Distinguishing base surge deposits and volcaniclastic fluviatile sediments: an ancient example from the Lower Devonian Snowy River Volcanics, southeastern Australia

A 500-m-long road cutting in the Lower Devonian Snowy River Volcanics (SRV), eastern Victoria, Australia, exposes phreatomagmatic units and volcaniclastic sediments. Based on bed geometry, sorting and sedimentary structures, it was possible to distinguish base-surge deposits from ephemeral fluvial deposits in this relatively well-exposed ancient succession. Where the base-surge deposits infill irregular topography, bed sets mantle the pre-existing surface but thicken into topographic lows. In contrast, where the fluvial deposits infill topographic depressions, beds onlap laterally against channel walls. In addition, curviplanar slide surfaces within the base-surge deposits generated by inter-eruptive slumping indicate rapid emplacement as a constructional tuff rampart (? maar). The base-surge deposits are always poorly sorted and commonly contain accretionary lapilli, reflecting their deposition from turbulent, low-particle-concentration, steam-rich pyroclastic currents. In contrast, the fluvial deposits are relatively well-sorted, reflecting hydraulic sorting and winnowing during tractional transport and deposition. There are significant differences in the types of sedimentary structures present. (1) Bedding in the base-surge deposits is entirely tabular, and beds can be traced laterally to the limits of the outcrop. In contrast, the fluvial deposits have abundant internal scour surfaces that result in beds/bedding intervals lensing out laterally over intervals of the order of 5-10 m. (2) Cross-beds with relatively high-angle foresets are restricted to the fluvial deposits. (3) Laterally persistent tabular beds that contain abundant, densely packed accretionary lapilli are restricted to the base-surge deposits. In summary, although base-surge deposits and ephemeral fluvial deposits can appear superficially similar, it is possible to apply facies models carefully to distinguish between them, even in ancient successions.