Geology and Petrology of the Cambria Felsite, a New Oligocene

W. G. ERNST Department of Geology, University of California, Los Angeles, Los Angeles, California 90024 C. A. HALL, JR.

Formation, West-Central California Coast Ranges

ABSTRACT 10-m.y quiescent interval, and middle plex (Turner, 1968; Turner and others, Tertiary plate tectonics. However, genera- 1970). Geologic mapping in the Cambria and tion of these igneous rocks was probably a We have discovered scattered outcrops Black Mountain areas, western San Luis thermal response of the western margin of (Fig. 1) of fine-grained tuff and crystalline Obispo County, California, has led to the continental crust-capped Americas plate rhyolite-dacite felsite in the Los Osos Valley, recognition of the two stratigraphically and to underflow of the Farallon plate, resulting ~25 km north of Morro Rock near petrologically distinct extrusive felsic igne- in Morro Rock and Cambria units, followed Cambria, and also in the vicinity of Black ous units: (1) the Obispo Formation; and (2) by a complex encounter with the East Pacific Mountain, —15 km east of Cambria, all in a new unit, named here the Cambria Felsite. Rise, resulting in the Obispo Formation. the southern part of the Santa Lucia Range, The Obispo Formation was deposited above San Luis Obispo County, California. We the Rincon Shale and below the Monterey INTRODUCTION TO THE PROBLEM present stratigraphic, petrographic, and Formation during middle Miocene time. Dacite volcanic plugs, lava domes, x-ray fluorescence analyses to support our The Cambria Felsite rests with angular intrusive sheets, and felsitic rocks cropping conclusion that the Cambria Felsite was unconformity on Franciscan rocks of out between Morro Rock and Islay Hill in deposited at about the same time as the Jurassic and (or) Cretaceous age, and occurs San Luis Obispo County, west-central emplacement of the Morro Rock—Islay Hill as clasts in the nonmarine Lospe Formation California Coast Ranges, were correlated by complex, and that these rocks are consan- of Oligocene age. Hall and others (1966) with the Obispo guineous. Textural and mineralogic intergradations Formation on the basis of the proximity of exist between the Cambria Felsite and the the plugs, domes, and sills to the Obispo tuff GENERAL GEOLOGY OF THE nearby upper Oligocene hypabyssal vol- and on the similarity of radiometric ages of CAMBRIA AREA canic necks, plugs, lava domes, and dikes of the dacite and tuff. More recently, Turner Mesozoic sedimentary and volcanic rocks the Morro Rock—Islay Hill complex; this (1970) completed additional radiometric and more than 5,000 m of Cenozoic suggests that the Cambria Felsite represents age determinations of the Obispo tuff and sedimentary, pyroclastic, and igneous rocks an effusive equivalent of the Morro changed his earlier determination of 20.9 ± of Oligocene to Holocene age crop out in the Rock—Islay Hill. Cambria tuffs are enriched 1.5 m.y. to 15.9 ± 0.6 m.y. He concluded Cambria—Black Mountain area of western in alkalis plus silica and are depleted in fer- that the sample of Obispo tuff he had dated San Luis Obispo County (Figs. 1 and 2). romagnesian constituents relative to samples previously (Hall and others, 1966) had been The Mesozoic rocks in this area include of the Morro Rock type. Chemical contrast:; contaminated in the laboratory, and that the the Franciscan rocks and a sandstone of in the two units probably arose either because series of volcanic plugs and sheetlike questionable Late Cretaceous age (Hsu, of a winnowing of airborne crystals from the intrusions from Morro Rock to Islay Hill 1969). The oldest Tertiary rocks in the glassy shards or because of magma fractiona - (Los Osos Valley) first described by Cambria—San Luis Obispo area are the tion in the conduits followed by pyroclastii; Fairbanks (1904), are not the same age as newly recognized Cambria Felsite, of eruption of the upper portions of the melt the Obispo Formation. Radiometric ages Oligocene age, and sheets, plugs, lava columns. range between 22.1 ± 0.9 m.y. and 26.5 ± domes, and minor felsite of the Morro No obvious correlation exists between the 0.8 m.y. for rocks within the Morro Rock—Islay Hill complex (Turner and two-stage silicic volcanism, separated by a Rock—Islay Hill hypabyssal intrusive com- others, 1970; Hall, 1973a, 1973b). This

Geological Society of America Bulletin, v. 85, p. 523—532, 4 figs.,Apri l 1974

523

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12V00'

Sari Simeon Point V rt ^v

%.SL071 ARVX , Cambria o D-54 A.Brn Fig. 2 H-1,2,3,4-72 3 5o 30" • 1 N Cayucos Point

O o. ~~Obispo Formation and volcanic and some pyroclastic rocks of the Sandholdt Formation~~

~~Cambria Felsite~~

~~Point Buchón 6 35°15' - 35° 15' Morro Rock-Islay Hill complex~~

~~10 miles 10 kilometers~~

121°15' 121°00' I20°45' 120°30' Figure 1. Index map, areal distribution, and sample locations for Tertiary rocks studied. Map is modified from 1:250,000 scale Geologic Map of California, San Luis Obispo Sheet (Jennings, 1959). In Los Osos Valley, several fine-grained rocks that are transitional between Morro Rock-Isl iy Hill complex and Cambria Felsite (CAH-9-72 through CAH—12-72) are petrographically comparable to extrusive series, but, for simplicity, are not distinguished from intru; ive hypabyssal series on map.

latter unit occurs as local hypabyssal Sandstone of Oligocene (Zemorrian) age. shale, siltstone, and some cherty shale (Fig. intrusives within the Franciscan rocks, Approximately 20 m or Rincon Shale of 2). The Monterey Formation in the Cambria chiefly in the Los Osos Valley, 15 to 30 km probable ear y Miocene Saucesian age area is, for the most part, equivalent in age to southeast of Cambria; it is locally transi- overlies the Vaqueros Sandstone in the the Sandholdt Formation (see Smith and tional to Cambria Felsite. In some poorly vicinity of Cambria. More than 500 m of Durham, 1S>68) and the Point Sal Formation exposed regions, where lithologically inter- Monterey Formation (Srindholdt Member (Woodring and Bramlette, 1950). The mediate rock types are present, it is not of the Monterey Formation) of Relizian and Monterey is in turn uncomformably over- always possible to distinguish unambigu- Luisian age (Smith and Durham, 1968) lain by ~100 m of fossiliferous sandstone ously between the Cambria and Morro unconformably overlies tie Cambria Felsite, and shale of the lower Pismo Formation of Rock units. Probably due mostly to the Lospe Formation, or the Vaqueros late Miocene (Mohnian) age. The youngest extensive post-Zemorrian erosion, the Sandstone near Black Mountain. Nearly 70 mappable units in the area are marine terrace Cambria Felsite is restricted to a few m of Obispo Formation (Relizian) overlies and alluvial deposits of probable Pleistocene irregular patches or bold outcrops, both in the Rincon Shale in the Cambria area. and Holocene age. the Cambria—Black Mountain area and in Diabase and a few scattered outcrops of tuff, Folds of Tertiary rocks are generally the Los Osos Valley. considered to be part of the Monterey rather discontinuous, narrow, elongated, Younger rocks in the Cambria-Black Formation, and in part equivalent in age to doubly plunging structures. Late Pliocene or Mountain area include the nonmarine Lospe the Obispo Formation, crop out near Black early Pleistocene folding preceded most of Formation that interfingers with or grades Mountain. The Obispo Formation is uncon- the faulting in this area. Elsewhere in the vertically into the marine Vaqueros formably overlain by ~5 00 m of Monterey region there is evidence of pre-Zemorrian

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folding and faulting (Page, 1970a; Hall, in TABLE 1. ESTIMATED MODES, MORRO ROCK - ISLAY HILL COMPLEX prep.). Sample No. An cont. Plag. Qtz. Bio. Hornbl. Iron Matrix Shards Embayed Remarks Oxides Quartz CAMBRIA FELSITE CAH-MR 23-41 25 3 5 2 tr 65 no yes

~~Type Locality CAH-1-72 11-27 25 10 6 0 1 58 (1)~~

~~The type locality of the Cambria Felsite :.s CAH-2-72 23-28 30 11 5 1 1 52 (1) within an area of 17 sq. km between San Simeon and Santa Rosa Creeks in the CAH-3-72 13-36 20 10 5 5 1 59 (1)~~

~~Cambria quadrangle, San Luis Obispo CAH-4-72 20-34 25 8 6 1 1 59 (1) County, California. The type area lies within sections 11, 12, 13,14, 23, and 24, T. 27 S., SLO-i'tA 18-29 38 2 tr 6 50 (1)~~

R. 8 E. Good exposures of the Cambria Fel- SL0-5^B 23-33 33 6 5 It 2 50 " (1),(2) site occur in the northwest corner of section 11, at Scott Rock in section 23, and along SL0-5^C 22-28 35 6 it 0 tr 55 (1) California State Highway 41 east of Coast SLO-5ta> 21-30 30 8 3 2 2 55 (1) Union High School in section 24 (Fig. 2). SLO-75A 28-1+5 35 it it tr 2 55 Lithology SLO-75B 24-34 35 2 1 tr 2 60 (2) The Cambria Felsite consists of ~ 115 m tl of hard, rhyolite-dacite crystalline felsite; SLO-75C 26-40 38 2 2 0 3 55 soft, white, poorly exposed tuff, locally SLO-76 30-37 20 12 it 0 It 60 ft characterized by dark bentonitic soil; and , locally, a reworked soft tuff unit. The SLO-77 28-35 25 11 7 0 2 55 (2) reworked tuff, possibly equivalent to the II SL0-78B 25-31 25 6 8 tr 1 60 Lospe Formation, locally contains clasts of Franciscan graywacke (70 percent), chert SLO-79A 24-34 35 tr It 10 1 50 " no (2) (20 percent), and blueschist (10 percent). A SLO-79B 23-35 28 1 It tr 60 heavily weathered outcrop of reddish- 7 (2) tl brown to black, massive basalt is associated SL0-91A 25-49 3o tr 5 0 5 60 with the Cambria Felsite ~1.7 km south- SLO-91B 36-M+ 36 tr it 0 3 east of Coast Union High School (Fig. 2). 57 If Most of the felsite does not show bedding SL0-92A 34-46 35 0 3 tr 2 60 features or strongly preferred orientation of SLO-92B 28-31 3o tr It tr 6 60 yes phenocrysts. The pattern of joint planes is 11 commonly random. Where stratification is SLO-93A 36-45 33 tr 3 6 3 55 present, the beds range in thickness from SLO-93B 36-1*5 3o 0 It 7 2 57 no <30 mm to 60 cm. Biotite flakes are par- tially aligned to form a weak foliation paral- Note: (1) clast in Lospe Formation (2) abundant carbonate in matrix. lel to the bedding. Some outcrops are stained reddish-brown or orange-brown along joints and bedding. The unit is typically white, light brown, or gray. The Cambria Felsite, therefore, is younger the Lospe Formation consists of —125 m of From 3 to 30 percent of the rock is than the Franciscan rocks of Jurassic or Cre- poorly sorted massive green or, less composed of phenocrysts, typically sub- taceous age and older than the Oligocene commonly, red conglomerate. The clasts equal amounts of quartz and oligoclase, and Lospe-Vaqueros rocks. make up 50 to 60 percent of the rock, range minor biotite. The individual quartz crystals In the area near Cambria, the Lospe from 3 mm to 15 cm in diameter, and are are anhedral to subhedral and are rarely Formation consists of ~200 m of conglom- predominantly graywacke (40 to 55 per- larger than 1 sq mm in cross-sectional area. erate, sandstone, and green and red silty cent), chert (10 to 30 percent), metavolcanic The plagioclase laths are subhedral to claystone. The conglomerate is composed of rocks (20 percent), serpentinite (3 percent), euhedral and are as much as 3 mm long. 1 to 10 percent cobbles and boulders and and blueschist (<1 percent). Cambria Felsite Glomeroporphyritic texture is common. about 65 percent pebbles in a poorly sorted, clasts or felsite clasts derived from the The plagioclase is complexly twinned and sandy matrix. The percentage compositions underlying felsite of the type that crops out normally zoned from sodic andesine to soclic of the clasts in the basal 20 m of the Lospe at Black Mountain and immediately below oligoclase. Biotite flakes are euhedral and Formation are Morro Rock—Islay Hill the Lospe conglomerate are rare or absent. generally <1 mm in diameter. The ground- complex plus Cambria Felsite, 10 to 40; The Lospe Formation is equivalent to an mass is aphanitic. chert, 25 to 90; vesicular basalt, 10 to 20; undetermined part of the nonmarine Sespe and Franciscan graywacke and metavol- Formation exposed ~250 km to the south, Stratigraphic Relations canic rocks, 1 to 20. Locally Cretaceous(P) in Santa Barbara County. The name Lospe The Cambria Felsite unconformably sandstone or graywacke clasts are present in Formation is used because of the similarity overlies Franciscan rocks and is unconform- the Lospe. The upper 180 m of the Lospe between the lithology of the rocks in the ably overlain by the Lospe, Vaquerías, Formation consists of lenticular beds of Cambria area and those of the Lospe Sandholdt, or Pismo Formations. The conglomerate, sandstone, and red and green Formation located —120 km to the south, in stratigraphic relations are best seen a few claystone. the Santa Maria basin (Woodring and kilometers southeast of Black Mountain. About 5 km southeast of Black Mountain, Bramlette, 1950).

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~~Qal GEOLOGIC MAP OF THE CAMBRIA AREA,~~

~~Alluvial deposits Sand, gravel , silt (stream terrace deposits included) SAN LUIS OBISPO COUNTY,~~

~~Qls Landslide deposits CALIFORNIA~~

~~Qpt~~

~~Pleistocene (?) terrace deposits Loosely consolidated white to buff sandstone and conglomerate~~

C Tmp sri s MILE Pismo Formation 2 KILOMETERS Tmpc, white weathering claystone, resistant to I <—1 I—I I—i i—I t=T weatheringiTmpd, diatomite or diatomaceous si It- stone, Tmpsl, white to brown weathering siltstone CONTOUR INTERVAL 200 FEET and claystone, locally resistant to weathering ^ Tmpcg, cobble conglomerate,clasts of Cambria Felsite and Franciscan rocks^Tmps,soft, brown, friable, locally pebbly quartzose sandstone 35035'35m UNCONFORMITY

~~Tmm N~~

~~Monterey Formation Tuffaceous siltstone, dolomitic siltstone,calcare ous siltstone [Sandholdt Member or FmJ~~

~~/•.Tm o :•.'••••~~

~~Obispo Formation Resistant to soft tuff, contains pumice and glass shards~~

~~< Tmr Rincon Shale White to light brown or buff siltstone and light greenish-gray claystone~~

~~Tov~~

~~Vaqueros Sandstone White to gray or brown sandstone and conglomerate. Locally calc. or fossiliferous and glauconitic~~

~~Tol I2I°05~~

~~Lospe (?) Formation Red, green, and brown sandstone, siltstone, claystone, and conglomerate UNCONFORMITY~~

~~Toe Cambria Felsite Hard to soft biotite crystalline felsite and tuff; Tv, basalt~~

~~Kss 35°32'30~~

~~Cretaceous Sandstone Brown,hard to soft, poorly bedded micaceous wacke Geologic contact-, dashed where approximately located, dotted where buried~~

Jf High-angle fault; dashed where approximately located,dotted where buried Franciscan Rocks and Serpentinite Jfgw, graywacke; ch,chert j bs, blue schist; mv, metavolcanic rocks; s, serpentinite Thrust or reverse fault; dashed where approximately located or inferred. Saw-teeth on Symbols upper plate X59I5 i t UCLA fossil locality number Syncline; dashed where trace approximately located,clotted where buried O SL0-7I CAH 5-70 Rock locality Anticline; dashed where trace approximately located, dotted where buried Geology by Mappable Bed Symbols —o—o— conglomerate C. A. Hall a W.G. Ernst sandstone Inclined Vertical siltstone Strike and dip of bedding Drafted by M. J. Guenther

~~I2I°02'30" I2I°00' CROSS SECTIONS~~

~~c % » r- o CQ-a. £ CL CL E E.EE E S; A' I—M—F= O Sea level Sea level Sea level -Sea level~~

-n

~~^ Oj/^—^ )\ o jf ojj. •2000 ft. -2000 ft. -2000 ft. ^ : -2000 ft h N45°E-~~

~~Scale 1000 3000 5000 7000 Feet~~

~~2 Kilometers~~

~~Figure 2. Geology of Cambria area, San Luis Obispo County, California.~~

~~ERNST AND HALL, FIGURE 2 Geological Society of America Bulletin, v. 85, no. 4~~

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TABLE 2. ESTIMATED MODES, CAMBRIA FELSITE Sandstone locally contains Crassostrea vaquerosensis (Loel and Corey) and Mytilus Sample No. An cont. Plag. Qtz. Bio. Hornbl. Iron Matrix Shards Embayed Remarks Oxides Q,tz. or Crenomytilus sp. The following taxa have been collected from the Vaqueros Sandstone CAH-5-70 15-21 13 tr 1 0 1 35 no no in the Cambria area, and a similar fauna is ti CAH-9-72 % 20 3 3 tr 0 1 93 tr present in the formation near Black

~~H Mountain: Cancdlaria cf. C. dalliana CAH-10-72 18-27 10 15 5 0 tr 70 yes (3) Anderson, Miltha sanctaecrucis (Arnold), H H CAH-11-72 23-24 5 15 4 0 1 75 (3) Crenomytilus expansus (Arnold),~~

~~11 Crassostrea eldridgei ynezana (Loel and CAH-12-72 It 3 0 1 " 23-35 17 75 (3) Corey), Crassostrea titan (Conrad), SLO-69 23-25 16 tr 3 0 1 30 no Panopaea generosa (Gould), Leptopecten~~

H andersoni (Arnold), Lyropecten magnolia SLO-7OA 19-24 tr 3 2 80 " 15 0 (1) (Conrad), and Vertipecten cf. V. perrini it II SLO-7OB 20-23 11 0 tr 0 1» 85 (1) (Arnold). These fossils from the Vaqueros Sandstone correlate with the "Vaqueros 15-20 tr 0 ri 11 SLO-7OC 15 0 5 80 Stage" (Addicott, 1972, 1973). The Rincón It SLO-7OD 18-24 10 tr 0 0 1 89 Shale, —80 km south of the Cambria area, contains foraminifers of Zemorrian age SLO-7OE T, 20 3 tr 11 tr 7 0 tr 90 (Kleinpell, 1938; Hall and Corbató, 1967). SLO-7IA 13-18 10 tr tr 0 tr 90 no The Vaqueros Sandstone underlies the Rincón Shale both there and in the Cambria 11 tt " SLO-7IB 18-21 0 1 0 3 85 region and ::s therefore assumed to be no SLO-78A ^ 20 tr 3 0 0 tr 97 H yes younger than Zemorrian. The Rincón Shale it is overlain by the Obispo Formation. The SLO-86 11-17 3 5 2 0 tr 90 ? tuff of the Obispo Formation yields it SLO-87 •V 25 2 1 0 0 0 97 no (2) radiometric ages of 15.3 ± 0.5 to 16.5 ± 0.8 m.y. (Turner, 1970). It is clear that the SLO-88 24 5 10 tr 0 tr 35 „ Vaqueros Sandstone must be older than 16 m.y. According to Turner (1970), the Note: (1) abundant carbonate in matrix; (2) obsidian; (3) flow. Zemorrian Stage is older than 22.5 m.y. Kleinpell and Weaver (1963) considered the Zemorrian Stage to be Oligocene in age, There is a gradational and, locally, an clasts in the conglomerate have a percentage Lipps (1967) correlated it with the European interfingering contact between the Lospe composition of chert, 30 to 90; graywacke, Chattian and lower Aquitanian Stages and Formation and the overlying 300 to 400 m 15 to 20; blueschist, 10; and hypabyssal considers it to be Oligocene. Evernden and of Vaqueros Sandstone within the Cambria pluton plus felske, 10 to 3 '). North of Santa others (1964) correlated the Zemorrian and Black Mountain areas. However, the Rosa Creek, a basal conglomerate within the Stage with the European Burdigalian and Lospe Formation is not present everywhere. Vaqueros Sandstone is 95 percent Cambria Aquitanian Stages. Berggren (1969) corre- Locally, the Vaqueros Formation is con- Felsite and 5 percent Franciscan chert; lated the Zemorrian with the lower part ol: glomeratic. South of the town of Cambria, barnacle fragments are locally common. the BurdigaLan, the Aquitanian, Chattian. that is, south of Santa Rosa Creek (Fig. 2), the The basal 7 to 20 m of the Vaqueros Rupelian, and part of the Sannoisian. Berggren (1969, Table 3) considered the TABLE 3. ESTIMATED MODES OBISPO FORMATION Zemorrian to be Oligocene in age and to have a base about 35 m.y. old (planktonic Sample No. An cont Plag Qtz. Bio. Hornbl. Iron Matrix Shards Embayed Remarks Oxides Qtz. foraminiferal zone P 19) and a top about 22.5 m.y. old (planktonic foraminiferal zone slo-43a •> 1 1 0 0 tr 98 yes no P 22). H H SLO-56A 20-22 5 5 tr 0 tr 90 The Lospe Formation is older than the M H SLO-56B 20-22 5 4 1 tr! tr 90 Vaqueros Sandstone. It underlies, or, rarely, ti interfingers with, the Vaqueros Sandstone SLO-56C lU-16 6 3 0 0 1 90 and is therefore probably older than the H it SLO-72 15-18 15 tr 0 0 tr 85 upper 22.5-m.y. age limit of the Zemorrian ti it SL0-73A ? 2 3 0 0 tr 95 Stage. Approximately 50 km to the south in h M the Nipomo (Hall and Corbató, 1967) and SLO-73B 15-17 13 2 0 0 tr 85 Arroyo Grande quadrangles (Hall, 1973a), it ti SLO-8OA 12-14 3 2 0 0 tr 95 (1) as well as in the Cambria area, the Lospe it Formation contains clasts of dacite derived 2 3 0 0 tr it (1) SLQ-80B 15-18 95 from the Morro Bay—Islay Hill complex, ti SLO-8IA 23-28 14 6 0 0 0 80 1 (2) which has been dated as between —22.1 ± it 0.9 and 26.5 ± 0.8 m.y. (Turner, 1968; SLO-8IB 24-30 20 5 0 0 tr 75 •? (2) Turner and cithers, 1970). Thus, the Lospe Formation is probably no older than —26 Note: (lì abundant carbonate in matrix; (2) devitrified glass. m.y.

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Correlation Based on petrographic and bulk chemical relations now to be presented, as well as on the above stratigraphic relations, the Cam- bria Felsite is correlated with late Oligocene volcanic events that resulted in the emplacement of the Morro Rock—Islay Hill complex. The extrusive felsite and the intrusive complex are locally transitional in the Los Osos Valley. Although Figure 1 shows all the Los Osos outcrops as members of the hypabyssal intrusive series, some are apparently flows or tuff and correlate SL0-93A SL0-54B petrographically with the extrusive series.

PETROLOGY OF THE TERTIARY IGNEOUS ROCKS OF V! L " V * >%€ SAN LUIS OBISPO COUNTY Petrography ; We examined 51 thin sections of rocks from the investigated area, using a flat-stage microscope. Estimated modes are listed in Tables 1 to 3. Table 4 summarizes com- t .. - »V Vf V-, Q „1 * » / v• , , f i 4 'f posite characteristics and average phase r. * , r a 1* •i * proportions. V* The rocks of the Morro Rock-Islay Hill complex are porphyries containing abun- CAH-10-72 dant euhedral phenocrysts; crystals range up to 2 or 3 mm in size and consist of j • , r ^ • : Vswwfi^TMl intermediate plagioclase, quartz, brown biotite, pyroxenes locally (C.A. Hopson, 1973, written commun.), magnetite, and, in many samples, green hornblende (Fig. 3, F SLO-93A). The andesine-oligoclase grains a show marked oscillatory zoning, bu;, in

general, they have calcic cores and more i Br sodic rims; the plagioclase occurs as broken «& I crystals and as glomeroporphyritic groups. i?» In most specimens, embayed quartz grains •V (some of which appear to be xenocrysts) reflect strong resorption by the melt (Fig. 3, NBONKS uiiot*. SLO-54B, SLO-77). The biotite and SLO-69 SLO-73A especially the hornblende have bee:i se- 00 0.2 0.4 06 0.8 1.0 1.2 1.4 16 16 2.0 mm verely oxidized and dehydrogenated; in I L J L _1_ many samples these minerals are rimmed by opaque selvages, chiefly of magnetic, or Figure 3. Photomicrographs of Tertiary igneous rocks studied. All samples are at same magnification and, except they are pseudomorphed by chlorite, clays, for SLO-77, shown in crossed nicols, all photomicrographs are in plane light. A, amphibole; B, biotite; PI, plagioclase; and iron oxides. The groundmass consti- Pum, pumice fragment; Q, quartz. Note felted mosaic in groundmass of Morro Rock-Islay Hill complex (SLO-93A, tutes about half of these rocks by volume SLO-54B, SLO-77) and Cambria Felsite (CAH-10-72, SLO-69), and shards in Obispo Formation (SLO-73A). and consists of a fine-grained felted to granular aggregation of the phases just TABLE 4. COMPOSITE CHARACTERISTICS AND AVERAGE MODES OF THE ROCK TYPES described plus, in some cases, minor potassic feldspar; the individual grains are anhedral Average Mode Percent Exhibiting No. of Extreme to subhedral and average 0.02 to 0.06 mm in Unit Iron Embayed diameter. Samples An range Plag. Stz. Bio. Horribl. Oxides Matrix Shards Qtz. The Cambria Felsite contains moderate Morro Rock- amounts of crystal grains in the 1- to 2-mm Islay Hill 23 11-1*9 30 5 It 2 2 57 0 7k size range (Fig. 3, CAH-10-72, SLO-69). complex Most of these crystals are slightly zoned Cambria 11-35 euhedra and glomeroporphyritic clots of Felsite 17 8 5 1 0 1 85 0 53 oligoclase, and less commonly quartz Obispo anhedra, some of which show the effects of Formation 11 12-3o 8 3 tr 0 tr 89 100 0 resorption. Employing the 20 i3i—i:n x-ray

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~~TABLE 5. BULK CHEMICAL ANALYSES OF MORRO ROCK - ISL/Y HILL COMPLEX~~

Sample dumber Oxide Wt. percent CAH- CAH- CAH- CAH- SLO- SLO- SL0- SL0- SL0- SL0- £L0- SL0- SL0- SL0- SL0- SL0- 1-72* 2-72* 3-72* 4-72* 75A 75B 75C 78B 79A 79B S1A 91B 92A 92B 93A 93B SIOG 68.67 68 AO 72.30 74.10 64.07 64.50 64.35 68.50 61.40 63.20 66.13 65.92 64.70 66.74 65.77 65.82

~~AI2O3 15.33 14.68 12.97 13.11 16.36 16.33 15.98 16.39 16.01 16.39 17.63 18.00 15.35 17.19 14.41 14.46~~

~~CaO 2.17 2.65 2.14 1.43 3.6o 3.89 4.54 2.27 4.47 3.75 5.90 3.86 5.66 2.89 3.47 3.41~~

~~NA20 3.84 4.36 3.4o 3.79 4.12 3.88 3.32 4.20 4.08 3.96 :i.92 4.06 3.22 2.97 3.63 3.67~~

~~KgO 5.61 4.53 4.19 3.71 2.86 2.83 3.00 3.04 2.52 2.63 :>.00 3.07 3.31 2.87 2.92 2.98~~

~~Fe asFegO^ 1.39 2.24 2.37 2.11 4.15 4.50 3.87 2.60 5.95 4.95 1.55 1.32 2.26 1.81 4.73 4.63~~

~~MgO 0.18 o.3l 0.17 0.21 1.10 1.81 1.53 0.55 1.86 1.55 C.24 0.21 0.90 0.53 1.38 1.39~~

~~TiOg 0.28 0.27 0.24 0.22 0.58 0.54 0.46 0.32 0.70 0.65 0.66 0.66 0.58 0.60 0.60 0.60~~

~~MnO •• •• •• 0.08 0.06 0.06 0.06 0.06 0.05 •• •• •• •• ••~~

~~Ign.Loss 1.74 1.97 1.55 0.74 2.95 1.47 2.78 1.46 2.38 2.23 2 72 2.23 3.71 4.23 2.U 2.43~~

~~Total 99-21 99.41 99.33 99.42 99.80 99.38 99.43 99.36 99-75 99-33 S»9.69 99.83 99.02 99.39 percent 99.87 99.89~~

*Clast in Lospe Formation. Analyst: G. Stummer.

diffraction method of Smith and Yoder and quartz. The plagioclase is virtually amounts of embayed quartz grains. Clearly, (1956), the plagioclases of CAH-10-72 and unzoned oligoclase and occurs as minute three petrographically distinct igneous rock CAH—12—72 are An22±05 and An2S±o5! euhedra and subhedra, or more commonly se'ies are present in the area studied, respectively, and appear to be of low as anhedral broken crystals. The quartz although matrix textures and phenocryst structural state. Hexagon al plates of heavily grains are anhedral fragments. Some sam- proportions are intergradational between oxidized brown biotite and fine, granular ples contain traces of biotite and magnetite, the Morro Rock and Cambria units. magnetite are the other megascopically but these are not common. The matrix visible phases. Hornblende has not been consists of glass shards, colkpsed pumice Bulk Chemistry identified. For the soft tuffs, the matrix lapilli, and microperlite, altered and re- X-ray fluorescence analyses for 16 rock evidently was initially hyaloclastic material. placed to a small but variable extent by samples of the Morro Rock-Islay Hill Subsequent to the accumulation of the cryptocrystalline carbonate, unidentified complex, 12 of the Cambria Felsite, and 8 of felsites, this mesostasis was strongly recrys- zeolite(s), and clay minerals. tuf ? within the Obispo Formation are listed

tallized and now consists of plagioclase, As is evident from these descriptions, the in Tables 5 to 7. Although H20 and C02 quartz, iron oxides, and minor chlorite; indi- three rock types arc readily distinguished were not determined separately, it is clear vidual grains are ~0.01 to 0.03 mm in diame- under the microscope. Morrc Rock—Islay from examination of the ignition losses and ter, except for finely fibrous intergrowths, Hill rock specimens ¿re the coarsest grained CaO + MgO contents that several speci- clots, and sprays of sodic plagioclase (and and most mafic of all; they carry abundant mens have been s ubjected to extensive post- K-feldspar, detectable by x-ray techniques phenocrysts, including partia.ly resorbed igntous carbonation (Cambria Felsite sam- but optically irresolvable) present in some quartz grains, markedly zoned plagioclase ple SLO—70A, and Obispo Formation sam- rocks. Relict shards have not been identified crystals, biotite flakes, and, commonly, ples SLO-8OA and SLO-8OB; see also esti- in any of the specimens examined; question- minor hornblende. On the other hand, the mated modes in Tables 2 and 3). able relict shards were found in SLO—86, highly felsic Obispo tuffs contain only minor Tie compositional distinctions of the however. Individual samples represent por- amounts of finer grained, more homogene- thre: units are presented graphically in tions of a complete gradation in matrix tex- ous plagioclase and ouartz set in a glassy, Figure 4. Except for the four clasts of the ture and mineralogy between the Morro shard-rich mesostasis. The Carr bria Felsite Moiro Rock—Islay Hill complex collected Rock—Islay Hill complex and the Cambria is intermediate between tie Morro frorr the Lospe Formation (CAH-1-72, Felsite. Rock—Islay Hill complex and Obispo tuff CAFI-2-72, CAH-3-72, and CAH-4-72), The Obispo Formation is a crystal- lithologies with respect to pheno- and the sample of obsidian collected at Black bearing vitric tuff (Fig. 3, SLO-73A; see cryst/matrix ratio, fe sic/mafic ratio, and Mountain (SLO-87), the various igneous Hall and others, 1966; Surdam and Hall, grain size. However, in contrast to the tuff units show marked group compositional 1968; Surdam and others, 1970). Mineral within the Obispo Formation, it is recrystal- conti asts. The clasts in the Lospe and the old grains typically make up about one-tenth of lized and contains no recognizable shards; obsidian probably have suffered strong the rock by volume, are —0.01 to 0.50 mm unlike the Morro Roc.c—Islay Hi. 1 complex, postij;neous alteration during sedimentation in size, and consist principally of plagioclase it lacks hornblende and carries lesser of the younger rocks. Although the Morro

~~80.0 (a) 12.0 (b)~~

~~• ' \ 76.0 .Cambri«<. a 10.0 ^ A\ \ Xvna ^ Obispo \ \ 72.0 8.0 OJ • \ CO O o~~

~~CO 68.0 LJP o 6.0 Obispo t / 64.0 + 4.0 \\ + • 'Morr o Rock- / + Islay Hill Cambria / + Morro Rock_ + + + 60.0, _L 2.0 X^ Islay Hill y 10.0 12.0 14.0 16.0 18.0 20.0 22.0 • DdP K AU0 2^3~~

~~0.0,0. 0 1.0 2.0 3.0 4.0 5.0 6.0 5.0 (C) Fe203~~

\ \ 4.0 • « \a a*\ x t> • \ 0.7 Morro \ + \ (d). \ 1=1 \Cambria 30 Rock\+_^/ 0.6 O Islay Hil 3 0J 0.5 o .-.X-X Obispo / 2.0 \ \9 -04 A—/ Morro Rock- \ VA A I^. i- 0.3 Obispo X X Islay Hill 'A* X X 1.0 0.2 I Ca_mbria \ 0.1 \ 0.0, LA Ai 1— 0.0, JL 1.0 2.0 1.0 4.0 50 6.0 7.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0

K2O CaO Figure 4. Compositional variations of Tertiary igneous rocks studied. Crosses, outcrop samples of Morro Rock-Islay Hill complex; Xs, clasts of Morro Rock-Islay Hill complex in Lospe Formation; open squares, Cambria Felsite; solid squares, Cambria obsidian; triangles, Obispo Formation; asterisk, abundant carbonate in sample.

Rock and Cambria units show systematic CAH-10-72, CAH-11-72, and CAH- Because of their acicular shapes, the rare chemical differences, they are, nevertheless, 12—72), which crop out as the carapace of pyroxene and hornblende in general proba- mineralogically and texturally intergrada- plugs in the Morro Bay area and are not bly would fall nearer to the vents compared tional. Reasons for the compositional distinguished from the hypabyssal intrusive to biotite flakes, which would be carried contrasts are discussed below. series in Figure 1 (see Hall, 1973b). longer distances and therefore would have However, the much lower proportions of accumulated in the Cambria Felsite. Petrologic Conclusions crystals in the Cambria Felsite compared to An alternative explanation, in accord Textural and field relations demonstrate those in the contemporaneous Morro with observations in the Cascade Mountains that the Morro Rock-Islay Hill complex is a Rock-Islay Hill complex suggest the possi- (Hopson, 1972) and elsewhere in the series of hypabyssal to penesurficial plugs, bility of winnowing of airborne material, western conterminous United States (C. A. lava domes, and sill-like bodies, whereas the promoting concentration of the less dense, Hopson, 1973, written commun.), involves Obispo Formation is dominantly of pyro- more silicic, alkalic ashy material in the postulated shallow-level liquid fractiona- clastic origin. Although some of the Cambria felsites. This might account for the tion in the conduits during ascent toward the Cambria felsites are clearly soft tuffs, most highly silicic and potassic nature of the surface. H20, silica, and alkalis evidently of this unit is so thoroughly devitrified that Cambria Felsite and for depletion in were concentrated toward the top of the magma column. This material would have determination of origin is problematical; a constituents such as iron oxide, Ti02, and few of the more coarsely crystallized fe .sites CaO, which are concentrated in the mafic been blown out of the vents initially, leaving probably are flows (such as samples minerals (hornblende and rare pyroxene). behind plugs or necks depleted in these

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~~Sample Number Oxide, wt. CAH- CAH- CAH- CAH- SL0- SL0- SL0- SL0- SLO- SLO- SLO- SL0- percent 5-70 9-72 11-72 12-72 69 70A* 70C 7 3J. 78A 86 87+ 88~~

~~Si02 73.60 75.22 74.16 73.67 72.95 64.12 73.50 71.50 70.68 76.23 71.68 78.11~~

~~AI2o3 15.46 13.42 14.22 13.77 16.00 12.25 16.90 17.55' 18.40 12.22 11.62 11.69~~

~~CaO O.76 0.46 0.27 0.85 0.80 5.60 0.35 0.6~~

~~Na20 4.25 3.57 1.54 2.95 4.40 3.88 1.58 3.6c. 3.24 3.85 3.96 1.83~~

~~K2o 4.06 4.88 6.33 5.35 4.50 4.70 5.75 4.0; 4.86 4.10 1.60 6.11~~

~~Fe as FegO^ 0.67 0.75 1.02 1.22 0.43 1.60 0.15 0.4S' 0.24 0.87 1.28 0.27~~

~~MgO 0.12 0.11 0.10 0.26 0.06 2.08 0.00 0.2Ì 0.04 o.4o o.l3 0.06~~

~~Ti02 0.10 0.07 0.10 0.10 0.10 0.10 0.10 0.1c 0.09 0.08 0.10 0.08~~

~~MnO 0.02 0.01 0.06 0.01 0.01 0.02~~

~~Ign. Loss 1.01 1.49 2.40 1.09 0.67 6.45 1.76 1.82 1.97 1.72 7.96 1.14~~

~~T°percent 100•05 99.97 100.14 99.26 99.92 100.84 100.10 100.08 99.90 99.69 99.53 99.46~~

*Abundant carbonate. tObsidian. Analyst: G. Stwmner.

constituents, in accord with the observed analysis) suggests that portions of Rocky motions (p. 3235-3336).] Volcanism, ac- chemical differences between the Cambria Butte are petrographically and chemically cording to Yeats (1968), presumably re- Felsite and the Morro Rock-Islay Hill intermediate in character between the sulted from the juxtaposition of a high complex. The lesser degree of resorption of Morro Rock—Islay Hill complex and the heat-flow regime, the East Pacific Rise, quartz observed in the Cambria Felsite is Cambria Felsite. Their geomorphic de- against a more normal thermal environ- probably a function of the duration of velopment suggests that the Rocky Butte ment, the continental slope and shelf immersion of crystals in the magma under bodies may represent remnants of volcanic sediments draped on the western margin of near-surface conditions; evidently, the necks similar to those in the Los Osos Val- ti e Americas plate. Cambria pyroclastic material was expelled ley, thus — if they are of the proper age — Such an explanation does not readily relatively rapidly, whereas intratelluric providing another possible source for the account for the two distinct episodes of crystals of quartz remaining within the Cambria Felsite. scmewhat similar igneous activity in west- Morro Rock—Islay Hill volcanic necks ern San Luis Obispo County (that is, events during times of quiescent magma emplace- PLATE TECTONICS w nich produced the upper Oligocene Morro ment had longer to become resorbed. CONSIDERATIONS Rock—Islay Hill complex and Cambria The Cambria Felsite lies ~25 to 30 km Based on the inferred overriding of the Ft lsite and the Miocene Obispo Formation), north-northwest of the Morro Rock—Islay East Pacific Rise by the wesiern margin of tvo episodes separated by a nonvolcanic Hill complex as now exposed. The plugs the Americas plate (Menard, 1964, Chap. hiatus of ~ 10 m.y. In this regard, it may be might extend subsea some 25 km to the 6), or the coming together of ridge and worth noting that the Morro Rock—Islay northwest, thus bringing the hypothesized trench and subsequent triple junction—type Hill complex and presumably also the source vents of the pyroclastic material transform faulting which led to the San Cambria Felsite have isotopic ages of 22.1 ± closer and more nearly upwind(?) relative to Andreas system (Wilson, 19 55 ; McKenzie 0.? m.y. to 26.5 ± 0.8 m.y. (Turner, 1968; the preserved site of accumulation. How- and Morgan, 1969; Atwater, 1970), Yeats Turner and others, 1970); this is about the ever, another series of pluglike felsic igneous (1968, Fig. 1) has recognized a wave of 24.6 ± 2.5 m.y. timing of the increased bodies lies 5 to 15 km to the north and Miocene volcanic activity that: proceeded te :tonic activity in the western Pacific island northeast (that is, presumably upwind) of from west to east with time, through the arcs and change in plate motions deduced the present exposures of the Cambria and California Coast Ranges we st of the San from the bend in the Emperor-Hawaiian Black Mountain pyroclastic rocks. The Andreas fault. [Hawkins (1970) has also chain (Jackson and others, 1972, p. pluglike igneous masses include Rocky documented mid-Miocene and Pliocene- 61 0—613). These igneous rocks could be an Butte, 12 km N. 20° E. from the most Pleistocene, chiefly basaltic volcanism of expression of the postulated Farallon plate northerly outcrop of Cambria Felsite shown southernmost California and Baja Califor- subduction during middle Tertiary time in Figure 2 (see Jennings, 1959). Reconnais- nia; he related this magma generation to (Atwater, 1970). The Obispo tuffs yield sance investigation (field study, sample col- dilation of the crust and upper mantle in radiometric ages of 15.3 ± 0.5 m.y. to 16.5 lection, and both chemical and petrographic response to late Cenozoic lithospheric plate + 0.8 m.y. (Turner, 1970), in reasonable

TABLE 7. BULK CHEMICAL ANALYSES OF OBISPO FORMATION and Si02 contents from 2.52 to 3.31 and from 61.40 to 68.50 weight percents, Sample Number Oxide respectively, and averaging 2.92 K20 and wt. percent SLO- SLO- SL0- SL0- SL0- SLO- SL0- SLO- 65.09 Si02 weight percents for the twelve 56A 56b 73a 73b 80A* 8ob* 8ia 8ib samples. (These values were not recalculated to 100 percent on an anhydrous basis.) If 68.50 68.50 67.60 69.47 55.1+0 1+0.00 67.96 68.28 Si02 one were to assume an area of magma generation associated with the Benioff zone, ai2o3 13.30 13.50 13.50 12.12 7.32 4.01 11.37 11.83 the Morro Rock-Islay Hill melts evidently CaO 0.78 0.70 1.81* 1.47 8.21 15.50 1.30 l.ll would have been derived from depths approaching 200 km, judging from the Na20 0.00 0.00 0.00 0.63 1.10 2.30 1.87 2.19 relations presented by Dickinson (1970, Fig. 3). (The presence of xenocrystic quartz and k2O 4.97 4.15 3.21 2.88 2.1* 3.39 5.81 6.1+1+ possibly also plagioclase, evidently derived when the magma ascended through the Fe as Fe203 1.80 1.9k 1.80 2.32 2.1+7 1.1+2 2.76 0.7I+ Franciscan, indicates that the chemical MgO 0.60 0.70 1.06 0.37 3.72 6.17 0.11 0.21 analyses in Table 5 are biased somewhat toward high silica; if the parental melt were Ti02 0.35 0.39 0.33 0.31+ 0.18 0.11 0.28 0.29 appreciably lower in Si02 for a given K20 MnO 0.01 0.01 0.01 o.o3 0.06 content, this would mean an even greater depth to the inclined seismic zone.) How Ign.Loss 9.90 10.17 11.31 9.86 19.80 27.1+6 8.53 8.65 such melts could have risen toward the surface without severe reaction with the mantle is unclear. Assuming a 45° dip for the Total 100.21 100.06 100.66 99.1+6 100.67 100.1+2 99.74 Percent 99.99 paleoseismic zone, the late Oligocene plate junction would have been situated an equivalent distance — 200 km — to the west * Abundant carbonate. of the present exposures. During the Analyst: G. Stummer. following 10 m.y., trench and spreading center would have had to collide as the triple accord with a postulated collision between Tertiary) mélange sections have been junction (ridge-trench-transform) passed the East Pacific Rise and the Americas plate, "underplated" along the front of the this section of the Americas plate on its assuming constant relative motion (Atwa- Americas plate, we might suppose that the migration northward (see, for example, ter, 1970, Figs. 5, 6, 17). Tertiary igneous rocks of the California McKenzie and Morgan, 1969; Atwater, However, judging from the radiometric Coast Ranges are related to the inclined 1970). The Obispo Formation may reflect data for metamorphosed Franciscan rocks seismic (Benioff, Wadati) zone marking the this later thermal event at the continental (Suppe and Armstrong, 1972), the Mesozo:.c convergent plate junction between the margin. trench mélange was associated with the Farallon plate and the Americas plate prior In summary, although we see no obvious much older Kula plate or its precursor to inception of the currently extant San correlation between inferred mid-Tertiary (Atwater, 1970, p. 3531-3532; Grow and Andreas transform fault system. Such plate tectonics and the episodic igneous Atwater, 1970), as well as a Late Creta- relations are characteristic of the Pacific activity in western San Luis Obispo County, ceous stage of the Farallon plate (Larson an d margin (see Dickinson, 1970, 1972). How- it is likely that the generation of these rocks Chase, 1972, Fig. 14; Larson and Pitman, ever, inasmuch as the late Mesozoic volcanic occurred as a complex crustal response to 1972, p. 3657). Rapid subduction of this front is marked by the Klamath—Sierra Farallon plate underflow (resulting in complex terminated long before the timing Nevada-Salinia-Peninsular Ranges vol- Morro Rock-Cambria units) and im- of the middle Tertiary triple junction, canic-plutonic terranes, a subsequent local pingement of the East Pacific Rise with this discussed above. Sections of the Francisca n destruction of the arc-trench gap, allowing section of the Americas plate, and conse- ferrane certainly were exposed to erosion juxtaposition of the Franciscan and Salinian quent high heat flow in the California Coast prior to deposition of the Vaqueros (also Kalmath) rocks (Page, 1970b), fol- Ranges (resulting in the Obispo Forma- Sandstone in the San Luis Obispo region lowed by a westward stepping of the Benioff tion). (Page, 1970a) and also before deposition of zone, would have been required in Tertiary the Cambria Felsite and the Lospe Forma- time; no obvious evidence for this exists in ACKNOWLEDGMENTS tion as well, at least in the area described in the offshore magnetic signatures, although a We thank the University of California, this report and shown in Figure 2. As Page pre—anomaly 13 "disturbed zone" occurs Los Angeles, for support during this study. has pointed out (1973, written commun.), south of the Murray fracture zone (Heirtzler W. R. Dickinson and B. M. Page, Stanford the Morro Rock-Islay Hill complex was and others, 1968; Atwater and Menard, University, and C. A. Hopson, University of emplaced along a remarkably straight line 1970). California, Santa Barbara, provided helpful (see Fig. 1); perhaps intense deformation of Sufficient chemical data for the Morro critical reviews. the Franciscan basement ended here before Rock-Islay Hill complex are available this late Oligocene igneous event. Thus, (Table 5) to employ the Dickinson and REFERENCES CITED rapid convergent motion at the western Hatherton (1967) method in an attempt to margin of the Americas plate may have estimate roughly the depth to such a Addicott, W. O., 1972, Provincial middle and late Tertiary molluscan stages, Temblor Range, ceased in the region some time before the postulated paleoseismic zone. Analyses for Calfornia, in Pacific Coast Miocene Bio- volcanic episodes described in this report. clasts in the Lospe were not used because of stratigraphic Symposium, Proc.: Tulsa, If we regard this evidence from the the possibility of postigneous alteration. Okla., Soc. Econ. Paleontologists and Franciscan basement as inconclusive, by Considering the fact that xenocrysts are Mineralogists, Pacific Sec., p. 1-26. assuming, for instance, that more recently present, the intrusive rocks are remarkably 1973, Oligocene molluscan biostratigraphy

~~subducted and as yet unexposed (lower homogeneous chemically, ranging in K20 and paleontology of the lower part of the~~

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type Temblor Formation, California: U. S. p. 3323-3338. 1970b, Sur-Nacimiento fault zone of Geol. Survey Prof. Paper 791, 48 p. Heirtzler, J. R., Dickson, G. O., Herron, E. M., California; continental margin tectonics: Atwater, T., 1970, Implications of plate tectonics Pitman, W. C., III, and Le Pichon, X., 1968, Geol. Soc. America Bull., v. 81, p. 667-690. for the Cenozoic tectonic evolution of Marine magnetic anomalies, geomagnetic Smith, J. R., and Yoder, H. S., Jr., 1956, western North America: Geol. Soc. America field reversals, and mctions of the ocean Variations in x-ray powder diffraction Bull., v. 81, p. 3513-3536. floor and continents: Jour. Geophys. Re- patterns of plagioclase feldspars: Am. Atwater, T., and Menard, H. W., 1970, Magnetic search, v. 73, p. 2119-2136. Mineralogist, v. 41, p. 632-647. lineations in the northeast Pacific: Earth and Hopson, C. A., 1972, Origin of coherent and Smith, P. B., and Durham, D. L., 1968, Middle Planetary Sci. Letters, v. 7, p. 445—450. divergent lava suites at O.uaternary andesitic Miocene Forarninifera and stratigraphic Berggren, W. A., 1969, Cenozoic chronostratig- volcanos, Cascade Mountains: Geol Soc. relations in the Adelaida quadrangle, San raphy, planktonic foraminiferal zonation, America, Abs. with Pre grams (Cordilleran Luis Obispo County, California: U.S. Geol. and the radiometric time scale: Nature, v. Sec.), v. 4, nc. 3, p. 172-173. Survey Bull. 1271 A, p. A1-A14. 224, no. 5224, p. 1072-1075. Hsii, K. J., 1969, Preliminary report and geologic Suppe, John, and Armstrong, R. L., 1972, Dickinson, W. R., 1970, Relation of andesites, guide to Franciscan mélanges of the Morro Potassium-argon dating of Franciscan granites and derived sandstones to arc Bay-San Simeon area, California: Califor- metamorphic rocks: Am. Jour. Sci., v. 272, tectonics: Rev. Geophysics and Space nia Div. Mines and Geology Spec. Pub. 35, p. 217-233. Physics, v. 8, p. 813-860. 46 p. Surdam, R. C, and Hall, C. A., Jr., 1968, -1972, Evidence for plate-tectonic regimes in Jackson, E. D., Silver, E. A., and Dalrymple, G. B., Zeolitization of the Obispo Formation, the rock record: Am. Jour. Sci., v. 272, p. 1972, Hawaiian-Emperor chain and its Coast Ranges of California [abs.]: Geol. Soc. 551-576. relation to Cînozoic circ ampacific tectonics: America Spec. Paper 101, p. 338. Dickinson, W. R., and Hatherton, T., 1967, Geol. Soc. America Bull., v. 83, p. 601-618. Surdam, R. C., Turner, D. L., and Hall, C. A., Jr., Andesitic volcanism and seismicity around Jennings, C. W., 1959, Geologic map of 1970, Distribution and genesis of authigenic: the Pacific: Science, v. 157, p. 801-803. California, San Luis Obispo sheet: Califor- silicates in the Obispo Formation: Geol. Soc. Evernden, J. F., Savage, D. E., Curtis, G. B., and nia Div. Mines and Geology, scale America, Abs. with Programs (Cordilleran James, G. T., 1964, Potassium-argon dates 1:250,000. Sec.), v. 2, no. 2, p. 151-152. and the Cenozoic mammalian chronology of Kleinpell, R. M., 1938, Miocene stratigraphy of Turner, D. L., 1968, Potassium-argon dates North America: Am. Jour. Sci., v. 262, p. California: Tulsa, Okla., Am. Assoc. Pe- concerning the Tertiary foraminiferal time 145-198. troleum Geologists, 450 p. scale and San Andreas displacement [Ph.D. Fairbanks, H. W., 1904, Description of the San Kleinpell, R. M.. and Weaker, D. W., 1963, thesis]: Berkeley, Univ. California, 99 p. Luis quadrangle: U.S. Geol. Survey Geo- Oligocène biostratigrap hy of the Santa 1970, Potassium-argon dating of Pacific logic Atlas of the United States, San Luis Barbara embayment, California: California Coast Miocene foraminiferal stages: Geol. Folio 101, 14 p. Univ. Pubs. Geol. Sci., v. 43,250 p. Soc. America Spec. Paper 124, p. 91-129. Grow, J. A., and Atwater, T., 1970, Mid-Tertiary Larson, R. L., and Chase, C. E., 1972, Late Turner, D. L., Surdam, R. C., and Hall, C. A., Jr., tectonic transition in the Aleutian Arc: Geol. Mesozoic evolution of the western Pacific 1970, The Obispo Formation and associated Soc. America Bull., v. 81, p. 3715-3722. Ocean: Geol. Soc. America Bull., v. 83, p. volcanic rocks in the central California Hall, C. A., Jr., 1973a, Geology of the Arroyo 3627-3644. Coast Ranges — K-Ar ages and bio- Grande quadrangle, San Luis Obispo Larson, R. L., and Pitman, W. C., III, 1972, chronologic significance: Geol. Soc. County, California: California Div. Mines World-wide correlation of Mesozoic America, Abs. with Programs (Cordilleran and Geology Map Sheet 24. magnetic anomalies, and its implications: Sec.), v. 2, no. 2, p. 155. 1973 b, Geologic map of the Morro Bay Geol. Soc. America Bull., v. 83, p. Wilson, J. T., 1965, Transform faults, oceanic South and Port San Luis quadrangles, San 3645-3662. ridges, and magnetic anomalies southwest Luis Obispo County, California: U. S. Geol. Lipps, J. H., 1967, Planktonic foramanifera, of Vancouver Island: Science, v. 150, p. Survey Misc. Field Studies Map 511. intercontinental correlation and age of 482-485. Hall, C. A., Jr., and Corbato, C. E., 1967, California mid-Ceno;:oic microfaunal Wooclring, W. P., and Bramlette, M. N., 1950, Stratigraphy and structure of Mesozoic and stages: Jour. Paleontology, v. 41, no. 4, p. Geology and paleontology of the Santa Cenozoic rocks, Nipomo quadrangle, 994-999 Maria district, California: U.S. Geol. Sur- southern Coast Ranges, California: Geol. McKenzie, D. P., a id Morgan W. V., 1969, The vey Prof. Paper 222,185 p. Soc. America Bull., v. 78, p. 559-582. evolution of triple junctions: Nature, v. 224, Yeats, R. S., IS'68, Southern California structure, Hall, C. A., Jr., Turner, D. L., and Surdam, R. C., p. 125-133. sea-floor spreading, and history of the 1966, Potassium-argon age of the Obispo Menard, H. W., 1964, Mariie geology of the Pacific Basin: Geol. Soc. America Bull., v. Formation with Pecten lompocensis Arnold, Pacific: New York, McGraw-Hill Book Co., 79, p. 1693-1702. southern Coast Ranges, California: Geol. 271 p. Soc. America Bull., v. 77, p. 443-446. Page, B. M., 1970a, Time of completion of Hawkins, J. W., 1970, Petrology and possible underthrusting of Franciscan beneath Great MANUSCRIPT R ECEIVED BY THE SOCIETY JUNE 1, tectonic significance of late Cenozoic Valley rocks west of Salinia btock, Califor- 1973 volcanic rocks, southern California and Baja nia: Geol. Soc. America Bull., v. 81, p. REVISED MANUSCRIPT RECEIVED OCTOBER 5, California: Geol. Soc. America Bull., v. 81, 2825-2834. 1973

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