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Summary of the Geology of the Rico Region, Colorado Walden P New Mexico Geological Society Downloaded from: http://nmgs.nmt.edu/publications/guidebooks/19 Summary of the geology of the Rico Region, Colorado Walden P. Pratt, 1968, pp. 83-87 in: San Juan, San Miguel, La Plata Region (New Mexico and Colorado), Shomaker, J. W.; [ed.], New Mexico Geological Society 19th Annual Fall Field Conference Guidebook, 212 p. This is one of many related papers that were included in the 1968 NMGS Fall Field Conference Guidebook. Annual NMGS Fall Field Conference Guidebooks Every fall since 1950, the New Mexico Geological Society (NMGS) has held an annual Fall Field Conference that explores some region of New Mexico (or surrounding states). Always well attended, these conferences provide a guidebook to participants. Besides detailed road logs, the guidebooks contain many well written, edited, and peer-reviewed geoscience papers. These books have set the national standard for geologic guidebooks and are an essential geologic reference for anyone working in or around New Mexico. Free Downloads NMGS has decided to make peer-reviewed papers from our Fall Field Conference guidebooks available for free download. Non-members will have access to guidebook papers two years after publication. Members have access to all papers. This is in keeping with our mission of promoting interest, research, and cooperation regarding geology in New Mexico. However, guidebook sales represent a significant proportion of our operating budget. Therefore, only research papers are available for download. Road logs, mini-papers, maps, stratigraphic charts, and other selected content are available only in the printed guidebooks. Copyright Information Publications of the New Mexico Geological Society, printed and electronic, are protected by the copyright laws of the United States. No material from the NMGS website, or printed and electronic publications, may be reprinted or redistributed without NMGS permission. Contact us for permission to reprint portions of any of our publications. One printed copy of any materials from the NMGS website or our print and electronic publications may be made for individual use without our permission. Teachers and students may make unlimited copies for educational use. Any other use of these materials requires explicit permission. This page is intentionally left blank to maintain order of facing pages. 83 SUMMARY OF THE GEOLOGY OF THE RICO REGION, COLORADO' By WALDEN P. PRATT U.S. Geological Survey, Denver, Colo. The Rico Mountains are an elliptical group of 12,000- surprise (and a tribute to Cross) that the current mapping foot peaks on the southwest edge of the San Juan Moun- at 1:24,000 has produced no radical changes from the tains, close to the vague boundary between the San Juans earlier mapping at 1:23,600. McKnight's more detailed and the Colorado Plateau. The principal tectonic feature work in the center of the district has resulted in some re- of the Rico Mountains is the Rico dome, in which the vision of both stratigraphic and structural interpretations, sedimentary rocks are bowed up sharply from their gentle but has not affected the basic concept of the Rico dome southwesterly regional dip (see fig. 1); at the center of the as developed by Cross. General aspects to be summarized dome are a monzonite stock and an upfaulted core of Pre- here are refinement of the older mapping in terms of cur- cambrian rocks, and the doming is accentuated by the in- rently used Jurassic units, some observations on the valid- flationary effects of numerous sills and laccoliths, some as ity of the Rico Formation, and clarification of the relative much as several hundred feet thick. The dome and moun- ages of the igneous rocks. tains are bisected by the valley of the Dolores River, which flows southward through the area, producing a relief of more than 3,000 feet and exposing a sedimentary sequence JURASSIC UNITS of some 11,000 feet of rocks from Precambrian quartzite Cross and Spencer divided the thick Jurassic continental to the Cretaceous Mancos Shale (table 1). Nestled in succession between the Dolores Formation and the Da- this valley in the midst of the Rico Mountains, at an ele- kota Sandstone into two formations, the La Plata Forma- vation of 8,800 feet, is the town of Rico, which was estab- tion or La Plata Sandstone, and the McElmo Formation. lished as a silver mining camp in 1879. The principal Colorado Plateau geologists have long since revised this mineral production of the district has been ores of silver, stratigraphy elsewhere in the San Juan basin, and the only zinc, lead, gold, and copper, produced mostly from veins major revision of Cross' mapping during the recent quad- and irregular replacement deposits in limestones of the rangle mapping has been to carry out the same revision of Ouray, Leadville, and Hermosa Formations. Uranium- the La Plata and McElmo in the Rico region. The units vanadium ores have been mined from deposits in the now used in the Rico region, and their correspondence Entrada Sandstone east of Rico, and in recent years sul- to the La Plata and McElmo, are shown below (table 2). furic acid was made at Rico from pyrite, for shipment by The lithologic descriptions and thicknesses are taken truck to nearby uranium mills; but in 1966 the Colorado directly from sections measured by Spencer (Cross and Game, Fish and Parks Department took note of acid being Spencer, 1900, p. 73-77) : the McElmo Formation on the inadvertently dispatched downriver without benefit of con- north side of the West Dolores River, 81/2 miles west- tainers, and the acid plant was shut down. As of 1962 the northwest of Rico, and the La Plata Formation in the total mineral production from the Rico district was nearly cliff exposure at Section Point, 4 miles east-northeast of $45 million. Rico. The Rico region was mapped by Whitman Cross and The Wanakah Formation cannot be effectively separated his associates in 1897-1908, in conjunction with their from the Entrada Sandstone in most of the Rico region study of the Rico mining district and their mapping for west of the Dolores River because of the lack of good the Rico and Engineer Mountain 15' folios (Cross and exposures, and it follows that any attempt to map the Spencer, 1900; Ransome, 1901; Cross and Ransome, 1905; members of the Wanakah is even more futile. However, Cross, 1910). This early work resulted in published geo- east of the Dolores River the exposures are better, and logic maps of the entire region at 1:62,500, and a 1:23,600 mapping is further facilitated by the presence of the Pony geologic map of about 34 square miles centered on Rico. Express Limestone Member. Occurring as it does in the In 1967 E. T. McKnight of the U.S. Geological Survey midst of a rather monotonous succession of continental completed a detailed study of the heart of the district, an elastics, this beautifully distinctive medium-gray thin- area of about 10 square miles. His study included prepara- bedded limestone can be walked out almost continuously tion of a geologic map at 1:9,600 based on both surface from Section Point, where the unit is 2 feet thick and and mine mapping. During 1964-67 I remapped the Rico sandy, to the east edge of the Hermosa Peak quadrangle, and Hermosa Peak 71/2' quadrangles at 1:24,000, making where it is 15 feet thick. The Pony Express pinches out full use of Cross' and McKnight's large-scale mapping. westward along a northerly trending line that before ero- The general stratigraphy and structure of the region sion would have passed a short distance west of Section were well discerned by Cross and Spencer, and it is no Point. The limestone is exposed on the west side of Bar- 1 Publication authorized by the Director, U.S. Geological Survey 84 NEW MEXICO GEOLOGICAL SOCIETY-NINETEENTH FIELD CONFERENCE low Creek about a mile south of the junction with the where in the Rico quadrangle. The unit may be present Dolores River (Bromfield, 1967, p. 13), but is absent in in the Hermosa Peak quadrangle but this part of the sec- the roadcut just north of the junction. Because of the bet- tion is not sufficiently well exposed to permit identification ter exposures east of the Dolores River, and the presence of a possible Junction Creek equivalent. of the Pony Express as a distinct marker bed, the Entrada and Wanakah have been mapped separately in the Her- RICO FORMATION mosa Peak quadrangle but not in the Rico quadrangle ex- Geologists working in the Paleozoic section of south- cept along its north edge, where they are fairly well western Colorado are generally aware of the controversy exposed. regarding the Rico Formation. The problem has been The Salt Wash Sandstone Member as mapped in most discussed by Baars (1962) and will not be reviewed here of the Rico quadrangle includes at its base a medium- except to summarize it for readers not familiar with the grained massive sandstone considered to be an equivalent area. The formation was proposed by Spencer (Cross and of the Junction Creek Sandstone. This unit varies in Spencer, 1900, p. 59-66) as a transitional unit between thickness from 56 feet, north of the West Dolores River the underlying gray, limestone-rich marine beds of the 61/2 miles northwest of Rico, to about 40 feet west of Elliott Mountain, to 70 feet west of Eagle Peak, and probably thickens southward but is not measurable else- Hermosa Formation (Pennsylvanian) and the overlying dome. Six miles to the southeast, where the same strata continental red beds, at that time considered as Dolores come to the surface again in the Hermosa Park area, the (Triassic) but shortly thereafter separated from the Do- Rico has lost its character; good criteria for locating either lores and named the Cutler Formation (Permian).
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