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118°15'34°37'30"

34°30'118°15'

34°30'118°07'30"

118°07'30"34°37'30"

Topographic base from U.S. Geological Survey Ritter Ridge 7.5-minute Quadrangle, 1958, Photorevised 1974UTM projection, Zone 11, North American Datum 1927

This geologic map was funded in part by the USGS National Cooperative Geologic MappingProgram, Statemap Award no. 08HQAG0102

PRELIMINARY GEOLOGIC MAP OF THE RITTER RIDGE 7.5' QUADRANGLELOS ANGELES COUNTY, CALIFORNIA: A DIGITAL DATABASE

VERSION 1.1By

Janis L. Hernandez

Digital Database by

Janis L. Hernandez, Carlos I. Gutierrez and George J. Saucedo

2013

Copyright © 2013 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."

STATE OF CALIFORNIA - EDMUND G. BROWN JR., GOVERNORTHE NATURAL RESOURCES AGENCY - JOHN LAIRD, SECRETARY FOR NATURAL RESOURCES

DEPARTMENT OF CONSERVATION - MARK NECHODOM, CONSERVATION DIRECTOR CALIFORNIA GEOLOGICAL SURVEYJOHN G. PARRISH, Ph.D., STATE GEOLOGIST PRELIMINARY GEOLOGIC MAP OF THE RITTER RIDGE 7.5’ QUADRANGLE, CALIFORNIA

Axis of anticline

Axis of syncline - Dashed where approximately located, dotted where concealed.

Landslide - Arrows indicate principal direction of movement.

65 Strike and dip of metamorphic foliation.

Vertical metamorphic foliation.

35 Strike and dip of igneous joint.

Vertical igneous joint.

Strike and dip of sedimentary beds.

Horizontal bedding.25

70

Vertical sedimentary beds

Strike and dip of overturned sedimentary beds

Strike and dip of overturned metamorphic foliation

45 Strike and dip of igneous foliation

Vertical igneous foliation

75

MAP SYMBOLS

27

Fault - Solid where accurately located; long dash where approximately located; short dash where inferred; dotted where concealed, queried where uncertain.

Thrust Fault - Barbs on upper plate; solid where accurately located; long dash where approximately located; short dash where inferred. Arrow and number indicate direction and angle of dip of fault plane.

Contact between map units - Solid where accurately located; long dash where approximately located; short dash where inferred; dotted where concealed, queried where uncertain.

?

?

Barrows, A.G., Kahle, J.E., and Beeby, D.J., 1985, Earthquake hazards and tectonic history of the San Andreas fault zone, Los Angeles County, California: California Division of Mines and Geology Open-File Report 85-10 LA, 236 p., 21 plates, scale 1:12,000.

Barth, A.P., Jacobson, C.E., Coleman, D.S., and Wooden, J.L., 2001, Construction and tectonic evolution of cordilleran continental crust: Examples from the San Gabriel and San Bernardino mountains, in Dunne, G., and Cooper, J., editors, Geologic excursions in the California deserts and adjacent Transverse Ranges: Pacific Section SEPM, Book 88, p. 17-53.

Carter, B.A., 1980, Structure and petrology of the San Gabriel anothosite-syenite body, Los Angeles County, California: California Institute of Technology, Ph.D. thesis, scale 1:31,250.

Dibblee, T.W., Jr., 1997, Geologic map of the Sleepy Valley and Ritter Ridge quadrangles, Los Angeles County, California: Dibblee Geological Foundation map DF-66, scale 1:24,000.

Ehlig, P.L., 1981, Origin and tectonic history of the basement terrane of the San Gabriel Mountains, central Transverse Ranges, in Ernst. W.G., editor, The geotectonic development of California (Rubey Volume 1): Englewood Cliffs, New Jersey, Prentice-Hall, p. 253-283.

______, 1975, Basement Rocks of the San Gabriel Mountains, South of the San Andreas Fault, southern California, in Crowell, J.C., editor, San Andreas Fault in southern California: California Division of Mines and Geology, Special Report 118, p. 177-186.

Evans, J.G., 1966, Structural Analysis and movements of the San Andreas fault zone near Palmdale, southern California: University of California, Los Angeles, Ph.D. thesis, scale 1:12,000.

Gay, T.E., Jr., and Hoffman, S.R., 1954, Los Angeles County, Governor, Hi-Grade, and Red Rover mines: California Journal of Mines and Geology, vol. 50, p. 497-500.

Jacobson, C.E., 1990, The 40Ar/39Ar geochronology of the Pelona Schist and related rocks, southern California: Journal of Geophysical Research, vol. 95, p. 509-528.

Jacobson, C.E., Barth, A.P., and Grove, M., 2000, Late Cretaceous protolith age and provenance of the Pelona and Orocopia schists, southern California: Geology, vol. 28, no. 3, p. 219-222.

Joseph, S.E., Criscione, J.J., Davis, T.E., and Ehlig, P.L., 1982, The Lowe Igneous Pluton, in Fife, D.L., and Minch, J.A., editors, Geology and Mineral Wealth of the California Transverse Ranges, South Coast Geological Society, p. 307-309.

Perez, F.G., 2003, Landslide inventory of the Ritter Ridge 7.5-minute quadrangle, Los Angeles County, California: California Geological Survey, unpublished mapping, scale 1:24,000.

Ponti, D.J., Burke, D.B., and Hedel, C.W., 1981, Map showing Quaternary geology of the central Antelope Valley and vicinity, California: U.S. Geological survey Open-File Report 81-737, scale 1:62,500.

Silver, L.T., 1971, Problems of crystalline rocks of the Transverse Ranges: Geological Society of America Abstracts with Programs vol. 3, p. 193-194.

______, 1966, Preliminary history of the crystalline complex of the central Transverse Ranges, Los Angeles County, California: Geological Society of America Special Paper 101, p. 201-202.

Streckeisen, A.L., 1976, To each plutonic rock a proper name: Earth Science Reviews, vol. 12, p. 1-33.

______, 1973, Plutonic rocks – Classification and nomenclature recommended by the IUGS Subcommission on Systematics of Igneous Rocks: Geotimes, vol. 18, p. 26-30.

Wilson, R.I., Perez, F.G., and Barrows, A.G., 2003, Earthquake-induced landslide zones of required investigation in the Ritter Ridge 7.5-minute quadrangle, Los Angeles County, California: California Geological Survey Seismic Hazard Zone Report 083, Section 2, p. 21 – 41.

AIR PHOTOSDepartment of County Engineers, Soil Survey Los Angeles County, photo numbers: 1-90 through 1-97, dated 3/25/68; 3-29 through 3-36, 3-55

through 3-60, and 3-115 through 3-123 dated 3/28/68; 4-18 through 4-26 dated 3/29/68; 5-112 through 5-119 dated 4/4/68; and 7-45 through 7-53 dated 4/25/68, black and white, vertical; approximate scale 1:24,000.

U.S. Geological Survey, 1994, Digital Orthophoto Quarter Quadrangle Photos, dated 5-29-94, entire quadrangle area, Ritter Ridge Quadrangle. (DOQQ and information concerning them can be obtained at http://www.usgsquads.com/aerialphotos.htm).

ACKNOWLEDGEMENTSThe author wishes to express her gratitude to Douglas M. Morton - U.S. Geological Survey (retired) for his invaluable guidance and assistance in both thin section examination and field review. His generous contribution of time spent reviewing thin sections and field review of the map area is greatly appreciated and provided an enhanced understanding of field observations, particularly with respect to the anorthosite-syenite-gabbro complex, mylonite, and Pelona Schist units.

SELECTED REFERENCES

0°40'12 MILS

13°231 MILS

GNMN

UTM GRID AND 2009 MAGNETIC NORTHDECLINATION AT CENTER OF SHEET

Scale 1:24,000

Contour Interval 40 feetDotted Lines Represent 10-foot ContoursNational Geodetic Vertical Datum of 1929

0

0

0

2

2

2Thousand Feet

Kilometers

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5

COTTONWOODCR

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99

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Forebay

Castaic Lake

R o s a m o n d L a k e Buckhorn

Lake

Piru Lake

Santa Clara River

Pyramid Lake

Little Rock Wash

Fillmore

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LakeHughes

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Littlerock

Mojave

Piru

QuartzHill

Rosamond

Saugus

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Acton

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48

166

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5Kilometers

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GRAPEVINE

PASTORIA

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BISSELL

ROSAMOND

ROSAMOND

LITTLE

FAIRMONT

NEENACH

LA LIEBRE

LEBEC

FRAZIER

MOUNTAIN

BLACKMOUNTAIN

BURNT

LAKE SUR

LANCASTERLANCASTER

PALMDALE

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SPRINGS

MOUNTAIN

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14

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RIDGERITTER

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QuartzSyenite

QuartzMonzonite

QuartzMonzodiorite

Syenite Monzonite Monzodiorite

Granite

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ar G

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te

Tonalite

Diorite

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60

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5

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CORRELATION OF MAP UNITS

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Holocene

Pleistocene

Miocene

TERTIARY

Oligocene

CENOZOIC

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PALEOZOIC

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Cretaceous

Triassic

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DESCRIPTION OF MAP UNITS

SURFICIAL UNITS

Artificial fill and disturbed areas (Holocene, historic) – Surfaces intensely modified by human construction and grading activities. Consists of man-made deposits of earth-fill soils derived from local sources. Mapped specifically along the California Aqueduct structure, debris catchment basins, and includes fill soils along freeway/road alignments.

Wash deposits (late Holocene) – Unconsolidated fine- to medium-grained sand, with some coarse sand, fine gravel, and silt. Deposits are generally pale-brown (10YR 5/3), angular to sub-angular grains, derived from local bedrock, or reworked from other local Quaternary sources. Subject to localized reworking and new sediment deposition during storm events.

Modern alluvium (Holocene) – Unconsolidated to weakly consolidated, mostly undissected, fluvial gravel, sand, and silt. Loose, yellowish-gray sand, silt, and pebble-cobble gravel. Consists predominately of moderately sorted coarse-grained to very coarse-grained arkosic sand.

Ponded alluvium (Holocene) – Loose to dense gravel, sand, silt, and clay. Unconsolidated to weakly consolidated, poorly to moderately sorted gravel, sand, silt and clay deposits in closed depressions or areas of decreased stream gradient. Mapped predominantly adjacent to strands of the San Andreas Fault. Locally included with alluvium and slope wash where map scale prevents subdivision.

Slope Wash (Holocene) – Loose sand and rubble from downslope movement of surficial materials. Unconsolidated sand and rubble transported predominantly by mass wasting and runoff and deposited directly downslope from local sources as nearly undissected talus cones and broad aprons. Slope wash is differentiated from alluvium especially where it masks contacts and faults.

Terrace deposits (Holocene) – Loose, unconsolidated, poorly sorted, fluvial deposits of gravel, sand, and silt locally adjacent to, but slightly above, present stream channels and washes.

Modern alluvial fan deposits (Holocene) – Unconsolidated to weakly consolidated, poorly sorted, rubble, gravel, sand, and silt deposits forming active, essentially undissected, alluvial fans. Includes small to large cones at the mouths of stream canyons and broad aprons of coarse debris adjacent to mountain fronts.

Lake deposits (Holocene) – Unconsolidated, poorly to moderately sorted, sand, silt, clay, and minor gravel deposits in closed depressions that normally contain water. Deposits occur in all lakes in the area and in closed depressions not directly associated with faulting.

Colluvium (Holocene to late Pleistocene) – Unconsolidated sand, gravel, and rock fragments flanking bedrock slopes. Deposited by down-slope creep or rain wash. Mapped where thick and continuous enough to obscure underlying bedrock.

Landslide deposits (Holocene to late Pleistocene) – Unconsolidated to weakly consolidated, jumbled rock debris. Recognizable by topographic expression or chaotic internal structure. Most of the landslides occur in metamorphic rocks, particularly in the Pelona Schist. Identification and delineation of landslides in the metamorphic terrain was made difficult by the complexity of the structures as manifested by the foliation, the intricate folding, and the highly jointed/broken-up nature of the rock exposures. Unpublished landslide mapping by Perez, 2003 was included on the geologic map, with questionable landslides designated as queried. Debris slides are the most common types of landslides in the area. Their location and distribution are strongly influenced by folding. The attitude of the foliation in the metamorphic rocks in this location indicates that most of the landslides are on dip slopes.

Younger alluvium (Holocene to late Pleistocene) – Unconsolidated, light-yellowish-brown sand and gravel of slightly dissected alluvial fans and associated washes.

Younger alluvial fan deposits (Holocene to late Pleistocene) – Unconsolidated to weakly consolidated, dark yellowish-brown, fine to medium arkosic sand with fine gravel. Gravels are primarily from granitic sources, with many subangular quartz fine gravel clasts. Unit is exposed as slightly dissected, elevated broad alluvial fans.

Older alluvium (late Pleistocene) – Unconsolidated to moderately consolidated, strong brown (7.5YR 4/6) fluvial pebbly fine-to coarse sand and silt. Color ranges from light-brown to dark-reddish-brown depending upon source of debris and extent of weathering. Unit is moderately gullied to deeply dissected. Includes sub-angular gneiss clasts and rounded Pelona Schist clasts with lesser diorite and granitic clasts. Unit is massive to poorly stratified with occasional cobble lenses and moderately developed soil profile with thin clay coatings on coarse sand grains.

Older alluvium with Pelona Schist clasts (late Pleistocene) – Dense, fluvial gravel, sand and silt. Fine- to coarse-grained sand, clast supported. Gravels are primarily angular to sub-angular Pelona Schist clasts. Average clast size 10-15 cm.

Older alluvium with Portal schist clasts (late Pleistocene) – Unconsolidated to weakly consolidated, poorly stratified moderately dissected, fluvial gravel, sand, and silt with angular to sub-angular Portal schist clasts, locally mixed with variable proportions of sub-angular to rounded gneissic and granitic debris of sedimentary rock fragments.

Older alluvium with Pelona and Portal schist clasts (late Pleistocene) – Dense, fluvial gravel, sand, and silt. Dissected remnants of alluvial gravel and sand of mostly Pelona and Portal schists.

Older alluvium with Pelona Schist and syenite clasts (late Pleistocene) – Dense, fluvial gravel, sand, and silt. Predominate clasts of Pelona Schist and syenite in the unit. Unconsolidated, poorly sorted, moderately dissected fluvial gravel, sand, and silt deposits found within the San Andreas fault zone near the western boundary of the quadrangle.

Older terrace deposits (late Pleistocene) – Weakly consolidated, poorly sorted, fluvial boulder and cobble gravel, and sand. Uplifted, weathered, and deeply dissected.

Older fan deposits (late Pleistocene) – Loose to moderately consolidated, poorly sorted, coarse gravel and boulder fan deposit. Clasts on south side of Sierra Pelona consist mostly of Pelona Schist.

Older colluvium with metamorphic debris (late Pleistocene) – Dense, cobble- to boulder-size blocks of Pelona Schist in a silty sand matrix. Clasts are commonly coated with caliche. Mapped as Qcm by Barrows and others, 1985.

Nadeau Gravel (middle to late Pleistocene) – Loose to moderately consolidated, poorly sorted, coarse fluvial gravel. Cobble to boulder gravel with a distinctive dark-reddish-brown, silty sand matrix. Clasts up to 45 cm, predominately dark-gray, angular to subrounded slabs, discs and cobbles of Pelona Schist, commonly with angular milky quartz fragments. Nadeau Gravel also contains syenite, leucocratic granitic rocks, and volcanic clasts from the Vasquez Formation.

Harold Formation, undivided (early to middle Pleistocene) – Light-brown, gray, and reddish-brown, silty and sandy to gravelly alluvial fan and playa deposits, with local lacustrine deposits. Unit represents low-gradient deposition with low relief. Unit is poorly to moderately consolidated, and commonly contains caliche nodules, and caliche coatings on clasts, and within cracks.

Harold Formation, Pelona Schist clast member (early to middle Pleistocene) – Light-brown, light- to dark-gray and reddish- brown alluvial fan and playa deposits. Poorly to moderately consolidated, massive to moderately well stratified, poorly sorted, fluvial gravel with 80% of the pebble- to cobble-size clasts, consisting of sub-rounded to sub-angular Pelona Schist.

Crushed granitic rocks (Quaternary to Tertiary) – Crushed granitic rock within the San Andreas fault zone. Consists of white, powdered, crumbly brecciated granite to granodiorite. Locally well-developed, but shearing not everywhere intensive enough to obscure original igneous textures, including foliation. Poorly exposed, and severely weathered in shear zones. Mapped as grc by Barrows and others, 1985.

TERTIARY SEDIMENTARY AND VOLCANIC UNITS

Ritter Formation (Plio-Pleistocene?) – Dark-yellowish-brown (10YR 4/4) to light-gray, arkosic sandstone and siltstone, moderately indurated, fine to coarse sand and fine to coarse gravel, with clasts occasionally up to 0.5 m diameter, most common size is 25 cm. Clasts composed mainly of sub-rounded to sub-angular diorite and gneiss complex from source west of Sierra Pelona (Barrows and others, 1985).

Anaverde Formation

Red arkose member (middle Pliocene) – Pale-red to moderate-red, medium- to thick-bedded, locally massive, coarse pebbly arkose. Pebbles are angular, blocky, and are granitic clasts with minor diorite.

Buff arkose member (middle Pliocene) – Tan to gray, medium-bedded to massive, medium- to very coarse-grained pebbly to cobbly arkose, with thin silty interbeds near the top.

Gray arkose member (middle Pliocene) – Gray to buff arkosic sandstone with lenses of pebbly to cobbly arkose and conglomerate. Unit is medium- to thick-bedded to massive and medium- to very coarse-grained.

Buff arkose with volcanic clasts member (middle Pliocene) – Tan to light-gray, coarse arkose with interbedded pebbly to boulder arkose and local clayey fine sand and silt beds. Moderately to well-consolidated, with cross bedding and cut and fill structures common. Pebble and boulder clasts are well-rounded, mostly granitic, with lesser quartz latite and dacite volcanic clasts.

Red volcanic arkose member (middle Pliocene) – Red to light-maroon, well-bedded, poorly sorted, coarse arkose with interbedded pebbly to boulder arkose with volcanic clasts, and local clayey fine sand and silt.

Clay shale member (middle Pliocene) – Light- to dark-gray, thin-bedded, argillaceous to silty clay shale. Locally contains numerous white layers of gypsum, up to 5 cm thick, and a few thin lenses of tan, fine-grained sandstone and sandy siltstone. Some clay layers reported to contain leaf fragments. Highly folded unit exposed at San Andreas fault zone, located in road cut north of Avenue S on Interstate 14.

Breccia member (middle Pliocene) – Reddish to dark-gray, massive, pervasively sheared sedimentary breccia with angular clasts of biotite-hornblende diorite. Locally sheared and brecciated zones contain layers of mylonite. Unit is exposed within the San Andreas fault zone near the western boundary of the quadrangle.

Vasquez Formation

Andesitic volcanic rocks (Oligocene to early Miocene?) – Dark-gray to dark-brown, hard, massive, very fine grained matrix, with fine to medium phenocrysts of plagioclase feldspar. Occasional flow banding, conglomeratic layering, and some silica-filled amydules. Unit also contains thin, discontinuous, light-tan travertine beds, approximately 1 m thick.

Tuff breccia (Oligocene) – Light-tan to light-green, hard, massive, contains small angular fragments of dark-brown andesite with minor pumice fragments, fine-grained matrix; forms lenses in upper Tva unit.

Basaltic and andesitic volcanic rocks (Oligocene) – Dark-gray, basaltic and andesitic volcanic rocks. Unit contains small phenocrysts of augite and olivine. Outcrop exposures are highly jointed and resistant, with weathered surfaces dark-gray to dark-brown. Deposited as sub-aerial flows and flow breccias.

Andesitic-basaltic rocks (Oligocene) – Dark-brown to dark-maroon, hard, erosion-resistant, with occasional quartz cavity fillings and amygdules. Unit is generally much more vesicular than other Vasquez units.

Basal conglomerate (Oligocene) – Light-gray, pink to reddish-maroon, unsorted cobble and basal pebble conglomerate, with mostly granitic source clasts.

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METAMORPHIC AND INTRUSIVE ROCKS – MESOZOIC AND/OR OLDER

Mylonite of Vincent Thrust (Late Cretaceous to Paleocene) – Gray to greenish-gray, light-greenish-brown where weathered, commonly marked by a light-greenish-gray seam of very fine grained actinolite-chlorite schist. Composed of small to large white feldspar augen, in dark, crudely laminated, non-granular lithified shear matrix. Forms broad zone of mylonized and partially recrystallized rocks. Intense stretching and laminar shear flow parallel to foliation, with flattening of grains perpendicular to foliation, where observed. Unit is structurally at the top of Pelona Schist unit, formed by thrust movement of plutonic and gneissic rocks over Pelona Schist at great depth in a metamorphic environment during Late Cretaceous to Paleocene time (Ehlig, 1981). Thickness and outcrop exposure of unit varies due to subsequent faulting and erosion. Folds and lineations of the mylonite appear parallel to the underlying Pelona Schist structures. In thin section, samples have much iron-bearing epidote, and some sections show feldspars are altered to muscovite mica (D.M. Morton, personal comm., 2009). Sr-Rb mineral isochron age of 59 Ma from the Vincent Thrust mylonite zone (Ehlig, 1981), and 55 Ma for 40Ar/39Ar muscovite ages (Jacobson, 1990).

Pelona Schist (Late Cretaceous to earliest Tertiary) – Mica schist; bluish-gray, weathers greenish-brown, fine- to medium- grained, coherent but fractured, highly foliated, cleaves into slabs with a silvery sheen from mica flakes, most foliation planar, but in some places foliation is linear; composed of mica (mostly muscovite), albite feldspar, and quartz, and in places variable amounts of chlorite and some actinolite; fuchsite is also present. Unit locally includes thin layers of quartzite and also may include small masses of greenish-gray serpentine and actinolite within local shear zones. Metabasalt is part of basal pelona schist unit. Typically greenschist facies of metamorphism, with lower amphibolite facies of metamorphism close to the Vincent Thrust. Outcrops are weakly to moderately resistant, thinly layered, with meta-chert and marble lamellae scattered through the sequence. Occasional talc is exposed on Sierra Pelona including lenticular masses of meta-serpentinite and talc-actinolite rocks. Pelona Schist is determined to have a Late Cretaceous protolith age (Jacobson and others, 2000). In thin section, meta-chert samples have spessartine garnets, and high strain texture. Some samples show intercalated quartzite and marble. Primary micas in samples show uniform orientation, however they also contain abundant secondary white mica with random crystal orientation. Metbasalt samples have common hornblende, and garnets, and approach amphibolite grade metamorphism (D.M. Morton, personal comm., 2009).

Portal schist (Late Cretaceous to earliest Tertiary) – Quartzo-feldspathic and biotite schist; schist is partially re-crystallized, and has strong foliation with quartz augen layering and quartz veins parallel to foliation. Amphibolite, marble, vein quartz and quartzite layering are common, with minor talc-actinolite schist. Unit is fine-grained, composed largely of albite, plagioclase and biotite. Noted as typically amphibolite facies of metamorphism. In thin section, samples have broken up fabric, and evidence of texture deformed after peak prograde metamorphism. Samples also have garnet crystals and hornblende with abundant twinned plagioclase, indicating higher grade of metamorphism than Pelona Schist (D.M. Morton, personal comm., 2009).

Holcomb Quartz Monzonite (Cretaceous) – Quartz monzonite to granodiorite composition, light-gray, massive, medium- grained, somewhat incoherent where weathered, composed of sodic plagioclase feldspar, potassic feldspar and quartz in nearly equal amounts and some biotite mica; exposed north of San Andreas Fault, in contact with unit QTgrc. Mapped as Holcomb quartz monzonite, by Barrows and others, 1985.

Quartzo-feldspathic and amphibolite gneiss (Early Cretaceous to Proterozoic) – Dark-gray to light- tan, with alternating white and dark-gray to black banding upper amphibolite grade gneiss. Consists of Proterozic, Paleozoic and/or Mesozoic metasedimentary rocks that were subject to tectonic deformation and overprint by Early Cretaceous plutonic rocks (R.E. Powell, personal communication, 2011). Unit is internally folded. Outcrops are somewhat incoherent where weathered, closely fractured. Composed mostly of feldspar, quartz, biotite, hornblende and accessory garnet. Metasedimentary layering and structures preserved in some localized quartz-rich zones, and biotite schist layers.

Mount Lowe intrusive suite - hornblende zone (Triassic) – Quartz-bearing diorite, hornblende diorite to granodiorite, light-gray, massive, medium-grained, somewhat incoherent where weathered; composed of up to 50 percent plagioclase feldspar and lesser amounts of alkali feldspar, quartz and hornblende, with hornblende as scattered dark clusters. Localized, possible diatreme exposure involving volcanic dikes intruding this unit at water tank outcrop in section 29 of the Acton quadrangle to the south. In thin section, samples have a little myrmekitic quartz, apatite, plagioclase, chlorite and secondary white mica (muscovite) and quite a bit of calcite. Samples also have many small to some large sphene crystals, and generally abundant epidote. Hornblende is a deep-green color, medium- to coarse-grained and is mostly altered to chlorite (D.M. Morton, personal comm., 2009). Mt. Lowe intrusive ages using zircon U/Pb are about 220 ±10 my reported by Silver, 1971, and Rb/Sr whole-rock age of 208 ±14 my as reported by Joseph and others, 1982.

Ferruginous syenite (Mesoproterozoic) – Syenite and quartz syenite, light-gray where fresh, stained rusty-brown from iron oxides where weathered. Fine- to coarse-grained, equigranular, highly fractured. Magnetite is common. Where exposed, soils are typical dark-reddish-brown. Outcrops are jointed and blocky. Unit is part of the San Gabriel Mountains anorthosite-syenite-gabbro complex. In thin section, samples have a distinctive alkali feldspar which has exsolved into hairline mesoperthite (Ehlig, 1975). This feldspar texture is also observed in thin section from samples collected for this study, and map units Yhd and Ydgn. Samples also have epidote and some zircon crystals. Samples include brecciated zones involving quartz, feldspars and zircons (D.M. Morton, personal comm., 2009). Age of the anorthosite body is reported as 1.19 Ga (Carter, 1980).

Hornblende diorite (Mesoproterozoic) – Hornblende diorite to gabbro: dark-gray to greenish-black, massive, medium- to coarse-grained, foliated. Composed of hornblende and minor plagioclase feldspar; locally contains green epidote veinlets. Unit appears to be related to the anorthosite-syenite-gabbro complex of the San Gabriel Mountains. In thin section, samples have apatite, and are chlorite-rich, with unique plagioclase exsolution lamellae texture, and very fine grained carbonate. Hornblende shows kink-banded crystal deformation (D.M. Morton, personal comm., 2009).

Quartz diorite to diorite gneiss (Mesoproterozoic) – Variable degrees of gneissic texture, with samples ranging from quartz diorite to diorite composition: Light-gray, hornblende-rich, mostly foliated with thinly laminated zones. Deeply weathered, highly jointed and sheared outcrops. Younger quartz vein presence suggests relation to the later sulfide auriferous mineralization at the Governor, Red Rover, and Puritan mines (Gay and Hoffman, 1954). Unit appears to be related to the anorthosite-syenite-gabbro complex of the San Gabriel Mountains. In thin section, samples have unique plagioclase exsollution lamellae texture. Samples are pervasively altered, with significant secondary white mica and calcite (D.M. Morton, personal comm., 2009).

Augen gneiss (Paleoproterozoic) – Olive-gray to olive-black, medium-grained, and strongly foliated. Composed of mostly feldspar, biotite and polycrystalline quartz, with minor hornblende. Unit has distinctive light-gray to light-pink large porphyroblasts of potassic feldspar, average size up to 4 cm long and 2 cm wide. Intrudes into quartzo-feldspathic and amphibolite gneiss unit. Zircons within the augen gneiss from the Soledad Basin have been dated at 1670 ±15 my by the U/Pb isotope method (Silver, 1966). In thin section, samples show mylonitic texture and have large phenocrysts of plagioclase, with thin, long biotite crystals that are deformed into mylonitic planes. Samples also have microcline tartan twinning with very fine grained white mica, with secondary epidote and about 20 percent biotite (D.M. Morton, personal comm., 2009).

Quartzo-feldspathic and amphibolite gneiss (Paleoproterozoic) – Dark-gray to light-tan, with alternating white and dark-gray to black banding. Outcrops are somewhat incoherent where weathered, closely fractured. Composed mostly of feldspar, quartz, biotite mica, and in places, minor hornblende. Locally intruded by younger granitic rocks, too small to map. The complex has been thrust over the Pelona Schist and exhibits a cataclastic texture near the thrust. In thin section, some samples have large amount of myrmekite with calcite vein filling, an abundance of biotite, coarse plagioclase and plagioclase augen, and secondary calcite. Samples also have large apatite crystals and have classic intergrowth deformation (D.M. Morton, personal comm., 2009).

Preliminary Geologic Map available from:http://www.conservation.ca.gov/cgs/rghm/rgm/preliminary_geologic_maps.htm

Revised: 08/12/2013