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 lay ers of well-rounded, fine- to coarse-grained, granular material that typica lly has the appearance of stratified, stream-deposited sandstone or conglom erate. Stratification is subparallel to the basal surface (detachment) and is typically inversely graded, although normal grading and banding of mater ial are present in some samples. The sandstone-conglomerate-like material o f the basal layer can be traced continuously from the detachment surface di rectly into the upper plate, where it is localized within elastic dikes. Al though many of the physical characteristics of these basal layers appear to be those associated with water-laid stream deposits, there is no direct ev idence of fluid involvement. We interpret the sandstone-conglomerate-like m aterial in the basal layer to be produced by mechanical sieving in a high-e nergy fluidized environment under a rapidly moving rock mass during a singl e 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.