Incorporation and redistribution of locally derived lithic fragments within a pyroclastic flow

The lower Miocene Peach Springs Tuff exposed in the Newberry Mountains, California, was deposited within a paleovalley trending S65°W. Exposures within the paleovalley contain lithic breccia intercalated with ash-rich ignimbrite. The clast assemblage of the lithic breccias matches the rock types of the paleovalley walls, and therefore the clasts were not derived from a distant eruptive vent. Flow direction, breccia bed thickness, grain-size data, and sedimentary textures indicate that the lithic breccias were deposited from density currents within the pyroclastic flow that moved down tributaries and into the main paleovalley to be intermingled with the ash-rich pyroclastic flow. A model is proposed whereby a turbulent boundary layer at the base of the pyroclastic flow is induced by surface roughness of the substrate and incorporates loose material from the substrate to produce a high-density ground layer that decouples from the lower-density, ash-rich pyroclastic flow. After decoupling has occurred, the high-density, lithic-rich ground layer moves independently from the ash-rich pyroclastic flow. The lithic breccia horizons have many characteristics of proximal (lag) breccias, and caution must be used when inferring distance from vent in ancient ignimbrites based on the occurrence of coarse breccias.