The geology of the East Coachella Tunnel of the Metropolitan Water District of Southern California

Citation

MacLellan, Donald D. (1936) The geology of the East Coachella Tunnel of the Metropolitan Water District of Southern California. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:04132010-082926337

Abstract

The following paper describes the geology of the Last Coachella Tunnel of the Metropolitan Water District of Southern California. This tunnel extends along the southern flank of the Little San Bernardino Mountains, in Riverside county, California. Four rock types were encountered in the excavation of the tunnel: a regionally metamorphosed series of schists and gneisses of sedimentary origin, here called the Berdoo series; granitoid rocks resulting from replacement, called the Thermal Canyon series; a grananitic intrusive called the Fargo granite; and bench gravels. The Berdoo series is divisible on the basis of lithology into three facies: a dark thin-bedded feldspathic and biotitic schist, a light gray feldspathic gneiss, and a medium gray coarse grained schist which is transitional between the first two. The Thermal Canyon series consist of massive granitoid rocks which are of the mineralogical composition and general aspect of quartz-diorites. Between these two rock series occurs a transition zone of from 1000 to 6000 feet in width within which, as examined in the field, the sedimentary textures and structures which characterize the Berdoo series appear to gradually give way to the massive granitoid textures of the Thermal Canyon series. Petrographic studies reveal the existence of a zone of mineralogical gradation between the two series within which the assemblage which constitutes the Berdoo series gradually becomes replaced through recrystallization by that which constitutes the Thermal Canyon series. The minerals of the recrystallized assemblage are not conformable to definite surfaces such as bedding, but instead show the fortuitous orientation of an igneous rock. Also, the plagioclase feldspars of the recrystallized assemblage are slightly more calcic than those of the Berdoo series. Chemical analyses show that the composition of the two rock series is very nearly the same, excepting for a progressive increase in lime, and corresponding decreases in ferric iron and potassa with advancing recrystallization. The results of field, petrographic, and chemical methods of investigation thus indicate that the Thermal Canyon series rocks resulted trough recrystallization of a portion of the Berdoo series rocks. Ascending hot solutions and gases of magmatic origin are postulated as the actuating agencies in the recrystallization. The Fargo granite occurs as an intrusive stock in the east central portion of the area, and consists of a pinkish gray coarsely crystalline rock of about the mineralogical composition of a quartz-monzonite. The chief structural feature of the area is faulting, of which two systems are recognizable, one which trends easterly, and one which trends northerly. Two successive periods of domical uplift in the south-eastern portion of the area are postulated as causes for the faulting. The geological aspects of tunnel excavation, with especial reference to the influence of structure, are discussed briefly.

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