Commercial production of ethanol in the San Luis Valley, Colorado. Final Report Page: 43 of 222
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shows the typical structural interpretation of the Alamosa Basin (San Luis Valley). The
maximum basement depths of the two grabens as interpreted from the literature
indicate that the Monte Vista graben basement would be a maximum of about 3050
meters (10,000 feet) deep to the west of the Alamosa horst while the Baca graben
basement would be at most 5790 meters (19,000 feet) deep about 16 kilometers
(10 miles) northwest of Great Sand Dunes National Monument. It has been suggested
(Burroughs, 1980) that one or more east-west tending saddles may exist in the Alamosa
horst to the north of the city of Alamosa.
The stratigraphic section, as depicted in Figure 3-3, of the basin is
composed of 4 to 5 major rock formations. At the top is the Alamosa Formation; next
is the Sante Fe Group which overlies and intertongues with the Los Pinos Formation;
below the Los Pinos is the Vallejo Formation which in turn overlies volcaniclastic and
a. Alamosa Formation
The Alamosa Formation is a series of blue clays interstratified with
water-bearing sands which divide most of the Alamosa basin into an unconfined (upper)
and confined (lower) aquifer. Figure 3-4 shows the approximate limits of the clay. The
clays are thickest in the northern end of the basin. The peripheral area outside the clay
limit is an aquifer recharge area whose waters charge both the confined and unconfined
aquifers. Burroughs (1980) reported logging 625 meters (2050 feet) of Alamosa Forma-
tion in the Mapco-Amoco well. The formation consisted in the first 55 meters
(180 feet) of loose, well-sorted sand. From 55 meters (180 feet) to 98 meters (320 feet)
blue-gray fresh water clays were encountered. From 98 meters (320 feet) to
543 meters (1780 feet) the lake clays changed to a greenish-gray. From 543 meters
(1780 feet) to 625 meters (2050 feet) the greenish-gray clays were mixed with very
b. Santa Fe Group
Rocks of the Santa Fe Group are found beneath the Alamosa Forma-
tion throughout most of the Alamosa basin. The Santa Fe Group is composed of pinkish-
orange clays interbedded with silty sands. The sand grains consist generally of quartz,
volcanic rock fragments (VRF), plutonic rock fragments (PRF) and metamorphic rock
fragments (MRF). The mix varies locally with one or the other becoming dominant
based on the respective sources. The San Juan Mountains are the source of VRFs, with
the Sangre de Cristos providing the PRF and MRF. The group thins from west to east in
conjunction with a corresponding thickening of the underlying Los Pinos Formation.
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Hewlett, E.M.; Erickson, M.V.; Ferguson, C.D.; Sherwood, P.B.; Boswell, B.S.; Walter, K.M. et al. Commercial production of ethanol in the San Luis Valley, Colorado. Final Report, report, July 1, 1983; United States. (digital.library.unt.edu/ark:/67531/metadc874948/m1/43/: accessed April 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.