Lithic-enriched segregation bodies in pyroclastic flow deposits of Laacher See Volcano (East Eifel, Germany)

Small-volume (ca. 0.6 km3) pyroclastic flow deposits at Laacher See contain lithic breccias and two types of ground layers that differ significantly in their structure and composition from the main body of flow units. Lithic breccia bodies, up to 3.5 m thick, containing up to 85 weight% lithic blocks, occur locally at various distances from the vent. The deposition of these breccias was apparently governed by the strong influence of paleomorphology on the dynamics of the pyroclastic flows. The breccias were deposited at three main changes in bottom gradient along the path of the pyroclastic flows. The accumulation of large lithics is explained: (a) by compression of flows on the rising bottom close to the vent; (b) by thinning of flows accelerating over a steep incline; (c) by deceleration of the pre-concentrated lower part of flows in hydraulic jumps; and (d) possibly by a stationary vortex at the inner bend of a valley curvature. Poorly sorted lithic-rich ground layers, laterally highly variable in internal structure and composition, are restricted to marginal regions of the pyroclastic flow deposits within deep and narrow valleys. They are interpreted as having formed due to the extreme roughness of the valley walls, enforcing irregular turbulent flow and intense fluidization of the flow head, in which density-dominated segregation of lithics occurred. Wellsorted lapilli-rich ground layers of constant lateral thickness were probably generated by a more regularly moving, less intensely fluidized head of pyroclastic flows in which size-dominated segregation was effective but density-segregation was minor. A model of the temporal and longitudinal evolution of a flow head is proposed. Close to the vent, the head is exclusively erosive. With increasing distance, erosive power declines and erosion is paralleled by ground layer formation under strong fluidization. Further from the vent, the head ceases to erode while fluidization is still sufficient for ground layer formation. When fluidization declines to a level ineffective for segregation, ground layers terminate while the head advances and only terminates when plug-flow dominates.