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

Abstract

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, curvi‐planar 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.

Primary versus secondary and subaerial versus submarine hydrovolcanic deposits in the subsurface of Jeju Island, Korea

“…The distinction between pyroclastic surge deposits and volcaniclastic fluvial sediments, which can be compared to PH TR and RH AE , respectively, has been attempted by Bull & Cas (2000). These authors proposed a number of criteria that can help distinguish between these deposits on the basis of bed geometry, depositional structures and sorting.…”

“…Resedimented hydrovolcanic deposits with grains that were fully reworked can be distinguished unambiguously from primary hydrovolcanic deposits not only by their distinctive sedimentary structures but also by their textural characteristics (cf. Bull & Cas, 2000). For example, the stratified sandstone of RH AE (Fig.…”

“…, 2001; Bulmer et al. , 2005), but relatively few studies have detailed mafic hydrovolcanic deposits (Bull & Cas, 2000).…”

Syn- and post-eruptive erosion, gully formation, and morphological evolution of a tephra ring in tropical climate erupted in 1913 in West Ambrym, Vanuatu

“…Some of the blocks up to 1.5 m in diameter with impact sags of 20-40 cm are ballistic bombs. These 0.5-1.5 m-thick accidental-lithic-rich beds are overlain by a metre-thick succession of cross-bedded to dune bedded ash and lapilli beds, interpreted to be high-to-low particle concentration base-surge deposited units (after Waters and Fisher, 1971;Chough and Sohn, 1990;Bull and Cas, 2000) (Fig. 5D).…”

Sedimentation history of the Palaeoproterozoic Dhanjori Formation, Singhbhum, eastern India

“…Fisher and Schmincke, 1984, p. 107-109; and Wright, 1987;Bull and Cas, 2000). The terrestrial depositional setting of this facies is constrained by the bounding sandstone-siltstone and volcaniclastic lithofacies of fluvial origin ( Fig.…”