USGS Scientific Investigations Map 3394 sheet 2 · 2020. 3. 17. · Tfcn Tla Tbg Tdc Tsav Twsn Ttra...
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Qal Qc Qf Qfo Qr Ql Qt Qlo Qm Tdup Tdua Tdu Ttra Thb Tw Tcr Thu Tcrn Thv Twsn Tfc Tfcn Tla Tdc Tsc Tscn Tbt Tsav Tas Tta Ttg Tgu Ted Tgs Tag Tbs Tbav Tbdk Tbfd Tbfs Tba Tbdh Tbfr Tblr Tbab Tbam Tbap Tbas Tia Tid Tir Tiap Tbdu Tbru Tbd Tbdn Tbrl Tfbd Tbr Tbrr Tbru Tbd Tbdn Tbrl Tbrn Tfbr Tbrb Tbmu Tbma Tbmp Tbmc Tyca Tyat Tyv Tydu Tyau Tyap Tyds Tya Tyaq Tyab Tyfr Tyd Tydb Tydt Tyir Tyid Tyia Tyip Tyig Tysr Tsa Trig Trid Tmp Tmd Tmdf Tmap Tma Tmr Tmds Tmv Trax Tra Trdx Trd Trv Trfr Trt Trdk Trds Trap Tras Ttma Ttrn Ttr Ttmb Ttmc Tbc Tjab Tja Tjdb Tjap Tjfr Tcv Tjv Tjd Tca Tcab Tcap Tcd Tjds Tjig Tjir Tjid Tjia Tev k s l Ml Mlb Dc Of Oh Om g m a u Tbg Outflow tuff Intracaldera tuff Bonanza Tuff Intracaldera landslide breccia IGNIMBRITE SHEETS AND RELATED ROCKS LAVAS AND RELATED ROCKS VOLCANIC SEDIMENTARY ROCKS INTRUSIONS Holocene Pleistocene QUATERNARY Oligocene TERTIARY TERTIARY TERTIARY SOUTH RIVER CALDERA (27.4 Ma) BACHELOR CALDERA (27.5 Ma) LA GARITA CALDERA CYCLE (28.0 Ma) PREVOLCANIC ROCKS Upper Paleozoic PRECAMBRIAN Wason Park Tuff Carpenter Ridge Tuff 1 Ar-Ar ages generalized from Lipman and others (2015) WESTERN SAN JUAN CALDERAS (28.3 Ma) CALDERA RELATIONS (age) 1 SURFICIAL DEPOSITS MIOCENE POSTCALDERA ROCKS GLACIAL DEPOSITS Sapinero Mesa Tuff LOCAL LAVAS AND VOLCANICLASTIC ROCKS NORTH PASS CALDERA CYCLE (32.2 Ma) BONANZA CALDERA CYCLE (33 Ma) Badger Creek Tuff (34.1 Ma) Lower Paleozoic MARSHALL CALDERA CYCLE (33.8–33.9 Ma) Miocene Miocene Hinsdale Formation POSTCALDERA ROCKS PALEOZOIC Huerto Andesite Fish Canyon Tuff Intrusions of uncertain age and affinity (precaldera and caldera) Andesite of Lone Tree Gulch Tuff of Big Dry Gulch Debris-flow deposit of Buffalo Pass Campground Saguache Creek Tuff Bedded rhyolitic tuff SEDIMENTARY ROCKS OF THE RIO GRANDE RIFT ZONE Dry Union Formation ROCKS OF THE SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD Andesitic conglomerate Travertine Granitoid intrusions Postcaldera lavas Tracy volcano ( <32.5?–33.5 Ma) Sargents Mesa volcano Rawley volcanic complex (~33.2–33.8 Ma) CONEJOS FORMATION, Middle units (~33.2–33.8 Ma) Postcollapse caldera-filling rocks Thorn Ranch Tuff Intracaldera breccia CONEJOS FORMATION, Upper units (~30 Ma) SAWATCH-RANGE IGNIMBRITE Jacks Creek volcano (~34.5 Ma) Early lavas and proximal breccias Proterozoic Volcaniclastic rocks Eocene? CORRELATION OF MAP UNITS [Dotted unit-box boundaries indicate gradational welding or other emplacement contacts within eruptive units. Some major formation names are labelled. Some units of a formation are divided by emplacement type] CONEJOS FORMATION, Lower units LIST OF MAP UNITS [Some unit exposures on the printed or plotted maps are too small to distinguish the color for unit identification. These units are labeled where possible; unlabeled units are attributed in the database] SURFICIAL DEPOSITS Alluvium (Holocene) Colluvium (Holocene) Landslide deposits (Holocene) Alluvial-fan deposits (Holocene) Talus (Holocene) Older alluvial-fan deposits (Pleistocene) Older landslide deposits (Pleistocene) GLACIAL DEPOSITS Rock glacier (Holocene) Till (Pleistocene; Pinedale glaciation) MIOCENE POSTCALDERA ROCKS Travertine (Miocene?) Dry Union Formation Dry Union Formation, undivided Dry Union Formation, Precambrian clasts dominant Dry Union Formation, volcanic clasts dominant Hinsdale Formation Mafic lavas OLIGOCENE VOLCANIC ROCKS IGNIMBRITES AND ASSOCIATED ROCKS OF THE CENTRAL SAN JUAN CALDERA COMPLEX Rocks Erupted from South River Caldera (27.4 Ma) Wason Park Tuff Rocks Erupted from Bachelor Caldera (27.55 Ma) Carpenter Ridge Tuff Nonwelded ignimbrite Welded rhyolite ignimbrite Rocks of La Garita Caldera Cycle (28.0 Ma) Huerto Andesite Olivine andesite and hornblende andesite-dacite, undivided Bedded breccia and conglomerate Fish Canyon Tuff Welded outflow ignimbrite sheet Nonwelded to partly welded ignimbrite Local Volcaniclastic Rocks of Saguache Paleovalley Andesitic conglomerate Older Ignimbrite Related to Western San Juan Calderas (28.3 Ma) Sapinero Mesa Tuff Conejos Formation, Upper Units (~30 Ma) Andesite of Lone Tree Gulch (Saguache Valley) Tuff of Big Dry Gulch Debris-flow deposit of Buffalo Pass Campground Distal conglomerate CALDERA-RELATED ROCKS PREDATING THE WESTERN AND CENTRAL CALDERA CLUSTERS North Pass Caldera Cycle (32.2 Ma) Saguache Creek Tuff Nonwelded crystal-poor rhyolite Partly to densely welded crystal-poor rhyolite Bedded rhyolitic tuff Rocks of the Bonanza Caldera Cycle (33 Ma) Granitoid intrusions Spring Creek intrusion Aplitic granite Porphyritic granite Porphyritic granite of Alder Creek Turquoise Mine intrusion Granodiorite Intrusive andesite Granitoid rocks, undivided Eagle Gulch Dacite Intrusions of uncertain age and affinity Rhyolite Dacite Plagioclase andesite Andesite Postcaldera lavas and volcaniclastic rocks Lake-bed sedimentary rocks Hayden Peak Dacite K-feldspar dacite Dacite lavas Finely porphyritic dacite lavas Porphyry Peak Rhyolite Rhyolite of Laughlin Gulch Andesitic rocks Upper dark andesite Breccia cone and spatter Andesite flows Sparsely porphyritic andesite lavas Plagioclase-rich andesite lavas Squirrel Gulch andesite Volcaniclastic rocks Bonanza Tuff Dacite Upper outflow dacite Main dacite Nonwelded dacite Intracaldera dacite fracture fill Rhyolite Upper rhyolite Main intracaldera rhyolite Lower rhyolite Brecciated rhyolite Rheomorphic rhyolite Nonwelded rhyolite Intracaldera rhyolite fracture fill Intracaldera landslide breccia Mixed clast or undivided megabreccia Andesite- and dacite-clast megabreccia Plagioclase-andesite-clast megabreccia Proterozoic-clast megabreccia VOLCANIC ROCKS PREDATING BONANZA CALDERA Conejos Formation, Middle Units Tracy Volcano (<32.5?–33.5 Ma) Intrusions Rhyolite Dacite Andesite Plagioclase andesite Granodiorite Caldera-filling(?) rocks Caldera andesite Dacite caldera-wall talus Andesite caldera-wall talus and landslide breccia Flank lavas Volcaniclastic rocks Upper dacite Upper andesite Plagioclase andesite Small-phenocryst dacite Andesite Quartz-xenocrystic andesite Andesite breccia Rhyolite Rhyolitic sedimentary rocks Dacite of Red Rock Canyon Dacite lavas Dacite breccia Sargents Mesa Volcano Andesite-dacite lava Rawley Volcanic Complex (~33.2–33.8 Ma) Intrusions Granodiorite Dacite Lavas and related rocks Volcaniclastic rocks Rhyolite Rhyolite tuff Sanidine-bearing dacite Dacite shatter breccia Dacite Small-phenocryst dacite Plagioclase andesite Andesite shatter breccia Andesite Andesite scoria Rocks of the Marshall Caldera Cycle (33.8–33.9 Ma) Postcollapse caldera-filling rocks Dacite lavas, undivided Small-phenocryst dacite Plagioclase andesite lavas Andesite lavas, undivided Rhyolite lava Sheep Mountain Dacite Volcaniclastic rocks Pitch-Pinnacle Formation Thorn Ranch Tuff Densely welded tuff Non-welded to partly welded tuff Megabreccia, andesite clasts dominant Megabreccia, clasts of Badger Creek Tuff Megabreccia, Proterozoic clasts dominant Sawatch-Range Ignimbrite (Erupted from Aetna Caldera) Badger Creek Tuff Conejos Formation, Lower Units Early lavas and proximal breccias Dacite Plagioclase andesite Andesite Andesite breccia Volcaniclastic rocks Jacks Creek Volcano (~34.5 Ma) Intrusions Rhyolite Dacite Andesite Granodiorite Lavas and related rocks Dacite Small-phenocryst dacite Plagioclase andesite Andesite Andesite cone breccia Rhyolite Volcaniclastic rocks PREVOLCANIC ROCKS LOWER TERTIARY SEDIMENTARY ROCKS Conglomerate and sandstone (Eocene?) PALEOZOIC SEDIMENTARY ROCKS Upper Paleozoic sedimentary rocks Sharpsdale Formation (Pennsylvanian) Kerber Formation (Pennsylvanian) Lower Paleozoic sedimentary rocks Leadville Limestone (Mississippian) Brecciated and silicified rocks Chaffee Group (Devonian and lower Mississipian) Fremont Dolomite (Upper Ordovician) Harding Quartzite (Middle Ordovician) Manitou Limestone (Lower Ordovician) Lower Paleozoic rocks, undivided PRECAMBRIAN ROCKS Aplite of Alder Creek Granitoid intrusive rocks, undivided Metamorphic rocks, undivided Precambrian crystalline rocks, undivided EXPLANATION OF MAP SYMBOLS Contact—Dashed where approximately located; dotted where grada- tional. Internal contacts (dash-dot) delineate some conspicuous lava flows or ash-flow cooling breaks within map units. No contact shown where boundary between map units is uncertain or indefinite Fault—Dashed where approximately located; dotted where concealed or occupied by intrusive rocks. Bar and ball on downthrown side, where movement known Low-angle (thrust?) fault—Teeth on upper block Crest of elliptical resurgent dome Quaternary slump block—Dashed where approximately located. Hachures on downthrown side Monoclinal hinge zone—Dotted where concealed. Triple hachures indicate direction of uplift Unconformity along caldera wall—Dashed where approximately located; dotted where concealed Bonanza (33.12 Ma) Marshall (33.9 Ma) Bonanaza caldera topographic rim—Dashed where approximately located; dotted where concealed. Erosionally modified Syncline Strike and dip of beds Inclined Vertical Strike and dip of foliation Inclined Vertical Trend of lineation—Showing bearing and plunge. Defined by elongate pumice lenses or flow structures. May be combined with foliation symbol Fracture-fill tuff Dacitic tuff (Tfbd) Rhyolitic tuff (Tfbr) Sample location—X-ray fluorescence (XRF) chemical analyses in tables 3, 5. 39 Ar/ 40 Ar ages shown in millions of years (Ma) on map and in tables 4, 6. Sample abbreviation at end of age: b, biotite; gm, groundmass separate; h, hornblende; no abbreviation, sanidine. (Table 5, 6 available at https://doi.org/10.3133/sim3394) XRF 40 Ar/ 39 Ar XRF and 40 Ar/ 39 Ar Hydrothermally altered rock—Areas of conspicuous supergene acid-sulfate alteration, mainly due to dissolution of pyrite, in vicinity of subvolcanic intrusions Qal Qc Qr Qm Tcrn Tcr Thu Thv Tfc Tfcn Tla Tbg Tdc Tsav Twsn Ttra Ql Qf Qt Qfo Qlo Tdu Tdup Tdua Tscn Tsc Tbt Tas Tgs Tag Ttg Tta Tgu Ted Tir Tid Tiap Tia Tbs Tbdh Tbdk Tbfd Tbfs Tbfr Tblr Tmd Tmdf Tmap Tma Tmr Tmds Tmv Tmp Trdk Trdx Trd Trv Trfr Trig Tsa Trid Tyd Tydb Trt Trds Trap Trax Tra Tras Tfbd Tbru Tbr Tbdu Tbd Tbdn Tbav Tba Tbap Tbas Tbrl Tbrb Tbrr Tbrn Tfbr Tyau Tyap Tyds Tyv Tydu m u a g Tyia Tyip Tyig Tyir Tyid Tbmp Tbmc Tbmu Tbma Tyat Tyca Tydt Tya Tyaq Dc Of Oh Ml Mlb s Tev k Om l Tjap Tja Tjab Tjd Tjds Tjdb Tca Tcab Tcv Tcd Tbc Tcap Ttmc Ttma Ttmb Tjia Tjig Tjir Tjid Tjfr Tjv Tyab Tyfr Tysr Tbab Tbam Ttr Ttrn Thb Tw 40 35 34.08 ±0.13b 34.08 ±0.13b S a n L u i s V a l l e y R C LA GARITA (27.5) SOUTH RIVER La Garita South La Garita (28.0) Lake City (23.0) Silverton (27.6) SJ (28.3) UC (28.6) Uncompahge (28.3) Creede (26.9) Platoro (29.8–28.4) Summitville (29.0) La Garita South (28.0) Lost Lakes (28.5) South River Marshall (33.9) NORTH PASS (32.2) Bonanza (33.2) Bachelor (27.55) COCHETOPA PARK (26.9) NM (26.9) G u n n i s o n v a l l e y CC RC Lower Conejos map area (Lipman, 1975) Del Norte map area (Lipman, 1976c) Central San Juan map area (Lipman, 2006) San Juan Basin Bonanza Caldera map area (this map) Cochetopa-North Pass map area (Lipman, 2012) Platoro map area (Lipman, 1974) Lake City map area (Lipman, 1976a) 0 50 KILOMETERS 10 20 30 40 0 10 20 30 MILES 108° 38° 37° 107° 106° Villa Grove The Gate v v v v PP HP SM AP FM PO UP SP MO Marshall Creek San Luis Valley Sangre de Cristo Range RM Salida Kerber Creek Bz Little KC WH MARSHALL CALDERA SaM Middle Creek hinge UAV Andesite Dacite Basaltic trachy andesite Trachyandesite Trachydacite Rhyolite K 2 O + Na 2 O, in weight percent SiO 2 , in weight percent 11 10 9 8 7 6 5 50 60 70 80 Silicic intrusions Intermediate intrusions Postcaldera silicic lavas Postcaldera intermed. lavas Bonanza Tuff Rawley complex Tracy volcano Jacks Cr volcano EXPLANATION SC T TR T BZ T BC T Post- caldera lavas Granitic intrusions Marshall caldera lavas Rawley complex Tracy volcano Jacks Creek volcano Marshall cycle Bonanza cycle Age, Ma 35 34 33 32 No. Mean Samples Age (Ma)* NORTH PASS CALDERA CYCLE Saguache Creek Tuff 6 32.25±0.05 BONANZA CALDERA CYCLE Resurgent intrusions Fine-grain granodiorite/andesite (sn, gm) 4 32.83±0.21 Aplitic granite 2 33.28±0.06 Late postcollapse lavas Dacite of Hayden Peak (gm) 2 32.66±0.18 Megacrystic dacite 2 32.76±0.02 Caldera-fill lavas (tilted) Sanidine dacite 3 33.03±0.03 Porphyry Peak Rhyolite 4 33.04±0.04 Andesite-dacite (bio, gm) 4 33.0–33.3 Bonanza Tuff Intracaldera 4 33.05±0.06 West-side dacite 13 33.12±0.03 East-side rhyolite (Gribbles Park Tuff) 6 33.14±0.09 Precaldera central volcanoes Sargents Mesa volcano (bio, gm) 4 32.99±0.15 Rawley volcanic complex (gm) 8 33.3–33.7 Precursor(?) rhyolitic lavas 2 33.73±0.09 OLDER VOLCANIC ROCKS Tracy volcano (sn, bio, gm) 10 31.6–33.7 Marshall caldera cycle Postcollapse lavas (bio, gm) 6 33.4–33.6 Thorn Ranch Tuff 5 33.93±0.10 Aetna caldera, Badger Creek Tuff 2 34.06±0.15 Pre-Bonanza Jacks Creek volcano (bio, gm) 8 34.2–35.3 *Ages are normalized to Fish Canyon Tuff at 28.02 Ma A B Unit (sanidine age, except as noted) NM AT 500 1,000 SP AT FC 5,000 km 3 SM CR WP NM SMT LJ WM GP SC (Bcr) B BC 38 34 36 30 32 26 28 22 0 0 24 Age, in millions of years Volume (individual tuff sheets), km 3 Sawatch Range Trend Southeast and West San Juan Loci Central San Juan Transition Lake City Latir VOLCANIC LOCI CP TR 16,000 Cumulative volume, km 3 12,000 8,000 4,000 4,000 km 3 /m.y. 2,000 km 3 /m.y. 1,000 km 3 /m.y. 250 km 3 /m.y. SP SOUTHWEST NORTHEAST Sapinero Mesa Tuff CONEJOS FORMATION (undivided) Volcanics of Cochetopa Hills (North Pass) Rawley volcano (Rawley Andesite, includes dacite and rhyolite) Postcaldera volcanics (Marshall) (Sheep Mountain Dacite, andesite, other dacite) Postcaldera volcanics (Bonanza) (upper andesite, dacite, Porphyry Peak Rhyolite, and others) CONEJOS FORMATION, upper lava units (mostly southwest of Bonanza map area) [No lavas preserved?] REGIONAL LAVA UNITS INTRACALDERA LAVA UNITS 28 30 32 34 36 Age, million years Jacks Creek Summer Coon Tracy Thorn Ranch Tuff (34 Ma) Bonanza Tuff (33 Ma) Wall Mountain Tuff (37 Ma) Saguache Creek Tuff (32 Ma) Fish Canyon Tuff (28 Ma) MARSHALL CALDERA BONANZA CALDERA R i n g F a u l t s 5 5 5 35 San Luis Valley (Rio Grande rift) Kerber Creek Sheep Mountain Antora Peak Whale Hill Pre-Bonanza andesite and dacite Porphyry Peak * * * * * * * * * * * * * * * * [Mapped 2006-11] Jacks Creek volcano Monoclinal hinge Sargents Mesa The Gate Findley Gulch Saguache Peak HH PC SC Caldera- floor lavas Resurgence crest Figure 14B Egd * * 106°15' 38°15' 38°22'30" 38°07'30" 106° 106°07'30" Flagstaff Mountain Post- Bonanza andesite Qal Qal Poncha Pass 15 20 60 45 35 25 30 40 35 45 65 75 80 30 55 25 20 35 55 Figure 14A 55 B O N A N Z A T O P O R I M S a g u a c h e C r e e k gr gd a T O P O R I M OLIGOCENE EXPLANATION Quaterary surficial deposits Post-Bonanza ignimbrites Resurgent intrusions Granite Granodiorite Andesite Eagle Gulch Dacite Post-Bonanza lavas—Andesite to rhyolite Bonanza Tuff—Rhyolite to silicic andesite Pre-caldera lavas—Andesite to rhyolite Andesite and dacite Rhyolite Sheep Mountain Dacite Thorn Ranch Tuff—Rhyolite to dacite Paleozoic sedimentary rocks Precambrian granitic rocks Bonanza caldera—Dashed where approximate; dotted where concealed Eroded topographic rim Ring faults—Bar and ball on downthrown block. Dashed where approximate; dotted where concealed Marshall caldera—Dashed where approximate Wall unconformity Fault—Bar and ball on downthrown block. Dashed where approximate; dotted where concealed Monoclinal hinge Resurgent crest Inclined bedding—Showing strike and dip Some caldera megabreccia sites Photograph Site—Includes figure number; C, cover. Bar indicates view direction Dike—Jacks Creek volcano Cross section location—See cross sections in figure 14 16 * 20 gr Qal gd Egd a 0 5 MILES 0 10 KILOMETERS 5 16 C 13 Fish Canyon T, 28.0 Saguache Creek T, 32.2 Bonanza Tuff, 33.15 Fine-grain andesite Malpais andesite, 33.0 Thorn Ranch Tuff (intracaldera), 33.9 Post-collapse lavas of Marshall caldera Rawley lavas, ~33.7–33.3 Jacks Creek volcano, ~35.3–34.2 Paleozoic rocks (?) Proterozoic granitic rocks MARSHALL CALDERA Topographic rim SOUTH NORTH Guffey Canon City Rosita Saguache Salida TR TR GP Marshall Creek Caldera Bonanza Caldera SAN JUAN VOLCANIC FIELD Thorn Ranch Tuff distribution [Eroded/covered] [Covered] [Eroded] [Eroded] Mostly rhyolite [60+ km] [80+ km] BONANZA TUFF (33.12 Ma) East Gulch Tuff distribution Mount Princeton batholith THIRTYNINE MILE VOLCANIC AREA Mount Aetna cauldron Grizzly Peak cauldron Buffalo Peaks Triad Ridge [60+ km] Mostly dacite 0 30 KILOMETERS 15 0 10 20 MILES 39°30' 106°30' 106° 105° 105°30' 39° 38° 38°30' [60+ km] TR GP EXPLANATION Caldera boundary Thorn Ranch Tuff Gribble’s Park/Bonanza Tuff Distance from Bonanza Caldera Bonanza Tuff, areal distribution—Dashed boundaries approximate WEST EAST Landslide breccia (mainly andesite) ~500 m Gribbles Park Tuff Prior intracaldera thickness, estimated: 300 m (Varga and Smith, 1984); now 2.5 km Intracaldera previously mapped as simple sequence: andesite-dacite-rhyolite, with many fault repeats (Burbank, 1932) Dacite Rhyolite INTRACALDERA A B Flagstaff Mountain Kerber Creek Brewery Creek Wagonbox Creek Slaughterhouse Creek g (~2.5 km intracaldera Bonanza Tuff; >1 km postcaldera lavas = ~4–5 km total caldera subsidence) Feet, above sea level SOUTHWEST CALDERA RING FAULTS 12,000 10,000 8,000 Resurgent granodiorite intrusion Intracaldera rhyolite and dacite tuff, ~2.5 km thick Andesite breccia (m) CALDERA RIM Burned Mountain Caldera-floor andesite-dacite (a) dacite (d) r m m r r d d a a d r Post-collapse lavas A a f Resurgent aplitic granite intrusion r 10,000 8,000 Near-vertical northeast flank, resurgent dome d m NORTHEAST 12,000 Whale Hill, 12,162’ BROAD CREST OF RESURGENT DOME (granitic intrusive rocks exposed just beyond plane of section) C A L D E R A F L O O R C A L D E R A F L O O R 0 1 KILOMETER 2 0 0.25 0.5 MILES EXPLANATION Alluvial-fan deposits Glacial moraine Caldera-collapse megabreccia Dacitic Bonanza Tuff Rhyolitic Bonanza Tuff Andesitic lava flows Precambrian rocks a d f g m r 0 2 KILOMETERS 1 0 0.5 1 MILE EXPLANATION Upper Paleozoic (clastic rocks) Sharpsdale/Kerber Formations Lower Paleozoic (carbonate rocks) Leadville Dyer Fremont Manitou Formation Feet, above sea level 9,000 7,000 8,000 Kerber Creek South caldera rim ~600 m (2,000 ft) displacement ~1.5 km (5,000 ft) displacement? Precambrian granite S L L L S Precambrian granite Precambrian granite SOUTH NORTH South flank resurgent dome 2,150 Meters,above sea level 2,450 2,750 D D F M M M F F B S L D F M 9300 9300 9500 9500 9500 9700 10,200 10,300 10,100 10,000 9900 9800 9700 9600 9400 9900 10,000 9800 9600 Tree Gulch Lane Schoolhouse Gulch A A A M A A A R R R A A R R R R R R M R R R M R M A A A A A A R R R R R R R A A A A R R A R A R R A R A A A R R R R R D D A A A R A A A A A A R R R A A A A A R R R M A A R A R R A A R A A A R R R A A A A R R A R A R A A A A A D A R R A A A A A A A D A A A R R R R R A A A A A A A A MBR FLOW-LAYERED RHYOLITE (Rheomorphic Bonanza Tuff?) DACITE ANDESITE R A Q Q Q Q Q 0 100 METERS 50 0 100 200 FEET 14.6' 14.4’ 14.2’ 14.8' 05.6' 05.4' 05.2' C B Figure 2. Map showing calderas (maroon, purple, green, and brown calderas discussed here; red, others) of the San Juan volcanic region (blue) and location of map area. Dotted lines, inferred locations of caldera margins where concealed by younger rocks. Caldera ages (in parentheses) are in Ma. Caldera abbreviations: CC, Cebolla Creek; NM, Nelson Mountain; RC, Rat Creek; SJ, San Juan; UC, Ute Creek. Published map areas are outlined and labeled. Figure 14. Cross sections, showing structures and stratigraphic relations across Bonanza caldera; locations shown on fig. 10. A. South- west topographic rim, across crest of resurgent dome (Whale Hill), to west margin of San Luis Valley (segment of Rio Grande rift zone) shows large ring-fault displace- ments, great thickness of intracaldera Bonanza Tuff, steep dips on flanks of resurgent dome, and inferred location of subcaldera granitoid intrusions. B. South caldera margin, showing displacements across ring faults. Figure 15. A, Geologic map of sheet-like mass of flow-layered rhyolite (see B, C, photo 10.3), interpreted as rheomorphic lower Bonanza Tuff (unit Tbrr), along upper Schoolhouse Gulch (GG). Letter symbols mark locations of GPS-located outcrops and areas of monolith- ologic float inferred to reflect immediately underlying bedrock: A, andesite; D, dacite; M, large blocks of andesite, in matrix of nonwelded rhyolite tuff; MBR, mega- and meso-breccia of andesite blocks surrounded by rhyolite matrix; Q, colluvium; R, flow-lay- ered rhyolite. Topographic contour interval, 50 feet; thick gray lines, graded gravel roads of the Elk Horn Ranch subdivi- sion. B, Highly irregular blocks (outlined in orange; dashed where approximate) of andesite laharic breccia, enclosed within flow-laminated rheomorphic rhyolite tuff. Although flow laminated, the rhyolite contains centimeter-size angular andesite fragments (hiking pole, ~0.9 m). C, Closer view of contact (dashed line) between block of andesite laharic breccia and enclosing dense rhyolite that remains fluidal at the contact. Several small fragments of andesite enclosed in the fluidal rhyolite are circled. Figure 5. Oblique view of Bonanza caldera area, viewed from the south. Line definitions: (long dashed where approximate, short dashed where concealed) orange, erosionally modified remnants of Bonanza topographic caldera rim; black, caldera-subsidence ring faults; yellow, crest of elliptical resurgent dome; green, margin of Marshall caldera. Topographic-name abbreviations: AP, Antora Peak; Bz, town of Bonanza; FM, Flagstaff Mountain; HP, Hayden Peak; MO, Mount Ouray; PO, Poncha Pass; PP, Porphyry Peak; RM, Round Mountain; SaM, Sargents Mesa; SM Sheep Mountain; SP, Saguache Peak; UAV, upper Arkansas Valley; UP, Ute Pass; WH, Whale Hill. Imagery from Google Earth (9/24/13). Figure 6. Alkali-silica diagram, showing compositional variations in rocks of the Bonanza map area. Volcanic and intrusive rocks of the Bonanza caldera cycle are modestly more alkalic than those from the preceding central volcanoes (Tracy, Jacks Creek). Figure 7. A. Summary of 40 Ar/ 39 Ar age determinations for igneous rocks of the Bonanza region, illustrating the narrow weighted-mean ages obtained for multiple samples of individual ignimbrites, lavas, and intrusions of the Bonanza cycle. Abbreviations: bio, biotite; gm, groundmass; sn, sanidine. Data from tables 3, 6. B. Eruptive chronology of the Bonanza region shows precursors to eruption of the Bonanza Tuff, including several central volcanoes and Marshall caldera, source of the Thorn Ranch Tuff at 33.9 Ma. Abbreviations: BC T, Badger Creek Tuff; BZ T, Bonanza Tuff; SC T, Saguache Creek Tuff; TR T, Thorn Ranch Tuff. Figure 8. Age-location-volume plot, showing southward progression of Tertiary ignimbrite-caldera volcanism in the Southern Rocky Mountain Volcanic Field (revised from Lipman, 2007). Colored vertical bars, volumes of individual ignimbrites, scale on left axis (data from table 1); shaded area, increasing cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT, Amalia Tuff; B, Bonanza Tuff; BC, Badger Creek Tuff; CP, Chiquito Peak Tuff; CR, Carpenter Ridge Tuff; FC, Fish Canyon Tuff; GP, Grizzly Peak Tuff; LJ, La Jara Canyon Tuff; Bcr, rhyolite of Barret Creek flow field; NM, Nelson Mountain Tuff; SC, Saguache Creek Tuff; SM, Sapinero Mesa Tuff; SMT, Snowshoe Mountain Tuff; SP, Sunshine Peak Tuff; TR, Thorn Ranch Tuff; WM, Wall Mountain Tuff; WP, Wason Park Tuff. Figure 9. Diagrammatic stratigraphic relations between regional ignimbrites and early interme- diate-composition lavas (Conejos Formation), northeast San Juan region. Thin-dashed lines delimit locally mapped lava units in relation to associated calderas. Thick-dashed lines (vertical), approx- imate age spans of mapped central volcanoes along east flank of the San Juan Mountains. Figure 10. Generalized geologic map of Bonanza and Marshall calderas, showing major stratigraphic units, structural features, and locations of cross sections (fig. 14). Margin of Marshall caldera (unconformity between caldera-fill and precaldera rocks), dashed where approximately located. Approximate topographic rim of Bonanza caldera inferred from distribution of intracaldera rocks and from erosionally modified present-day morphology. Rectangular grid, boundaries of 7.5' quadrangle maps. Abbreviations: HH, Houghland Hill; PC, Peterson Creek; SC, Spring Creek. Figure 11. Diagrammatic cross section through concealed south margin of Marshall caldera, showing inferred relations of caldera-filling volcanic units. Possible remnants of the topographic wall are marked by an east-west alignment of paleohills of Precambrian rocks. Post-collapse lavas and volcani- clastic rocks of this caldera cycle are onlapped by lavas of the Rawley volcanic complex, then by younger ignimbrites (Bonanza, Saguache Creek, and Fish Canyon Tuffs) and interleaved lavas. Ages, million years (Ma); T, Tuff. Figure 12. Areal extent of preserved erosional remnants of Bonanza Tuff (modified from McIntosh and Chapin, 2004). Eastern areas are dominated by crystal-poor rhyolite, while western area consists mainly of crystal-rich dacite. Figure also shows inferred distribution of the eastern outflow of Thorn Ranch Tuff and East Gulch Tuff. GP, Gribbles Park/Bonanza Tuff; TR, Thorn Ranch Tuff. Figure 13. Generalized regional thickness and compositional variations in the Bonanza Tuff. Outflow ignimbrite is mainly early erupted rhyolite to the east, mainly later-erupted dacite to the west. Intracaldera tuff contains multiple compositional zones and is ponded to thickness of at least 2.5 km, as exposed on west flank of resurgent dome. Intracaldera tuff was previously mapped as a simple andesite-dacite-rhyolite sequence with many fault repeats (Burbank, 1932). Intracaldera tuff thickness was previously estimated at 300 m (Varga and Smith, 1984) but is currently considered 2.5 km thick. A. Photograph showing densely welded intracaldera dacite, with flattened pumice lenses as long as 0.5 m that define a down-dip lineation, east side road above Squirrel Gulch. B. Photograph showing lower rhyolite, densely welded to its base without any intervening tephra deposits, deposited against caldera-wall andesite at Windy Point (above white dashed line). See figure 10 for photo location. Printed on recycled paper ISSN 2329-1311 (print) ISSN 2329-132X (online) https://doi.org/10.3133/sim3394 Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, Colorado By Peter W. Lipman 2020 Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government This map or plate also is offered in an online, digital version. Users should be aware that, because of differences in rendering processes and pixel resolution, some slight distortion of scale may occur when viewing the online version on a computer screen or when printing it on an electronic plotter, even when it is viewed or printed at its intended publication scale For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS Digital files available at https://doi.org/10.3133/sim3394 and https://doi.org/10.5066/P911OL4Q. Suggested citation: Lipman, P.W., 2020, Geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey Scientific Investigations Map 3394, pamphlet 73 p., 2 sheets, scale 1:50,000, https://doi.org/10.3133/sim3394. Associated data for this publication: Robinson, J.E., and Lipman, P.W., 2020, Database for the geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/P911OL4Q. U.S. Department of the Interior U.S. Geological Survey Scientific Investigations Map 3394 Sheet 2 of 2 Pamphlet accompanies map
Transcript of USGS Scientific Investigations Map 3394 sheet 2 · 2020. 3. 17. · Tfcn Tla Tbg Tdc Tsav Twsn Ttra...
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IGNIMBRITE SHEETS AND RELATED ROCKS LAVAS AND RELATED ROCKS VOLCANIC SEDIMENTARY ROCKS INTRUSIONS
Holocene
PleistoceneQUATERNARY
Oligocene
TERTIARY
TERTIARY
TERTIARY
SOUTH RIVERCALDERA(27.4 Ma)
BACHELORCALDERA(27.5 Ma)
LA GARITA CALDERACYCLE (28.0 Ma)
PREVOLCANIC ROCKS
UpperPaleozoic
PRECAMBRIAN
Wason Park Tuff
Carpenter Ridge Tuff
1Ar-Ar ages generalized from Lipman and others (2015)
WESTERN SAN JUANCALDERAS (28.3 Ma)
CALDERARELATIONS
(age)1
SURFICIAL DEPOSITS
MIOCENE POSTCALDERA ROCKS
GLACIAL DEPOSITS
Sapinero Mesa Tuff
LOCAL LAVAS AND VOLCANICLASTIC ROCKS
NORTH PASS CALDERACYCLE (32.2 Ma)
BONANZA CALDERA CYCLE (33 Ma)
Badger Creek Tuff (34.1 Ma)
LowerPaleozoic
MARSHALL CALDERACYCLE (33.8–33.9 Ma)
Miocene
Miocene
Hinsdale FormationPOSTCALDERA
ROCKS
PALEOZOIC
Huerto Andesite
Fish Canyon Tuff
Intrusions of uncertain age andaffinity (precaldera and caldera)
Andesite of Lone Tree Gulch
Tuff of Big Dry Gulch
Debris-flow deposit ofBuffalo Pass Campground
Saguache Creek Tuff
Bedded rhyolitic tuff
SEDIMENTARY ROCKS OF THE RIO GRANDE RIFT ZONE
Dry Union Formation
ROCKS OF THE SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD
Andesitic conglomerate
Travertine
Granitoid intrusions
Postcaldera lavas
Tracy volcano ( <32.5?–33.5 Ma)
Sargents Mesa volcano
Rawley volcanic complex (~33.2–33.8 Ma)
CONEJOSFORMATION,Middle units
(~33.2–33.8 Ma)
Postcollapse caldera-filling rocks
Thorn Ranch Tuff Intracaldera breccia
CONEJOS FORMATION,Upper units (~30 Ma)
SAWATCH-RANGEIGNIMBRITE
Jacks Creek volcano (~34.5 Ma)Early lavas and proximal breccias
Proterozoic
Volcaniclastic rocks
Eocene?
CORRELATION OF MAP UNITS
[Dotted unit-box boundaries indicate gradational welding or other emplacement contacts within eruptive units. Some major formation names are labelled. Some units of a formation are divided by emplacement type]
CONEJOSFORMATION,
Lower units
LIST OF MAP UNITS[Some unit exposures on the printed or plotted maps are too small to distinguish the color for unit identification. These units are labeled where possible; unlabeled units are attributed in the database]
SURFICIAL DEPOSITS
Alluvium (Holocene)
Colluvium (Holocene)
Landslide deposits (Holocene)
Alluvial-fan deposits (Holocene)
Talus (Holocene)
Older alluvial-fan deposits (Pleistocene)
Older landslide deposits (Pleistocene)
GLACIAL DEPOSITS
Rock glacier (Holocene)
Till (Pleistocene; Pinedale glaciation)
MIOCENE POSTCALDERA ROCKS
Travertine (Miocene?)
Dry Union Formation
Dry Union Formation, undivided
Dry Union Formation, Precambrian clasts dominant
Dry Union Formation, volcanic clasts dominant
Hinsdale Formation
Mafic lavas
OLIGOCENE VOLCANIC ROCKS
IGNIMBRITES AND ASSOCIATED ROCKS OF THE CENTRAL SAN JUAN CALDERA COMPLEX
Rocks Erupted from South River Caldera (27.4 Ma)
Wason Park Tuff
Rocks Erupted from Bachelor Caldera (27.55 Ma)
Carpenter Ridge Tuff
Nonwelded ignimbrite
Welded rhyolite ignimbrite
Rocks of La Garita Caldera Cycle (28.0 Ma)
Huerto Andesite
Olivine andesite and hornblende andesite-dacite, undivided
Bedded breccia and conglomerate
Fish Canyon Tuff
Welded outflow ignimbrite sheet
Nonwelded to partly welded ignimbrite
Local Volcaniclastic Rocks of Saguache Paleovalley
Andesitic conglomerate
Older Ignimbrite Related to Western San Juan Calderas (28.3 Ma)
Sapinero Mesa Tuff
Conejos Formation, Upper Units (~30 Ma)
Andesite of Lone Tree Gulch (Saguache Valley)
Tuff of Big Dry Gulch
Debris-flow deposit of Buffalo Pass Campground
Distal conglomerate
CALDERA-RELATED ROCKS PREDATING THE WESTERN AND CENTRAL CALDERA CLUSTERS
North Pass Caldera Cycle (32.2 Ma)
Saguache Creek Tuff
Nonwelded crystal-poor rhyolite
Partly to densely welded crystal-poor rhyolite
Bedded rhyolitic tuff
Rocks of the Bonanza Caldera Cycle (33 Ma)
Granitoid intrusions
Spring Creek intrusion
Aplitic granite
Porphyritic granite
Porphyritic granite of Alder Creek
Turquoise Mine intrusion
Granodiorite
Intrusive andesite
Granitoid rocks, undivided
Eagle Gulch Dacite
Intrusions of uncertain age and affinity
Rhyolite
Dacite
Plagioclase andesite
Andesite
Postcaldera lavas and volcaniclastic rocks
Lake-bed sedimentary rocks
Hayden Peak Dacite
K-feldspar dacite
Dacite lavas
Finely porphyritic dacite lavas
Porphyry Peak Rhyolite
Rhyolite of Laughlin Gulch
Andesitic rocks
Upper dark andesite
Breccia cone and spatter
Andesite flows
Sparsely porphyritic andesite lavas
Plagioclase-rich andesite lavas
Squirrel Gulch andesite
Volcaniclastic rocks
Bonanza Tuff
Dacite
Upper outflow dacite
Main dacite
Nonwelded dacite
Intracaldera dacite fracture fill
Rhyolite
Upper rhyolite
Main intracaldera rhyolite
Lower rhyolite
Brecciated rhyolite
Rheomorphic rhyolite
Nonwelded rhyolite
Intracaldera rhyolite fracture fill
Intracaldera landslide breccia
Mixed clast or undivided megabreccia
Andesite- and dacite-clast megabreccia
Plagioclase-andesite-clast megabreccia
Proterozoic-clast megabreccia
VOLCANIC ROCKS PREDATING BONANZA CALDERA
Conejos Formation, Middle Units
Tracy Volcano (<32.5?–33.5 Ma)
Intrusions
Rhyolite
Dacite
Andesite
Plagioclase andesite
Granodiorite
Caldera-filling(?) rocks
Caldera andesite
Dacite caldera-wall talus
Andesite caldera-wall talus and landslide breccia
Flank lavas
Volcaniclastic rocks
Upper dacite
Upper andesite
Plagioclase andesite
Small-phenocryst dacite
Andesite
Quartz-xenocrystic andesite
Andesite breccia
Rhyolite
Rhyolitic sedimentary rocks
Dacite of Red Rock Canyon
Dacite lavas
Dacite breccia
Sargents Mesa Volcano
Andesite-dacite lava
Rawley Volcanic Complex (~33.2–33.8 Ma)
Intrusions
Granodiorite
Dacite
Lavas and related rocks
Volcaniclastic rocks
Rhyolite
Rhyolite tuff
Sanidine-bearing dacite
Dacite shatter breccia
Dacite
Small-phenocryst dacite
Plagioclase andesite
Andesite shatter breccia
Andesite
Andesite scoria
Rocks of the Marshall Caldera Cycle (33.8–33.9 Ma)
Postcollapse caldera-filling rocks
Dacite lavas, undivided
Small-phenocryst dacite
Plagioclase andesite lavas
Andesite lavas, undivided
Rhyolite lava
Sheep Mountain Dacite
Volcaniclastic rocks
Pitch-Pinnacle Formation
Thorn Ranch Tuff
Densely welded tuff
Non-welded to partly welded tuff
Megabreccia, andesite clasts dominant
Megabreccia, clasts of Badger Creek Tuff
Megabreccia, Proterozoic clasts dominant
Sawatch-Range Ignimbrite (Erupted from Aetna Caldera)
Badger Creek Tuff
Conejos Formation, Lower Units
Early lavas and proximal breccias
Dacite
Plagioclase andesite
Andesite
Andesite breccia
Volcaniclastic rocks
Jacks Creek Volcano (~34.5 Ma)
Intrusions
Rhyolite
Dacite
Andesite
Granodiorite
Lavas and related rocks
Dacite
Small-phenocryst dacite
Plagioclase andesite
Andesite
Andesite cone breccia
Rhyolite
Volcaniclastic rocks
PREVOLCANIC ROCKS
LOWER TERTIARY SEDIMENTARY ROCKS
Conglomerate and sandstone (Eocene?)
PALEOZOIC SEDIMENTARY ROCKS
Upper Paleozoic sedimentary rocks
Sharpsdale Formation (Pennsylvanian)
Kerber Formation (Pennsylvanian)
Lower Paleozoic sedimentary rocks
Leadville Limestone (Mississippian)
Brecciated and silicified rocks
Chaffee Group (Devonian and lower Mississipian)
Fremont Dolomite (Upper Ordovician)
Harding Quartzite (Middle Ordovician)
Manitou Limestone (Lower Ordovician)
Lower Paleozoic rocks, undivided
PRECAMBRIAN ROCKS
Aplite of Alder Creek
Granitoid intrusive rocks, undivided
Metamorphic rocks, undivided
Precambrian crystalline rocks, undivided
EXPLANATION OF MAP SYMBOLSContact—Dashed where approximately located; dotted where grada-
tional. Internal contacts (dash-dot) delineate some conspicuous lava flows or ash-flow cooling breaks within map units. No contact shown where boundary between map units is uncertain or indefinite
Fault—Dashed where approximately located; dotted where concealed or occupied by intrusive rocks. Bar and ball on downthrown side, where movement known
Low-angle (thrust?) fault—Teeth on upper block
Crest of elliptical resurgent dome
Quaternary slump block—Dashed where approximately located. Hachures on downthrown side
Monoclinal hinge zone—Dotted where concealed. Triple hachures indicate direction of uplift
Unconformity along caldera wall—Dashed where approximately located; dotted where concealed
Bonanza (33.12 Ma)
Marshall (33.9 Ma)
Bonanaza caldera topographic rim—Dashed where approximately located; dotted where concealed. Erosionally modified
Syncline
Strike and dip of bedsInclined
Vertical
Strike and dip of foliationInclined
Vertical
Trend of lineation—Showing bearing and plunge. Defined by elongate pumice lenses or flow structures. May be combined with foliation symbol
Fracture-fill tuffDacitic tuff (Tfbd)
Rhyolitic tuff (Tfbr)
Sample location—X-ray fluorescence (XRF) chemical analyses in tables 3, 5. 39Ar/40Ar ages shown in millions of years (Ma) on map and in tables 4, 6. Sample abbreviation at end of age: b, biotite; gm, groundmass separate; h, hornblende; no abbreviation, sanidine. (Table 5, 6 available at https://doi.org/10.3133/sim3394)
XRF
40Ar/39Ar
XRF and 40Ar/39Ar
Hydrothermally altered rock—Areas of conspicuous supergene acid-sulfate alteration, mainly due to dissolution of pyrite, in vicinity of subvolcanic intrusions
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40
35
34.08±0.13b
34.08±0.13b
San Luis Valley
R CLA GARITA (27.5)
SOUTH RIVER
La Garita South
La Garita(28.0)Lake City
(23.0)
Silverton(27.6) SJ
(28.3)
UC(28.6)
Uncompahge(28.3)
Creede(26.9)
Platoro(29.8–28.4)
Summitville(29.0)
La GaritaSouth(28.0)
LostLakes(28.5)
South River
Marshall(33.9)
NORTHPASS(32.2)
Bonanza(33.2)
Bachelor (27.55)
COCHETOPA PARK(26.9)
NM(26.9)
Gunnison valley
CC
RC
Lower Conejos map area
(Lipman, 1975)
Del Nortemap area
(Lipman, 1976c)
Central San Juanmap area (Lipman, 2006)
San Juan Basin
Bonanza Caldera map area (this map)
Cochetopa-North Passmap area (Lipman, 2012)
Platoro map area (Lipman, 1974)
Lake City map area (Lipman, 1976a)
0 50 KILOMETERS10 20 30 40
0 10 20 30 MILES
108°
38°
37°
107° 106°
Villa Grove
The Gate
v v v v
PP
HP
SMAP
FM
PO
UP
SP
MOMarshall Creek
San Luis Valley
Sangre de Cristo RangeRM
Salida
Kerber Creek
Bz
Little KC
WH
MARSHALL CALDERASaM
Middle Creek hinge
UAV
Andesite
Dacite
Basaltictrachy
andesite
Trachyandesite
Trachydacite
Rhyolite
K 2O +
Na 2O,
in w
eigh
t per
cent
SiO2, in weight percent
11
10
9
8
7
6
550 60 70 80
Silicic intrusions
Intermediate intrusions
Postcaldera silicic lavas
Postcaldera intermed. lavas
Bonanza Tuff
Rawley complex
Tracy volcano
Jacks Cr volcano
EXPLANATION
SC T
TR T
BZ T
BC T
Post-caldera
lavas
Granitic intrusions
Marshallcaldera
lavas
Rawleycomplex
Tracyvolcano
Jacks Creekvolcano
Mar
shal
lcy
cle
Bona
nza
cycl
e
Age,
Ma
35
34
33
32 No. Mean Samples Age (Ma)*
NORTH PASS CALDERA CYCLE Saguache Creek Tuff 6 32.25±0.05
BONANZA CALDERA CYCLE Resurgent intrusions Fine-grain granodiorite/andesite (sn, gm) 4 32.83±0.21 Aplitic granite 2 33.28±0.06 Late postcollapse lavas Dacite of Hayden Peak (gm) 2 32.66±0.18 Megacrystic dacite 2 32.76±0.02 Caldera-fill lavas (tilted) Sanidine dacite 3 33.03±0.03 Porphyry Peak Rhyolite 4 33.04±0.04 Andesite-dacite (bio, gm) 4 33.0–33.3 Bonanza Tuff Intracaldera 4 33.05±0.06
West-side dacite 13 33.12±0.03 East-side rhyolite (Gribbles Park Tuff) 6 33.14±0.09 Precaldera central volcanoes Sargents Mesa volcano (bio, gm) 4 32.99±0.15 Rawley volcanic complex (gm) 8 33.3–33.7 Precursor(?) rhyolitic lavas 2 33.73±0.09
OLDER VOLCANIC ROCKS Tracy volcano (sn, bio, gm) 10 31.6–33.7 Marshall caldera cycle Postcollapse lavas (bio, gm) 6 33.4–33.6 Thorn Ranch Tuff 5 33.93±0.10 Aetna caldera, Badger Creek Tuff 2 34.06±0.15 Pre-Bonanza Jacks Creek volcano (bio, gm) 8 34.2–35.3
*Ages are normalized to Fish Canyon Tuff at 28.02 Ma
A B
Unit (sanidine age, except as noted)
NMAT
500
1,000
SP
AT
FC5,000 km3
SM CR
WP
NM
SMT
LJ
WM
GP
SC
(Bcr)
B
BC
38 3436 3032 26 28 2200
24
Age, in millions of years
Volu
me
(indi
vidu
al tu
ff sh
eets
), k
m3
Sawatch Range Trend
Southeastand
West San JuanLoci
Central San JuanTransition Lake CityLatirVOLCANIC
LOCI
CP
TR
16,000
Cum
ulat
ive
volu
me,
km
312,000
8,000
4,000
4,000 km3/m.y. 2,000 km3/m.y.
1,000 km3/m.y.
250 km3/m.y.
SP
SOUTHWEST NORTHEAST
Sapinero Mesa Tuff
CONEJOSFORMATION(undivided)
Volcanics of Cochetopa Hills (North Pass)
Rawley volcano(Rawley Andesite,
includes dacite and rhyolite)
Postcaldera volcanics (Marshall)(Sheep Mountain Dacite, andesite, other dacite)
Postcaldera volcanics (Bonanza) (upper andesite, dacite, Porphyry Peak Rhyolite, and others)
CONEJOS FORMATION, upper lava units(mostly southwest of Bonanza map area)
[No lavas preserved?]
REGIONAL LAVA UNITS INTRACALDERA LAVA UNITS
28
30
32
34
36
Age,
mill
ion
year
s
JacksCreek
SummerCoon
Tracy
Thorn Ranch Tuff (34 Ma)
Bonanza Tuff (33 Ma)
Wall Mountain Tuff (37 Ma)
Saguache Creek Tuff (32 Ma)
Fish Canyon Tuff (28 Ma)
MARSHALLCALDERA
BONANZACALDERA
Ring
Fau
lts
5
5
5
35
San Luis Valley
(Rio Grande rift)
Kerber Creek
SheepMountain
AntoraPeak
Whale H
ill
Pre-Bonanzaandesite
and dacite
Porp
hyry
Peak
** *
**
*
*
*
*
**
*
*
*
**
[Mapped 2006-11]
Jacks Creekvolcano
Monoclinal
hinge
SargentsMesa
TheGate
Findley Gulch
SaguachePeak
HH
PC
SC
Caldera-floorlavas
Resurgence crest
Figu
re 1
4B
Egd
* *
106°15'
38°15'
38°22'30"
38°07'30"
106°106°07'30"
FlagstaffMountain
Post-Bonanzaandesite
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QalPoncha
Pass
15
20
60
45
35 2530
40
35
45
65
75
80
30
55
25
20
35
55
Figure 14A
55
BONANZA T O PO RIM
Saguache Creek
gr
gd
a
TOPO RIM
OLIG
OCEN
E
EXPLANATIONQuaterary surficial deposits
Post-Bonanza ignimbrites
Resurgent intrusions
Granite
Granodiorite
Andesite
Eagle Gulch Dacite
Post-Bonanza lavas—Andesite to rhyolite
Bonanza Tuff—Rhyolite to silicic andesite
Pre-caldera lavas—Andesite to rhyolite
Andesite and dacite
Rhyolite
Sheep Mountain Dacite
Thorn Ranch Tuff—Rhyolite to dacite
Paleozoic sedimentary rocks
Precambrian granitic rocks
Bonanza caldera—Dashed where approximate; dotted where concealed
Eroded topographic rim
Ring faults—Bar and ball on downthrown block. Dashed where approximate; dotted where concealed
Marshall caldera—Dashed where approximate
Wall unconformity
Fault—Bar and ball on downthrown block. Dashed where approximate; dotted where concealed
Monoclinal hinge
Resurgent crest
Inclined bedding—Showing strike and dip
Some caldera megabreccia sites
Photograph Site—Includes figure number; C, cover. Bar indicates view direction
Dike—Jacks Creek volcano
Cross section location—See cross sections in figure 14
16*
20
gr
Qal
gd
Egd
a
0 5 MILES
0 10 KILOMETERS5
16
C
13
Fish Canyon T, 28.0Saguache Creek T, 32.2
Bonanza Tuff, 33.15
Fine-grain andesite
Malpais andesite, 33.0
Thorn Ranch Tuff (intracaldera), 33.9
Post-collapse lavas of Marshall caldera
Rawley lavas, ~33.7–33.3
Jacks Creekvolcano,
~35.3–34.2
Paleozoic rocks (?)
Proterozoic granitic rocks
MARSHALL CALDERATopographic rimSOUTH NORTH
Guffey
CanonCity
RositaSaguache
SalidaTR
TR
GP
MarshallCreek
CalderaBonanzaCaldera
SAN JUANVOLCANIC
FIELD
Thorn Ranch Tuff distribution
[Eroded/covered][Covered]
[Eroded]
[Eroded]
Mostly
rhyolite
[60+ km]
[80+ km]
BONANZA TUFF (33.12 Ma)
East Gulch Tuff distribution
Mount Princeton batholith
THIRTYNINE MILEVOLCANIC AREA
Mount Aetna cauldron
Grizzly Peakcauldron
BuffaloPeaks
TriadRidge
[60+ km]Mostlydacite
0 30 KILOMETERS15
0 10 20 MILES
39°30'106°30' 106° 105°105°30'
39°
38°
38°30'
[60+ km]
TR
GP
EXPLANATION
Caldera boundary
Thorn Ranch Tuff
Gribble’s Park/Bonanza Tuff
Distance from Bonanza Caldera
Bonanza Tuff, areal distribution—Dashed boundaries approximate
WEST EAST
Landslide breccia(mainly andesite)
~500
m
Gribbles Park Tuff
Prior intracalderathickness, estimated: 300 m
(Varga and Smith, 1984);now 2.5 km
Intracaldera previouslymapped as simple sequence:
andesite-dacite-rhyolite,with many fault repeats
(Burbank, 1932)
Dacite
Rhyolite
INTRACALDERA
A BFlagstaff
Mountain
Ker
ber C
reek
Brew
ery
Cre
ek
Wag
onbo
x C
reek
Slau
ghte
rhou
se C
reek
g
(~2.5 km intracaldera Bonanza Tuff; >1 km postcaldera lavas = ~4–5 km total caldera subsidence)
Feet, above sea level
SOUTHWEST
CALDERA RING FAULTS
12,000
10,000
8,000
Resurgentgranodiorite intrusion
Intracaldera rhyolite and dacite tuff,
~2.5 km thick
Andesite breccia (m
)
CALDERA RIM Burned MountainCaldera-floor andesite-dacite (a)
�dacite (d)
r
m
m
rr d
da
a
d
r
Post-collapselavas
A
afResurgent
aplitic granite intrusion r
10,000
8,000
Near-vertical northeast flank,resurgent dome
dm
NORTHEAST
12,000
Whale Hill, 12,162’
BROAD CREST OF RESURGENT DOME(granitic intrusive rocks exposed just beyond plane of section)
CALDERA FLOOR
CALDERA FLOOR
0 1 KILOMETER2
0 0.25 0.5 MILES
EXPLANATION
Alluvial-fan deposits
Glacial moraine
Caldera-collapse megabreccia
Dacitic Bonanza Tuff
Rhyolitic Bonanza Tuff
Andesitic lava flows
Precambrian rocks
a
d
f
g
m
�
r
0 2 KILOMETERS1
0 0.5 1 MILE
EXPLANATION
Upper Paleozoic (clastic rocks)
Sharpsdale/Kerber Formations
Lower Paleozoic (carbonate rocks)
Leadville
Dyer
Fremont
Manitou Formation
Feet, abovesea level
9,000
7,000
8,000
Ker
ber C
reek
South calderarim ~600 m (2,000 ft)
displacement
~1.5 km (5,000 ft) displacement?
Precambriangranite
S
L L L S
Precambriangranite
Precambriangranite
SOUTH NORTHSouth flank
resurgent dome
2,150
Meters,above sea level
2,450
2,750D
D
F
MM
M
F
F
B
S
L
D
F
M
9300
9300
9500
9500
9500
9700
10,20
010,300
10,100
10,000
9900
9800
9700
9600
9400
9900
10,000
9800
9600
Tree Gulch Lane
Scho
olho
use
Gul
ch
A
A
A
M
AA
A
RRR
A
A RR
R RR
RM
R
R
RM
R
M
AA
A
A
A
ARR
R
RR
RR
AAA
A RR
AR
AR R A
RA
A A
RRR
R
R
D
D
A
A
A
RA
AA
A AAR
RR
A
A
A
A
A
R R
RM
A
ARA
RRAA
RA
A
A
R
RR
A
A
A
A
R
R
A
R
A
R
A
A
AA
A
D
A
RR
AA
A
A
A
AA
D A
AA
R
RR
R
RA
AA
A A
A
A
A
MBR
FLOW-LAYEREDRHYOLITE
(RheomorphicBonanza Tuff?)
DACITE
ANDESITE
R
A
Q
Q
Q
Q
Q
0 100 METERS50
0 100 200 FEET
14.6'
14.4’
14.2’
14.8'
05.6' 05.4' 05.2'
C
B
Figure 2. Map showing calderas (maroon, purple, green, and brown calderas discussed here; red, others) of the San Juan volcanic region (blue) and location of map area. Dotted lines, inferred locations of caldera margins where concealed by younger rocks. Caldera ages (in parentheses) are in Ma. Caldera abbreviations: CC, Cebolla Creek; NM, Nelson Mountain; RC, Rat Creek; SJ, San Juan; UC, Ute Creek. Published map areas are outlined and labeled.
Figure 14. Cross sections, showing structures and stratigraphic relations across Bonanza caldera; locations shown on fig. 10. A. South-west topographic rim, across crest of resurgent dome (Whale Hill), to west margin of San Luis Valley (segment of Rio Grande rift zone) shows large ring-fault displace-ments, great thickness of intracaldera Bonanza Tuff, steep dips on flanks of resurgent dome, and inferred location of subcaldera granitoid intrusions. B. South caldera margin, showing displacements across ring faults.
Figure 15. A, Geologic map of sheet-like mass of flow-layered rhyolite (see B, C, photo 10.3), interpreted as rheomorphic lower Bonanza Tuff (unit Tbrr), along upper Schoolhouse Gulch (GG). Letter symbols mark locations of GPS-located outcrops and areas of monolith-ologic float inferred to reflect immediately underlying bedrock: A, andesite; D, dacite; M, large blocks of andesite, in matrix of nonwelded rhyolite tuff; MBR, mega- and meso-breccia of andesite blocks surrounded by rhyolite matrix; Q, colluvium; R, flow-lay-ered rhyolite. Topographic contour interval, 50 feet; thick gray lines, graded gravel roads of the Elk Horn Ranch subdivi-sion. B, Highly irregular blocks (outlined in orange; dashed where approximate) of andesite laharic breccia, enclosed within flow-laminated rheomorphic rhyolite tuff. Although flow laminated, the rhyolite contains centimeter-size angular andesite fragments (hiking pole, ~0.9 m). C, Closer view of contact (dashed line) between block of andesite laharic breccia and enclosing dense rhyolite that remains fluidal at the contact. Several small fragments of andesite enclosed in the fluidal rhyolite are circled.
Figure 5. Oblique view of Bonanza caldera area, viewed from the south. Line definitions: (long dashed where approximate, short dashed where concealed) orange, erosionally modified remnants of Bonanza topographic caldera rim; black, caldera-subsidence ring faults; yellow, crest of elliptical resurgent dome; green, margin of Marshall caldera. Topographic-name abbreviations: AP, Antora Peak; Bz, town of Bonanza; FM, Flagstaff Mountain; HP, Hayden Peak; MO, Mount Ouray; PO, Poncha Pass; PP, Porphyry Peak; RM, Round Mountain; SaM, Sargents Mesa; SM Sheep Mountain; SP, Saguache Peak; UAV, upper Arkansas Valley; UP, Ute Pass; WH, Whale Hill. Imagery from Google Earth (9/24/13).
Figure 6. Alkali-silica diagram, showing compositional variations in rocks of the Bonanza map area. Volcanic and intrusive rocks of the Bonanza caldera cycle are modestly more alkalic than those from the preceding central volcanoes (Tracy, Jacks Creek).
Figure 7. A. Summary of 40Ar/39Ar age determinations for igneous rocks of the Bonanza region, illustrating the narrow weighted-mean ages obtained for multiple samples of individual ignimbrites, lavas, and intrusions of the Bonanza cycle. Abbreviations: bio, biotite; gm, groundmass; sn, sanidine. Data from tables 3, 6. B. Eruptive chronology of the Bonanza region shows precursors to eruption of the Bonanza Tuff, including several central volcanoes and Marshall caldera, source of the Thorn Ranch Tuff at 33.9 Ma. Abbreviations: BC T, Badger Creek Tuff; BZ T, Bonanza Tuff; SC T, Saguache Creek Tuff; TR T, Thorn Ranch Tuff.
Figure 8. Age-location-volume plot, showing southward progression of Tertiary ignimbrite-caldera volcanism in the Southern Rocky Mountain Volcanic Field (revised from Lipman, 2007). Colored vertical bars, volumes of individual ignimbrites, scale on left axis (data from table 1); shaded area, increasing cumulative eruptive volume (right axis). Inset, slopes corresponding to different cumulative eruption rates. Abbreviations: AT, Amalia Tuff; B, Bonanza Tuff; BC, Badger Creek Tuff; CP, Chiquito Peak Tuff; CR, Carpenter Ridge Tuff; FC, Fish Canyon Tuff; GP, Grizzly Peak Tuff; LJ, La Jara Canyon Tuff; Bcr, rhyolite of Barret Creek flow field; NM, Nelson Mountain Tuff; SC, Saguache Creek Tuff; SM, Sapinero Mesa Tuff; SMT, Snowshoe Mountain Tuff; SP, Sunshine Peak Tuff; TR, Thorn Ranch Tuff; WM, Wall Mountain Tuff; WP, Wason Park Tuff.
Figure 9. Diagrammatic stratigraphic relations between regional ignimbrites and early interme-diate-composition lavas (Conejos Formation), northeast San Juan region. Thin-dashed lines delimit locally mapped lava units in relation to associated calderas. Thick-dashed lines (vertical), approx-imate age spans of mapped central volcanoes along east flank of the San Juan Mountains.
Figure 10. Generalized geologic map of Bonanza and Marshall calderas, showing major stratigraphic units, structural features, and locations of cross sections (fig. 14). Margin of Marshall caldera (unconformity between caldera-fill and precaldera rocks), dashed where approximately located. Approximate topographic rim of Bonanza caldera inferred from distribution of intracaldera rocks and from erosionally modified present-day morphology. Rectangular grid, boundaries of 7.5' quadrangle maps. Abbreviations: HH, Houghland Hill; PC, Peterson Creek; SC, Spring Creek.
Figure 11. Diagrammatic cross section through concealed south margin of Marshall caldera, showing inferred relations of caldera-filling volcanic units. Possible remnants of the topographic wall are marked by an east-west alignment of paleohills of Precambrian rocks. Post-collapse lavas and volcani-clastic rocks of this caldera cycle are onlapped by lavas of the Rawley volcanic complex, then by younger ignimbrites (Bonanza, Saguache Creek, and Fish Canyon Tuffs) and interleaved lavas. Ages, million years (Ma); T, Tuff.
Figure 12. Areal extent of preserved erosional remnants of Bonanza Tuff (modified from McIntosh and Chapin, 2004). Eastern areas are dominated by crystal-poor rhyolite, while western area consists mainly of crystal-rich dacite. Figure also shows inferred distribution of the eastern outflow of Thorn Ranch Tuff and East Gulch Tuff. GP, Gribbles Park/Bonanza Tuff; TR, Thorn Ranch Tuff.
Figure 13. Generalized regional thickness and compositional variations in the Bonanza Tuff. Outflow ignimbrite is mainly early erupted rhyolite to the east, mainly later-erupted dacite to the west. Intracaldera tuff contains multiple compositional zones and is ponded to thickness of at least 2.5 km, as exposed on west flank of resurgent dome. Intracaldera tuff was previously mapped as a simple andesite-dacite-rhyolite sequence with many fault repeats (Burbank, 1932). Intracaldera tuff thickness was previously estimated at 300 m (Varga and Smith, 1984) but is currently considered 2.5 km thick. A. Photograph showing densely welded intracaldera dacite, with flattened pumice lenses as long as 0.5 m that define a down-dip lineation, east side road above Squirrel Gulch. B. Photograph showing lower rhyolite, densely welded to its base without any intervening tephra deposits, deposited against caldera-wall andesite at Windy Point (above white dashed line). See figure 10 for photo location.
Printed on recycled paper
ISSN 2329-1311 (print)ISSN 2329-132X (online)https://doi.org/10.3133/sim3394
Geologic Map of the Bonanza Caldera Area, Northeastern San Juan Mountains, ColoradoBy
Peter W. Lipman2020
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government
This map or plate also is offered in an online, digital version. Users should be aware that, because of differences in rendering processes and pixel resolution, some slight distortion of scale may occur when viewing the online version on a computer screen or when printing it on an electronic plotter, even when it is viewed or printed at its intended publication scale
For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225, 1–888–ASK–USGS
Digital files available at https://doi.org/10.3133/sim3394 and https://doi.org/10.5066/P911OL4Q.
Suggested citation: Lipman, P.W., 2020, Geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey Scientific Investigations Map 3394, pamphlet 73 p., 2 sheets, scale 1:50,000, https://doi.org/10.3133/sim3394.
Associated data for this publication: Robinson, J.E., and Lipman, P.W., 2020, Database for the geologic map of the Bonanza caldera area, northeastern San Juan Mountains, Colorado: U.S. Geological Survey data release, https://doi.org/10.5066/P911OL4Q.
U.S. Department of the InteriorU.S. Geological Survey
Scientific Investigations Map 3394Sheet 2 of 2
Pamphlet accompanies map
