LARGE-ROCK AVALANCHE DEPOSITS, EASTERN BASIN AND RANGE, UTAH - EMPLACEMENT, DIAGENESIS, AND ECONOMIC-POTENTIAL

Large-rock avalanche deposits are a common component of the basin fill within the extensional tectonic terrain of the Basin and Range; these deposits recently have been interpreted to host oil and gas within th e Railroad Valley area of eastern Nevada. Large blocks of brecciated b ed rock are a primary component of these avalanche deposits and are po tentially excellent oil and gas reservoirs. Our work provides further insight into the emplacement and economic potential of these deposits. Exposed large-rock avalanche deposits of the Miocene Oak City Formati on on the western margin of the Canyon Range, Utah, contain coherent b reccia blocks up to 3.5 km long, 1 km wide, and 200 m thick. These dep osits were derived from the near-vertical dipping bed rock of the adja cent Canyon Range and now are exposed as much as 5.5 km from the range front within the Sevier Desert basin. Emplacement was relatively rapi d, as indicated by three well-developed breccia facies within the carb onate breccia blocks. Stratigraphically, from the base the facies incl ude (1) matrix-rich breccia; (2) jigsaw breccia, and (3) crackle brecc ia. The deposits were cut and segmented by a series of syn-depositiona l normal faults that developed during late Miocene and post-Miocene ex tension. Primary porosity was reduced by cement soon after burial. Cat hodoluminescence cement patterns indicate that initially the basinward breccia blocks were more deeply buried relative to the water table th an the breccia blocks proximal to the Canyon Range. After initial ceme ntation, the basinward blocks were uplifted relative to the water tabl e. Secondary porosity approaches 8% in the carbonate blocks and is gre ater than 14% within the jigsaw breccia. The size and porosity of thes e breccia blocks indicate their potential as reservoir targets.