Great Sand Dunes National Monument is part of a geologic system that includes the San Luis Valley and the surrounding mountains. The structures created during the formation of these features now determine the characteristics of the dune field and the regional hydrology. Most of the data used to identify the structures come from gas exploration well logs and geophysical surveys.
Prior to the formation of the San Luis Valley was the Laramide Orogeny (60 to 70 million years ago), which uplifted Paleozoic (245 to 570 Ma) sedimentary rocks. In some areas, erosion completely removed the sedimentary rocks, exposing the underlying Precambrian (older than 570 Ma) metamorphic rocks, while in other areas over 6,000 feet of sedimentary rocks remained. The most extensive sedimentary formations were the Pennsylvanian-age Minturn Formation (285 to 320 Ma) and the Pennsylvanian/Permian Sangre de Cristo Formation (250 to 300 Ma). Both are mostly shale, sandstone, and conglomerate. The erosional period was followed by deposition of the Vallejo/Blanco Basin Formation during the early Tertiary Period (35 to 66 Ma). It contains massive beds of mudstone interbedded with sand and gravel and is up to 2,000 feet thick (Greis and Brister, 1989). The features of the modern San Luis Valley began to form when crustal extension initiated the Rio Grande rift, a system of down-dropped crustal blocks (grabens) that extends from Leadville, CO, to El Paso, TX. The San Juan Mountains were the first to develop. Initial rifting resulted in a volcanic field that was active from 17 to 35 million years ago (Lipman, 1975). It consists of lava and ash flows interbedded with volcaniclastic rocks. The majority are part of the Conejos Formation. Above the Conejos Formation are various ashflow Tuffs and basalt flows. These flows extended all the way across what is now the valley floor, thinning with distance eastward.
As rifting continued, crustal blocks began to shift. The blocks that rose (horsts) now comprise the Sangre de Cristo Mountains while the blocks that dropped are now the San Luis Valley. This process began 18 million years ago and continues today (Chapin, 1971). The non-uniform decline of the valley graben was more extreme on the east side, least in the center, and intermediate on the west side, defining three distinct areas known as the Baca Graben, Alamosa Horst, and Monte Vista Graben, respectively. Cross sections constructed by the Colorado Water Resources and Power Development Authority show 16,000 feet of vertical displacement between the upper Precambrian contacts of the Sangre de Cristo Mountains and the Baca Graben.
The subsidence associated with the graben created a depositional basin within the San Luis Valley that is filled with unconsolidated sediments of the Santa Fe Group. The formations within the group are the Los Pinos, Santa Fe, and Alamosa. The Los Pinos Formation was deposited on the west side of the Valley from 4.4 to 26.8 million years ago (Lipman, 1975) and is composed of volcaniclastic sediments from the San Juan volcanic field. The Santa Fe Formation was deposited on the east side of the Valley from 1.8 to 12 million years ago (Lockman-Balk and Bruning, 1971). It is finer grained than the Los Pinos and is intertongued with it near the center of the Valley. Hanna and Harmon (1989) suggest that the upper Arkansas River Valley is the source of Santa Fe Formation sediments while B. S. Brister (personal commun., 1988) believes there were structural barriers that would have prevented sediment transport from the Arkansas to the San Luis Valley. The Alamosa Formation represents a change in deposition beginning about 5 million years ago when playa lakes developed in the northern half of the Valley (Hanna and Harmon, 1989). The change may have been caused by the Taos Plateau basalt flows which accumulated in the southern half of the Valley, 2.0 to 4.5 million years ago (Lipman and Mehnert, 1979) and blocked drainage from the north. The clay and silt lake sediments are interbedded with sand and gravel and can be up to 1,600 feet thick in the Baca Graben (Greis and Brister, 1989) but are generally less than 1,000 feet thick.
The active dune field is definitely a young geologic feature, but an absolute age has yet to be determined. U.S. Geological Survey geologists estimate that the dunes are between a couple of thousand years and 12,000 years old, and will use the optical thermoluminesence method on their dune field and sand sheet samples in an attempt to determine a more precise age. The eolian system could be much older than 12,000 years. Drill cores reveal at least 130 feet of eolian sand beneath the Valley floor.
The composition of the sand indicates that the majority of it originated in the San Juan Mountains. It was then transported to Valley floodplains by the Rio Grande and tributaries and carried by prevailing southwesterly winds to the Sangre de Cristo Mountain front. Details of sand movement into the present dune field are not completely understood, but recent studies indicate that sand sheet vegetation is a barrier to sand movement from sand sheet to dune field. The possibilities are that sand enters the dune field: 1) when the sand sheet isn't so heavily vegetated; 2) as clusters of dunes migrating across the sand sheet; 3) very gradually even when the sand sheet is vegetated; 4) a one-time "catastrophic" eolian event following formation of extensive glacial outwash; or 5) a combination of any of these possibilities.
Activity of the dunes is subject to numerous controlling factors, including aridity, sand supply, wind and its variability, topography of the Sangre de Cristo Mountains, and surface flow in streams on the dune field perimeter.
The shape of the Sangre de Cristo Mountains acts as a sand trap and creates a unique setting for the eolian deposits. Prevailing winds traveling across the San Luis Valley are directed to the deposition site because they exit the valley through Medano, Mosca, and Music Passes, and a change in the trend of the mountain front funnels wind into the three passes. The active dune field is the sand deposit located farthest downwind and is situated in a mountain front indentation adjacent to the passes.
Prevailing southwesterly winds are most common in the spring, but during the rest of the year, the wind can blow in any direction. The second most common wind direction comes from the northeast whenever low air pressure systems are east of the Sangre de Cristo Mountains. These winds frequently diminish within a 10 mile zone west of the mountain front. Other wind directions are usually fluctuations in the two major directions caused by turbulent flow around the mountains.
Wind patterns change within the dune field and that results in differences in dune types. The majority of the dune field is exposed to winds from multiple directions. Dune types that develop here are reversing and star dunes. Both tend to grow vertically as opposed to migrating horizontally. The east-central edge of the dune field is sheltered from the northeastern wind by the adjacent mountains, forming migrating barchan and transverse dunes (Valdez, 1992).
Two Creeks flow along the perimeter of the dune field and influence the shape of it. Sand Creek flows along the northwest edge and Medano flows along the east and southeast edges. Their role is to erode sand from downwind parts of dune field and transport and deposit it further upwind. This may be responsible for the crescent shape of the dune field. Each stream builds a lobe of the crescent with the Medano lobe being much bigger since its contact with the downwind part of the dune field is greater.
The dune field is relatively complex because of interactions among the controlling factors and variance in those interactions in different parts of the dune field. In response to those interactions, three distinct regions have developed, the Sand Creek star dunes, Medano Creek ridge, and the central dune field (Valdez, 1992). The Sand Creek star dunes are in the northwest part of the dune field, at the base of Mount Herard. Winds that blow over this area are deflected northward or eastward toward either Music Pass or Medano Pass. Wind patterns become very complex and consequently dune ridges developed in many directions. The Medano Creek ridge is found on the southeast part of the dune field and is parallel to Medano Creek. The thickest sand deposits are found here (up to 750 feet) partially because Medano Creek supplies large amounts of sand to this area. It is unique because two sets of dune ridges are superimposed on each other. There are north-south trending dunes with northeast trending dunes in the interdunal troughs. Characteristics of the central dune field are large north-south trending dunes with well-developed troughs. It is the simplest area because winds are bimodal, for the most part, and it lacks an external source of sand.
Mineral Development Mineral-development activities that potentially could affect water quality of the Monument are in three categories:
There are other mines and prospects within and near the Monument that are thought not to represent any potential for water quality degradation. These and other mines and prospects in the Monument and vicinity are detailed further in a separate report supplied to Monument management (Chatman, 1995c).
Mineral Development on the Baca Grant The Baca Grant is a 100,000-acre contiguous tract of private land that is immediately adjacent to Great Sand Dunes National Monument. The 1,832.65 acres of the Baca Grant that are in the southeasternmost corner of the tract have been incorporated into the Monument as an inholding. Surface and mineral ownership of the Baca Grant (and the inholding) is fee simple absolute. This land and minerals status was verified both through literature (Luis Maria Baca Mining and Development Company, 1935), and through a May 3, 1995 search of U.S. Bureau of Land Management records. The Baca Grant ownership leased the grant's minerals to one mineral exploration company and some surprising and potentially developable mineral discoveries, detailed below, have resulted.
Lexam Explorations USA, Inc., Wheat Ridge, CO, has been conducting gold exploration of a geophysical target (low-angle, west-dipping detachment fault zone) on the Baca Grant since 1990. In the process of core drilling the gold targets in this cataclastic zone, the company accidentally discovered that the zone also has acted as a trap of live oil in Precambrian-age gneiss immediately underlying the detachment zone. Oil was found in 27 of the 42 gold exploration drill holes. The oil, a biodegraded crude, was examined and found to have originated from Cretaceous sediments, heretofore unrecognized in the eastern San Luis Valley. The potential of this discovery has expanded with followup exploration and the firm now (May 1995) has suspended its gold exploration on the Baca Grant in favor of concentration solely on development of the petroleum potential.
There are four petroleum exploration targets that have been defined by Lexam Explorations, Inc. It is notable that the southernmost target includes most of the 1,833-acre inholding in the northwesternmost part of the Monument. Two exploratory wells are planned for the Deadman Creek target, which is 4 miles further to the northwest. Both wells will penetrate the Precambrian-age rocks below the confining high-clay cataclastic part of the detachment zone; one well is to be 4,500-feet deep and the other 6,500-feet deep. Both are to be spudded in during the summer of 1995 (James Donaldson, Lexam Explorations, Inc, written commun. to Colorado Oil and Gas Conservation Commission, May 18, 1995).
The four targets are defined by not only the presence of live oil, but also the presence of several Mesozoic sedimentary rock formations that are petroleum reservoirs and/or petroleum source rocks in other localities in the West. These rocks were thought to have been removed from the San Luis Valley through erosion many ages ago. They include several wellknown rock formations, including Upper Mancos Shale, possibly the Niobara Formation; Lower Mancos Shale; sandstone of the Dakota Formation; and the Morrison Formation. All unconformably overlie Precambrian-age crystalline rocks that are very common in the Sangre de Cristo Mountain range. These rocks are within a heretofore not widely known structural unit that lies between the Sangre de Cristo Mountains fault block on the east and the San Luis Valley graben (which is deeply buried by Tertiary and recent sediments of the Santa Fe Formation) on the west. The structural unit's upper margin is defined by the detachment zone. Once the effort to explain the oil shows began, public domain gravity data were utilized to partially define this intermediate structural unit. It's size and location were further refined through a privately contracted aero-magnetic survey. The structural unit has roughly a 16-mile by 4-mile area and is about 3 miles west of the main basin bounding fault of the Sangre de Cristo Mountain front.
The Mesozoic sedimentary rocks, at this point in the exploration program, are used to define target areas for potential petroleum reservoirs, even though they are actually above the detachment zone. They are in connection with the cataclastic layer of the detachment zone through a system of normal faults that also may be important traps. It is thought that many of these Mesozoic formations are deeply buried (about 16,000-feet deep;) petroleum source rocks or reservoir rocks on a western extent of the cataclastic layer. But according to the currently available data the only known reservoir rocks are the Precambrian crystalline rocks immediately below the cataclastic zone.
The petroleum exploration is modeled on geophysical, structural, and lithologic analogs in Nevada. In the Railroad Valley area of Nye County, NV, are the Grant Canyon and the Bacon Flats petroleum fields, from which 19,000,000 barrels of oil have been produced from a very small area (a 300-acre field). In the Pine Valley area of Nevada, in Eureka County, the Blackburn field has produced about 3,000,000 barrels of oil; it also is a field of a very small area. These analogs favor the existence of both petroleum source and reservoir rocks on the hanging wall of the detachment fault zone along the Sangre de Cristo Mountains. Exploration is not advanced enough to speculate on economic viability (unless noted otherwise, data from a presentation by Tom Watkins, Lexam Explorations, Inc., to the Colorado Scientific Society, Denver, CO, May 1995).
A gold prospecting program was undertaken on the Baca Grant by Lexam Explorations, Inc. in 1990, using Battle Mountain Gold Company's San Luis Mine, located near the town of San Luis, CO, as a model. The San Luis Mine hosts both oxidized and sulfidic, refractory, disseminated gold of hydrothermal origin in a 24 to 27 million-year-old low-angle, detachment-style fault zone associated with the Rio Grande rift. The same type of clay layer that acts as the petroleum trap on the Baca Grant oil shows also acted as barrier to upward-migrating auriferous hydrothermal fluids at the San Luis Mine. Those hydrothermal fluids therefore tended to pool below the clay zone. Refractoriness of the ore is due to both incorporation of the gold into pyrite mineral grains and silica flooding. Other sulfide minerals, present in minor quantities, include sphalerite and galena. These were prospected for lead and zinc decades ago. Some copper is present, which is suggestive of an original content of chalcopyrite in the deposit. The complete milling and recovery processes, which utilize grinding, sulfide ore roasting, and cyanide leaching, are on-site (Alan Wallace, U.S. Geological Survey, Central Mineral Resources, Denver, CO, personal commun., May 17, 1995). The San Luis Mine was operating as of May, 1995. In order to have a sense of the scale of this operation, it should be noted that the operation in 1994 was profitable, during which time 73,000 oz gold were produced at an average total operating cost of $317/oz. In 1995, production is targeted at 68,000 oz gold; 17,000 oz have been produced during the first quarter of the year (Rocky Mountain Pay Dirt, Feb. 1995). The San Luis Mine deposit contains 12 million short tons at a grade of 0.04 oz Au/st (data from presentation by Tom Watkins of Lexam Explorations, Inc., to Colorado Scientific Society, Denver, CO, May 9, 1995).
The same anomalous geophysical zone on the Baca Grant in which Lexam Explorations, Inc. discovered oil was also the company's initial disseminated gold deposit exploration target. Forty-two exploratory drill holes were completed around Deadman Creek in and near the foothills (data from presentation by Tom Watkins of Lexam Explorations, Inc., to Colorado Scientific Society, Denver, CO, May 9, 1995). Gold exploration completed to date has not been of enough success that a reserve estimate could be substantiated. Gold mineralization encountered has been was weak and spotty (Jim Cappa, Colorado Geological Survey, personal commun., May 1995). No mine plan for gold production from the site was ever filed with the State of Colorado (Jim Stevens, Colorado Division of Minerals and Geology, Denver, CO, personal commun., May 1995). The only data available to characterize the actual grade of gold metallization in the area come from a U.S. Bureau of Mines field investigation in Deadman Creek on the Baca Grant, work done in the process of evaluating mineral resource potential peripheral to the proposed Sangre de Cristo Wilderness. About ½ mile south of the Deadman Creek drainage channel are two, interconnected, near-surface adits with at least four portals and over 400 total feet of underground excavations. It appears that these adits intersect the clay zone of the Lexam Explorations, Inc. detachment fault gold target. Of the eight U.S. Bureau of Mines samples from that zone, five contain gold in the range of 0.01 oz gold per short ton to 0.06 oz gold per short ton. All concentrations were determined through fire assay. Most samples also contain about ½ oz silver per short ton and some have elevated arsenic content (300 to 600 parts per million) (Ellis et al., 1983; U.S. Bureau of Mines field notes, Denver, CO, June 19, 1979).
These are gold concentrations that are sufficiently high to attract exploration interest, but that are also low enough to make profitable mining difficult unless delineated reserve tonnages are high and/or the deposit is highly amenable to beneficiation.
It is important to note that, while the gold exploration has currently been suspended, the price of gold has been somewhat volatile in past years and an a substantial increase in the price of gold will almost certainly bring about a return of the exploration program. The main gold exploration target is the same area as the Deadman Creek oil exploration target, but the structural zone that is thought to be the gold host probably extends from Crestone, CO, all the way to the 1,832-acre AWDI inholding in the Monument. The structural and mineralogic similarities with the gold deposit of the San Luis Mine suggest the possibility, unproven as yet due to lack of exploration, that the structural zone is continuous between San Luis Mine and the Baca Grant. If such is the case, the zone would extend beneath the Great Sand Dunes National Monument.
In past years and as recently as 1937, there was lode mining for various metals along much of a 70-mi distance of the Sangre de Cristo Mountains' western slope. Archeological remains of Spanish-exploration era mining south of Crestone, CO have been found. The more recent era of mining was started about 1870, as vuggy, gold-bearing quartz veins were noted in the Baca Grant. Few of the mines were very large or had very high quantities of ore mined from them, although some $7 million to $8 million worth of precious metals were produced during the American-era of mining, which peaked between 1880 and 1904. The Independent (or Independence) Mine was one of the largest operations (Parker, 1952; Gabelman, 1953; Benedict and Coggin, 1973; Ellis et al., 1983). As is the usual case with mining districts, there is a periphery around the main mining areas in which even smaller and less productive mine and prospect excavations are made. Such is the case within and around the Monument.
In addition to small mines and prospects within the boundary of Great Sand Dunes National Monument, there are numerous mine and prospect excavations upstream of and in watershed that drain into the Monument. The potential for water quality in the Monument experiencing any detrimental effects from these areas are assessed below.
Pulps (pulverized, powdered rock of less than 100- mesh size particles, that are ready for laboratory assays) of nearly all the U.S. Bureau of Mines rockchip samples collected from these mineralized zones and prospects have been obtained for the National Park Service. Also obtained are field notes describing these sites and samples, laboratory assay reports, and an index, tying the ordered sample numbers used in a related U.S. Bureau of Mines mineral resource report (Ellis et al., 1983) to the more random numbering scheme found in the field notes. Most of these data are compiled in Chatman (1995c).
Most of the lode mining within the Monument occurred in the Cold Creek watershed on a small, unnamed tributary of Cold Creek. The main (southernmost) working does contain a 2,300 cubic yard dump that is adjacent to the unnamed Cold Creek tributary drainage channel, but reports (U.S. Bureau of Mines field notes, Denver, CO, July 10, 1982) suggest little metallized or sulfidized rock is present in that dump and two rock-chip samples are lacking anomalous levels of hazardous substances (U.S. Bureau of Mines file data, Denver, CO). The mine is not, therefore, likely to be contributing to water quality degradation via such mechanisms as acid mine drainage or heavy metal loading.
Three other groups of prospect excavations in this watershed, all north of the main Cold Creek drainage channel, are thought to be the source of at least some of the iron-oxide cementation of sediment that occurs occasionally in Sand Creek. Iron-oxides alone are not detrimental, but in this geologic environment the mineral group potentially could carry heavy-metal ion concentrations or be indicative of sulfide minerals, which, in turn, can generate acids when exposed to water. The sites should be investigated further. Mitigating the potential for water quality degradation problems are the facts that there is little surface flow in Cold Creek, and rock-chip samples from the sites are generally devoid of concentrations of toxic materials.
Other lode-type mineral excavations in the Monument are thought not to be potentially detrimental to water quality, based on some combination of the following factors:
That group of excavations includes those in Sawmill Canyon; on the Wellington tract; and at Denton Spring. The sites are detailed further in Chatman (1995c).
The patented Myrtle K Mine and mill site, upstream from the Monument on Sand Creek , is on a narrow fault zone that contains sulfide forms of iron and copper minerals and a 2,100 cubic yards dump, a boiler, and a five-stamp mill, all immediately adjacent to the creek. Samples of the fault and a shear zone at the site contain slightly elevated levels of arsenic, boron, copper, manganese, and zinc, based on semi-quantitative analyses (U.S. Bureau of Mines field notes, Denver, CO, June 14, 1979; U.S. Bureau of Mines file data, Denver, CO). The site should be investigated further.
Mine and prospect sites upstream from the Monument in other watersheds are mostly without consequence relative to Monument water quality due to some combination of small workings size, lack of metallization or sulfidization, neutralizing bedrock, or distance from drainage channels. This includes workings in Medano Creek, Buck Creek, Morris Gulch, Evans Gulch, South Arrastre Creek, and a working in the Sand Creek watershed, about 1/2 mile downstream from the Myrtle K Mine. Some historical information is suggestive of copper loading in Mosca Creek, possibly related to releases of mineral processing reagents, but recent, limited water quality testing provides no indication of degraded water quality in Mosca Creek (sites detailed further in Chatman (1995c)). Should poor water quality be detected in the future, the metallized part of the Mosca Creek watershed, including the old prospect workings, should be investigated more closely.
An upstream site from the Monument that should be examined now is the easternmost working (an adit) in North Arrastre Creek. An abandoned, small mill was reported to the south of the adit. Information is lacking about the milling method employed or the presence of tailings and reagents. Contents of the steel drum reported at the adit opening are unknown (U.S. Bureau of Mines field notes, Denver, CO, May 14, 1982). Condition of the site should be verified with a field examination, and a decision made at that time about the need for water quality testing upstream and downstream from the mine.
The 1928 report that gold associated with magnetite grains was detected in dune field sand sparked a financially unsuccessful gold rush. Claims were staked on lands now within the Monument, not only on Medano Creek, but also in the dune field. One claimant had claims occupying 11 mi2 of the dunes. Mining ended by 1938, but some prospecting (collection and assay of dune sand samples for gold content) continued to 1941. The major detriment was the low gold concentration. Another, more recent attempt to retrieve placer gold in the region occurred in 1980 at some unknown site on Medano Creek, most likely inside the Rio Grande National Forest. That work ended due to the lack of access routes (U.S. Bureau of Mines file data, Denver, CO, including interview with Paul Maddrell (undated but post-1963 material); U.S. Bureau of Mines field notes, Denver CO, May 15, 1982, including interview with Bob Schultz, Ranger, U.S. Department of Agriculture, Forest Service; Alamosa Daily Courier, 1938; Voynick, 1992).
The abandoned placer claim sites appear to represent no water quality issue for the Monument. All equipment and materials have been removed. Water quality testing done since 1990 reveals no anomaly of mercury, the most likely contaminant that a placer gold operation might release into the creek (used to amalgamate small placer gold particles).
Chapin, C. E. 1971. The Rio Grande Rift, Part I: Modifications and Additions. Guidebook of the San Luis Basin , Colorado . New Mexico Geological Society. 22" d Field Conference. pp. 191.
Chatman, M. L. 1995c. A Preliminary Literature Search, Inventory, and Assessment of Mines and Prospects in and near the National Monument with Emphasis on Potential Water Quality Degradation: Great Sand Dunes National Monument , Colorado . Unpublished report in Great Sand Dunes National Monument files. 17 pp., 1 appendix.
Ellis, C. E., B. J. Hannigan, and J. R. Thompson. 1983. Mineral Investigation of Sangre de Cristo Wilderness Study Area, Alamosa, Custer, Fremont , Huerfano, and Saguache Counties , Colorado . U.S. Bureau of Mines Open File Report MLA 65-83. 190 pp.
Gries, R. R. and B. S. Brister. 1989. New Interpretation of Seismic Lines in the San Luis Valley South-Central Colorado . Water in the Valley. Colorado Ground-Water Association. Eighth Annual Field Trip. pp. 241-254.
Hanna, T. M. and Harmon E. J. 1989. An Overview of the Historical, Stratigraphic, and Structural Setting of the Aquifer System of the San Luis Valley .
Lipman, P. W. 1975. Evolution of the Platoro Caldera Complex and Related Volcanic Rocks, Southeastern San Juan Mountains , Colorado . United States Geological Survey Professional Paper 852. 128 pp.
Lipman, P. W. and H. H. Mehnert. 1979. The Taos Plateau Volcanic Field, Northern Rio Grande Rift , New Mexico . Rio Grande Rift: Tectonics and Magmatism. American Geophysical Union . pp. 289-311.
Lochman-Balk, C. and S. E. Bruning. 1971. Lexicon of Stratigraphic Names used in South Central Colorado and Northern New Mexico . Guidebook of the San Luis Basin , Colorado . New Mexico Geological Society. 22nd Field Conference. pp. 101-111.
Luis Maria Baca Mining and Development Company. 1935. A Synopsis of the Operation, Development, and Possibilities of the Project of the Luis Maria Baca Mining and Development Company. Unpub. manuscript by Luis Maria Baca Mining and Development Company, Crestone, CO, in file data of U.S. Bureau of Mines, Denver, CO. 7 pp.
Parker, C. O., III. 1952. A History of Gold Mining in the Sangre de Cristos. The Mines Magazine, , 2 May 1952 . pp. 25-27, 43.
Rocky Mountain Pay Dirt . 1995. BMG's San Luis Operation Improved During 1994. Pay Dirt Magazines, Bisbee, AZ. Feb. 1995, p. 12A.
Valdez , A. D. 1992 (in press). Sand Supply and Wind Regime as Related to Dune Field Development at the Great Sand Dunes National Monument , Colorado . Great Sand Dunes Researchers Symposium. Submitted to National Park Service.