Diatom Stratigraphy of the Lower Pliocene Part of the Sisquoc Formation, Harris Grade Section, California

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1986

Diatom stratigraphy of the lower Pliocene part of the Sisquoc Formation, Harris Grade section, California

John A. Barron USGS

Jack G. Baldau Ocean Drilling Program, Texas A&M University, College Station, Texas 77843-3469

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Barron, John A. and Baldau, Jack G., "Diatom stratigraphy of the lower Pliocene part of the Sisquoc Formation, Harris Grade section, California" (1986). USGS Staff -- Published Research. 268. https://digitalcommons.unl.edu/usgsstaffpub/268

This Article is brought to you for free and open access by the US Geological Survey at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in USGS Staff -- Published Research by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Diatom stratigraphy of the lower Pliocene part of the Sisquoc Formation, Harris Grade section, California John A. Barron' and Jack G. Baldauf2 ' U.S. G~ologicuiSunqt ,Menlo Park, California 94025 ZOceanDrilling Program, Texas A&lM University, Callege Station, Taas 77843-3469

ABSTRACE Lower Pliocene diatoms were studied from the Sisquoc Formation and lowermost Foxen Mudstone, exposed along Hams Grade north of Lompoc, California, to refine the diatom biostratigraphy of post-Montercy Formation sediments in California. Sixty-seven diatom taxa were identified in the 25 samples examined from the 790-m thick (2950-ft) section. The diatoms are assignable to the uppermost N~rzschiareinholdii Zone and Thalassros~raoestrupi~ Zone of Damn (1 98 11, and five tcntative subzones for local correlation are proposed. Regional correlations and taxon occurrence are discussed, and the base of the Aritzschia reinholdii Zone is redefined as at the last occurrence of Thalassionema schraderi.

INTRODUCTION Basin have included those of Porter (19321, Woodring et al. (1943), Woodring and Bramlette (1 950), Dibblee (1 950), and Middle Miocene to lower Pliocene strata of coastal California Wornardt (1 963). Each of these studies made use of the Har- are well known for their extensive deposits of diatomaceous ris Grade section which is exposed along former California rocks (Ingle 198 1). Typically, these deposits contain herni- State Highway 1 as it crosses the Purisima Hills north of pelagic diatomaceous shales of the Monterey Formation (up- Lompoc (text-fig. 1). At this locality, the lower portion of the per lower Miocene to upper Miocene) overlain by more mas- Sisquoc is truncated by a fault. The section's base, therefore, sive, detritus-rich diatomaceous mudstones of the Capistrano is takcn as the axis of the Purisima Anticline of Woodring Formation, the Malaga Mudstone Member of the Monterey and Bramlette (1950). The section extends from this anti- Formation, the Sisquoc Formation, or the Purisima For- clinal axis northward for 3 km across the crest ofthe Purisima mation (uppermost Miocene to lower Pliocene) (Ingle 198 1; Hills. Woodring and Bramlette (1 950) measured 899 m (2950 Barron 1986). ft) of Sisquoc Formation overlain by 244 m (800 ft) of the Studies by Barron (1 97Sa, 1975b, 1976a, 1976b 1976c, 198 l), lower Foxen Mudstone in this section. Wornardt (1963) also Rowel1 (198 I), Baldauf and Barron (19821, and Barron and recorded 244 m (800 ft) of Foxen Mudstone in this section, Keller (1983) have established a workable diatom biostra- but he reported the thickness of the Sisquoc Formation as tigraphy for the Monterey Formation. In contrast, few de- 1067 m (3500 ft) (table 1). tailed biostratigraphic studies exist for the overlying diatomaceous units, especially the Sisquoc Formation. Diatoms from the Harris Grade section are discussed by Woodring et al. (1943), Simonsen and Kanaya (1961), and Classid exposures of the Monterey and Sisquoc Formations Wornardt (1 963,1967). Simonsen and Kanaya's (1 96 1)study are located in the Johns-Manville Quarry south of Lompoc, was restricted to the genus Denriculopsis, but Woodring et California (text-fig. 1). Although Barron (1975a, 1975b), fol- al. (1943) and Wornardt (1963, 1967) followed the practice lowing Dibblee (1 9501, assigned the majority of the quarry of earlier diatom biostratigraphers by concentrating on dia- section to the Sisquoc Formation, this sequence propcrly toms visible at x 250, and failing to document the strati- belongs to the Monterey Formation (Woodring and Brarn- graphically useful smaller forms (5-30 pm), such as Thalas- lette 1950; Wornardt 1967; Barron 1976c; Isaacs 198 1). Only siosira, which dominate Sisquoc Formation assemblages the three uppermost samples (ARL-2, -4, and -8) studied by (Barron 1975a, 1975b). Recent studies have stressed the im- I Barron (1975a, 1975b) are actually assignable to the Sisquoc portance of smaller forms in latc Neogene biostratigraphy of Formation. the middle latitude northeastern Pacific (Burckle and Opdyke In the Santa Maria Basin as much as 1524 m (5000 A) of 1977, 1985; Barron 1981; Dumont 1984; Whiting and Sisquoc Formation are exposed (Woodring and Bramlette Schrader 1985). These studies indicate the need to document 1950). Woodring and Bramlette (1950) recognized both a such forms in the Harris Grade section and other exposures I basinal facies and a marginal facies in the Sisquoc Formation. of the Sisquoc Formation in order to improve the strati- The fine-grained basinal facics consists mostly of diatoma- graphic correlation and resolution of onshore sections. ceous mudstone but includes some porcelaneous mudstone Although the base of the Sisquoc Formation is not exposed and claystone. This facies is exposed throughout the central in the Hams Grade section, the Sweeney Road section east Santa Maria Basin and along the Santa Barbara coast of Lompoc contains about 700 m (2300 ft) of lower Sisquoc (Woodring and Bramlette 1950; Dibblee 1966). The marginal Formation where it unconfomably overlies the Monterey facies is predominantly composed of sandstone, the Tina- Formation (Dibblee 1950; Barron 1974). A reconnaissance quaic Sandstone Member, and it is exposed only in the north- study of the diatoms of the Sweeney Road section was com- east portion the Santa Basin (Woodring and Bram- of Maria pleted by Barron (1974), and a more thorough study is pres- I lette 1950). ently under way by Durnont (1984) as part of a Master's Geologic studies ofthe Sisquoc Formation in the Santa Maria Thesis at the University ofCalifornia, Berkeley. Earlier work I

rnicropalwnlology, wl. 32, no. 4, pp. 357-371, pls. 1-3, 1986 I J. A. Barron, J. G. Baldauf Diatom stratigaphy of the lower Pliocene. Sisquoc Formation. California

TEXT-FIGURE 1 Location of Hams Grade section in Santa Barbara County, California, and location of samples studied along Harris Grade. H-I, lowest sample I studied (barren of diatoms) is 7 m (20 ft) stratigraphically above axis of Purisima Anticline.

(John Ruth, oral communication 1973) suggested that the TABLE 1 upper part of the Sweeney Road section correlates with the Comparison of lithologic units of Woodring and Bramlette (1950) lower part of the Harris Grade section, a conclusion which and Wornardt (1963). I is supported by our reconnaissance studies. The Hams Grade study when combined with Dumont's (1 984) ongoing study Woodring and Bramlette of the Sweeney Road section, helps to refine the latest Mio- (1950) Wornardt (1963) I cene to early Pliocene diatom biostratigraphy of California. Lower 244 m (800 ft) mud- 244 m (800 ft) clayey Foxen stone, clayey silt- siltstone, thin beds Mudstone stone and thin beds of silty sandstone, MATERIALS AND METHODS of silty sandstone. and a few layers of A total of 25 samples were collected from the Harris Grade diatomaceous shale. I section during the 1983 and 1984 field seasons (text-fig. 1). Upper 236 rn (775 ft) light 274 m (900 ft) light The stratigraphic intervals of these samples were estimated Sisquoc colored diatoma- colored diatom- by their measured bedding attitudes and horizontal distance Formation ceous mudstone. mudstone, shale, Forms bare outcrops, and diatomite. I along the highway, as well as by comparison with the sample between which are intervals of Simonserl and Kanaya (196 1) and Woodring and stands of brush. Bramlette (1950). Sisquoc 76 m (250 R) clayey 152 m (500 ft) clayey Because the Sisquoc Formation contains little or no calcar- Formation diatomaceous rnud- diatomite, shale. stone containing few eous material, the samples were not processed in acid. Rath- diatoms. Supports er, strewn slides were prepared using the following technique. growth of grass. I A sample measuring approximately 1 cub crn was placed into Sisquoc 434 m (1425 A) light 457 m (1 500 ft) light a Cdram vial. The vial was filled with distilled water and Formation colored diatoma- colored diatoma- the sample was disaggregated mechanically with a stirring ceous mudstone. ceous mudstone. rod and by agitation of the vial. Strewn slides were prepared Forms bare outcrops, following the techniques of Schrader and Gersonde (1 978). between which are I stands of brush. Di- At least two slides (cover glass size 30 x 22 mrn) were ex- atomite may be lam- I amined in their entirety under the light microscope at a mag- inated. nification of x 500. Species identifications were confirmed Lower 9 1 m (300 ft) porcela- 182 m (600 ft) porce- at x 1250. The relative abundance of each species was es- Sisquoc neous shale with laneous shale in- timated in the following manner: abundant, two or more Formation thin, cherty layers. terbedded with dia- specimens per field of view (x 500); common, one specimen Thickness uncettain tomaceous in two fields of view; kw, one specimen in each horizontal due to folding and mudstone. travene (length 30 mm); and rare, if a specimen was en- faulting at crest of anticline, countered less frequently. I Micropaleonra~,vol. 32, no. 4, 1986

Sample preservation is judged by the abundance of finely and Opdyke, 1985). Consequently, it is inappropriate to use silicified forms such as Actinocyclus ehrenbergii Ralfs, Coc- the D. kamtschatica Zone in California, and Barron (1976a) coneis spp., and Nitzschia spp. compared with the abundance proposed using refined versions of Schrader's (1973) North of more robust forms such as Coscinodiscus marginatus Eh- Pacific Diatom (NPD) Zones X and IX for this latest Miocene renberg, C. oculus-iridis Ehrenberg, Lithodesrnium cornige- to early Pliocene interval in California. Later, Barron (198 1) rum Brun, Rhizosolenia barboi Brun, and Thalassionema renamed NPD Zones X and 1X the Nitzschia reinholdii and nitzschioides Grunow. Thalassiosira oesfrupii Zones, respectively. The Nitzschia reinholdii Zone is defined by Barron (1 98 1) as DIATOM BIOSTRATIGRAPHY the interval from the first occurrence of Nitzschia reinholdii Diatom wnation Kanava ex Barron et Baldauf to the first occurrence of Tha- lassioiira oestrupii (Ostenfeld) Proshkina-Lavrenko. Nitz- Text-figure 2 correlates the diatom zones of Koizumi (1973) schio reinholdii sporadic and throwout the N. and Barron (1980) for the northwest Pacific with the diatom mrse rernholdii Zone, and is a poor marker for the base the N. 'Ones Barren for the northean The base of rejnho/dij zone(Bamn 198 1; Bamn and Kellcr ,983). The the Denticulopsis kamtschatica Zone of Koirumi (1973) is last occurrence of Thalassion~moschraderi Aldba, on the defined at the first occurrence of Denticulopsrs kamtschatica other hand, is a useful stratigraphic marker throughout the (Zabelina) Simonsen its is defined the first Oc- middle-latitude North Pacific(Akiba 1982; Maruyama 1984; currence of D. seminae var. fossilis (Schrader) Simonsen. Barnon ,985). The last occurrence of Thalass,onema schra- Barren (1980) proposed three subzones for the D. kamtscha- dcri correlates with the upper part of Chron (ca

iica Zone based on the last occurrences of Rouxia ca1l:fornica 6+7 Ma) according to 9110) and Koizumi ( 985), and M. Peragallo and Cosmiodiscus insignis JousC. is slightly below the first occurrence of N. reinholdii (ca 6.5 In the northwest Pacific and Bering Sea, D. kamtschatica first ~a).-~onsequentl~,the last occurrence of T. schraderi ap- occurs in the uppermost Miocene (ca 6.5 Ma); however, its pears to be a better marker for the base of the Nitzschia first occurrence in California sections was delayed until the reinholdiizone than the first occurrence of N, reinholdii. The Pliocene (ca 5.2 Ma) (Barron, 1976a; Harper, 1977; Rurckle base of the Nirzschia reinkoldiiZone of Barron (1 98 1), there-

1 Y o Z ONE ZONE 0 + KOlZUMl1973, BARRON 1981. l" 2 this paper BARRON 1980 I-z i 2J 4 D.seminee 1b f1 D. seminae 2 var. fossilis var. fossifis LT - L. common - Ma Wa D. seminae D. kamtschatica D. seminae a var. fmsilis - 3 var. fossilis D. kam tscha tic3 D. kamtschatica 3- u 5- 0 0 A- F. 0.seminae var. fossilis d - a 4- a: c T. oes trupii W I 4-- L. C. insignis 2 b D. kamtschatica 5- L. R. californica F. T. oestrupii --

a N. reinholdii r 6- $ a 2 2 F . D. kamtschatica F . N. reinholdii -+ Z b - L. T. schraderii T. antiqua D. hustedtii 7 TEXT-FIGURE 2 Comparison of diatom zonation of Koizumi (1973) and Barron (1980) for northwestern Pacific with diatom zonation of Barron (1981) for middle-latitude northeastern Pacific. F = first occurrence datum; L = last occurrence datum. I J. A. Barron, J. G.BaldauJ Diatom strafigraphy of the lower Pliocene. Sisquoc Formation. California

fore, is herein redefined at the last occurrence of T. schraderi. zone 4 wherein Lifhodesmium cornigerum and L. minus- It should be pointed out that Thalassionema hirosakiensis culum Grunow range stratigraphically above the last occur- (Kanaya) Schrader as identified by Barron (1 98 1) and Rarron rences of T. praeoesfrupii and T. hyalinopsis. Finally, Samples and Keller (1983) in California sections is assigned to T. H- I 1A through H- 15 are distinguished by the range of Den- schraderi by Akiba (1982). The first occurrence of Thalas- ticulopsis kamtschatica and T. oestrupii above the range of siosira oestrupii, Barron's (198 1) marker for the top of the Lithodesmium species (= Subzone 5). N. reinholdii Zone, is retained. Further studies are required to determine whether these var- NPD Zone X is divisible into two sub7.ones by the last con- ious subzones are useful throughout the Santa Maria Basin sistent occurrence of Rouxia californica (Barron 1976a), so of California. Subzone 1 has been recognized in the lower the Nitzschia reinholdii Zone can be similarly subdivided. Capistrano Formation and in the Purisima Formation (Bar- The last consistent occurrence of R. californica is correlated ron and Keller 1983 and Barron unpublished data). with the uppermost part of polarity Chron 6 (ca 6 Ma), based on the paleomagnetic studies of Madrid et al. (in press) on the Purisima Formation at Santa Cruz in northern California, Age of the Harris Grade section a result consistent with the extrapolations of Barron (1 98 1) Thalassiosira oestrupii first occurs in Sample H-3C and ranges and Rarron and Keller (1983). The last occurrence of R. to the top of the Hams Grade section. The first occurrence I ca/[fornicarecorded by Barron (1 980) off Japan, however, of Thalassiosira oestrupii falls in the lower reversed event of approximates the Miocene/Pliocene boundary (ca 5.2 Ma), the Gilbert Reversed Polarity Chron in the Equatorial Pacific, attesting to the diachroneity of this datum level reported by slightly above the Miocene/Pliocene boundary (Burckle 1978; Harper (1 977). Barron et al. 1985). The isochroneity of this datum level in the middle ~atitudeNorth Pacific has been demonstrated by The Thalassiosira oestrupii Zone is the interval between the Burckle (1 978) and Barron (1 980, 198 1) implying that Sam- first occurrence of Thalassiosira oestrupii and the first oc- ple H-3C lies slightly above the Miocene/Pliocene boundary. currence of Denticulopsis seminaevar.fossilis, and it is there- Koizumi and Tanimura (1 985) reported that the first occur- fore equivalent to Subzones b and c of the D. kumrschatica rence of T. oestrupii coincides with the top of polarity Chron Zone (Barron 1981). 5 at northwest Pacific DSDP Site 578: however, these two events fall at the break between Cores 578-12 and 578-13, Harris Grade biostretlgraphy so the correlation is implied. The occurrences of diatom taxa encountered in the 23 sam- Underlying Samples H-2a through H-3B at Harris Grade ples from the Harris Grade section are shown on table 2. contain Thalassiosira praeoestrupii, a diatom which first oc- Samples H-1 (39 m) and H-2 (58 m) are barren of diatoms, curs at the top of Chron 5 in California according to the which is to be expected as they are from the lower 100-m- paleomagnetic studies of Madrid et al. (in press) and Dumont thick porcelaneous shale unit of Woodring and Bramlette et at. (1986) at Santa Cruz. Because Berggren et al. (1985) (1950). Sample H-15 (905 rn) is from the basal Foxen Mud- placed the Miocene/Pliocene boundary at the top of polarity stone, approximately 6 m above the contact with the Sisquoc Chron 5, Sample H-2A, the lowermost Harris Grade Sample Formation. containing diatoms, would be earIiest Pliocene in age. The first occurrence of Tha/assiosiraoes~mpiiin Sample H-3C Lithodesrnium cornigerum also occurs in Sample H-2A. Ear- (160 m) marks the base of the T. oestrupii Zone. Recause ly diatom workers (Woodring et al. 1943; Hanna 1930; Wor- Denriculopsis seminae var. fossilis does not occur in the over- nardt 1963, 1967) considered L. cornigerum to be a marker lying samples, the entire upper portion of the Harris Grade for the Pliocene, but this has been poorly substantiated by section is assigned to the T. oestrupii Zone. The presence of correlations with the international geologic time scale. Nirzschia reinholdii and the absence of both Rowcia califor- Schrader (1973) reported L. cornigerum in his North Pacific nica and Thalassionema schraderi in Samples H-2A (100 m) Diatom Zone IX, which is equivalent.to the Thalassiosira through H-3B (1 51 m), places the lower diatom-bearing in- oestrupii Zone or early Pliocene. Barron (1 976a) noted the terval of the Hams Grade section in Subzone b of the Nitz- first occurrence of L. cornigemm in his sample N- 14a from schia reinholdii Zone. I the lower Capistrano Formation at Upper Newport Bay in I Further biostratigraphic subdivision of the Harris Grade sec- southern California, about 10 m stratigraphically above sam- tion is suggested by the range of selected species. At least ple N- 14 where T. uestrupii sensu Barron (1976a) first occurs. I I five subzones can be recognized. Samples H-2A though H-3B Thalassiosira oestrupii sensu Barron (1976a) is actually T. I lack Thalassiosira oestrupii but contain T.praeoesfrupii Du- praeoestrupii (see Taxonomic Notes and Barron and Keller mont, Baldauf and Barron (1986) and Lirhodesmium cor- 1983), so near coincidence of the first occurrences of T. nigerum, and are assignable to Subzone 1 within the upper- praeoestrupii and L. cornigerum in the lowermost Capistrano most part of the Nitzschia reinholdii Zone. Samples H-3C Formation at Upper Newport Bay is suggested. The near I through H-6 contain the stratigraphic overlap of T. aestrupii coincidence of the first occurrences of these two species has f and T. nativa Sheshukova-Poretzkaya and constitute a lower also been observed in the Sweeney Road section of the Sis- subzone (= Subzone 2) of the T. oestrupii Zone. In Samples quoc Formation by M. Dumont (oral communication 1985). 1 I H-9 through H-8, T. praeoestrupii and T. hyaltnopsis Barron Arends (oral communication 1985), on the other hand, has range above the last occurrence of T. nativa (= Subzone 3). observed L. cornigerum ranging into the latest Miocene part Samples H-9A through H-1 1 are tentatively assigned to Sub- of the N. reinholdii Zone in southern California.

J. A. Barmn. J. G. BaldauJ Diatom stratigraphy of the lower Pliocene, Sisquoc Formation, California

The upper age constraint on the Hams Grade section samples imens is examined in its entirety before a sieved slide is is provided by the absence of Denticulopsis serninae var. examined. fossilis. a diatom which first occurs in the uppermost part of the Gilbert Reversed Polarity Chron (ca 3.7 Ma) in the mid- Woodring et al. (1943) identified 220 species and varieties dle-latitude North Pacific (Koizumi and Tanimura, 1985). of diatoms in samples collected from the lower portion of Because D, seminae var. fossilis is recorded in DSDP Sites the Harris Grade section. Their samples were not arranged 467 and 469 off southern California by Barron (1981), it stratigraphically.They characterized the basinal facies of the seems unlikely that it was ecologically excluded from the Sisquoc Formation by the following diatom species: Melosira Santa Maria Basin area. Accordingly, the entire Harris Grade clavig~raGrunow, M, recd~nsSchmidt, Stephanopyxis tur- section appears to be earlv Pliocene in age. ris var. cylindrus Grunow, Endictya robusta (Greville)Hanna & Grant, Coscinodiscus aeginensis Schmidt, C. intersectus The early Pliocene age assignment of the Harris Grade sec- Brun, C. obscurus Schmidt, C. stellaris Roper, C. vetwtis- tion is supported by the occurrence of Delmontian Stage simus Pantacsek, Actinocyclus ehrenbergii, Actinoptychu. foraminifers in the upper part of the Sisquoc exposed in the marmoreus Brun, A. perisetosus BN~,Lithodesmium cor- section (Woodring and Bramlette 1950). According to War- nigerum, Xanthiopyxis ovalis Lohman, and Thalassionema ren (in Kleinpell 1980), the Delmontian Benthonic Fora- nitzschioides. They also noted that pelagic species compose miniferal Stage does not range younger than about 4.0 Ma about two-fifths of the observed taxa. or early Pliocene. Wornardt (1 963) tabulated I20 diatom species and varieties Stanley and Surdham (1984) equate an unconforrni~sepa- from 17 samples collected from Harris Grade and illustrated rating the Sisquoc Formation with the overlying Foxen Mud- them in a later paper (Wornardt 1967). He distinguished a stone to drop in global sea-level shown by Vail and a 4.2 Ma lower flora characterized by Actinocycltu ehrenbergii, Hardenbol(1979), based on correlations of mollusks, benthic Bacteriastrum vartans Lauder, Coscinodiscus eccentricus foraminifers and gross lithology. Similarly, Barron (198 1, [= Thalassiosira antique (Grunow) Cleve Euler], C. margin- 1986) observed that diatom-rich sediments, which are prev- atus, C. lineatus [= T. leptopus (Grunow) Hasle et Fryxell], alent in the middle Miocene through earliest Pliocene of C. radiatus Ehrenberg, Lithodesmium cornigerum, Nitzschia California, do not range above 4.0 to 4.5 Ma. The absence pliocena [= N. reinholdii], and Rhaphoneis ischaboenis of diatoms described from both the equatorial Pacific (Nitz- [= Delphineis sachalinemis (Sheshukova-Poretzkaya) Bar- schiu jouseae Burckle and N. cyhdrira Burckle) and the ron et BaIdaufl. This was followed by a middle flora wherein northwest Pacific (Cosmiodiscus insignis and Coscinodiscus Coscinodiscus mteromphalus Ehrenberg, C, obscurus, Bid- Iemperei Brun) with datum levels between 4.5 and 4.0 Ma dulphia aurita (Lyngbye) Brkbisson [= Odontella aurita in thc Harris Grade section prevents confirmation of the (Lyngbye) Agardh], and Hemidiscur simplicissimlcs Hanna suggestions by Stanley and Surdham (1984) and Bamn (1 986) et Grant make their first appearance. Lithodesmium corni- that the top of the Sisquoc Formation is older than 4.0 Ma. gerum and R. ischaboensis become rare near the upper limit Although it is possible that the entire Harris Grade section of the middle flora. Wornardt's (1967) upper flora contains predates the first occurrence of N.jomeae (4.5 Ma), it is more the first occurrences of Arachnoidiscus ehrenbergii, Aster- likely that transitional paleotemperatures and the inshore site omphalus arachne (Bdbisson) Ralfs, Coscinodisclls lineatus of deposition of the Harris Grade section combined to ac- var. convexus (Ehrenberg) Wornardt (= Thalassiosir~lep- tiveIy excIude N. jouseae from the area. This conclusion is topus) and Coscinodiscus robustus var. herculus J. Brun, while supported by the absence of N. jouseae in the lower Pliocene including the last appearances of L. cornigerum and R. is- parts of DSDP Sites 467 and 469 off southern California. chaboensis. Most of the lower Pliocene at DSDP Site 173 is absent due to a hiatus (Keller 1979; Barron and Keller 1983), so that The diatom occurrences in table 2 do not reveal Wornardt's Site 173 cannot be used as a lower Pliocene reference section (1967) three Hams Grade floras. Although some of Wor- for comparison with the Harris Grade section. It is hoped nardt's (1967) taxa were not recognized in our study (note that study of the lower Pliocene diatomaceous sections of that Hemidiscus simplicissimws was not separated from H. DSDP Sites 32 and 34 now under way by Baldauf and Du- cuneijbrmis Wallich), those that can be compared do not mont (in prep.) will allow refined age resolution of the lower show the restriction in the Hams Grade section that Wor- Pliocene of California. nardt (1967) claimed. An exception is Lithodesmium mrnigemm, which is also absent from the upper part of the Comparison with earlier shrdies section in our study. Earlier studies by Woodring et al. (1943) and Wornardt (1 963, Wornardt (1967) compared the Harris Grade diatom assem- 1967) emphasized larger diatom taxa visible at lower power blage to the type Delmontian diatom assemblage, which he (about x 250) which wwe concentrated in part by sieving. considered to be larest Miocene in age. It is now known that The different techniques employed by us and our emphasis the type Delmontian section near Monterey is latest middle on planktonic species of Thalassiosira, Nitzschia, and Den- Miocene to earliest late Miocene in age (Barron 1976b). Based ticulopsis make direct comparison of our results with those on this comparison with the type Delmontian section, as well of Woodring et al. (1943) and Wornardt (1963, 1967) diffi- as molluscan correlations and superpositional relationships, cult. Modern diatom biostratigraphic studies emphasize Wornardt (1967) assigned the Harris Grade section to the smaller taxa and are completed at x 500 or greater. Gener- lower Pliocene. This age assignment is supported by our I ally, at least one unsieved slide containing 500 or more spec- refined diatom correlations. i Simonsen and Kanaya (196 I) studied the Deflticulopsisspecies Wornardt (1 967), Barron (1 975a, 1976a, 198 11, and Whiting in 10 samples from the Harris Grade section; seven from the and Schrader (1 985). Citation to figures in this paper appear SiquocFormation and three from the overlying Foxcn Mud- in parenthesis. A formal taxonomic section does not appear stone. They noted the hrst occurrence of Denticulapsis kam- here. Comments are added for taxa that occur in the Sisquoc rschatica in their sample 17, approximately 686 m 12250 ft) Formation but are absent from the Monterey Formation. above the sectton base. Simonsen and Kanaya (1 96 1) placed Actinocyclus crrwaruZus Janisch in Schmidt et al. the Miocene/Pliocene boundary immediately below their 17 ActinqcIus divisus (Grunow) Hustedr sample based on provincial benthonic foraminifera1 stm- Ac:inocyclus ehmnbergii Ralfs in Pritchard tigraphy. That sample approximates our Sample H- 1 1 A, Acrinocj~lusehrenkqii var. tenellus (Brtbisson) Husted~ where D,kamtschatica is relatively common, but we record Actinoc~~lrrsellipticus Grunow D. karntschatica down to our Sample H-5, over 450 m (1 476 Acrinucycius ingens Rattray ft) Iowcr in the section. Actinoptychns senorius (Ehrcnberg) Ehrenberg. Synonym: A. undu- lnrtrs (Bailey) Ralfs. Bucrcriasrrrrm vsriams Lauder, in Schmidt et al. 1874-1 959, pl. 328, Barron ( 1 97 5b) dehned a Thalassiosira hyalinopsis Assem- figs. 1-5; Barron 1975a, pl. 4, fig. 8. Rpmarks: Although specimens blage Zone in samples from the basal Sisquoc Formation (see of Bacrcriasrrum arc present in rht uppermost Monterey Formation Barron 1976b) in the Johns-Manville Quarry, and he noted at Upper Newpart Bay (Rarron unpublished data), they are usually its presence (Barnon 1975b) in the Sisquoc Formation along restricted to post-Monterey rocks in Cal~fomia. Sweeney Road east of Lumpoc, Barron (1 9 75b) characterized Chaetoceros cinctus Gran the assemblage as containinggreater than 5% Thalassianema Cocnneis costata Gregory nitzsckioides, Thalassiost ra antiqua, T. Iineara Jousk (= T. Cocconeis scurellum Ehrenberg leptoplrs), and T, hyalinopsis, but he also noted the presence Coccaneis ritrea Bnm of Bacteriastrum deiicatulum Cleve, B. varians, Thalassio- Coscinodiscus asteromphoIus Ehren berg si~kryophiln (Grunow)Jorgensen, T, oe.~trupii.and Cosci- Coscirrodiscus marginatus Ehrenberg nodiscus sufililis Ehrenberg, taxa which are not found in the Coscinodiscus nodulifer Schmidt Coscinodiscus oculus-iridis Ehrenberg Monterey Formation. This assemblage was not observed in Coscinodiscus mdiatus Ehrenberg the Harris Grade section, because the basal portion of the Sisquoc Formation is missing. Coscinpdiscus subtilis Ehrenberg;Lohman 1938, pl. 2 1, hg. 6; Barron 1975a, pl. 7, figs. 17, 18 (pl. 2, fig. 8). Remarks: This species occurs Additional taxa that are absent fmm the Monterey Forma- in post-Montcrey rocks in California, including the Sisquoc For- t~onbut present in the Sisquoc Formation and other post- mation (Barron I975a; this paper), the Etchcgoin and San Jmqujn Monterey rocks in California include: Llenticulopsis kam- Formations (Lohrnan 1938), and the Purisima Format~on(Barron rschatica, Nepltidiscus ovaiis Lohman, Nitzschia granulara unpublished data). Gmnow, Rhaphoneis fatula Lohman, Rossrella sp. cf. R. Cminodicss sp. cf. C. wtustissimus Pantocsek. Pernark Speci- tatsunokuchie~tsis(Koizumi) Gersonde, Thalassiostra jack- mens observed are similar to C. wtuslissimus Pantocsek of Lohman sonu Koizuma et Batron, T. rnultipora Whiting et Schrader, 1938, pl. 20, fig. 7 (pl. 2, fig. 6). T. praeoestrupii, T, symbolophora Schrader, and T. sp. cf. T. zabelinae lousi.. CymBeIia sp. cf. C. prostrata (Berkeley) Grun Delphineis sachalincnsis (Sheshukova-Potetzkaya) Barron el Bal- CONCLUSION dauf, n. comb. Rasionym: Rhaphoneis sachalin~nsisSheshukova- Porctzkaya, 1967, pl, 42, fig. 2. Jilustra~ion~:Schrader, 1973, pl. 23, The diatoms from 23 samples of the Sisquoc Formation and fig. 20, pl. 25, figs. 5, 7. 8, 14-16; Barron 1975a, pl. 12, hg. 3. the basal Foxen Mudstone along Harris Grade north of Lom- Xemarh: Specimens possess typical Delphlnns apices. which are poc, California, are assignabk to the uppermost Nitzschia penetrated by two finc pores. mnholdii Zone and Thalassiosira oestrupii Zones of Barron Delphineis simbirskiana (Grunow)Barron et Baldauf, n. comb. Basi- (1,98 1) and are early Pliocene in age. The Hams Grade dia- onjvn: Rhaphoneis simbir.~kianaGrunow in Panlocsek 1886, p. 36, tom assemblage can also be locally subdivided into five sub- pl. 18, fig. 162. Illusrrarions: Barron 1 975a, pl. 12, fig. 4 (pl. 3, fig. zones: Subzone 1 = the occurrence of T. pruew.stmpii and 6). Reniarks: Specimens possess the typical Dejphineisapices, which Lithodesrnium cornigerum prior to the first occurrence of T. are penetrated by two fine pores. oesrrupii; Subzone 2 = the stratigraphic overlap of T. oes- tmpii and T, nativa; Subzone 3 - the range of T, pracoes- Demtlculopsis hustedtii (Simonsen et Kanaya) Sirnonsen trugii and T. hyalinopsis above the last occurrence of T. Denticulopsis kamtscrhatica (Zabelina) Simonsen (pi. 2, fig. 4). Re- natlva; Subzone 4 = the range of Lithodesmiurn minusculum tnarks: The true forms with no more than six pseudoseptae in 10 Grunow and L,cornigemm above the last occurrenws of T. urn do not range berow the Pliocene in California (Harper 1977; pr#eaestrupii and T. hyulinopsis; and Subzone 5 = the range Barron 1976a). This species is obswved in the Sisquoc Formation of T. oesrrupii and Denticulopsis kamtsckatrca above thc last (Simonsen and Kanaya 1961; this paper), the Capislrano Formation occurrence of Lirhod~smiumspecies. (Barron 1976a), and the Purisima Formation (Addicott et al. 19781, Xlenticulopsis lauta (Bailey) Simonsen FLORA LIST Grammatophora angnIo$a Ehrenberg

Listed are citations for the diatom taxa shown on table 2. Hemidiscus cuneifomris Wallich. Remark H. simplicissimus Han- I These taxa are illustrated and discussed in Lohman (1 938), na el Gmnt is tabulated in this manuscript with If. ~une{furtnis. I J. A. Barron, J. G. Baldauf Diatom srratigraphy of the Iower Pliocene, Sisquoc Formation, California

I Hemidiscus ovalis Lohman 1938. pl. 22, fig. 9; Andrews 1980, pl. izumi 1972; Simonsen 1974; Akiba and Yanagasawa 1986) have 2, fig. 10, pl. 5, fig. 9 (pl. 2, fig. 3). Synonym: Hemidiscus weissJugi designated a type specimen, and N. reinholdii remains invalid ac- sensu Koizumi 1972, pl. 43, fig. 14. R~marks:11. avalis was de- cording to Article 37 of the International Codc of Botanical No- scribed from the San Joaquin Formation and has not been observed menclature (Sancetta et al. 1985). in the Monterey Formation. Andrews (I 980) figured it from lower Pliocene strata exposed in Petemburg, Virginia, and suggests that it Wornardt (1967) provided some of the earliest good iltustratlons of is a brackish water form, N. wjnkoldii, but he referred to the specimens as N. pliocena (Em) Womardt. NevertheIess, Womardt's (1 967) illustrations have been Hyalodiscus dentatus Kototkevin referred to A! r~iinholdiiKanaya by Koimmi (1 9721, Schrader (1 9731, and most other modern workers. Both of Womardt's (1 967) illus- Lithdesmium cornigerum Brun (pl. 3, figs. 1, 3, 5). Remarks: Two trated specimens are labeled with hypotype numbers and are stored forms are observed in the Harris Grade flora, a form with long at the Califomla Academy of Sciences, San Francisco. Therefore, it extended apices as recorded by Hanna 1930. pl. 14, figs. 9, 10 and is proposed that figure 21 2 of Wornardt (1 967)(CAS no. 3858) from Wornardt 1967, fig. 131 (pl. 3, figs. 3, 5) and a form with shorter locality 2729 5-29 (CAS), Hams Grade, Santa Barbara County, Cal- apices, which is equivalent to L. cornigerum of Barron 1976a, pl. 1, ifornia, be the holotype for N. reinholdii Kanaya, cmend. Koizumi fig. 14 (pl. 3, fig. 1). Both forms are included logether in this manu- (1 972), Schmder (1 973, ex Barren et Baldauf. F~gurc2 13 of Wor- script, because they are found together in the same populations and nard1 (1 967) (CAS no. 3859) from the =me locality as the holotype appear to have the same stratigraphic range. Lithodcsrnium cornr- is proposed as an isotype. gerum is not present in the Montcrey Formation; however, it occurs In the Siquoc Format~on,the lower Capistrano Formation (Barron Dcscriprion: From Schrader (1 973. p. 708)-"Valvcs elliptical with 1976a), the Etchegoin Formation (Lohman 1938), and the Purisirna sllghtly convex margins. 4245 pm long, 9-12 pm wide. Apices Formation (Barron unpublished data). slightly acute, broadly rounded. Transapical costae 10-1 2 in 10 urn, I between the costae (intercostal membranes) two transapiwl rows, I Lithodesmium minusculum Grunow in Van Heurck near the costae, about 20 in 10 pm. Rows puncute, about 22 in 10 Nuvi~ulahennedyi Wm. Smith urn. Punctat in decussatc arrangement forming oblique rows. Trans- NavicuIa iym Ehrenberg apical costae and rows slightly rounded near the apices. Two apical Navicuia optima Hanna lines near the raphe-lacking margin at the interior side of the valves Nifeschia f0~8ili~(Frenguelli) Kanaya et Koi7umi forming the inner openings of the inner pores. Inner pores small elliptical. Raphe marginal, keel wrong. About 13 keel pmctae in 10 Nitz$chia gnrnulafa Grunow; Lohman 1938, pl. 22, hg. 10; hmn pm." 1976a, pl. 3, fig. 2. Remarks: This post-Montcrey diatom has been observed in the Capislrano Formation (Barron 1976a), the Etche- Remarks: The similarity of N. reinholdii to N. marina Grunow is goln, San Joaquin, and Tulare Formations (Lohman 1938), and the discussed by Kanaya and Koimmi (I 970), Koimmi (1972), Schrader I Capistrano Formation (Barron unpublished data). (1 973). Baldauf (1 984). and Akiba and Yanagasawa (1986). Basi- cally. N. reinholdii margins that are somewhat convex than i NitzscRia mnboldii Kanaya ex Barron et Baldaui'(p1. 2, fig. 7) has more I those of N. marina, and the raphe of N. reinholdii is not inturned Vdidarion: Kanaya first proposed N~tzschiareinholdii In Kanaya at the apices as it is in n! marina. Baldauf (1 984) illustrated forms and Koizumi (1970), but he failed to designate a type specimen and that are intermediate between the two species. I gave a very brief description in Japanese. More complete descrip- tions were given by Koizumi (1972, p. 35 I) and Schrader (1 973, p. Nltzschia rolandii Schrader 708), but both these workers continued to refer the species to Kanaya Odontella aun'ta (Lyngbye) Agardh. Synonym: Biddulphia aurila and faiIed to des~gnatea type. Sirnonsen (1974, p. 54) recognized that Kanaya could not k considered the author, and he considered (Lyngbye) Brkbisson. i Schrader (1 973) as lhe author of thc spccies. Unfortunately, neither Portilia sulcata mrenkrg) Cleve. Synonym: Melosira sulcata (Eh- Schrader (1 973) nor other workers discussing h! r~iinholdii(e.g. Ko- ren berg) Kiitzing.

PLATE 1 1 Thalmsiosira praeoestmpii Dumont, Baldauf and Barron 6 Thafassiosira nativa Sheshukova-Poretzkaya sensu I Sample H-7, diameter 33 pm. Schrader Sample H-4A, diameter I9 pm. 2 Thalassiosira sp. aK T, plicala Schrader Sample H- 1, diameter 48 pm. 7 Thalmsiosira anrigua (Grunow) Cleve-Euler 3 Thalussiosira oestrupii (Ostenfeld) Proshkina-Lavrenko Wnnhg electron mimsc(1pe photwph. Sample H-3C. Sample H-1 I, diameter 19 pm. Scale bar = I0 pm. 4 Thalassiuslra hyalinops~sBarron 8 Thalassiosira sp. afT T. kryophilu (Grunow)Jsrgensen Approaching T. nativa sensu Schrader (1973) in mor- Sample H-11,diameter 48 pm. phology. Sample H-2A, diameter 23 pm. 9 Thalas~iosa'rtahyaiinopsis Barron 5 Thalassiosira hyali~opsisBarrm Scanning electron microscope photograph. Sample H-3C. I I Sample H-2A, diameter 25 pm. Scale bar = 10 pm. I ! L A. Burron, J, G.BaIdauf PLATE 1

rnicropaleontology,volume 32, number 4 J. A. Barrun. J. G. Baldauj Diatom stratigraphy ofthe lower Pliocene, Sisguoc Formation, CaI$ornia

Pnmlic sulcata var. biseriata (Grunow)Cleve Thalmsiosira plicato sensu Whiting and Schrader 1985, pl. 2, figs. PamIia sulcata var. comnata (Ehrenberg) Cleve 20, 21. Remarks: This species is not present in the Monterey For- Rhaphoneis amphiceros Ehrenberg mation. Rhaphlraeis amphicem vat. elongala Peragallo Tb~fmsio~iraleptopus (Grunow) Hasle et Fryxell (pl. 3, 8gs. 2, 4?). Rhuphoneis an~uiularisLohman 1938, pl. 22, figs. 6-8. Synonym: Thalassiusira lineata Jousk sensu Barron 1975a, pl. 14, Rhaphoneis farrrla Lohrnan 1938, pl. 22, fig. 5 (pl. 3, fig. 8). Remark fig. 2. This post-Monterey diatom is present in the Sisquoc Fomation(this Thalassiosira multipotn Whiting et Schrader 1985, p. 258, pl. 2, ligs. paper), the San Joaquin Formation (Lohman 1938), and the Puri- 1 1, 16. 17 (pl. 2, hg. 1). Remarkr:This species has alsobeen observcd sima Formation (Ramon unpublished data). in the Purisima Formation (Baron unpublished data), but it does Rhapkoneis gernm~raEhrenbtrg; Andrtws 1975, pl. 2, figs. 28,29; not occur in the Monterey Formation. pl. 4, figs. 56-60; pl. 5, figs. 69, 70. Thalmsiosira natim Sheshukova-Poretzkaya sensu Schrader 1973, pl. 11, figs. 23, 24. Damn 1975~1,pl. 14, figs. 44(pl, 1, fig. 6). Rhizosolenia barboi Brun Rhizosolenia hebetara f. hiemalisGran Thalmsiosira o~slrupii(Ostenfeld) Proshkina-Lavrenko; Schrader RhizosoIenia ~tyi$omisBrightwell 1973. pl. 11, figs. 16-22, 26-33, 36, 39-45; not Bamn 1976a, pl. R. (Kouumi) 2, fig. 19 (pl. 1, fig. 3). Remarks: This Pliocene to Holocene diatom Rossiella sp, cf. tnisunokndiensis Gersonde. Syn- is present in the Sisquoc Formation (this paper), the Capisttano onym: Rhophonei.7 tatlsunokurhierrsis Koizumi 1972, pl. 42, fig. 3, Koizumi, Formation (Barron and Keller 1983), the Malaga Mudstone Member 4; Cussla tatsunokuchie~is(Koizumi) Schrader; 1975, pl. (Rowell 198 1); and the Purisima Formation (Barron unpubIished 4, figs. 33-35. Remarks: R. raisunokuchi~nsilsis not present in the data). Monterey Formation, but it has been observed in the Sisquoc For- mation (this paper) and the Etchegoin Formation (Barron unpub- ThnImsim;rn sp. aft T. pliwta Schrader 1 974, pl. 3, figs. 44, 7-9 lished data). (pl. 1, fig. 2). Thal~siosirapraeoesimpiiDumont, Baldauf and Barton (pl. I, fig. Srqkunogonia polymntha Farti 1) Synonym: Thalassiosira oestrupii sensu Barron 197Ba, p1. 2, fig. Stephanopyxis tmrris (Greville et Amott) Ralfs 19. Remarks: In this species, the central accessory pore (strutted Thalassionema nitzschioides Grunow in Van Heurck process) is separated from the submarginal pore (process) by at least Thalassionemtr rrit~whiuidesvar. pclrva et Kolbe Reiden four areolae (Dumont et aI. 1986). In T. oesiruprr, the separation Thnlussiasira antiqua (Grunow) Cleve-Euler (pl. 1, fig. 7) of the two pores is 2-3 arcolae. Thalassiosira eccentricu (Ehrenberg) Cleve Thalm~iosirasymbolophota Schrader 1 974, pl. 4, figs. 1-8 (pl. 2, fig. Thalassioxim mcenirica (Ehrenberg) Cleve, fine form (pl. 3, fig. 7). 2). Rrmarkr: This diatom has not been observed in the Monterey R~rnarh:Tabulated here forms with small areolae are (9-10 in 10 Formation. pm) and a center which is slightly indented. ThuIassiosira sp, cf. T. gabelinae Joust; Shradw 1973, pl. 14, figs. 1, ~halassiosimhyalinnpsis Barron 197Sa, pl. 13, figs. 12, IS (pl. I, 2. Remarks: This typically cold-watcr species has not been observed nativa figs. 4, 5,9). Remarks: This species intergrades with T. She- in the Monterey Formation. s h ukova-Poretzka ya sensu Schrader 1 973. Thalussiosira hyultnopsis occurs in the Sisquoc Formation (this paper), and it is sparse and Thalassiothrix longissima Cleve et Grunow;Schrader 1 973, pl. 23, sporadic in the Capistrano Formation (Barron 1976a), the Etchegoin figs. 7, 17, 18. Formation (Barron unpublished data), and the hrisima Formation (Barron unpublished data). Thalmsiosirojacksonii Koizurni et Barron in Koizumi 1980, pl. 1, We thank Michael Dumont of Chevron, U.S.A., Peter Ra- figs. 11-1 4 (pl. 2, fig. 5). Remarks: This species does not wur in mirez of the U.S. Geological Survey, and Robert Arends of the Monterey Formation. UNOCAL for helpful discussions during this study. We also Thatmsior~t-usp. aK T. ktyuphjle (Gmnow) Jargensen; Koizumi thank Sean M. Stone of the U.S. Geological Survey and Mi- 1972, pl. 43, fig. 9; Ramn 1975a, pl. 14. fig. 1 (pl. 1, fig. 8). Synonym: chael Durnont for assistance in the field. This manuscript

PLATE 2 1 Thalossiosira multipom Whiting et Schrader 5 Thalassiosira jacksonii Koizumi et Barron Sample H-1 1, diameter 48 pn. Sample H-9, diameter 28 #m. 2 Thalassiosiru symbolophora Schrader 6 Coscinodiscus sp. cf. C. vet ustissimus Pantocsek Sample H-1 1, diameter 32 m. Low and high focus. Sample H-8,diameter 5 1 pm. 3 Hemidism ovalis Lohman 7 Nirzschia reinholdii Kanaya ex Barron et Baldauf Sample H-8,length 22 pm. Sample H-3A, length 52 pm. 4 Dentlculop~iskamtschatica (Zabelina) Simonsen 8 Coscinodism subtilis Ehrenberg Sample H-11, length 13 pm. Sample H-8,diameter 63 pm. - -. - -

J. A. Barron, J. G. Baldauf

micro pale onto lo^, voIume 32, number 4 J. A. Barron, J. G. Bulduuf Diatom s~ratigrnphyof the lower Pliocenp, Sisquoc Formation. California

was reviewed by David Bukry of the Minerals Management raphy of Upper Newport Bay, Newport Beach, California. Marine Service, Robert Arends, Itaru Koizumi of Osaka University, Micropaleontology, 1:27-63. and an anonymous reviewer. This is Ocean Drilling Program -, 1976b. Middle Miocene-lower Pliocene marine diatom and contribution no. ODP-P85-000. silicoflagellate correlations in the California area. In: Fritsche, A. E., et al., Eds., The Neogene Symposium, Pacific Section, Society REFERENCES of Economic Paleontologists and Mineralogists Meeting, San Fran- cisco, California, 1 17-1 24. ADDTCOTT, W. O., BARRON, J. A., and MILLER, J. W., 1978. , 1976c. Marine diatom and silicoflagellatebiostratigraphy of Marine late Neogene sequence near Santa Cruz, California. U.S. the type Delmontian Stage and the type Bolivina obliqua Zone, Geological Survcy, Open-File Repot?, 78-466:97-109. California. Journal offescarch, U.S.Geological Survey,4(3):339- AKIBA, F., 1982. Taxonomy and biostratigraphic significance of a 35 I. new diatom, Thalassionema schradcri. Bacillaria, 5:43-6 1. -, 1980. Lower Miocene to Quaternary diatom biostratigtaphy Leg AKIBA, F., and YANAGASAWA, Y., 1986. Taxonomy, morphol- of 57, off northeastern Japan, Deep Sea Drilling Project. In: ogy and phylogeny of the Neogene diatom zonal marker species Scientific Party, Initial Reports of the Deep Sea Drilling Project, in the middle-to-high latitudes of the North Pacific. In: Kagami, Volumes 56, 57:641-685. Washington, D.C.: U.S. Government H., Karig, D. E., Coulbourn, W. T., et al., Inilia1 Reports of the Printing Office. Decp Sea Drilling Project, Volume 87:483-554. Washington, D.C.: -, 198 1. Late Cenozoic diatom biostratigraphy and paleocean- U.S.Government Printing Office. ography of the middle-latitude eastern North Pacific, Deep Sea Drilling Project Leg 63. In: Yeats, R., Haq, B. U., et al., Initial ANDREWS, G. W., 1975, Taxonomy and stratigraphic occurrence Reports of the Deep Drilling Project, Volume 63: 507-538. ofthe marine diatom genus Rhaphoneis. Nova Hedwigia, Beihefte, Sea Washington, 53:193-222, pls. 1-5. D.C.: U.S. Government Printing Office. -, 1980. Neogene diatoms from Petersburg, Virginia. Micro- , 1985. Miocene to Holocene planktic diatoms. In: Bolli. H. paleontology, 26(1):17-48, pls. 1-6. M., Saunders, J. B., and Perch-Nielscn, K., Plankton stratigraphy, 763-805. Cambridge: Cambridge University Press. BALDAUF. J. G., 1984. Cenozoic diatom biostratigraphy and pa- -, 1986. Paleoceanographicand tectonic controlson deposition leoceanography of the Rockall Plateau region, North Atlantic, Deep Sea Drilling Project Leg 8 1. In: Roberts, D. G., Schnitker, of the Monterey Formation and related siliceous rocks in Cali- D., el al., Initial Reports of the Deep Sea Drilling Project, Volume fornia. Palaeogeography, Palaeoclimatology, Palaeoecology, 53: 8 1:439478. Washington, D.C.:U.S. Government Printing Office. 27-45. BARRON, J. A., and KELLER, G., 1983. Paleotemperature oscil- BALDAUF, J. G., and BARRON, J. A., 1982. Diatom biostratig- lations in the middle and late Miocene of the northeastern Pacific. raphy and paleoecology of the type section of the Luisian Stage, Micropaleontology, 29(2): 150-1 8 1, pls. 1-3. central California, Micropaleontology, 28(1):59-84. G., BARRON, J. A., 1974. The late Miocene-early Pliocene marine BARRON, J. A., KELLER, and DUNN, D. A., 1985. A multiple diatom assemblage of southern California. Ph.D. dissertation, microfossil biochronology for the Miocene. In: Kennett, J. P., Ed., The Miocene Ocean: paleoceanography and biogeography. Geo- University of CaIifornia, Los Angeles, 289 pp. logical Society of America Memoir, 163:21-36. , 1975a. Late Miocene-early Pliocene marine diatoms from BERGGREN, W. A., KENT, D. V., FLYNN, and VAN COU- southern California. Palaeontographica, 15 I(B):97-170, pls. 1-1 5. J. J., VERTNG, J. A., 1985. Cenozoic geochronology. Geological So- -, 1975b. Marine diatom biostratigraphy of the upper Mio- ciety of America, Bulletin, 96: 1407-141 8. cene-lower Pliocene strata of southern California. Journal of Pa- I leontology, 49A19-632. BURCKLE, L. H., 1978. Early Miocene to Pliocene diatom datum levels for the equatorial Pacific. In: Proceedings, Second Working , 1976a. Revised Miocene and Pliocene diatom biostratig- Group Meeting, Biostratigraphic Datum-Planes of the Pacific

PLATE 3 I Lirhodesmium cornigenirn Brun 5 LithodPsmiurn cornigerum Brun Form with short apices. Scanning electron microscope Form with long apices. Sample H-8,base 49 pm. I photograph. Sample H-3C.Scale bar = 10 pm. 2 Thalassiosira leptopus (Grunow) Hasle et Fryxell 6 Delphineis simhirskiana (Grunow) Barron et Baldauf Scanning electron microscope photograph. Sample H-3C, Sample H-4A, length 25 pm. scale bar = 10 urn. 7 Tkalassiosira eccenfrica(Ehren berg) Cleve 3 Lithodesmium cornigerum Brun Fine form. Sample H-1 1, diameter 26 pm. Form with long apices. Sample H-3A. Base 48 pm. I 4 Thalassiosira sp. cf, T. lepropus (Grunow) Hasle et Fryxell 8 Rhuphoneis fatula Lohman Sample H- 11, diameter 28 pm. Sample H-8, length 42 pm. J. A. Barron, J. G. BaIdauj PLATE 3

micropaleontology, volume 32, number 4 369 Y J. A. Barron, J. G. Baldau? Diatom stratigraphy of the lower Pliocene. Sisquoc Formation. CaIifDrnia

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