Preliminary Geologic Map of the Eagle Lake 30’ x 60 ... · Preliminary Geologic Map of the Eagle...

Preliminary Geologic Map of the Eagle Lake 30’ x 60’ Quadrangle, Lassen County, California

Scale 1:100,000

By

Thomas L.T. Grose1, George J. Saucedo2, and David L. Wagner2

Digital Preparation by

Carlos I. Gutierrez2, Anita Carney2, Karen Toman-Sager2, and Jason D. Little2

2014

Prepared in cooperation with:

__________________________________ 1Colorado School of Mines 2California Geological Survey

STATE OF CALIFORNIA THE NATURAL RESOURCES AGENCY DEPARTMENT OF CONSERVATION CALIFORNIA GEOLOGICAL SURVEY EDMUND G. BROWN JR. JOHN LAIRD MARK NECHODOM JOHN G. PARRISH, Ph.D. GOVERNOR SECRETARY FOR RESOURCES DIRECTOR STATE GEOLOGIST

CALIFORNIA GEOLOGICAL SURVEY JOHN G. PARRISH, Ph.D.

STATE GEOLOGIST

Copyright © 2014 by the California Department of Conservation. 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 particular purpose.

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California Geological Survey

Table of Contents

Introduction ................................................................................................................................................................5

Geologic Summary .....................................................................................................................................................5

Volcanic Geology ..................................................................................................................................................5

Basins .....................................................................................................................................................................6

Faulting ..................................................................................................................................................................7

Compilation ..............................................................................................................................................................10

Base Material .......................................................................................................................................................10

Acknowledgements ..................................................................................................................................................10

Description of Map Units .........................................................................................................................................11

Surficial Deposits .................................................................................................................................................11

Volcanic Rocks ....................................................................................................................................................11

Volcanic Units Associated with Fissures, Cones, or Uncertain Sources .........................................................11

Volcanic Units Associated with a Source Volcano .........................................................................................18

Tertiary Sedimentary Rocks .................................................................................................................................25

Pre-Cenozoic Rocks .............................................................................................................................................25

Sources of Mapping for the Eagle Lake 30’ x 60’ quadrangle .................................................................................26

References Cited .......................................................................................................................................................27

Plate Preliminary Geologic Map of the Eagle Lake 30’ x 60’ Quadrangle, Lassen County, California ............... (see PDF)

Figures Figure 1. Index map showing the location of the Eagle Lake 30’ x 60’ quadrangle, local cities and faults, as well

as the geomorphic provinces of northeastern California. Modified from Jennings and Saucedo, 2002 .... 7

Figure 2. Index map of the Eagle Lake 30’ x 60’ quadrangle showing the locations of source volcanoes. Symbols and numbers shown in red correspond to volcanoes listed in Table 1. Index map also includes geographic features referenced in the Description of Map Units ............................................................... 8

Figure 3. Index map showing the 15-minute and 7.5-minute quadrangles within the Eagle Lake 30' x 60' quadrangle ................................................................................................................................................ 26

Figure 4. Index map of the Eagle Lake 30’ x 60’ quadrangle showing the locations of volcanic rocks dated by radiometric methods (numbers correspond to Location Number in Tables 2 - 4) ................................... 29

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Preliminary Geologic Map of the Eagle Lake 30’ x 60’ Quadrangle, California

Tables Table 1. Source volcanoes and associated geologic units. Numbers correspond to locations of source volcanoes

displayed on Figure 2. ................................................................................................................................9

Table 2. Summary of K-Ar ages on volcanic rocks within the Eagle Lake 30’ x 60’ quadrangle, California. .......30

Table 3a. K-Ar analytical data for unpublished age determinations presented in this report. See Figure 4 and Table 2 for sample locations. .............................................................................................................................31

Table 3b. 40Ar/39Ar analytical data. See Figure 4 and Table 2 for sample locations. ...............................................32

Table 4. Major-element chemical analyses on volcanic rocks within the Eagle Lake 30’ x 60’ quadrangle, California. See Figure 4 and Table 2 for sample locations. .....................................................................33

Table 5. Terminology used in the description of volcanic rocks. .............................................................................35

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Preliminary Geologic Map of the Eagle Lake 30’ x 60’ Quadrangle, Lassen County, California By Thomas L.T. Grose1, George J. Saucedo2, and David L. Wagner2

Introduction In 1989, the California Geological Survey (then Division of Mines and Geology) Regional

Geologic Mapping Project began a project in northeastern California to improve the level of detail of geologic data and provide a better understanding of the framework geology of an area previously covered by only small-scale reconnaissance mapping. This project has developed new geologic map data that may be used as the foundation for geologic hazard investigations, natural resource evaluations, and other earth science related studies in this part of northeastern California. The map was compiled and digitized primarily from new geologic mapping that was conducted between 1989 and 1991 and also includes unpublished mapping.

This compilation was prepared by the Department of Conservation, California Geological Survey and was supported in part by the U.S. Geological Survey (USGS) Cooperative Geologic Mapping Program - COGEOMAP (Agreement No. 14-08-0001-A0640) the predecessor to STATEMAP. This map is being published as part of the California Geological Survey’s 1:100,000-scale Regional Geologic Map Series and replaces an earlier version released as Open-File Report 92-14.

Geologic Summary The Eagle Lake map area includes the south-central part of the Modoc Plateau, a large volcanic

province in northeastern California (Figure 1). In the western part of the map, the Modoc Plateau and Cascade Range gradually grade into each other through a zone roughly ten miles wide trending N30W. The transition zone is characterized by NNW – NW faults that impart a northwesterly grain to the topography and by mixed volcanic activity. In contrast, the central and eastern part of the Eagle Lake map area is dominated by several, variously eroded, shield and stratovolcanoes. Faulting is absent over most of the area.

Volcanic Geology The Eagle Lake map area is composed almost entirely of Neogene volcanic flows, pyroclastics,

volcaniclastics, and shallow intrusions. All are sourced from volcanoes or dikes that occur within or immediately adjacent to the map boundaries. The numerous volcanic formations that have been delineated are appropriate for the scale of mapping of 1:62,500 or one inch equals one mile. Some of the formations consist of essentially one lithology, e.g. basalt flows. Most of the formations, however, consist of more than one lithology, e.g. andesitic flows, tuffs, and mudflow breccias (lahars), that in physical or genetic composite comprise a distinctive, mappable, layered assemblage. Contacts between the formations consist of gradational interbedding, facies changes in lithology, unconformities, faults, and intrusions. The eruptive source or sources of some formations is indicated where field relations are reasonably clear, but in many cases the source may be buried by younger volcanics or be otherwise uncertain. The lateral extent of the formations nowhere exceeds several miles, and most are clearly centered around their sources, e.g. an individual volcano.

1 Colorado School of Mines 2 California Geological Survey

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More than twenty-five shield and stratovolcanoes of andesitic and basaltic composition occur in the Eagle Lake map area. Their ages generally decrease from about 14 Ma in the east to about 1 Ma in the west. Volcano elevations range from 5000 to 8000 feet. The older and initially higher volcanoes are more eroded by radial stream drainage whereas the younger, originally lower, shield volcanoes have very little, if any, drainage development.

The McDonald Peak – Three Peaks volcano cluster is the largest in the region and located just north of the Madeline Plains. It consists of a dual central vent ringed by eight satellite andesitic and basaltic volcanoes. The second largest is the andesitic Snowstorm Mountain which is composed of a deeply eroded central vent with scattered small plugs in the southern flank. Part of a large silicic volcanic complex, called Hayden Hill (# 19; Figure 2, and Table 1), occurs in the northwest corner of the map. The area is important for two reasons: 1) it has been the locale of repeated economic gold mining, the latest in the 1990s, and 2) the complex is believed to represent part of a highly faulted volcano-tectonic depression or caldera of mid-Miocene age. Elsewhere in the Eagle Lake map area, and at the “other end of the volcanic spectrum”, are four low-amplitude basaltic shield volcanoes: Heavey Mountain (# 1), Logan Mountain (# 3), Secret Valley shield (# 7), and Spanish Springs shield (# 8). The last two are so low-amplitude as to be barely noticeable in the field. Large volumes of basalt were also extruded from fissure dikes localized along faults rather than derived from central vents of shield volcanoes. An outstanding example is the flows of Brockman Flat which fill in the western part of the Eagle Lake depression.

Obviously, many varieties of volcanic vents with all gradations between them are present in the Eagle Lake map area. In composite, they display a full range of volcanic phenomena that testify to a westward migrating andesitic arc of Cascade affinity followed by basaltic back-arc volcanism of a westward expanding Basin and Range Province.

Basins Two large topographic basins occur in the map area: Eagle Lake Basin and the Madeline Plains.

The Eagle Lake Basin is an active volcano-tectonic depression that was shaped by late Quaternary faulting and basaltic fissure eruptions locally along the western and eastern margins. Ponded lava flows and lake sediments form the floor of the basin. The Madeline Plains is a very different kind of basin and one of uncertain origin. Many andesitic and basaltic central volcanoes encircle the Plains, making a highly indented and irregular margin with the flat valley floor. Remnant shoreline deposits rim areas around the margin and extensive lake beds underlie the flat valley floor. These deposits testify to a large, isolated, shallow lake, called Lake Madeline, which occupied the Madeline Plains basin in Late Pleistocene up to about 10,000 years ago.

Three smaller basins occur in the Eagle Lake map area: Grasshopper Valley, Willow Creek Valley, and Secret Valley. Grasshopper Valley is mostly bound by faults and surrounded by eroded volcanoes including the young, uneroded, Heavey Mountain shield volcano (#1; Figure 2, and Table 1). Topographically the valley appears to be a westward extension of the Madeline Plains as well as a northern extension, with en echelon right step, of the Eagle Lake depression.

Willow Creek Valley is a flat-floored tectonic depression of mixed structural origin. It is bound on the southwest by a late Quaternary fault, on the east by a minor fault, and on the north by basinward dipping andesite and basalt flows. The floor of Willow Creek Valley is anomalously flat suggesting a recent lake which may have impounded intermittent inflow from Eagle Lake and outflow to the Honey Lake Valley, located just south of the map.

Most of Secret Valley is included in a southeast part of the Eagle Lake map. It resembles the Madeline Plains in that it is surrounded by several eroded central volcanoes, its margin is irregularly indented, and the flat valley floor in the middle of the basin suggests young lake bed deposition. Both the Madeline Plains and Secret Valley are, in a simple sense, intervolcano areas that have remained topographically low in the midst of surrounding areas that are topographically high because of in situ volcano buildup.

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Physiographically, the Eagle Lake map area is an outstanding natural laboratory that exhibits a wide range of erosional degradation that correlates with the different volcanic rocks and the nature and age of the volcanic activity. Faulting and deposition have also significantly influenced the topography.

Faulting Most of the faults occur in the transition zone between the Modoc Plateau and the Cascade

Range. Large central volcanoes are few in this area. In the central and eastern parts of the map, faults are few and far between, but large volcanoes are abundant. The faults have a northwest to north-northwest trend, and a few trend north-south. All appear to be dip slip and high angle. A significant right-slip component of displacement is indicated by a regional, left-stepping, en echelon fault pattern. The swarm of faults in the transition zone are a northwesterly extension of the Walker Lane belt, a broad zone of right-lateral and normal faults in the western basin and range, where it appears to merge and disappear (?) into the Cascade Range farther north.

Figure 1. Index map showing the location of the Eagle Lake 30’ x 60’ quadrangle, local cities and faults, as well as the geomorphic provinces of northeastern California. Modified from Jennings and Saucedo, 2002.

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Table 1. Source volcanoes and associated geologic units. Numbers correspond to locations of source volcanoes displayed on Figure 2.

Source Volcano Associated Geologic Units

1. Heavey Mountain Thab, Theb, Tjsb 2. Lava Peak Tlpa 3. Logan Mountain Tlma 4. Antelope-Fox Mountains Tama 5. Campbell Mountain Tca 6. Secret Valley Shield Tsvb, Tsvt, Tlfb, Tlrt 7. Spanish Springs Shield Tsbu, Tsbl 8. Roop Mountain Trma 9. Observation Peak Topa, Tscb 10. Ashurst Mountain Taob 11. Cave Mountain Tcab 12. Susanville Peak Tspa 13. Dow Butte Tdba 14. Steel Post Reservoir Tspv, Tfrf, Tbrb 15. Horse Mountain Thfp, Thf, Thlt, Tpip, Tppf 16. York vents Typa 17. South Madeline Mountain Tsma 18. Whaleback Mountain Twma 19. Ambrose Basalt Field Tarb 20. Black Mountain Tbmb 21. Slate Mountain Tsmf 22. Hayden Hill Silicic Center Tdsd, Tshd, Tdst, Tart, Thrl, Tpct 23. Cottonwood Mountains Tcoa 24. South Knob Tskb 25. Whittinger Mountain Twmf, Tdva 26. Anderson Mountain Taaf 27. West Ravendale Mountain Twra, Tfp 28. McDonald Peak-Three Peaks Tmrf, Tpsf, Ttpf, Ttpn, Ttpw, Ttps, Tub, Tomb, Tsff, Tmst 29. Snowstorm Mountain Taf, Tsrf, Tsri, Tsu, Ttti, Tsl, Tafi, Tsi 30. Spanish Springs Peak Tssa 31. Shinn Mountain Tshv, Tsha 32. Five Springs Mountain Tfsa 33. Rush Creek Mountain Trua 34. Cherry Mountain Tmai, Tcma 35. Juniper-Dill-Twin Buttes Ttba, Tjrb 36. Sugarloaf Tsub 37. Button Mountain Tbua

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The southeastern end of the Likely fault zone occurs in the northeastern part of the map. It seems to end in an oblique transtensional pull-apart that coincides with the McDonald Peak–Three Peaks volcano cluster. Several widely spaced, small faults in the eastern part of the map could be construed to reflect a weak continuation of the Likely fault zone.

The Eagle Lake volcano-tectonic depression marks a large, transverse, structural anomaly within the regional northwesterly tectonic trends of the transition zone. The depression trends roughly north-northeast with a strong north-south fault along the western margin. This fault has been active in late Quaternary time and is associated with a line of late Quaternary cinder cones. Another late-Quaternary, right oblique-slip, northwest-trending fault crosses the north-south fault and extends greater than two miles into the Eagle Lake depression. The major eruptive source of the Brockman Flat lavas occurs at the intersection of the two fault zones. The projection of the northwesterly fault across the depression aligns with a major fault zone along the west margin of the granite-cored Gallatin Uplift. The fault-bounded Gallatin Range represents the farthest north exposure of Sierra Nevadan granitic basement rocks which presumably underlie the Modoc Plateau region.

Most of the eruptive centers in the Eagle Lake map area do not appear to be associated with particular faults. However, it is probable that many through-going fault zones are present, but are simply buried beneath later volcanic deposits.

Compilation The Preliminary Geologic Map of the Eagle Lake 30’ x 60’ Quadrangle, California was

compiled and digitized from new and existing, published and unpublished, geologic maps covering the area. New geologic mapping was completed during field seasons between 1989 and 1991 in the eight 15-minute quadrangles covering the area. Geologic maps of the Antelope Mountain, Fredonyer Peak, Grasshopper Valley, Hayden Hill, Karlo, Observation Peak, Ravendale, and Shinn Mountain quadrangles (Fig. 3, page 20) were published by the California Geological Survey (Division of Mines and Geology) as individual Open-File Reports. The original fieldwork was supported in part by USGS - COGEOMAP funds, the predecessor to STATEMAP. The geologic maps were put into digital format by hand, using stable base material on a digitizing tablet or by scanning and digitizing on screen. The individual maps were digitized at a scale of 1:62,500 with minor modifications to produce this seamless digital map.

The original digital work for the individual quadrangles, as well as the merged database file, was completed using ArcInfo® (Versions 8.3 and 9.3), a commercial GIS software package by Environmental Systems Research Institute (ESRI). For publication purposes the merged coverage was converted into the ESRI Geodatabase format. The merged geology, structure, and annotation files along with base layers and shaded-relief images were combined using the ArcMap application within ArcGIS® 10.1 and 10.2 (ESRI).

Base Material The base for the Eagle Lake 30’ x 60’ quadrangle consists of topographic map and shaded-relief

raster images. The topographic base map is taken from the USGS digital raster graphic (DRG) of the Susanville 1:100,000-scale quadrangle available from the Cal-Atlas Geospatial Clearinghouse (http://www.atlas.ca.gov/download.html). The shaded-relief image was derived from 30-meter resolution elevation data obtained from the National Elevation Dataset (NED; http://ned.usgs.gov/)

Acknowledgements The authors wish to thank Chris Wills and Carlos Gutierrez of CGS for their thoughtful review.

We would also like to thank Jerry Wampole, Sarah Watkins, and Matt O’Neal of CGS for their assistance in the preparation of this map for publication.

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Description of Map Units [Approximate stratigraphic relationships]

Surficial Deposits Qhl Lake deposits (Holocene)—Silt, sand, and clay; light-gray, massive- to thin-bedded,

horizontal deposits around the margins of Eagle Lake.

Qhp Playa deposits (Holocene)—Clay, silt, and sand; thin deposits in small local basins.

Qhs Sand deposits (Holocene)—Sand, silt, and clay; sheet and local dune deposits, mainly Eolian and lacustrine. Includes bar and beach deposits around Horse Lake at high stand. May include relict beach deposits along the east margin of the Madeline Plains.

Ql Older lake deposits (Holocene and Pleistocene)—Silt, clay, and sand; includes unconsolidated lacustrine deposits of ancient Lake Madeline and light-gray, tan, massive- to thin-bedded, horizontal lacustrine deposits of Eagle Lake, at maximum stand of about 5140 feet; also includes playa deposits in small enclosed basins

Qp Older playa deposits (Holocene and Pleistocene)—Silt, clay, and sand; light-gray, tan, massive- to thin-bedded; occurs in small playa lake basins.

Qlmd Near-shore and deltaic deposits of Lake Madeline (Holocene and Pleistocene)—Sand, silt, and gravel.

Qa Alluvium (Holocene and Pleistocene)—Sand, silt, clay, and gravel; fluvial deposits along modern drainages containing local colluvial wedges. Locally may include lacustrine deposits.

Qc Colluvium and alluvium (Holocene and Pleistocene)—Sand, silt, volcanic cobbles and soil in swales and as cone-shaped deposits at the base of slopes. Locally may include fan deposits.

Qf Alluvial fan deposits (Holocene and Pleistocene)—Gravel and sand; largely consisting of volcanic cobbles. Locally interbedded with lake deposits.

Qta Talus debris (Holocene and Pleistocene)—Coarse blocky slope debris. Occurs mainly along fault scarps.

Qls Landslide deposits (Holocene and Pleistocene)—Rotational slump blocks along margins of volcanic flows. Includes a major inferred landslide east of Cold Springs Valley that probably moved in Pliocene time.

Qpf Older alluvial fan deposits (Pleistocene?)—Volcanic gravel and sand. Occur as isolated flat-topped hills south of Rush Creek.

Tt Tufa deposits (Pliocene) –

Volcanic Rocks [Volcanic rocks are grouped either by their association to fissures, cones or uncertain sources, or association to a source volcano, and then listed in general order of increasing age; see Figure 2 for the locations of source volcanoes; see Figure 4 or geologic map for sample locations (Location Number); see Tables 2, 3a, 3b, and 4 for the results of radiometric and chemical analyses; see Table 5 for an explanation of terminology used in the description of volcanic rocks.]

Volcanic Units Associated with Fissures, Cones, or Uncertain Sources Qbb Basalt of Brockman Flat (Pleistocene)—Dark-gray to black, slightly to moderately

diktytaxitic, intergranular, with essential labradorite, olivine, and clinopyroxene. Occurs as

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constructional horizontal pahoehoe and aa flows from several local cinder cones along west and north-west margin of Eagle Lake volcano-tectonic depression.

Qbbo Older basalt of Brockman Flat (Pleistocene)—As Qbb above, but slightly more weathered constructional forms.

Obbv Basaltic agglutinate and flows (Pleistocene)—Vent facies of linear eruptive fissure, the local eruptive source of the Basalt of Brockman Flat (Qbb).

Qwb Basalt of Westwood (Pleistocene)—Red and brown vent cinders and scoria in constructional cone. Occurs in southwestern-most corner of map.

Qbma Mafic andesite of Black Mountain (Pleistocene)—Gray to black, aphanitic, pilotaxitic, with local small glomerocrysts of labradorite and olivine. Constructional blocky lava flows and local tumuli and summit vent facies. Represents earliest volcanic activity associated with the modern Eagle Lake volcano-tectonic depression. K-Ar age: Location No. 1 – 0.17±0.07 Ma (Grose and McKee, 1986; Youngkin, 1980).

Qcla Hypersthene andesite of Crater Lake Mountain (Pleistocene)—Dark-gray to black, aphanitic to sparsely microporphyritic with hypersthene and less abundant olivine phenocrysts; hyalo-ophitic. Occurs as vesicular to dense flows on a relatively large, slightly eroded shield (Macdonald, 1965).

Qscb Basalt of Stony Creek (Pleistocene)—Light- to dark-gray, with glomeroporphyritic olivine and plagioclase phenocrysts in a fine-grained, commonly microvesicular, ground-mass. Occurs as multiple flows erupted from several local vents. May be equivalent to the mafic andesite of Shinn Ranch (Tsrm) to the north. K-Ar age: Location No. 2 – 0.80±0.34 Ma (This report).

QTmb Basalt of Merrill Flat (Pliocene or Pleistocene)—Light- to dark-gray, aphanitic, dense to moderately diktytaxitic, subophitic, locally sparsely finely olivine-phyric, with clinopyroxene. Occurs as horizontal flows infilling between older volcanic highs.

Tp Intrusive plugs, undifferentiated (Pliocene and (or) Miocene)—Intrusive core of volcano or smaller mass.

Tsrm Mafic andesite of Shinn Ranch (Pliocene?)—Dark-gray to black, commonly with glomeroporphyritic olivine and plagioclase phenocrysts; SiO2 is 56%. Groundmass is often microvesicular. Tsrm is a single flow that erupted from a fissure vent (NE1/4 sec. 10, T.32N. R.16E.). It may be equivalent to a lithologically similar flow along Stony Creek to the south (Qscb).

Tlrb Basalt of Lava Rock Reservoir (Pliocene?)—Medium-gray to black, moderately diktytaxitic, sparsely olivine microporphyritic. Occurs as thin horizontal flows.

Tcla Mafic andesite of Colman Lake (Pliocene)—Medium- to dark-gray, sparsely microporphyritic and glomeroporphyritic with olivine and calcic plagioclase set in a uniform intergranular matrix of clinopyroxene, olivine, and plagioclase, locally weakly diktytaxitic. Flow-on-flow sequence, some columnar jointing. K-Ar age: Location No. 4 – 2.3±0.1 Ma (Grose and McKee, 1986).

Tsra Mafic andesite of Stone Ranch (Pliocene?)—Gray, variously porphyritic with labradorite and olivine, locally strongly platy. Flows and vent facies. Estimated age is 2.5 Ma relative to vent and flows 3 miles to south-southeast. Associated with east margin of Eagle Lake volcano-tectonic depression.

Twca Mafic andesite of West Champs Flat (Pliocene)—Gray, aphanitic to moderately microporphyritic with plagioclase phenocrysts in inter-granular matrix with clinopyroxene and

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lesser olivine. Occurs as dense to highly vesicular flows from several local vents. Minor silica in vesicles.

Tsb Basalt of Squaw Valley (Pliocene)—Light-to medium-gray, strongly finely diktytaxitic with olivine, augite, and plagioclase in intergranular matrix; massive flows generally horizontal. K-Ar age: Location No. 6 – 3.0±0.5 Ma (Grose and McKee, 1986).

Tlha Mafic andesite of Little Harvey Mountain (Pliocene)—Gray, aphanitic to sparsely micro-porphyritic, with hypersthene and lesser olivine, hyaloophitic. Occurs as flows limited to Little Harvey Mountain.

Tbda Hypersthene andesite of Big Dry Lake (Pliocene)—Gray to brown, variously porphyritic with labradorite, hypersthene, and olivine; pilotaxitic, locally flow-banded and platy. Mostly flows and flow breccias, minor pyroclastic layers. Forms west-tilted horst divide between Eagle Lake depression and Willow Creek Valley. K-Ar age: Location No. 8 – 3.1±0.1 Ma (Grose and McKee, 1986).

Trca Mafic andesite of Roney Corral (Pliocene)—Light- to dark-brownish-gray, aphanitic, locally weakly diktytaxitic to generally dense, with pyroxene and minor olivine. Occurs as eroded horizontal flows from unknown source.

Tdrb Basalt and mafic andesite of Ducasse Reservoir (Pliocene?)—Medium-gray to black, dense, stony, aphanitic to variously porphyritic with olivine and plagioclase, commonly grading into, and interstratified with, diktytaxitic intergranular basalt. Estimated nearly equal volumes of basalt and mafic andesite. Cindery vent facies common. Occurs as nearly horizontal infilling of the Madeline Plains depression.

Tsst Tuff of Summit Spring (Pliocene)—White to tan, crystal vitric, with minor pumice lapilli layers, and rare hornblende crystals. Appears to mainly overlie Tssb.

Tssb Basalt and andesite of Summit Spring (Pliocene?)—Light- to dark-gray, aphanitic to moderately diktytaxitic, vesicular, rarely porphyritic, irregularly gray color laminated on some weathered surfaces, locally splotchy; minor thin pyroclastic interbeds. Forms west-dipping beds on volcanic upland.

Ttlb Basalt of Twin Lakes (Pliocene)—Light- to dark-gray, variously porphyritic with olivine and plagioclase, vesicular to dense, locally diktytaxitic and interbedded with cinders, air-fall tuffs, and volcaniclastics. K-Ar age: Location No. 9 – 3.2±0.1 Ma (Bean, 1980; Grose and McKee, 1986).

Tcsb Basalt of Camp Stanford (Pliocene)—Light- to medium-gray, glomeroporphyritic with olivine and calcic plagioclase in intergranular matrix. Mostly dense blocky flows that originated from a local vent. K-Ar age: Location No. 10 – 3.3±0.2 Ma (Bean, 1980; Grose and McKee, 1986).

Tbca Mafic andesite of Bridge Creek (Pliocene)—Light- to dark-gray, aphanitic to moderately finely porphyritic with local glomerocrysts of plagioclase set in intergranular to pilotaxitic matrix, contains varying amounts of orthopyroxene and clinopyroxene and olivine. Occurs as lenticular platy to massive flows from many local plugs and dikes. K-Ar age: south of map area – 3.4±0.2 Ma (Grose, 1989b; Grose and McKee, 1986).

Tmma Hornblende-pyroxene andesite of Merrill Mountain (Pliocene)—Light- to dark-gray, variously finely porphyritic with hornblende, pyroxene, and locally glomerocrysts of plagioclase and pyroxene; dense to strongly vesicular flows from Merrill Mountain vent.

Tdvb Basalt of Dixie Valley (Pliocene)—Dark-gray, variously porphyritic with olivine and calcic plagioclase in glassy to intergranular matrix. Vesicular to dense blocky flows.

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Ticb Basalt of Indian Creek (Pliocene)—Light- to dark-gray, moderately microporphyritic to finely porphyritic with olivine and clinopyroxene in intergranular to subophitic matrix. Moderately diktytaxitic flows.

Tsca Mafic andesite of Southeast Champs Flat (Pliocene)—Dark-gray to black, mostly aphanitic with sparsely microporphyritic, hyalopilitic and microvesicular textures. Occurs locally around two vents.

Teca Andesite of East Champs Flat (Pliocene)—Medium-tan-gray to black, aphanitic to sparsely microporphyritic with pyroxene and rare olivine and plagioclase set in pilotaxitic and hyaloophitic matrix. Occurs overlying rhyolite unit Tmr. Appears to originate from two local vents.

Tlla Mafic andesite of Long Lake (Pliocene)—Light-gray, aphanitic to glomeroporphyritic with labradorite, augite, enstatite, and minor olivine in pilotaxitic matrix. Occurs as dense flows.

Tcha Andesite of Coyote Hole (Pliocene)—Medium-gray to black, aphanitic to moderately porphyritic with olivine and plagioclase; dense and platy flows. Originated locally from two vents.

Tja Mafic andesite of Jauriga Spring (Pliocene?)—Similar to Tsa. Flows and vent facies from two closely spaced plugs.

Ttb High-alumina olivine basalt of Troxel Point (Pliocene)—Medium-gray to black, variously diktytaxitic, vesicular, and columnar jointed. Tabular to lensic flow-on-flow sequences. Local interbedding with tuffs and volcaniclastics of Ttbp. May be earliest volcanic activity associated with Eagle Lake volcano-tectonic depression.

Tmr Rhyolite (Pliocene)—Light-gray, weathers white, moderately microvitrophyric with sanidine, sodic plagioclase, and biotite in hyalopilitic groundmass; massive to fine flow lamina. Occurs as massive uniform flows and/or flow domes. K-Ar age: Location No. 11 – 3.8±0.3 Ma (Grose and McKee, 1986).

Tncb Basalt of North Champs Flat (Pliocene)—Light- to medium-gray, weathers orange-red, strongly finely diktytaxitic with augite and olivine, massive with uncommonly uniform texture and composition. K-Ar age: Location No. 14 – 4.3±0.7 Ma (Grose and McKee, 1986; Tuppan, 1981).

Tgpi Mafic andesite plugs of Gallatin Peak (Pliocene)—Brown to gray, aphanitic, pilotaxitic, locally porphyritic with olivine, locally miarolitic cavities. Associated with cinder and agglomerate vent facies. K-Ar age: Location No. 15 – 4.6±0.2 Ma (Grose and McKee, 1986; Youngkin, 1980).

Tcra Basalt and mafic andesite flows of Craemer Reservoir (Pliocene?)—Brown-gray to dark-gray, microvesicular to slightly diktytaxitic to dense. Locally sparsely finely porphyritic with olivine phenocrysts.

Timt Tuff of Indian Mountain (Pliocene and (or) Miocene)—White, light-tan and gray, crystal vitric, pumice-lapilli, mostly massive. May be partially correlative with Tsft.

Tjcd Dacite of Juniper Creek (Pliocene and (or) Miocene)—Light-gray, moderately finely porphyritic with plagioclase, sanidine, hornblende, and quartz. Dense to highly vesicular. Occurs in northwest corner of map.

Tima Hypersthene andesite of Indian Mountain (Pliocene and (or) Miocene)—Black, moderately finely porphyritic with plagioclase and hypersthene in hyaloophitic matrix, dense and glassy appearing. Probably originated from local source, a faulted volcanic dome.

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Tsft Tuff of Silva Flat (Pliocene and (or) Miocene)—White, gray, and tan, variously lithic crystal vitric; fine-grained to lithic obsidian cinder and pumice, hornblende locally common. Distinctive rounded pebbles of quartzite occur irregularly. Interbedding transitional upward into Tssb and downward into Tsfb.

Tsfb Basalt and tuff of Silva Flat (Pliocene and (or) Miocene)—Medium- to dark-brown and gray, aphanitic to finely sparsely porphyritic with olivine in intergranular matrix, weakly to strongly diktytaxitic. Minor interbeds of tuff similar to Tsft including exotic quartzite pebbles. May be sourced at Hayden Hill.

Tecf Hornblende andesite flows of East Champs Flat (Pliocene and (or) Miocene)—Light- to medium-gray, sparsely finely microporphyritic with hornblende laths 1 mm long in pilotaxitic ground mass. Occurs as massive flows underlying Tncb.

Ttbp Basalt and andesite flows and pyroclastics of Troxel Point (Pliocene and (or) Miocene)—Interbedded with laharic breccias and coarse volcaniclastics. Large volume of diktytaxitic basalt in lower part. Partial facies relationship with Ttb above and Tbha laterally to the north.

Tbfp Mafic andesite flows and pyroclastics of Bailey Creek (Pliocene and (or) Miocene)—Gray, sparsely to moderately microporphyritic and finely porphyritic with olivine, plagioclase, and lesser pyroxene phenocrysts in thin flows interbedded with flow breccia and coarse tuffs. Probably originally interstratified with Typa and Thla.

Ti Intrusive basalt or mafic andesite (Pliocene and (or) Miocene)—Includes a distinctive coarse, plagioclase porphyritic intrusive, exposed along the northwest margin of Painters Flat near Harrison Spring.

Tff Basalt and andesite flows of Fredonyer Peak (Pliocene and (or) Miocene)—Minor flow breccia and tuff interbeds. Thin to thick discontinuous and lenticular bedding, with moderate lateral lithologic variation. Flowed from dikes and plugs in summit area of Fredonyer Peak and North Fredonyer Peak. Over 1200 foot thickness preserved.

Thla Andesitic flows and pyroclastics of Horse Lake Mountain (Pliocene and (or) Miocene)—Minor basaltic layers; complexly bedded, lensic units, highly variable laterally, about equal volume of flows and coarse to fine pyroclastics. Generally gradational contacts with units above and below. Preserved widely beneath Tff.

Ttmp Andesitic pyroclastics and flows of Tunnison Mountain (Miocene)—Highly variable, interbedded and facies relationships between mostly tuffaceous units toward the north and flows, tuffs, and minor coarse volcaniclastic lenses toward the south. Gradational and interbedded with Thla above. Laterally interstratifies with Typa. May be partially equivalent to Tbha and Tgft units. (modified from Peterson and Goeldner, 1987). Mostly correlative with Ttma on Susanville geologic map (Grose et al., 2013).

Ttma Mafic andesite of Tunnison Mountain (Miocene)—Gray, massive, vesicular to dense with local platy jointing. Mostly flows interbedded with lesser pyroclastics (Peterson and Goeldner, 1987). Probably is lateral facies of Thla.

Tcst Tuff of Cottonwood Spring (Miocene)—Gray to reddish-brown, pumice lapilli, eutaxitic, ash-flow, locally moderately welded. May correlate in part with Tstf and Tswt.

Trpt Tuff of Rye Patch Canyon (Miocene)—A light-colored, pumiceous, water-laid tuff. Contains sand and gravel interbeds.

Tcsf Basalt and pyroclastics of Cottonwood Spring (Miocene)—Medium-gray to black flows, aphanitic to abundantly finely porphyritic with olivine and plagioclase interbedded with tuffs,

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pumice and black glass lapilli with local eutaxitic texture, and andesite and basalt fragments. May correlate with Tstf and Tswt.

Tsmp Pyroclastics of Slate Mountain (Miocene)—Variously white, gray, and brown heterogeneous tuffs and volcaniclastics with minor interbedded andesite flows; bedded volcanic sandstone, laharic debris flows, and fine air-fall tuff.

Tasb Basalt and andesite of Abbott Spring (Miocene)—Gray to black, aphanitic, pilotaxitic, and locally diktytaxitic. Locally has yellow lamina streaks; weathers to red soil and supports little vegetation.

Tblb Basalt of Bullard Lake (Miocene)—Black, moderately microporphyritic with plagioclase and olivine set in intergranular to pilotaxitic matrix. Probably originated locally from two dikes.

Tlob Basalt of Long Reservoir (Miocene)—Light- to medium-gray, variously porphyritic, diktytaxitic, with interbeds of platy mafic andesite.

Tbwa Mafic andesite of Blue Water (Miocene)—Greenish-gray to black, variously porphyritic with two pyroxenes, olivine, and seriate plagioclase in intergranular matrix. Probably originated locally and interbedded gradationally with Tsmf to south.

Thrf Basalt flows of Hagata Reservoir (Miocene?)—Dark-gray to black, vesicular and locally amygdaloidal, flows ±3 m thick with minor interbeds of andesite and pyroclastics at local vents.

Tfpb Basalt of Four Pine Spring (Miocene)—Medium-gray to brown, variously porphyritic with olivine; originated from two prominent fissure dikes.

Tbha Andesite and basalt of Bald Hills (Miocene)—Gray to black, aphanitic to variously porphyritic with plagioclase, olivine, pyroxene and rarely hornblende. Occurs as massive to platy flows, lenticular and locally interbedded with pyroclastics and fluvial volcanic sandstones. Occurs in east-tilted fault blocks. Laterally equivalent in part with Ttbp and may represent partial facies equivalent of Ttmp. K-Ar age: Location No. 17 – 6.9±0.6 Ma (Grose and McKee, 1986; Tuppan, 1981).

Tseb Basalt of section eleven (Miocene)—Dark-gray to black, aphanitic to variously finely porphyritic with olivine, plagioclase, and pyroxene.

Tdob Basalt of Dodge Reservoir (Miocene)—Black diktytaxitic basalt; SiO2 is 50.26%. Probably erupted from sources north of the quadrangle.

Tlgb Basalt of Loveland Gulch (Miocene)—Light-gray, aphanitic to variously porphyritic with olivine and calcic plagioclase set in pilotaxitic to sub-ophitic groundmass. K-Ar age: Location No. 18 – 7.4±0.3 Ma (Grose and McKee, 1986; Tuppan, 1981).

Tlgt Tuff of Loveland Gulch (Miocene)—Variously colored and weathered in limited exposure; vitric matrix with lapilli of pumice, andesite, and basalt fragments, some eutaxitic. Appears to underlie Tlgb.

Tcsa Mafic andesite of Chicken Spring (Miocene)—Medium-gray-black, variously porphyritic with plagioclase, olivine, and minor pyroxene. Massive relatively thick flows that rest on irregular surfaces of Tstf and Tswt. K-Ar age: Location No. 19 – 7.4±0.3 Ma (Garman, 1991).

Tswt Welded tuff of Stratton Spring (Miocene)—White to dark-gray, fragmental, vitroclastic, fine-grained, lapilli breccia, with resistant layers of eutaxitic, lithic obsidian welded tuff breccia interbedded with non-welded to moderately-welded lapilli pumice, crystal vitric tuff. Silicic to intermediate bulk composition. Interstratifies downward into Tstf. K-Ar age: Location No. 20 (0.5 miles north of map area) – 9.9±0.5 Ma (Garman, 1991).

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Tgft Andesite flows and silicic tuffs of Grasshopper Ridge (Miocene?)—Lensic andesite flows in lower part, mostly aphanitic, pilotaxitic; overlain by white pumice lapilli tuff, crystal tuff and locally heterolithic volcanic rubble. Tuffaceous strata here may correlate with Tstf, Tswt and other units to northwest and with Ttmp to the south.

Tstf Intermediate tuffs and flows of Stratton Spring (Miocene)—White, gray, tan, brown, tuff breccia; massive to crudely bedded, mostly latitic with minor well-bedded pumice lapilli tuff intertonguing with gray porphyritic latite. Contains minor moderately welded discontinuous units. Appears to interstratify with Tskb and andesite flows locally.

Tas Mafic andesite and basaltic vent complex (Miocene)—Red, brown, and gray, aphanitic to porphyritic, dense to highly vesicular. Scoria, cinders, and irregular flows of large and complex vent area. Probably represents a deeply eroded vent complex, most likely facies and possible source of some flows of Tma and Tdva.

Tha Andesite of Highway 139 (Miocene)—Gray, finely porphyritic with pyroxene and locally hornblende. Massive flows variously mixed with andesitic pyroclastics partially correlative with Tamb.

Tbb Basalt of Buckhorn Reservoir (Miocene)—Dense olivine-rich basalt that weathers to a distinctive red ochre. In places contains pyroxene phenocrysts with a distinctive bluish coating.

Tgf Mafic andesite flows of Gilman Spring (Miocene)—Medium- to dark-gray, aphanitic to variously porphyritic with pyroxene and minor olivine. Forms extensive flow-on-flow sequence with slight westerly dip (Grose, 1989a).

Tbft Tuff of Bull Flat (Miocene?)—White, gray, tan tuff and lapilli tuff, apparently horizontal underlying Tgf unit (Grose, 1989a).

Tmaf Mafic andesite flows (Miocene)—Gray to black, mostly sparsely porphyritic with fine olivine, plagioclase, and pyroxene phenocrysts to aphanitic. Locally glomeroporphyritic, intergranular, dense, but locally vesicular and slightly diktytaxitic. Some interbeds of basalt, mafic flow breccias, and associated pyroclastics. Primarily massive flows that are vesicular and platy near their margins. Often has distinctive green alteration blotches around vesicles. Exposed in the eastern part of the quadrangle on many of the major peaks which are stratovolcanoes. K-Ar age: Location No. 26 – 12.84±0.22 Ma (This report). Also includes:

Tmat Tuff interbeds—Occurs within flows of Tmaf.

Tasa Mafic andesite of Al Shinn Canyon (Miocene)—Gray, sparsely porphyritic with fine olivine phenocrysts; SiO2 is 53%. Massive, well-layered flows which have well-developed platy structure at their margins. K-Ar age: Location No. 29 – 12.47±0.50 Ma (This report).

Tvtb Andesitic or basaltic tuff breccia (Miocene?)—Near vent facies, well-bedded yellow tuff with clasts of vesicular andesite or basalt.

Tdrt Dacite and rhyolite tuff (Miocene)—Gray to white pumiceous tuff; in places is a distinctive blue color, lightly welded pumiceous tuff that has been quarried for building stone. Most commonly is a light-gray, massive tuff breccia. Cross-bedding and interbedded gravel is common. Well rounded pebbles of obsidian are weathered out of the clay. The tuff is easily eroded so it is not well exposed and is often obscured by slope wash from overlying volcanic rocks. 40Ar/39Ar ages: Location No. 31 – 13.31±0.25 Ma; Location No. 32 – 13.73±0.42 (This report).

Tamb Andesite breccia of Antelope Mountain (Miocene)—Gray-brown, mauve, and red, auto-brecciated, two-pyroxene andesite breccia, flows, and polylithic laharic debris interbedded with

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minor flows, tuffs, and cinders. Correlative with units to southeast of map area that have K-Ar ages of 10.0±0.4 and 10.4±0.4 Ma (Grose and McKee, 1986; Grose and Porro, 1989b).

Tscm Mafic andesite of Smoke Creek (Miocene)—Light-gray, abundantly porphyritic with 3-5 mm phenocrysts aligned in trachytic texture; SiO2 is 53.5%. Appears to be the oldest volcanic unit in the quadrangle.

Volcanic Units Associated with a Source Volcano

Heavey Mountain Shield Volcano Thab Mafic andesite and basalt of Heavey Mountain (Pliocene)—Medium-gray to black,

aphanitic to variously sparsely to moderately microporphyritic to finely porphyritic with olivine, plagioclase, and rarely clinopyroxene phenocrysts, set in intersertal, hyaloophitic, or pilotaxitic matrix. Some olivine basalt flows are moderately diktytaxitic. K-Ar age: Location No. 3 – 2.1±0.3 Ma (Grose and McKee, 1986; Tuppan, 1981).

Theb Early basalt of Heavey Mountain (Pliocene)—Light- to dark-gray, aphanitic to moderately finely olivine porphyritic, diktytaxitic. Earliest known flows from Heavey Mountain.

Tjsb Basalt of Juniper Spring (Miocene?)—Gray to black, aphanitic, pilotaxitic to variously porphyritic with olivine, locally diktytaxitic; mostly dense flows.

Lava Peak

Tlpa Mafic andesite of Lava Peak (Pliocene)—Medium-gray, aphanitic to sparsely microporphyritic with olivine in a pilotaxitic matrix. Platy flows extruded from monogenic plug and cinder cone called Lava Peak. K-Ar age: Location No. 7 – 2.9±0.2 Ma (Bean, 1980; Grose and McKee, 1986; and Tuppan, 1981).

Logan Mountain Shield Volcano Tlma Mafic andesite of Logan Mountain (Pliocene)—Dark-gray to black, aphanitic to finely

porphyritic, hyaloophitic, with calcic plagioclase, hypersthene and lesser clinopyroxene and olivine. Occurs as dense to vesicular flows on Logan Mountain shield. Very similar to Crater Lake Mountain, though slightly older.

Antelope-Fox Mountain Stratovolcano Tama Andesite of Antelope Mountain (Pliocene)—Light- to dark-gray, aphanitic to moderately

finely porphyritic, microseriate, inter-granular to hyalopilitic, with clinopyroxene and rare olivine. Occurs as massive to platy flows from several vents and dikes on Antelope and Fox Mountains.

Campbell Mountain Shield Volcano Tca Mafic andesite of Campbell Mountain (Pliocene)—Light- to dark-gray, aphanitic to sparsely

finely porphyritic with olivine and plagioclase phenocrysts; dense to slightly vesicular flows.

Secret Valley Shield Volcano Tsvb Basalt of Secret Valley (Pliocene)—Dark-gray, weakly to strongly diktytaxitic, moderately

finely olivine, augite, and plagioclase porphyritic with dense flow interiors; locally with 1 cm long plagioclase phenocrysts and glomerocrysts. Flows <10 m thick, some with columnar joints. Commonly interbedded with thin, white silicic tuff (Tsvt). Wide-spread occurrence in

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south-central part of map and continuing to the south. K-Ar age: Location No. 5 – 2.5±0.2 Ma; similar flows, south of map area, yield a K-Ar age of 4.9±0.5 Ma (Grose and McKee, 1986; Grose and Porro, 1989a; Youngkin, 1980).

Tsvt Tuff and pumice lapilli tuff (Pliocene)—White, massive- to thin-bedded, locally interbedded with Basalt of Secret Valley (Tsvb).

Tlfb Basalt of Little Fredonyer Hill (Pliocene?)—Gray, black, brown, and red vent facies, dikes, plugs, and basal flow. Variously porphyritic, vesicular to dense, with olivine and pyroxene. May be a major source of Basalt of Secret Valley occurring in the Karlo 15-minute quadrangle.

Tlrt Tuff of Lava Rock Reservoir (Pliocene)—White to gray, fine-grained to lapilli pumiceous, crystal-vitric tuff, tuffaceous silt, sand, and gravel; massive- to thin-bedded, flat-lying; mostly lacustrine with minor air fall tuff. Irregular basal contact; interbedded at top with Basalt of Secret Valley (Tsvb). May be partly equivalent to Thlt and Tsbl.

Spanish Springs Shield Volcano Tsbu Basalt of Spanish Springs (Pliocene?)—Dark-brown-gray to black, massive to slightly

diktytaxitic, ophimottled to intergranular with abundant pyroxene and olivine. Extensive horizontal flows grading downward to Tsbl. May have originally been co-extensive with Tsvb.

Tsbl Basalt, mafic andesite, and tuff of Spanish Springs (Pliocene?)—Gray to black flows interbedded with silicic tuff. Massive and locally platy andesitic; locally diktytaxitic. May correlate with Tjrb and Tdrb. Flows occur in Secret Creek canyon.

Roop Mountain Shield Volcano Trma Mafic andesite of Roop Mountain (Pliocene)—Medium- to dark-gray, aphanitic to crystal

splotchy, locally sparsely finely porphyritic with olivine and pyroxene. Occurs as flow-on-flow sequence from several summit vents. Slightly older than K-Ar age: south of map area – 3.9±0.2 Ma (Grose, 1989b; Grose and McKee, 1986).

Observation Peak Stratovolcano Topa Andesite of Observation Peak (Pliocene)—Dark-gray, fine-grained andesite with sparse

olivine phenocrysts; SiO2 is 61%. K-Ar age: Location No. 12 – 4.02±0.11 Ma (This report).

Tscb Basalt of Smoke Creek (Pliocene)—Flows of dark-gray to black, aphanitic, vesicular basalt; SiO2 is 46 to 48%. Sparse phenocrysts of olivine and plagioclase, slightly to strongly diktytaxitic. Vesicle filling of drusy, translucent material that weathers to a chalky white color. Tscb erupted from a source on the west slope of Observation Peak and flowed south-east along the course of Smoke Creek. Nearly 400 feet of flows are exposed in the gorge of Smoke Creek in the east-central part of the quadrangle. Tscb is probably equivalent to the Basalt of Secret Valley (Tsvb) and the Basalt of Spanish Springs (Tsbu) found to the west.

Ashurst Mountain Fissure Flows Taob Olivine basalt of Ashurst Mountain (Pliocene)—Medium- to dark-gray, moderately finely

porphyritic and glomeroporphyritic with olivine and calcic plagioclase phenocrysts set in intergranular matrix, dense to vesicular flows that originated from several fissure vents. K-Ar age: Location No. 13 – 4.1±0.2 Ma (Bean, 1980; Grose and McKee, 1986).

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Cave Mountain Volcano Tcab Mafic andesite and basalt of Cave Mountain (Pliocene)—Medium- to dark-gray, aphanitic

to variously moderately finely porphyritic to locally splotchy plagioclase glomeroporphyritic, intergranular to hyalopilitic. Occurs as blocky to strong platy-jointed flows.

Susanville Peak Fissure Flows Tspa Mafic andesite of Susanville Peak (Pliocene)—Light- to dark-brownish-gray, aphanitic to

fine-grained, with olivine and pyroxene intergranular with local sparse fine phenocrysts. Flows originated from dikes in summit area of Susanville Peak 2 miles south of map area.

Dow Butte Volcano Tdba Andesite of Dow Butte (Pliocene)—Light-gray to black, aphanitic to moderately

microporphyritic with plagioclase and olivine and minor glomerocrysts of plagioclase set in intergranular to pilotaxitic matrix. Occurs as massive to platy flows from local sources.

Steel Post Reservoir Volcanoes Tfrf Mafic andesite and basalt flows of Fredonyer Reservoir (Pliocene?)—Gray to black,

massive with local columnar jointing and slightly diktytaxitic texture. Variously porphyritic with olivine, plagioclase and uncommonly pyroxene. Probably derived from late vent in Tspv.

Tbrb Basalt of Branham Reservoir (Pliocene?)—Dark-gray to black, massive to slightly diktytaxitic. One to three distinct linear flows.

Tspv Andesitic and basaltic pyroclastics, flows, and intrusions of Steel Post Reservoir (Pliocene and (or) Miocene)—Gray to black, andesitic to basaltic flows interbedded with white to brown coarse vent facies, mostly bedded and locally intruded by plugs and radial dikes. Unit is mostly locally derived from two small volcanoes and appears to interfinger with Thla and Tsma. Resembles Typa unit.

Horse Mountain Stratovolcano Thfp Mafic andesite of Horse Mountain (Pliocene)—Medium-gray, vesicular, aphanitic to

moderately microporphyritic with olivine and pyroxene phenocrysts. Massive to moderately platy flows. Interbedded with flow breccia and andesitic pyroclastics. Flowed from plugs and dikes of Horse Mountain volcano.

Thf Olivine basalt of Horse Mountain (Pliocene)—Black, dense, moderately finely porphyritic with olivine and plagioclase; blocky fracture. Probably derived locally from vent beneath Horse Mountain. Overlies Thlt and underlies Thfp.

Thlt Tuff of Horse Lake (Pliocene)—White to gray, crystal-vitric with rare biotite, pumice and lithic fragments locally, massive to variously bedded. Overlain by Thf and Thfp units and interbedded with, and overlain by, Tsvb unit. May correlate with Tlrt unit.

Tpip Pyroclastic rocks of Pony Mountain (Pliocene and (or) Miocene)—Red, tan, and yellow; mainly cinders, suggesting local source vent of generally andesitic composition at Pony Mountain.

Tppf Mafic andesite of Pony Mountain (Pliocene and (or) Miocene)—Medium-gray to black, dense to slightly vesicular flows, aphanitic to moderately finely porphyritic with olivine phenocrysts. Interbedded with andesitic pyroclastics.

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Volcanic Vents of York Typa Mafic andesite flows, vent facies, and intrusions of York Place (Pliocene and (or)

Miocene)—Gray, brown-red, black, dense to vesicular lensic flows which range from aphanitic dense to variously intergranular, pilotaxitic, to hyaloophitic with olivine and plagioclase phenocrysts. Red scoriaceous, massive to bedded vent facies intruded by vertical dikes. Mostly locally derived and interfingering with Ttmp and Thla. Resembles Tspv unit.

South Madeline Mountain Stratovolcano Tsma Mafic andesite flows and pyroclastics of South Madeline Mountain (Pliocene and (or)

Miocene)—Light- to dark-gray, aphanitic to variously porphyritic flows with olivine and plagioclase as common phenocrysts. Abundant interbedded flow breccia and tuff and minor black olivine basalt flows. Originated from summit vent area.

Whaleback Mountain Stratovolcano Twma Mafic andesite of Whaleback Mountain (Pliocene and (or) Miocene)—Medium-gray to

black, aphanitic to variously finely porphyritic with plagioclase, pyroxene, and olivine phenocrysts. Dense to moderately vesicular massive to platy flows that originated from local dikes and vents.

Ambrose Basalt Field Tarb Basalt of Ambrose Reservoir (Miocene)—Light- to dark-gray. Aphanitic to finely moderately

porphyritic with olivine and plagioclase, dense to moderately diktytaxitic, originated locally from at least two vents.

Black Mountain Stratovolcano Tbmb Basalt and mafic andesite of Black Mountain (Miocene)—Dark-gray to black, vesicular to

dense, sparsely finely porphyritic with olivine phenocrysts; intergranular. Forms flow-on-flow sequence probably originated from Black Mountain.

Slate Mountain Fissure Flow Tsmf Andesite of Slate Mountain (Miocene)—Gray, purple, and black, variously porphyritic with

plagioclase and two pyroxenes. Thick, massive flows with interbeds of tuffs and volcaniclastics. K-Ar age: Location No. 16 – 6.4±0.4 Ma (Grose and McKee, 1986; Tuppan, 1981).

Hayden Hill Silicic Center Tdsd Dacite of Dago Spring (Miocene)—Black, massive to highly vesicular, variously porphyritic

and glassy. Domal forms with subhorizontal jointing are common. May be cogenetic with Tdst.

Tshd Dacite of Snyder Hole (Miocene)—Dark-gray to black, vitric, variously porphyritic and vesicular.

Tdst Tuff of Dago Spring (Miocene)—White to gray to tan, vitric crystal, pumice and lithic fragments locally; fine-grained to tuff breccia. Contains well-bedded volcanic sandstones and conglomerates and tuffaceous diatomite and mudstone.

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Tart Tuff of Anderson Ranch (Miocene)—White and tan, heterolithic tuff breccias, pumice lapilli, crystal vitric ash-flow and ash-fall tuffs interbedded with volcaniclastic sandstones, siltstones, and mudstones. Locally intruded by dacite plugs. Altered at Hayden Hill.

Thrl Lahar of Hatfield Reservoir (Miocene)—Variously colored, chaotic and unsorted to bedded, locally diatomaceous, commonly with subrounded lithic clasts of claystone, sandstone, basalt, dacite, tuff, and charcoal fragments.

Tpct Tuff of Preston Canyon (Miocene)—White, gray, and brown variable tuffs, air-fall, ash-flow surge, lithic and pumice lapilli, massive to complexly bedded; contains restricted volcanic sands.

Cottonwood Mountains Shield Volcano Tcoa Mafic Andesite of Cottonwood Mountains (Miocene)—Gray, fine-grained mafic andesite

with sparse olivine phenocrysts; a sugary granular texture is often diagnostic; erupted from vents along the crest of the Cottonwood Mountains. K-Ar age: Location No. 21 – 7.86±0.17 Ma, (This report).

South Knob Volcano Tskb Basalt and mafic andesite of South Knob (Miocene)—Dark-gray to black, aphanitic to

sparsely finely porphyritic with olivine, mostly pilotaxitic and locally diktytaxitic. Thin flows of basalt and andesite from local sources, marginally interfinger with, and thin into, tuffs of Tstf.

Whittinger Mountain Stratovolcano Twmf Andesite and basalt flows and pyroclastics of Whitinger Mountain (Miocene)—Medium-

gray to black, aphanitic to variously porphyritic with olivine, plagioclase, and minor clinopyroxene and hypersthene; interbedded flow breccia and minor tuffs. Three major plugs with dikes and vent facies. May be source of Tdva.

Tdva Mafic andesite of Dry Valley Ridge (Miocene)—Dark-gray, sparsely to moderately microporphyritic to finely porphyritic, mainly with olivine and lesser glomeroporphyritic plagioclase and pyroxene; tabular to lensic beds 1-6 m thick with local strong platy structure.

Anderson Mountain Stratovolcano Taaf Andesite flows and pyroclastics of Anderson Mountain (Miocene?)—Gray, variously

porphyritic with olivine, plagioclase, and pyroxene in intergranular to subophitic matrix. Some flows are stony and aphanitic. Flows interbedded with cogenetic fragmental layers. Derived from summit vent facies, plug, and dikes.

West Ravendale Mountain Shield Volcano Twra Andesite of West Ravendale Mountain (Miocene)—Light- to dark-gray, dense, aphanitic to

variously vesicular and locally slightly diktytaxitic, variously sparsely microporphyritic with olivine and plagioclase, locally with tan-red and green-gray 1-3 mm splotches. Mostly dense-platy flows with lesser equivalent interbedded pyroclastics and minor basalt flows. K-Ar age: Location No. 24 – 11.13±0.16 Ma (This report).

Tfp Andesitic flows and pyroclastics (Miocene)—Near horizontal, poorly exposed, overlain irregularly by colluvial and lacustrine clastic deposits on near-shore shelf of ancient Lake Madeline. Unit is probably a downslope extension of Twra.

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McDonald Peak-Three Peaks Stratovolcano Center Tmrf Basalt flows of Mendiboure Reservoir (Miocene)—Medium-gray to black, dense and

variously vesicular, aphanitic to moderately finely porphyritic with olivine and glomerocrysts of olivine, pyroxene, and plagioclase. Flow-on-flow sequence with some andesitic and pyroclastic interbeds.

Tpsf Mafic andesite flows of Poulsen Spring (Miocene)—Dark-gray to black, sparsely finely porphyritic to microporphyritic with plagioclase and olivine phenocrysts; dense, locally vesicular and slightly diktytaxitic.

Ttpf Basaltic flows and pyroclastics of Three Peaks (Miocene)—Dark-gray to black, aphanitic, dense, variously vesicular; occasionally sparsely microporphyritic with olivine, hyaloophitic. Locally derived from four clusters of plugs:

Ttpn Basalt of Three Peaks North volcano

Ttpw Basalt of Three Peaks West volcano

Ttps Basalt of Three Peaks South volcano (Miocene)—Three small basaltic volcanoes peripheral to Three Peaks central volcano. Dark-gray to black, dense to variously vesicular, aphanitic to variously porphyritic with olivine phenocrysts, hyaloophitic. Interbeds of basaltic pyroclastics and vent facies. K-Ar age (Ttpw): Location No. 23 – 10.88±0.28 Ma (This report).

Tub Basalt from small unnamed volcano (Miocene)—Lithology is similar to Ttfp and associated units.

Tomb Basalt from Olson Mountain volcano (Miocene)—Summit source one-half mile north of map. Lithology is similar to Ttfp and associated units but more flows and less pyroclastics.

Tsff Basalt flows of Skeleton Flat (Miocene)—Dark-gray to black, aphanitic to sparsely porphyritic with olivine and plagioclase, dense to variously vesicular. Mainly flow-on-flow sequence from small volcano, with some mafic andesite flows.

Tmst Tuff of Moran Siding (Miocene)—Light-gray, pumice-lapilli, crystal vitric, massive to faintly bedded. May correlate with Tdrt.

Snowstorm Mountain Shield Volcano Taf Andesite flow (Miocene?)—Brown to dark-gray, aphanitic to moderately finely porphyritic

with olivine and pyroxene phenocrysts; dense, massive, platy, thick flow. Probably interbedded with Tsu unit.

Tsrf Andesitic flows and pyroclastics (Miocene)—In part flowed from Saddle Rock intrusive (Tsri).

Tsri Mafic andesite of Saddle Rock (Miocene)—Plugs, dikes, and pyroclastic vent facies. Sparingly microporphyritic with olivine phenocrysts to aphanitic. Largest single plug observed in the area. K-Ar age: Location No. 22 – 10.47±0.32 Ma (This report).

Tsu Mafic andesite of Snowstorm Mountain (Miocene)—Flows with minor pyroclastic interbeds. Minor basalt flows, dense to slightly microvesicular and diktytaxitic. Flowed from vents on Snowstorm Mountain.

Ttti Mafic andesite dikes, sills, and plugs of Two Tree (Miocene?)—Gray to black, dense, mostly aphanitic, intergranular and locally finely porphyritic. Associated with vent pyroclastics and flows.

23


Tsl Andesite and mafic andesite flows (Miocene)—Minor pyroclastic interbeds and many small plugs, dikes, and proximal vent facies breccias. Complex unit underlies Snowstorm Mountain and Saddle Rock volcanics.

Tafi Pyroxene andesite (Miocene?)—Brown to black, moderately finely porphyritic, dense, massive, platy, locally glassy. Distinctive light-brown weathering. A complex of flows with minor intrusions and domes(?).

Tsi Basalt and mafic andesite dikes and plugs (Miocene?)—Generally black, dense, aphanitic with labradorite and andesine; at summit source area of Snowstorm Mountain.

Spanish Springs Peak Stratovolcano Tssa Mafic andesite of Spanish Springs Peak (Miocene)—Dark-gray to black sparsely porphyritic

with fine olivine and plagioclase phenocrysts. Primarily massive flows that are vesicular and platy near their margins. Often has distinctive green alteration blotches around vesicles. Overlies (at least in part) Trpt.

Shinn Mountain Stratovolcano Tshv Vent facies of Shinn Mountain (Miocene)—Tuff and tuff breccia.

Tsha Mafic andesite of Shinn Mountain (Miocene)—Dark-gray to black, sparsely porphyritic with fine olivine and plagioclase phenocrysts; SiO2 is 55 to 56%. Primarily massive flows that are vesicular and platy near their margins. Often has distinctive green alteration blotches around vesicles. K-Ar ages: Location No. 27 – 12.09±0.21 Ma; Location No. 28 – 12.15±0.22 Ma (This report).

Five Springs Mountain Stratovolcano Tfsa Mafic andesite of Five Springs Mountain (Miocene)—See description of Tmaf. Equivalent

to Tgf on Susanville geologic map (Grose et al., 2013).

Rush Creek Mountain Stratovolcano Trua Mafic andesite of Rush Creek Mountain (Miocene)—See description of Tma.

Cherry Mountain Stratovolcano Tmai Mafic andesite intrusives (Miocene?)—

Tcma Mafic andesite of Cherry Mountain (Miocene)—See description of Tmaf.

Juniper-Dill-Twin Buttes Volcanoes Ttba Mafic andesite of Twin Buttes (Miocene)—Medium- to dark-gray, abundantly porphyritic

with fine plagioclase phenocrysts and sparse, fine to coarse pyroxene (?) phenocrysts; SiO2 is 55.29%. Occurs with fragmental vent facies deposits. K-Ar age: Location No. 25 – 11.90±0.20 Ma (This Report).

Tjrb Basalt and mafic andesite of Juniper Ridge (Miocene)—Light- to dark-gray, dense, stony, to slightly, diktytaxitic, aphanitic to variously microporphyritic with olivine, pyroxene, and plagioclase, intergranular to hyaloophitic. Flow-on-flow sequence horizontal to slight east dip. May correlate with Tdrb. K-Ar age: Location No. 30 – 13.03±0.31 Ma (This report).

24


Sugarloaf Shield Volcano Tsub Basalt of Sugarloaf (Miocene)—Dark-gray to black olivine- pyroxene basalt; pyroxene

phenocrysts up to 1 cm across with phenocrysts of white plagioclase and sparse intergranular olivine; SiO2 is 50.58% to 51.25%. K-Ar age: Location No. 33 – 14.53±0.33 Ma (This report).

Button Mountain Shield Volcano Tbua Mafic andesite of Button Mountain (Miocene)—Gray, trachytic mafic andesite; abundant

aligned phenocrysts of plagioclase, minor intergranular olivine. K-Ar age: Location No. 34 – 14.99±0.45 Ma (This report).

Tertiary Sedimentary Rocks Tvg Siliceous pebble-cobble conglomerate (Miocene?)—Rounded clasts of metavolcanics and

quartzite preserved in two small patches resting on crystalline rocks south-east of Eagle Lake. Correlative with conglomerate and volcanic units to the south and southwest of map area (Grose, 1989b).

Pre-Cenozoic Rocks Kgd Hornblende-biotite granodiorite (Cretaceous)—White to gray, medium-grained, massive,

mostly uniform but locally with aplitic, pegmatitic and seriate textures and elongate mafic xenoliths. In sharp to gradational contact with foliated roof pendants.

pKm Schist, gneiss, and amphibolite (Cretaceous or older)—Massive to strongly foliated, fine- to medium-grained. Probably derived from sedimentary and volcanic protoliths.

25


Sources of Mapping for the Eagle Lake 30’ x 60’ quadrangle (For complete citation see the reference section following this list.) Antelope Mountain Quadrangle: Grose, T.L.T., 1991. Grasshopper Valley Quadrangle: Grose, T.L.T., and Abrams, R.H., 1993. Karlo Quadrangle: Grose, T.L.T., and Abrams, R.H., 1991. Ravendale Quadrangle: Grose, T.L.T., and Saucedo, G.J., 1993.* Fredonyer Peak Quadrangle: Grose, T.L.T., and Youngkin, M.T., 1991. Hayden Hill Quadrangle: Grose, T.L.T., Hazlett, D.P., Tuppan, E.J., and Bean, S.M., 1993. Shinn Mountain Quadrangle: Wagner, D.L., and Saucedo, G.J., 1991. Observation Peak Quadrangle: Wagner, D.L., and Saucedo, G.J., 1993.*

*For additional fault data in the Observation Peak and Ravendale 15-minute quadrangles see: Bryant, W.A., 1991.

Figure 3. Index map showing the 15-minute and 7.5-minute quadrangles within the Eagle Lake 30' x 60' quadrangle.

26


References CitedBean, S.M., 1980, Volcanotectonics and geothermal potential in the Big Jack Lake area, Lassen County,

California: Colorado School of Mines, M.S. thesis no. 2383, 103 p.

Bryant, W.A., 1991, Fault Evaluation Report FER-218, Likely Fault zone, Lassen and Modoc counties: California Division of Mines and Geology (unpublished report on file in Bay Area Regional Office), 16 p.

Garman, C.E., 1991, Geology and tectonism of the northeast Hayden Hill area, Lassen County, California: Colorado School of Mines, M.S. thesis no. 3987, 84 p.

Grose, T.L.T., 1989a, Geologic map of the Wendel 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 89-35, scale 1:62,500.

Grose, T.L.T., 1989b, Geologic map of the Westwood 15-minute quadrangle, Lassen and Plumas counties, California: California Department of Conservation, Division of Mines and Geology Open-File Report 89-34, scale 1:62,500.

Grose, T.L.T., and McKee, E.H., 1986, Potassium-argon ages of late Miocene to late Quaternary volcanic rocks in the Susanville-Eagle Lake area, Lassen County, California: Isochron/West, no. 45, p. 5-11.

Grose, T.L.T., and Porro, C.T.R., 1989a, Geologic map of the Litchfield 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 89-32, scale 1:62,500.

, 1989b, Geologic map of the Susanville 15-minute quadrangle, Lassen and Plumas counties, California: California Department of Conservation, Division of Mines and Geology Open-File Report 89-33, scale 1:62,500.

Grose, T.L.T., 1991, Geologic map of the Antelope Mountain 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 91-21, scale 1:62,500.

Grose, T.L.T., and Abrams, R.H., 1991, Geologic map of the Karlo 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 91-23, scale 1:62,500.

Grose, T.L.T., and Youngkin, M.T., 1991, Geologic map of the Fredonyer Peak 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 91-22, scale 1:62,500.

Grose, T.L.T., and Abrams, R.H., 1993, Geologic map of the Grasshopper Valley 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 93-07, scale 1:62,500.

Grose, T.L.T., Hazlett, D.P., Tuppan, E.J., and Bean, S.M., 1993, Geologic map of the Hayden Hill 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 93-06, scale 1:62,500.

Grose, T.L.T., and Saucedo, G.J., 1993, Geologic map of the Ravendale 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 93-08, scale 1:62,500.

Grose, T.L.T., Saucedo, G.J., and Wagner, D.L., 2013, Preliminary geologic map of the Susanville 30’ x 60’ quadrangle, California: California Geological Survey, scale 1:100,000.

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Jennings, C.W., and Saucedo, G.J., 2002, Simplified fault activity map of California: California Geological Survey Map Sheet 54, scale 1:2,500,000.

Macdonald, G.A., 1965, Geologic map of the Harvey Mountain quadrangle, Lassen County, California: U.S. Geological Survey Geologic Quadrangle Map GQ-443, scale 1:62,500.

Peterson, J.A., and Goeldner, C.A., 1987, Geologic map of the Tunnison Mountain Wilderness Study Area, Lassen County, California: U.S. Geological Survey Miscellaneous Field Studies Map MF-1945, scale 1:62,500.

Tuppan, E.J., 1981, The volcanics and tectonics of the Slate Mountain area, Lassen County, California: Colorado School of Mines, M.S. thesis no. 2527, 130 p.

Wagner, D.L., and Saucedo, G.J., 1991, Reconnaissance geologic map of the Shinn Mountain 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 91-24, scale 1:62,500.

Wagner, D.L., and Saucedo, G.J., 1993, Reconnaissance geologic map of the Observation Peak 15-minute quadrangle, Lassen County, California: California Department of Conservation, Division of Mines and Geology Open-File Report 93-09, scale 1:62,500.

Youngkin, M.T., 1980, Late Cenozoic volcanism and tectonism of the Eagle Lake area, Lassen County, California: Colorado School of Mines, M.S. thesis no. 2371, 106 p.

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Table 2. Summary of K-Ar ages on volcanic rocks within the Eagle Lake 30’ x 60’ quadrangle, California.

Location Number

Map Unit

Sample Number

Material Dated

Latitude (North)

Longitude (West)

Description of Sample Age (Ma) Reference

1 Qbma YAD-2 Whole rock 40°36.8' 120°43.5' Mafic andesite 0.17±0.07 Grose and McKee, 1986

2 Qscb GJS-1 Whole rock 40°35.5' 120°13.3' Basalt 0.80±0.34 This report

3 Thab TAD-2 Whole rock 40°47.2' 120°45.6' Mafic andesite 2.1±0.3 Grose and McKee, 1986

4 Tcla GAD-2 Whole rock 40°30.7' 120°43.0' Mafic andesite 2.3±0.1 "

5 Tsvb YAD-3 Whole rock 40°37.1' 120°37.6' Mafic andesite 2.5±0.2 "

6 Tsb GAD-8 Whole rock 40°41.6' 120°58.6' Diktytaxitic basalt 3.0±0.5 "

7 Tlpa BAD-1 Whole rock 40°50.7' 120°57.2' Mafic andesite 2.9±0.2 "

8 Tbda YAD-1 Whole rock 40°36.0' 120°38.5' Andesite flow 3.1±0.1 "

9 Ttlb BAD-5 Whole rock 40°50.8' 120°57.3' Olivine basalt 3.2±0.1 "

10 Tcsb BAD-3 Whole rock 40°47.7' 120°57.4' Olivine basalt 3.3±0.2 "

11 Tmr GAD-7 Whole rock 40°41.2' 120°52.8' Rhyolite flow 3.8±0.3 "

12 Topa DW-103 Whole rock 40°46.4' 120°10.1' Andesite 4.02±0.11 This report

13 Taob BAD-2 Whole rock 40°44.5' 120°58.8' Olivine basalt 4.1±0.2 Grose and McKee, 1986

14 Tncb TAD-3 Whole rock 40°45.8' 120°54.3' Diktytaxitic basalt 4.3±0.7 "

15 Tgpi YAD-5 Whole rock 40°33.9' 120°44.5' Mafic andesite plug 4.6±0.2 "

16 Tsmf TAD-1 Whole rock 40°49.5' 120°50.6' Andesite flow 6.4±0.4 "

17 Tbha TAD-4 Whole rock 40°45.6' 120°49.0' Andesite flow 6.9±0.6 "

18 Tlgb TAD-5 Whole rock 40°47.9' 120°52.0' Mafic andesite 7.4±0.3 "

19 Tcsa CEGar1 Whole rock 40°59.8' 120°47.4' Mafic andesite 7.4±0.3 Garman, 1991

20 Tswt CEGar2 Whole rock 41°0.1' 120°45.3' Welded tuff 9.9±0.5 "

21 Tcoa GJS-2 Whole rock 40°55.9' 120°8.3' Mafic andesite 7.86±0.17 This report

22 Tsri K-215 Whole rock 40°36.5' 120°16.3' Mafic andesite 10.47±0.32 "

23 Ttpw GJS-41 Whole rock 40°56.1' 120°30.2' Basalt 10.88±0.28 "

24 Twra GAD-245 Whole rock 40°46.5' 120°26.2' Andesite 11.13±0.16 "

25 Ttba GJS-40 Whole rock 40°46.8' 120°18.3' Mafic andesite 11.90±0.20 "

26 Tmaf DW-110A Whole rock 40°49.3' 120°8.0' Mafic andesite 12.84±0.22 "

27 Tsha DW-82B Whole rock 40°39.6' 120°8.9' Mafic andesite 12.09±0.21 "

28 Tsha DW-76 Whole rock 40°41.5' 120°12.8' Mafic andesite 12.15±0.22 ”

29 Tasa DW-91A Whole rock 40°42.6' 120°0.8' Mafic andesite 12.47±0.50 "

30 Tjrb GJS-14A Whole rock 40°49.4' 120°17.8' Andesite 13.03±0.31 "

31 Tdrt GJS-31 Plagioclase 40°57.2' 120°13.0' Rhyolite 13.31±0.25 "

32 Tdrt DW-119 Plagioclase 40°52.4' 120°6.6' Rhyolite 13.73±0.42 "

33 Tsub DW-121A Whole rock 40°54.6' 120°9.1' Basalt 14.53±0.33 "

34 Tbua DW-114 Whole rock 40°50.3' 120°2.6' Mafic andesite 14.99±0.45 "

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Table 3a. K-Ar analytical data for unpublished age determinations presented in this report. See Figure 4 and Table 2 for sample locations.

Location Number

Sample Number

Material Dated

% K+ average

(n=2)

Weight (grams)

40Ar* (mol/gm) %40Ar* Age (Ma)

± 1σ

1 See reference cited in Table 1 for analytical data on this sample.

2 GJS-1 Whole Rock 0.6896 2.28732 9.611 x 10-13 4.5 0.80 ± 0.34

3-11 See references cited in Table 1 for analytical data on these samples.

12 DW-103 Whole Rock 2.0368 2.6708 1.423 x 10-11 44.2 4.02 ± 0.11

13-20 See references cited in Table 1 for analytical data on these samples.

21 GJS-2 Whole Rock 1.4745 2.30005 2.02 x 10-11 33.3 7.86 ± 0.17

22 K-215 Whole Rock 1.144 1.04036 2.083 x 10-11 55.2 10.47 ± 0.32

23 GJS-41 Whole Rock 0.9641 2.98703 1.83 x 10-11 38.9 10.88 ± 0.28

24 GAD-245 Whole Rock 2.2019 1.94692 4.26 x 10-11 85.6 11.13 ± 0.16

25 GJS-40 Whole Rock 1.5046 2.18272 3.12 x 10-11 56.6 11.90 ± 0.20

26 DW-110A Whole Rock 2.0042 1.34092 4.48 x 10-11 69.5 12.84 ± 0.22

27 DW-82B Whole Rock 2.257 2.1247 4.749 x 10-11 50.2 12.09 ± 0.21

28 DW-76 Whole Rock 2.028 1.47841 4.287 x 10-11 70.5 12.15 ± 0.22

29 DW-91A Whole Rock 1.7522 1.41198 3.802 x 10-11 15.9 12.47 ± 0.50

30 GJS-14A Whole Rock 2.1011 2.06487 4.77 x 10-11 43 13.03 ± 0.31

31 40Ar/39Ar Age determination - see Table 2a.

32 40Ar/39Ar Age determination - see Table 2a.

33 DW-121A Whole Rock 1.434 2.30718 3.63 x 10-11 57.7 14.53 ± 0.33

34 DW-114 Whole Rock 0.8406 1.89862 2.19 x 10-11 51.3 14.99 ± 0.45

Decay constants: λε + λε' = 0.581 x 10-10yr-1 ; λß = 4.962 x 10-10yr-1; λ = 5.543 x 10-10yr-1

40K/Ktotal = 1.167 x 10-4 (40Ar* refers to radiogenic component)

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Table 3b. 40Ar/39Ar analytical data. See Figure 4 and Table 2 for sample locations.

Location Number

Material Dated

Sample Number J 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar %(39Ar)Ca %(36Ar)Ca %40Ar* Age

(Ma) ±1σ

31 Plagioclase GJS-31 5.46E-03 1.61 2.49 0.002 0.17 31.6 74.5 11.84 0.53

1.46 1.25 0.0006 0.08 54.7 94.5 13.58 0.32

1.44 1.93 0.001 0.13 47.5 88.6 12.53 0.5

9.55 2.61 0.0282 0.18 2.4 14.9 14.01 1.32

2.13 2.74 0.0035 0.18 20.3 61.3 12.81 1.66

1.54 2.32 0.0013 0.16 46.9 86.9 13.14 0.27

2.08 2.27 0.0028 0.15 20.6 67.9 13.9 0.27

Average: 13.12

Standard Deviation: 0.25

Weighted Average: 13.31

Uncertainty: 0.25

32 Plagioclase DW-119 3.30E-04 36.83 12.19 0.053 0.82 6.35 60.3 13.3 1.15

54.75 13.31 0.107 0.92 3.5 43.9 14.37 0.7

77.86 13.51 0.188 0.91 2.03 30 13.98 1.12

280.58 13.2 0.892 0.89 0.41 6.4 10.82 2.96

103.38 14.54 0.284 0.98 1.44 20 12.39 1.32

94.51 14.17 0.248 0.95 1.55 23.7 13.42 1.82

Average: 13.05

Standard Deviation: 1.28

Weighted Average: 13.73

Uncertainty: 0.42

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Table 4. Major-element chemical analyses on volcanic rocks within the Eagle Lake 30’ x 60’ quadrangle, California. See Figure 4 and Table 2 for sample locations.

Location Number 1 2* 3 4 5 6 7 8 9 Sample Number YAD-2 GJS-1 TAD-2 GAD-2 YAD-3 GAD-8 BAD-1 YAD-1 BAD-5 Map Unit Qbma Qscb Thab Tcla Tsvb Tsb Tlpa Tbda Ttlb

Oxide Whole-rock analyses (weight percent) SiO2 56 49.97 53.67 54.26 52 48.32 57.1 58.5 52 Al2O3 18 17.8 18.35 17.92 17 17.69 17.93 16.3 16.6 Fe2O3 6.5 9.62 - - 9.1 - - 6.7 - FeO - - 7.4 7.44 - 9.04 6.85 - 8.35 MgO 4.6 6.85 5.22 5.53 5.6 9.27 2.65 2.7 3.6 CaO 7.4 9.87 7.51 7.89 9.1 11.35 5.68 6.2 5.42 Na2O 3.7 3.63 4.16 3.7 3.5 2.44 4.05 3.4 4.15 K2O 2 0.78 1.69 1.7 1.1 0.17 1.69 2.35 1.65 TiO2 0.86 1.07 1.62 1.39 1.2 0.87 0.86 0.87 1.25 P2O5 - 0.28 0.5 0.57 - 0.14 0.14 - 0.3 MnO - 0.16 0.13 0.13 - 0.17 - - -

Total 99.06 100.03 100.25 100.53 98.6 99.46 96.95 97.02 93.32

Location Number 10 11 12* 13 14 15 16 17 18 Sample Number BAD-3 GAD-7 DW-103 BAD-2 TAD-3 YAD-5 TAD-1 TAD-4 TAD-5 Map Unit Tcsb Tmr Topa Taob Tncb Tgpi Tsmf Tbha Tlgb

Oxide Whole-rock analyses (weight percent) SiO2 49 72.2 61.38 52.9 48.82 53 62.44 57.3 52.19 Al2O3 17.4 14.12 16.23 17.93 17.05 19 16.79 17.46 16.67 Fe2O3 - - 6.19 - - - - - - FeO 8.41 1.38 - 8.2 9.24 - 5.64 6.86 8.59 MgO 2.65 0.5 3.5 2.6 9.83 2.7 2.75 4.3 7.36 CaO 6.25 1.86 5.88 8.87 11.11 8.8 5.27 7.21 8.31 Na2O 3.4 3.13 3.43 4.15 2.41 3.4 3.91 3.61 2.98 K2O 0.99 4.13 2.17 1.28 0.23 1.2 2.48 1.57 0.81 TiO2 1.31 0.17 0.84 1.2 0.87 0.78 0.82 0.9 0.86 P2O5 0.42 0.1 0.28 0.12 0.1 - 0.24 0.32 0.23 MnO - 0.04 0.09 - 0.17 - 0.11 0.12 0.14

Total 89.83 97.63 99.99 97.25 99.83 88.88 100.45 99.65 98.14

*Total Fe reported as Fe2O3 for these samples.

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Table 4 (continued). Major-element chemical analyses on volcanic rocks within the Eagle Lake 30’ x 60’ quadrangle, California. See Figure 4 and Table 2 for sample locations.

Location Number 19 20 21 22 23 24* 25* 26* Sample Number CEGar1 CEGar2 GJS-2 K-215 GJS-41 GAD-245 GJS-40 DW-110A Map Unit Tcsa Tswt Tcoa Tsri Ttpw Twra Ttba Tmaf

Oxide Whole-rock analyses (weight percent) SiO2 58.56 55.29 54.06 Al2O3 16.68 18.18 16.64 Fe2O3 6.78 7.69 9.22 FeO - - - MgO Not Not Not Not Not 3.88 4.42 5.08 CaO available available available available available 6.21 7.84 7.86 Na2O 3.86 3.85 3.12 K2O 2.32 1.38 2.02 TiO2 0.93 0.92 1.32 P2O5 0.67 0.3 0.53 MnO 0.11 0.12 0.16

Total 100 99.99 100.01

Location Number 27* 28* 29* 30* 31 32* 33* 34 Sample Number DW-82B DW-76 DW-91A GJS-14A GJS-31 DW-119 DW-121A DW-114 Map Unit Tsha Tsha Tasa Tjrb Tdrt Tdrt Tsub Tbua

Oxide Whole-rock analyses (weight percent) SiO2 55.89 54.89 52.71 59.7 65.41 51.25 Al2O3 18.23 18.67 16.93 16.59 16.89 15.83 Fe2O3 7.88 7.81 9.2 6.9 6.23 9.84 FeO - - - - - - MgO 3.53 4.34 5.39 3.47 Not 2.44 9.18 Not CaO 7.08 6.85 8.11 6.77 available 3.23 8.87 available Na2O 3.7 3.87 3.5 3.01 1.71 2.57 K2O 2.29 2.1 1.63 2.24 2.81 1.11 TiO2 1 0.99 1.51 0.92 1.02 0.95 P2O5 0.3 0.34 0.82 0.3 0.15 0.27 MnO 0.12 0.13 0.2 0.1 0.09 0.13

Total 100.02 99.99 100 100 99.99 100

*Total Fe reported as Fe2O3 for these samples.

Chemical analyses on samples labeled DW and GJS as well as GAD-245 were performed by Dennis Fox, Department of Geology, San Jose State University.

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Table 5. Terminology used in the description of volcanic rocks.

DESCRIPTIVE TERMINOLOGY

Porphyritic volcanic rock features Abundantly porphyritic -- phenocrysts >20% (volume) Moderately porphyritic -- phenocrysts 5-20% Sparsely porphyritic -- phenocrysts <5% Coarsely porphyritic -- phenocrysts >5 mm (size) Finely porphyritic -- phenocrysts 1-5 mm Microporphyritic -- phenocrysts <1 mm Volcanic rock names Basalt -- <52% SiO2 (weight %) Mafic andesite -- 52-57% Andesite -- 57-63% Dacite -- 63-68% Rhyodacite -- 68-72% Rhyolite -- >72% Example - sparsely finely olivine porphyritic mafic andesite; abundantly coarsely plagioclase, sanidine porphyritic dacite.

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