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The Purgatory Conglomerate of Rhode Island is part of the Pennsylvanian Narragansett Basin deposited on the composite Avalon terrane of Eastern Massachusetts and Rhode Island. The conglomerate experienced two generations of Alleghanian folding, and subsequent local shearing. Deformation in the clast-supported conglomerate was accommodated by Dissolution-Precipitation Creep (DPC) 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 focused on clast rotation and quartz overgrowth evolution with increasing strain, and the relationship between the two, in the Purgatory Conglomerate. It was the subject of the MS thesis of Eric McPherren, who has been working on the project since May 2008 and completed his thesis and MS degree in September 2010. 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. During the two years that followed, Eric conducted all the lab work, finished field work and wrote his MS thesis.

Eric has gained invaluable experience through field work, lab work and collaborations. Drs. Jiang from the University of Western Ontario and Mosher from the University of Texas in Austin and myself 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). After the first field season and initial structural analysis of the field data it became apparent that the clast rotation patterns were not as complicated as expected. 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 depositional orientation of the cobbles was already fairly close to the orientation of the finite strain ellipse.

It became clear that the most valuable information on the deformation mechanism of the Purgatory Conglomerate was to be gained from detailed investigation of DPC processes that occurred within the conglomerate during the deformation and that was the focus of the remainder of the study. First, Eric performed hot-cathode luminescence on quartz at the University of Toronto in collaboration with Dr. Spooner (the owner of the one and only hot-cathode luminescence setup in North America) in order to investigate which quartz grains in the quartz strain shadows (developed during the deformation) were newly grown during deformation (authigenic) and which were pre-existing (detrital) grains. He then measured quartz c-axis orientations within one strain shadow using a U-stage at Boston College, as a preliminary study to investigate whether the strain shadows had any significant c-axis patterns indicating certain quartz deformation or rotation patterns. His preliminary data showed random c-axis orientations. He then performed extensive EBSD analyses at Bowdoin College, in collaboration with Dr. Beane, on various quartz domains within the strain shadows, where quartz precipitation occurred during deformation, as well as in the cobbles, where quartz dissolution took place. He investigated samples from three sites that display an increase in strain and metamorphic grade from west to east. Quartz grain c-axis orientations of various domains within the cobbles and strain shadows indicate that Crystallographic Preferred Orientation (CPO) patterns are absent in both domains of dissolution and of precipitation irrespective of the degree of strain or metamorphic grade. The existence of discrete mica selvages along the cobble margins suggests that quartz dissolution only occurred along the cobble surface and did not affect the grains, or result in a CPO, within the cobble’s interior. Quartz precipitation within the strain shadows did not result in a CPO, probably because the strain shadows are truly localized regions of low strain with little to no differential stress, allowing quartz grain growth in random orientations. Thus, the quartz precipitation process or rock cementation process is independent of the regional acting stresses, at least within the strain shadow.

In 2009, Eric took the lead in presenting a class project at the Geological Society of America (GSA) 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. In March 2010 he presented his own thesis work at the same meeting and benefited from discussion with some of his collaborators and other scientists. This Fall (2010) he will present his work at the annual GSA meeting in Denver and he is using the Fall semester to write up his work as a publication in the Journal of Structural Geology (to be submitted in December 2010). Overall the project was successful as it lead to publishable results, several conference presentations and the completion of a MS thesis.