The volcanic geology of the Rancho Peñas Azules area, Chihuahua, Mexico / by Richard K. Spruill.
| Author/creator | Spruill, Richard K. author. |
| Other author | Mauger, Richard L. (Richard Leroy), 1936- degree supervisor. |
| Other author | East Carolina University. Department of Geology. |
| Format | Theses and dissertations |
| Production | 1976. |
| Description | 99 leaves : illustrations ; 28 cm |
| Supplemental Content | Access via ScholarShip |
| Subjects |
| Subject | A section of volcanic and volcaniclastlc rocks in the Rancho Pęas Azules area is approximately 4650 meters thick and is divisible into eight Informally named formations. From oldest to youngest, the formations are the Tres Cuchillas, Lavas Azules, Cumbres, Quintas Tuff, Capilla Amarilla Tuff, Picos Gemelos Andesite, Rancher̕a Roja Tuff, and Rancher̕a Tuff. The Tres Cuchillas and Lavas Azules Formations comprise the Pęas Azules Volcanic Group (4000+ meters), and the upper six formations constitute the Upper Volcanic Group (700+ meters). The stratigraphic section is known to be early Tertiary; however, the exact ages of the units are not known at the present time. The Pęas Azules Volcanic Group is composed of steeply dipping interbedded basaltic andesites, welded tuffs, and laharic deposits and is overlain, in angular unconformity, by a nearly flat lying ignimbrite sequence. This older sequence is well exposed in the relatively flat area surrounding Rancho Pęas Azules. In thin section the basaltic andesites contain plagioclase, hornblende, and/or pyroxene phenocrysts in a pilotaxitlc groundmass. The laharic breccias and conglomerates are composed of boulder-size fragments of andesite and devitrified tuffs, and formed by rapid erosion of topographically high lava flows and ignimbrites. The welded tuff units of the Tres Cuchillas Formation are exposed in a series of conspicuous hogbacks in the vicinity of Rancho Pęas Azules. The laharic and pyroclastic rocks are capped by 3000+ meters of basaltic andesite, the Lavas Azules Formation, which may have accumulated in a sinking graben. An unusual tuff lava is present near the central part of the flow. The rock is believed to have formed by eruption and flow of a vertically differentiated magma body. Clots of a silica-enriched upper part of the magma body now occur as flattened and elongated lenses in a flow-banded matrix of basaltic andesite. Extensive block faulting and erosion of the older sequence took place before extrusion of the ignimbrites and andesite flows of the younger sequence. The nearly flat lying units of the Upper Volcanic Group form resistant ridges which almost completely surround Rancho Pęas Azules. The Cumbres Formation is composed of interfingering basaltic andesite, flows and red-bed deposits of varying coarseness. The red beds are the result of talus-type accumulation of breccia and fluvial deposits from rapid erosion of fault scarps. A thick, short, and spherulitic dacite flow 1 kilometer northeast of Rancho Pęas Azules occupies approximately the same stratigraphic position as the red beds of the Cumbres Formation. The Quintas Tuff, Capilla Amarilla Tuff, Rancher̕a Roja Tuff, and Rancher̕a Tuff are rhyolitic pyroclastic deposits with varying degrees of welding. The tuffs are generally rich in sanidine and contain small amounts of quartz and plagioclase. The Quintas Tuff has a basal vitrophyre zone which is overlain by a zone with well developed spherulites. Tlie eruption of pyroclastic materials was locally interrupted by outpourings of the Picos Gemelos Andesite, a topographically prominent, porphyritic basaltic andesite. The Quintas Tuff, Picos Gemelos Andesite, and Rancher̕a Tuff display conspicuous, widely spaced (2-5 meters) columnar joints, faulting followed cessation of volcanic activity. |
| General note | "Presented to the faculty of the Department of Geology ... in partial fulfillment of the requirements for the degree Master of Science in Geology." |
| General note | Advisor: Richard L. Mauger |
| Dissertation note | M.S. East Carolina University 1976 |
| Bibliography note | Includes bibliographical references (leaves 97-99). |
| Genre/form | dissertations. |
| Genre/form | Academic theses. |
| Genre/form | Academic theses. |
| Genre/form | Thèses et écrits académiques. |
Availability
| Library | Location | Call Number | Status | Item Actions |
|---|---|---|---|---|
| Joyner | University Archives | ASK AT SPECIAL COLLECTIONS DESK | ✔ Available | Request Material |
| Electronic Resources | Access Content Online | ✔ Available |