Scientific Investigations Map 3123 U.S. Department of the Interior Sheet 1 of 2 U.S. Geological Survey Pamphlet and CD-ROM accompany map 107° Iris NW (IW) 38°30' 106°52'30" Iris (IR) follows old paleovalley 106°45' that localized Tws deposition Tfcn Houston Gulch (HG) Tfcn Tcv 106°37'30" Doyleville (DV) Jm QTg Tws p_g 106°30' Sargents (SS) QTf Qc Qal Twsn Tcv Tfc p_m 106°22'30" Tsc Twsn p_m Tfc Tfc 38°30' Qc Jj Qc Tcuv Twsn Twsn Jm Jj Tcv Jj Tfc Tsc p_m Qc Tev Ql Tcr Qc QTg Qc p_g Kd QTg Tev COCHETOPA-NORTH PASS CALDERA MAP AREA BONANZA CALDERA STUDY AREA Tfc Kd p_g 38°30’ Twsn Tfc p_m Qc Jm Twsn p_g Jm Gunnison, QTf Tcr p_m Tfc Gunnison, sheet 5 Chaffee QTf p_m Jm sheet 4 QTf p_m Jj p_g Jm Jj Qt Qc Km Iris NW Iris Houston Gulch Doyleville Sargents Pahlone Peak Mount Tcv Jm (IW) (IR) (HG) (DV) (SS) (PP) (MO) (PO) Qc Qc Qc Jj Jj Tfcn Jj Tcv Ql Olson, 1976c Olson, 1976b Olson, 1976a Tfc Tcv Saguache, sheet 2 Qc Tsc Tfc QTg Jm Qc Kd Ql Jm Tsc Km Sawtooth Razor Creek Qc QTf Tcr Ql Spring Hill (ST) West Baldy Sargents Mesa Chester Bonanza Whale Hill Qc p_g Kd Dome (RD) Qc Creek (SH) (WB) (SM) (CH) (BZ) (WH) Tcv Jm Jm Kd Bonanza Qc Ql p_g Hedland and Olson and Olson and Olsen, 1974 Steven, 1976a Steven, 1976b Saguache, sheet 1 Tfc Tcv Tcv Tfc p_g Caldera p_m Twsn Tfc Km Jj Km Cochetopa Park Tfc Tfc Jm Tcv Caldera Qc Qc Twsn Qc Tcuv Qc Rock Creek Cold Spring Cochetopa North Trickle Lake Mountain Klondike Mine Graveyard Gulch p_m Qc Park Park Park Pass Mountain Northeast (KM) (GG) Tfcn QTf Tws Jj Tcv Kd Tfc p_g Jj Jm Kd (RC) (CS) (CP) unmapped(NP) (TM) (LMNE) p_m Ql Tfc NP Jm Tfcn Tcv Jj Tcuv Tcv Kd Tcuv Tfc Jm Caldera Mesa-capping Tfc Qc Tcv Jm Tsc, at higher Qt Jj Tfc Tfcn Qc than Qc younger Tfc Twsn Jj Tfcn Qc Tfcn Tcv Modern canyon at Qc Tcv Qc Iris NW (IW) Iris NW Tfcn Tsc Jj Tcv Qt Tcv right angle to volcanic-filled Tsc Kd Elk Park Saguache Park Grouse Creek Lake Mountain Laughlin Gulch Saguache Hickey Bridge paleovalley Tcv (SP) (EP) (SA) (GC) (LM) (LG) (SG) (HB) Tsc unmapped N-trending post- Tscl QTg Qc Qal Tws paleovalley, Saguache, sheet 3 Saguache, sheet 4 (SS) Sargents filled with Tfc QTf Jm Qc Tcv? Jj Thick mesa cap of Tsc, previously Qc Tcv 38° Kd Tcv mapped as tuff of 107° 106° Qal Tca Ql Tsc Tfc MAPPING CREDIT Tcv Jm CENTRAL SAN JUAN MAP AREA p_g Tcv Ql Ql p_m Tsc (Lipman, 2006) Paleohill capped Km Tfc by Tsc, abutted Tcv Ql by Tfc Km Km Jm Twsn Tbc Qc Ql Tcv Tcv P. Lipman, P. Lipman, Pre-Tertiary geology modified Tbc Qc Tws Steep Tsc Tfc Km 2000–2007 1986–89, 1995–99 from published USGS maps Tfc Tsc Km Tsc unconformity, Tsc Qc Tscn (J. Olson and others, 1974–76) Qc Jj Kd Jm Tfc against Tws Qc In-place Tbc directly on Pzl, Tcv Figure 3. Northeastern San Juan region, showing areas of geologic mapping responsibilities, 7.5' topographic quadrangles (black lines), abbreviations just east of map Qc Kd Tcv Qf Tcv Qc Jm Jj Tscl Ql used on maps and in text, and 1:50,000-scale county maps (blue dashed lines and labels). Published USGS 7.5' geologic map quadrangles indicated by p_g Qfo citation below quadrangle name and abbreviation. In-progress study of the Bonanza caldera, east of this map area, provides documentation for some Qc High-standing Complex slumping paleohill of Tsc, Tsc on underlying p_g features referenced in this report. Qal abutted by Tfc Qc Mancos Shale (Km) Tfc Km Tcv Km Tbfa Jj Ql Tca Jm Tcv Tcv Qc Tca Tfcn Tcap Jj Tfc Tcd Qc Km p_m Ql Qc Tfc Tcv p_g Km Tca Km Tca Tcd Tca Ql Tcv Km Tfc Tca Jj Jm Tca Kd Qc Qc Ql Qc Tfc Qc Tcv Km Tbfa Km Ql Km Km Qf Qal Tca Tca Tfc Ql Kd Ql Qc Tca Jj Tca Tfc N-trending dikes, Kd similar in composition Tca Concentrated Tcap boulder float of Qc and age to Needle Cr 33.60± Pzl; no outcrop 15 Tbfa intrusion 0.09b Tci |l Qc 38°22'30" 32.27± Tsc Tbfa Tci Tcap 0.05 p_m Kd Tfc Qt Tcfr 5 34.28±0.18h Tcd 25 Large (to 0.5 m) Tbfc Tct Tca Qc 33.67± 32.29±0.05 pumice blocks p_g Km Tcv Ql Tcv Tcv 0.19b in Tscn Qc Tsc Tbfc 25 Qc 38°22'30" Ql 5 Ql Jm 29.85±0.04 Tcd Tca Jj Jm Jj Tfc Ql Tbfa Tcv QTg Jm Kd Km p_g Tev Qc Jj Jm Qc Tca Ql Tcv Tcd Tscn Jj Tca Tcap Kd Tfc Tcr Tca Qc Multiple ages p_g p_m Kd Qc (29.8, >34 Ma) Ql Km Km Ql of Tcfr? p_m Jm Tcv Tws? Tbfa Qc Tfc Tca Tev QTl Qal p_m Kd Km 29.63±0.06 Km Qc Km Qc Km p_g QTg? Tca Tcfr Tfc Tca Jj Qc Qlo Tfc Ql Ql Km Tcv Twm Jm Tsc Jm 34.39±0.23h QTg? Large blocks Tcr p_m Tcv Jj Tci (to 10 m), both Qc Jm Ql Tbd and Tsc Tca Low Jj Tbfc Tct Tct Tcv Thb? Tcv paleovalley Tcd Tcd Tci fill of Tbd Jj Ql p_g Tca Kd Abundant basalt Tcv Qc Qc Tcfr Qc Tfc Twsn Tcv fragments in 29.79±0.05 Tci Tfc Twm 27.14 rhyolite plug Tcv Tcv ±0.05 Qc Angular Tfc 20 Km Qc Qc 60 Tct Tca andesite Qc Kd Km Tbfc Qc Tcfr QTl Ql Qc Jm Jm Ql QTg? breccia Tcd Jj Twsn Kd Tcdi Ql Angular Qc Kd 27.08±0.05 Ql Qc Ql Tct andesite Tbrl 33.18 Tcv ±.06 Qc Qc Jj 34.42 Qc breccia Ql Tcv Tca ±0.21h Tcv p_m Qc Tca p_m p_g Qc Tcfr Tcd Coarse Tbd Tcv conglomerate, 33.19 Tscn Jm Tcv Qc Tct ±0.05 Tcv Km 33.98 Tca =g and Tca Tca 36.95±0.15gs, Qal Tca Tbfc Tir ±0.08 Qc boulders Tba? Tbd Tca 36.85±0.07 Km Tbfc Tbfc Tbrl Jj Tcv Qal Qc Ql Tct Tcd Twsn Thb? Km Tcfr Qc Tcv Thb? 20 Kd Qc Tca Qc Tcd Qc Tbrl Qal Tcv QTl Tbd Tbfa Jj Jm Tscn Twm Jm Kd Tcfr Qt Twm Qc Qc Ridge-capping Kd Twm Jj Ql Tci andesite; tentatively Tcdi Km Tca Qc 34.10±0.05 Qc Tbd 10 Qc Tbfa NEEDLE CREEK INTRUSION Tca Tca Thb (Olson and others, 1975) Jj Jm Tbfa Tca Qc Qc Jj Tbfa Tca Qc Tcv 30.08 Tcd Qc Twm Km Tca ±0.09 Tcfr Ql Tca? Tcfr Tbd Tfc Ql Tscc Tcv Tsc Jm 15 Qt Qc Kd Jj Tcv 20 Tca? Scabs of Tbd, Tcv Tcv Jm Kd Tca Tcfr banked high Tba Jj Tca Tcv Tca Km Tfc Tcv Km Tcv Tcd Qc on paleovalley Qc Tba Poorly exposed Ql Qc 29.83 wall 20 Tbd Qt Jm Tcv p_g Tca Qc ±0.05 Qc Tsc, seemingly Tca Tcfr Ql Tca Tfc overlain by Tca? Rheomorphic ramp Tcfr Tca Twm Ql structures in mesa- Qc Tid? Jm Tscn Tcv Jm Tci Qm Tca Jj capping Twn Qlo Tid? Ql Tcv 30 Ql Qc Tcv Tca? Qt Tbd Tcv Kd Qc Qc Tca Jj Outer caldera Tcv Alternatively, thick Ql Kd Jm fault(?); only Tcv Qc Km Qc Qm Tca Tca Tcv valley-filling lava Qc Tcd Tia Kd Tca modest dome? Jm Kd Qc Tcfr Tbd displacements 10 Tcr Tcfr Twm Kd Tfc Tcd 5 Tcv Jj Tca Km Tcv Tpcf Tca Tca Tca Tbfa Qc Jm Tca QTg? Tia Ql Jm Qc 5 Tcfr Tid Spring Hill Creek (SH) Tcv Tcd 15 Qc 5 Qt Qc Qc Tca Tbd Qc Tscl Twm Tpba? 5 p_g Tcd Qc Jj Kd Tpcf 30 Tcv 30 Qc Qc Qc Tca Tcv Kd Tcv Jj Slumped Tbd Qt Kd 15 40 Tcv Ql Tcv Tcht Tcht 15 35 Tca Tci Tbd Qt Mesa (SM) Sargents 25 Qc 33.01±0.10b Qc Kd Qc Tcv Qc Long Branch Ql 35 Jm Tpba Tca intrusion (at roof 32.98±0.21h Km 32.26 Qc Tpcf Qc Ql 5 25 Jm Qgo 5 Qc level) Tca ±0.07 25 Tcht Qc Tcfr Tca Qm 15 25 Tpba Qc Qc Tcfr Tcv Qc 70 Tca Tcv Kd Tbrl Tid Tca Tscn Tcfr Jj Tca Tpba Tca Tca 15 Qc Tca Qc Jm Tcht 40 20 15 Tca Qc Tcfr Tpcf 5 Tci Ql 50 45 25 Tpba 10 Qt 65 30 37.13 5 Tbd Tca 20 ±0.09 10 Qc Qc 381 Tcht Tcv Tca Tcd 50 10 Tcv Tcv Tca p_g Tci Km ±60 Qc Ql Qc Ql Tca 5 Kd 25 Tcv p_g Qc Qc Twsn Tcd Tcfr Tcv 10 Slumped SAWTOOTH Jm Qc 35 Tcd 10 Tcd Tca 20 brecciated slabs MOUNTAIN 60 Qc Coherent outer Tcfr Tpba Tcr Tca Kd p_g of andesite INTRUSION Ql caldera floor, and Tca Tcfr 30 Tpba Tca Tcv Qc Tca Tcv Jm Twm (or) downsagged 34.61 Tpcf Tcht Tci Tfc Tbd Jj ±16h caldera margin? Qc 10 Qt Qt Qt Qc Tws Tcv Ql Tca 20 Tca Tca Tcht Tcv Qc Tca? Tcdi Jj Qm Slumped Tpba Tca Tcv brecciated Ql Tca Tcd Jj Jj Tfc Tca slabs of Tpcf Qc 27.07±0.06 Qc 10 Tcfr andesite Tcv Tcht 25 Tcfr Qc Tcd Tcfr Qc Ql Tpbf Clasts of Tbrl Qc Tci Qf Qc Qc Deep Precambrian Tcv paleovalleys Tca rocks to Qal Qc 50 Qc Tpcf fill of Tbd Twsn Tws Tca Tcv Tfcn Tcv 0.5 m, in 55 unit Tcht Tpba? Tca Tpbr Ql Tcv 15 32.00+0.09b Tci Qgo 60 Tpba Tfcn Tpbr? 25 p_g Qc Tpba Tca 15 Tpba 20 Qf Jj Qc 20 Qg Qc Qc Tcv Qc Tca Broad paleovalley Tpbs Conspicuous knob: unclear Tpcf fill (Tfc, Tcr) Tcd Tscn Tpba Qt Qc Qc whether Tpbf or in-place Tca Tcfr Kd Tca Tpcf Tpff Qm Tca Ql Deposition of Tca Ql Tci Tci Qf brecciated Tfc? Tcd Tcv thick-welded 50 Ql Tca Tbd Qc Ql 25 Tfc Tws controlled Qc Tcv Tcv Tca Tscn Tcr Kd Jm Kd by N-trending Tpcf Tpt Tpt Qc Tpba Tpbf Tcfr Tcv paleovalley Tpcf Tsc Tcfr Tcv Tpbm Tcfr Qc Tcd Tcd Qfo Tcv Jj Qc 5 Tca Tbd Tca Tca Tfc Tpbf Qc Tcv Tpba Qg Tcfr Ql Tws Qc Tpt Tscn Tcfr Tcv Tcfr Tca Tfc 35 45 Tpbf p_g Tfc Qc Tcv Qc 20 Tpt Tcfr Tcv Jj Tca Tca Tpcf 80 Tpt Tpcf Tscn Tcd Qc 80 55 Qfo Tca Tpcf? Qc 38°15'30" Ql Tpt 50 Qf Qc Qc 70 Tcd Tcr Qc Qc Tpcf Tpba Tca 32.40±0.05 Ql Tcv Tpt 10 50 Tpt Tca Tcfr Tca Tcdm Tts Tws Tcd Qc Qfo Qc Tcv Tcfr Tca 60 20 Tcd Tfc Rheomorphic Qf Tpcf 5 Tpt 75 Qf Tpff Tpba Tca Tpbd Qf Tpt Tts, banked Tcv 38°15'30" Tca Qc Qc Qt Tcdu Qt against steep Tts Ql Tbd Qc 50 Qc Qc Qc Tcv Tca Tcsa Qg Near-vertical paleotopo- Tcv Qc 30 Tpt Tca 20 ramp structures, Meter-size graphy May include 20 Qc 10 20 Tpbf Tcd Tscl Megablock near top of blocks of Tcr Tpba? some slumped Tcd 32.55 Qc Qc 40 Qt 70 ±0.05 Tcfr slabs of Qf Tcdm flow 50 30 in landslide Tfc deposit Tpbm Tts Tca Tfc Tcv dacite 45 45 5 32.14±0.05 Tlr Tca Tbd 25 15 Tbd Tcd Ql Qg 60 20 Qc Tca Tcv 30 85 Tcd 32.07±0.17b 32.48±0.05 25 Tsc Tscl Tcd Black obsidian 45 40 20 Tca Qt 25 5 20 80 Tcv Tst? 30 Tpt Qg nodules (Apache 25 50 Tca Tpt 60 30 65 10 tears) in Tcdu Tpt Qf Tpcf 20 Tpbm Tcv Tcfr Tca basal perlite 5 Qt Tcdm 40 50 15 Tpcf 45 5 20 65 Tcdx 27.48 Tcd Tscl 5 ±0.05 Tca Tcv Tpbf 75 5 Qc Tci Tca Tcv Tfc Tcdm Tcd Tcv Tcv Tcfr Tcfr 20 Qc 15 20 Tca Paleolandslide of Tsc Tsc Qc Tpt 50 Tca Tca Tnpd Tsc Tcv Qgo 45 Tcdu 45 Tpt Tst into same valley as Tsc Qt 30 Tbd Tca 40 Tct Tsc earlier localized Tbd 35 Tsc Tca Tpcf Qal Qc 40 20 50 Tpbf 26.91±0.20ob 5 50 Tnpd Tcr 45 Topographically Qc Jumbled pre- Tcv Tcv 80 Tct Tcfr Tscn 45 Abrupt change Tcdc No obvious structural irregular scabs of Tcd p_g caldera units (caldera Tcr Tpt 5 40 60 70 75 Tsc, high on 32.10 Qc Tca Tcv Tfc upward from control for NE 40 Tcdl megabreccia?) Qg Qfo Tnr 50 Qf caldera rim ±0.30 Massive finely 20 Tfc crystal-poor to topographic grain 70 32.45 brecciated rhyolite 20 5 Qc Tct ±0.05 Tbd Tca Sparse clasts Tnpd 20 Tcd Qc Tca Tfc Tpt crystal-rich Tnr Qc 80 65 Tfc Qc Qf 60 26.90±0.04 of gray crystal- 32.44 Tsc Qc 10 Qc Tcv Qf Tcdc Ql Tlr Qc 75 35 Tscn rich ignimbrite ±0.08 Tdb 10 Tfcb Tcdm 75 Tcv Qal Tcv Tca Qf Qc (Tts?) Qc Qf Qc 10 Qfo 33.17±0.06 Vitrophyre along Tpt 26.81 Qc Tca Qc 15 Tpt 35 Tca flow contact Tcd Tca 10 ±0.04 65 Tcfr 10 Tscn 5 Tpt Qfo Tcv Tct Tcv Tfc Ql Tcv Tcv Tpt 35 26.99±0.14 Tca Qc Tcv 10 40 60 Tcd Tpt 25 5 60 60 Qc 45 80 Tcdl Qc Tct 40 Tnpd p_g 10 Tcv Tfc 10 Qc 26.95±0.06, Tca Qt Qt 26.93±0.04 Tpt 25 50 50 25 Qc 32.48±0.17, 20 Qc Tfc Tca Tfc 10 40 75 25 Tfc Tcv Qc Tfcb 25 26.88±0.05 32.21±0.07 Tca Tnpd Qc 15 Qc 55 25 20 Tsc Tsc Tcv Qt 55 15 Qgo Vapor-phase Qc 30 Tsc Qc 30 Tdb Qc Tbd Tws 35 Near-vertical ramp Ql Tcv Tws 10 crystallized 65 60 Tscn Tca 15 Tcr Tnn 25 25 structures, near top of Paleovalley fill of nonwelded Tpt 5 Tca Qc Tws Tcv Qc Tcdl flow Tcv Qc Tnpd Tbd, parallel dacite tuff 35 Qc Tnpr Tcv present drainage Tfcb 10 50 Tst 30 Tfc 5 Qc 15 60 Tfc Trdn? Qc Tcdm Ql Tnr Tnn 75 Tsc 40 20 Inclined columnar Tcv Tcv 26.88±0.06 55 Qt Transi- cooling joints near flow Tcv Tws Qfo 10 27.59±0.05 Qf Qt Tpff Tscn tional, base Qc 45 35 Tlr-Tld Qc Tca? 30 20 Ql 32.31± Tdb Densely welded Tfc, 15 Tfcb Tfcb 30 40 Tlr Tcv Tbd Tcr 45 20 25 60 0.13b Tst banked steeply Tca Qc Qc Megablocks Tsc 10 Qc 60 25 30 Tpt Tcd 60 Tld Tst against fault scarp, Tpbf 25 25 Qc Tnpr (to 20 m) of Qc p_g no apparent offsets 5 35 Tnpd Tdb Qt Tfcb Ql Tndn 30 Ttsn? Ql Tld Tcd 35 5 Tsc 15 Qc Tfc Tcd 25 Tca 15 5 Tdb 5 Qc Tca Qc Tfc Tfcb Qc Tcv Tcv Qf 25 Tdbm Tcr Qc Tca Tfc Tca Trdn? Tsc Tnpd 15 Qf 5 Tsc Tct Qc Odd closed basin; possible Tsc RALSTON CREEK FAULT ZONE Tfc Qf Tfcb 5 Tcfr maar of Hinsdale Fm.? Tfc Ql Tfc Angular clasts of 10 Qc Qg Tpbr Tsa Tcd dacitic ignimbrite Qal 15 Tlav Rock Creek Park (RC) Tnr (Tnd or Tld?) in 10 Tcd Tfc unit Tpt Tfc Qc Tcv Tfc Tndn Qc Tsa Tnpr Tdb Tcrn Tcd Tws Tcr 5 Tid Tnpa 32.15 Tca Tca Tfc Tpt Resembles ±0.10 Tcr Tca Tpt Tca, but Tsc

5 Mountain (TM) Trickle Tca overlies Tsc Qc Qt Faults that displace 32.22±0.12b Qc Tfc Tfc unconformably Qgo Tnpr 20 Qc Tfc Tfc Ql 26.83±0.05 Tscn Tia Qc covered by Tcr Ql Tnpr Tfc Tnpd 30.31± Tws Qg Qal Qc 0.17gm Tdb Tnpr 5 Flow-layered Tbpv Tscl Qc Tfc lava, Tsc Tld 30.20± 45 Tci Tst 32.15 30.16±0.23gm Tca Tlav composition Tlr ±0.06 0.14gm QTg Twsb 35 Exposed Tfc-Tcr contact Kd 10 Qf Tpt p_g Tnpr Tlr Qc Tcd BLUE 27.53±0.04, Tcr Tci 25 30.54±0.29gm Tbt Tcd Tfc Ql Tnpd 27.60±0.06 Tcv Tfc Tca Tfc CREEK Tsa Tbpf Tlav INTRUSION Tld Tlr 30.17± Qal Tahh Ql Trdn? Tid 32.22 20 Tbg Tfc Tca Tca Qc Tbpi 0.11gm 27.35 Thick Tcr, Tpbf 15 ±0.05 Tsa Tfc Tws Tcr Qgo Qc Tpbf ±0.05 Tsc ponded within Tci Tca Tlr Tcap 5 Qt 5 Tcr Tcr Tcrn Qc graben Qc Tid Tdb Tw Tcr Qg Tpff 30.41± Qc Qc Tst Tbpf Maximum late fault 0.79h 32.19±0.13 27.35 Tw Ql displacement where ±0.05 5 5 aligned with p_g 5 Qg Complexly Qc Tca Ql caldera wall Tcfr slumped/rotated Tcc Qc Tscn Tcr Tnd Qf Tcfr Tld blocks Large inclined Qc Tpff? Tfc 50 29.98± columnar joints, Qt Thu? Tpt 32.17 against caldera Qt ±0.06 Tst? 10 31gm Tla 27.44 27.41 Tcc p_g Tdb wall Tbt Qt Tcfr 10 60 Tst 20 25 ±0.05 ±0.04 Tcr Tsc Dipping Tcr, depositional Tev Taha Tcr Tw 25 27.92±0.19h against graben margin Tnn Qf Qc Tpbf Tca Tnpa? Tcrn Tnr Tdb Qt Qc Tsa Tdb Qt Tsa Qt Tpbf Tca Tdb Ttsn Tcd Qg Sparse clasts of Tsc Tfc Qal Tcfr? Qc Tsa Tsc 5 Tcfr in conglomeratic Tsv Tcr Qc Tcd Tnpd Tsa Tcr Ql Tfc Qfo Tfc Tct Tsa Qal Ql Qc Ql Tcc Qf Tsd Tca Tcap 50 20 Ql Tca Tnpd ZONE Ql Qc Tpt Tndn Tnpd Tfc 5 Tdb Tcv Tfc Qc Abundant clasts of Tsc Tfc Tcd Tsa Thick Tfc, ponded Tnn Qfo Qc Tcd in conglomeratic Tsv Qt Tcv Large coherent Tcr steeply against Tca Compaction(?) faults, 25 20 Thu? Abundant clasts Tpbm Tcrn 20 Ttsn early Quaternary lavas: early caldera of Tnd, Tnr along buried caldera wall Qc Tsa Tdb Tdb Qt slump block Tla Qc Tcd? Tld Tca LOS PINOS GRABEN wall? Tpt Tbg Qc Tcfr Tlr Tsa Tsc 5 Brecciated sequence Qal Trr of andesite-dacite; Tcr Tsc Tfc Tca 15 Qf 26.93±0.08 Tfc Qc 10 Qc Tcr Qc incipient caldera Tct Tsa Qc Qc FAULT Qt Qc QTl? Tndn Qc landslides? Tld Tsa Tw Qc Qc Tcv Tpbf Tlr Tbt 38°07'30" Tcc Qm Tnn Tbt Tcd Qm Tcfr Tlr Tfc Qc 107° COCHETOPA CREEK GRABENQl 5 Tpbf Tcfr Tld Qc Tld Ql Rock Creek Park (RC) Tnd Tsc Tca Tscn 106°52'30" Tca Qc Ql Base from U.S. Geological Survey in cooperation 1 Elk Park (EP) 9 / 2° 106°45' with Division of Planning, Department of Saguache Park (SA) Tca Ttsn local affairs, 1:50,000 county map series Geology mapped by P.W. Lipman 2000–2006; Tsc 38°07'30" Tbt (topographic): Chaffee; Gunnison, sheets 4,5; assisted by J. Robinson (2000) and P. Christe (2001) Tfc Tdb? Tsc Saguache, sheets 1–4. COLORADO Digital database by Joel Robinson Qc Tbt Ql Tsc Tdb Tsc Tscn Lambert Conformal Conic projection TRUE NORTH Edited by J.L. Zigler Tscn Tla MAGNETIC NORTH Qc Digital cartographic production by Kathryn Nimz CREEK Ql Tsc APPROXIMATE MEAN AREA OF MAP 30.47± DECLINATION, 2012 Manuscript approved for publication April 28, 2010 0.08b Tbg CONTOUR INTERVAL 80 FEET Tcr Tca Tscn Qt Qc Qc 20 Tcrn Tsc Tscn Qc 25 Tdb Qfo Ttsn? Tcr Tsc Tfc Qc Tbt Qal Tw Tbt Tbt Qt Tcr Tfc Silicified- Good exposure, wood section between Tbg Qt Tcrn logs (to 1 32.49 Tsc and Tfc ±0.07 m long) Tfc Tdc 20 Tdb Qc SHEEP Qc Tcr 27.57±0.21 Tbt Tscn SOUTHWEST Tws Tsc Tdb Tscn Qc Sparse probable Tw Qc Tcv? clasts of Tsc in Paleovalley fill of Tfc conglomeratic Tsv Tws Tw, oblique to Tbt Qc present valley NORTHEAST Tca? Tcrn Ql Tfc Tab Massive chaotic breccia of uncertain Tab Qc , Tsa 13,365' Tcv Qc origin Tfc Qc , (west rim, Ql Tcr Ql 13,971' Tnpd Tca Bonanza caldera) Tca Tws could lie Qc Tdb Tsa Tscn Razor Creek Dome Mesa Mountain, 12,944' within caldera North Pass (Continental Divide) La Garita San Luis caldera complex Ql Ql 11,530' Green Mountain (on east rim, ) Tnpd Tws (high peaks are postcaldera lava flows) 32.75±0.08b, Tcr Tws (resurgent uplift within caldera) Tca Qc Tab , 11,132' Tca 32.82±0.18h Tsc Tdb Tfc Tcr Tws Tsc Ql Tdc Tcv Tca Tws Uncertain location Tfc Tfc of caldera margin; Thu age of Tnpd too W Pass Creek NE-trending lobe of Nelson Mountain Tuff, erupted from within San Luis caldera complex old? Tfc Qc Tfc Cochetopa Creek Tscn Tab 27.64±0.12gm, Qc 28.32±0.31gm Tcv Tfc Los Pinos Creek Tsc Tfc Ql Twsn Thu Agency Peak, 11,179' Thu Tdc Tfc Qt Tfc Tst Qc Tcv Ql Tsc Ql Tab Twsn Tca Tfc Twsn 27.96± Qc Tca 0.11gm

Grouse Creek (GC) Ql Thu Tdc Tcr Tfc Tsc 32.15 Qal ±0.05 Tcv Thu Lake Mountain (LM) Tfc Tfc Qc Sparse clasts of Tld in Tca Tsc distal conglomeratic Tdc Tca Tsc Slumped fill from Qt Tbt La Garita caldera Tsa Tcr Ql Tsc Tfc Tfc Tcv Tfc Qc Tsc Ql Qal Qc Ql Ql Qc Tcr Tcfr

Tsa Ql Tfc Tfc Qc Sawtooth Mountain, 12,147 ft Qc Thu Tscn Tfc Qt Ql Tfc Twsn Qc Tcv Tfc Tfc Qc Panoramic view of Cochetopa Park caldera, looking east and south from summit of Sawtooth Mountain. Sawtooth Mountain, Razor Creek Dome, Green Mountain, Cochetopa Hills, and Agency Peak are surviving erosional remnants of the topographic rim of Cochetopa Park caldera. Nelson Mountain Tuff, a large ignimbrite erupted at 26.90 Ma from eruptive vents within the San Luis caldera complex, flowed to northeast along a pre-existing graben (partly defined by the drainage of Los Pinos Creek), into Cochetopa Park caldera as it subsided concurrently with this eruption. Cochetopa Ql Tcr Tcrn Ql Dome is a postcaldera sequence of rhyolitic lava flows erupted from within the Cochetopa Park caldera after it subsided. West Pass and Cochetopa Creeks have eroded soft caldera-filling tuffaceous sedimentary rocks, exhuming the Oligocene caldera morphology. In distance to south is northern segment Tsa 27.50± Qc Tfc Qt 0.11gm of La Garita caldera, source of the Fish Canyon Tuff at 28.02 Ma; the north rim of this 75-km-long subsidence structure is the Continental Divide along the Cochetopa Hills, which form a ridge between Cochetopa Park and La Garita calderas. Digital photograph by Andreas Sbisa, July 2009. Tsa Tsa Tsd Tfc Qc Tfc Qc Tcrn Qt Qc Tsa 27.81± Tal Twsn 0.16gm Qc Tfc Tsa Tsd 27.51.± 0.16gm, Qc Tal Tfc Tnpd? 27.46± Uncertain Qc Tsc Ql 0.11gm Thu boundary Tnpd? Qc between Tcr Ql Tcv? Tsa and Tcv Alternatively, Tcd? Tca Tfc Qt Ql Tfc Qc Qc Tcd Tca Tcr Tscn Qc Qc Tcd? Tcv? Tcd Thu Geologic Map of the Cochetopa Park and North Pass Calderas, Northeastern , Colorado Any use of trade, product, or firm names in this publication is for descriptive 60 Qc Tcd Qc purposes only and does not imply endorsement by the U.S. Government Tca Tcv? Dark dacite For sale by U.S. Geological Survey, Information Services, Box 25286, Qc Tfc Tcap flow with large Federal Center, , CO 80225, 1–888–ASK–USGS Tca sanidine Tcr unambiguously By Tcv phenocrysts Ql overlies andesite flows Digital files available at http://pubs.usgs.gov/sim/3123/ Tca (to 2 cm) (Thu equivalent) Tfc Tca Tcd Peter W. Lipman Suggested Citation: Lipman, P.W., 2012, Geologic map of the Cochetopa 38°00' Tcr Tca Qc Tfc Tfc Park and North Pass calderas, northeastern San Juan Mountains, Colorado: 106°37'30" Tca Tfc Qc U.S. Geological Survey Scientific Investigations Map 3123, 48 p., 2 sheets, 38°00' Printed on recycled paper Grouse Creek (GC) 106°30' 2012 scale 1:50,000. (Available at http://pubs.usgs.gov/sim/3123/.) Lake Mountain (LM) 106°22'30"