones that are semiconsoli- formation reaches a thickness of about 11,000 feet. dated and poorly sorted make up, with minor sandy siltstoiie and pebbly sandstone, almost the entire QUATERNARY TERRACE DEPOSITS formation. Abundant molluscan fossils of middle Terrace deposits consisting of poorly sorted sand, Miocene age commonly occur near the base of the for­ gravel, and rubble are well developed within the mapped mation but are also found at higher horizons. area, which includes the mouth of Santiago Creek. In the El Modeno area two or more thin beds of white These deposits range in color from light gray to reddish vitric tuff are interbedded in the sandstone of the brown. Topauga formation. These tuff beds are not found The six levels of deposits along Santiago Creek are east of the mapped area. found at the following approximate elevations above PUENTE FORMATION the stream bed: 10 to 15 feet, 40 feet, 60 feet, 150 feet, The sedimentary rocks of upper Miocene age in the 210 feet, and 300 feet. This group of terrace deposits Santa Ana Mountains have been called the Puente was mapped downstream from the head of Santiago formation by English (1926, p. 26), using the name Creek by Schoellhamer and others (1954). assigned by Eldridge and Arnold (1907) to similar Along the south side of Burruel Ridge occur several strata in the Puente Hills north and west of the El isolated terrace deposits (Qtu) at elevations from 350 Modeno area. to 550 feet above the stream bed. Sedimentary rocks In the El Modeno area, strata of the Puente forma­ of the Puente formation have been thrust over high tion rest with apparent conformity upon the El Modeno isolated remnants of terraces (Qtd] north of Santiago volcanics. North of Santiago Creek and elsewhere Creek. strata of the Puente formation overlap rocks of the El Modeno volcanics, and the Topanga, Vaqueros, and STRATIGRAPHIC POSITION AND CORRELATION OF THE VOLCANIC ROCKS Sespe formations. In one locality, about 2 miles east of the mapped area, about 700 feet of strata of the The stratigraphic position of the El Modeno volcanics Pueiite, Topanga, and, perhaps, of the Vaqueros for­ in the standard California section is known within mations are missing below the base of the Soquel mem­ close limits. The underlying Topanga formation con­ ber of the Puente formation. tains a large molluscan fauna which includes Turritella Schoellhamer and others (1954) have divided the ocoyana and other guides to Pacific coast middle Puente formation into four members in the Santa Ana Miocene strata. Interbedded near the base of the Mountains. In ascending order these are: (1) the La volcanic series is a marine siltstone bed that contains Vida member, consisting of gray to black laminated fish scales which are probably characteristic of the siltstoiie which locally contains phosphatic nodules, and middle Miocene or lowest part of the upper Miocene. interbedded lenticular feldspathic sandstone and con­ On Burruel Ridge, north of Santiago Creek near the glomerate beds; (2) the Soquel member, consisting of north end of structure section A-A' (pi. 47), the basalt massive to moderately well bedded coarse-grained to flow member is overlain by a silicified claystone bed granular poorly sorted buff sandstone with interbedded that contains a rich foraminiferal fauna including siltstoiie and local conglomerate beds; (3) the Yorba Bulimina, montereyana, Epistominella gyroidinaformis, member, consisting of thin-bedded hackly chocolate- Baggina robusta, Valvulineria californica obesa, and brown to pinkish gypsiferous and diatomaceous silt- Nonion costiferum. R. M. Kleinpell has recently ex­ stone and local strata of sandstone and conglomerate; amined this collection and considers it to represent the and (4) the Sycamore Canyon member (which does not Siphogenerina reedi zone of his Luisian (middle Miocene) appear on the geologic map, pi. 46), consisting of inter­ stage (oral communication). Resting conformably on bedded conglomerate, medium- to coarse-grained buff the andesite flow and flow breccia member of the El arkosic sandstone, and siltstone. Modeno volcanics is the La Vida member of the Puente The La Vida member commonly contains a forami- formation, which contains Foraminifera diagnostic of niferal fauna of early late Miocene age that includes the lower part of KleinpelPs Mohnian stage. The El VOLCANIC ROCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 327

Modeno volcanics thus appear to be middle to early Pleistocene in age. Low-angle reverse faulting thrusts late Miocene in age. sedimentary rocks of the Puente formation over Tentative correlation is made with the Glendora Quaternary terrace deposits on the south side of Burruel volcanics of Shelton (1946). The Glendora volcanics Ridge, near the north edge of the mapped area. crop out about 25 miles north of the El Modeno area (fig. 55). They are predominantly andesitic massive DISCONFORMITIES IN THE VOLCANIC SEQUENCE flows and tuff breccias, but they are stratigraphically Two disconformities of local nature occur in the and lithologically more complex than the El Modeno volcanic sequence in the east-central part of the mapped volcanics. The Glendora volcanics are probably some­ area. One occurs near the center of structure section what older than the El Modeno, as fossiliferous sedi­ B-B' where the palagonite tuff and tuff breccia member mentary rocks of the Topanga formation (middle rests on a siltstone of the Topanga formation, with the Miocene) are interbedded with and locally overlie basalt flow member missing. Just to the south, near the volcanic rocks (Shalton, 1955, p. 79-80). the northeast end of structure section C—C', the STRUCTURE andesite flow and flow breccia member rests upon a similar siltstone (not mapped), with the basalt flow The structure of the El Modeno area is characterized and palagonite tuff and tuff breccia members missing. by faulting rather than folding. The great displace­ These two members reappear in the complete sequence ment of some of the faults and the relatively small size east of Peters Canyon. of most of the fault blocks render structural interpre­ tation hazardous. MODE OF DEPOSITION OF THE VOLCANIC ROCKS FOLDS Submarine accumulation of at least a part of the The volcanics near the east edge of the mapped area basalt flow member is indicated by pillows with siltstone are involved in a faulted syncline which lies between seams (fig. 57) and by the overlying fossiliferous marine the large westward-plunging anticline that was pene­ siltstone. The alteration characteristic of the rock trated by the National Securities Irvine 1 well and a strongly suggests deposition in a hydrating environment. horst block of rocks of the Sespe and Vaqueros forma­ Most of the palagonitic tuffs were deposited in water tions which trends northward from Peters Canyon deep enough to effect fairly uniform sorting over a re­ Reservoir, as shown in structure section A-A' plate latively large area; winnowing by winds probably facili­ 47. The main mass of El Modeno volcanics shown on tated the sorting to some extent. The angularity of the structure section B-B' is on the west side of a second fragments precludes thorough reworking of former sub- but smaller anticlinal fold that is faulted on both aerial deposits. Alteration of glass to palagonite, which limbs. Evidence in the mapped area indicates that is characteristic of these tuffs, is considered by Peacock folding occurred after the formation of Miocene strata and Fuller (1928) and MacDonald (1949, p. 59) to be but prior to the formation of the Quaternary terraces. postdepositional, owing to ordinary weathering. Erup­ Sedimentary rocks of Pliocene age which crop out tion of hot tuffaceous material directly into water or north and west of the mapped area are folded but the water-saturated sediments would accelerate this dehy­ terrace deposits are undeformed. This evidence indi­ dration. Peacock and Fuller consider the formation of cates that most of the diastrophism occurred during chlorophaeite to be a deuteric process, initiated at the late Pliocene or early Pleistocene time. time of deposition. In the northern part of the central volcanic area sev­ FAULTS eral large blocks with distinct cooling cracks (fig. 62), Several large faults traverse the area from southeast occur in the andesite flow and flow breccia member, each to north-northwest. These are all steep normal block lying in a matrix of vesicular lava or palagonitic faults downdropped to the west and reflect the north­ tuff. This is strong evidence for deposition of hot west-trending regional structural pattern of the Santa avalanche deposits characteristic of some volcanic Ana Mountains. The large fault that trends north- explosions. At the same stratigraphic horizons in the northwest through the mapped area has a displacement southern part of the area, these features are missing and of 1,600 to 2,000 feet. Subsurface data indicate that in their place are dense flows with occasional poorly this fault continues west-northwest of El Modeno for developed pillow structure. a distance of 12 to 15 miles beneath the alluvial cover It is concluded from their lithologic and petrographic of the Los Angeles basin, where it is called the Norwalk characteristics that the El Modeno volcanics were fault by oil company geologists. The smaller faults formed by both submarine and subaerial deposits of have similar trends and relative displacements. The igneous material of intermediate composition. No faults truncate the fold structures and are probably evidence is at hand concerning the source of these 328 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY volcanic rocks. The uniformity of the sequence and altered to crumbly palagonitic masses, interpreted as distribution of individual lithologic units throughout evidence of submarine accumulation. the series indicates a source capable of extruding large Petrographically, the intrusive rocks are characterized quantities of material over a wide area. Subsurface by chloritized ferromagnesian minerals. These minerals data suggest that the present area of outcrop lies at the are commonly represented by pyroxene and olivine, eastern margin of the area of deposition and therefore occasionally by accessory biotite, and rarely by horn­ probably at some distance from the position of the blende. Albitized feldspars are also seen. The rocks source vents. usually exhibit a diabasic, often coarsely ophitic texture. The extrusive rocks are characterized by such alteration SUBSURFACE DATA products as chlorophaeite and palagonite; chlorite is Twenty-eight wells drilled for oil in the Los Angeles rare. The presence of more than 10 percent of glass is basin south of the Whittier fault and east of the San generally reliable evidence of extrusive origin, as pointed Gabriel River have penetrated volcanic rocks similar out by Durrell (1953). to those represented at El Modeno (see table 4 and The composition of the extrusive and intrusive rocks fig. 56). Samples from these wells are commonly of the eastern Los Angeles basin south of the Whittier amygdaloidal, usually deeply altered and often pyritized. fault may be quite similar. In most of the samples of Generally they consist of fragmental or brecciated ex­ each rock examined, the plagioclase was found to range trusive deposits of intermediate composition. The between andesine and sodic labradorite in composition, stratigraphic relations and lithologic character of the except in the albiti:ed intrusive rocks. Augite, or its volcanic rocks penetrated by wells are apparently alteration products, were present in all the crystalline similar to those of the outcrops in the El Modeno area. samples; olivine, in perhaps half the samples of each type. As mentioned above, minor amounts of horn­ DISTINCTION BETWEEN EXTRUSIVE AND INTRUSIVE blende and biotite occur in the intrusive rocks but ROCKS apparently never total more than 5 percent. However, Several characteristics of the extrusive rocks of Mio­ these minerals have been seen only in rocks believed to cene age of the eastern Los Angeles basin may serve to be intrusive. The intrusive rocks may be somewhat distinguish them from the dike and sill rocks that are more alkalic in composition and slightly younger than often observed in the subsurface, intruding sedimentary the extrusive rocks. While there is no structural evi­ rocks of Miocene age and older. dence indicating a common source, it is conceivable that the few distinctive characteristics of each type are The stratigraphic position of the extrusive rocks seems to be sharply limited upwards, as they are not known attributable entirely to their mode of emplacement, and that they are consanguineous and contemporaneous or to occur above the base of the La Vida member of the Puente formation. Dikes and sills do not crop out nearly so. south of the Whittier fault zone, between the El Modeno DISTRIBUTION OF THE VOLCANIC ROCKS -area and the San Gabriel River, 10 miles to the west. The distribution of the volcanic rocks indicates that The stratigraphic position of these intrusive rocks is they were deposited over an area of about 770 square somewhat uncertain, but they apparently intrude no miles south of the Whittier fault (see fig. 56 and table 4). rocks younger than the lower member of the Puente for­ Only a few wells are known to have been drilled through mation. the volcanic rocks into older sedimentary rocks. In Lithologic distinctions between extrusive and intru­ these few wells the average thickness of the volcanic sive rocks are limited mainly to the state and type of rocks suggests that they may thicken basinward (west­ alteration; textural distinctions are reliable only in the ward) from El Modeno at a rate near 100 feet per mile. case of tuffaceous or bedded pyroclastic deposits. The There is no clear evidence in the subsurface data of an intrusive rocks are commonly hard and dense although unconformity beneath the volcanic rocks, and well the ferromagnesian minerals are often replaced, whereas cores are not numerous enough to indicate which units the extrusive rocks are commonly vesicular and severely of the sequence thicken westward. TABLE 4.—Log of wells drilled for oil south of the Whittier fault and east of the San Gabriel River, Calif., which penetrated middle or upper Miocene volcanic rocks [Measurements involving location, elevation, depth, interval, and thickness are infeet. Wells are now abandoned except wells 19 and 27, which were producing in 1951, and well 13, which was idle in 1953. Rocks examined microscopically are indicated with an asterisk (*)]

i*KM**| Data on Volcanic Rocks No. on Eleva­ Total Year Fig. 56 Operator Well Location tion depth begun Thick-; Well^bottomed in— Remarks Interval ness Overlain_by — pene- } trated

1 Amerada Petroleum Anaheini Comm. 325 N. and 2325 E. of south­ 118± 8,946 1949 8348±-8615± 207± Middle Miocene (?) Vaqueros and Sespe Fragmental volcanic rocks with Corp. 48-8. west corner sec. 8. T. 4 S., sedimentary rocks. (?) formations tin- interbedded sedimentary rocks. II. 10 W. differentiated. Sandstone and shale of middle Miocene age from 8200± to 8348±, and8615±to8850±. 2 Bandini Petroleum Segorstrom 1...... 330 N. and 060 E. of intersec­ 32± (i, 882 1047 5571-6882+ 1311+ Upper, Miocene sedi­ *JIypersthene augite basalt and Co. tion Huntzinger and Har­ mentary rocks. andesite tuff breccia. Data from bor Blvds., sec. 34, T. 5 S., Shelton (1954). R. 10W. 3 Continental Oil Co. Santa Ana Comm. 2310 S. and 1660 E. of North­ 69± 4,411 1935 4317-4411 94+ Pliocene (?) sand­ * Calcic andesite and palagonitic 1. west corner sec. 14, T. 5 S., stone and con­ tuff interbedded with sandstones R. 10W. glomerate. of middle Miocene age. 4 350 S. and 300 E. of north­ 52± 2,224 1925 2158-2224 66+ Middle Miocene sed­ Volcanic rocks _ .. Upper and middle Miocene sedi­ west corner sec. 30, T. 5 S., imentary rocks. mentary rocks 1750 (?) to 2158; R. 9W. fraginental volcanic rocks 2158 to 2224. 5 General Petroleum 330 S. and 330 W. of north­ 62± 11, 422 1944 11, 134-11, 422 288+ Pliocene sediment­ *Vesicular basalt at 11,276. Middle Corp. east corner sec. 34, T. 3 S., ary rocks. Miocene megafossils and Fora- R. 11 W. minifera in 2-foot limestone bed in volcanic rocks at 11,244. Data in part from Schoellhainer and Woodford (1951). 6 General Petroleum LaMirada46-l_.__ 275 N. and 2350 W. of south­ 85± 12,629 1946 12, 582±-12, 629 47+ Middle Miocene *Porphyritic basalt. Lower Plio­ Corp. east corner sec. 16, T. 3 sandstone. cene sedimentary rocks 5800± to S., R. 11 W. 11,197; upper Miocene sedi­ mentary rocks 11,197 to 12,300; middle Miocene sedimentary rocks 12,300 to 12,582. Datafrom Schoellhainer and Woodford (1951). 7 General Petroleum 992 N. and 330 W. of south­ 68± 12, 600 1945 12, 080-12, 094 134+ Upper Miocene *Altered basalt. Lower Pliocene tel Corp. east corner sec. 21, T. 3 12, 480-12, 600 sedimentary rocks. sedimentary rocks 7060 to 11,524; S., R. 11 W. upper Miocene sedimentary rocks 11,524 to 11,898; middle Miocene sedimentary rocks 11,898 to 12,080; volcanic rocks and mter­ bedded middle Miocene sedi­ mentary rocks 12,080 to 12,600. 8 General Petroleum 1525 S. and 2325 W. of north­ 120± 11, 606 1950 11, 525-11, 606 81+ Upper (?) Miocene Fragmental basalt. Upper Mio­ Corp. east corner sec. 22, T. 3. sedimentary rocks. cene sedimentary rocks 11,080 S., R. 11 W. to 11,525. 9 General Petroleum Was 1 330 S. and 990 W. of north­ 57± 5,955 1939 5655±-5955 300+ Middle Miocene sed­ Volcanic rocks _ .... *Vesicular flow or tuff, 5655 to 5667; Corp. east corner sec. 21, T. 5 imentary rocks. porphyritic basalt, 5794 to 5804: S., R. 10W. lower Pliocene sedimentary rocks 5550± to 5610; volcanic rocks and interbedded middle Miocene sedimentary rocks 5610 to 5955. Data from Schoellhainer and Woodford (1951), and thin sec­ tions. 10 Girard, P. M_...... Mark Fisher 1 _ .. 2300 N. and 600 E. of south­ 42± 5655 1943 5200-5650 450+ Lower Pliocene sedi­ Volcanic rocks _ _. *Fraginental (?) basaltic volcanic west corner sec. 27, T. 5 mentary rocks. rocks. Data from Shelton (1954). S., R. 10W. 11 McKee Oil Co.— ... Kokx Comm 8-1 ... 2460 N and 1510 E. of south­ 274± 4005 1946(?) 2145±-2176± 31± La Vida member of Vaqueros and Sespe Sandstones with Turritella ocoyana west corner sec. 16, T. 4 Puente formation. formations undif- 3410 to 3475; Vaqueros and Sespe S., R. 9 W. ferentiated. formations undifferentiated be­ low 3700. 12 2325 S. and 1000 W. of north­ 54± 4455 (?) 4368-4450 (?) 90± Pliocene sediment­ Middle Miocene (?) Fragmental volcanic rocks at 4368. east corner sec. 23, T. 4 S., ary rocks. sedimentary rocks. Poor data. R. 10 W. 13 Seguro Petroleum 1000 N. and 124 W. of south­ 64± 9110 1947 7570-7605 35 Upper Miocene sed­ Middle Miocene(?) Top of Miocene sedimentary rocks Co. east corner sec. 34, T. 5 S., imentary rocks. sedimentary rocks. at 4100. R. 11 W. 14 Shell Oil Co., Inc... . 200 S. and 220 E. of north­ 152± 5944 1937 4915-5944 1029+ Middle Miocene sed­ Volcanic rocks.. _ .. *Altered olivine basalt. Lower west corner sec. 23, T. 4 S., imentary rocks. Pliocene sedimentary rocks ? to R. 10 W. 4600±; upper Miocene sedi­ mentary rocks 4600± to 4770; middle Miocene sedimentary rocks 4770 to 4915. Data from oc Schoellhainer and Woodford to (1951). 330 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY

o o3 a o 2*1 O !ard,darkdensepyriticgray andesite,riticdeeplyaltered. basalt,pyroclasticolivineat Miocene(?)top;sedi­upper ragmentalvolcanicrocks.Upper Miocenesedimentaryrocks2315 grainedinelightporphy-gray Fragmentalvolcanicrocksinter- «.-sa*.IflPI gj° ^",§^.9« i -yo^ba^^o-!MMI H SSoiS«l |^ ll1 idleinexamined1953.Rocksras |2 *•%%%* ||S3a 2§5p«g^ 1^ .^^^l -Itg rocksmentary8140.8108to T>>s °*S Q "ol^l °4i S ^fl-Hg §| laS"-Hs| --X.W-H-SM Remarks ^Illwl^a S"^55 ^Silil'S^I? "Sf !§•* ^BaSl l§i~ U; I? 1 W .P^I>25l o|ll| 1&S| S ||JS |a§loIS |a^w^Sl'Si to3006.

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b£ £j2 oa cS °s •rtS SC3 SC^^rS^flr^^'S ^• ai 'aj.s-.2«S ^cTo 1^° w ^^ oa roc:sedimentary VidamemberLa formati(Puente MioceneUpper sedimentaryroi VidamemberLa formaticPuente producinhichwere byOverlain— UpperMiocene §ESo *SS 5. *Ss ja. §>^a* P §^*j £ ^^^SPflo^^S gal §! §>> §>>°^2 §t «,§ „§ si S| -i? ^§§s s si si nicRocks o^-^ GJ^Sa S^*SS^cu^"^^atjHuafi"^ rdfi''^*-i?i^ard'ta M n ^ 41 + -H + g 4i4i + -H+4i-H -H OS O 00 i—i c^iot— cboos^o^o ^ N^ > fl % §"1 00 CS) Oi t^-O^CCO CCOS l>- i C^ 1—1 ^ T-f "^ *"* Is .23 g EH -^ Oi +i o3 "w S "S s § -H 0 S S35!^S-+i§«-H -H I s 1 p -aC3 s ^f^^i^^s § 1 1 on CC N CO I O ~n 1 t H 00 °° S ~H M S -fl a? &••-« M o i K « gj g S S ° if 1 -§1 old » 00 00 ^ CO ^OOt~-.J$<0 £ W Q o> bo OS CO C? co cS^cSSScScnc? * •a a ^•0 s ^^? S I 2 li§||||| j It 00 i° 1 1 p,- t~ en = cc oo •*<•*< ^. «* if ?•§ ^a -H -H -H i! 4Ji^^^)i«S J ll'3 3Oi Q _O g CO ° s~ ^ IM CO-H-I-'N .2iT ^a /, r flod -^od *aT"g^. Aw ^,* ^^r ^ ^^ ^^ fl^ ^^ i/ fij t^^, "£„, 'Stc "flrj "flf4 '3'*l"flE-H"fl« tH EH' !H ";tl u mflcO S-* 1 §EH §^ §^i °^ w ° ^ °^- S . S^ fi^- s^ fl^- °^; °^i 1 'Sco" "®e> ^-sT o ,_^ om ocS o^o^o^o^OoT ° 50- °oT °c» .CO ^" H cj w^ ^d ^cj W d ^' d ^ N. H § H 0- Srd^^ pa o ^ 1 H cj o" ^OJ Qca^^^" lo w o S QSog ^o o « S |° o CO h » g li *drt H2 . *d ^8 ts ">1 S8is iji |ijis- ft H ^ -|0» ^ -4-3 ^ ^ -fcJ " ^7 "TO '^ i_* 4-S W ' 4J i-^ r/) "t^ ^ OQ "en Oi .4^Oi^4J)Oi . "^ Oi OQ|J ° aJ^oD "' ^W« W. ^^.^W^^CQ.qj^ 05. CQ JS • M • ^- q^ » o &K 0 Sf^« &P= »OoOQ i— i q»i OQ ^^P^OC^OQOCjPHio^OQcD^PH OQ^WocsPH ?PH OS 5 o^rt .a OS 2 CO S S S^g§SSS2 S a ; ; 'S a i 1—1 S8 ^d»so d og .tes ; °a 'i 4«J •§ ^ O^P^t-1 1—I CJ, . - a • a oa oSS ^^ ^-SSS S o "a! a i 0 ^^O ^C3 5^ fl fi.fl S O o ^* .^^fefe "S r ^"^S^^^'fi^dS 3 , H 5 ri B ' ^fl*S^S °- 1 £ .S P^Fl P^M 'dS S ^H C3 4J ^S'1' MS& ^ "3 oRu^d t-||-^i o^tbjc ^ .-i^t* SflS .& i ^ "S fl OT-HQ^M « t—' ST-H^ ^ fl^ ^* H ^M»iH i — i § s EH ^fe C3««§^OEHD O *s "S "S "S "3 =33 i 'o 'o '0 ^S "° "° 1 v -° .000 ,2.2 d d O O I w W w 'S 4J X rs nvolvingl OilCo,rd U D CordOil rj r\ iWWWfTrrtO O ^4 • 3 • ornia. O.2 O.2 ornia. *- Si e a ^5 ^5 ^5 ll D d ° 03- a a °-s OS s g I-H 5- S- " Sa 2'a t-i o ^ O hq -S'w 1 |l os g £ |o |o |o £^ °i| °,| O iS"^ ^ss ^s '§••' rrj"i "S^Ci.fH F-f ^'d^'d'^'d ^)flfl^4 S^H 1 §0 1° S° So |o §-00 jjj J 2~§ 2« S-o! Ba 'So ao a So 09 03 ODEHEHEHEHEHEHt> t> a^o « cc t~ 00 OS OTHNCC^^SOt— CO w§ §•« 1— I t—i *—i T-H C^NC^NC^C^iNC^ N 1 d.Sf VOLCANIC ROCKS, EL MODENO AREA, ORANGE COUNTY, CALIF. 331

REFERENCES CITED Ross, C. S., and Hendricks, S. B., 1945, Minerals of the mont- Chayes, Felix, 1949, A simple point counter for thin section morillonite group: U. S. Geol. Survey Prof. Paper 205-B, analysis: Am. Mineralogist, v. 34, p. 1-11. p. 23-79. Durrell, Cordell, 1953, Tertiary igneous rocks of the Santa Schoellhamer, J. E., and Woodford, A. O., 1951, The floor of the Monica Mountains: Pacific Petroleum Geologist, v. 7, no. Los Angeles basin, Los Angeles, Orange, and San Ber- 12, p. 2-3. nardino Counties, Calif: U. S. Geol. Survey Oil and Gas Eldridge, G. H., and Arnold, Ralph, 1907, The Santa Clara Iiiv. Map OM 117. Valley, Puente Hills, and Los Angeles oil districts, southern Schoellhamer, J. E., Kinney, D. M., Yerkes, R. F., and Vedder, California: U. S. Geol. Survey Bull. 309. J. G., 1954, Geology of the Northwest Santa Ana Moun­ English, W. A., 1926, Geology and oil resources of the Puente tains, Orange, Riverside, and San Bernardino Counties, Hills region, southern California: U. S. Geol. Survey Bull. Calif: U. S. Geol. Survey Oil and Gas Inv. Map OM 154. 768. Shelton, J. S., 1946, Geology of northeast margin of San Gabriel Fuller, R. E., 1938, Deuteric alteration controlled by the jointing basin, Los Angeles County, Calif: U. S. Geol. Survey Oil of lavas: Am. Jour. Science, v. 35, p. 161-171. and Gas Inv. Prelim. Map 63. George, W. 0., 1924. The relation of the physical properties of ———— 1954, Miocene volcanism in coastal southern California, natural glasses to their chemical composition: Jour. Geol­ in Jahns, R. H. ed., Geology of southern California: Calif. ogy, v. 32, p. 353-372. Div. Mines Bull. 170, chap. 7, p. 31-36. Kleinpell, R. M., 1938, Miocene stratigraphy of California: ———— 1955, Glendora volcanic rocks, Los Angeles basin, Cali­ Tulsa, Okla., Am. Assoc. Petroleum Geologists. fornia: Geol. Soc. America Bull., v. 66, p. 45-89. Larsen, E. S., 1948, Batholith and associated rocks of Corona, Elsinore and San Luis Rey quadrangles, southern California: Vedder, J. G., 1950, The Eocene and Paleocene of the Northwest Geol. Soc. America Mem. 29. Santa Ana Mountains: M. A. Thesis, Claremont Graduate Loel, Wayne, and Corey, W. H., 1932, The Vaqueros formation, School. lower Miocene of California; Pt. 1—Paleontology: Univ. Wentworth, C. K., and Williams, Howel, 1932, The classification Calif. Pub., Dept. Geol. Sci. Bull., v. 22, p. 31-410. and terminology of the pyroclastic rocks: Natl. Research MacDonald, G. A., 1949, Petrography of the Island of Hawaii: Council Bull. 89, p. 19-53. U. S. Geol. Survey Prof. Paper 214-D. Woodring, W. P., and Popenoe, W. P., 1945, Paleocene and Peacock, M. A., and Fuller, R. E., 1928, Chlorophaeite, sider- Eocene stratigraphy of northwestern Santa Ana Mountains, omelane, and palagonite from the Columbia River plateau: Orange County, Calif: U. S. Geol. Survey Oil and Gas Inv. Am. Mineralogist, v. 13, p. 360-383. Prelim. Chart 12.

INDEX

Page Ac-know ledgments_____ —______313 Halloysite———————— — ——— — — ————--- — _— — — 323 Alteration, augite______318,323 Hot avalanche deposits______—— —— ______327 Alteration, glass______318,319,323,327 Hypersthene.-.------—-—-- — ------— -—- — ---- 319,323 hydrothermal ______321 olivine-—_—— — —— — —— ——— — —— _ — ———————— — — __ 323,324 Intrusive and extrusive rocks, distinction and stratigraphic position__.______328 olivine basalt-_---______---______--______316,317,324 La Vida member of Puente formation, character and distribution_ _ ___ 313,326 Amerada Petroleum Corp., subsurface data_--_____-______329 glass fragments in______323 Anauxite______325 plagioclase in______324 Andesite, basaltic ______318,323 tuff in.______322-323,324 calcic. ______320 Lava flows, andesite flow and flow breccia member______320 Andesite dikes______321 Lithologic distinction, intrusive and extrusive rocks....______328 Andesite flow and flow breccia member, character and distribution. 313,320-321,321 plagioclase in.______324 Los Angeles basin, eastern, underlying igneous rocks______313,329-330 stratigraphic position.______... 313,326 wells drilled into Miocene volcanic rocks.______315,328-330 Apatite...______324 Augite_.______318,319,321,323 McKee Oil Co., subsurface data______.___ 329 Magnetite.--_--__----_---_____------_------_---_------_ 324 Bandini Petroleum Co., subsurface data_.______329 Methods of study____.______313 Basalt, plagioclase in.______324 Miocene age, El Modeno volcanics______326 Basalt dikes______322 Glendora volcanies______327 Basalt flow member of El Modeno volcanics, character and distribution. _____ 313, La Vida member... ———- ——— — —— — — — —— —— — — — —— 326 316-319,321 Sespe formation.______325 stratigraphic position.______326 Topanga formation, __._.______326 Bedded palagonite tuff______319 Vaqueros formation...______— ______325 Burruel Ridge, claystone bed______317-318 Molluscan fauna, Santiago formation______— ______325 reverse faulting______327 Silverado formation..______325 terrace deposits______326 Topanga formation.______326 tuff in La Vida member______323 Vaqueros formation-.______325 Morton & Sons, subsurface data______— ______329 Cerro Villa Heights, outcrop______317 Chlorite—— --___--- — — __-- — _—______—______—_—_ ——__ — 321 Norwalk fault. 327 Chlorophaeite, definition-______313 occurrence and description...______316,318,321,322,323,324,327 Oil wells, subsurface data—- —— -- ———-- —— — — _--- — — — 328,329-330 Clay minerals.______323,324,325 Olivine, occuiTence and composition.-______316,318,322,324 Claymont, clay bed__.______325 Olivinebasalt.. —— —————— —— — - — - —— — ——— — -— 316,323,328 Claystone bed, basalt flow member______317-318 Composition, extrusive and intrusive rocks______313,316,328 gonite—— —— —- ———— ————— — —————— — ——— —— — 313,319,323,327 Continental Oil Co., subsurface data______329,330 Palagonite tuff and tuff breccia member, character and distribution.... 313,319,321 Cooling cracks______320,327 Palagonitization———— — — ———— — ———— —— ———— 321,323,327 Correlation, volcanic rocks______326 Paleocene age, Silverado formation.______325 Panorama Heights.,..______...______317,319 Descriptive geology______316-326 Peters Canyon, rock samples.______316 Deuteric processes.______319,324,327 sequence.-.—______327 Diastrophism, age______327 Petrographic characteristics, extrusive and intrusive rocks______328 Differentiation-______324 Petrographic description, basalt dikes.______322 Dikes and sills--______.______321,322,328 basalt flow member______------_-______------_--__ — __--_.______._ 318 Disconformities, volcanic sequence-.___-___--____--______-______-___-__ 327 bedded palagonite tuff. ______319-320 La Vida member....______— _____ ..___ ——— ______323 El Modeno volcanics, age and correlation.______313,326-327 lava flows------___ —. —— ———————————. — - 320-321 character and distribution------______316-321,326 Topanga tuffs.___--._-____-______-_„___ _— ——— __. — — —. — — — -.- 316 Eocene age, Santiago formation______325 Petrography of El Modeno volcanics-______—__— — _—_— __-.-_.__ 323 Sespe formation.-______325 Phillipsite______------.---_--__-__ . —— — - — _ —— —— —— ——— —— —— 321 Pillow structure in lava flows____-----.--.----_--_ -______--_------__ 317,320,327 Faults..______321,327 Plagioclase, average composition.__- — ____. ____-____-__-__ —— ______324 Feldspar, determination______313 Previous investigations—_-_-_—- — — — _— __ — __ — ______314 Ferromagnesian minerals, alteration,______313,321,324 Puente formation, character and distribution ______— ------—— ______326 Fish scales.-..______317,326 Puente Hills.... —— — — ——— ———— ——.——. —— —— - — — -_——- 326 Flow breccias.______313,320 Pyroclastie rocks, terminology—— — — — — — ______—— _ —— ____ 313 Folds..______327 Pyroxene___ — ___ — — _____—-_——_—__——— — _ — 318,319,321,322,323 Foraminiferal fauna.______318,326 Fossils______325,326,329 Rock samples studied, keys...——-. — — — ——— -————————————— 314-316 Gale, Hoyt 8., subsurface data..______329 Santa Ana Mountains, geology—- — —— - —— — — — ——— — —— — - 324,325,326 General Petroleum Corp. subsurface data______329 Santiago Creek area, geology— __-___._.___ _„___.______317,320,322,325,326 Girard, P. M., subsurface data__.______.______^______329 Santiago formation, character and distribution...______325 Glass.______324 basalt dares.—— — ———— —— ..— — — ———— ——— — — — ——— 322 alteration to palagonite___-----______327 Sedimentary rocks, geology— ——, ______— ______—.._ 324-326 Glass fragments, La Vida foimation______323 Seguro Petroleum Co., subsurface data______— — ___— ._._-_-____.__.. 329 Topanga formation. ______316 Sespe formation. See Vaqueros formation. Glendora volcanics, comparison with.______319,327 She!' Oil Co., Inc., subsurface data._._ .._.______—_..-—— ..... 329 333 334 INDEX

Page Page Silica content of glass, andesite flow mornber _..___.______324 Topanga formation, dikes—Continued basalt flow member..... ______318,324 volcanic rocks, character and distribution—______313 tuffs of La Vida member--. ______..____.___.______324 petrography-...__----_—_-—______323 tuffs of Topanga flow.-____...___. ______. ______316,324 Stratigraphic position and correlation.... — ______327 Silverado formation, character and distribution... ______324-325 Tridyniite—----. —... —.——_..————. — — — — ..- — _-— 319,321 Soquel member, Puente formation__..-—______..__--______326 Trustees Development Association, subsurface data___-_-_..-______330 Source of volcanic rocks-... -_-_-_-_-___..___--_._._-______..______.. 327-328 Tufi's, El Modeno volcanics___--_---_------_------__-_ ..____._. 319 Standard Oil Company of California, subsurface data______330 Puente formation,..---__-._.___------.------_------_-_--_ 316,322-323 Stratigraphic position, volcanic rocks.__ _.._- ______326 Topanga formation.____ .._____.___.... —... ._-_--_--___-____ 322-323 Structure..----_.______. 327 basalt flow member...,.-- — _---- — - — ----- — _ — — __.______--__.- 317 Union Oil Company of California, subsurface data___-_- —____ —— __ ——.___ 330 Subsurface data, rocks penetrated in wells ______328, 329-330 Vaqueros and Sespe formations undifferentiated, character and distribution. - 325,326 Superior Oil Co., subsurface data...._——-.- ______330 Volcanic breccia --...... ___- ______-_.____----...___ ... ______321,322 Sycamore Canyon member of Puente formation, description______.-___ 326 Volcanic rocks, mode of deposition. ___-_------______-_.___._._--.. 327 Terminology, pyroclastic rocks...__.___.______313 source.____..-____---__.--__-______-..__.___ 327-328 TeiTace deposits. ___.______-_.-______326 Stratigraphic position and correlation. _.______326-327 Texture, andesite dikes_____..___.._..______------.---_-______321 subsurface data.,-.---.-.---.-.--. _._..__._..______329-330 andesite flow and flow breccia member_.__.-_-__-_ __.______---______320 undifferentiated, character and distribution ______321,325,326 basalt dikes _ __.__.______.______322 Volcanic sequence, disconformities__._.------.,.______327 basalt flow member....--_—..-._. ______316-317 Weir Canyon, basalt dikes...-_-.._.-.------______322 palagonite tuff and tuff breccia member._.___ _-__._--___._--_-__.______319 Wells drilled into Miocene volcanic rocks, eastern Los Angeles basin___ 315, 324, 328, The Texas Co., subsurface data,______.__. . 330 329-330 Thickness of formations in area. _ _.. --_-______.- 317, 319, 320, 322,324, 325, 326 Wellsite. ______321 Tidewater Associated Oil Co., subsurface data...______330 Whittier fault zone._-..---._-.._-_..-_----. ----- ______328 Topanga formation, dikes..-_-___.-__-______._.___._____ ..... ______322 sedimentary rocks..______.__._.______.. 326 Yoiba member of Puente formation, description.__.______.___.. 326 structural relations...... -___.---____._____ -______-----___.______327 tuffs.-... —-,--_--..---- ...-...^--...... 316-317,324 Zeolite—. — ——————.—.,———--——-—— - ———- 321,324 o