The eruption, pyroclastic flow behaviour, and caldera in-filling processes of the extremely large volume (> 1290 km3), intra- to extra-caldera, Permian Ora (Ignimbrite) Formation, Southern Alps, Italy

Abstract

The Permian Ora Formation (277–274Ma) preserves the products of the Ora caldera ‘super-eruption’, Northern Italy. The stratigraphic architecture of the exceptionally well preserved intra-caldera succession provides evidence for caldera collapse at the onset of the eruption, a multiple discharge point, fissure eruption style, and progressive, incremental caldera in-filling by numerous pyroclastic flow pulses within the caldera. The ignimbrites of the Ora Formation are voluminous (>1290km3), crystal-rich (~25 to 55%), and ubiquitously welded. The Ora Formation has been divided into four members (a–d), which also define the principal eruption phases. The eruption proceeded in four main stages: (1) early caldera collapse and vent opening, producing locally distributed, basal co-ignimbrite lithic breccia (member a); (2) vent clearing, which produced the eutaxitic, lithic-rich ignimbrite and minor thin ground and ash-cloud surge deposits (member b); (3) waxing and steady eruption, which produced the dominant eutaxitic, coarse-crystal-rich ignimbrite, with local lithic-rich and fine-crystal-rich ignimbrite and minor surge deposits (member c); and (4) waning eruption, recorded by the eutaxitic, fine-crystal-rich ignimbrite, with local lithic-rich ignimbrite deposits (member d).The incremental filling and late-stage outpouring of pyroclastic material from the caldera is recorded by vertical and lateral lithofacies deposit variation and some correlation between stratigraphic sections. These findings reveal a structure to the outwardly monotonous, >1300m thick, intra-caldera fill and thinner (<230m) outflow successions. These data together with the gradational contacts between the main ignimbrite lithofacies, support the hypothesis that pyroclastic material was erupted from multiple source regions in various parts of the caldera, during quasi-steady, low eruption column collapse and pyroclastic flow forming events. Field study revealed the absence of a Plinian fallout deposit, suggesting a lack of a high, buoyant, Plinian precursor eruption phase. This caldera was initiated immediately by a low collapsing column phase, producing the main, thick ignimbrite succession. Simultaneously, catastrophic volcano-tectonic caldera collapse and decompression of the magma chamber occurred, facilitated by the regional extensional environment in the Permian and pre-existing crustal weaknesses. The Ora pyroclastic flow system is suggested as having been a hot and poorly expanded, high particle concentration, granular density current. The confined nature of the majority of the erupted products to the intra-caldera setting, reduced the formation of the full array of facies commonly expected in ignimbrites in extra-caldera settings.