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General Description SUMMER COON LA VENTANA GEOLOGIC AREA (S. Nicolais, 12/5/17, has edited, enhanced and illustrated the original version submitted to the Rio Grande Forest Plan by San Luis Valley Ecosystem Council and Rocky Mountain Wild, ) Proposed Designated Area 22,400 acres Rio Grande National Forest Divide Ranger District Fig. 1 Google Earth View of the Summer Coon Volcanic Center with the major radial dikes and central intrusive complex La Ventana Central Intrusive Complex General Description The proposed Summer Coon La Ventana Geologic Area offers a unique opportunity to see well preserved outcrops that represent many of the significant features of the Summer Coon stratovolcano (a composite, conical volcano built up by many layers (strata) of hardened lava, tephra (ejected material), pumice, mud flows and volcanic ash. The proposed area incorporates a nearly perfect pattern of radial dikes, and the La Ventana Natural Arch, which was eroded through the wall of one of the most prominent dikes. The area also includes significant, ecological, botanical and cultural values in addition to its notable geological importance and educational values. The proposed area needs to be preserved and protected. The proposed Summer Coon La Ventana Geologic Area of Alternative “D” (Fig. 4) expands the existing 8,441-acre Elephant Rocks Special Interest Area of Alternative “A” (Fig. 4) to 22,400 acres to incorporate the entirety of the Summer Coon volcano complex, particularly the well-developed radial dikes originating from the area’s center out to and including the varied flank deposits. The existing SIA was designated in the prior Forest Plan (Alternative A, Fig.4) for just a portion of the Summer Coon volcanic features as well as rare botanical features in the form of a Forest Service sensitive species, the rock-loving Neoparrya. The La Ventana Natural Arch is a traditional cultural property considered sacred to the Ute and Jicarilla Apache. Extension of the SIA to include the La Ventana Natural Arch expands the represented values to include cultural as well as geologic and ecological values. The expanded boundary (Fig. 4) encompasses lower elevation 8,880-foot ecosystem types representative of the foothills surrounding the San Luis Valley, with grassland, pinyon-juniper and ponderosa pine woodlands transitioning to higher elevation Douglas-fir and aspen at over 11,000 feet. Geologic Values Summer Coon is an eroded Oligocene-aged (34 million-year-old) stratovolcano located in the eastern San Juan Mountains of Colorado, on the western edge of the San Luis Valley, about 6.5 miles north of the town Del Norte. The volcano has well developed radial dikes, but no ring dikes. One of the most prominent dikes on the north side contains La Ventana, natural arch /window eroded into the long, narrow wall of the dike. Some twenty major (Figs. 1,2,3) silicic dikes (in addition to hundreds of small mafic dikes) have orientations radial to the central intrusive complex of the volcano. The central intrusive complex appears as a group of low hills running north-northwest in the center of the volcano. The hills are surrounded by an approximately circular alluvium-filled valley about two miles in diameter. Although the volcano has been deeply and heavily eroded, the dipping extrusive deposits preserved around the flanks can be used to reconstruct the paleo topography of the volcano. The volcano is tilted slightly to the east as a result of the subsidence of the Rio Grande Rift basin to the east beginning 26 million years ago. A reconstruction and estimate of the volcano’s paleo topography was done (S Nicolais, 6/2017) by removing the easterly tilt (as mapped by USGS), yielding an original, (assumed symmetrical), depositional slope angle on the flanks of 20 -22.5 degrees. The result is a volcanic edifice of approximately 6,300 -6800 ft. above the circular valley surrounding the central intrusive complex. The Summer Coon volcano is part of the larger San Juan Mountains Volcanic complex that erupted from 35-30 million years ago. This was followed by 22 major ash-flows between 30-26 million years ago and is now characterized by a series of calderas throughout the San Juan mountains (Lipman, 2006). Erosion has exhumed the former stratovolcano down to its base, revealing a complete basal section of the 8-10-mile diameter cone. A geology driving tour along Saguache County Road A32 provides an educational introduction to readily accessible and interesting stops. (Noblett and Loeffler, 1987), and could be expanded into an engaging interpretive tour for forest visitors. A very enthusiastic group of SLV residents and a few tourists participated in geologic road tour, sponsored by the La Vereda Chapter of the of the Old Spanish Trail Association and the Rio Grande County Museum that was done in June of 2017 (Led by S. Nicolais, geologist). More tours have been requested and are tentatively planned for next year indicating a significant interest by locals and potentially tourists which could help promote economic development for Del Norte and surrounding area. Educational Values The area has tremendous educational value. Due to its uniqueness, excellent exposures, and diversity of lithologies, the Summer Coon Volcanic Center has been the subject of numerous scientific studies and graduate theses. It also serves as a geology field laboratory and mapping project area for teaching university geology students about the structure, stratigraphy and overall characteristics of a stratovolcano complex. In summer of 2017, a group of University Wyoming students spent a week mapping the geology of the volcanic complex. Ecological values The Summer Coon La Ventana Geologic Area includes the Elephant Rocks Potential Conservation Area (Alternative A) identified by Colorado Natural Heritage Program. This PCA is a complex of volcanic boulders, rock outcrops, and shrublands separating the prairie of the valley floor from the San Juan Mountains and contains both rare plant and animal species, which results in its rank of High Biodiversity Significance. Specific biodiversity elements present include a medium sized population of the rock-loving Neoparrya – an herb that is restricted to south-central Colorado; a rare milkvetch (Astragalus cerussatus) with only 20 known occurrences; and a silky pocket mouse subspecies population found here that is restricted to the San Luis Valley and is rare within its range. (CHNP Potential Conservation Area Report, 2015) The Eagle Mountain PCA is also located within the proposed Summer Coon La Ventana Geologic Area. The PCA includes the cliffs around Eagle Mountain and Eagle Rock, and is identified as of General Biodiversity Interest because of its nesting habitat for peregrine falcons. (CHNP Potential Conservation Area Report, 2015) . Cultural Values The Natural Arch is considered sacred to the Jicarilla Apache and Ute people. The Forest Service presently manages it as a traditional cultural property because of significance to the indigenous peoples. The Rio Grande NF’s Assessment of Areas of Tribal Importance describes the arch’s significance in cultural ceremonies and as a rendezvous location. (Rio Grande NF Assessment 12, 2015) Boundary, Size, and Access The proposed Summer Coon La Ventana Geologic Area consists of 22,400 acres, which incorporates the entirety of the existing 8,441-acre Elephant Rocks Special Interest Area (Alternative A). The geologic area is contiguous on the east with a portion of BLM’s popular Penitente Canyon Special Recreation Management Area. The geologic area is defined by La Garita Creek on the north, by the national forest boundary on the east and south, and generally by Old Woman Creek or private land along the west. Saguache County road A32 provides access directly to the interior of the area, and Forest Road #659 leads to the La Ventana Natural Arch. The expanded boundary better captures the geologic, ecological, and cultural features than the previous Elephant Rocks Special Interest Area boundary and is more appropriate for conserving this unique landscape. It is important to note that from geologic standpoint the actual Elephant Rocks outcrops are really unrelated and much younger than the Summer Coon volcanic complex and are ash-flow tuff erupted from the much younger (27.8 million years) La Garita caldera. Information Resources Topic Data Source Geology Noblett & Loeffler, Summer Coon Volcano Geology, Colorado College, 1987, Mertzman,S. A. Jr., 1971,The Summer Coon volcano, eastern San Juan Mountains, Colorado,New Mexico Geological Society Guide, 22nd Field Conference, p. 265-273. Lipman, P.W ,2006, Geologic map of the central San Juan caldera cluster, southwestern Colorado, IMAP 2799, 37p. Cultural Rio Grande NF Assessment 12, Areas of Tribal Importance, 2015 Biodiversity CHNP PCA Reports, 2015 Fig 2. USGS Geologic Map of the Del Norte Area , Eastern San Juan Mountains Colorado, by Peter Lipman, 1976 Del Norte Fig 3. SCHEMATIC GEOOGIC MAP of the SUMMER COON VOLCANIC CENTER (Map from Wohletz, Kenneth, and Grant Heiken. Volcanology and Geothermal Energy. Berkeley: University of California Press, 1992. Fig. 4 Map prepared by:Alison Gallensky,Rocky Mountain Wild, 11/14/17 La Ventana Natural Arch (Photo S. Nicolais 2017) One of the radial Dikes (Photo John McEvoy) East – West radial dike on west side (view to south) Continuation of the East - West dike complex (view to east) (Photos S. Nicolais 2017) Dipping deposits on the north flank. Thin lava bed (Arrow) in middle of outcrop separating two north flank, dipping volcanic mud flow/sedimentary deposits. (Photos S. Nicolais 2017) Old Spanish Trail /Rio Grande County Museum Geologic tour group June 17, 2017 Photo by Braden Nicholson 2017 .
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