GEOLOGIC MAP OF THE BADGER MOUNTAIN SE, …ds830fh4331/Plate 5.pdf · Miab is composed of numerous...
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Coble 2012 Base from U.S. Geological Survey Badger Mtn. SE, NV 7.5’ topographic quadrangle, 1966. Universal Transverse Mercator Projection, Zone 11 MAP LOCATION NEVADA 15° TRUE NORTH MAGNETIC NORTH APPROXIMATE MEAN DECLINATION, 2007 PLATE 5 Samp le Unit lat. long. Age ± error M SWD n M odel MC330 Mlwc 41.5421 -119.3713 14.551 ± 0.065 4.45 10 II MC237 Mm 41.5899 -119.2638 14.842 ± 0.032 0.95 11 TG MC333 Mlmm 41.6111 -119.3616 15.562 ± 0.017 1.56 11 TG MC238 Mlch 41.5807 -119.2714 15.585 ± 0.017 1.10 11 TG MC314 Miab 41.5755 -119.3260 15.733 ± 0.023 0.94 10 TG MC429B Miab 41.5846 -119.3734 15.838 ± 0.019 1.03 10 TG 40 Ar/ 39 Ar ages for units in the Badger Mountain SE quadrangle TABLE 1 All ages are calculated using λ = 5.543x10 -10 a -1 (Steiger and Jäger, 1977) and referenced to a model age of 28.02 Ma for Fish Canyon Sanidine (Renne et al., 1998). Individual analyses are total fusion of single sanidine grains extracted from porphy- ritic rhyolites and lavas or total fusion of devitrified rhyolite or microcrystalline mafic groundmass concentrates. TG–Total gas weighted mean age, 2σ standard error, II–Inverse isochron intercept age, 1σ standard deviation of York fit. Qal Qt Mlwc DESCRIPTION OF MAP UNITS REFERENCES CORRELATION OF MAP UNITS Qal Qt NEOGENE PLIOCENE? QUATERNARY TERTIARY CENOZOIC Mt Mm Mp Mlwc Alluvium (Quaternary) – Unconsolidated alluvial sand and gravel in modern stream beds. Landslide deposits (Quaternary) – Slope failure deposits of displaced or chaotically mixed bedrock blocks. Mm Alkali Olivine Basalt, Basalt, and Basaltic-Trachyandesite Lavas or Plugs (ca. 15.5- 14.5 Ma) – Mafic lavas, plugs, and dikes that weather to shades of dark brown or red. Individual lava flows are typically 3 to 6 m thick, and are vesicular, scoriaceous, and typically strongly oxidized. Fresh surfaces are glassy or microcrystalline. The phenocryst content ranges from 0 to 8%. Basalt and alkali basalts contain small (~1 mm diameter) olivine phenocrysts, whereas more evolved compositions are dominated by 1 to 5% clinopyroxene and/or plagioclase phenocrysts, with minor olivine. Mm also typically contains 1 to 3% partially oxidized and exsolved, subhedral Fe-Ti oxides. Mm lavas locally overlie Miab and Mlmm. Trachyandesite to Trachyte Lavas (ca. 15.5- 14.5 Ma) – Individual lava flows are typically 3 to 10 m thick; aggregate thickness increases to the south toward High Rock Canyon (up to 120 m). Fresh surfaces are typically glassy and black in color, giving them the appearance of basalt. The flows weather shades of orange or brown. Mt lavas are weakly porphyritic, and typically contain 1-5% of the following phases in various proportions: clinopyroxene, plagioclase, olivine, and Fe-Ti oxides in a glassy or microcrystalline matrix. Plagioclase is typically resorbed, and there can also be xenocrysts of quartz present, suggesting that there was mixing by entrainment of rhyolitic magma. Magma mingling textures are common in small dikes of Mt lavas south of Badger Mountain and in Wall Canyon. These lavas were mapped as Andesite and Dacite flows (Tad) by Ach (1988) and Ach et al. (1991). Mp Phreatomagmatic Tuff (Miocene) – Phreatomagmatic deposits related to intrusion of Mt and Mm into wet sediments and/or shallow lakes or ponds. Individual beds are cm-scale, normally graded, and can display meter-scale cross bedding. Dark brown to black lapilli range from 1 to 30 cm in size, are subrounded, and typically scoriaceous. Accretionary lapilli can also be present. Locally Mp is interbedded with lacustrine sediments or thin surge deposits. Peperite is also present. Mp is altered to palagonite in some exposures. Intermediate clasts within the tuff are interpreted to be vesicular clast of Mt. Mp was previously mapped as Palagonite Tuff (Tpt) by Greene (1984) and Basaltic Tuffaceous Rocks (Tslb) by Ach (1988) and Ach et al. (1991). Mcfp Reworked Pumiceous Deposits (Miocene) – Sedimentary deposits consisting of unconsolidated and variably reworked rounded pumice lapilli in an ashy matrix. Weathered and fresh surfaces are various shades of grey. The unit is weakly laminated, is generally un-graded, but small-scale normal grading is present locally. Pumice-rich horizons appear to have formed by floating of larger pumice lapilli to the lake surface. This unit is always associated with, and typically surrounds postcaldera rhyolitic domes or flows, and represents reworking of the deposits of phreatomagmatic eruptions associated with emplacement of rhyolitic lavas into water or wet sediments. Previously grouped into an undifferentiated sedimentary package termed the High Rock Sequence by Bonham (1969), and as tuffaceous sedimentary rocks by Greene (1984). Miab Tuff of Alkali Flat and Tuff of Badger Mountain (ca. 15.9-15.7 Ma) – Nonwelded and incipiently welded ignimbrite predomi- nantly exposed south and east of Badger Mountain. Miab is composed of numerous thin flows which are probably related to emplacement of Mlmm lavas. Colors range from white, tan, to light-grey when fresh, and brown to rust-red when weathered. Typically contains 60-80% rounded white to tan pumice lapilli 1- to 5-cm in size in an ashy matrix . Weakly welded ignimbrite become more strongly welded west of the mapping area near Wall Canyon. Increase in the degree of welding correlates with changes of color in the devitrified groundmass from tan to pink, light-purple, light-blue, and blue-green. Welded Miab exhibits a planar texture defined by flattened pumices, which have a large aspect ratio (> 5:1). The ignimbrite contains 36-50% phenocrysts. Quartz and sanidine are clear in non-welded and incipiently welded deposits and become smoky and adularescent-blue, respectively, in more strongly welded deposits. Phenocrysts include sanidine (16-22%), quartz (20-27%), and trace amounts of sodic amphibole, resorbed ferrohedenbergite, and Fe-Ti oxides. I distinguish between the petrographically similar 16.0 Ma Soldier Meadow Tuff based on younger 40 Ar/ 39 Ar ages and major- and trace-element geochemistry. Mlch Crystal-poor Rhyolite Lavas of Cherry Spring (ca. 16.0-15.6 Ma) – Weakly porphyritic rhyolite dikes and lava flows can be glassy, perlitic when hydrated, devitrified, or spherulitic where massive. Outcrops are strongly silicified, small in volume, and commonly have mm-scale flow banding with stretched vesicles. Weathered surfaces are dark-brown or rust colored; fresh surfaces are pale-red, pink, brown, rust or grey. This lava contains sanidine (5%) up to 2 mm in size. A small lava flow near Butcher Flat contains distinctive euhedral 0.5-2 mm long feldspar that make up approximately 15-20% of the rock. Because the lavas are pervasively silicified, mafic phases are strongly oxidized and are largely absent, and the lava appears glassy on fresh surfaces with secondary quartz or chalcedony filling vesicles and voids. This lava is distinguished from Mlxp in the Bear Buttes Quadrangle by its limited aerial extent, lower total phenocryst content, and a paucity of quartz and zircon. The range in 40 Ar/ 39 Ar ages suggest multiple distinct flows, but I group them here based on their small volume, similar stratigraphic relation and trace-element compositions. They are locally overlain by Miab. Mcf Lacustrine Caldera-Fill Deposits (Miocene) – Caldera-fill sediments deposited in a quiescent shallow lake environment. These deposits are locally interbedded with thin Mp, Mcfp or ash layers. Individual beds are thinly laminated (mm-scale), fine-grained, diatomaceous, white to light-yellow in color, and locally cross-bedded where reworked. Petrified logs or wood chips are a hallmark feature of this unit. When silicified, the lacustrine sediments are replaced by silica (chert), and break into semi-transparent chips of various colors. Previous mapping by Bonham (1969) grouped Mp, Mcf, and Mcfp deposits into an undifferentiated sedimentary package termed the High Rock Sequence. Mcf and Mcfp can be correlated with tuffacous sedimentary rocks mapped by Greene (1984), and termed the Virgin Valley Formation by Merriam (1910) in Virgin Valley Caldera to the north. Ach, A.J., 1988, Geologic map of the Yellow Hills East Quadrangle, Washoe and Humboldt Counties, Nevada, U.S. Geological Survey Miscella- neous Field Studies Map MF-2029. Ach, A.J., Bateson, J.T., Turrin, B.D., Keith, W.J., Noble, D.C., and Swisher, C.C., 1991, Geologic map of the High Rock Lake Quadrangle, Washoe and Humboldt Counties, Nevada, U.S. Geological Survey Miscellaneous Field Studies Map MF-2157 Bonham, H.F., and Papke, K.G., 1969, Geology and mineral deposits of Washoe and Storey Counties, Nevada: Nevada Bureau of Mines and Geology Bulletin, v. 70, p. 1-140. Greene, R.C., 1984, Geologic appraisal of the Charles Sheldon Wilderness Study Area, Nevada and Oregon: U.S. Geological Survey Bulletin, 1538-A, p. 13-34. Merriam, J.C., 1910, Geologic history, prt 1 of Tertiary mammal beds of Virgin Valley and Thousand Creek in northwest Nevada: California University Department of Geology Bulletin, v. 6, p. 21-53. Noble, D. C., McKee, E. H., Smith, J. G., and Korringa, M. K., 1970, Stratigraphy and geochronology of Miocene rocks in northwestern Nevada: U.S. Geological Survey Professional Paper 700-D, p. D23-D32. Mlmm Mahogany Mountain Rhyolite Lavas (ca. 15.9-15.5 Ma) – Informally named the Rhyolite of Badger Mountain by Greene (1984), I suggest the name Mahogany Mountain Rhyolite Lava to avoid confusion with the older Badger Mountain Caldera. Weathered exposures of Mlmm are rust to reddish-brown and exhibit thick, centimeter- to meter-scale flow-banding. Well-developed pressure ridges can be mapped from air photos for several kilometers. The lava is typically strongly porphyritic with distinctive adularescent- blue sanidine (18-22%) and black- to smoky quartz (16-20%). Ferrohedenbergite, sodic amphiboles, zircon, and resorbed Fe-Ti oxides are present in trace amounts within a granophyrically recrystallized groundmass that is blue to light-blue-green in color. Microcrystalline trachytic enclaves several cm in size can be found near vents. A newly identified weakly porphyritic lava capping the southwest side of Badger Mountain contains sanidine (1-2%), lacks quartz, and contains only trace amounts of Fe-Ti oxides. This crystal-poor member of Mlmm is compositionally similar to the crystal-rich member, and the weathering color and centimeter- to meter-scale flow-banding are also similar. Wall Canyon Rhyolite Lava (14.55±0.07 Ma) – Aphyric to weakly porphyritic lava domes and flows that erupted through lacustrine sediments (Mcf and Mcfp) and phreatomagmatic deposits (Mp). The lavas are light grey to blueish on fresh devitrified surfaces and vesicular near the tops of flows. Obsidian nodules are locally present. Small (0.25-0.5mm) euhedral sanidine are the most abundant phenocrysts (<1%), but trace amounts of quartz and Fe-Ti oxides are also present. The lavas exhibit mm- to cm-scale flow laminae. Aphanitic dark-colored magmatic enclaves are locally abundant near vents. Talus deposits (Quaternary) – Unconsolidated debris sourced locally, typically along eroded normal fault scarps. Ql Ql Mcf Mcfp Mlmm Mt ? ? Mlch Mism ! MC237 14.84 ± 0.03 MC238 15.59 ± 0.02 MC314 15.73 ± 0.02 MC429B 15.84 ± 0.02 MC333 15.56 ± 0.02 MC330 14.55 ± 0.06 Contact: Solid where located to ±15 meters, dashed where located within ±30 meters. Dotted where concealed by alluvium Normal Fault: Ball and bar on downthrown side. Solid where located to ±15 meters, dashed where located within ±30 meters. Dotted where concealed by alluvium. Location of inferred dikes Strike and dip of sedimentary bedding Strike and dip of flow planes in mafic lava flows Strike and dip of flow planes in silicic lava flows Strike and dip of compaction foliation in ignimbrites EXPLANATION 10 10 10 10 10 Dip and dip direction of fault planes Approximate location of vents ! ! ! ! ! 40 Ar/ 39 Ar age in million years and sample number (parentheses) (100) 10.00 ± 0.10 ! Aphyric rhyolitic lava Overlay Patterns Alteration: Areas with pervasive hydrothermal alteration and/or silicification. Crystal-rich rhyolitic lavas (>10% phenocrysts). Crystal-poor rhyolitic lavas (<10% phenocrysts).
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119º 17’ 30”119º 20’
119º 15’ 30”41º 30’
41º 32’ 30”
119º 15’41º 30’
41º 35’
41º 37’ 30”119º 22’ 30”
119º 20’119º 17’ 30”
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CONTOUR INTERVAL 20 FEETNATIONAL GEODETIC VERTICAL DATUM OF 1983
7000 FEET1000 10000 2000 3000 4000 5000 6000
.5 1 KILOMETER1 0
SCALE 1:24 0001/ 21 0 1 MILE
GEOLOGIC MAP OF THE BADGER MOUNTAIN SE, 7.5’ QUADRANGLE, HUMBOLDT COUNTY, NEVADABy
Matthew A. Coble
2012
Base from U.S. Geological Survey Badger Mtn. SE, NV 7.5’ topographic quadrangle, 1966.Universal Transverse Mercator Projection, Zone 11
MAP LOCATION
NEVADA
15°
TRU
E N
OR
TH
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NET
IC N
ORT
H
APPROXIMATE MEANDECLINATION, 2007
PLATE 5
Sample Unit lat. long. Age ± error MSWD n Model
MC330 Mlwc 41.5421 -119.3713 14.551 ± 0.065 4.45 10 II
MC237 Mm 41.5899 -119.2638 14.842 ± 0.032 0.95 11 TG
MC333 Mlmm 41.6111 -119.3616 15.562 ± 0.017 1.56 11 TG
MC238 Mlch 41.5807 -119.2714 15.585 ± 0.017 1.10 11 TG
MC314 Miab 41.5755 -119.3260 15.733 ± 0.023 0.94 10 TG
MC429B Miab 41.5846 -119.3734 15.838 ± 0.019 1.03 10 TG
40Ar/39Ar ages for units in the Badger Mountain SE quadrangle
TABLE 1
All ages are calculated using λ = 5.543x10-10a-1 (Steiger and Jäger, 1977) and referenced to a model age of 28.02 Ma for Fish Canyon Sanidine (Renne et al., 1998). Individual analyses are total fusion of single sanidine grains extracted from porphy-ritic rhyolites and lavas or total fusion of devitrified rhyolite or microcrystalline mafic groundmass concentrates. TG–Total gas weighted mean age, 2σ standard error, II–Inverse isochron intercept age, 1σ standard deviation of York fit.
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DESCRIPTION OF MAP UNITS
REFERENCES
CORRELATION OF MAP UNITS
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Alluvium (Quaternary) – Unconsolidated alluvial sand and gravel in modern stream beds.
Landslide deposits (Quaternary) – Slope failure deposits of displaced or chaotically mixed bedrock blocks.
Mm Alkali Olivine Basalt, Basalt, and Basaltic-Trachyandesite Lavas or Plugs (ca. 15.5- 14.5 Ma) – Mafic lavas, plugs, and dikes that weather to shades of dark brown or red. Individual lava flows are typically 3 to 6 m thick, and are vesicular, scoriaceous, and typically strongly oxidized. Fresh surfaces are glassy or microcrystalline. The phenocryst content ranges from 0 to 8%. Basalt and alkali basalts contain small (~1 mm diameter) olivine phenocrysts, whereas more evolved compositions are dominated by 1 to 5% clinopyroxene and/or plagioclase phenocrysts, with minor olivine. Mm also typically contains 1 to 3% partially oxidized and exsolved, subhedral Fe-Ti oxides. Mm lavas locally overlie Miab and Mlmm.
Trachyandesite to Trachyte Lavas (ca. 15.5- 14.5 Ma) – Individual lava flows are typically 3 to 10 m thick; aggregate thickness increases to the south toward High Rock Canyon (up to 120 m). Fresh surfaces are typically glassy and black in color, giving them the appearance of basalt. The flows weather shades of orange or brown. Mt lavas are weakly porphyritic, and typically contain 1-5% of the following phases in various proportions: clinopyroxene, plagioclase, olivine, and Fe-Ti oxides in a glassy or microcrystalline matrix. Plagioclase is typically resorbed, and there can also be xenocrysts of quartz present, suggesting that there was mixing by entrainment of rhyolitic magma. Magma mingling textures are common in small dikes of Mt lavas south of Badger Mountain and in Wall Canyon. These lavas were mapped as Andesite and Dacite flows (Tad) by Ach (1988) and Ach et al. (1991).
Mp Phreatomagmatic Tuff (Miocene) – Phreatomagmatic deposits related to intrusion of Mt and Mm into wet sediments and/or shallow lakes or ponds. Individual beds are cm-scale, normally graded, and can display meter-scale cross bedding. Dark brown to black lapilli range from 1 to 30 cm in size, are subrounded, and typically scoriaceous. Accretionary lapilli can also be present. Locally Mp is interbedded with lacustrine sediments or thin surge deposits. Peperite is also present. Mp is altered to palagonite in some exposures. Intermediate clasts within the tuff are interpreted to be vesicular clast of Mt. Mp was previously mapped as Palagonite Tuff (Tpt) by Greene (1984) and Basaltic Tuffaceous Rocks (Tslb) by Ach (1988) and Ach et al. (1991).
Mcfp Reworked Pumiceous Deposits (Miocene) – Sedimentary deposits consisting of unconsolidated and variably reworked rounded pumice lapilli in an ashy matrix. Weathered and fresh surfaces are various shades of grey. The unit is weakly laminated, is generally un-graded, but small-scale normal grading is present locally. Pumice-rich horizons appear to have formed by floating of larger pumice lapilli to the lake surface. This unit is always associated with, and typically surrounds postcaldera rhyolitic domes or flows, and represents reworking of the deposits of phreatomagmatic eruptions associated with emplacement of rhyolitic lavas into water or wet sediments. Previously grouped into an undifferentiated sedimentary package termed the High Rock Sequence by Bonham (1969), and as tuffaceous sedimentary rocks by Greene (1984).
Miab Tuff of Alkali Flat and Tuff of Badger Mountain (ca. 15.9-15.7 Ma) – Nonwelded and incipiently welded ignimbrite predomi-nantly exposed south and east of Badger Mountain. Miab is composed of numerous thin flows which are probably related to emplacement of Mlmm lavas. Colors range from white, tan, to light-grey when fresh, and brown to rust-red when weathered. Typically contains 60-80% rounded white to tan pumice lapilli 1- to 5-cm in size in an ashy matrix . Weakly welded ignimbrite become more strongly welded west of the mapping area near Wall Canyon. Increase in the degree of welding correlates with changes of color in the devitrified groundmass from tan to pink, light-purple, light-blue, and blue-green. Welded Miab exhibits a planar texture defined by flattened pumices, which have a large aspect ratio (> 5:1). The ignimbrite contains 36-50% phenocrysts. Quartz and sanidine are clear in non-welded and incipiently welded deposits and become smoky and adularescent-blue, respectively, in more strongly welded deposits. Phenocrysts include sanidine (16-22%), quartz (20-27%), and trace amounts of sodic amphibole, resorbed ferrohedenbergite, and Fe-Ti oxides. I distinguish between the petrographically similar 16.0 Ma Soldier Meadow Tuff based on younger 40Ar/39Ar ages and major- and trace-element geochemistry.
Mlch Crystal-poor Rhyolite Lavas of Cherry Spring (ca. 16.0-15.6 Ma) – Weakly porphyritic rhyolite dikes and lava flows can be glassy, perlitic when hydrated, devitrified, or spherulitic where massive. Outcrops are strongly silicified, small in volume, and commonly have mm-scale flow banding with stretched vesicles. Weathered surfaces are dark-brown or rust colored; fresh surfaces are pale-red, pink, brown, rust or grey. This lava contains sanidine (5%) up to 2 mm in size. A small lava flow near Butcher Flat contains distinctive euhedral 0.5-2 mm long feldspar that make up approximately 15-20% of the rock. Because the lavas are pervasively silicified, mafic phases are strongly oxidized and are largely absent, and the lava appears glassy on fresh surfaces with secondary quartz or chalcedony filling vesicles and voids. This lava is distinguished from Mlxp in the Bear Buttes Quadrangle by its limited aerial extent, lower total phenocryst content, and a paucity of quartz and zircon. The range in 40Ar/39Ar ages suggest multiple distinct flows, but I group them here based on their small volume, similar stratigraphic relation and trace-element compositions. They are locally overlain by Miab.
Mcf Lacustrine Caldera-Fill Deposits (Miocene) – Caldera-fill sediments deposited in a quiescent shallow lake environment. These deposits are locally interbedded with thin Mp, Mcfp or ash layers. Individual beds are thinly laminated (mm-scale), fine-grained, diatomaceous, white to light-yellow in color, and locally cross-bedded where reworked. Petrified logs or wood chips are a hallmark feature of this unit. When silicified, the lacustrine sediments are replaced by silica (chert), and break into semi-transparent chips of various colors. Previous mapping by Bonham (1969) grouped Mp, Mcf, and Mcfp deposits into an undifferentiated sedimentary package termed the High Rock Sequence. Mcf and Mcfp can be correlated with tuffacous sedimentary rocks mapped by Greene (1984), and termed the Virgin Valley Formation by Merriam (1910) in Virgin Valley Caldera to the north.
Ach, A.J., 1988, Geologic map of the Yellow Hills East Quadrangle, Washoe and Humboldt Counties, Nevada, U.S. Geological Survey Miscella-neous Field Studies Map MF-2029.
Ach, A.J., Bateson, J.T., Turrin, B.D., Keith, W.J., Noble, D.C., and Swisher, C.C., 1991, Geologic map of the High Rock Lake Quadrangle, Washoe and Humboldt Counties, Nevada, U.S. Geological Survey Miscellaneous Field Studies Map MF-2157
Bonham, H.F., and Papke, K.G., 1969, Geology and mineral deposits of Washoe and Storey Counties, Nevada: Nevada Bureau of Mines and Geology Bulletin, v. 70, p. 1-140.
Greene, R.C., 1984, Geologic appraisal of the Charles Sheldon Wilderness Study Area, Nevada and Oregon: U.S. Geological Survey Bulletin, 1538-A, p. 13-34.
Merriam, J.C., 1910, Geologic history, prt 1 of Tertiary mammal beds of Virgin Valley and Thousand Creek in northwest Nevada: California University Department of Geology Bulletin, v. 6, p. 21-53.
Noble, D. C., McKee, E. H., Smith, J. G., and Korringa, M. K., 1970, Stratigraphy and geochronology of Miocene rocks in northwestern Nevada: U.S. Geological Survey Professional Paper 700-D, p. D23-D32.
Mlmm Mahogany Mountain Rhyolite Lavas (ca. 15.9-15.5 Ma) – Informally named the Rhyolite of Badger Mountain by Greene (1984), I suggest the name Mahogany Mountain Rhyolite Lava to avoid confusion with the older Badger Mountain Caldera. Weathered exposures of Mlmm are rust to reddish-brown and exhibit thick, centimeter- to meter-scale flow-banding. Well-developed pressure ridges can be mapped from air photos for several kilometers. The lava is typically strongly porphyritic with distinctive adularescent-blue sanidine (18-22%) and black- to smoky quartz (16-20%). Ferrohedenbergite, sodic amphiboles, zircon, and resorbed Fe-Ti oxides are present in trace amounts within a granophyrically recrystallized groundmass that is blue to light-blue-green in color. Microcrystalline trachytic enclaves several cm in size can be found near vents. A newly identified weakly porphyritic lava capping the southwest side of Badger Mountain contains sanidine (1-2%), lacks quartz, and contains only trace amounts of Fe-Ti oxides. This crystal-poor member of Mlmm is compositionally similar to the crystal-rich member, and the weathering color and centimeter- to meter-scale flow-banding are also similar.
Wall Canyon Rhyolite Lava (14.55±0.07 Ma) – Aphyric to weakly porphyritic lava domes and flows that erupted through lacustrine sediments (Mcf and Mcfp) and phreatomagmatic deposits (Mp). The lavas are light grey to blueish on fresh devitrified surfaces and vesicular near the tops of flows. Obsidian nodules are locally present. Small (0.25-0.5mm) euhedral sanidine are the most abundant phenocrysts (<1%), but trace amounts of quartz and Fe-Ti oxides are also present. The lavas exhibit mm- to cm-scale flow laminae. Aphanitic dark-colored magmatic enclaves are locally abundant near vents.
Talus deposits (Quaternary) – Unconsolidated debris sourced locally, typically along eroded normal fault scarps.
Ql
Ql
Mcf Mcfp
Mlmm
Mt
? ?
Mlch
Mism
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MC23714.84 ± 0.03
MC23815.59 ± 0.02
MC31415.73 ± 0.02
MC429B15.84 ± 0.02
MC33315.56 ± 0.02
MC33014.55 ± 0.06
Contact: Solid where located to ±15 meters, dashed where located within ±30 meters. Dotted where concealed by alluviumNormal Fault: Ball and bar on downthrown side. Solid where located to ±15 meters, dashed where located within ±30 meters. Dotted where concealed by alluvium.
Location of inferred dikes
Strike and dip of sedimentary bedding
Strike and dip of flow planes in mafic lava flows
Strike and dip of flow planes in silicic lava flows
Strike and dip of compaction foliationin ignimbrites
EXPLANATION
10
10
10
10
10
Dip and dip direction of fault planes
Approximate location of vents
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40Ar/39Ar age in million years and sample number (parentheses)
(100)10.00 ± 0.10!
Aphyric rhyolitic lava
Overlay Patterns
Alteration: Areas with pervasive hydrothermal alteration and/or silicification.
Crystal-rich rhyolitic lavas(>10% phenocrysts).
Crystal-poor rhyolitic lavas(<10% phenocrysts).
