Stratified granular media beneath large slide blocks: Implications for mode of emplacement

We have examined the bottoms of many large slide blocks and found basal layers of well-rounded, fine- to coarse-grained, granular material that typically has the appearance of stratified, stream-deposited sandstone or conglomerate. Stratification is subparallel to the basal surface (detachment) and is typically inversely graded, although normal grading and banding of material are present in some samples. The sandstone-conglomerate-like material of the basal layer can be traced continuously from the detachment surface directly into the upper plate, where it is localized within elastic dikes. Although many of the physical characteristics of these basal layers appear to be those associated with water-laid stream deposits, there is no direct evidence of fluid involvement. We interpret the sandstone-conglomerate-like material in the basal layer to be produced by mechanical sieving in a high-energy fluidized environment under a rapidly moving rock mass during a single catastrophic emplacement event. These sandstone-conglomerate layers found along detachment surfaces may prove to be important in determining whether a detached block is simply a gravity slide or the upper-plate remnant of a low-angle normal fault associated with significant crustal extension.