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Masters Thesis

Deposition and diagenesis of terrigenous rocks of the middle member of the Miocene Barstow Formation, Mud Hills, California

This study documented the composition and diagenesis of sandstone in a well dated interval of the Barstow Formation in order to provide information on changes in provenance related to middle Miocene tectonic events in the central Mojave desert of California and to provide insight into the nature of diagenetic fluids and controls on fluid flow paths. The middle member of the Barstow Formation was chosen for the study because it is bounded by two dated tuffs units, the Rak Tuff, 16.3� 0.3 Ma (MacFadden et al., 1990) and the Skyline Tuff, 14.128�0.013 Ma (Rasbury et al., 1999). A 425-mthick section of this rock unit was measured on the south limb of the Barstow syncline in Owl Canyon. Composition of the sandstone units was determined by x-ray diffraction and point-counting of thin sections. The results of this study indicate that the mostly likely source of non-volcanic quartz, feldspar, and plutonic rock fragments in the middle member is Mesozoic granodiorite that was exposed in uplifted areas of the lower plate of the Waterman Hills detachment fault south of the Barstow Basin. Volcanic rock fragments were derived from lower Miocene volcanic rocks that covered parts of these highlands. Volcanic quartz and feldspar were derived from contemporaneous ashfalls in the drainage basin. Rock fragments with strong foliation, interpreted to have formed from shearing associated with extension along the detachment fault, occur in abundance only in the top 200 m of the measured section. This portion of the section is bounded by two dated horizons, a 14.76 Ma tufa and the 14.1 Ma Skyline Tuff (Rasbury et al., 1999) and thus provides an age when unroofing of the detachment zone reached sheared rocks. The most striking feature of diagenesis in the epiclastic rocks of the middle member is the dissimilarity in diagenesis between strata throughout the measured section, even between strata that are stratigraphically separated by only a few meters. These differences indicate that pervasive movement of an individual diagenetic fluid did not occur throughout sandstone units in the middle member. The mudstone that separates the epiclastic rocks probably isolated the porous conduits during early and late diagenesis. It is also possible that post-depositional deformation disrupted the lateral continuity of fluid flow and focused fluid into different strata on different sides of faults.

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