Geologic Map of the , , and Upper Area,

By Robert L. Christiansen, Michael A. Clynne, and L.J. Patrick Muffler

Pamphlet to accompany Geologic Investigations Series I–2723

2002

U.S. Department of the Interior U.S. Geological Survey INTRODUCTION event and subsequent eruption. Mixing of hot and its with cool host dacitic magma and its The Lassen Peak, Chaos Crags, and upper Hat Creek phenocrysts caused partial resorption of the host dacitic phe- map area lies near the southern end of the in nocrysts, quenching of the mixed magma to form undercooled (fig. 1, see map). The map area includes inclusions, and circulation and disaggregation of inclusions parts of the three elements that together form the Lassen back into the host dacitic magma (Clynne, 1999). These proc- volcanic center (Clynne, 1990): the Lassen dacitic dome field, esses sometimes created multiple generations of disequilib- the Central andesitic field, and the underlying rium assemblages. Because phenocryst deeply eroded and partly altered Brokeoff andesitic stratocone abundance, character, and assemblage can vary within indi- (fig. 2, see map). The Lassen volcanic center is the south- vidual domes, intradome variation can be greater than ernmost active long-lived center of the present-day Cascades some interdome variation. . Additionally, andesitic, mafic-andesitic, and The following unit descriptions, intended mainly to basaltic have extruded through regionally distributed facilitate use of this map in the field, are based mainly on meg- monogenetic vents and short-lived shield volcanoes at the ascopic criteria. Typically the unit descriptions do not in- edges of the map area. Figure 2 also shows the local setting clude details of subtle variations in the appearance of the and the names of localities and other features referred to in rocks. For example, most Lassen have two popula- the Description of Map Units. tions of dacitic phenocrysts—one normal and a second partly This geologic map contributes to understanding the resorbed—but it is usually difficult to distinguish resorbed youngest major volcanic events in the evolution of the Lassen phenocrysts in hand specimen. The abundance, distribution, dacitic dome field and provides the basis for a revised as- and character of undercooled mafic-magma inclusions (called sessment of its hazards by emphasizing the young- simply “mafic inclusions” below), though commonly varied est eruptive products of the dome field. Although specific within individual dacites, can sometimes be used to distin- interpretation of these youngest events and eruptive prod- guish units. ucts will be undertaken in separate publications, the most Common mineral names are used in the descriptions significant events are briefly noted here. The most recent even though they may not always be compositionally accu- eruptive activity, mainly steam-blast eruptions, occurred in- rate. For example, many orthopyroxenes may actually be termittently between May, 1914, and June, 1917, and cli- bronzite rather than hypersthene; the name is used maxed during a week of magmatic eruptions in May, 1915. for all calcic . Mineral rock-name modifiers are A separate large-scale geologic map of the Lassen Peak sum- given in order of increasing abundance. Phenocryst abun- mit area emphasizes some features of the May, 1915 activ- dances are estimated, based on the most common lithology ity. The next youngest eruptions, about 1,100-1,000 years or range of lithologies in a unit; “sparse” means less than ago, formed the six Chaos Crags volcanic domes (fig. 2) and 1%, and “trace” means much less than 1%. Oxide minerals, related pyroclastic deposits; a rapid sequence of major slope primarily titanomagnetite, ubiquitous but generally sparse, failures on the northernmost of these domes about 300 years are not included in the descriptions. ago produced a large avalanche deposit known as Chaos All units shown on the geologic map are of Quaternary Jumbles. The map also shows other, older dacite to age. All 14C ages referred to in the text are given as reported domes and flows of the Lassen dome field (including Lassen by the analyst in radiocarbon years BP (relative to 1950), Peak, the largest dome) as well as some mainly andesitic not corrected to calendar years. Symbols in parentheses on lavas of Central Plateau, Brokeoff, and regional provenance. the map indicate units buried beneath the unit identified by Also shown are various surficial sedimentary deposits, in- the color and pattern. The initial letter of each map symbol cluding moraines and till sheets representing three glacia- indicates the dominant lithology of the unit, as follows: tions of late age, laid down contemporaneously with much of the volcanic activity. Volcanic units:

All the dacites in the Lassen dome field belong to a b, (<53% SiO2); all basalts in the map area single suite of lavas that formed through a continuum of are diktytaxitic low-K2O types designated as magma-mixing processes, resulting in the varied appearance tholeiitic and composition of many individual rock units. Thus, lavas m, mafic (<57% SiO2) with distinct compositions may be quite similar in appear- a, andesite (<63% SiO2) ance while different occurrences of the same eruptive unit d, dacite (<68% SiO2) may appear quite different. Lassen dacitic evolved r, (<72% SiO2) and rhyolite ( 72% SiO 2) by the introduction of mafic magma into a reservoir p, pyroclastic deposits (pf, pyroclastic-flow containing phenocrysts. The fundamental variables deposits) controlling the character of the erupted magma were rela- Sedimentary units: tive proportions of mafic and silicic components in the re- c, colluvium, including talus sulting mixtures and the length of time between a mixing f, fluvial deposits, including most alluvium

1 l, lacustrine deposits Formed by debris flows onto all sides from o, glacial outwash deposits the steep upper slopes of Lassen Peak s, and avalanche deposits after the major eruption of May 22, t, till 1915 w, mudflow and other debris-flow deposits p22 Pumice-fall deposit of May 22, 1915, from Lassen

Peak—65-68% SiO2. Generally well-sorted ACKNOWLEDGMENTS unconsolidated thick to thin beds consist- ing of blocks, , and ash of dacitic Data critical to the preparation and stratigraphic inter- pumice; blocks prismatically fractured and pretation of this map were provided by a number of U.S. commonly disintegrated in place. Bedding Geological Survey scientists. Dates by K/Ar and 40Ar/ 39Ar generally indistinct. Pumice blocks typically geochronology were produced by G.B. Dalrymple, B.D. banded between dark-gray andesite and light- Turrin, and M.A. Lanphere; 14 C dates were provided by D.A. gray to nearly white dacite; lapilli and coarse Trimble. D.E. Champion provided equally critical paleomag- ash commonly yellowish-brown with faint netic data. Tracey Felger helped throughout the process of banding. Thickness where deposit is distin- digital map compilation. Sherman Wu arranged for produc- guished on the map generally exceeds 2 m. tion of the large-scale base map of the Lassen Peak summit Mapped only on the northeast and east side area. Permission for this study in Lassen Volcanic National of Lassen Peak; not mapped separately where Park was arranged through the enthusiastic cooperation of preserved only in small patches or where the , especially Park Superintendents thickness is less than a few decimeters William Stephenson, Gilbert Blinn, and Marilyn Parris and wf22 Fluid debris-flow deposits of May 22, 1915, from Chief Park Naturalists Richard Vance, Scott Isaacson, —Largely nonsorted uncon- Elizabeth Knight, and Russ Lesko. solidated nonbedded deposits containing lapilli and blocks of dacite flow of May 19- 20 (unit d19), dacite dome of May 14-19 DESCRIPTION OF MAP UNITS (unit d14), and dacite of Lassen Peak (unit dlp) as large as 3 m across in a sandy to silty matrix. Also contain conspicuous banded pumice of the May 22 eruption (unit p22) p17 Pyroclastic deposit of May and June, 1917, from but less abundantly than in the viscous debris- Lassen Peak—Generally poorly sorted partly flow deposits of May 22, 1915 (unit wv22). indurated thin to thick beds of lithic ash, Individual, apparently rootless, small flows lapilli, and blocks. Matrix generally very fine, emerge from the pyroclastic-flow deposit of indurated, and pale yellowish-brown. Consists May 22 (unit pf22) and merge downslope mainly of dacite of Lassen Peak (unit dlp). into a generally sheetlike deposit as thick Thins abruptly from several meters on the as 2-3 m, tapering to imperceptibility at the crater rim to a few centimeters at the mapped margins. Occur in the Devastated Area (fig. edge of the deposit. Ejected in steam-blast 2) on the northeast side of Lassen Peak and eruptions from a crater in the northwestern in the drainages of Lost Creek and Hat Creek part of the Lassen Peak summit area, mainly pf22 Pyroclastic-flow deposit of May 22, 1915, from during May, 1917. Best displayed on the Lassen Peak—Largely nonsorted unconsoli- summit-area geologic map dated nonbedded deposit containing lapilli wv22 Viscous debris-flow deposits of May 22, 1915, and blocks of dacite of May 19 (unit d19), from Lassen Peak—Largely nonsorted un- dacite of May 14 (unit d14), and dacite of consolidated nonbedded deposits containing Lassen Peak (unit dlp) as large as 3 m lapilli and blocks, the largest of which are diameter in a sandy to silty matrix. Also greater than 1 m across, in a sandy to silty contains conspicuous banded pumice of the matrix. Contain conspicuous clasts of banded May 22 eruption (unit p22) and abundant pumice of the May 22 eruption (unit p22) fragments of wood, some of it charred. in addition to dacite of May 19 (unit d19), Ranges in thickness from a feather edge to dacite of May 14 (unit d14), and dacite of at least 3 m. Occurs in the Devastated Area Lassen Peak (unit dlp). Recognized mainly on the northeast side of Lassen Peak and by generally lobate form with scarp-like grades into fluid debris-flow deposits of May margins as much as 2 m high; thickness 22 (unit wf22), as noted in description of ranges from a few decimeters to about 3 m. that unit. The original margins of this deposit

2 largely coincide with the edge of the trees avalanche deposit of May 19 (unit s19). De- that were left standing after the eruption posits occur in the Devastated Area on the of May 22 (magenta dash-dot line on the map) northeast side of Lassen Peak (fig. 2), in Lost d19 Dacite flow of May 19-20, 1915, on Lassen Creek, and eastward across a low divide into

Peak—64-65% SiO2. - the drainage of Hat Creek. Deposits com- hornblende dacite with uneroded rough block- monly rooted within the avalanche deposit lava flow surface having 5-10 m of relief. of May 19 (unit s19); near the margins, Groundmass dark-gray to black and typically numerous standing trees predating the de- glassy. Phenocrysts: 20% 3-5 mm posits are scarred from collisions with across, but conspicuous composite crystals boulders and logs carried in the debris flows as large as 12 mm; 3% hornblende 1-5 mm s19 Avalanche deposit of May 19, 1915, from Lassen across, rarely as large as 12 mm; 3% biotite Peak—Nonsorted unconsolidated nonbedded 1-3 mm across; 2% generally 1-3 mm debris consisting of dacite dome of Lassen across, rarely as large as 6 mm; trace of ~0.75- Peak (unit dlp), dacite dome of May 14-19 mm . Abundant microphenocrysts of (unit d14), pumice of Chaos Crags (unit pc), and hypersthene as much as 0.5 mm and much wood debris, including numerous across. Plagioclase, biotite, hornblende, and decayed logs lying in positions pointing quartz commonly partly resorbed. Mafic downslope, away from the summit of Lassen inclusions range in size from a few centi- Peak. Generally lies on soil developed in meters to about 50 cm, form about 5% of underlying deposits, including pumice of the rock, and contain partly resorbed host- Chaos Crags. Deposited from a mixed snow rock phenocrysts (conspicuously, - and debris avalanche originating in cirque rimmed quartz) and 0.75-mm olivine. Most at top of northeast face of Lassen Peak. inclusions have pyroxene and plagioclase Deposits occur in the Devastated Area on microlites containing interstitial glass, are the northeast side of Lassen Peak (fig. 2) microvesicular, and lack crenulate margins; and eastward across a low divide into the inclusions that have crenulate margins and drainage of Hat Creek. The margins of the lack partly resorbed host-rock phenocrysts avalanche (cyan dash-dot line on the geo- are rare. Erupted from vent at the summit logic map) were marked by the trees left of Lassen Peak and flowed about 300 m down standing before the eruption of May 22 and the western and northeastern flanks; the recorded in photographs taken on that day northeast lobe was swept away in the eruption before the eruption of May 22 and incorporated into pyroclastic- p19 Pyroclastic deposit of May 19, 1915, on Lassen flow and fluid debris-flow deposits of May Peak—Nonsorted unconsolidated nonbedded 22 (units pf22 and wf22). Best displayed on deposit of blocks to fine lithic ash, consisting the summit-area geologic map entirely of dacite dome of May 14-19 (unit f19 Flood deposits of May 19-20, 1915, in Hat d14) and dacite of Lassen Peak (unit dlp). Creek—Sorted to nonsorted unconsolidated Ranges in character from a deposit about generally nonbedded sand to boulders and 4 m thick to a field of discontinuous blocks. other debris in Hat Creek downstream from Ejected explosively during opening of a the bedrock gorge just north of the Lassen summit crater through the dacite dome of Volcanic National Park Boundary. Scored May 14-19 and preserved only in summit by numerous braided stream channels, most area of Lassen Peak. Best displayed on the of them abandoned. The amount of sediment summit-area geologic map decreases progressively downstream, but the d14 Dacite dome of May 14-19, 1915, on Lassen

affected area remains complexly scoured and Peak—64-65% SiO2. Remnants of a small marked by chaotic jams of boulders, logs, dacite dome of porphyritic biotite-hornblende and other debris dacite. Groundmass dark-gray to black and w19 Debris-flow deposits of May 19-20, 1915, from typically glassy. Phenocrysts: 20% plagio- avalanche deposit—Nonsorted unconsoli- clase 3-5 mm across, but conspicuous com- dated generally nonbedded sand to boulders posite crystals as large as 12 mm; 3% as large as 3 m diameter. Consist almost hornblende 1-5 mm across, rarely as large entirely of dacite of Lassen Peak (unit dlp), as 12 mm; 3% biotite 1-3 mm across; 2% dacite dome of May 14-19 (unit d14), and quartz 1-3 mm across, rarely as large as 6 some logs derived from the underlying mm; trace of ~0.75-mm olivine. Abundant

3 microphenocrysts of augite and hypersthene biotite 1-3 mm across (rarely larger); 4% to 0.5 mm. Partly resorbed plagioclase, biotite, hornblende 2-5 mm across (less commonly hornblende, and quartz commonly are more as large as 15 mm); 2% quartz 2-5 mm across. abundant than in the slightly younger lava Quartz is conspicuous and more abundant flow (unit d19). Mafic inclusions from a few than in any other rocks of the Lassen vol- millimeters to about 50 cm across form about canic center except dacite of Lassen Peak 5% of the rock and contain partly resorbed host- (unit dlp); pyroxene-rimmed quartz pheno- rock phenocrysts (conspicuously, pyroxene- crysts are particularly conspicuous. Abun- rimmed quartz) and ~0.75-mm olivine. Most dances of partly resorbed phenocrysts, inclusions have coarse-grained pyroxene and millimeter- to centimeter-sized fragments of plagioclase microlites and interstitial glass, mafic inclusions, and larger mafic inclusions are microvesicular, and lack crenulate mar- are much more abundant in the upper four gins; inclusions having crenulate margins domes (units rcc through rcf) than in the and lacking resorbed host-rock phenocrysts lower domes (units rca and rcb) or the py- are rare; millimeter- to centimeter-sized roclastic deposits (unit pc). Contains sparse inclusions more abundant in the olivine and calcic-plagioclase xenocrysts than in the lava flow (unit d19). The dome, derived from disaggregated mafic inclu- emplaced between about May 14, 1915 and sions. Textures of mafic inclusions vary the night of May 19-20, 1915, filled a crater from porphyritic to sparsely phyric; porphy- in the summit area of Lassen Peak that had ritic inclusions contain phenocrysts inher- been excavated by earlier steam-blast ex- ited from their rhyodacite host, and sparsely plosions; the dome was disrupted by a single phyric inclusions generally contain olivine large explosion on the evening of May 19- and calcic-plagioclase phenocrysts 1-2 mm 20, 1915, to form the slightly younger long inherited from their mafic parent. A pyroclastic deposit (unit p19). Remnants of few are multiple-generation mafic inclusions this dome, confined to the summit area of (inclusions that contain inclusions). Most Lassen Peak, are best shown on the summit- common inclusions have hornblende-plagio- area geologic map clase groundmasses; others have pyrox- scj Avalanche deposits of Chaos Jumbles, from ene± hornblende-plagioclase groundmasses. dome C of Chaos Crags—Nonsorted un- Mafic inclusions abundant (to 10% of the consolidated nonbedded coarse to fine rubble rocks) in domes C-F (units rcc-rcf); com- containing blocks as large as 5 m across. monly as large as 20 to 50 cm across, less Consists almost entirely of pink oxidized commonly to 1 m across blocks of rhyodacite dome C of Chaos Crags ccf Talus, emplaced hot from dome F of Chaos (unit rcc); also contains rare gray prismatically Crags—Laterally sorted nonbedded blocks jointed blocks from dome C as well as andesite of rhyodacite from dome F of Chaos Crags from Brokeoff volcano (unit abk). Deposit (unit rcf). Largest blocks, as large as 4 m, is lobate, has run up onto adjacent topographic are prismatically jointed and commonly dis- highs, and has transverse and longitudinal integrated in place after deposition. Formed debris ridges on its surface and scarp-like by hot rockfalls from dome F during its margins generally 1-5 m high. Formed by emplacement catastrophic collapse of dome C of Chaos sce Avalanche deposit from partial collapse of dome Crags (unit rcc). Trees drowned by Man- E of Chaos Crags—Nonsorted unconsoli- zanita Lake, which formed as this avalanche dated nonbedded deposit consisting of fine dammed Manzanita Creek, dated as 275±25 granular matrix to blocks as large as about 14C years (Clynne and Muffler, 1989) 2 m, some of them prismatically jointed, rcf Rhyodacite dome F of Chaos Crags—68% SiO2. consisting entirely of rhyodacite from dome Porphyritic hornblende-biotite rhyodacite; E of Chaos Crags (unit rce); emplaced hot. one of 6 lava domes emplaced sequentially Many large blocks internally fractured. (units rca through rcf) that are lithologically Formed by partial collapse of dome E of nearly identical. Generally dense rhyodacite Chaos Crags before emplacement of dome ranges from white or light-gray and glassy F in the resulting avalanche scar

to light- to medium-gray and devitrified. rce Rhyodacite dome E of Chaos Crags—68% SiO2. Phenocrysts: 25% plagioclase 2-5 mm across Porphyritic hornblende-biotite rhyodacite. (composite crystals as large as 1 cm); 4% Lithologically nearly identical to rhyodacite

4 dome F (see description of unit rcf) but Lithologically similar to rhyodacite dome morphologically distinct; dome F partly fills F (see description of unit rcf), except as the scar left by partial collapse of dome E follows: weakly pumiceous; mafic inclusions to produce a hot avalanche deposit (unit sce) generally less abundant (generally < 1%) and cce Talus, emplaced hot from dome E of Chaos smaller (10-20 cm across) than in domes C Crags—Laterally sorted nonbedded blocks through F of rhyodacite from dome E of Chaos Crags ccb Talus, emplaced hot from dome B of Chaos (unit rce). Largest blocks, as large as 4 m Crags—Laterally sorted nonbedded blocks across, are prismatically jointed and com- of rhyodacite from dome B of Chaos Crags monly disintegrated in place after deposi- (unit rcb). Largest blocks, as much as 4 m tion. Formed by hot rockfalls from dome E across, are prismatically jointed and com- during its emplacement monly disintegrated in place after deposi- scd Avalanche deposit from partial collapse of dome tion. Formed by hot rockfalls from dome B D of Chaos Crags—Nonsorted unconsoli- during its emplacement

dated nonbedded deposit consisting of fine rca Rhyodacite dome A of Chaos Crags—70% SiO2. granular matrix to blocks as large as about Porphyritic hornblende-biotite rhyodacite. 2 m across, some of them prismatically Lithologically indistinguishable from rhyo- jointed, consisting mainly of rhyodacite from dacite dome B (see description of unit rcb). dome D of Chaos Crags (unit rcd) but This small dome is partly mantled by py- containing some pumiceous blocks from roclastic deposits (unit pc) and largely buried dome B (unit rcb). Many large blocks in- by dome B ternally fractured. Formed by partial collapse pc Pumiceous pyroclastic-flow and fall deposits of of dome D of Chaos Crags Chaos Crags—Several individually recog- rcd Rhyodacite dome D of Chaos Crags—68% SiO2. nized units can be distinguished in strati- Porphyritic hornblende-biotite rhyodacite. graphic sections but cannot be shown Lithologically nearly identical to rhyodacite separately at the scale of the geologic map. dome F (see description of unit rcf) Petrographically the pumice of this unit ccd Talus, emplaced hot from dome D of Chaos resembles the dacite of domes A and B (units Crags—Laterally sorted nonbedded blocks rca and rcb); present in addition to the of rhyodacite from dome D of Chaos Crags predominant pumice are dense glassy blocks (unit rcd). Largest blocks, as much as 4 m and lapilli of rhyodacite that lithologically across, are prismatically jointed and com- resemble domes A and B, as well as sparse monly disintegrated in place after deposi- accidental lithic inclusions. Fall deposits of tion. Formed by hot rockfalls from dome D generally well-sorted unconsolidated medium- during its emplacement to thin-bedded nearly white rhyodacitic rcc Rhyodacite dome C of Chaos Crags—68% SiO2. pumice typically mantle the surface and are Porphyritic hornblende-biotite rhyodacite. locally reworked as colluvium. Deposits range Lithologically indistinguishable from rhyo- from near-source blocks as large as about dacite dome F (see description of unit rcf). 1 m to distal small lapilli and ash and from A small area designated as rcc(?) between as much as 35 m thick at vent cones near domes D and E (units rcd and rce) appears south margin of Chaos Crags dome cluster to preserve a short segment of an otherwise to less than 1 m thick distally. Most pum- buried crater rim and dome flank that pre- ice fall postdates emplacement of rhyodacite date the adjacent domes and is inferred to dome A (unit rca) but predates dome B (unit be part of dome C rcb). Beneath these fall deposits are nonsorted ccc Talus, emplaced hot from dome C of Chaos unconsolidated pumiceous pyroclastic flows. Crags—Laterally sorted nonbedded blocks Three distinct pyroclastic-flow units are of rhyodacite from dome C of Chaos Crags recognized. The stratigraphically highest (rcc). Largest blocks, as much as 4 m pyroclastic-flow unit is characterized by across, are prismatically jointed and com- coarse-celled pumice that forms breadcrusted monly disintegrated in place after depo- blocks as large as 4 m across in a white to sition. Formed by hot rockfalls from dome gray glassy ash matrix. This uppermost C during its emplacement pyroclastic flow occurs widely in the area rcb Rhyodacite dome B of Chaos Crags—70% SiO2. around Chaos Crags and locally in the Porphyritic hornblende-biotite rhyodacite. drainages of Manzanita Creek and Lost Creek,

5 overlies dome A (unit rca) but underlies dome underlying swampy ground within or adjacent B (unit rcb), and includes charcoalized wood to seasonally flooded areas. Mapped only dated as 1,062±14 14C years (Clynne and in two small areas: on Table and Muffler, 1989). The lower two pyroclastic- near upper Manzanita Creek. Age may range flow units resemble one another, each having throughout the Holocene fine-celled pumice blocks as much as 30 cm ch Colluvium and talus of Holocene age—Nonsorted across in a pumiceous ash matrix. Each is to laterally sorted unconsolidated nonbedded generally gray to pale yellow but has a pink coarse to fine rubble. Mainly occurs as talus top as much as 2 m thick. A pink laminated at the base of cliffs or steep slopes of bedrock fine-ash deposit commonly 5-20 cm thick but locally occurs as slopewash or thin local overlies the upper of these two flow units. debris flows. Deposit mapped only where These two lower pyroclastic-flow units it obscures underlying bedrock completely predate dome A (unit rca), are exposed mainly or consists mainly of different lithology than in the upper Manzanita Creek and upper Lost underlying bedrock. Age ranges throughout Creek drainages, and include charcoalized the Holocene wood dated as 1,124±15 14C years (Clynne (sh) Landslide deposits of Holocene age—Nonsorted and Muffler, 1989) unconsolidated nonbedded fine to coarse whe Debris-flow deposits from the northeast side rubble in small slumps on unconsolidated of Lassen Peak—Poorly sorted, generally slopes. Shown only as small concealed units unconsolidated, massively bedded rubble east of Chaos Crags and east of Emigrant ranging from silt-sized to large blocks. Con- Pass (fig. 2). Age ranges throughout the sists entirely of dacite of Lassen Peak (unit Holocene dlp). Commonly pinkish in color but ranges from pink to gray, reflecting the lithology LATE PLEISTOCENE of source dacite on different parts of the Lassen Peak dome. Thickness ranges from tal Late till of Anklin Meadows—Diamicton con- less than 1 m to as much as 6 m. Formed taining boulders as much as 4 m across, con- as mudflows from the steep high northeastern sisting of locally derived dacite. Occurs as slopes of Lassen Peak (northeast corner of small moraines at elevations of about 7,600- the summit-area geologic map) and depos- 9,000 ft on the east and south sides of Lassen ited in the drainage of upper Lost Creek. Soil Peak; also present (buried beneath pumice underlying the deposit dated as 8,130±100 of Chaos Crags, unit pc) in Crescent Cra- 14C years; soil developed on the surface of ter and at elevation about 7,600 ft in a small the unit dated as 7,550±50 14C years (Turrin cirque east of Chaos Crags. This unit is

and others, 1998) equivalent to most of unit A3 of Turrin and the Till or protalus-rampart debris of early Holo- others (1998). It is undated but older than cene age—Poorly sorted unconsolidated 8-ka debris-flow deposits from northeast side nonbedded diamicton consisting of silt to of Lassen Peak (unit whe) and till of early boulders of dacite of Lassen Peak (unit dlp). Holocene age (unit the); probably of latest Forms two small moraines or ramparts at Pleistocene age, about 12 ka elevations between 7,800 and 9,000 ft near ta Till of Anklin Meadows—Widespread diamicton the base of Lassen Peak on the southeast containing boulders commonly as much as and northeast sides. Older than 8-ka debris- 2 m across, locally as large as 4 m, consisting flow deposits from northeast side of Lassen of varied volcanic lithologies. In the valleys Peak (unit whe) and younger than probably of Manzanita Creek and Lost Creek, where 12-ka late till of Anklin Meadows (unit tal). the glaciers descended from Lassen Peak,

This unit is equivalent to unit A4 of Turrin boulders consist largely of 27-ka dacite of and others (1998) Lassen Peak (unit dlp). Occurs as sheetlike fh Fluvial deposits of Holocene age—Moderately till deposits and large moraines at elevations well-sorted unconsolidated lenticular- as low as about 6,100 ft in the valleys of bedded sand and gravel in modern stream Manzanita Creek and Hat Creek and as low channels. Age ranges throughout the Holo- as about 5,500 ft in the valley of Lost Creek.

cene Equivalent to most of units A1 and A2 of Turrin lh Lacustrine deposits of Holocene age—Well-sorted and others (1998); correlated with last major unconsolidated silt to fine sand, typically glacial advance of the Pleistocene (oxygen-

6 isotope stage 2), probably between about 25 glacial ice in the valley of Hat Creek after and 17 ka partial glacial retreat from terminal moraine oa Outwash deposits coeval with till of Anklin of the till of Raker Peak (unit tr) Meadows—Moderately sorted unconsoli- pflp Pyroclastic-flow deposit from partial dome col-

dated gravel and sand, commonly contain- lapse of Lassen Peak—68% SiO2. Poorly ing boulders as large as 1-2 m across sorted unconsolidated nonbedded volcanic consisting of the same lithologies as the till ash to blocks as large as 3 m across, consist- of Anklin Meadows (unit ta) in the same ing of porphyritic, gray hornblende-biotite drainages. Occur as partial valley fills and dacite petrographically identical to dacite of alluvial fan in the drainages of Lost Creek Lassen Peak (unit dlp); blocks commonly and Hat Creek prismatically jointed. Forms incised valley fa Alluvium coeval with till of Anklin Meadows— fill in upper Lost Creek, at the northeast base Moderately well-sorted unconsolidated of Lassen Peak. Thickness ranges from feather lenticular-bedded sand and gravel. Mapped edge to as much as 50 m. Formed by dome- only in a few drainages without permanent collapse pyroclastic flows during emplace- streams or in terraces above modern stream ment of the Lassen Peak dacite dome. Dated channels, adjacent to or between the lower by 40Ar/39Ar method as 28.3±2.7 ka; paleo- courses of Manzanita, Lost, and Hat Creeks magnetic correlation suggests an age of 27±1 clp Talus of Pleistocene age on Lassen Peak— ka (Turrin and others, 1998)

Generally nonsorted unconsolidated non- dlp Dacite dome of Lassen Peak—66-70% SiO2. bedded coarse to fine rubble. Consists of Porphyritic hornblende-biotite dacite. Outer dacite of Lassen Peak (unit dlp) and cov- portion of the dome commonly oxidized to ers much of the surface of the Lassen Peak grayish pink; locally exhibits a dacite dome. Some of the talus was depos- carapace. Exposed interior is platy to mas- ited as hot blocks during emplacement of sively jointed. Phenocrysts: 20% plagioclase the dome, recognized by the presence of pris- 1-5 mm across, some composite crystals as matically jointed blocks of dacite as large large as 1 cm; 4% quartz, more abundant as 5 m across. Much was deposited later, than in any other dacite in the map area, during glaciation, , and rockfalls typically 1-2 mm across, rarely as large as from cliffs and other outcrops on Lassen Peak 5 mm; 3% biotite 1-2 mm across; 2% during the Pleistocene hornblende 1-5 mm across, rarely as large trl Post-maximum till of Raker Peak, consisting as 15 mm; sparse augite typically 0.25 to of Lassen Peak avalanche debris— about 1 mm across, composite crystals as Diamicton containing boulders as much as large as 3 mm; trace olivine generally less 1.5 m across, consisting virtually entirely than 1 mm. Commonly contains partly re- of dacite of Lassen Peak (unit dlp) but derived sorbed phenocrysts of plagioclase, horn- secondarily from avalanche debris that spread blende, biotite, and quartz. Microlites and across glacial ice (unit slp). Forms a small microphenocrysts of plagioclase, augite, hy- moraine at the northeast base of Raker Peak. persthene, and hornblende disaggregated from Deposited considerably after maximum mafic inclusions abundant in light- to medium- advance of ice that deposited most till of gray, glassy to aphanitic groundmass. Mafic Raker Peak (unit tr) inclusions particularly large and abundant, slp Avalanche debris spread across glacial ice from ranging from a few centimeters to 1.5 m Lassen Peak—Nonsorted unconsolidated across; 20- to 50-cm inclusions more abun- nonbedded angular rubble containing blocks dant than in any other dacite of the map area. commonly as large as 1-2 m across, but Textures of mafic inclusions range from fine- locally (on the east side of Raker Peak) as grained and porphyritic to coarse-grained and large as 6 m; matrix is coarse grit. Consists aphyric; sparse multiple-generation inclu- entirely of dacite of Lassen Peak (unit dlp). sions (inclusions that contain inclusions). Exposed high on the east side of Raker Peak, Porphyritic inclusions contain partly resorbed north of Raker Peak on the west side of the phenocrysts from host dacite; some also Hat Creek drainage as low as about 5,400 contain augite and olivine phenocrysts as large ft, and on the east side of the Hat Creek as a few millimeters. Fine-grained inclusions drainage at the foot of Badger Mountain. typically have hornblende-plagioclase ground- Interpreted as having been emplaced on mass but may have pyroxene-hornblende-

7 plagioclase groundmass. Most inclusions that olivine-augite andesite. Phenocrysts: 5% look aphyric contain remnants of augite and plagioclase generally 0.5 mm across but olivine phenocrysts in hornblende-plagioclase locally as large as 1.5 mm; 3% olivine 1- groundmass. Fragments of mafic inclusions 2 mm across; 2% augite 0.25-0.5 mm across. that range in size from a few millimeters Abundant cumulophyric clots ranging in size to a few centimeters are abundant. This large from 1 to 10 mm consist of all three min- (~ 2 km3) volcanic dome is interpreted as erals. The lava flow is unglaciated and a single emplacement unit because of its generally devoid of soil or vegetation but single remanent magnetic direction and lack supports a few mature trees near the mar- of internal contacts. Glaciated, with a promi- gins. Bear Wallow Butte is the southernmost nent cirque on the northeast side. Contem- of a youthful alignment of volcanoes north poraneous with pyroclastic-flow deposit from of the map area called Tumble Buttes; the partial dome collapse of Lassen Peak (unit unit is present in the map area only near the pflp), of 27±1 ka north edge. 40Ar/39Ar age is 35.1±3.1 ka (M.A. tr Till of Raker Peak—Widespread diamicton with Lanphere, written commun., 2001)

boulders commonly as large as 2 m, locally rk Rhyodacite flow of Kings Creek—70% SiO2. to 4 m, of varied volcanic lithologies. Promi- Porphyritic hornblende-biotite rhyodacite nent boulder lithologies in drainages of about 75 m thick, having steep lateral Manzanita Creek and Lost Creek include margins; glaciated, but much flow morphol- dacite of hill 8283 (unit d83) and rhyodacite ogy still preserved. Basal and lateral mar- of Eagle Peak (unit re); in drainage of Hat gins glassy, commonly perlitic, in some Creek contains prominent andesite of Badger instances brecciated; flow top pumiceous. Mountain (unit abm), andesite of Hat Moun- Flow layering and spherulitic and lithophysal tain (unit ahm), and dacite of Reading Peak zones common. Flow breccia preserved (unit dr). In all drainages notably lacks dacite locally. Phenocrysts: 15% plagioclase 1-5 of Lassen Peak, in contrast to the younger mm across, largest are composite grains; till of Anklin Meadows (unit ta) and, there- 2% biotite 1-2 mm across; 1% hornblende fore, predates formation of the Lassen Peak 1-15 mm across; sparse quartz 1-2 mm across; dome. Occurs as sheetlike till deposits and sparse 0.5-mm hypersthene. Contains only moraines at elevations as low as about 5,500 sparse partly resorbed equivalents of the ft. As discussed by Turrin and others (1998), dacitic phenocrysts, fragments of inclusions, predates 27±1 ka (age of dacite of Lassen and crystals derived from disaggregation of Peak, unit dlp) and probably predates 35±1 mafic inclusions. Mafic inclusions sparse, ka (age of rhyodacite of Kings Creek, unit rk) typically smaller than about 10 cm across or Outwash deposits coeval with till of Raker (a few to 20 cm). Mafic inclusions have Peak—Moderately to poorly sorted uncon- coarse-grained hornblende-plagioclase solidated gravel and sand, commonly con- groundmasses, partly resorbed phenocrysts taining boulders as large as 1-2 m across derived from dacite host magma, and small consisting of the same lithologies as the till olivine phenocrysts. Occurs east and southeast of Raker Peak (unit tr). Occurs as a large of Lassen Peak. Overlies pyroclastic-flow alluvial fan and downstream deposits in the deposit of Kings Creek (unit pfk) and overlain drainage of Hat Creek by dacite of Lassen Peak (unit dlp). Inferred fr Alluvium coeval with till of Raker Peak— to be coeruptive with underlying pyroclas- Moderately to well-sorted unconsolidated len- tic flows (unit pfk) at 35±1 ka. Erupted from ticular-bedded sand and gravel. Occurs in a vent now buried by Lassen Peak drainages without permanent streams. pfk Pyroclastic-flow deposit of Kings Creek—Poorly Mapped only in 2 small areas on the sur- sorted unconsolidated containing face of the andesite flow of Raker Peak (unit pumice blocks as large as about 1 m across. arp), in one area adjacent to Lost Creek, and Pumice consists of porphyritic biotite- in one area near Manzanita Creek downstream hornblende rhyodacite petrographically from identical to rhyodacite flow of Kings Creek abw Andesite flow of Bear Wallow Butte—57% SiO2. (unit rk); immediately preceded emplacement Thick block-lava flow having margins as of that lava flow. Four separate pyroclas- high as 40 m. Consists of microvesicular, tic flows or flow units are exposed in the dark-gray, glassy to aphanitic porphyritic banks of Hat Creek near Emigrant Pass (fig.

8 2). Similar deposits are exposed downstream by glaciers and, although heavily forested, in Hat Creek, upstream in the west fork of retain their talus-covered primary morphology

Hat Creek and, outside the map area, in Kings ahm Andesite flow of Hat Mountain—58-62% SiO2. Creek Meadows. Dated by 40Ar/39Ar method Large flow of porphyritic augite andesite, as 32.0±17.1 ka; paleomagnetic correlation generally about 100 m thick; vent at Hat suggests an age of 35±1 ka (Turrin and others, Mountain, about 3 km east of the map area, 1998) is marked by a breached cone of aggluti- fk Sediments beneath pyroclastic-flow deposit of nated cinders and spatter. Generally mas- Kings Creek—Well-sorted unconsolidated sively jointed and microvesicular; weathers to partially indurated sands, silts, and into rounded boulders 0.5 to several meters interbedded peat. A few beds are pebbly. in diameter. Contains a disequilibrium Thickness about 4 m. Deposited by low- phenocryst assemblage of partly resorbed energy streams and quiet-water overbank plagioclase, quartz, hornblende, and biotite? flows or in ponds. Radiocarbon age calibrated and unresorbed augite and calcic plagioclase by U-Th is 37.6±0.2 ka (Turrin and others, in varied proportions. Includes two end- 1998) member andesite types: a typical type and psf Pyroclastic-flow and proximal fall deposits of a less common type. The typical type con- Sunflower Flat—Poorly to moderately sorted tains strongly resorbed felsic phenocrysts: unconsolidated volcanic ash containing blocks 4-8% plagioclase 1-2 mm across, sparse-1% of porphyritic biotite-hornblende rhyodacite augite as large as 1 mm across, sparse-1% petrographically identical to rhyodacite domes quartz 1-2 mm across rimmed by tiny augite of Sunflower Flat (unit rsf). Maximum thick- crystals and commonly having black to ness greater than 3 m. Occurs as pyroclas- brownish specks of glass and biotite, and tic flows from Sunflower Flat to Lost Creek sparse-1% 1-4 mm pseudomorphs of horn- and as proximal fall deposits forming a blende (and biotite?). The less common ande- pumice crater southeast of Sunflower Flat. site type is more porphyritic and has the same Underlies most rhyodacite domes of Sun- phenocryst assemblage but also has many flower Flat (unit rsf) but at least partly overlies additional less strongly resorbed felsic oldest dome. Radiocarbon age calibrated by phenocrysts. Groundmass medium- to dark- U-Th is 41±1 ka (D.A. Trimble, written gray; typically aphanitic in more porphyritic commun., 1993) samples but glassy in less porphyritic samples. rsf Rhyodacite domes of Sunflower Flat—69% SiO2. Mafic inclusions are generally small (less Porphyritic biotite-hornblende-rhyodacite than about 10 cm across) and sparse, though forming 8 domes extruded in an approxi- locally more abundant; they contain the same mately linear northwest trend north of Chaos phenocryst assemblage as in the host rock. Crags. Range from dark-gray or black, dense, Small augite phenocrysts are abundant in and glassy to light-gray or pink, micro- mafic inclusions; plagioclase, hornblende, vesicular, microlite-choked, and partially and quartz phenocrysts are sparse and devitrified. Phenocrysts: 12% plagioclase 1- strongly resorbed or rimmed. Chemical com- 5 mm across, composite crystals as large as position varied, but typically contains 58-

8-10 mm; 2% hornblende 1-12 mm across 59% SiO2; more porphyritic rocks contain

(large phenocrysts are conspicuous); sparse as much as 62% SiO2. Mapped only near biotite 1-2 mm across; sparse quartz 1-2 mm east edge of the map area. The unit is not across. Microphenocrysts (1-2%) of pyroxene precisely dated, but the entire edifice has (hypersthene>>augite) abundant but generally been glaciated. On the Central Plateau east smaller than 0.5 mm though a few are as of the map area it overlies andesite of Fairfield much as 1 mm across. Abundant partly Peak (82 ka) and andesite of Crater Butte resorbed plagioclase phenocrysts and pla- (92 ka). Its presence in till of Anklin gioclase phenocrysts containing vermicular Meadows demonstrates that andesite of Hat glass inclusions like those in rhyodacite of Mountain is older than about 27 ka; pres- Krummholz (unit rkr). Common fragments of ence in till of Raker Peak (unit tr) suggests mafic inclusions range in size from a few that andesite of Hat Mountain may be older millimeters to a few centimeters; rare larger than about 35 ka. A low-precision K-Ar age inclusions are as much as 10 cm across. The of 25±21 ka (A.L. Cook, written commun., summits of the domes have not been overridden 1983) is consistent with the stratigraphy

9 acr Andesite scoria deposit on west rim of Cres- . Massive jointing forms large rectan-

cent Crater—57% SiO2. Porphyritic hyper- gular blocks. Locally vaguely flow-layered. sthene-olivine-augite andesite scoria a few Phenocrysts: 10% plagioclase mostly 2-3 mm meters thick. Phenocrysts: 10% plagioclase across, common composite crystals as large ranges from 0.25 to about 1 mm across; 5% as 1 cm across, typically with gray cores; augite 0.5-1 mm across; 2% olivine 0.25- 1% hornblende generally 1-2 mm across, 0.5 mm across; 1% hypersthene 0.5-1 mm sparsely as large as about 1 cm; sparse biotite across. Common cumulophyric clots as large 1-2 mm across; sparse quartz as large as 2 as 5 mm contain varied proportions of pla- mm across; sparse hypersthene as large as gioclase, olivine, augite, and hypersthene. 0.5 mm across; most plagioclase partly The deposit is present at only one locality, resorbed, with vermicular glass inclusions. beneath Chaos Crags pumice (unit pc) on west Mafic inclusions sparse and small (gener- rim of Crescent Crater. The eruptive source ally less than about 10 cm across); contain is unknown partly resorbed felsic phenocrysts derived dcr Dacite dome and flow of Crescent Crater—66- from the host and sparse small phenocrysts

69% SiO2. Porphyritic pyroxene-hornblende of olivine and augite. Groundmasses of mafic dacite in a petrographically and composi- inclusions contain acicular microphenocrysts tionally zoned dome and thick lava flow. The of pyroxene, hornblende, and plagioclase. lava flow is typically flow-layered; both the Occurs north of Lassen Peak and east of flow and the dome have massive to blocky Chaos Crags. The dome and flow are gla- jointing. Groundmass of the lava flow dark- ciated and mostly obscured by a thick blanket gray to black, glassy, and typically perlitic; of pumice from the Chaos Crags eruption groundmass of the dome medium-gray where (unit pc). Dated by 40Ar/39Ar method as 43±2 fresh but typically oxidized to purplish or ka (M.A. Lanphere, written commun., 1998) light-reddish. Phenocrysts: 15% plagioclase r27 Rhyodacite flow of Section 27 on the north flank

ranges from 2 mm to about 1 cm across of Lassen Peak—70% SiO2. Porphyritic (largest are composite crystals); 3% horn- biotite-hornblende rhyodacite forms thick lava blende 1-5 mm across; 1% hypersthene 0.5- flows whose original flow morphology and 1 mm across; sparse biotite 1-2 mm across; pumiceous carapace are partly preserved; trace quartz 1-2 mm across. Hornblende and locally spherulitic or flow-layered. Unit has biotite fresh in the lava flow but strongly a basal flow breccia. Dark-gray to black and oxidized in the dome; partly resorbed pla- perlitic groundmass where dense; white to gioclase phenocrysts more abundant in the colorless where pumiceous. Phenocrysts: 12% dome than in the lava flow; abundant augite plagioclase 1-3 mm across, common com- plus hypersthene microphenocrysts in the posite crystals as large as 6 mm; 3% horn- dome, only hypersthene microphenocrysts blende 1-3 mm across, rarely larger; 1% in the flow. Abundant porphyritic mafic in- hypersthene as large as 1 mm across; sparse clusions, particularly in the dome, commonly biotite generally 1-2 mm across; trace quartz; as large as 50 cm across contain 1- to 2- rare augite xenocrysts. Mafic inclusions mm olivine, augite, hypersthene, and cal- generally small and sparse and as large as cic plagioclase phenocrysts as well as partly 10-12 cm across; common fragments of mafic resorbed host-lava phenocrysts; some inclu- inclusions range in size from a few milli- sions have hornblende-plagioclase ground- meters to about 1 cm. Inclusions typically mass, others have pyroxene-plagioclase contain augite and calcic plagioclase phe- groundmass. Occurs northeast of Lassen nocrysts in addition to dacitic phenocrysts Peak; overlies rhyodacite of Krummholz (unit derived from host rhyodacite. Extrusive vent rkr, 43 ka) probably in the area now occupied by the rkr Rhyodacite dome and flow of Krummholz—69% dacite dome of Lassen Peak (unit dlp). The

SiO2. Thick flow of porphyritic biotite- lava flows are slightly to heavily glaciated. hornblende rhyodacite and a small vent dome. The unit may contain several flows of similar Groundmass light- to dark-gray and glassy; lithologies and ages. Age unknown; much perlitic and spherulitic varieties occur. Top, younger than dacite of hill 8283 (unit d83, bottom, and margins of flow are character- 261 ka) and older than rhyodacite of Krum- istically dense and glassy but devitrified mholz (unit rkr, 43 ka) and dacite of Cres- where flow surface removed by glacial cent Crater (unit dcr)

10 axe Andesitic mixed lava of Eagle Peak—61-68% centimeters across. Nearly contemporane-

SiO2. Mixed lithology of rhyodacite of ous with 66-ka pyroclastic-flow deposit of Eagle Peak (unit re) and augite-hypersthene Eagle Peak (unit pfe) andesite. Blotchy or color-banded, dark- pfe Pyroclastic-flow deposit of Eagle Peak—71%

brownish-gray, microvesicular to pumiceous. SiO2. Poorly sorted matrix-supported non- The least-mixed material approaches lithol- indurated white ash, lapilli, and yellowish- ogy of rhyodacite of Eagle Peak (unit re) weathering pumice blocks; sparse but but contains sparse mafic phenocrysts in a widespread pumice blocks as large as sev- brownish groundmass. The most-mixed eral meters across. Thickness generally about material is porphyritic andesite containing 2 m. Pumiceous pyroclastic flow erupted from about 10% phenocrysts, mainly plagioclase, a vent now covered by the Eagle Peak dome. hornblende, biotite, and quartz similar to The pumice is porphyritic biotite-hornblende those in rhyodacite of Eagle Peak, though rhyodacite similar to the rhyodacite of Eagle hornblende and biotite are somewhat oxi- Peak (unit re). Contains matrix-free mafic dized; also contains abundant 0.5-mm calcic- inclusions as lithic fragments. Occurs mainly plagioclase and sparse 0.5- to 1-mm pyroxene west of Manzanita Lake, but a similar deposit phenocrysts (hypersthene>>augite) from the several meters thick covers the summit of andesitic magma and cumulophyric clots of Ski Heil Peak and also includes dense frag- pyroxene and plagioclase. Most material is ments of rhyodacite of Eagle Peak. Dated intermediate between the two types. Poorly by 40Ar/39Ar as 66±4 ka (B.D. Turrin, written exposed in a small area of mostly oxidized commun., 1994) and altered rock between Ski Heil Peak and sd Avalanche deposit from dacite older than Eagle Peak. Precise age unknown but prob- pyroclastic-flow deposits of Eagle Peak— ably contemporaneous with last extrusion Nonsorted unconsolidated nonbedded angular of rhyodacite of Eagle Peak (unit re) rubble containing dacite blocks commonly re Rhyodacite dome and flow of Eagle Peak—71% as large as 1-2 m; matrix is coarse grit.

SiO2. Thick lava flow and vent dome of Lithology of blocks resembles dacite of hill porphyritic hornblende-biotite rhyodacite, 8283 (unit d83) and dacite of Vulcans Castle about 100 m thick; has steep lateral mar- (unit dvc). Crops out locally in upper Man- gins and well-preserved flow morphology zanita Creek near the west base of Lassen (despite glaciation). Basal and lateral mar- Peak and more abundantly northwest of gins glassy, commonly perlitic, locally brec- Manzanita Lake, where the deposit is largely ciated; flow top pumiceous. Commonly covered by pyroclastic-flow deposit of Eagle flow-layered and spherulitic or lithophysal. Peak (unit pfe) and where only the large ava- Light-colored to white and typically glassy lanche blocks protrude. West of the map area, or perlitic groundmass. Phenocrysts: 12% overlies 199-ka tholeiitic basalt of Eagle plagioclase 1-3 mm across, sparsely as large Canyon (unit be). At least partly emplaced as 6 mm; 1-2% biotite 0.5-2 mm across; 1- hot (as indicated by prismatically jointed 2% hornblende generally 1-4 mm across, blocks) by partial collapse of dacite domes rarely as large as 8 mm; 1% quartz as large in the vicinity of Lassen Peak considerably as 4 mm across. Sparse partly resorbed before the growth of the Lassen Peak dome plagioclase, hornblende, and biotite pheno- (unit dlp) crysts. Abundant mafic inclusions have a wide atu Andesite flow of Tumble Buttes older than Bear

variety of textures; generally fine- to coarse- Wallow Butte—58% SiO2. Porphyritic grained, nearly aphyric, and 10-15 cm across olivine-augite andesite aa to block lava 5- but size may be as great as about 50 cm. 10 m thick. Black glassy to aphanitic ground- Finer-grained inclusions commonly have 0.5- mass having large vesicles of irregular shapes to 1-mm phenocrysts of calcic plagioclase and smooth walls, commonly containing and phenocrysts (some of them partly re- opaline silica. Phenocrysts: 2-3% olivine 1- sorbed) of olivine and sparse augite as well 2 mm across; 2-3% plagioclase mostly 0.5- as partly resorbed host-rhyodacite pheno- 1 mm across but a few as large as 1.5 mm; crysts. Inclusion groundmasses generally trace augite as large as about 0.5 mm across. hornblende-plagioclase; some are pyroxene- Abundant cumulophyric clots as much as 5 hornblende-plagioclase. Abundant fragments mm across of all three phenocryst miner- of mafic inclusions are as large as a few als, dominated by plagioclase. Present in the

11 map area only near the north edge. Erupted xenocrysts. About 30% plagioclase micro- from a vent subsequently buried by the phenocrysts, generally as much as about 0.5 andesite of Bear Wallow Butte. The flow mm across but sparsely as large as 1 mm. lies below the limits of glaciation and has Rough unglaciated flow surfaces have con- a poorly developed soil and forest cover siderably better developed soil and more meh Mafic andesite flow of Eskimo Hill—54% SiO2. forest cover than the mafic andesite of Red Olivine-augite mafic andesite block-lava flow Lake Mountain (unit mrl). Mapped only at from a scoria cone satellitic to Red Lake northwest edge of the map area. Age un- Mountain. Microvesicular medium-gray known but underlies andesite of Red Lake aphanitic groundmass. Phenocrysts: 25% Mountain and overlies andesite of Raker Peak plagioclase generally 0.5-1 mm across, (unit arp) of 270 ka sparsely as large as 1.5 mm; 8% bright green tb Till of Badger Mountain—Diamicton contain- diopsidic augite generally 1 mm across, ing boulders as large as 2 m, consisting of sparsely as large as 2 mm; 5% olivine mostly varied volcanic lithologies. Prominent boulder 1 mm across, sparsely as large as 2 mm; lithologies include andesite of Raker Peak common small clots of olivine and augite. (unit arp) and andesite of Badger Mountain Rough unglaciated flow surfaces; sparsely (unit abm); less prominent but still relatively vegetated although mature trees are present common lithologies include basalt of Nobles in some areas. Present near the west edge Trail (unit bnt) and various older dacites, of the map area, northwest of Manzanita Lake. including the dacite of Reading Peak (unit Underlies pyroclastic-flow deposit of Eagle dr). Occurs as sheetlike till deposits and Peak (unit pfe, 66 ka); overlies mafic andesite moraines adjacent to the Hat Creek and Lost of Red Lake Mountain (unit mrl) Creek drainages at elevations as low as about mrl Mafic andesite flows of Red Lake Mountain— 5,000 ft and in a small area south of Man-

54-59% SiO2. Older olivine-augite and younger zanita Lake. Equivalent to units B2 and B1 medium-gray microvesicular hypersthene- of Turrin and others (1998) olivine-augite mafic andesite aa to block- lava flow. Phenocrysts: older flows: 25% MIDDLE PLEISTOCENE plagioclase generally 0.5-1 mm across, sparsely as large as 1.5 mm; 8% diopsidic rdm Rhyodacite flow of Dersch Meadows—69%

augite generally less than 1 mm across, sparsely SiO2. Thick lava flow of porphyritic biotite- as large as 2 mm; 5% olivine mostly about hypersthene-hornblende rhyodacite. Interior 1 mm across, sparsely as large as 2 mm; of flow light-gray to pink, conspicuously younger flows: 20% plagioclase generally flow-layered, generally devitrified; spherulites 0.5-1 mm across, sparsely as large as 1.5 common; flow base dark-gray, dense, and mm; 8% diopsidic augite generally less than glassy, typically perlitic, locally brecciated. 1 mm across, sparsely as large as 2 mm; 2% Phenocrysts: 15% plagioclase 1-3 mm across, olivine mostly about 1 mm across, sparsely sparsely as large as 7 mm; 2% hornblende as large as 2 mm; 1% hypersthene to 0.5 1-5 mm across, rarely as large as 8 mm; 1% mm across. Abundant small clots of olivine hypersthene 0.25-1 mm across; sparse bi- and augite. Rough, unglaciated flow surfaces otite as large as 1 mm; trace quartz 1 mm have weak soil and sparse vegetation but some across. Mafic inclusions sparse, rarely exceed mature trees. Mapped only at west edge of 5 cm; contain sparse bright green augite phe- the map area, northwest of Manzanita Lake. nocrysts. Partly resorbed phenocrysts and Underlies pyroclastic-flow deposits of Eagle fragments of mafic inclusions sparse. Mapped Peak (unit pfe, 66 ka) only at east edge of the map area, south of mrm Mafic andesite flow of Red Mountain—54% SiO2. Emigrant Pass (fig. 2). Vent location un- Olivine-augite mafic andesite aa to block- known, but probably in area east of Para- lava flow. Microvesicular medium-gray dise Meadows; lava flowed north to Dersch aphanitic groundmass. Phenocrysts: 7% Meadows and the east fork of Hat Creek. diopsidic augite generally less than 1 mm Dated by 40Ar/39Ar as 193±11 ka (B.D. Turrin, across, sparsely as large as 2 mm; 3% olivine written commun., 1994) phenocrysts mostly about 1 mm across, bec Tholeiitic basalt flow of Eagle Canyon—48%

sparsely as large as 2 mm. Common small SiO2. Thin lava flows of olivine basalt, wide- clots of olivine and augite; sparse quartz spread west of the map area. Medium-gray

12 holocrystalline, diktytaxitic tube-fed pahoehoe phenocrysts of calcic plagioclase, augite, and having tumuli on upper surface and consisting olivine present in some inclusions, as well of several flow units. Hexagonal-block joint as partly resorbed host-dacite phenocrysts; pattern at flow top; massive jointing in mafic inclusions have hornblende-pyroxene- interior. Abundant round vesicles at upper plagioclase groundmasses. Small fragments and lower surfaces. Phenocrysts: 1-5% olivine of mafic inclusions abundant. Only one small 1-3 mm across; 0-5% plagioclase 1-2 mm area of dacite of Reading Peak occurs within across; cumulophyric clots of olivine and the map area, low on the eastern flank of plagioclase as large as 1 cm. Phenocrysts Lassen Peak; it is similar to and correlated more abundant in proximal localities, less with the glaciated domes exposed on Reading abundant in distal localities (beyond the map Peak, 2-3 km farther southeast. The age of area). Sparse (but locally abundant) distinc- dacite of Reading Peak was determined by tive clusters as much as 2 cm across of 2- K-Ar to be 212±5 ka (G.B. Dalrymple, written to 5-mm plagioclase crystals that radiate from commun., 1991)

an olivine-plagioclase cumulophyric clot. dsh Dacite dome of Ski Heil Peak—66% SiO2. Occurs in only one part of the map area, near Porphyritic augite-hornblende dacite. Light- west edge, just south of California Highway gray to pinkish, aphanitic, massively jointed, 44; vent location uncertain, but cinders and devitrified, and commonly oxidized. Pheno- agglutinate exposed in roadcuts near there crysts: 10% plagioclase generally 1-5 mm suggest that the vent was nearby. Basalt of across, composite crystals as large as 1 cm; Eagle Canyon flowed down the drainage of 1% hornblende 1-3 mm across, sparsely as Manzanita Creek beyond the map. Macdonald large as 5 mm; 1% augite 0.5-2 mm across, (1963) and Macdonald and Lydon (1972) commonly in small cumulophyric clots; 1% called this flow (and several other tholei- biotite generally 1-2 mm across; sparse quartz itic basalt flows) Shingletown basalt; Helley generally 1-2 mm across. Hornblende, bi- and others (1981) established the name otite, and some plagioclase phenocrysts “olivine basalt of Eagle Canyon” for this unit strongly resorbed. Common fragments of but did not recognize its entire distribution mafic inclusions, 1-2 cm across; sparse larger or its possible source area. Dated by 40Ar/39Ar mafic inclusions to 20 cm. Occurs at south as 199±2 ka (B.D. Turrin, written commun., end of the map area. Summit area of the dome 1994) mainly covered by pyroclastic flow of Eagle dr Dacite domes of Reading Peak—65-69% SiO2. Peak (unit pfe). K-Ar age is 244±10 ka (G.B. Complex of lava domes of porphyritic bi- Dalrymple, written commun., 1991)

otite-hornblende dacite. Light- to medium- bnt Tholeiitic basalt flow of Nobles Trail—48% SiO2. gray, aphanitic, massively jointed, devitrified, Thin aphyric basalt lava flow with medium- and locally oxidized. Phenocrysts: 10-12% gray coarsely diktytaxitic groundmass of plagioclase 1-5 mm across, sparse composite plagioclase, olivine, clinopyroxene, and crystals as large as 1 cm, 2% hornblende mostly Fe-Ti oxide microphenocrysts. Consists of 1-3 mm across, sparsely as large as 5 mm; multiple flow units of tube-fed pahoehoe. sparse-1% biotite as large as 1 mm; rare- Hexagonal-block joint pattern at top; mas- sparse quartz as large as 2 mm; rare augite sive jointing in interior. Abundant spheri- as large as 1 mm in small clusters. Pyrox- cal vesicles at upper and lower surfaces. ene microphenocrysts (hypersthene>>augite) Phenocrysts: trace olivine as large as 1 mm common but rarely exceed 0.5 mm across across. Occurs near Hat Creek east and and also occur locally as small cumulophyric northeast of Raker Peak; vent location clots. Most plagioclase phenocrysts are unknown, probably buried beneath younger weakly to strongly resorbed, and almost all lavas in the vicinity of the Central Plateau hornblende and biotite has been pseudo- (fig. 2). Age unknown, but overlies ande- morphed by aggregates of Fe-Ti oxides, site of Raker Peak (unit arp, 270 ka) and plagioclase, and pyroxene. Abundant maf- underlies till of Badger Mountain (unit tb)

ic inclusions contain a wide variety of tex- dvc Dacite dome of Vulcans Castle—66% SiO2. tures from porphyritic to coarse-grained Porphyritic biotite-hornblende dacite. Light- aphyric; their size and character varies from to medium-gray but commonly oxidized to location to location. Most inclusions <20 cm pink; aphanitic, massively jointed, devitri- across but many as large as 50-60 cm. Sparse fied, locally flow-layered. Brecciated and

13 glassy dome margins preserved along north- flank of the dome. Abundant mafic inclu- ern flank. Phenocrysts: 10% plagioclase 2- sions ranging from 10 to about 50 cm across 6 mm across, largest are composite; 1% vary from fine- to coarse-grained and are hornblende 1-3 mm across; 1% biotite 1-2 generally sparsely phyric, but some contain mm across; 1% quartz 1-2 mm across; sparse sparse to abundant partly resorbed host phe- augite phenocrysts 0.5-1 mm across and small nocrysts. Groundmasses of inclusions composite augite crystals; sparse 0.5-mm hy- either pyroxene-plagioclase or hornblende- persthene microphenocrysts. Abundant mafic plagioclase. Abundant fragments of mafic inclusions contain a variety of textures; inclusions 5 mm to 2 cm across, commonly generally less than about 10 cm across. contain pyroxene microphenocrysts and Occurs at southwest edge of the map area. cumulophyric clots of microphenocrysts. The Underlies dacite dome of Ski Heil Peak (unit hill near the north base of Lassen Peak labeled dsh) of 244±10 ka; includes small area of 8283 ft in elevation marks the vent for lava dome-collapse breccia northwest of Cres- that emplaced a thick flow northeast toward cent Cliff; highly sculpted by glacial ero- Lost Creek. Extensively mantled by pumi- sion and mass wasting ceous pyroclastic-flow and fall deposits of dpl Dacite flow of the Lassen Peak Trail Parking Lot— Chaos Crags (unit pc). K-Ar age 261±5 ka

64% SiO2. Moderately porphyritic horn- (G.B. Dalrymple, written commun., 1991) blende-augite dacite. Medium-gray, dum Dacite flow of upper Manzanita Creek—66-67%

aphanitic, massively jointed, devitrified, SiO2. Porphyritic pyroxene-biotite-hornblende and commonly oxidized; commonly flow- dacite dome. Light- to medium-gray, splotchy, layered, especially at the parking lot for aphanitic, massively jointed, and devitrified. the trail up the south side of Lassen Peak Phenocrysts: 12% plagioclase 2-5 mm (fig. 2). Phenocrysts: 5% plagioclase 1- across, sparse composite crystals with dark 5 mm across, sparsely as large as 8 mm; cores, as large as 1 cm; 2% hornblende 1% augite as large as 3 mm; 1% hornblende 1-5 mm across; 1% biotite as large as 2 1-2 mm across; sparse biotite generally 1 mm; rare quartz as large as about 1 mm. mm across; trace quartz 1 mm across. Most Common pyroxene microphenocrysts plagioclase, hornblende, and biotite partly (hypersthene>>augite) rarely exceed 0.5 resorbed. Sparse mafic inclusions as much mm across, locally as small composite crys- as 10 cm across; most are deformed and tals. Mafic inclusions abundant, as large as partly disaggregated to merge with the about 20 cm; include both coarse-grained groundmass. Abundant small fragments of equigranular inclusions that lack host-lava mafic inclusions a few millimeters to a few phenocrysts and fine-grained inclusions that centimeters across. Overlain by dacite dome contain partly resorbed host-lava phenocrysts; of Ski Heil Peak (unit dsh) both types contain sparse 0.5- to 1-mm d83 Dacite dome and flow of hill 8283—66% SiO2. phenocrysts of augite and olivine; ground- Porphyritic pyroxene-hornblende dacite dome masses are hornblende-plagioclase. Abun- and thick lava flow. Light- to medium-gray, dant fragments of mafic inclusions a few aphanitic, having patchy microvesicular areas. millimeters to a few centimeters across. Extensively glaciated, but a few remnants Present only at the north base of Lassen Peak. of pumiceous carapace preserved. Weakly Glacially eroded. Underlies dacite of hill flow-layered with aligned phenocrysts. 8283 (unit d83)

Proportions of phenocrysts vary, especially arp Andesite flows of Raker Peak—57-59% SiO2. in the lava dome. Phenocrysts: 12% plagio- Thick andesite lava flows and , as clase 1-6 mm across, sparse composite crys- well as an agglutinated scoria vent cone at tals with dark cores, as large as 1 cm; 1% summit of Raker Peak. Typically light-gray, hornblende 1-5 mm across; sparse biotite aphanitic, and microvesicular but dark-gray generally less than 2 mm across; sparse-1% to black where glassy. Typical exposures are quartz generally less than 2 mm across. rounded boulders or squarish joint-bounded Abundant pyroxene microphenocrysts blocks; flow interiors dense and have massive (augite>>hypersthene) generally less than to platy jointing. Lithologically varied, having 0.5 mm across. Felsic phenocrysts both partly disequilibrium phenocryst assemblage. resorbed and unresorbed. Augite and olivine Most common lithology consists of lava xenocrysts common, especially on the western flows 50-100 m thick of porphyritic olivine-

14 pyroxene andesite. Phenocrysts: 10-15% pla- by tills of Badger Mountain (unit tb) and gioclase 1-6 mm across, generally partly re- Raker Peak (unit tr). 40Ar/39Ar age is 297±1 sorbed; 4-6% augite 0.5-3 mm across; ka (B.D. Turrin, written commun., 1994)

2-4% olivine 1-5 mm across; 1-2% hyper- rl Rhyodacite flow of Loomis Peak—69% SiO2. sthene 0.5-1 mm across; 1% hornblende 0.5- Thick lava flow of porphyritic biotite- 3 mm across; sparse quartz as large as 2 mm, hypersthene-hornblende rhyodacite. Varies generally having pyroxene rims. Both hyper- from white to pink, dense to pumiceous, and sthene and augite also occur together with devitrified to dense dark-gray or black perlitic plagioclase in cumulophyric clots as large glass containing spherulites and lithophysae. as 5 mm across. Hornblende phenocrysts Extensively glaciated but locally still exhibits replaced by dark-colored aggregates of a poorly preserved pumiceous carapace; base plagioclase, pyroxene, and Fe-Ti oxides; of flow dense and perlitic; flow interior plagioclase strongly resorbed. Flows near massively jointed, flow-layered, and devit- the base of the unit contain only sparse rified and having common spherulitic, phenocrysts of the minerals described above. lithophysal, and oxidized zones. Phenocrysts: Mafic inclusions absent, but inclusions of 12% plagioclase generally 1-3 mm across volcanic and metamorphic crustal rocks occur (but a few as large as 5 mm and composite locally. Glaciated except for the northern crystals as large as 8 mm); 2% hornblende distal end and the summit area. K-Ar age generally 1-3 mm across (but a few as large of a sample collected near the exposed base as 5 mm); 1% hypersthene as large as 0.75 of the unit is 270±18 ka (G.B. Dalrymple, mm; trace biotite 0.5-1 mm across. Particu- written commun., 1991) larly abundant mafic inclusions contain a wide rmz Rhyodacite flow of Manzanita Chute—69% SiO2. variety of textures ranging from fine-grained Thick lava flow of moderately porphyritic dark porphyritic and having partly resorbed biotite-hornblende rhyodacite. Largely host-rhyodacite phenocrysts to coarse- unglaciated, weathered surface has a well- grained and aphyric. Most inclusions 10- preserved flow morphology; the upper surface 20 cm across but some as large as 50 cm; has well defined crescentic flow ridges and coarse-grained aphyric inclusions having a pumiceous carapace; basal part of flow is hornblende-plagioclase groundmasses are dense and perlitic. Groundmass ranges from commonly larger; porphyritic mafic inclu- white pumiceous perlitic glass containing sions containing sparse 1-mm olivine, augite, spherulites to dark-gray or black dense perlitic and calcic-plagioclase phenocrysts, partly glass. Phenocrysts: 10% plagioclase generally resorbed host-rhyodacite phenocrysts, and fine- 1-3 mm across, sparsely as large as 5 mm; grained pyroxene-plagioclase or pyroxene- 2% hornblende generally 1-3 mm across, hornblende-plagioclase groundmasses are sparsely as large as 5 mm; 1% biotite 0.5- commonly smaller. Mafic inclusions that 1 mm across. Partly resorbed phenocrysts contain older generations of inclusions are rela- absent. Mafic inclusions containing a vari- tively common. Fragments of mafic inclusions, ety of textures generally sparse but locally partly resorbed phenocrysts, and small crystals common; most are 10-30 cm across, aphyric, disaggregated from inclusions are abundant. fine- to coarse-grained, and have hornblende- Occurs near southwest edge of map area, plagioclase groundmasses; less abundant are where it overlies of Brokeoff smaller porphyritic mafic inclusions con- Volcano (unit abk, about 400 ka) and un- taining sparse 1-mm augite and olivine phe- derlies the rhyodacite flow of Manzanita nocrysts, partly resorbed host-rhyodacite Chute (unit rmz, 297 ka)

phenocrysts, and fine-grained hornblende- av Andesite flow of Viola—57-59% SiO2. Thick plagioclase groundmasses. Sparse small block-lava flow of sparsely porphyritic augite- fragments of mafic inclusions. Mapped only olivine andesite. Light- to medium-gray, at west edge of the map area, southwest of aphanitic. Original flow surface poorly pre- Manzanita Lake. The vent dome is peak 7241, served, marked by subrounded vesicular about 1 km southwest of the map area. The boulders. Flow interior, where exposed, unit overlies rhyodacite flow of Loomis Peak thickly to thinly platy-jointed. Distinctly (unit rl), andesite flow of Viola (unit av), disequilibrium phenocryst assemblage con- and andesite flow and of lower sisting of olivine, augite, hornblende, pla- Manzanita Creek (unit amc); overlain only gioclase and quartz. Phenocrysts: 4-8%

15 plagioclase generally 2-5 mm across, many 470 to 400 ka (Clynne and Muffler, 1989), strongly resorbed; trace-2% augite 0.25-1 but those in the vicinity of Raker Peak may mm across; 1% hornblende (+biotite?) 3-5 be older mm across completely pseudomorphed by amc Andesite flow and cinder cone of lower Manza- fine-grained aggregates of plagioclase, nita Creek (part of Brokeoff volcano)—

pyroxene, and Fe-Ti oxides; trace olivine 63% SiO2. Thick lava flow of porphyritic 0.1-0.35 mm across, generally corroded and augite-hypersthene andesite. Unglaciated and partly converted to iddingsite; trace quartz. retains much original flow morphology. Proximal parts of the flow contain twice as Upper surface of the block-lava flow char- many phenocrysts as distal parts. Present in acterized by rounded vesicular boulders 0.5- the map area only near west edge, south of 1 m across. Generally aphanitic, light-gray Manzanita Creek. The vent is marked by an where weathered (typical), medium- to dark- eroded cinder cone (hill 6924 about 1.5 km gray where fresh (rare); distal part of the southeast of Deep Hole) west of the map flow, outside the map area, is commonly flow- area. K-Ar age 313±8 ka (G.B. Dalrymple, layered, partially oxidized, and devitrified. written commun., 1991) Phenocrysts: 20% plagioclase 1-3 mm across; rrp Rhyolite dome of the west side of Raker Peak— 6% hypersthene 0.5-2 mm across; 3% augite

74% SiO2. Moderately porphyritic hornblende- 0.5-2 mm across; sparse hornblende, gen- biotite rhyolite. White to light-gray, glassy, erally smaller than 1-2 mm. Abundant commonly spherulitic. Glacial erosion along cumulophyric clots of plagioclase and py- the southern margin of the dome exposes roxenes 5 mm to 1 cm across. Mafic inclu- prominent columnar joints. Phenocrysts: 10% sions sparse and small, generally less than plagioclase 1-3 mm across, commonly partly about 5 cm across. Shown only in west part resorbed; 2% biotite as large as 1 mm, of the map area. Erupted from vent marked unresorbed and unoxidized; 1% hornblende by small cinder cone (black-stippled area) as large as 1 mm, unresorbed and unoxidized; near Manzanita Lake campground; the eroded sparse quartz as large as 0.25 mm. Abun- cone of andesitic scoria has a thick soil cover. dant coarse-grained equigranular mafic inclu- This unit is overlain by andesite of Viola

sions (57.5% SiO2) generally 10 to about 30 (unit av, 313 ka); correlation with the later cm across. Abundant small fragments of stages of Brokeoff volcano suggests an age mafic inclusions a few millimeters across. between about 470 and 400 ka Inclusions have pyroxene-hornblende- plagioclase groundmasses and sparse pla- EARLY PLEISTOCENE AND EARLY PLEISTOCENE(?) gioclase phenocrysts; inclusions locally deformed by flowage into even layers a few abm Andesite flows of Badger Mountain—57 and 60-

millimeters thick; these layers commonly 63% SiO2. Moderately to highly porphyritic extend over several square meters and provide pyroxene andesite lavas forming a small planar discontinuities that separate the rocks . Flows are generally thin (but into large blocks. Overlain by andesite of locally as thick as 10 m); interflow brec- Raker Peak (unit arp) cias sparse. Glaciated on the western mar- abk Andesite flows and debris flows of the com- gins, and cut by normal faults. Generally

posite Brokeoff volcano—61-63% SiO2. dark-gray to black, glassy to aphanitic. Lithol- Small areas of various porphyritic augite- ogy varied, including two types corresponding hypersthene andesite lavas from the Brokeoff to upper and lower stratigraphic positions. volcano have been mapped together. Litholo- Phenocrysts (upper flows): 20-30% plagio- gies generally similar, typically medium- to clase as large as 1 mm; 3% hypersthene dark-gray and aphanitic, containing 10-30% generally as large as 1 mm, sparsely as large phenocrysts of plagioclase, hypersthene, and as 2 mm; 2% augite generally as large as augite (generally hypersthene>augite); pla- 1 mm, sparsely as large as 2 mm. Pheno- gioclase dominates the assemblage. Pheno- crysts (lower flows): 15-30% plagioclase; crysts typically 1-3 mm across, locally as 3-4% augite; sparse-2% hypersthene; sparse- large as 5 mm; olivine sparsely present as 1% olivine 0.5 mm. Abundant cumulophyric partly resorbed phenocrysts. Common clots of plagioclase and pyroxene±olivine cumulophyric clots of plagioclase and py- generally 2-5 mm across. The two types of roxene. Most of these flows are approximately lavas correspond to two chemical groups:

16 the hypersthene-augite andesites low in the consisting of all three phenocryst types,

section contain about 57% SiO2, and the dominated by plagioclase, generally 2-5 mm augite-hypersthene andesites above contain across. A few small NNW-trending faults

60-63% SiO2. Occurs at northeast edge of cross the edifice. The unit has normal rema- the map area. There is a single conventional nent magnetic polarity and probably is K-Ar age of 708±21 ka (G.B. Dalrymple, younger than 780 ka. Age relations to written commun., 1994) andesite flows of Badger Mountain (unit btb Tholeiitic basalt flows of Twin Bridges—49% abm) uncertain

SiO2. Light-gray, holocrystalline, diktytaxitic. Multiple thin flow units, typically a few REFERENCES CITED decimeters to a few meters thick; vesicu- lar flow tops. Maximum exposed thickness Clynne, M.A., 1990, Stratigraphic, lithologic, and major element about 30 m, base not exposed. Original flow geochemical constraints on magmatic evolution at Lassen vol- surfaces poorly preserved. Phenocrysts: sparse canic center, California: Journal of Geophysical Research, v. to 1% olivine 0.5-1.5 mm across, locally in 95, p. 19,651-19,669. small glomeroporphyritic clots as large as Clynne, M.A., 1999, A complex magma mixing origin for rocks 3 mm. Present in the map area only near the erupted in 1915, Lassen Peak, California: Journal of Petrol- northeast corner. Location of the vent is ogy, v. 40, p. 105-132. unknown. Several small faults and one major Clynne, M.A., and Muffler, L.J.P., 1989, Lassen Volcanic National fault, all NNW-trending, break this unit. Park and vicinity, in Chapin, C.E., and Zidek, J., eds., Field Overlain by andesite flows of Badger excursions to volcanic terranes in the western ; Volume II, Cascades and Intermountain West: New Mexico Mountain (unit abm) and has normal rema- Bureau of Mines and Mineral Resources Memoir 47, p. 183- nent magnetic polarity; thus probably be- 194. tween 708 and 780 ka Helley, E.J., Harwood, D.S., Barker, J.A., and Griffin, E.A., 1981, atb Andesite flows of Table Mountain—62% SiO2. Geologic map of the Battle Creek fault zone and adjacent parts Porphyritic augite-hypersthene andesite lava of the northern Sacramento Valley, California: U.S. Geologi- flows forming a small shield volcano. Largely cal Survey Miscellaneous Field Studies Map MF-1298, scale unglaciated but no vent or pyroclastic 1:62,500. material preserved. Dark-gray to black, Macdonald, G.A., 1963, Geology of the Manzanita Lake quadrangle, glassy, and vesicular, having conspicuously California: U.S. Geological Survey Geologic Quadrangle Map rubbly flow tops and dense flow interiors; GQ-248, scale 1:62,500. typical exposures consist of vesicular round- Macdonald, G.A., and Lydon, P.A., 1972, Geology of the Whitmore quadrangle, California: U.S. Geological Survey Geologic Quad- weathered boulders. Phenocrysts: 20% pla- rangle Map GQ-993, scale 1:62,500. gioclase as large as 0.75 mm; hypersthene Turrin, B.D., Christiansen, R.L., Clynne, M.A., Champion, D.E., generally as large as 0.75 mm, rarely as large Gerstel, W.J., Muffler, L.J.P., and Trimble, D.A., 1998, Age of as 1.5 mm; augite generally as large as 0.75 Lassen Peak, California, and implications for the ages of late mm, rarely as large as 1.5 mm. Abundant Pleistocene glaciations in the southern Cascade Range: Geo- cumulophyric clots and crystal clusters logical Society of America Bulletin, v. 110, p. 931-945.

17