CLARENCE A. HALL DONALD L. TURNER RONALD C. SURDAM

POTASSIUM-ARGON AGE OF THE OBISPO FORMATION WITH PECTEN LOMPOCENSIS ARNOLD, SOUTHERN COAST RANGES,

Abstract: Fossil pelecypods, Peclen (Amusiuni) plagioclase obtained from the middle of the tuff lompocensis Arnold and Pecten (Lyropecten) crassi- is 20.9 + 1.5 X 106 years. The age of plagioclase in cardo Conrad, were discovered in rhyohtic tuff and the Morro Rock andesite- volcanic neck is breccia of the Obispo Formation, San Luis Obispo 23.5 ± 1.8 X 10« years. County, California. The potassium-argon age of

Introduction Dating of Rhyolitic Tuff of Obispo Formation Pecten lompocensis Arnold and Peclen crassi- Three fragments of Pecten lompocensis were cardo Conrad were discovered at two localities found in a fine-grained, white rhyolitic tuff within the Obispo Formation, San Luis Obispo containing abundant fragments of pumice. The County, California. To our knowledge, the oc- fossil fragments were a few inches apart strati- currence of marine pelecypods in rhyolitic tuff graphically. Although their measurements in and breccia has not previously been reported. inches are only 3 X 3, 1 X 3, and 1x2, the Because the specimens of P. lompocensis were ribs and rib spacing are so distinctive that there collected from an uncontaminated tuff, the is little doubt as to the fossil's identification. potassium-argon dating method was used in The fossil locality (Fig. 1) is 29,000 feet north this unique opportunity to date a fossil of and 1200 feet east of the southwest corner of narrow stratigraphic range and to provide in- the Nipomo quadrangle (U. S. Geol. Survey, formation on the age of part of a volcanic 1952 Edition). The sample of rhyolitic tuff that formation having widespread correlatives in was dated, however, was taken from a small the Coast Ranges of California. quarry 500 feet to the west in order to insure a The Obispo Formation lies above the Vaque- fresh sample. It is from the same stratigraphic ros and Rincon formations, may locally inter- interval in which P. lompocensis is found. The finger with these formations and the Point Sal age of the tuff, determined from a concentrate Formation, and lies below the Monterey Shale. of plagioclase microphenocrysts, is 20.9 + 1.5 Rhyolitic tuff and breccia cropping out near X 106 years. San Luis Obispo were first named the Obispo The tuff is characterized by 30-40 per cent Tuff Member of the Monterey Shale by Bram- microphenocrysts, averaging 0.3 mm in di- lette (1946, p. 22-23). These rocks can be traced ameter, which are set in a partly devitrified from the San Luis Obispo area southward to glassy groundmass. The percentage composition the region from which we obtained the marine of the microphenocrysts is albite-oligoclase, pelecypods. In this region the tuff is inter- 40; glass shards (?), 40; quartz, 15; mafic miner- bedded with basalt and andesite lavas and with als, 3-5; and some magnetite (?). The plagio- calcareous siltstone or claystone. Because of clase grains are fresh, with sharp angular bound- the stratigraphic relations and the facts that aries; the quartz grains exhibit some corroded the tuff is commonly subordinate to the other edges. The mafic minerals are altered beyond lithologies and the unit is lithologically dis- specific identification, and the glassy matrix has tinct from the Monterey Shale, Bramlett's partly devitrified to mordenite. "Obispo tuff member" is amended to Obispo Pecten (Lyropecten) crassicardo Conrad is Formation. known from lower (? ) to

Geological Society of America Bulletin, v. 77, 443-446, 1 fig., April 1966 443

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upper Miocene rocks of California (Arnold, Pecten crassicardo in tuff and breccia suggests 1906, p. 92; Keen and Bentson, 1944, p. 89). that there was no post-mortem transportation. and Pecten (Amusiurri) lompocensis Arnold, from the lower Miocene (? Oligocene) (Arnold, Dating of Morro Roc^ Volcanic Plug 1906, p. 92; Keen and Bentson, 1944, p. 89). Morro Rock, one of the volcanic plugs in the Arnold reported P. lompocensis from the Va- San Luis Obispo area, approximately 25 miles queros Formation, which Kleinpell and Weaver northwest of the fossil-bearing tuff, was radio- (1963) regard as being of Oligocene age. Klein- metrically dated in order to determine the age pell and Weaver (1963, p. 12), however, con- sider one of the occurrences reported by Arnold to be from the Monterey Shale, which south- west of Lompoc they regard as lower Relizian. They also report P. lompocensis from lower Relizian calcareous in the eastern San Rafael Mountains. Besides P. lompocensis and P. crassicardo, we have found Trachycardium vaquerosensis, Conus sp., and echinoid spines near the top of the Obispo Formation. These fossils are in dolomite and calcareous siltstone interbedded with tuff and are stratigraphically above, and a few miles south, of the P. lompo- censis locality. That part of the Obispo Formation con- taining Pecten lompocensis is 20.9 + 1.5 m.y. old, according to the present calculation; hence it is within the Saucesian Stage, according to Evernden and others (1964, p. 167). Kleinpell and Weaver (1963, Fig. 5) place the formation within the upper Saucesian Stage of the lower Miocene Series on the basis of foraminiferal and stratigraphic evidence. Thus, P. lompocen- sis may range in age from the Oligocene to early or perhaps middle Miocene, and at least a part of the Obispo Formation is approximately 21 Figure 1. Map showing fossil locality, which m.y. old. is also the locality from which rhyolitic tuff was radiometrically dated, southern Coast In the tuffaceous deposits northwest of the Ranges, California fossil sites, toward San Luis Obispo, pumice fragments increase in grain size and abundance. Within the San Luis Obispo area close to a relationship between the tuff and the volcanic series of volcanic plugs, i.e., from Morro Rock necks. The Morro Rock sample dated is an to , some rocks contain clasts of pum- andesite-dacite porphyry. Its age is 23.5 +1.8 ice and perlitic glass 8 inches or more in diam- X 106 years, although Fairbanks (1904) as- eter. This distribution of volcanic and pyro- signed the volcanic plugs to the Cretaceous clastic rocks suggests that pyroclastic ma- System. terial fell south and southwest of the sites of Approximately 50 per cent of the rock is extrusion, the coarse-grained ejecta falling composed of phenocrysts with the following close to the volcanoes. Southeast of the fossil percentage composition: strongly zoned an- sites, toward the Cuyama River, are larger de- desine (Anso-4o), 65; biotite and clay, 15; horn- posits of interbedded tuffaceous siltstone, fine- blende, 10; quartz, 5; and magnetite and apa- grained tuff, and andesitic lava. A few miles to tite, 5. The groundmass, making up the other the southeast, and away from the sites exposed 50 per cent of the rock consists of plagioclase to heavy deposition of pyroclastic debris, microlites, glass, and hornblende. A concentrate marine invertebrates could have lived in a rela- of plagioclase phenocrysts was used for dating. tively normal marine environment. The abun- The ages obtained for the Morro Rock dacite dance of unworn and articulated valves of and for the fossil-bearing tuff are close enough

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to suggest the possibility that Morro Rock and rocks are significantly different. This implies possibly other nearby plugs may have been that there may have been more than one stage sources for the Obispo tuff. Fairbanks (1904, of extrusion. Work is in progress therefore to p. 6) concluded, however, on the basis of obtain several more dates from the tuff and partial chemical analyses of the rocks from other volcanic plugs in order to shed more light Morro Rock and , sev- on these problems. eral miles southeast of Morro Rock, that the

References Cited Arnold, Ralph, 1906, The Tertiary and Quaternary pectens of California: U. S. Geol. Survey Prof. Paper 47, 264 p. Bramlette, M. N., 1946, Monterey formation of California and origin of its siliceous rocks: U. S. Geol. Survey Prof. Paper 212, 55 p. Evernden, J. F., and Curtis, G. H., 1965, The potassium-argon dating of Late Cenozoic rocks in East Africa and Italy: Current Anthropology, v. 6, no. 4, p. 343-385 Evernden, J. F., Savage, D. E., Curtis, G. H., and James, G. T., 1964, Potassium-argon dates and the Cenozoic mammalian chronology of North America: Am. Jour. Sci., v. 262, p. 145-198 Fairbanks, H. W., 1904, Description of the San Luis quadrangle: U. S. Geol. Surv. Geol. Atlas, Folio 101, Hp. Keen, A. Myra, and Bentson, H., 1944, Check list of California Tertiary marine : Geol. Soc. America Special Paper 56, 280 p. Kleinpell, R. M., and Weaver, D. W., 1963, Oligocene biostratigraphy of the Santa Barbara Embayment, California: Univ. Calif. Pub. Geol. Sci., v. 43, 250 p.

DEPT. GEOLOGY, UNIVERSITY OF CALIFORNIA, Los ANGELES, CALIFORNIA DEPT. GEOLOGY AND GEOPHYSICS, UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA DEPT. GEOLOGY, UNIVERSITY OF CALIFORNIA, Los ANGELES, CALIFORNIA MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 15, 1965

Appendix: Data for Roc/( Samples Dated by Potas- Sample KA-1772 (University of California, Berke- sium'Argon Method ley) X0 = 0.584 X 10-'° yr-1; X = 4.72 X 10-10 yr-> Mineral: Plagioclase, + 250-150 Tyler-mesh size e Data: Sample wt., 9.64380 gm;K = 0.404 per cent; Sample KA-1768 (University of California, Berke- total radiogenic argon, 16.4 X 10"11 moles + 5 ley) per cent; Ar40 radiogenic = 37 per cent; t = 23.5 Mineral: Plagioclase, + 150-35 Tyler-mesh size ± 1.8 X 10" years Data: Sample wt., 6.17561 gm; K = 0.924 per cent; Locality: Morro Rock, east face, T. 29 S., R. 10 E., total radiogenic argon, 21.3 X 10~n moles + 5 San Luis Obispo County, California. per cent; Ar40 radiogenic = 75 per cent; t = 20.9 Stratigraphy: Volcanic neck surrounded by sea + 1.5 X 106 years water and beach sand. Other volcanic necks Locality: Quarry near the center of sec. 31 (pro- nearby have intruded rocks of the Franciscan jected), T. 12 N., R. 14E., northwest of the town group. of Nipomo and Mehlscaw Creek, San Luis Collectors: R. C. Surdam and E. R. Hall Obispo County, California, approximately 500 feet north of BM 431 on Los Berros Road Dating was done by D. L. Turner at the Univer- Stratigraphy: Near middle of Obispo Formation sity of California, Berkeley, potassium-argon lab- Age: late Saucesian oratory. Procedures used are described by Evernden Collectors: R. C. Surdam and E. R. Hall and Curtis (1965).

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