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Date Permissions Signed


Date of Award

Summer 1975

Document Type

Masters Thesis

Degree Name

Master of Science (MS)



First Advisor

Beck, Myrl E.

Second Advisor

Lipman, Peter W.

Third Advisor

Brown, Edwin H.

Fourth Advisor

Hailwood, E. A.


Geologic Setting

The San Juan volcanic field of southwestern Colorado consists of 2.5 X 104 km2 of volcanic rocks ranging in age from early Oligocene to Pliocene. Major volcanic activity in the San Juan field began about 35 million years ago with widespread eruption of rhyodacitic to andesitic lavas from scattered volcanic centers (Lipman and others, 1970). This type of volcanism peaked about 33 million years ago and then began to decline. About 30 million years ago, volcanic activity once again increased in the area. This time the eruptions were ash flows of intermediate to felsic composition. The ash-flow eruptions were accompanied by caldera subsidence of the source areas. At least 14 caldera structures (figure one) have been recognized in the Sam Juan volcanic field (Steven and others, 1974). In most calderas collapse was apparently followed soon by resurgent doming, elevating thick intracaldera accumulations of welded tuffs. Such intracaldera tuffs are up to several hundred meters thick where flow was blocked by caldera walls. The related outflow sheets are usually less than 30 meters thick, but they cover as much as several thousand square kilometers. At the close of the Oligocene the composition of the volcanics changed again, from the intermediate-through-silicic suite of pyroclastlcs and lavas to a bimodal assemblage of basalt and rhyolite flows. Bimodal volcanism continued intermittently until the late Pliocene.

Because of the extensive mineralization associated with the volcanic field, the area long has been subject of geologic Interest (Cross and others, 1905; Burbank, 1932; Larsen and Gross, 1956). Much of the area has recently been remapped in detail by personnel of the United States Geologic Survey (Lipman, 1975; Lipman, 1974; Lipman and others, 1973, Mehnert and others, 1973; Steven and others, 1974). Other work in the area is currently in press or in progress.

Although the stratigraphy and structure of the San Juan volcanic field is now well known, only reconnaisance paleomagnetic studies have been made. Previous studies include field polarity determinations (Lipman and Steven, 1970), a paleomagnetlc reconnaisance of the Platoro caldera (Diehl, 1972; Diehl and others, 1974) and a paleomagnetlc study of some of the formations In the area by Tanaka and Kono (1973, 1974).


Tanaka and Kono (1973, 1974) and Diehl and others (1974) have shown that many San Juan rocks have high magnetic stability. For this reason, and because the stratigraphy is well known and many absolute age determinations have been made, my research was undertaken to provide precise paleomagnetlc directions for the mid-Tertiary.

The data derived during this investigation provide a very precise paleomagnetlc pole for the Ollgocene-Miocene boundary of cratonic North America and also describe the secular variation of the geomagnetic field during San Juan volcanism. The magnetic stratigraphy developed herein, combined with that of Diehl and others (1974) and Tanaka and Kono (1973), provides a detailed reversal sequence which may facilitate future geologic mapping and correlation of mineralization with Igneous activity in the San Juan volcanic field.




Southwestern Colorado


Western Washington University

OCLC Number


Subject – LCSH

Paleomagnetism--Colorado--San Juan Region.

Geographic Coverage

San Juan Region (Colo.); San Juan Volcanic Fields (Colo.)




masters theses




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