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Pleistocene volcanic ash, Poncha Springs Area, Chaffee Co., Colorado, USA

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Latitude & Longitude (WGS84): 38° 35' 9'' North , 106° 6' 23'' West
Latitude & Longitude (decimal): 38.58583,-106.10639
Köppen climate type:Dfc : Subarctic climate

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Mineral List

Regional Geology

This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.

Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org

Pliocene - Miocene
2.588 - 23.03 Ma

ID: 2882150
Dry Union Fm

Age: Neogene (2.588 - 23.03 Ma)

Stratigraphic Name: Dry Union Formation

Description: Siltstone, sandstone, and conglomerate. Includes Wagontongue Fm (Miocene) in South Park

Lithology: Major:{siltstone,sandstone,conglomerate}, Minor:{claystone}

Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. [133]

2.588 - 5.333 Ma

ID: 3186821
Cenozoic sedimentary rocks

Age: Pliocene (2.588 - 5.333 Ma)

Comments: Rio Grande Rift

Lithology: Sedimentary rocks

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Miocene - Pliocene
5.333 Ma

ID: 3300358
Dry Union Formation

Age: Pliocene (5.333 Ma)

Stratigraphic Name: Dry Union Formation

Description: Locally tan or orange-brown, gray, reddish-gray, or greenish-gray mostly unconsolidated to moderately consolidated, weakly to well stratified, locally cross-stratified, minor clay and silt, silty sand, sand, and gravel. Clasts composed mostly of Proterozoic gneiss and igneous rocks and Tertiary volcanic rocks, generally less than 10 cm in diameter, but locally much larger. Unit locally contains ash-rich sandstone beds. A detailed sedimentological study of the Dry Union formation in the southern part of the upper Arkansas River valley was carried out by Xu (2001). Com- monly, unit is poorly exposed and is mantled by unmapped deposits of colluvium (Qc) and other mass-movement deposits and, locally, by sheetwash alluvium. Named for expo- sures in Dry Union Gulch, about 7 km south of Leadville (Tweto, 1961). Originally called “lake beds” by early miners that excavated shafts and adits in western part of the Leadville mining district (Emmons and others, 1886; Emmons and Irving, 1907; Emmons and others, 1927). Extensively exposed in southern part of upper Arkansas River valley; but in the northern part is mostly overlain by Quaternary surficial deposits. Generally dips gently west due to general down-dropping of west side of valley along faults. Unit Nd includes reddish-brown to yellowish brown, crudely bedded to well-stratified, cobbly pebble, fluvial gravel that commonly contains quartzite clasts beneath terrace deposits composed of outwash gravel of middle Pleistocene age (Qg3) between Malta and Iowa Gulch in the Leadville South quadrangle (McCalpin, Funk, and others, 2012), where it is correla- tive with the Malta Gravel of Tweto (1961), but here interpreted as upper part of the Dry Union Formation. Thickness probably locally more than 1,600 m in southern part of upper Arkansas River valley (Tweto, 1979a) and possibly as much as 1,200 m in the northern part (Tweto and Case, 1972). Chiefly fluvial deposits and fan deposits are recognized in the northern part of the Arkansas River graben in the Granite quadrangle. The fan deposits locally include alluvium of minor tributary streams, sheetflood deposits, and debris-flow deposits (Shroba and others, 2014). Two distinct facies (northern and south- ern) are recognized in the southern sub-basin of the Arkansas River graben (Shannon and McCalpin, 2006). North of the Salida-Maysville fault system, the Dry Union Formation commonly consists of white, pinkish-white, orange-tan, and light greenish- gray, very fine-grained siltstone, minor sandstone, and thin, discontinuous lenses of pebble to boulder conglomerate. Clasts are as long as 2 m (Shannon and McCalpin, 2006). Beds are as thick as several meters. Unit Nd locally forms extensive eroded, badland topography. This northern facies contains white, fine-grained, massive ash beds as much as about 1 m thick that are mapped at a few localities. Ash beds were sampled at two localities about 1.0–1.5 km south of Salida, yielding ages of 9.30±0.04 Ma and 15.6±0.3 Ma, respectively. These ages and reported mammalian fos- sils ages (using the current subdivision of Neogene time [U.S. Geological Survey, 2010]), the ages of ash- and fossil-bearing beds is inferred to be late Miocene or possibly early Pliocene (Van Alstine, 1974; Shannon and McCalpin, 2006). In addition to very fine- grained siltstone, conglomerate, and minor sandstone, two distinct subfacies are identified south of the Salida-Maysville fault: clay subfacies and slide-block subfacies (Nds). South of Salida-Maysville fault, age of exposed beds may be slightly older than north of the fault, consistent with the down-to-the-north offset on the fault; ash beds in the clay subfacies (Ndc) south of the fault (described below) suggest middle Miocene age. Conglomerate clasts south of the fault typically are much larger than clasts north of the fault. Some clasts south of the fault are as large as 6 m (Shannon and McCalpin, 2006)

Reference: Kellogg, K.S., R.R. Shroba, C.A. Ruleman, R.G. Bohannon, W.C. McIntosh, W.R. Premo, M.A. Cosca, R.J. Moscati, T.R. Brandt. Geologic map of the upper Arkansas River valley region, north-central Colorado. doi: 10.3133/sim3382. U.S. Geological Survey Scientific Investigations Map 3382. [186]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

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