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The Purgatory Conglomerate of Rhode Island experienced two generations of Alleghanian folding, and subsequent local shearing. Deformation in the clast-supported conglomerate was accommodated by pressure solution creep and clast rotation, two processes that influence porosity and permeability. Porosity and permeability are processes that are pertinent to the petroleum industry, because they control the distribution and flow of oil and gas. This study focuses on clast rotation and quartz overgrowth evolution with increasing strain, and the relationship between the two, in the Purgatory Conglomerate. It is the subject of the MS thesis of Eric McPherren, who has been working on the project since May 2008. In the summer of 2008, Eric spent most of his time in the field, in order to carry out (1) geological mapping to investigate the regional distribution of strain in the conglomerate, (2) an analysis of cobble orientations, and (3) sampling for optical microscopy, cathodoluminescence (CL), C-axes measurements on the universal stage (U-stage) and Electron Backscattered Diffraction (EBSD) analysis. Eric has gained invaluable experience thus far, through field work, lab work and collaborations. Drs. Jiang from the University of Western Ontario and Mosher from the University of Texas in Austin have visited in the field. Dr. Mosher conducted her PhD research on the Purgatory conglomerate. Dr. Jiang is an expert on numerical modeling of deformation (e.g. rotation patterns of planar and linear fabrics during progressive deformation). Furthermore, we planned on using a newly installed Scanning Electron Microscope (SEM) in his department for CL and EBSD analyses. However, we learned that quartz CL is much better visible through hot-cathode luminescence using an optical microscope, than on and SEM. Furthermore, due to technical complications, the SEM is not yet capable of EBSD analyses. For those reasons, CL was performed at the University of Toronto in collaboration with Dr. Spooner (the owner of the one and only hot-cathode luminescence setup in North America) and EBSD analyses are being carried out at Bowdoin College, in collaboration with Dr. Beane. In 2009, Eric took the lead in presenting a class project at the Geological Society of America northeastern Section meeting, together with three classmates. The results from his thesis project were too preliminary at the time of abstract submission and this way he still gained experience in presenting his work at a professional meeting and in taking the lead of a group project, so that he will be better prepared to present his work at the same meeting in 2010.

Based on field results, the study area is divided in three structural domains, where strain increases, and the long axes of the cobbles are progressively better aligned with the regional 010°-trending sub-horizontal fold hinge lines, from the SW to the NE. Whereas the cobbles do rotate with increasing strain, this rotation is not as great as expected, because the initial orientation of the long axes of the cobbles was already fairly close to an 010°-trending subhorizontal orientation. The intermediate and short axes of the cobbles are also better aligned with increasing strain. Three main questions are being investigated using optical microscopy, CL, U-stage and EBSD methods: (1) Which quartz grains in the strain shadows of the cobble are detrital and which are newly grown? (2) What is the relationship between cobble rotation and quartz overgrowth formation? Do the strain shadows have asymmetric shapes as a result of cobble (and early quartz overgrowth) rotation? (3) Do quartz grains within certain domains in the overgrowth display crystallographic preferred orientations (CPO) so that older grains have a different CPO than younger grains in the overgrowth? At first sight, through optical microscopy, it was difficult to distinguish between detrital quartz and newly grown quartz. Through CL methods, the two types can be distinguished to some extent, as detrital quartz shows in blue, with bright red metamorphic overgrowths, and newly grown quartz as a result of pressure solution turns dark red. A CPO was not immediately apparent in optical microscopy. Eric then tried to analyse quartz C-axis fabrics in one domain in one strain shadow using the U stage. Surprisingly, the pattern was almost perfectly random. Recently (i.e. last week) we visited Bowdoin College to study EBSD patterns in several domains of the strain shadow of the sample. A very weak CPO was detected in one of the domains. In the next few months, Eric will perform EBSD analyses on domains of a few more samples to determine whether the quartz c-axes patterns in the strain shadows is mostly random, or whether some samples do show CPO patterns. If so, he will investigate the variation of these patterns with location in the strain shadow and with cobble orientation.

In the upcoming year, Eric will finalize laboratory analysis, additional sampling (if necessary) and he will write his thesis. If results allow, he will also publish his results in a peer-reviewed journal such as the Journal of Structural Geology.