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

Variations in eruptive style and depositional processes associated with explosive, phonolitic composition, caldera-forming eruptions: The 151 ka Sutri eruption, Vico Caldera, central Italy

Vico Volcano in central Italy, experienced a complex eruptive history (419–95 ka). The Vico volcanic edifice was constructed by voluminous (50 km3) leucitite composition effusive lava flows and minor explosive activity over a 50 ka period (305–258 ka). The summit of the edifice was destroyed during the Sutri eruption (151 ka) that resulted in the formation of the 8 km caldera depression. A revised stratigraphy for the eruptive products produced by the Sutri eruption (Sutri Formation) is proposed, based on extensive field observations, detailed stratigraphic logging and petrographic analysis; the Sutri A unit consists of a fallout deposit of limited dispersal; the Sutri B unit consists of a small volume, variably welded pumice-rich ignimbrite; the Sutri C unit is a lithic and spatter clast-rich co-ignimbrite lag-flow breccia; the Sutri D unit is a fines-depleted pumice-rich ignimbrite; Sutri (E1–E3) subunits consist of spatter clast-rich ignimbrite; Sutri E4 (sbx) is a spatter clast-rich co-ignimbrite lag-flow breccia; the Sutri E4 (lsbx) subunit is a lithic, pumice and spatter clast-rich co-ignimbrite lag-flow breccia; the climactic Sutri E5 unit is a variably zeolitised spatter and pumice-rich ignimbrite (with lithic-rich base; Sutri E4 flsbx) to the west). The reconstruction of the Sutri eruption leading up to and including formation of the Vico Caldera based on the new stratigraphy of the Sutri Formation, consists of three distinct phases: Phase 1—Initial Plinian activity produced a fallout deposit (Sutri A) deposited to the south, followed by partial eruption column collapse that generated a pyroclastic flow directed to the south/southeast (Sutri B). Vent widening in response to vent-wall rock instabilities and increased magma discharge rate overloaded the eruption column with dense, lithic debris resulting in southward collapse and deposition of a thick co-ignimbrite lag breccia in the deflation zone (Sutri C). A change in clast supply at vent from lithic clast-dominated to pumice clast-dominated produced a fines-depleted ignimbrite, Sutri D (plbx) prior to cessation of activity in the south. Phase 2—Shortly following the initial Plinian phase or partially overlapping, decompression of the magma chamber in response to partial magma chamber roof block collapse promoted explosive fragmentation of magma at the base of the conduit (as pumice) and led to the opening of a new vent in the north. Sustained mixing of highly vesiculated, grey pumice clasts and dense, poorly vesiculated, black spatter clasts occurred both in the conduit and in the eruption column and subsequent column collapse produced a small volume pyroclastic flow directed to the north, dominated by dense, black spatter clasts (Sutri E1–E3). Phase 3—Sustained eruptive activity continued in the north and increased magma discharge rate likely then resulted in great instability of the magma chamber roof and surrounding wall rocks of the progressively draining magma chamber. The eruption column collapsed radially depositing vast quantities of lithic clasts, spatter clasts and some pumice clasts into the deflation zone as co-ignimbrite lag-flow breccias (Sutri E4 sbx, lsbx), signalling the onset of caldera collapse. Eruption column collapse also produced a radially distributed, pyroclastic flow that deposited a moderate volume ignimbrite (Sutri E5) with a lithic-rich base (Sutri E4 flsbx) at proximal localities in the west. Final caldera collapse occurred shortly following emplacement of Sutri E5 presumably in response to evacuation of the magma chamber and final collapse of the magma chamber roof. We infer from the eruption dynamics that progressive collapse of the magma chamber roof occurred culminating in final climactic and post-eruptive caldera collapse

Volcanological constraints on the post-emplacement zeolitisation of ignimbrites and geoarchaeological implications for Etruscan tomb construction (6th – 3rd century B.C) in the Tufo Rosso a Scorie Nere, Vico Caldera, Central Italy

We examine the role of physical volcanological processes including eruption style (magmatic versus phreatomagmatic) as well as transport and depositional processes (pyroclastic fall versus pyroclastic flow) in promoting the ideal hydrologic conditions necessary for large scale, homogeneous, post-emplacement zeolitisation of ignimbrites, associated with caldera forming eruptions. We consider the Tufo Rosso a Scorie Nere (TRSN) of Vico Caldera (151 ka), in central Italy. The TRSN exhibits pervasive, homogenous alteration of high alkali tephriphonolitic and phonolitic glass to zeolite minerals (chabazite and phillipsite) in all regions of the study area and at all stratigraphic levels with the exception of the basal 1 m. Based on detailed lithological studies, we propose that a large geothermal field around the vent area was destroyed during the closing stages of the Sutri eruption. Subsequent incorporation and entrapment of superheated geothermal fluids into the ensuing pyroclastic flow during eruption column collapse greatly influenced the emplacement temperature and provided the necessary water required for post-emplacement zeolitisation of the TRSN. We suggest that the absence of zeolitisation at the base of the ignimbrite is directly related to transport conditions reflecting cooler regions in the lower portions of the deposit where the flow came into contact with the underlying substrate. We also consider the geoarchaeological implications of enhanced strength and cohesiveness provided by the zeolite rock framework on Etruscan tomb location and burial architecture in the Vico region. The TRSN contains literally hundreds of hypogeum-style Etruscan tombs at a number of sites across the study area. This study focuses on two sites in particular, the Norchia Necropoli and the San Guiliano Necropoli. Considering the enhanced mechanical properties of zeolitised ignimbrites we infer that physically the TRSN would still have been a relatively soft rock, suitable for the carving of tombs. However, we infer the increased strength and cohesiveness provided by the zeolite framework enhanced the conservation potential of these tombs, preserving them for over two thousand years