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USGS Scientific Investigations Map 3394 Sheet 2 U.S. Department of the Interior Scientific Investigations Map 3394 U.S. Geological Survey Sheet 2 of 2 Pamphlet accompanies map CORRELATION OF MAP UNITS LIST OF MAP UNITS 05.6' 05.4' 05.2' Tbas Squirrel Gulch andesite Ttmc Megabreccia, Proterozoic clasts dominant Figure 15. A, Geologic map of [Some unit exposures on the printed or plotted maps are too small to distinguish the A A A 10,300 [Dotted unit-box boundaries indicate gradational welding or other emplacement contacts within eruptive units. Some major formation sheet-like mass of flow-layered 10,200 A Tbav Sawatch-Range Ignimbrite (Erupted from Aetna Caldera) A A color for unit identification. These units are labeled where possible; unlabeled units Volcaniclastic rocks rhyolite (see B, C, photo 10.3), A R R names are labelled. Some units of a formation are divided by emplacement type] R R are attributed in the database] Tbc Badger Creek Tuff A Bonanza Tuff interpreted as rheomorphic AM R A M SURFICIAL DEPOSITS GLACIAL DEPOSITS lower Bonanza Tuff (unit Tbrr), A R 10,000 SURFICIAL DEPOSITS Dacite Conejos Formation, Lower Units Q along upper Schoolhouse Gulch R R A A R R Tbdu M A R R Qal Qc Ql Qf Qt Qr Qal Alluvium (Holocene) Upper outflow dacite Early lavas and proximal breccias (GG). Letter symbols mark A R A A Holocene 14.8' R QUATERNARY locations of GPS-located R AR Tcd Dacite R R R A Qlo Qfo Qm Pleistocene Qc Tbd Main dacite 10,100 R A R Colluvium (Holocene) outcrops and areas of monolith- A A A R R R R M A Tcap Plagioclase andesite ologic float inferred to reflect A 9900 Tbdn Nonwelded dacite 10,000 R A Ql Landslide deposits (Holocene) immediately underlying R R R R Tca R R R Tfbd Andesite bedrock: A, andesite; D, dacite; A FLOW-LAYERED R A MIOCENE POSTCALDERA ROCKS Qf Alluvial-fan deposits (Holocene) Intracaldera dacite fracture fill R 9900 R RHYOLITE M, large blocks of andesite, in R Rhyolite Tcab Andesite breccia 9800 (Rheomorphic R A Qt matrix of nonwelded rhyolite A Bonanza Tuff?) Talus (Holocene) R A SEDIMENTARY ROCKS OF THE RIO GRANDE RIFT ZONE Tbru Upper rhyolite tuff; MBR, mega- and 9700 A R R 9800 Tcv Volcaniclastic rocks A R Qfo A R A A A Older alluvial-fan deposits (Pleistocene) meso-breccia of andesite R RR Ttra Tdup Tbr Main intracaldera rhyolite A A R R Jacks Creek Volcano (~34.5 Ma) blocks surrounded by rhyolite 9600 A Tdu Miocene TERTIARY Qlo Older landslide deposits (Pleistocene) A A A Tdua matrix; Q, colluvium; R, flow-lay- R R Travertine Tbrl Lower rhyolite Intrusions 14.6' A M ered rhyolite. Topographic Q A A A A A 9700 Dry Union Formation GLACIAL DEPOSITS Tjir Rhyolite A MBR Tbrb Brecciated rhyolite contour interval, 50 feet; thick 9500 Qr Rock glacier (Holocene) gray lines, graded gravel roads ANDESITE Tjid Dacite A A Q Tbrr Rheomorphic rhyolite of the Elk Horn Ranch subdivi- A AA ROCKS OF THE SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD Qm Till (Pleistocene; Pinedale glaciation) R CALDERA Tjia Andesite sion. B, Highly irregular blocks A Tbrn Nonwelded rhyolite A (outlined in orange; dashed Tree Gulch Lane R RELATIONS A MIOCENE POSTCALDERA ROCKS A R 1 Tjig Granodiorite where approximate) of andesite 9600 (age) IGNIMBRITE SHEETS AND RELATED ROCKS LAVAS AND RELATED ROCKS VOLCANIC SEDIMENTARY ROCKS INTRUSIONS Tfbr Intracaldera rhyolite fracture fill R Ttra Travertine (Miocene?) laharic breccia, enclosed within A A A Schoolhouse Gulch Lavas and related rocks R Intracaldera landslide breccia flow-laminated rheomorphic A Dry Union Formation 9400 D Tjd Dacite rhyolite tuff. Although flow D A Tbmu Mixed clast or undivided megabreccia 9500 Tdu A A A POSTCALDERA Thb Dry Union Formation, undivided Tjds laminated, the rhyolite contains 14.4’ Small-phenocryst dacite 9300 A ROCKS Miocene Tbma Andesite- and dacite-clast megabreccia Tjdb centimeter-size angular R A Hinsdale Formation Tdup Dry Union Formation, Precambrian clasts dominant R andesite fragments (hiking pole, A A Tjap Plagioclase andesite A A Tbmp Plagioclase-andesite-clast megabreccia ~0.9 m). C, Closer view of Tdua Dry Union Formation, volcanic clasts dominant A Tja Andesite contact (dashed line) between Q Tbmc Proterozoic-clast megabreccia SOUTH RIVER Tw Hinsdale Formation block of andesite laharic A CALDERA Tjab Andesite cone breccia breccia and enclosing dense Wason Park Tuff Thb Mafic lavas VOLCANIC ROCKS PREDATING BONANZA CALDERA Q (27.4 Ma) rhyolite that remains fluidal at Conejos Formation, Middle Units Tjfr Rhyolite D the contact. Several small 9300 OLIGOCENE VOLCANIC ROCKS A fragments of andesite enclosed Tracy Volcano (<32.5?–33.5 Ma) Tjv Volcaniclastic rocks 9500 Tcrn IGNIMBRITES AND ASSOCIATED ROCKS OF THE CENTRAL SAN JUAN in the fluidal rhyolite are 14.2’ D DACITE BACHELOR 050 100 METERS CALDERA COMPLEX Intrusions PREVOLCANIC ROCKS circled. CALDERA Tcr 0 100 200 FEET A Rocks Erupted from South River Caldera (27.4 Ma) Tyir Rhyolite (27.5 Ma) Carpenter Ridge Tuff LOWER TERTIARY SEDIMENTARY ROCKS Tw Wason Park Tuff Tyid Dacite Tev Conglomerate and sandstone (Eocene?) B Rocks Erupted from Bachelor Caldera (27.55 Ma) Tyia Andesite PALEOZOIC SEDIMENTARY ROCKS Thu Thv Carpenter Ridge Tuff Tyip Plagioclase andesite Upper Paleozoic sedimentary rocks Huerto Andesite Tcrn Nonwelded ignimbrite LA GARITA CALDERA Tyig Granodiorite s Sharpsdale Formation (Pennsylvanian) Tfc CYCLE (28.0 Ma) Tcr Welded rhyolite ignimbrite Caldera-filling(?) rocks k Kerber Formation (Pennsylvanian) Tfcn Rocks of La Garita Caldera Cycle (28.0 Ma) Tyca Caldera andesite Lower Paleozoic sedimentary rocks Fish Canyon Tuff Huerto Andesite Tydt Dacite caldera-wall talus Ml Leadville Limestone (Mississippian) Tsav Thu Olivine andesite and hornblende andesite-dacite, undivided Tyat Andesite caldera-wall talus and landslide breccia Mlb Brecciated and silicified rocks Andesitic conglomerate Thv Bedded breccia and conglomerate Flank lavas Dc Chaffee Group (Devonian and lower Mississipian) Twsn Fish Canyon Tuff WESTERN SAN JUAN Tyv Volcaniclastic rocks Of Fremont Dolomite (Upper Ordovician) CALDERAS (28.3 Ma) Sapinero Mesa Tuff Tfc Welded outflow ignimbrite sheet Tydu Upper dacite Tfcn Nonwelded to partly welded ignimbrite Oh Harding Quartzite (Middle Ordovician) Tyau Upper andesite Local Volcaniclastic Rocks of Saguache Paleovalley Om Manitou Limestone (Lower Ordovician) Tyap Plagioclase andesite C Tla Tsav Andesitic conglomerate l Lower Paleozoic rocks, undivided Andesite of Lone Tree Gulch Older Ignimbrite Related to Western San Juan Calderas (28.3 Ma) Tyds Small-phenocryst dacite PRECAMBRIAN ROCKS Twsn Sapinero Mesa Tuff Tya Andesite a Aplite of Alder Creek Tbg Conejos Formation, Upper Units (~30 Ma) Tyaq Quartz-xenocrystic andesite g Granitoid intrusive rocks, undivided CONEJOS FORMATION, Tuff of Big Dry Gulch Tla Andesite of Lone Tree Gulch (Saguache Valley) Upper units (~30 Ma) Tyab Andesite breccia m Metamorphic rocks, undivided Tdc Tbg Tuff of Big Dry Gulch Tyfr Rhyolite u Precambrian crystalline rocks, undivided Debris-flow deposit of Debris-flow deposit of Buffalo Pass Campground Buffalo Pass Campground Tysr Rhyolitic sedimentary rocks Tdc Distal conglomerate EXPLANATION OF MAP SYMBOLS Dacite of Red Rock Canyon CALDERA-RELATED ROCKS PREDATING THE WESTERN AND CENTRAL Contact—Dashed where approximately located; dotted where grada- CALDERA CLUSTERS Tyd Dacite lavas LOCAL LAVAS AND VOLCANICLASTIC ROCKS tional. Internal contacts (dash-dot) delineate some conspicuous lava flows or ash-flow cooling breaks within map units. No contact North Pass Caldera Cycle (32.2 Ma) Tydb Dacite breccia shown where boundary between map units is uncertain or indefinite Saguache Creek Tuff Sargents Mesa Volcano Fault—Dashed where approximately located; dotted where concealed or Tsc Tscn Tscn Nonwelded crystal-poor rhyolite Tsa Andesite-dacite lava occupied by intrusive rocks. Bar and ball on downthrown side, where movement known NORTH PASS CALDERA Saguache Creek Tuff Tsc Partly to densely welded crystal-poor rhyolite Rawley Volcanic Complex (~33.2–33.8 Ma) CYCLE (32.2 Ma) Low-angle (thrust?) fault—Teeth on upper block Tbt Tbt Bedded rhyolitic tuff Intrusions Bedded rhyolitic tuff Crest of elliptical resurgent dome Rocks of the Bonanza Caldera Cycle (33 Ma) Trig Granodiorite Quaternary slump block—Dashed where approximately located. Granitoid intrusions Trid Dacite Hachures on downthrown side SOUTHWEST NORTHEAST Spring Creek intrusion Lavas and related rocks Monoclinal hinge zone—Dotted where concealed. Triple hachures Fish Canyon Tuff (28 Ma) Tas Trv Volcaniclastic rocks TERTIARY Aplitic granite indicate direction of uplift 28 Tas Tgs Tag Ttg Tta Tgu Ted Sapinero Mesa Tuff Tgs Porphyritic granite Trfr Rhyolite Unconformity along caldera wall—Dashed where approximately Granitoid intrusions Oligocene located; dotted where concealed Tag Porphyritic granite of Alder Creek Trt Rhyolite tuff Bonanza (33.12 Ma) Turquoise Mine intrusion Tbdh Tbs Trdk Sanidine-bearing dacite Marshall (33.9 Ma) Ttg Granodiorite Tbdk Trdx Dacite shatter breccia CONEJOS FORMATION, upper lava units Bonanaza caldera topographic rim—Dashed where approximately (mostly southwest of Bonanza map area) Tta Intrusive andesite located; dotted where concealed. Erosionally modified Tbfd Trd Dacite 30 Tbav Tbfs Tgu Granitoid rocks, undivided Syncline Trds Small-phenocryst dacite Tbfr Tblr Ted Eagle Gulch Dacite Strike and dip of beds BONANZA CALDERA Volcaniclastic rocks Trap Plagioclase andesite CYCLE (33 Ma) Tbab Tbam Tba Tbap Tbas Intrusions of uncertain age and affinity 40 Inclined Tir Tid Tiap Tia Trax Andesite shatter breccia Volcanics of Cochetopa Hills (North Pass) Postcaldera lavas Tir Rhyolite Vertical Bonanza Tuff Intrusions of uncertain age and Tra Andesite Saguache Creek Tuff (32 Ma) affinity (precaldera and caldera) Tid Dacite Strike and dip of foliation 32 Tras Andesite scoria Postcaldera volcanics (Bonanza) Tiap Plagioclase andesite 35 Inclined Summer (upper andesite, dacite, Porphyry Peak Rhyolite, and others) Tbdu Rocks of the Marshall Caldera Cycle (33.8–33.9 Ma) Coon Tia Andesite Vertical Bonanza Tuff (33 Ma) Tbru Tbru Postcollapse caldera-filling rocks Postcaldera lavas and volcaniclastic rocks Trend of lineation—Showing bearing and plunge.
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