Role of a Rigid Bedrock Substrate on Emplacement of the Blue Diamond Landslide, Basin and Range Province, Eastern Spring Mountains, Southern Nevada

Abstract

Rock avalanches are high velocity mass movements of fragmented rock that experience unusually long runouts. Emplacement of rock avalanches have been studied for over a century, yielding many explanations for their enhanced mobility. Whereas most of these hypotheses address intrinsic mechanisms, few have addressed the influence of runout path substrate material. This study investigates the role of a rigid bedrock substrate in the emplacement of the Blue Diamond landslide deposit. To better understand the role of the runout path material, facies analyses were conducted at exposed remnants of the landslide breccia. Observed sedimentary fabrics and internal morphologies were identified and mapped following standard facies models for largerock avalanche deposits. The presence of basal fabrics and morphologies rules out a frictionless emplacement because they represent prolonged interactions at the rock avalanche-substrate interface. This suggests that the rock avalanche endured significant basal shear resistance during motion; however, conservative H/L estimates indicate a low to moderate basal resistance was experienced. Aided by outcrop mapping, clast count data from 120 stations across the deposit shed light on intrinsic mechanisms that occurred in the slide mass. Correlations between matrix and carbonate count frequencies show that areas with more matrix experience substantial amounts of fragmentation and comminution. It is likely that basal shear stresses were transmitted upward through the rock mass during emplacement which is seen in the vertical transition from matrixiii rich to matrix-poor breccia facies. Observations of proportional breakage and enhanced mobility may best be explained by the process of dynamic fragmentation. Uncertainties surrounding the originally interpreted genesis and distribution of the landslide deposit led to an alternate emplacement hypothesis. I propose that the landslide breccia was derived from slightly north of the previously suggested source area and slid into the Blue Diamond Hill site where it was deposited onto Moenkopi Formation. Afterword and over some time, the underlying Moenkopi Formation experienced gravity gliding as it slid from the Blue Diamond Hill site to its current position in the Blue Diamond area, taking the landslide breccia with it. However, more field observations are needed to validate this secondary emplacement hypothesis. iv