Pumice clasts in cross stratified basalt-dominated sandstones and conglomerates. Characteristics and depositional significance: Huarenchenque Fm (Neuquén, Argentina)

Abstract

The Huarenchenque Fm is a Pleistocene clastic sedimentary unit that crops out in the SW of the Neuquén Basin, Argentina. This unit is made up of cross stratified basalt-dominated sandstones and conglomerates with pumice clasts in a fluvial context. The roundness of these pumice clasts, the numerous fragments of shards and their association with polymodal basalt-rich clasts in cross-bedded conglomerates strongly suggest that these clasts were transported together during hydraulic flow. Pumice clasts are usually buoyant in cold water whenever their density is lower than 1 g/cm 3 . However, since their density exceeds that of water during sediment transport, they form part of the sedimentary record. A flume was employed to yield fresh insight into the behavior of these clasts mixed with sands during transport and accumulation. The hydraulic equivalence of both materials was calculated by evaluating the density of the pumice clasts and that of the quartz sands. The pumice clasts sank as a result of the interaction of pumice-rich pyroclastic flows with a moving mass of shallow water. However, they retained their buoyancy after recovery from the sedimentary record. The sinking process is therefore rapid and reversible.