Assessing the presence of pulverization textures along the Buck Ridge fault, NE of Anza, California

Ariella Goldstein

Ariella Goldstein
Advisor: Dr. Gary Girty

Friday May 4th, 2017
CSL 422 – 3:30 to 3:50 pm

Abstract
The Buck Ridge fault, one of three major faults of the San Jacinto fault zone, near Anza, California, transects a variety of rock types including metamorphic rocks of the pre-mid Cretaceous Burnt Valley complex, and alluvial sediments of the Pleistocene Bautista Formation.  The purpose of my research is to understand (1) the characteristic properties of alluvial sandstones of the Bautista Formation outside the Buck Ridge fault zone, (2) how those properties are modified within the fault zone, and (3) the role that the sandstone matrix and porosity may have played in controlling the pulverization process.

Within the study area, the Bautista Formation consists of interstratified beds of poorly to very poorly sorted sandstones and cobble conglomerates derived from debris flows.  Bed thickness is commonly ~0.5- >1 m.  To determine the key properties of sandstones outside the damage zone of the Buck Ridge fault, 9 samples were collected, thin sectioned, and stained to distinguish plagioclase from K-feldspar.  In each thin section, 300 points were counted at a spacing of 1 mm.  In addition, porosity was determined from bulk and grain densities.  Sandstones are feldspathic with an average QFL composition of Q40.3F37.7 L22.0.  Detrital matrix ranges from 18.3% to 26.3%, and porosity varies from 19% to 30%.  In the 9 samples point counted, 6 contain between 38%-48% igneous and 0% metamorphic fragments while 3 contain between 16% and 20% igneous and 5% to 10% metamorphic fragments.  The plagioclase to total feldspar ratio varies from 0.87 to 0.94 in samples containing only igneous fragments and from 0.5 to 0.78 for those containing between 5% and 31% metamorphic fragments.  The average of the proportion of quartz grains containing 0-3 fractures/grain, 4-10 fractures/grain, and >10 fractures/grain is 73.2%, 18.1%, and 8.8% respectively.  Most quartz grains are undulatory to nonundulatory and monocrystalline.  Overall, the above data indicate that the source of the debris flows in the Bautista Formation was primarily granodiorite with a minor but significant contribution from metamorphic rock.

In contrast to the above properties, quartz, plagioclase, and amphibole grains within sandstones within the ~25 m thick zone of damaged conglomeratic sandstone adjacent to the Buck Ridge fault, are characterized by in situ intragranular fragmentation.  Under the microscope, there is little to no evidence of shear, and individual intragranular fragmented grains resemble a jig-saw puzzle consisting of more than 10 fractures.  In the case of quartz, the proportion of grains containing 0-3 fractures/grain, 4-10 fractures/grain, and >10 fractures/grain is 32.9%, 21.5%, and 45.6%, respectively.  Relative to these proportions in undamaged sandstones noted above, there is a dramatic increase in the proportion of quartz grains with >10 fractures.  Hence, the results of my work document the first occurrence of pulverization at the grain scale along the Buck Ridge fault.