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SAN JUAN VOLCANIC FIELD (SJVF), COLORADO
((( Western and Eastern half )))
2011...Rev. 2

Cenozoic Calderas Colorado
Cenozoic Calderas in U.S....Credit...P.W. Lipman, USGS, modified by W-G-H, 2011.

This large Volcanic Field is located in south-central Colorado and adjacent parts of New Mexico and is part of the larger Southern Rocky Mountian Volcanic Field (SRMVF). The following towns are within its location....Creede, Gunnison, Lake City, Platoro, Silverton, South Fork. In New Mexico, its Questa and Red River. The Continental Divide snakes through the SJVF, which is an erosional remnant of an extensive volcanic field that formed on a dissected dome structer created during the Laramide orogeny.

We have divided the SJVF up into two parts, the western part was explored in 2005, 2006 & 2007. We will explore the Eastern part in 2011. Here are some basic terms for those not familar with them Caldera volcanoes.

A particularly deep negative gravity anomaly exists in the Southern Colorado Rocky Mountains where the overall density of rock, below the Colorado Mineral Belt (COMB) formation, is somewhat low. Two northeast-trending basement batholiths are postulated from gravity lows of >-300MGals in a zone of Laramide and older magmatism that extends greater than 200 km across central Colorado. This coincides with one of the most negative Bouguer gravity anomalies in the US.

The West San Juan Volcanism, had three main general stages...
1. Volcanism
2. Caldera Subsidence
3. Mineralization

A few basic questions we will try to understand this year are;
Why is the SJVF roughly "T" shaped?
What is the relationship to the "RGR"?
What is the relationship to the Colorado Plateau?
What is the relationship to the COMB?
What is the relationship to the Uncomphage Plateau?
What caused the long lasting activity?

SJVF Overview Map

The San Juan volcanic field, located in southwestern Colorado, contains two distinct phases of volcanism. The earlier volcanism is Oligocene in age, and consists of largely intermediate composition lavas and breccias. A few ash flow tuffs also constitute this phase of volcanism. Later volcanism is Miocene–Pliocene in age, and is basaltic in composition. The Oligocene volcanism probably was related to subduction along the western margin of North America, and the Miocene-Pliocene volcanism is best interpreted as a partial melt of the lower crust that was erupted onto the surface

It should be noted that peak volcanism occurred around 33Ma, then caldera eruptions peaked at ~28Ma. The Lake city caldera at ~23Ma was the last. The last big event was mineralization of the previous volcanics between ~22 to ~10 ma.

***** Ancestral Rocky Mountains *****

In the southern Rocky Mountains, near present-day Colorado, these ancestral rocks were disturbed by mountain building approximately 300Ma, during the Pennsylvanian. This mountain building produced the Ancestral Rocky Mountains. The uplift created two large mountainous islands, known to geologists as Frontrangia and Uncompahgria, located roughly in the current locations of the Front Range and the San Juan Mountains. They consisted largely of Precambrian metamorphic rock forced upward through layers of the limestone laid down in the shallow sea. The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving extensive deposits of sedimentary rock.

Simplified geological history of the SJVF area
Ice age
500,000 to 10,000 years ago
Three glacial periods,

Miocene and early Pliocene rifting
Rifting was accompanied by quiet
eruptions of basalt
Establishment of the Rio Grande rift
28–26 Ma,
Bimodal assemblage dominated by basalt and
rhyolite, concurrently with the onset of regional extension
SJ Caldera
30–26 Ma from caldera sources.
caldera subsidence
First, SJ volcanoes
~35–30 Ma
~30–22 Ma The earliest volcanic
activity in the SJVF
Precaldera stratovolcanoes.
San Juan highlands, emplacement of the
Colorado Mineral Belt. The calderas of the SJVF formed within a cluster of precaldera stratovolcanoes.
Laramide Orogeny
80–55Ma, early Cenozoic

Cretaceous seaway
Late Jurassic and Early Cretaceous time ~160–96Ma
Western Interior Seaway, Sevier orogeny



Late Triassic & Early Jurassic
~210 Ma
Great deposits of windblown sands
Early  Mesozoic
250–230 Ma
Erosion of the Ancestral Rocky Mountains continued.
Ancestral Rockies
320-270Ma
Uplifts and Basins, Uncompahgria
Early Paleozoic
540Ma
Shallow seas, sandstones,
carbonates, clay and shales
Time between Precambrian & Paleozoic
~1.2 billion years
Weathering and erosion stripped off Precambrian rocks
Precambrian...Yavapai tectonostratigraphic
province,  (new lithosphere)
1.8-1.7Ga
Also intrusions at  ≈1.7, 1.4 &1.1Ga


* San Juan Volcanic Field ...USGS I-2799

***** General SJVF Geology Info *****
San Juan Mountains: Credit the USGS... "About 28 million years ago, a series of volcanic ash flows that originated from the San Juan Mountains blanketed much of southern Colorado. The tremendous caldera eruptions of the San Juans were characterized by turbulent, flowing clouds of hot incandescent ash, gasses and tiny shards of volcanic glass. Such plinian-type eruptions are sometimes referred to as nuée ardentes or "glowing avalanches". As the turbulent ash clouds settled out, the burning-hot ash and glass shards welded together to form a dense, erosion-resistant rock called welded tuff. The various layers of welded tuff serve as cap rocks that protect the softer rocks beneath them and give the mesas of Curecanti their flat top (mesa means "table" in Spanish)".

***** Western half of the SJVF Geology*****
San Juan Mountains: Credit the USGS... "The mountains that you see while driving along Red Mountain Pass were shaped over billions of years through multiple episodes of mountain building and uplift, ancient seas, volcanic upheavals, and icy glaciers. In the San Juan Mountains we see evidence of rock formations that span a vast amount of geologic time. From recent landslide features and relatively young volcanic events to billion-year-old basement rocks, we see a large part of the geologic time scale represented in these rocks. The San Juan Mountains are mostly composed of rocks that erupted from Tertiary volcanoes beginning about 40 million years ago. The volcanic activity continued sporadically for another 30 million years. Lava flows covered vast areas and mixed with older rocks to form conglomerates and breccias. Underlying the volcanic rocks, the older geologic sequence is visible in the Ouray area and south along the Uncompahgre River Canyon. Small outcrops are present in Ironton Park. Precambrian rocks (>600 million years before present) in much of Colorado are igneous or extensively metamorphosed rocks, but the Precambrian Uncompahgre Formation near Ouray comprises former sedimentary rocks that have been only moderately metamorphosed, and retain much of their original character. Sandstones have become quartzites; shales and mudstones have become slate. In Ouray and south to the vicinity of Bear Creek, sedimentary rocks from the Paleozoic era outcrop on the canyon walls. The rocks unconformably overlie the Uncompahgre Formation. Paleozoic rocks include the Ouray Limestone of Devonian age, the Leadville Limestone of Mississippian age, the Pennsylvanian Molas and Hermosa Formations, and the Permian Cutler Formation. The horizontally bedded, tan rocks to the east of the hot springs are part of the Ouray Limestone. Unconformably overlying the Paleozoic sequence are the Mesozoic rocks of the Triassic Dolores Formation, Jurassic Wanakah and Morrison Formations, and the Cretaceous Dakota Formation. These rocks are exposed north and west of Ouray, but are not visible south along the Uncompahgre River Canyon. After the Mesozoic sediments were deposited, a time of uplift and erosion ensued and the Tertiary Eocene Telluride Conglomerate was deposited. Another period of erosion removed most of the Telluride Conglomerate in this area. Subsequent volcanic activity began in the area of the San Juan Mountains. The San Juan Formation volcanic material was erupted from stratovolcanoes (like Mount St. Helens) and was deposited unconformably above the older formations exposed at the surface. The San Juan Formation volcanics are predominately andesites. Subsequent to deposition of the San Juan Formation, the Silverton Volcanic Group was deposited. The early members of this volcanic sequence are contemporaneous with caldera development in this area. Its members, from oldest to youngest, are the Eureka Member (rhyolite), Burns Member (andesite, rhyodacite), Henson Formation (andesite), and a pyroxene andesite member. The San Juan, Uncompahgre, Silverton, and Lake City Calderas and their associated volcanic deposits record a 15-20 million-year history of volcanism in the Ouray-Silverton area."

---------------------All known (25ea) SJVF Caldera's that W-G-Hs* could find ---------------------
Note, there could more undiscovered Calderas buried under this gigantic volcanic pile.

* Bachelor Caldera, (27.5Ma), - N37°49'03" and W106°54'46"
* Blue Creek, concealed caldera, (27.11Ma)
* Bonanza Caldera, (33.2 Ma), concealed or inferred
* Cebolla Creek Caldera, (~27Ma)
* Cochetopa Park Caldera, (26.9Ma) - N38°12' and W106°45'
* Creede Caldera, (26.9Ma), - N37°45'34" and W106°56'20"
* La Garita Caldera, (27.8Ma), ~size: 35 x 75 km.
* La Garita, North Caldera (Saguache), N37°57' and W106°48'
* La Garita, Central Caldera, N37°45'34" and W106°56'20"
* La Garita, South Caldera, N37°34' and W107°00"
* Lake City Caldera, (23.0Ma), - N38°01'31" and W107°23'04"
* Lost Lake Caldera, (28.2Ma), concealed or inferred
* Marshall Creek Caldera, (33.8Ma) N36° 18', & W106°,24'
* Mount Hope, Caldera, (28.4) concealed or inferred
* Nelson Mountain Caldera, (~25.7Ma), - N37°58' and W106°56'
* North Pass Caldera, (32.2Ma)
* Platoro Caldera, (29.4Ma), - N 37°21'08" and W106°31'52"
* Rat Creek Caldera, (~26.5Ma)
* San Juan Caldera, (28.3 Ma) , concealed or inferred
* San Luis Caldera, (~26–27Ma)
* Silverton Caldera, (~27-28Ma)
* South River Caldera, (27.25Ma)
* Summitville Caldera, (28.8Ma)
* Uncompahgre Caldera, (28.1 Ma)
* Ute Creek Caldera, (28.3 Ma), concealed or inferred

* Photographs ...U.S.G.S. Geologic Investigations Series I-2799

***** Misc. *****

* USGS Bulletin 2213 ... Resource Potential and Geology of the Grand Mesa, Uncompahgre, and Gunnison (GMUG) National Forests and Vicinity, Colorado. **Large PDF Warning**
* Summitville Mine...Helen M. Lang West Virginia University
* The Summitville Mine and its Downstream Effects...USGS OFR 95-0023

References Cited

Note 1, * W-G-H=Western Geo Hikes
** Hunt , C. B., 1956, Cenozoic Geology of the Colorado Plateau: USGS, PP 279.
Note; Modified by W-G-Hs