A Portion of the San Andreas Rift in Southern California. A Miocene Mammalian Fauna from Beatty Buttes, Oregon

Citation

Wallace, Robert Earl (1946) A Portion of the San Andreas Rift in Southern California. A Miocene Mammalian Fauna from Beatty Buttes, Oregon. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/3C57-B890. https://resolver.caltech.edu/CaltechTHESIS:08302018-143742731

Abstract

[Major thesis]: A Portion of the San Andreas Rift in Southern California:

A portion of the San Andreas rift along the south side of the Mohave Desert between Palmdale and Elizabeth Lake has been mapped and described in this paper. The details of geology and geomorphology have been analyzed and their significance in regard to the interpretation of the genesis of the rift has been discussed.

The area includes four principal geomorphic and geologic divisions; a portion of the Mohave Desert, two parallel ridge zones and an intervening trough which marks the rift. The two ridge zones are principally old crystalline rooks whereas the trough zone is underlain principally by a long narrow strip of Tertiary sediments. These sediments outcrop in a low "center-trough" ridge and most are part of the Anaverde formation which, on the basis of paleobotanical evidence, is believed to be of Pliocene age. Other Tertiary formations are the Vasquez volcanics, presumably Oligocene, the Pleistocene Harold beds, and an arkose of undetermined age possibly older than the named Tertiary formations of the area. The presence of this long, narrow strip of sediments between two crystalline masses was a problem for which no conclusive evidence was found.

Two structural trends are prominent in the area. The older rooks of the ridge zones are distributed in more or less bands striking east or slightly north of east. This trend is cut by the rift which in this area has a strike of approximately N. 65° W. There is a suggestion that the east-trending structures are older than the rift and related to a different period of diastrophism.

Terrace deposits and offset streams suggest a horizontal displacement along the rift of the order of 5 - 6 miles since Pleistocene time. To north side of the rift moved southeast with respect to the south side. A block of Pelona schist north of the rift has an apparent horizontal displacement of 9 miles in the opposite direction to that indicated by recent faulting and stream offsets. This relationship is believed to be only apparent and possibly the result of vertical displacements and stripping.

Terraces on the ridge blocks and in the rift zone indicate a great amount of "juggling" of the blocks in and adjacent to the rift. Considerable vertical displacement along the rift is also indicated. The drainage along the rift and in the surrounding areas has frequently been interrupted, reversed, and otherwise changed by the repeated tectonic activity along the rift. Wind gaps, stream captures and unadjusted streams are features which have resulted.

[Minor thesis]: A Miocene Mammalian Fauna from Beatty Buttes, Oregon:

Fossil mammalian remains were first discovered in tuff beds of the Beatty Buttes area in 1938 by Dr. Warren D. Smith and Lloyd Ruff, of the University of Oregon. Shortly thereafter, the occurrence was brought to the attention of Dr. Chester Stock by Dr. Smith. During the summer field seasons of 1940 and 1941 collecting parties from the California Institute of Technology examined the region and obtained much additional material.

The original locality at Beatty Buttes lies approximately 22 miles southwest of Blitzen, Oregon, and nearly 100 miles south of Burns, Oregon (see map, fig. I). Here the tuff beds in which the fossil mammals are found border the north side of the volcanic cone of Beatty Buttes. Most of the material was collected in an area lying approximately 5 miles from the highest point of the cone and nearly due north of it. At a second locality, here designated Corral Buttes, occur fossiliferous deposits that are unquestionably like those exposed at Beatty Buttes. Fossils were discovered in these beds by K. A. Richey and J. C. Stock in 1941. Corral Buttes lies approximately 20 miles northwest of Beatty Buttes.

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