JON S. GALEHOUSE Dept. Geology, San Francisco State College, San Francisco, California 94132
Provenance and Paleocurrents of the
Paso Robles Formation, California
Abstract: A provenance and paleocurrent study of present site of the Temblor Range and into a marine the middle and upper Pliocene continental Paso basin at the present site of the southern San Joaquin Robles Formation in the Salinas Valley area, Cali- Valley. The Salinas River was a relatively unim- fornia, adds to the knowledge of the late Cenozoic portant stream during Paso Robles deposition. geologic history of the California Coast Ranges. The Near the end of the Pliocene, uplift in the Tem- geographical distribution of heavy minerals and blor Range and southwestward tilting of the Gabi- pebbles in the Paso Robles Formation and in lan Mesa brought about the end of Paso Robles streams presently draining proposed source areas deposition by defeating the southeast-flowing drain- leads to the same conclusions as do measurements of age and creating the conditions for its capture by foreset beds, pebble imbrication, and channels in the the modern Salinas River. Lithologic differences be- formation. Uplift in the Santa Lucia and La Panza tween juxtaposed beds of the Paso Robles Forma- ranges initiated Paso Robles deposition in early tion on either side of the San Andreas fault suggest Pliocene time. The paleodrainage was southeast- that about 25 miles of right-lateral movement has ward from the Santa Lucia Range and northward occurred since deposition of the formation. from the La Panza Range, continuing across the
CONTENTS Introduction 952 2. Physiographic features of Salinas Valley area 953 Acknowledgments 952 3. Major geologic units of Salinas Valley area . 953 Physiographic setting 953 4. Present drainage of Salinas Valley area . . . 954 Geologic setting 953 5. Sample localities in Paso Robles Formation . 954 General description 954 6. Sample localities of heavy-mineral analyses of Method of sampling 954 Paso Robles Formation 957 Lithology 954 7. Relationship of heavy-mineral percentages Thickness 955 and median grain size 959 Age 955 8. Distribution of sphene in heavy fraction of Heavy minerals 956 Paso Robles Formation and peripheral General discussion 956 streams 959 Laboratory techniques 956 9. Distribution of hornblende in heavy fraction Vector analysis 957 of Paso Robles Formation and peripheral Variation in composition with grain size . . . 958 streams 960 Intrastratal solution 958 10. Proportional data from vector analysis. . . 961 Geographical distribution and source area. . . 959 11. Distribution of garnet in heavy fraction of Vertical variation 963 Paso Robles Formation and peripheral Pebbles 965 streams 962 General discussion 965 12. Distribution of epidote in heavy fraction of Sampling procedure and field methods. . . . 965 Paso Robles Formation and peripheral Geographical distribution and source area . . 967 streams 962 Maximum clast size 969 13. Distribution of apatite in heavy fraction of Paleocurrent indicators 970 Paso Robles Formation and peripheral Implications concerning late Cenozoic geologic streams 963 history 971 14. Distribution of zircon in heavy fraction of Drainage changes and tectonic uplift .... 971 Paso Robles Formation and peripheral Movement along the San Andreas fault . . . 974 streams 963 Summary 976 15. Vertical variation in hornblende and garnet References cited 976 percentages at locality 167 965 16. Distribution of combined percentages of Figure hornblende and garnet in heavy fraction 1. Index map showing location of Paso Robles of Paso Robles Formation 965 Formation and area investigated .... 952 17. Distribution of combined percentages of
Geological Society of America Bulletin, v. 78, p. 951-978, 30 figs., 1 pi., August 1967 951
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sphene and apatite in heavy fraction of 28. Paleogeography 973 Paso Robles Formation 966 29. Distribution of sphene in heavy fraction of 18. Sample localities of pebble counts and out- Paso Robles Formation in late Pliocene crops containing concentrations of por- time 974 celanite pebbles in Paso Robles Formation 966 30. Distribution of hornblende in heavy fraction 19. Distribution of concentrations of porcelanite of Paso Robles Formation in late Pliocene pebbles in Paso Robles Formation and time 975 peripheral streams 967 20. Distribution of silicic basement pebbles in Plate Facing Paso Robles Formation and peripheral streams 968 1. Two paleocurrent indicators in the Paso 21. Distribution of sandstone pebbles in Paso Robles Formation, California 954 Robles Formation and peripheral streams 968 22. Distribution of chert pebbles in Paso Robles Table Formation and peripheral streams . . . 969 1. Fossils collected from three localities in the 23. Distribution of maximum-sized clasts in Pancho Rico Formation 956 Paso Robles Formation 970 2. Compositional variability of the Paso Robles 24. Site averages of foreset and pebble-imbrica- Formation accounted for by six end tion azimuths in Paso Robles Formation . 971 members 957 25. Site averages of channel trends in Paso 3. Heavy-mineral composition of end members Robles Formation 971 in the Paso Robles Formation 958 26. Averages of paleocurrent measurements in 4. Heavy-mineral composition of sample locality Paso Robles Formation 972 403 in the Paso Robles Formation . . . 964 27. Moving averages of paleocurrent measure- 5. Heavy-mineral composition of sample locality ments in Paso Robles Formation .... 972 167 in the Paso Robles Formation . . . 964
author's interest in the Paso Robles Formation INTRODUCTION and suggesting an investigation of it. The au- During late Cenozoic time, certain areas in thor is also indebted to A. K. Lehre and L. the California Coast Ranges were uplifted thou- Lewis of the University of California, Berkeley, sands of feet while others were downwarped a for assistance in the field; M. F. Franklin and similar amount. Horizontal shifts of tens of J.Witteof Scripps Institution of Oceanography miles occurred on lateral faults. This geologic for assistance in the laboratory and with com- history is inferred in large part from the char- puter programming; T. W. Dibblee, Jr., of the acter and distribution of late Cenozoic conti- nental formations which crop out in many areas of the Coast Ranges. As not many detailed studies have been made of these formations, much of this history can be stated only in gen- eral terms. A study of the provenance and paleocurrents of one of these continental deposits, the Paso Robles Formation, was undertaken in order to supply some of the details necessary for a more exact determination of the late Cenozoic geo- logic history (Fig. 1). This investigation, which has determined the distribution of highland source areas and the directions of major drain- age at the time the Paso Robles Formation was deposited, has added to our knowledge of the paleogeography of the Salinas Valley, several of the Coast Ranges, and the San Andreas fault. This paper is essentially the author's Ph.D. dissertation at the University of Cali- fornia, Berkeley. Some of the research, how- ever, was done at Scripps Institution of Ocea- nography. ACKNOWLEDGMENTS Figure 1. Index map showing location of the Thanks are expressed to M. N. Christensen Paso Robles Formation and the area in- and Tj. H. van Andel for stimulating the vestigated
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United States Geological Survey for the use of south, this investigation deals only with the unpublished maps; S. Adegoke of the Univer- essentially contiguous beds outlined in Figure sity of California, Berkeley, for identification 3, which include the type-Paso Robles Forma- of the fossils in the Pancho Rico Formation; J. tion originally described by Fairbanks (1898, Moriarty of Scripps Institution of Oceanogra- p. 565). phy for drafting the figures; and B. Galehouse In general, the Paso Robles Formation occu- for help in preparing the manuscript. pies a topographically low area (about 1000- 2500 feet elevation) surrounded by highlands (about 2000-5000 feet elevation). The major streams draining the area are shown in Figure 4, which also shows the outline of the Paso Robles Formation. GEOLOGIC SETTING The subparallel San Andreas and Nacimiento faults cut the southern portion of the California Coast Ranges (Fig. 2). In the area between the faults, Cretaceous to Recent sediments overly
Figure 2. Physiographic features of the Salinas Valley area
Tj. H. van Andel and Scripps Institution of Oceanography kindly provided the use of their settling tube and electronic computer. M. N. Christensen, Tj. H. van Andel, and C. Wahr- haftig critically read the manuscript. The work was supported in part by funds from the Penrose Bequest of The Geological Society of America, the National Science Figure 3. Major geologic units of the Salinas Foundation, and the University of California, Valley area. Generalized from Jennings Berkeley. (1958; 1959), Jennings and Strand (1958), and Smith (1964) PHYSIOGRAPHIC SETTING The Paso Robles Formation crops out nearly basement rocks of the Santa Lucia Granodiorite continuously over about 1000 square miles of of Late Cretaceous age which are intruded into the upper Salinas Valley in California. The metamorphic rocks of the Sur Series. Northeast main physiographic features in this portion of of the San Andreas fault and southwest of the the coast ranges are shown in Figure 2. Al- Nacimiento fault the Cretaceous to Recent sed- though the name "Paso Robles Formation" iments are on Franciscan basement rocks of has also been used for continental beds that oc- Jurassic and Cretaceous age. Distribution of the cur discontinuously over a wide area from Sa- major geologic units is shown in Figure 3. linas in the north to the Santa Maria area in the The Paso Robles Formation is generally flat-
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lying, with dips of less than 10 degrees. Near the San Andreas fault and the basin margins, how- Lithology ever, some of the beds approach the vertical The Paso Robles Formation is a continental and in a few places are overturned. The forma- deposit consisting of gravel, sand, silt, clay, and tion overlies the early Pliocene Pancho Rico limestone. Most of the formation is light- Formation with apparent conformity through- brownish orange, massive bedded, and uncon- out much of the northwestern half of the depo- solidated. Nearly all the beds were deposited by sitional basin, intertonguing with it in some streams, with coarser sand and gravel occurring places (Durham and Addicott, 1965, p. 1). In as channel deposits and finer sand, silt, and clay occurring as flood-plain deposits. Both of these types of deposits are poorly sorted. There are also some lake beds consisting of sand, silt, clay, and limestone. An estimate was made of the number of strat- igraphic feet exposed and the percentage of
SANTA LUCIA RANGE
Figure 4. Present drainage of the Salinas Valley area
the southeastern portion of the basin, the Paso Robles Formation lies mainly on the late Mio- cene Santa Margarita Formation (Durham, 1965, p. D107). Near the margins of the basin, the Paso Robles Formation usually rests with angular unconformity on rocks of early Plio- cene to Cretaceous age.
GENERAL DESCRIPTION SAMPLE LOCALITIES Method of Sampling IN THE Throughout the 1000-square-mile outcrop PASO ROBLES area of the Paso Robles Formation, 387 out- FORMATION
crops were investigated. An attempt was made 0 MILES 10 to distribute the sampling as uniformly as possi- ble over the area (Fig. 5). The lack of sampling Figure 5. Sample localities in the Paso Robles in some areas is due to a lack of outcrops. Expo- Formation. (Detailed descriptions of the sures of the Paso Robles Formation occur sample localities in Galehouse, 1966, Ph.D. mainly in roadcuts and stream banks. thesis, App. 1-A)
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Figure 2. Channel of porcelanite gravel underlain and overlain by fine sandstone TWO PALEOCURRENT INDICATORS IN THE PASO ROBLES FORMATION, CALIFORNIA
GALEHOUSE, PLATE 1 Geological Society of America Bulletin, volume 78
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each lithologic type present at each outcrop listed it as Pliocene in age. He has been mis- investigated. An average lithology for the por- quoted, however, as having called it upper tion of the formation sampled, calculated by Pliocene and lower Pleistocene (Schombel, weighting each outcrop according to the num- 1940, M.A. thesis, Univ. Calif., Berkeley; im- ber of feet exposed, is about 25 per cent gravel, plied in Jenkins and others, 1943, p. 681). 67 per cent sand, 7 per cent silt and clay, and 1 Early papers following Fairbanks' original work per cent limestone. Details of various lithologic refer to the Paso Robles Formation as Pliocene aspects of the outcrops studied are discussed (English, 1918, p. 232; Reed, 1925, p. 591), as further on. do some recent ones (e.g., Church, 1963, p. 60). Reed (1933, p. 231) apparently later changed Thickness his mind and said it may be partly or wholly The total thickness of the Paso Robles For- Pleistocene. Subsequently, more than a score mation is unknown. It has been estimated to be of authors have said the Paso Robles Formation as little as 300 feet, the maximum thickness is Plio-Pleistocene in age. The formation has found to be exposed in an outcrop, and as much also been listed as entirely Pleistocene (Dibb- as 2000 feet (Church, 1963, p. 60; Peryam, lee, 1962, p. 8; Colvin, 1963, p. 57; Hughes, 1950, M.A. thesis, Univ. Calif., Berkeley; 1963, p. 95). Hughes, 1963, p. 94; Reed, 1925, p. 591; 1933, The problem concerning the age of the Paso p. 231; Taliaferro, 1941, p. 148; 1943, p. 461; Robles Formation stems from the fact that it Fairbanks, 1904, p. 4; English, 1918, p. 232; apparently contains no diagnostic fossils and no Bramlette and Daviess, 1944; Dibblee, 1962, beds suitable for radiometric dating. p. 8; Gribi, 1963, p. 16; Manning, 1963, p. 108). The Plio-Pleistocene age cited by most au- The larger estimates are based mainly on well thors is based, in part, on the belief that late records from the central portion of the basin, Pliocene diastrophism in the California Coast some of which list the total thickness at more Ranges initiated the deposition of coarse clastic than 3000 feet. Because the Paso Robles For- continental beds such as the Paso Robles For- mation usually rests on the Pancho Rico For- mation (e.g., Taliaferro, 1943, p. 461). It is also mation in the northwestern portion of the based on a lithologic correlation of the Paso basin and on the Santa Margarita Formation in Robles Formation of the Salinas Valley with the southeastern portion, and because both of the San Benito Gravels, the Tulare Formation, these formations contain much sandstone, it is and the Paso Robles Formation in the Santa difficult to identify the contact with the over- Maria basin (Jennings and Strand, 1958; Jen- lying Paso Robles Formation on electric logs. nings, 1958; 1959). All three of these units con- Consequently, the maximum thickness of the tain either fossil or radiometric evidence prov- Paso Robles Formation has not been satisfacto- ing that they are at least in part as young as the rily determined. Pleistocene and have no portions older than late This investigation of the Paso Robles For- Pliocene (Kerr and Schenck, 1925, p. 476; mation does not cover the entire thickness of Wilson, 1938, Ph.D. thesis, Univ. Calif., the formation. Near the margins of the basin, Berkeley; Janda, 1965, p. 131; Woodring and the outcrops sampled are mainly from a portion others, 1940, p. 104; Woodring and Bramlette, of the formation stratigraphically close to the 1950, p. 108). contact with the older formations. Near the This correlation of the Paso Robles Forma- center of the basin, the outcrops sampled may tion of the Salinas Valley with the three previ- be from a stratigraphically higher portion of the ously mentioned units is not valid because the formation. In spite of this, the coherent relation lower portion of the Paso Robles Formation is of the distributions of heavy minerals and peb- older than any of the other units. The youngest bles to current indicators and lithology of the beds underlying the Paso Robles Formation are source areas suggests that the variations de- those of the Pancho Rico Formation. The scribed herein are real lateral variations within Pancho Rico Formation has been considered as the formation rather than vertical variations young as middle Pliocene (Taliaferro, 1943, p. (see "Vertical Variation," further on). 460; Jennings, 1958), but recently, however, it Age was dated as early Pliocene (Durham and Addi- cott, 1964; 1965). The present author collected Fairbanks (1898, p. 565; 1904, areal geology fossils, identified by S. Adegoke (Table 1), at map) named the Paso Robles Formation for three localities in the Pancho Rico Formation beds exposed near the town of Paso Robles and (see Gatehouse, 1966, Ph.D. thesis, Univ. Calif.,
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Berkeley, App. 1-C). Two of the pelecypods, sitional basin after the source rocks have been Lyropecten terminus and Macoma jacalitosana broken down to particles small enough to be Arnold, are restricted to the early Pliocene. transported. Some heavy minerals are victims Each of the three fossil localities contains one of of chemical weathering at the outcrop and are these two pelecypods and each is within a few never transported into the depositional basin. stratigraphic feet of the contact with the Paso Robles Formation. This evidence, along with Laboratory Techniques the fact that in places the two formations inter- The grains in each sample larger than —101 tongue, shows that the lowermost beds of the and smaller than +40 were removed and the Paso Robles Formation are early Pliocene in remaining sample split, one portion being ana- age- Most, if not all, of the Paso Robles Forma- TABLF. 1. FOSSILS COLLECTED FROM THREE LOCALITIES tion of the Salinas Valley is Pliocene in age and IN THE PANCHO Rico FORMATION, CALIFORNIA will be considered in this paper as belonging Identified by S. Adegoke mainly to the middle and late Pliocene. This conclusion is based on the early Pliocene age of Localities its lowermost beds, the lack of obvious uncon- Fossil 177 184 482 formities within individual outcrops of the for- mation, and the fact that large-scale movement Echinoidea seems to have occurred on the San Andreas Astrodapsis sp. indet. X Pelecypoda fault since deposition of the Paso Robles For- Anadara sp. cf. A. irilineata (Conrad) X mation (see "Movement along the San Andreas Lyropecten terminus (Arnold) X fault," further on). Consequently, the use of Macoma sp. cf. M. jacalitosana the name "Paso Robles Formation" for conti- Arnold X X Protothaca tenenima (Carpenter) X nental beds in the Santa Maria basin should be Solen sp. cf. 5. sicarius Gould X discontinued. These beds are mostly Pleistocene S. sp. indet. X in age (Woodring and Bramlette, 1950, p. 108) Gastropoda and were not deposited in the same depositional ? Calyptraea sp. indet. X Nassarius sp. cf. N. (Caesta) basin as the type-Paso Robles Formation of the gramtnatus (Dall) X upper Salinas Valley. Sinum sp. indet. X Cirripedia HEAVY MINERALS Balanus (Balanus) gregarius (Conrad) X B. sp. indet. X General Discussion The distribution of heavy minerals in the Paso Robles Formation helps delineate the lyzed for heavy minerals and one portion for broad pattern of rivers that deposited the for- grain size. The portion reserved for heavy- mation. Of the 14 heavy minerals noted, six mineral analysis was placed in bromoform (spe- predominate: sphene, hornblende, garnet, epi- cific gravity 2.89), the heavy and light fractions dote, apatite, and zircon. These can be traced separated and weighed. In general, the heavy to their sources in the highlands surrounding fraction comprises about 1 per cent of the total the Paso Robles depositional basin. sand fraction of the Paso Robles Formation. Heavy-mineral analyses were run on 197 The —10—\-l
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The portion of the samples reserved for water drainage basins. Vector analysis is one grain-size analysis was placed in an automatic method that was used for determining the de- recording settling tube. The resulting data gree of mixing and the compositions of the were processed by a computer, which, for each source assemblages (see Imbrie and van Andel, sample, calculated the median, mean, variance, 1964, for a detailed discussion of the use of vec- tor analysis in processing heavy-mineral data). The other method is a comparison of heavy- mineral assemblages in the Paso Robles Forma- SANTA LUCIA RANGE tion with assemblages in modern streams pe- ripheral to the Paso Robles deposition basin (see first portion of section, "Heavy minerals"). The analysis of numerous variables (14 differ- ent heavy-mineral species) in 191 samples pro- duces a data matrix which is large and complex in structure. Vector analysis provides a mathe- matical model and computing technique which shows that a simpler structure exists consisting of a number of mixtures of heavy-mineral as- semblages which is less than the total number of individual variables. A very large proportion of the total variability was shown to result from the mixing of these assemblages in varying proportions. Vector analysis showed that six assemblages in the heavy minerals of the Paso Robles Formation account for 99.5 per cent of the total variability (Tables 2 and 3). These were defined in terms of actual sample compo- sitions (end members). The computer then cal- culated the proportion of each remaining sam- ple that could be attributed to each of the end members (see Galehouse, 1966, Ph.D. thesis, App. III).
TABLE 2. COMPOSITIONAL VARIABILITY OF THE PASO ROBLES FORMATION, CALIFORNIA, ACCOUNTED FOR BY Six END MEMBERS OF HEAVY MINERAL ANALYSES Per cent of OF THE compositional Cumula- PASO ROBLES FORMATION End Sample variability tive member number accounted for per cent 0 MILES 10 1 129 60.3 60.3 Figure 6. Sample localities of heavy-mineral 2 313 24.2 84.5 analyses of the Paso Robles Formation 3 92 5.5 90.0 4 157 4.6 94.6 5 443 3.3 97.9 standard deviation, and modes (see Galehouse, 6 32 1.6 99.5 1966, Ph.D. thesis, App. IV) and plotted a fre- quency curve and a cumulative curve (see van Andel, 1964, p. 268-270). Vector analysis of the heavy-mineral compo- sitions of samples of the Paso Robles Formation Vector Analysis objectively determined the significant compo- This study assumes that most or all the varia- nents involved and gave the assurance that sig- tion in heavy-mineral composition within the nificant variables were not overlooked. As Paso Robles depositional basin represents mix- shown in Tables 2 and 3, one particular mineral ing of heavy-mineral assemblages introduced is predominant in each of the six end members; into the depositional basin from various head- two of the end members (those with sphene and
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hornblende dominant, respectively) account the five other minerals, except that the zircon for 84.5 per cent of the total variability. percentage may be affected to a slight degree In applying the computer results to this (see discussion of zircon in this section). Conse- heavy-mineral study, the question was investi- quently, as shown graphically in Figure 7, the gated concerning the relation of the composi- heavy-mineral variability in the Paso Robles tions of the end members selected by the com- Formation is not significantly related to grain puter with the compositions of the proposed size. source areas. The end members do correspond closely in composition to assemblages of heavy Intrastratal Solution minerals in modern streams peripheral to the In order to see if tntrastratal solution affected Paso Robles depositional basin. the composition of the heavy minerals in the Paso Robles Formation, the percentage of vari- Variation in Composition with Grain Size ous mineral species that are etched was noted. In order to test the possibility that variation Hornblende and augite are the only minerals in the heavy-mineral composition of a certain that are etched fairly commonly. In the samples
TABLE 3. HEAVY-MINERAL COMPOSITION OF END MEMBERS (IN PER CENT) IN THE PASO ROBLES FORMATION, CALIFORNIA
e u c u u. s c -3 u 0 "Oc H i. 'c»
portion of the Paso Robles Formation is related from the peripheral streams, between 1 and 2 to grain size, the product-moment correlation per cent of the hornblende grains and about 10 coefficients (r) of the percentages of the six per cent of the augite grains are etched. In the most common heavy minerals in the formation samples from the Paso Robles Formation, about were calculated. Using the t test, with N = 201, 25 per cent of the hornblende grains and about all the following values of r are significant at the 75 per cent of the augite grains are etched. If 1 per cent level. For sphene, r is —0.033; for we assume that the streams depositing the Paso hornblende, —0.086; garnet, —0.239; epidote, Robles Formation contained about the same 0.189; apatite, 0.261; and zircon, 0.106. Positive percentages of etched grains as the peripheral numbers indicate a correlation between increas- streams of today, then some intrastratal solution ing mineral percentage and decreasing median must have occurred in the Paso Robles Forma- grain size, whereas negative numbers indicate a tion. correlation between increasing mineral percent- In general, however, the degree of etching of age and increasing size. For sphene, r2 is 0.0011; individual grains of hornblende and augite in for hornblende, 0.0057; garnet, 0.0571; epidote, the formation is not very intense. Most of these 0.0357; apatite, 0.0681; and zircon, 0.0112. grains are elongated parallel to (110) and only This means that in the case of sphene, 0.11 per the ends of the grains are etched, resulting in a cent of the variation in mineral percentage is loss of volume estimated at less than 5 per cent predictable from the variation in median grain for hornblende and about 25 per cent for au- size. In other words, the grain size of the sample gite. If significant numbers of hornblende or has not appreciably affected the percentage of augite grains were completely dissolved, then sphene found in the sample. This is also true of the proportion of the remaining grains showing
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etching and the intensity of the etching would Discussion of the distribution of heavy min- be greater than that found in the formation. erals in the Paso Robles Formation is primarily Therefore, although intrastratal solution did concerned with the main distributive prov- occur to some extent, it did not affect the over- inces. The maps of particular heavy minerals all composition of the heavy minerals in the show, in addition, samples containing moderate Paso Robles Formation. concentrations of the mineral, which are not within the main province. These samples usu- ally cannot be traced to specific source areas. 70 - . • . ' .' - The sources may have been local (see the follow- 60 " • ' •-.'''' ing discussion), and the sampling of the pe- ripheral streams was not on a fine enough scale 40 i " .. . • ' . ' 1 ' ' '•'.'• ' . '.' 5J for all these local source areas to be noticed. u 30 " '-.'• ., ' ' * LO - ' : . -.'>'. "•.' ''':-• - . ;.;'. '.:\^-^:'. 60 L • • ' 50 - « 40 -...-• \ E , . „ S 30 - /..'. ;.: • • | •; •' .'.. • '•. ' s ^ 20 - .• • ''• V/.. . . :.•'• ' 10 '..' iV;.!:ii-^i--! ' .•.''•;';.-,:,.•.,'., ..•;•'/-. 70 60 o 50 g 40 • .-. . ... s S « 30 s 2. .' 10 ; : : -••. ;. ,tv:V;C # ^' "..'• -^'' :^k'-^:^.'' 0-5 10 15 2025 30 35 05 10 15 2-0 25 3 0 3-5
Figure 7. Relationship of heavy-mineral per- centages and median grain size. 0 equivalent to —logo diameter, in millimeters
Geographical Distribution and Source Area The geographical distribution of heavy min- erals in the Paso Robles Formation is depicted by contour maps showing percentages of the predominant mineral in each of the end mem- bers. Because each of the six end members is nearly monomineralic (see Table 3), this method gives distribution patterns nearly identical to the method using proportional data from vec- tor analysis. The distribution pattern resulting from vector analysis is shown for sphene and hornblende in order to illustrate this similarity. Figure 8. Distribution of sphene in the heavy fraction of the Paso Robles Forma- The maps also show the percentage of each tion and peripheral streams important heavy mineral in the heavy minerals of each sample from peripheral streams. Be- cause each stream sample represents the entire SPHENE: Nearly all samples of the Paso upstream drainage area, the percentage of each Robles Formation which contain a high propor- heavy mineral found in each stream sample is tion of sphene in their heavy fraction are in the depicted by a single pattern over the entire southeastern portion of the depositional basin headward portion of the drainage basin. (Fig. 8). The main area with high concentra-
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tions extends northward from the north margin draining these mountains do contain moderate of the La Panza Range, strongly suggesting to high percentages of garnet (Fig. 11). These that the source area was in these mountains. streams drain mainly Late Cretaceous sand- Samples from streams draining the plutonic stones and middle Miocene porcelaneous rocks area of the La Panza Range have very high per- (Fig- 3) centages of sphene in their heavy fraction, con- firming this suggestion (Figs. 3 and 8). Just north of the town of Paso Robles is a small area with high values of sphene which is not contiguous with the main sphene area in the Paso Robles Formation. The source of this sphene is probably a small adjacent plutonic body to the west, containing nearly 60 per cent sphene in its heavy minerals. As shown in Figure 8, there is generally a narrow area in which the percentage of sphene in the Paso Robles Formation goes from high to low values, indicating a limited amount of mix- ing of the sphene assemblage with adjacent as- semblages. HORNBLENDE: Samples of the Paso Robles Formation which contain a high proportion of hornblende in their heavy fraction are found mainly in the northwestern portion of the depo- sitional basin (Figs. 9 and 10). The main area with high concentrations trends northwest- southeast and has high values at both ends, sug- gesting a source area either to the northwest or to the southeast. Samples from streams draining mainly the plutonic and metamorphic rocks of the Santa Lucia Range have extremely high percentages of hornblende, giving strong evi- dence in favor of a northwesterly source (Figs. 3 and 9). A second area of high concentration of horn- blende, located in the northern portion of the Carrizo Plains, has not been related to a specific source area. This hornblende may have come from the same streams which deposited the main concentration of hornblende, i.e., streams Figure 9. Distribution of hornblende in the flowing southeastward from the Santa Lucia heavy fraction of the Paso Robles Forma- Range may have curved southward and also tion and peripheral streams deposited these rocks in the Carrizo Plains. On the other hand, older rocks south of the area Because garnet has a distribution and source investigated may have been the source. The area similar to that of hornblende, this portion hornblende in this portion of the Paso Robles of the Paso Robles Formation is considered to Formation would then have been deposited by be within the Hornblende-Garnet heavy-min- northward-flowing streams. The distribution of eral province. On the other hand, there is no garnet indicates that this latter explanation is garnet associated with the hornblende in the probably the correct one. northern Carrizo Plains (Figs. 9 and 11). This GARNET: The geographical distribution of lack of associated garnet is evidence that the garnet in the heavy fraction of the Paso Robles hornblende in the Carrizo Plains did not come Formation (Fig. 11) is quite similar to the dis- from the streams that deposited garnet in the tribution of hornblende (Fig. 9) and gives main area of hornblende concentration. strong evidence in support of a source area in EPIDOTE: There are three main areas in the the Santa Lucia Range. Some of the streams Paso Robles Formation with moderate to high
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amounts of epidote (Fig. 12). The epidote con- ing more than 20 per cent epidote in their centration on the northeast side of the San heavy fraction, 17 are from streams draining Andreas fault was derived from nearby rocks to either Franciscan rocks or rocks lying on a the northeast. Streams draining these rocks are Franciscan basement (Figs. 3 and 12). extremely rich in epidote (Fig. 12). Samples APATITE: Most of the samples of the Paso
SANTA LUCIA RANGE
HORNBLENDE PROJECTIONS |^^| .76 - 1.00
^ ^ 26 - .50 I;.' •'] <.26 0 MILES 10
Figure 10. Proportional data from vector analysis: distributions of sphene and hornblende
from a large area near the town of Paso Robles Robles Formation containing moderate or high contain mostly moderate amounts of epidote. concentrations of apatite in their heavy fraction Their distribution indicates a source area to the He in an area extending northward from the west or southwest. Figure 12 shows that there is north margin of the La Panza Range (Fig. 13). a peripheral stream sample to the southwest This geographical distribution, similar to that which contains a moderate amount of epidote. of sphene (Fig. 8), likewise suggests a source The third main concentration of epidote is a area in the La Panza Range. Samples from few miles south of San Ardo and has not been streams draining the La Panza Range do con- related to a specific source area. tain moderate concentrations of apatite (Fig. Of the 19 peripheral stream samples contain- 13). The portion of the Paso Robles Formation
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containing both sphene and apatite is within portion of the formation on the northeast side the Sphene-Apatite heavy-mineral province. of the fault, which has a source area to the A small group of samples containing moderate northeast (see "Epidote," "Sandstone peb- amounts of apatite occurs a few miles south of bles," and "Maximum clast size"). The finer- San Ardo and is probably derived from the grained samples contain larger percentages of zircon; samples with more than 20 per cent zircon have a median grain size of about 3.0$, whereas those with less than 20 per cent aver- age half a
Figure 11. Distribution of garnet in the heavy fraction of the Paso Robles Forma- tion and peripheral streams
nearby Monterey Formation in the foothills of the Santa Lucia Range. ZIRCON: Only eight of the 197 heavy-min- Figure 12. Distribution of epidote in the eral samples of the Paso Robles Formation con- heavy fraction of the Paso Robles Forma- tain more than 20 per cent zircon. Six of these tion and peripheral streams eight samples form a Zircon province on the northeast side of the San Andreas fault (Fig. 14) for which no specific source area is known. heavy-mineral counts of these samples, even Although grain size does not affect zircon per- though it may be present in the source rocks. centage in the Paso Robles Formation as a As shown by the correlation coefficient and whole (see "Variation in composition with Figure 7, in many samples of the Paso Robles grain size"), it may have some influence in the Formation the median grain size is quite fine
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and yet little or no zircon is present. Therefore, La Panza ranges, along with the younger rocks even though grain size has some effect on zircon resting on them, were the main sources for the percentage, there must be a difference in source heavy minerals in the Paso Robles Formation, area for the fine-grained samples with much not the Franciscan and related rocks. Mica, zircon and those with little. which is much more common in the granitic OTHER MINERAL SPECIES: Samples of the Paso Robles Formation contain no heavy-min- eral species in appreciable amounts in the counts of 100 identifiable grains other than the six already discussed. A few samples from pe- ripheral streams draining Franciscan rocks or rocks lying on a Franciscan basement, however,
Figure 14. Distribution of zircon in the heavy fraction of the Paso Robles Forma- tion and peripheral streams
basement rocks in the Santa Lucia and La Panza ranges than in Franciscan basement rocks in the southern Diablo range, is also common in Figure 13. Distribution of apatite in the the Paso Robles Formation (see Galehouse, heavy fraction of the Paso Robles Forma- tion and peripheral streams Ph.D. thesis, 1966, App. II). Vertical Variation contain abundant augite, kyanite, or glauco- The foregoing discussion concerning the dis- phane. The fact that these three minerals are tribution of heavy minerals in the Paso Robles very rare in the Paso Robles Formation is in Formation is based on the assumption that the harmony with the conclusion that the plutonic primary variation in heavy minerals within the and metamorphic rocks of the Santa Lucia and formation is lateral rather than vertical. Ad-
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mittedly, the heavy-mineral distribution at the stream were draining mainly the plutonic depth was not investigated through lack of and metamorphic rocks, the heavy minerals in suitable samples. Consequently, the only meth- the Paso Robles Formation are richer in horn- od of checking for vertical variation was by de- blende; if the stream were draining mainly the tailed investigations of thick outcrops. Late Cretaceous sandstones and middle Mio- At sample locality 403 (see Gatehouse, Ph.D. cene porcelaneous rocks, the heavy minerals are thesis, 1966, App. I for detailed locations), richer in garnet. Therefore, the total horn- about 225 stratigraphic feet of the Paso Robles blende-plus-garnet percentage is more mean- Formation was sampled at about 50-foot inter- ingful than either single percentage. Figure 15 vals, and heavy-mineral counts were made shows the vertical variation in hornblende and (Table 4). There is remarkably little vertical garnet percentages compared to the vertical variation at this locality. The three most com- variation in the hornblende-plus-garnet per- centage at locality 167. Although the percent- age of garnet varies by a factor of eight and the TABLE 4. HEAVY-MINERAL COMPOSITION (IN PER percentage of hornblende by a factor of three, CENT) OF SAMPLE LOCALITY 403 IN THE PASO ROBLES the combined percentage remains nearly con- FORMATION, CALIFORNIA stant throughout the interval. Sample interval is 50 feet. Figure 16 shows the geographical distribution of the combined hornblende and garnet per- centage in the heavy minerals of the Paso
c u o c o 11 Sample no., from G i u top to bottom o E 'S. "S. TABLE 5. HEAVY-MINERAL COMPOSITIO•IOIN (IN PER I O W (/J H N CENT) OF SAMPLE LOCALITY 167 IN THE P'AS. O ROBLES 403-A 2 10 7 21 54 1 5 FORMATION, CALIFORNIA 403-B 2 14 2 18 60 2 2 Sample interval is 25 feet except between 167-F and 403-C 1 12 8 20 55 4 167-H where it is 50 feet. 403- D 12 1 22 58 1 5 1 403-F. 2 8 6 24 58 1 1
y a. Sample^no. OJ u o cV e u
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mination of source area and dispersal patterns. fraction were then identified. The number of The fact that the major suites of heavy miner- porcelanite pebbles found during the count was als in the formation have compositions and noted separately. Dark amber-colored chert geographical distributions that can be traced to pebbles, which also characterize the Monterey particular source areas is perhaps the best evi- Formation, are included in the "porcelanite" dence that vertical variation is of minor im- category as are pebbles containing both chert portance. and porcelanite. Light-brown chert pebbles
PEBBLES
General Discussion SANTA LUCIA 0 SAN ARDO The compositions of pebble fractions in 174 RANGE outcrops of gravel in the Paso Robles Forma- tion (Fig. 18) were examined in detail and 124
40 30 60 70 80 90 Percentage Figure 15. Vertical variation in hornblende and garnet percentages at locality 167
pebble counts made from those outcrops which contain quantities of pebbles other than por- celanite (Galehouse, 1966, Ph.D. thesis, App. V). In 50 outcrops, the only pebble type found was white to light-gray porcelanite, which char- acterizes and was probably derived from the nonclastic beds in the Monterey Formation. An additional 65 outcrops contain areas in which all the pebbles are porcelanite. Porcelan- HORNBLENDE - ite pebbles were segregated from the other GARNET varieties of pebbles because they have a lower ,1-100% specific gravity (about 1.5-1.6) than do pebbles of most other lithologies and consequently be- have differently during stream transport. They can continue to be carried by a stream with a competency that has fallen too low to transport other varieties of pebbles of a similar size. Figure 16. Distribution of the combined per- centages of hornblende and garnet in the Sampling Procedure and Field Methods heavy fraction of the Paso Robles Forma- tion In order to compare quantitatively the non- porcelanite pebble lithologies at one outcrop with those at others, a single size fraction was which contain no admixtures of porcelanite and always investigated. The pebbles were dug which may or may not have come from the from the outcrop and poured through sieves. Monterey Formation are placed in a "chert" The diameter of the intermediate axis of the category as are all the red and green chert peb- largest clast was noted. Lithologies of 100 of the bles. nonporcelanite pebbles in the 1- to 2-inch size Pebbles derived from coarsely banded gneiss
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such as that found in the Sur Series in the these streams do not contain any pebbles. Santa Lucia Range often do not show any Streams depositing the Paso Robles Formation, banding and cannot readily be distinguished however, must have had sufficient competency from pebbles derived from silicic plutonic to carry large amounts of pebbles. It is possible that a difference in competency existed be- tween these streams and the present streams due to a difference in climate or gradient, or
SANTA LUCIA RANGE
SAMPLE LOCALITIES
OF
• PEBBLE COUNTS Figure 17. Distribution of the combined per- , CONCENTRATIONS OF centages of sphene and apatite in the heavy PORCELANITE PEBBLES fraction of the Paso Robles Formation 0 MILES 10
rocks. Therefore, all silicic plutonic and gneissic Figure 18. Sample localities of pebble counts pebbles are included in one category labeled and outcrops containing concentrations of porcelanite pebbles in the Paso Robles For- "silicic basement." mation The various types of arenites or wackes (see Williams and others, 1954, p. 293) are not dis- tinguished but are included in one category that the weathering processes then produced labeled "sandstone." more pebble-sized particles than they do now. Counts were also made of the various litholo- Since relatively few streams are now transport- gies of pebbles in the bedloads of 29 of the 75 ing pebbles, maps showing pebble percentages peripheral streams which were sampled for in the peripheral streams do not provide suf- their heavy-mineral composition. Many of ficient information to form interpretations con-
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cerning the source areas for the pebbles in the therefore, the nonclastic beds in the Monterey Paso Robles Formation. Thus, the distribution Formation were the principal source of the of the various lithologies in the peripheral rocks pebbles in the formation. Almost all the gravels themselves (Fig. 3) must also be considered in in the northern and western portions of the determining the source area. Paso Robles depositional basin are rich in por- celanite pebbles (see Fig. 19, which also shows the distribution of porcelanite pebbles in the peripheral streams). In general, the portion of the Paso Robles Formation containing concen- trations of porcelanite pebbles in its gravels also contains high percentages of hornblende or garnet in its heavy minerals. The Santa Lucia Range, the source area for most of the horn- blende and garnet, was probably the source area for most of the porcelanite pebbles as well. The Monterey Formation, which is character- ized by porcelanite beds, covers an area of sev- eral hundred square miles in these mountains, between the basement rocks and the Salinas River (Fig. 3). Porcelanite pebbles in the Paso Robles grav- els, in the western portion of the basin, (south of the town of Paso Robles) were derived from the Monterey beds immediately to the west and southwest. They are thought to be of local origin because some are quite angular. In gen- eral, porcelanite pebbles become rounded quite rapidly during stream transport (probably in a few miles or less). There are also porcelanite-rich gravels in the portion of the formation in the northern Car- rizo Plains. Some of these pebbles are also angu- lar and must have had a nearby source. Perhaps the Monterey beds in the Temblor Range which are now to the south and east may have been closer during Paso Robles deposition (see "Movement along the San Andreas fault," further on, and Dibblee, 1962, p. 8). The main area where the Paso Robles gravels Figure 19. Distribution of concentrations of lack appreciable amounts of porcelanite coin- porcelanite pebbles in the Paso Robles For- cides, in general, with the area of the Sphene- mation and peripheral streams Apatite heavy-mineral province (Fig. 19). The conclusion that the heavy minerals in this prov- ince were derived from the La Panza Range is Geographical Distribution and Source Area further substantiated by this lack of associated In general, the pebble distributions further porcelanite pebbles and by the fact that out- substantiate the conclusions concerning source crops of the Monterey Formation are not ex- area and dispersal patterns which were reached tensive in these mountains (Fig. 3). from a consideration of the heavy-mineral data SILICIC BASEMENT PEBBLES: Much of the alone. Of the 15 different varieties of pebbles area of the Paso Robles Formation within the noted in the Paso Robles gravels, only four are Hornblende-Garnet and Sphene-Apatite heavy- common: porcelanite, silicic-basement, sand- mineral provinces contains moderate to high stone, and chert. concentrations of silicic basement pebbles in the PORCELANITE PEBBLES: Porcelanite pebbles nonporcelanite portions of its gravels (Fig. 20). are more than twice as common as any other The basement rocks in the Santa Lucia and La type of pebble in the Paso Robles Formation; Panza ranges were the source of these pebbles
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(Figs. 3 and 20) as well as the source for the two SANDSTONE PEBBLES : Many of the gravels in previously mentioned heavy-mineral provinces. the Paso Robles Formation are rich in sand- There are different provenances for the Paso stone pebbles (Fig. 21). The entire area periph- Robles gravels on either side of the San Andreas eral to the depositional basin, however, is rich fault. The gravels adjacent to the fault on the in sandstone beds of Jurassic, Cretaceous, or Tertiary age (Figs. 3 and 21). Consequently, the distribution of sandstone pebbles in the for- mation is not diagnostic of provenance. The distribution, however, contradicts none of the conclusions previously mentioned concerning the source areas for the formation and does give supporting evidence for one of them.
Figure 20. Distribution of silicic basement pebbles in the Paso Robles Formation and peripheral streams
southwest side contain about 20-60 per cent silicic basement pebbles, whereas gravels on the northeast side contain, at most, only 2 per cent Figure 21. Distribution of sandstone pebbles silicic basement pebbles. in the Paso Robles Formation and periph- None of the sampled peripheral streams in eral streams the Diablo or Temblor ranges contains more than 7 per cent silicic basement pebbles in its gravel load. This observation supports the con- The gravels of the Paso Robles Formation on clusion that these two ranges were not source the northeast side of the San Andreas fault are areas for the main portion of the Paso Robles particularly rich in sandstone pebbles, the non- Formation on the southwest side of the San porcelanite portions averaging about 80 per Andreas fault. cent, higher than any other area in the forma-
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tion. This is additional evidence supporting the pebbles found in the Paso Robles Formation, conclusion that the Paso Robles Formation on however, were probably derived from similar- opposite sides of the fault had different source looking cherts which occur in the Franciscan areas. The source for the sandstone-rich gravels Formation throughout California. on the northeast side of the fault was probably Because the distribution of heavy minerals the Miocene sandstones bordering on the north- and other pebbles in the chert-rich portion of east side of the formation. the Paso Robles Formation points to a source area in the Santa Lucia Range, it is likely that the red and green chert pebbles were also de- rived from rocks in these mountains, e.g., the Franciscan rocks on the southwest side of the Nacimiento fault (Fig. 3). If this is the correct explanation, in order to drain these Franciscan rocks, the major streams flowing from these mountains and depositing the Paso Robles gravels must have had headwater systems as ex- tensive as that of the present Nacimiento River. Chert pebbles are not readily lost by attrition and would persist during the long transport distance from the Franciscan rocks in the Santa Lucia Range to the Paso Robles depositional basin. On the other hand, some of the chert pebbles in the Paso Robles Formation are fairly well rounded, may be second- or third-cycled, and may have come from post-Franciscan con- glomerates in the Santa Lucia Range. OTHER PEBBLE TYPES: Volcanic, quartz, mudstone, schist, limestone, pyroclastic, con- glomerate, serpentinite, amphibolite, basic plu- tonic, and breccia pebbles are also found in vari- ous outcrops of the Paso Robles Formation. They are not very abundant and are of little use in determining dispersal patterns and source areas for the Paso Robles Formation. In three sample localities, however, limestone pebbles comprise more than 20 per cent of the nonpor- celanite pebble fraction. All three of these localities are on the northeast side of the San Andreas fault. Maximum Clasl Size Figure 22. Distribution of chert pebbles in The distribution of the largest sized clasts in the Paso Robles Formation and peripheral the Paso Robles Formation, by itself, does not streams give a clear enough pattern for a determination of the downstream directions in the deposi- tional basin (Fig. 23). The distribution does, CHERT PEBBLES: The distribution of the however, help substantiate some of the conclu- chert pebbles, which were obviously not de- sions reached from a consideration of other rived from the Monterey Formation, is shown factors. in Figure 22. From the percentages of chert in Large clasts are found near the margin of the the pebbles of the peripheral streams, no obvi- depositional basin adjacent to the La Panza ous source for the chert in the Paso Robles For- Range and were probably derived from it. The mation can be postulated. As mentioned previ- largest clasts in two localities on the northeast ously, some of the light-brown chert pebbles in side of the San Andreas fault are in the 2-3-foot the formation may have been derived from the range. Debris this large was probably trans- Monterey Formation. The red and green chert ported only a short distance and was most likely
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derived from the Miocene sandstones bordering p. 1035-1036). The same grid units and the on the northeast side of the formation. same method of averaging as those described in the preceding paragraph were used. PALEOCURRENT INDICATORS FORESET AND PEBBLE IMBRICATION AZIMUTHS: Interpretations concerning the paleocurrent In general, the foreset beds and imbricated pefa- system in the Paso Robles Formation have been made by considering the geographical distribu- tions of its heavy minerals and pebbles. A more direct method for determining paleocurrent di- rections is to measure current indicators in the field. The Paso Robles Formation contains current indicators in fair abundance; 94 of the 387 sample localities investigated contain either foreset beds, channels, imbricated pebbles, or a combination of these (PL 1). Foreset beds and channels are about equally abundant. Pebble imbrication was recognized in only 7 of the out- crops. FIELD METHODS AND AVERAGING TECHNIQUES : At each outcrop containing current indicators, measurements were made of each different kind of indicator. Where current indicators occur in more than one horizon or vary laterally within a horizon, additional measurements were re- corded. Generally, the variation in an indicated direction within an outcrop is not large (see Galehouse, 1966, Ph.D. thesis, App. VI). For each site containing either foreset beds or imbricated pebbles, an average current azimuth was calculated by means of a vector summation, with each individual measurement weighted equally (Fig. 24). In outcrops containing chan- nels, an average current trend was calculated (Fig. 25). In order to show the general current pattern better, local variations were "smoothed out" in two different ways. A grid with units of 10 square miles was placed over a map of the Paso Robles Formation, and all the measurements within each unit of the grid were averaged, the resulting value being plotted at the center of the grid unit (Fig. 26). In order to use all the Figure 23. Distribution of maximum-sized measurements, however, a method had to be clasts in the Paso Robles Formation found for averaging channel measurements, which give only a trend with foreset and imbri- bles in the Paso Robles Formation show that in cated pebble measurements, which give an azi- the southern half of the depositional basin, the muth. Therefore, for each channel measure- depositing streams were flowing north and east, ment, an interpretation was made as to which away from the La Panza Range (Fig. 24). In the direction along the trend the paleocurrent was northern half of the basin, the general flow was flowing. These interpretations are based on a to the east, away from the Santa Lucia Range. consideration of associated azimuth indicators The patterns of flow determined from these cur- and the dispersal patterns suggested by the dis- rent indicators are quite similar to the ones de- tributions of heavy minerals and pebbles. termined through a consideration of the distri- The second method used for showing the butions of heavy minerals and pebbles. general current pattern is that of the moving CHANNEL TRENDS : The trends of channels in average (Fig. 27), described by Pelletier (1958, the Paso Robles Formation show a more con-
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sistent pattern than do the foresets and imbri- terns, determined from the distribution of cated pebbles (compare Figs. 24 and 25). The heavy minerals, and the paleocurrent directions, general trend of the channels further substanti- determined from the field measurements of cur- ates the conclusions concerning the paleocur- rent indicators. The fact that all the different rent directions, reached through a considera- methods used for determining the paleocurrent tion of the heavy-mineral and pebble distribu- tions and the foreset beds and imbricated peb- bles. SANTA LUCIA AVERAGES OF FIELD MEASUREMENTS: Figure RANGE 26 shows the average of all the field measure-
SITE AVERAGES —- OF CHANNEL TRENDS *-_ HOBNBLENOC- (MflNET SITE AVERAGES --*• SWeNE-APWITE OF FORESET AND PEBBLE IMBRICATION AZIMUTHS Figure 25. Site averages of channel trends in HOftNBLEWC- GARNET the Paso Robles Formation L SPHENE-ARM1TE 0 MILES 10 system in the Paso Robles Formation point to Figure 24. Site averages of foreset and peb- the same conclusions helps establish the validity ble-imbrication azimuths in the Paso Robles of each particular method. Formation IMPLICATIONS CONCERNING LATE ments of paleocurrent direction; Figure 27 CENOZOIC GEOLOGIC HISTORY shows the moving average of these measure- ments. The outline of the Hornblende-Garnet Drainage Changes and Tectonic Uplift and Sphene-Apatite heavy-mineral provinces is During early Pliocene time there was proba- included in the figures in order to illustrate the bly a marine connection between the Santa very close agreement between the dispersal pat- Maria, Paso Robles, and Coalinga areas (Fig.
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28, left), as indicated by the early Pliocene fos- areas. This conclusion is based on the proposed sil assemblages in these areas (Durham and Ad- age, provenance, and paleocurrents of the Paso dicott, 1965, p. 17-19). In addition, the Co- Robles Formation as well as on differences be- alinga area of the San Joaquin marine basin was tween the middle and late Pliocene marine probably connected to the sea in the Monterey faunas of the two areas (Woodring and Bram- lette, 1950, p. 103). At present the Paso Robles Formation is drained by the Salinas River, which flows north- westward to Monterey Bay along the Salinas Valley structural trough (Fig. 4). The geo-
AVERAGES OF „ PALEOCURRENT MEASUREMENTS
-. HOWBLENDE- GARNET
* SPHENE-AfiftTtTE MOVING 0 MILES 10 AVERAGES OF PALEOCURRENT Figure 26. Averages of paleocurrent meas- MEASUREMENTS urements in the Paso Robles Formation
Bay area via the San Benito trough, which trended northwestward along the present trace Figure 27. Moving averages of paleocurrent of the San Andreas fault (Hoots and others, measurements in the Paso Robles Formation 1954, p. 123-124). Consequently, the Santa Lucia and Gabilan ranges, if emergent, formed an island during early Pliocene time. Uplift in graphical distribution of heavy minerals and the Santa Lucia and La Panza ranges toward pebbles in the formation and measurements of the end of the early Pliocene initiated Paso current indicators lead to the conclusion that an Robles deposition and ended the marine con- exceedingly different drainage pattern existed nection between the Santa Maria and Coalinga during deposition of the formation (e.g., Figs.
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26 and 27). Streams flowing southeastward and the entire Salinas River drainage basin was from the Santa Lucia Range and northward less than one-fourth as extensive as it is now. from the La Panza Range deposited most of the The paleodrainage and provenance determina- formation. These two main sets of streams tions also indicate that the Santa Lucia and La probably flowed into a major stream which Panza ranges were positive features contribut- flowed eastward across the present site of the ing coarse debris to the Paso Robles depositional Temblor Range into a marine basin at the pres- basin during middle and late Pliocene time, ent site of the southern San Joaquin Valley whereas at least a portion of the Temblor (Fig, 28, right). The course of this major stream Range was low during Paso Robles deposition
0 30 IMilesJ
Paso Drain; Pattern
! San \ t Joaquin i Paso Robl Basin ,' San Andreas Fault i Miles BakerMe s field SantajMarU San Andreas Fault LATE PLIOCENE Figure 28. Early Pliocene and late Pliocene paleogeography. Modified from Reed (1933, p. 252), Woodring and Bramlette (1950, p. 103), Hoots and others (1954, p. 124), Durham and Addicott (1965, p. 17-19), and Seiden (1964, p. 51)
may have been along the boundary between the and was not uplifted until post-Paso Robles Hornblende-Garnet and Sphene-Apatite heavy- time. In fact, the beginning of uplift in the mineral provinces. Temblor Range, along with southwestward Ideas concerning a northern or western source tilting of the Gabilan Mesa, probably brought for at least part of the Paso Robles Formation about the end of Paso Robles deposition, de- have been expressed previously, based in part feating the streams draining the Paso Robles on the fact that coarse debris is found along the area and initiating a period of interior drainage northwestern margin of the basin and on the which may be represented by lake beds in the assumption that drainage lines existed along the upper portion of the Paso Robles Formation. present site of the San Antonio and Nacimiento The Salinas River then began to capture the rivers during Paso Robles deposition (e.g., headwaters of these defeated streams. The Car- Reed, 1925, p. 606; Taliaferro, 1943, p. 461; rizo Plains are probably a relic of this period of Weidman, 1959, Ph.D. thesis, Univ. Calif., interior drainage, not yet captured by the Berkeley; Baldwin, 1963, map II; Christensen, Salinas River or its tributaries. 1966). As pointed out previously (see "Chert peb- This paleodrainage pattern indicates that in bles"), some of the streams flowing from the Pliocene time the headwaters of the Salinas Santa Lucia Range during Paso Robles deposi- River were no further south than King City tion probably had extensive headwater systems.
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In fact, the San Antonio and Nacimiento rivers blende in the silt and sand portion of these were probably major headwaters of the middle rocks as well as silicic basement pebbles in the and late Pliocene drainage that flowed south- conglomeratic portions. However, because this eastward into the San Joaquin Valley. They is not the case, the northeast portion must not presently bend sharply to flow northwestward nave been adjacent to the southwest portion (Fig. 4), a result of piracy by the Salinas River. during deposition. In order to separate the con- Movement along the San Andreas Fault The heavy minerals in the Paso Robles For- mation on the southwest side of the San An- dreas fault are different from those on the northeast side of the fault. The distributive provinces of the two most common heavy min- erals in the Paso Robles Formation, sphene and hornblende, are located entirely on the south- west side of the fault and terminate abruptly at the fault (Figs. 8 and 9). The two heavy-min- eral provinces that occur in the formation on the northeast side, Epidote and Zircon, also end abruptly at the fault (Figs. 12 and 14). The marked difference in heavy minerals shows that the source areas on either side of the fault were quite different. The pebbles in the Paso Robles Formation also indicate different provenances for the por- tions of the formation on either side of the San Andreas fault. A large percentage of silicic- basement pebbles is found in the gravels adja- cent to the fault on the southwest side, but es- sentially none are found in the gravels on the northeast side (see "Silicic basement pebbles" and Fig. 20). On the other hand, the gravels on the northeast side of the fault are extremely rich in sandstone pebbles, much richer than 61-100% those in any area on the southwest side (see 41-60 % "Sandstone pebbles" and Fig. 21). In addition, 21-40 % the gravels of the northeast portion of the Paso I.'.v.;| 0-20 % Robles Formation are the only ones with appre- ciable amounts of limestone pebbles and they contain clasts up to 3 feet in diameter, which were probably derived from nearby rocks to Figure 29. Distribution of sphene in the heavy fraction of the Paso Robles Forma- the northeast (see "Maximum clast size" and tion in late Pliocene time Fig. 23). Most of the streams depositing the Paso Robles Formation flowed either northward tiguous beds of the Paso Robles Formation on from the La Panza Range or southeastward either side of the San Andreas fault, the south- from the Santa Lucia Range and then flowed west side of the fault must be shifted about 25 across the present site of the Temblor Range miles to the southeast (assuming the predomi- into the San Joaquin basin (Fig. 28, right). If nant lateral movement on the fault has been during deposition the portion of the Paso right lateral). The distributions of sphene and Robles Formation on the northeast side of the hornblende in the Paso Robles Formation have San Andreas fault were in juxtaposition with been contoured as if this amount of movement the portion of the formation on the southwest has taken place in order to illustrate that prov- side of the fault (as it is now), the streams flow- inces on the southwest side are not as abruptly ing from the La Panza and the Santa Lucia terminated by beds on the northeast side (Figs. ranges would have deposited sphene or horn- 29 and 30).
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The idea that beds of the Paso Robles For- rera, 1951, M.A. thesis, Univ. Calif., Berkeley; mation on either side of the San Andreas fault Peryam, 1950, M.A. thesis; Dibblee, 1962). with different heavy mineralogy, pebble lithol- Granted that the Paso Robles beds on either ogy, and provenance have been brought into side of the San Andreas fault are essentially syn- juxtaposition by means of large-scale lateral chronous, one may argue that they represent movement is admittedly based on a number of different stratigraphic intervals within this for- mation and that the lithologic variations are vertically rather than laterally controlled. In view of the discussion, "Vertical variation," vertical variation is assumed to be minor, al- though it cannot be ruled out definitely. Finally, one may argue that a major trunk stream was flowing along the San Andreas fault and that lithologic differences are simply due to the various tributaries draining various lithol- ogies on either side of the fault. The fact that much of the Paso Robles Formation on both sides of the fault is flood plain deposit is evi- dence against this contention. It is most proba- ble in a major drainage system, during an ex- tensive flood stage, that the detritus from vari- ous tributaries will be mixed to some degree and will eventually be deposited on both sides of the trunk stream. This is not the case in the Paso Robles Formation; distributive provinces of various heavy minerals and pebbles end abruptly at the fault. The 25 miles of right-lateral movement along the San Andreas fault in post-Paso Robles time proposed in this paper is slightly more than twice as much as that proposed by Dibblee (1962, p. 8). At least this much movement is necessary, however, in order to eliminate prob- lems concerning the distribution of heavy min- erals and pebbles. If the Paso Robles Formation is entirely Pliocene in age, 25 miles of move- ment is not unreasonable. In this case, the rate of movement since deposition of the formation would be about 8 miles per million years. This is only slightly higher than the average rate of Figure 30. Distribution of hornblende in the 6.4 miles per million years necessary to give the heavy fraction of the Paso Robles Forma- displacement of 160 miles on the San Andreas tion in late Pliocene time fault in the last 25 million years, as suggested by Crowell (1962, p. 50). Durham (1965) gives assumptions. The first is that the Paso Robles evidence for 11 miles of right-lateral movement beds on either side of the fault are essentially on a fault in the area of the town of Paso Robles synchronous. There is no direct evidence for which occurred after the deposition of at least a this as fossils or radiometric dates are lacking. portion of the Paso Robles Formation. Present- The Paso Robles Formation on the northeast day measurements of right-lateral creep along side of the fault rests unconformably on rocks of the San Andreas fault near Hollister average late Miocene to Cretaceous age, as it does about 12 mm per year, which extrapolates to throughout much of its margin areas on the about 7.5 miles per million years (Tocher, 1960, southwest side of the fault (Fig. 3). All the p. 396). workers who have mapped these Paso Robles One problem in reconstructing the late Ce- beds on either side of the fault have considered nozoic geologic history of this area of the Cali- them synchronous (e.g., Taliaferro, 1941; Her- fornia Coast Ranges concerns the marine con-
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nection of the Pliocene basin of the southern an end). Continued movement along the San San Joaquin Valley. As pointed out previously, Andreas fault during the Pliocene would then in early Pliocene time, the marine San Joaquin account for the gradual restriction of the San basin was connected with the Santa Maria basin Joaquin basin and the resulting brackish-water and the Monterey Bay area (Fig. 28, left). The fauna. beginning of Paso Robles deposition, however, Although the evidence presented in this marked the end of the connection between the paper for about 25 miles of right-lateral move- San Joaquin basin and Santa Maria basin. In ment on the San Andreas fault in post-Paso middle and late Pliocene time, the only con- Robles time is not conclusive, it does agree with nection of the San Joaquin basin with the open the findings of other investigators and adds to ocean was the San Benito trough, bearing the data suggesting large right-lateral displace- northwestward along the present trace of the ment on the San Andreas fault in Tertiary time. San Andreas fault. The fossils in the San Joaquin basin indicate that the water was becoming pro- SUMMARY gressively brackish during the middle and late This detailed study of the provenance and Pliocene (Grant and Gale, 1931; Adegoke, paleocurrents of the Paso Robles Formation has 1966, Ph.D. thesis, Univ. Calif., Berkeley; resulted in a more explicit statement of the late Woodring and others, 1940), suggesting a grad- Cenozoic geologic history in this portion of the ual restriction of circulation of the water in the California Coast Ranges. Uplift in the Santa San Joaquin basin with the open ocean. Large- Lucia and La Panza ranges initiated Paso Robles scale, right-lateral movement on the San An- deposition in early Pliocene time. The paleo- dreas fault may account for this gradual re- drainage was southeastward from the Santa striction. Lucia Range and northward from the La Panza Assuming that during the Pliocene the fault Range, continuing across the present site of the was moving at an average rate of 6-8 miles per Temblor Range and into a marine basin at the million years, Coalinga would have been close present site of the southern San Joaquin Valley. to Salinas during the early Pliocene (Fig. 28, The Salinas River was a relatively unimportant left, has been drawn assuming that 65 miles of stream during Paso Robles deposition. right-lateral movement has occurred since early Near the end of the Pliocene, uplift in the Pliocene time). This results in a fairly open Temblor Range and southwestward tilting of connection between the San Joaquin basin and the Gabilan Mesa brought about the end of the open ocean. As the fault continued to move Paso Robles deposition by defeating the south- in Pliocene time, the southwest side moved east-flowing drainage and creating the condi- northwest relative to the northeast side. This tions for its capture by the modern Salinas moved the San Joaquin basin further from the River. Lithologic differences between juxta- open ocean,, resulting in its being connected to posed beds of the Paso Robles Formation on the ocean by a long narrow trough during the either side of the San Andreas fault suggest that late Pliocene (Fig. 28, right, has been drawn as- about 25 miles of right-lateral movement has suming that 30 miles of right-lateral movement occurred along the fault since deposition of the has occurred since late Pliocene time, i.e., since formation. shortly before Paso Robles deposition came to
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MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 10, 1966
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