SanLuisObispo layout4 cs2 · SAN LUIS OBISPO 7.5' QUADRANGLE SAN LUIS OBISPO COUNTY, CALIFORNIA: A...
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SNOW, SECRETARY FOR NATURAL RESOURCES DEPARTMENT OF CONSERVATION - DEREK CHERNOW, ACTING DIRECTOR CALIFORNIA GEOLOGICAL SURVEY JOHN G. PARRISH, Ph.D., STATE GEOLOGIST Copyright © 2010 by the California Department of Conservation California Geological Survey. All rights reserved. No part of this publication may be reproduced without written consent of the California Geological Survey. "The Department of Conservation makes no warranties as to the suitability of this product for any given purpose." GEOLOGIC MAP OF THE SAN LUIS OBISPO 7.5' QUADRANGLE SAN LUIS OBISPO COUNTY, CALIFORNIA: A DIGITAL DATABASE VERSION 1.0 By Mark O. Wiegers Digital Database by Mark O. Wiegers and Carlos I. Gutierrez 2010 This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program, Statemap Award no. G09AC00193 35°22'30'' 120°37'30'' 35°15' 120°37'30'' 35°22'30'' 35°15' Topographic base from U.S. Geological Survey San Luis Obispo 7.5-minute Quadrangle, 1994 UTM projection, Zone 10, North American Datum 1927 120°45' 120°45' 5 Kilometers 5 Miles 1 1 41 58 229 227 166 101 101 101 Morro Bay Los Osos San Luis Obispo Avita Edna Atascadero Santa Margarita Pozo Huasna Arroyo Grande Oceano Pismo Beach Nipomo Grover City California Valley Cayucos Beach San Luis Obispo Morro Bay North Atascadero Santa Margarita Wilson Corner Camatta Ranch La Panza Ranch La Panza NE Santa Lake Margarita Pozo Summit La Panza Valley California Lopez Mountain Port San Luis Pismo Beach Arroyo Grande NE T ar Spring Ridge Caldwell Mesa Los Machos Hills Oceana Branch Mountain Nipomo Huasna Peak Chimney Canyon Miranda Pine Mtn Morro Bay South Mapping completed under STATEMAP FY 2008-09 FY 2009-10 SYMBOL EXPLANATION Strike and dip of bedding plane. 30 Synclinal axis - Solid where accurately located, dotted where concealed. Anticlinal axis - Solid where accurately located. Contact between map units - Solid where accurately located, dashed where approximately located, dotted where concealed. Fault - Solid where accurately located, dashed where approximately located, dotted where concealed. Aerial photo lineaments along the Los Osos Fault (Lettis and Hall, 1994). t = tonal contrast; v = vegetative lineament; ld = linear drainage. v t ld Linear features indicitave of faulting along the Los Osos Fault mapped by Treiman (1989) and shown on the Alquist-Priolo Earthquake Fault Zone Map, San Luis Obispo Quadrangle. t = tonal contrast; v = vegetative lineament. v t Dibblee, T.W., Jr., 1974, Geologic map of the San Luis Obispo 15-minute quadrangle, California: U.S. Geological Survey Open-File Report 74-223, scale 1:62,500. Hall, C.A. and Prior, S.W., 1975, Geologic map of the Cayucos - San Luis Obispo Region, San Luis Obispo County, California: U.S. Geological Survey Miscellaneous Field Studies, Map MF-686, scale 1:24,000. Hart, E.W., 1976, Basic Geology of the Santa Margarita area, San Luis Obispo County, California: California Division of Mines and Geology Bulletin 199, 45 p., scale 1:24,000. Lettis, W.R., Kelson, K.I., Wesling, J.R., Angell, M., Hanson, K.I. and Hall, N.T., 1994, Quaternary deformation of the San Luis Range, San Luis Opisbo County, California, in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., editors, Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 111-132. Lettis, W.R. and Hall, N.T., 1994, Los Osos Fault Zone, San Luis Obispo County, California, in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., editors, Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 73-102. McClean, H., 1994, Geologic Map of the Lopez Mountain quadrangle, San Luis Obispo County, California: U.S. Geological Survey Geological Quadrangle Map GQ-1723, scale 1:24,000. Page, B.M., 1972, Oceanic crust and mantle fragment in subduction complex near San Luis Obispo, California: Geological Society of America Bulletin, v. 83, p. 957-972. Seiders, V.M.,1982, Geologic map of an area near York Mountain, San Luis Obispo County, California: U.S. Geological Survey Miscellaneous Investigation Series Map I-1369, scale, 1:24,000. Snow, C.A. and Shervais, J.W., 2001, Geologic map of the Cuesta Ridge Ophiolite: Utah State University, 2001 EDMAP project. Stanley, R.G., Wilson D.S. and McCrory, P.A., 2000, Locations and ages of middleTertiary volcanic centers in coastal California: U.S. Geological Survey Open-File Report 2000-154, 27 p. Treiman, J.A., 1989, Los Osos Fault Zone, San Luis Obispo County: California Division of Mines and Geology Fault Evaluation Report FER-200. SELECTED REFERENCES } Holocene } Pleistocene } Pliocene Miocene } QUATERNARY TERTIARY } CORRELATION OF MAP UNITS Qls Qls (sd) af Oligocene } CRETACEOUS JURASSIC } } Qa Qya Qoa QUATERNARY DEPOSITS TERTIARY AND OLDER ROCKS Tf Tsc Tdb KJfm KJfss KJfch KJfmv KJt Jch Jbd Jsd Ku Franciscan Complex Qyf Qyc bs gw ch mv } ? ? Qf Td Great Valley Sequence and Coast Range Ophiolite Cuesta Ridge Ophiolite and Sediments Tps Tm To Tr Trs Tv dol Jos sc Qya 2 Qya 1 KJfmv+sc af Qa Artificial fill (Historic) - Mine tailings of loose sand, gravel and bedrock excavated from inactive chromite mines on the southwest side of Cuesta Ridge. Other small fills, such as highway and railroad embankments, not mapped. Qya Qyf DESCRIPTION OF MAP UNITS QUATERNARY DEPOSITS Very Young Surficial Deposits Alluvial flood-plain deposits (late Holocene) - Active and recently active flood-plain deposits. Consists of unconsolidated sandy, silty, and clay-bearing alluvium. Qf Alluvial fan deposits (late Holocene) – Small active alluvial fans at the mouths of steep mountain streams. Consists of cobbles, gravel, sand, and silt. Young alluvial flood-plain deposits, undivided (Holocene to late Pleistocene) – Unconsolidated sand, silt, and clay-bearing alluvium deposited on flood-plains and along valley floors. Locally divided by relative age (2 = youngest, 1 = oldest): Young alluvial fan deposits (Holocene to late Pleistocene) – Fine-grained alluvial fan and flood-plain sediments deposited on gently-sloping margins of Los Osos Valley. Consists primarily of clay with interbedded lenses of sand and minor gravel. Qyc Young colluvial deposits (Holocene to late Peistocene) – Poorly sorted sandy and silty slope wash deposits. Qoa Old alluvial flood-plain deposits (late to middle Pleistocene) – Fluvial sediments preserved above active flood plains and channels. These deposits are moderately consolidated, slightly dissected and capped by moderate to well-developed pedogenic soils. Consists of gravel, sand, silt, and clay-bearing alluvium. KJfss KJfm Mélange – Chaotic mixture of fragmented rock masses embedded in a penetratively sheared matrix of argillite and crushed metasandstone. Individual rock masses contained in the matrix range from less than a meter to kilometers in scale. Blocks large enough to be shown on map include high grade blueschist (bs), greenstone (mv), graywacke (gw) and chert (ch). Penetrative deformation of matrix postdates metamorphism of enclosed rock masses. Sandstone and shale – Fine- to medium-grained, massive to thin-bedded graywacke sandstone interbedded with shale and siltstone. Composed of 60% to 70% quartz, 20% to 30% feldspar, 5% biotite, and 10% shale fragments embedded in a muddy matrix. Rocks are generally moderately to intensely sheared, often obscuring original stratification. This unit lacks exotic blocks characteristic of mélange. Metavolcanic rocks – Primarily greenstone, metamorphosed from basalt. Includes massive to pillowed basalt flows, breccia, tuff and diabase. Commonly deeply weathered. Locally contains zones of silica carbonate rock (KJfmv+sc) near fault contacts with Jos. Young Surficial Deposits Young alluvial flood-plain deposits, Unit 2 Qya 2 Young alluvial flood-plain deposits, Unit 1 Qya 1 Old Surficial Deposits Tr Trs Franciscan Complex (Cretaceous to Jurassic) Jos Serpentinized ultramafic rocks – Pervasively sheared serpentinite occurring as leticular fault-bounded bodies in Franciscan mélange. Considered to be dismembered bodies of the Coast Range Ophiolite tectonically interleaved with mélange during subduction. Locally, hydrothermally altered to silica-carbonate rock (sc). KJfch Chert – Red and green radiolarian chert associated with greenstone. Commonly veined and recrystallized, locally bleached to yellow or white. Deposited in deep oceanic setting on greenstone prior to influx of sandstone and shale. Locally interbedded with thin layers of argillite. TERTIARY AND OLDER ROCKS Tps Pismo Formation, basal sandstone member (Pliocene to Miocene) – White to buff, fine-grained friable sandstone. Contains barnacle and sea scallop shell fragments. Exposed in limited areas on the southwest side of Los Osos Valley. (Upper members not exposed in the map area.) Tm Monterey Formation (Miocene) – Laminated to thin-bedded, shale, siliceous shale, porcelaneous shale, siliceous claystone and soft diatomaceous siltstone. Dark-brown, weathers white to buff. Tdb To Diabase and Basalt (Miocene) – Intrusive sills and dikes and extrusive basalt flows. Fine- to coarse-grained. Typically deeply and spheroidally weathered. Occurs as dikes, sills and flows in the upper part of the Rincon Shale and at the base of the Monterey Formation. Obispo Formation (Miocene) – Soft, white, gray and light-brown crystalline, fine- to coarse-grained vitric tuff. Locally interbedded with porcelaneous siltstone. Contains subangular clasts of pumice in a white ashy matrix. Tv Vaqueros Sandstone (Oligocene) – Gray to brown, medium to coarse sandstone, poorly to well indurated, with silty, calcareous matrix. Clasts are well-rounded to subrounded with a typical composition of 40% to 60% quartz, 5% to 15% feldspar, 30% to 50% rock fragments. Contains fossiliferous zones with broken mollusk shells and large oysters. Tf Felsite (Oligocene) – Light-gray and grayish-orange flow-layered felsite and felsic tuff and tuff breccia. Exposed in a small area in the northern part of the map area. May correlate with felsites mapped in the Morro Bay North quadrangle (Seiders, 1982). Unnamed sandstone and conglomerate (Oligocene) – Massive non-marine sandstone and matrix supported pebble, cobble, and boulder conglomerate. Clasts include volcanic rocks, chert and biotite-rich sandstone probably derived from Upper Cretaceous conglomerates. Mapped as the Lospe Formation by Hall and Prior (1975). Tsc Ku Sandstone with mudstone, siltstone and conglomerate (Upper Cretaceous) – Brown to greenish-brown thick-bedded, biotitic, quartzofeldspathic sandstone with interbedded mudstone, micaceous siltstone and conglomerate lenses. Submarine fan assemblage. Contains granitic detritus and K-feldspar and lacks Franciscan detritus. Stucturally overlies Franciscan rocks and the Toro Formation. Mapped as the Atascadero Formation in adjoining areas (McLean, 1994; Seiders, 1982; Hart, 1976). KJt Toro Formation (Lower Cretaceous to Upper Jurassic) – Thin-bedded, greenish-brown to brown micaceous shale interbedded with thin sandstone beds. Sandstone occurs rarely in beds up to 5 m thick. Contains calcareous lenses and concretions. Buchia fragments occur locally in thin sandstone beds (Hall and Prior, 1975). In the Cuesta Ridge area, depositionally overlies chert and basalt of the Coast Range Ophiolite (Page, 1972). Morro Rock – Islay Hill volcanic intrusive complex (Oligocene) – Porphyritic dacite. Approximately 50% of rock consists of phenocrysts with a typical composition of 65% andesine, 15% biotite and clay, 10% hornblende, 5% quartz, and 5% magnetite, apatite, and zeolites. Groundmass consists of altered plagioclase, biotite, glass, quartz and hornblende (Hall and Prior, 1975). These rocks are exposed in a series of volcanic plugs and lava domes that form distinctive peaks between Morro Bay and San Luis Obispo. Flow banding is common. Radiometric dates range from 27 to 23 Ma (Stanley and others, 2000). Rincon Shale (Oligocene and Miocene) – Dark-brown to orange-brown siltstone and silty claystone, poorly to well-bedded, weathers white to light-brown. Locally contains zones of dolomite (dol). Contains calcareous and dolomitic lenses and concretions. Lithologically similar to rocks that have been assigned as the basal member of the Monterey Formation but contains fossils known to be older (Hall and Prior, 1975). Differentiated from Monterey Formation by absence of chert and pocelaneous shale. Trs – Fine- to coarse-grained, white to buff arkosic sandstone, locally interbedded with siltstone and claystone. Qls Qls (sd) Landslide deposits (Holocene and Pleistocene) – Highly fragmented to largely coherent landslide deposits. Notable landslides include deep-seated rock slides and earth flows in Franciscan mélange and large rock fall/debris flow complexes on the flanks of prominent dacite peaks. Qls (sd) - Very large, ancient, deeply-dissected landslide complex in serpentinite and diabase dike-and-sill complex on the southwest side of Cuesta Ridge. Td Great Valley Sequence (Cretaceous to Jurassic) Jch Chert – Brownish-black to olive-brown impure chert. Beds 2 to 15 cm thick intercalated with black, flaky, siliceous shale partings. Rock breaks into blocky, joint-bounded blocks with black manganese oxide coatings on some surface. Depositional on basaltic dike-and-sill complex. Overlain by marine shales of the Toro Formation. Jbd Basalt and intrusive dike-and-sill complex – Dark-brown extrusive basalt breccia and pillow lava intruded by diabase dikes and sills (oceanic crust remnant). Extensive fracturing and deep weathering typically obscures original structure of basalt. Intrusive dikes and sills are composed primarily of diabase, basalt, gabbro and quartz gabbro. Dikes and sills are locally more voluminous than host basalts and are considered to be feeders to overlying extrusive basalts. Oceanic crust fragment of Page (1972). Jsd Sepentinite and intrusive dike-and-sill complex – Serpentinite extensively intruded by altered diabase dikes and sills (mantle remnant). Relict minerals and textures of peridotite locally present where serpentinization is incomplete. Intrusive dikes and sills are primarily diabase typically altered to hydrous calcium aluminum silicates. Unit is in fault contact with overlying basalt dike-and-sill complex. Dikes and sills considered to be feeders to overlying extrusive basalts. Unit is tectonically underlain by Franciscan mélange. Mantle fragment of Page (1972). Coast Range Ophiolite (Jurassic) Cuesta Ridge Ophiolite Sequence dol sc KJfmv KJfmv+sc KJfmv Revised: 9/27/2010 Preliminary Geologic Map available from: http://www.conservation.ca.gov/cgs/rghm/rgm/preliminary_geologic_maps.htm
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STATE OF CALIFORNIA - ARNOLD SCHWARZENEGGER, GOVERNORTHE NATURAL RESOURCES AGENCY - LESTER A. SNOW, SECRETARY FOR NATURAL RESOURCES
DEPARTMENT OF CONSERVATION - DEREK CHERNOW, ACTING DIRECTOR CALIFORNIA GEOLOGICAL SURVEYJOHN G. PARRISH, Ph.D., STATE GEOLOGIST
Copyright © 2010 by the California Department of ConservationCalifornia Geological Survey. All rights reserved. No part ofthis publication may be reproduced without written consent of theCalifornia Geological Survey.
"The Department of Conservation makes no warranties as to thesuitability of this product for any given purpose."
GEOLOGIC MAP OF THESAN LUIS OBISPO 7.5' QUADRANGLE
SAN LUIS OBISPO COUNTY, CALIFORNIA: A DIGITAL DATABASEVERSION 1.0
By Mark O. Wiegers
Digital Database by
Mark O. Wiegers and Carlos I. Gutierrez2010
This geologic map was funded in part by the USGS National Cooperative Geologic MappingProgram, Statemap Award no. G09AC00193
35°22'30''120°37'30''
35°15'120°37'30''
35°22'30''
35°15'
Topographic base from U.S. Geological SurveySan Luis Obispo 7.5-minute Quadrangle, 1994UTM projection, Zone 10, North American Datum 1927
120°45'
120°45'
5Kilometers
5Miles
1
1
41
58
229
227
166101
101
101
Morro Bay
Los Osos San LuisObispo
AvitaEdna
Atascadero
SantaMargarita
Pozo
HuasnaArroyoGrande
Oceano
PismoBeach
Nipomo
GroverCity
CaliforniaValley
Cayucos
Beach
San Luis
Obispo
Morro Bay
North
Atascadero
Santa
Margarita
Wilson
Corner
Camatta
Ranch
La Panza
Ranch
La Panza
NE
Santa LakeMargarita
Pozo
Summit
La Panza
ValleyCalifornia
Lopez
Mountain
Port
San Luis
Pismo
Beach
Arroyo
Grande
NE
TarSpring
Ridge
Caldwell
Mesa
Los
Machos
Hills
Oceana
Branch
Mountain
Nipomo
Huasna
Peak
Chimney
Canyon
Miranda
Pine Mtn
Morro Bay
South
Mapping completed underSTATEMAP
FY 2008-09
FY 2009-10
SYMBOL EXPLANATION
Strike and dip of bedding plane.30
Synclinal axis - Solid where accurately located, dotted where concealed.
Anticlinal axis - Solid where accurately located.
Contact between map units - Solid where accurately located, dashed where approximately located, dotted where concealed.
Fault - Solid where accurately located, dashed where approximately located, dotted where concealed.
Aerial photo lineaments along the Los Osos Fault (Lettis and Hall, 1994). t = tonal contrast; v = vegetative lineament; ld = linear drainage.
vt ld
Linear features indicitave of faulting along the Los Osos Fault mapped by Treiman (1989) and shown on the Alquist-Priolo Earthquake Fault Zone Map, San Luis Obispo Quadrangle. t = tonal contrast; v = vegetative lineament.
vt
Dibblee, T.W., Jr., 1974, Geologic map of the San Luis Obispo 15-minute quadrangle, California: U.S. Geological Survey Open-File Report 74-223, scale 1:62,500.
Hall, C.A. and Prior, S.W., 1975, Geologic map of the Cayucos - San Luis Obispo Region, San Luis Obispo County, California: U.S. Geological Survey Miscellaneous Field Studies, Map MF-686, scale 1:24,000. Hart, E.W., 1976, Basic Geology of the Santa Margarita area, San Luis Obispo County, California: California Division of Mines and Geology Bulletin 199, 45 p., scale 1:24,000.
Lettis, W.R., Kelson, K.I., Wesling, J.R., Angell, M., Hanson, K.I. and Hall, N.T., 1994, Quaternary deformation of the San Luis Range, San Luis Opisbo County, California, in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., editors, Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 111-132.
Lettis, W.R. and Hall, N.T., 1994, Los Osos Fault Zone, San Luis Obispo County, California, in Alterman, I.B., McMullen, R.B., Cluff, L.S., and Slemmons, D.B., editors, Seismotectonics of the Central California Coast Ranges: Geological Society of America Special Paper 292, p. 73-102.
McClean, H., 1994, Geologic Map of the Lopez Mountain quadrangle, San Luis Obispo County, California: U.S. Geological Survey Geological Quadrangle Map GQ-1723, scale 1:24,000.
Page, B.M., 1972, Oceanic crust and mantle fragment in subduction complex near San Luis Obispo, California: Geological Society of America Bulletin, v. 83, p. 957-972.
Seiders, V.M.,1982, Geologic map of an area near York Mountain, San Luis Obispo County, California: U.S. Geological Survey Miscellaneous Investigation Series Map I-1369, scale, 1:24,000.
Snow, C.A. and Shervais, J.W., 2001, Geologic map of the Cuesta Ridge Ophiolite: Utah State University, 2001 EDMAP project.
Stanley, R.G., Wilson D.S. and McCrory, P.A., 2000, Locations and ages of middleTertiary volcanic centers in coastal California: U.S. Geological Survey Open-File Report 2000-154, 27 p.
Treiman, J.A., 1989, Los Osos Fault Zone, San Luis Obispo County: California Division of Mines and Geology Fault Evaluation Report FER-200.
SELECTED REFERENCES
}Holocene
}Pleistocene
}Pliocene
Miocene
} QUATERNARY
TERTIARY}CORRELATION OF MAP UNITS
Qls
Qls(sd)
af
Oligocene}CRETACEOUS
JURASSIC
}}
Qa
Qya
Qoa
QUATERNARY DEPOSITS
TERTIARY AND OLDER ROCKS
Tf
Tsc
Tdb
KJfm
KJfss KJfch KJfmv
KJt
Jch
Jbd
Jsd
Ku
FranciscanComplex
Qyf Qyc
bs
gw
ch
mv
}
?
?
Qf
TdGreat Valley Sequence
andCoast Range Ophiolite
Cuesta RidgeOphiolite and Sediments
Tps
Tm
To
TrTrs
Tv
dol
Jossc
Qya2
Qya1
KJfmv+sc
af
Qa
Artificial fill (Historic) - Mine tailings of loose sand, gravel and bedrock excavated from inactive chromite mines on the southwest side of Cuesta Ridge. Other small fills, such as highway and railroad embankments, not mapped.
Qya
Qyf
DESCRIPTION OF MAP UNITS
QUATERNARY DEPOSITS
Very Young Surficial Deposits
Alluvial flood-plain deposits (late Holocene) - Active and recently active flood-plain deposits. Consists of unconsolidated sandy, silty, and clay-bearing alluvium.
Qf Alluvial fan deposits (late Holocene) – Small active alluvial fans at the mouths of steep mountain streams. Consists of cobbles, gravel, sand, and silt.
Young alluvial flood-plain deposits, undivided (Holocene to late Pleistocene) – Unconsolidated sand, silt, and clay-bearing alluvium deposited on flood-plains and along valley floors. Locally divided by relative age (2 = youngest, 1 = oldest):
Young alluvial fan deposits (Holocene to late Pleistocene) – Fine-grained alluvial fan and flood-plain sediments deposited on gently-sloping margins of Los Osos Valley. Consists primarily of clay with interbedded lenses of sand and minor gravel.
Qyc Young colluvial deposits (Holocene to late Peistocene) – Poorly sorted sandy and silty slope wash deposits.
Qoa Old alluvial flood-plain deposits (late to middle Pleistocene) – Fluvial sediments preserved above active flood plains and channels. These deposits are moderately consolidated, slightly dissected and capped by moderate to well-developed pedogenic soils. Consists of gravel, sand, silt, and clay-bearing alluvium.
KJfss
KJfm Mélange – Chaotic mixture of fragmented rock masses embedded in a penetratively sheared matrix of argillite and crushed metasandstone. Individual rock masses contained in the matrix range from less than a meter to kilometers in scale. Blocks large enough to be shown on map include high grade blueschist (bs), greenstone (mv), graywacke (gw) and chert (ch). Penetrative deformation of matrix postdates metamorphism of enclosed rock masses.
Sandstone and shale – Fine- to medium-grained, massive to thin-bedded graywacke sandstone interbedded with shale and siltstone. Composed of 60% to 70% quartz, 20% to 30% feldspar, 5% biotite, and 10% shale fragments embedded in a muddy matrix. Rocks are generally moderately to intensely sheared, often obscuring original stratification. This unit lacks exotic blocks characteristic of mélange.
Metavolcanic rocks – Primarily greenstone, metamorphosed from basalt. Includes massive to pillowed basalt flows, breccia, tuff and diabase. Commonly deeply weathered. Locally contains zones of silica carbonate rock (KJfmv+sc) near fault contacts with Jos.
Young Surficial Deposits
Young alluvial flood-plain deposits, Unit 2 Qya2
Young alluvial flood-plain deposits, Unit 1Qya1
Old Surficial Deposits
Tr
Trs
Franciscan Complex (Cretaceous to Jurassic)
JosSerpentinized ultramafic rocks – Pervasively sheared serpentinite occurring as leticular
fault-bounded bodies in Franciscan mélange. Considered to be dismembered bodies of the Coast Range Ophiolite tectonically interleaved with mélange during subduction. Locally, hydrothermally altered to silica-carbonate rock (sc).
KJfchChert – Red and green radiolarian chert associated with greenstone. Commonly veined and
recrystallized, locally bleached to yellow or white. Deposited in deep oceanic setting on greenstone prior to influx of sandstone and shale. Locally interbedded with thin layers of argillite.
TERTIARY AND OLDER ROCKS
Tps Pismo Formation, basal sandstone member (Pliocene to Miocene) – White to buff, fine-grained friable sandstone. Contains barnacle and sea scallop shell fragments. Exposed in limited areas on the southwest side of Los Osos Valley. (Upper members not exposed in the map area.)
Tm Monterey Formation (Miocene) – Laminated to thin-bedded, shale, siliceous shale, porcelaneous shale, siliceous claystone and soft diatomaceous siltstone. Dark-brown, weathers white to buff.
Tdb
To
Diabase and Basalt (Miocene) – Intrusive sills and dikes and extrusive basalt flows. Fine- to coarse-grained. Typically deeply and spheroidally weathered. Occurs as dikes, sills and flows in the upper part of the Rincon Shale and at the base of the Monterey Formation.
Obispo Formation (Miocene) – Soft, white, gray and light-brown crystalline, fine- to coarse-grained vitric tuff. Locally interbedded with porcelaneous siltstone. Contains subangular clasts of pumice in a white ashy matrix.
Tv Vaqueros Sandstone (Oligocene) – Gray to brown, medium to coarse sandstone, poorly to well indurated, with silty, calcareous matrix. Clasts are well-rounded to subrounded with a typical composition of 40% to 60% quartz, 5% to 15% feldspar, 30% to 50% rock fragments. Contains fossiliferous zones with broken mollusk shells and large oysters.
Tf Felsite (Oligocene) – Light-gray and grayish-orange flow-layered felsite and felsic tuff and tuff breccia. Exposed in a small area in the northern part of the map area. May correlate with felsites mapped in the Morro Bay North quadrangle (Seiders, 1982).
Tcg Unnamed sandstone and conglomerate (Oligocene) – Massive non-marine sandstone and matrix supported pebble, cobble, and boulder conglomerate. Clasts include volcanic rocks, chert and biotite-rich sandstone probably derived from Upper Cretaceous conglomerates. Mapped as the Lospe Formation by Hall and Prior (1975).
Tsc
Ku Sandstone with mudstone, siltstone and conglomerate (Upper Cretaceous) – Brown to greenish-brown thick-bedded, biotitic, quartzofeldspathic sandstone with interbedded mudstone, micaceous siltstone and conglomerate lenses. Submarine fan assemblage. Contains granitic detritus and K-feldspar and lacks Franciscan detritus. Stucturally overlies Franciscan rocks and the Toro Formation. Mapped as the Atascadero Formation in adjoining areas (McLean, 1994; Seiders, 1982; Hart, 1976).
KJt Toro Formation (Lower Cretaceous to Upper Jurassic) – Thin-bedded, greenish-brown to brown micaceous shale interbedded with thin sandstone beds. Sandstone occurs rarely in beds up to 5 m thick. Contains calcareous lenses and concretions. Buchia fragments occur locally in thin sandstone beds (Hall and Prior, 1975). In the Cuesta Ridge area, depositionally overlies chert and basalt of the Coast Range Ophiolite (Page, 1972).
Morro Rock – Islay Hill volcanic intrusive complex (Oligocene) – Porphyritic dacite. Approximately 50% of rock consists of phenocrysts with a typical composition of 65%
andesine, 15% biotite and clay, 10% hornblende, 5% quartz, and 5% magnetite, apatite, and zeolites. Groundmass consists of altered plagioclase, biotite, glass, quartz and hornblende (Hall and Prior, 1975). These rocks are exposed in a series of volcanic plugs and lava domes that form distinctive peaks between Morro Bay and San Luis Obispo. Flow banding is common. Radiometric dates range from 27 to 23 Ma (Stanley and others, 2000).
Rincon Shale (Oligocene and Miocene) – Dark-brown to orange-brown siltstone and silty claystone, poorly to well-bedded, weathers white to light-brown. Locally contains zones of dolomite (dol). Contains calcareous and dolomitic lenses and concretions. Lithologically similar to rocks that have been assigned as the basal member of the Monterey Formation but contains fossils known to be older (Hall and Prior, 1975). Differentiated from Monterey Formation by absence of chert and pocelaneous shale. Trs – Fine- to coarse-grained, white to buff arkosic sandstone, locally interbedded with siltstone and claystone.
Qls
Qls(sd)
Landslide deposits (Holocene and Pleistocene) – Highly fragmented to largely coherent landslide deposits. Notable landslides include deep-seated rock slides and earth flows in Franciscan mélange and large rock fall/debris flow complexes on the flanks of prominent dacite peaks. Qls(sd) - Very large, ancient, deeply-dissected landslide complex in serpentinite and diabase dike-and-sill complex on the southwest side of Cuesta Ridge.
Td
Great Valley Sequence (Cretaceous to Jurassic)
Jch Chert – Brownish-black to olive-brown impure chert. Beds 2 to 15 cm thick intercalated with black, flaky, siliceous shale partings. Rock breaks into blocky, joint-bounded blocks with black manganese oxide coatings on some surface. Depositional on basaltic dike-and-sill complex. Overlain by marine shales of the Toro Formation.
Jbd Basalt and intrusive dike-and-sill complex – Dark-brown extrusive basalt breccia and pillow lava intruded by diabase dikes and sills (oceanic crust remnant). Extensive fracturing and deep weathering typically obscures original structure of basalt. Intrusive dikes and sills are composed primarily of diabase, basalt, gabbro and quartz gabbro. Dikes and sills are locally more voluminous than host basalts and are considered to be feeders to overlying extrusive basalts. Oceanic crust fragment of Page (1972).
Jsd Sepentinite and intrusive dike-and-sill complex – Serpentinite extensively intruded by altered diabase dikes and sills (mantle remnant). Relict minerals and textures of peridotite locally present where serpentinization is incomplete. Intrusive dikes and sills are primarily diabase typically altered to hydrous calcium aluminum silicates. Unit is in fault contact with overlying basalt dike-and-sill complex. Dikes and sills considered to be feeders to overlying extrusive basalts. Unit is tectonically underlain by Franciscan mélange. Mantle fragment of Page (1972).
Coast Range Ophiolite (Jurassic)Cuesta Ridge Ophiolite Sequencedol
sc
KJfmvKJfmv+sc
KJfmv
Revised: 9/27/2010
Preliminary Geologic Map available from:http://www.conservation.ca.gov/cgs/rghm/rgm/preliminary_geologic_maps.htm
