RESEARCH· ·1970·

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HANNA B69 B ' Este-ro Bay . 0 c E A N San Luis Obispo Bay 2.-Generarlized geologic and aeromagnetic map of the Cape San Martin-San Luis Obispo aTea. Geology from San l!~IGURE Luis Obispo and Santa Cruz sheets of ·the geologic map of California (Jer.nings, 1958; Jennings and Strand, 1959). Well data from Smith (1964). Magnetic contours show total-intensity magnetic field of the earth relative to arbitrary datum. Contour interval, 40 gammas. Flight elevation, 6,500 foot barometric. A regional field o1' 9 gammas in the direction N. 16° El. has been removed.
B70 GEOPHYSICS lies occur over relatively small outcrops of serpentinite, tnany of which extend several miles deep in the earth~s ernst. Larger ultramafic bodies, such as Burro l\1ountain (Burch, 1968), 11 miles east-southeast of Cape San Martin, have little or no magnetic expression. The southeast segment of the anomaly train extends from Estero Bay to just south of San Luis Obispo. Here the most conspicuous single anomaly is associated with a majpr serpentinite body at Cuesta, 5 miles north .of San Luis Obispo. The inferred subsurface extent of the Cuesta body exceeds that of all other serpentinite bodies in. the map area. The occurrence of positive anomalies over the terrane of Franciscan rocks suggests that most ultramafic rocks are expressed largely by their intensity of induced magnetization; these positive features serve as indicators of the degree of serpentinization of ultramafic rocks as well as the amount of these rocks in the subsurface. Absence of magnetic expression over outcrops of ultramafic rocks indicates either that the rocks are relatively fresh and unserpentinized or, if serpentinized, that they lack significant volume at depth. As an illustration, comparison of the strong anomaly associated with the Cuesta body with the weak magnetic expression of the Burro Mountain body indicates that the Burro Mountain mass contains much less serpentinite than the Cuesta mass. Positive magnetic features over Salinian terrane are associated with metamorphic rocks 12 and 18 miles northeast of Cape San Martin, plutonic rocks 3 miles northwest of Paso Robles, and plutonic rocks 2 to 16 miles east of Santa Margarita. In contrast to anomalies over Franciscan rocks, these magnetic features have lower gradients and are greater in areal extent. The two isolated anomalies northeast of Cape San Martin may reflect the presence of mafic intrusive rocks within the metamorphic rock complex or perhaps a more highly mafic part of the metamorphic rock assemblage. The areally restricted, elliptical anomaly northwest of Paso Robles is almost totally confined to the plutonic rock exposure; however, weak extensions of this anomaly 8 miles to the northeast and 20 miles to the northwest define areas of near-surface plutonic rocks buried by less· than half a mile of sedimentary cover. The broad magnetic high east of Santa Margarita is maximum over the northwest part of the igneous rock massif, indicating tbat the greatest subsurface distribution of mafic plutonic rocks is concentrated in this part of the massif. A single magnetic high, incompletely defined by the magnetic data, occurs over Cenozoic sedimentary rocks 11 miles southeast of San Ardo, suggesting a major topographic rise of mafic Salinian basement, generally supported by basement well data.· The location and configuration of the unexposed boundary between Salinian and Franciscan rocks south of the exposed contact near Cape San Martin and north of the well-defined contact near Santa l\1argarita is reflected to some extent in the magnetic data. A distinct northwest-trending magnetic trough extends for nearly the entire length of the map area near the northeast edge of the anomaly train within Franciscan rocks. Just northeast of the magnetic trough, the magnetic field increases with varying slope toward exposures of Salinian basement, except where the field is depressed by thick Cenozoic rock cover between Cape San l\1artin and San Ardo. Although there is much ambiguity in determining the exact location of the magnetic basement. barrier on the basis of the weak magnetic gradient, an approximate estimate of the shallow subsurface contact is between the heavy "2200" contour line defining the northeast edge of the elongate negative trough and the adjacent light "2240" contour line to the northeast. The contact between magnetically contrasting basement rocks is deeply buried between Nacimiento Lake and Santa l\1argarita, as evidenced by a flattening of the magnetic gradient. A more definite contact between magnetically contrasting rocks is the Rinconada-San 1\1arcos fault system, which outlines the southwestern extents of plutonic rock exposures east of Santa l\1argarita and northwest of Paso Robles. This contact, marked by high magnetic gradients, appears to be steeply dipping at the margins of exposed plutonic rocks, although the subsurface extension of the feature is probably shallower than the major crustal contact between Salinian basement and Franciscan rocks. The g;radients suggest a continuation of the Rinconada-San Marcos fault contact along the Espinosa fault to a point 8 ·miles west-southwest of San Ardo where the 1nagnetic pattern becomes diffuse. BOUGUER GRAVITY DATA Complete Bouguer gravity data in the rectangular study area (fig. 3) have been cmnpiled from surveys by the U.S. Geological Survey, California Division of Mines and Geology, University of California at Riverside, and Stanford University. Gravity coverage north of lat 36° N., representing about 10 percent of the area, consists of a compilation made by Bishop and Chapman ( 1967). Coverage south of lat 36° N., representing .the remaining 90 percent of the area, was compiled from surveys of Burch, Grannell, and Hanna ( 1970) together with lo.cal gravity control of Thompson ( 1963) in the Burro Mountain area. Most of the ·gravity stations were established with LaCoste-Romberg geodetic gravity meters at a spacing ~f 1 to 2
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. ., GEOLOGICAL SURVEY RESEARCH·
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CONTENTS GEOLOGIC STUDIES Petrology
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GEOLOGICAL SURVEY RESEARCH 1970 Geo
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B2 PETROLOGY AND PETROGRAPHY for bi
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B4 PETROLOGY AND PETROGRAPHY has be
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B6 PETROLOGY AND PETROGRAPHY. 32
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B8 PETROLOGY AND PETROGRAPHY TABLE
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BlO · PETROLOGY AND PETROGRAPHY EX
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B12 PETROLOGY AND PETROGRAPHY TABLE
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GE,OLOGICAL SURVEY RESEA·RCH 1970
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B16 PETROLOGY AND PETROGRAPHY Sprin
Page 23 and 24: B18 PETROLOGY AND PETROGRAPHY TABLE
Page 25 and 26: B20 PETROLOGY AND PETROGRAPHY Sigva
Page 27 and 28: B22 PETROLOGY AND PETROGRAPHY EXPLA
Page 29 and 30: B24 PE~ROLOGY AND PETROGRAPHY schis
Page 33 and 34: B28 PETROLOGY AND PETROGRAPHY sg•
Page 35 and 36: B30 PETROLOGY AND PETROGRAPHY Detri
Page 37 and 38: B32 PETROLOGY AND PETROGRAPHY place
Page 39 and 40: B34 PETROLOGY AND PETROGRAPHY 66 '
Page 41 and 42: B36 PETROLOGY AND PETROGRAPHY Post-
Page 45 and 46: B40 PETROLOGY AND PETROGRAPHY did n
Page 47 and 48: B42 PETROLOGY AND PETROGRAPHY Creta
Page 49 and 50: td :t 106'20' r--- 10' 106'00' 38'3
Page 51 and 52: B46 STRUCTURAL GEOLOGY FIGURE 4.-Vi
Page 53 and 54: B48 STRUCTURAL GEOLOGY FIGURE 6.-An
Page 55 and 56: B50 STRUCTURAL GEOLOGY clined slide
Page 57 and 58: GE,OLOGICAL SURVEY RESEARCH 1970 CA
Page 59 and 60: B54 GEOPHYSICS (1950), Domzalski (1
Page 61 and 62: B56 GEOPHYSICS STRATIGRAPHY AND LIT
Page 63 and 64: B58 GEOPHYSICS STRATIGRAPHY AND LIT
Page 65 and 66: B60 GEOPHYSICS value. A normal free
Page 67 and 68: B62 GEOPHYSICS Hammer, Sigmund, 193
Page 69 and 70: B64 110°45' GEOPHYSICS L I I \ "'\
Page 71 and 72: GE·OLOGICAL SURVEY RESEA·RCH 1970
Page 73: B68 GEOPHYSICS A' San 0 p A c I F I
Page 77 and 78: B72 GEOPHYSICS 36° 00' A' 40___./
Page 79 and 80: B74 GEOPHYSICS igneous massif is co
Page 81 and 82: B76 GEOPHYSICS A 50 Bouguer gravity
Page 83 and 84: GE·OLOGICAL SURVEY RESEARCH 1970 R
Page 85 and 86: B80 GEOPHYSICS FIGURE 1.-Bouguell'
Page 87 and 88: B82 GEOPHYSICS J I 37·~~------~~--
Page 89 and 90: B84 GEOPHYSICS B 600 B' 500 400 (/)
Page 91 and 92: GE,OLOGICAL SURVEY RESEA·RCH 1970
Page 93 and 94: EXPLANATION Pikes Peak • batholit
Page 95 and 96: DISTRIBUTION OF URANIUM IN URANIUM-
Page 97 and 98: B92 GEOCHRONOLOGY Fleischer, R. L.,
Page 99 and 100: B94 ECONOMIC GEOLOGY 103° 35' w 1/
Page 101 and 102: 'B96 ECONOMIC ·GEOLOGY 0 500 FEET
Page 103 and 104: B98 ECONOMIC GEOLOGY TABLE !.-Summa
Page 105 and 106: BlOO ECONOMIC GEOLOGY surface seems
Page 107 and 108: GE 1 0LOGICAL SURVEY RESEA·RCH 197
Page 109 and 110: B104 EJrucJh of the four samples wa
Page 111 and 112: B106 ECONOMIC GEOLOGY R. 56 E. R. 5
Page 113 and 114: B108 ECONOMIC GEOLOGY outcrop under
Page 115 and 116: BllO Brady, of the Museum of Northe
Page 117 and 118: Bll2 PALEONTOLOGY FIGURE 1.-ProtobZ
Page 119 and 120: B114 PALEONTOLOGY c .,;# I I ' ' lO
Page 121 and 122: B116 PALEONTOLOGY ments, the larges
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B120 L xMH 'xE soo~~B_R~I __ T_I __
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B122 PALEONTOLOGY Tetrataxis sp. Te
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GE~OLOGICAL SURVEY RESEARCH 1970 TR
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Bl26 PALEONTOLOGY TABLE 2.-Ranges o
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B128 PALEONTOLOGY l
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BI30 PALEONTOLOGY Ocmt/rrence.-P. m
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Bl32 PALEONTOLOGY proximately 5.7 m
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Bl34 PALEONTOLOGY a b c d e f g - D
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Bl36 PALEONTOLOGY --- 1963. The ori
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B138 PALEONTOLOGY core are m1ssmg,
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B140 PALEONTOLOGY death assemblage:
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Bl42 STRATIGRAPHY EXPLANATION l:.\.
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B144 STRATIGRAPHY ver. We do not in
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B146 STRATIGRAPHY age. A stratigrap
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B148 5 10 15 20 X X Plntygonus Pla.
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GE~OLOGICAL SURVEY RESEARCH 1970 CL
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B152 STRATIGRAPHY at a depth of 58
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••••• 0 .... ••••
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B156 STRATIGRAPHY REFERENCES Brown,
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B158 STRATIGRAPHY In what follows,
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Bi60 STRATIGRAPHY TABLE !.-Stratigr
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B162 SEDIMENTATION with increasing
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llj ...... ~ TABLE 2.-Specific grav
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B166 TABLE 3.-Settling time for a 1
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Bl68 GEOMORPHOLOGY 74° 44° YERMON
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B170 GEOMORPHOLOGY have also been r
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B172 GEOMORPHOLOGY end of Long Isla
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GEOLOGICAL SURVEY RESEARCH 1970 DET
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~ooo ______________ B176 ANALYTICAL
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B178 terranes where the search for
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B180 Reproducibility Replicate anal
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B182 ANALYTICAL METHODS in the aque
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GEOLOGICAL SURVEY RESEARCH 1970 CHE
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B186 ANALYTICAL METHODS 3.31 percen
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Bl88 ANALYTICAL METHODS 3" 2%" Oute
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GEOLOGICAL SURVEY RESEA~RCH 1970 TR
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B192 GROUND-WATER RECHARGE I T I 14
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B194 GROUND-WATER RECHARGE Therefor
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GEOLOGICAL SURVEY RESEARCH 1970 PRE
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B198 GROUND-WATER RECHARGE the basi
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B200 GROUND-WATER RECHARGE t \ \)
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B202 GROUND-WATER RECHARGE Water is
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B204 GROUND-WATER CONTAMINATION rv
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B206 GROUND-WATER CONTAMINATION TAB
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B208. GROUND-WATER CONTAMINATION fa
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GEOLOGICAL SURVEY RESEARCH 1970 MEA
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B212 SURFACE WATER plotted, and the
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GEOLOGICAL SURVEY RESEA·RCH 1970 T
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B216 SURFACE WATER however, if the
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B218 SURFACE WATER TABLE 4.-Variati
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B220 RELATION· BETWEEN GROUND eral
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B222 -0.010 f -0.005 (i) RELATION B
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GEOLOGICAL SURVEY RESEARCH 1970 THE
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'B226 RELATION BETWEEN GROUND WATER
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B228 RELATION, BETWEEN GROUND WATER
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B230 RELATION BETWEEN GROUND WATER
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GEOLOGICAL SURVEY RESEARCH 1970 HYD
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B234 EROSION AND SEDIMENTATION side
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B236 EROSION AND SEDIMENTATION s·o
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B238 EROSION AND SEDIMENTATION Cont
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B240 EROSION AND SEDIMENTATION 2 3
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GEOLOGICAL SURVEY RESEARCH 1970 SPE
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B244 GEOCHEMISTRY OF WATER REFERENC
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B246 HYDROLOGIC TECHNIQUES The upla
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B248 HYDROLOGIC TECHNIQUES type in
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GEOLOGICAL SURVEY RESEARCH 1970 ,,.
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B252 formula may also be useful in
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GEO'LOGICAL SURVEY RESEARCH 1970 CO
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B256 randomly fluctuating component
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B258 HYDROLOGIC TECHNIQUES 1.0 ~ Qj
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~ B260 HYDROLOGIC TECHNIQUES 1-3, f
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I B262 HYDROLOGIC TECHNIQUES REFERE
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,' B264 Page Gravitational sliding,
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., / )
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