Flow-direction and palaeoslope indicators in rheomorphic ignimbrites: Structural studies on Pantelleria & Gran Canaria

Pyroclastic density currents (PDCs), alongside other density currents (i.e. turbidity currents, lahars, dust storms or avalanches), are principal mechanisms for lateral sediment transport across varying landscapes. The runout and deposition of density currents is widely influenced by topography, however determination of internal processes and mechanisms is hindered by the currents’ opaque and highly-energetic nature. In addition, the aggraded deposits (e.g. ignimbrites and turbidites) greatly differ in their ability to record the required information such as flow indicators to determine current transport directions. While common sedimentary aggraded deposits (i.e. lowgrade ignimbrites or turbidites) only exhibit few types of flow indicator which are also prone to remobilisation, it is high-grade rheomorphic ignimbrites that exhibit a wide range in flow indicator types that are ‘frozen’ into solid rock, being incapable of remobilisation once solidified. Unique to such rheomorphic ignimbrites is that the pyroclasts constitute a combination of sedimentary and shear-induced processes during and after deposition by the overriding density current and gravity. Therefore, this study presents the most up to date compilation of published as well as new and previously undescribed flow indicators in rheomorphic ignimbrites with individual descriptions, characterisations, interpretations and examples by a consistent nomenclature and standard reference system. Furthermore, a new  classification scheme is proposed to describe the characteristics (‘lithofacies’) of rheomorphic ignimbrites. Although previously recognised that rheomorphic ignimbrites record a multitude of information on progressive syn- and post-depositional deformation, it is here, for the first time, that a basis for the comparison and characterisation of the structural architecture of highgrade ignimbrites is provided, building on and complementing previously published lithofacies and domains. It is subsequently demonstrated that sedimentary and shear-induced flow indicators within basal vitrophyres (i.e. lowermost 10 cm) can precisely record current flow directions, while flow indicators higher in ignimbrite sheets are likely to have undergone post-emplacement gravitational slumping and thus record the varying palaeotopography. The directional difference of flow indicators within these vertical levels in rheomorphic ignimbrites records the PDC flow direction and the palaeotopography, exemplary a directional difference at 90° records an unconstricted deposit at a longitudinal slope to the PDC flow direction. Basal vitrophyres in rheomorphic ignimbrites might therefore offer a robust way of determining prevailing current directions, which is leading towards an enhanced understanding of the internal processes of density currents with topography and enabling the reconstruction of ancient palaeogeography at large-scale caldera collapse events.