Facies architecture of alluvial fan systems: insights from the Early Permian Orobic Basin (North Italy)

The Lower Permian continental succession of central Southern Alps (Italy) comprises a number of volcaniclastic facies, ranging from alluvial fans to playa-lake sediments, deposited in a narrow, fault-controlled basin (Orobic Basin, OB) under hot, semi-arid climate conditions. Such deposits are formally known as Pizzo del Diavolo Formation (PDV). Although most sectors of the OB have been the subject of a number of recent studies, depositional and evolutionary models have been proposed for its central part only. This work presents new data from the marginal areas and proposes a more detailed and integrated view of the articulated depositional setting of this 50 km long graben.In the north-western sector of the OB (Pizzo dei Tre Signori massif), wedge-shaped units of crudely bedded conglomerates (100’s of metres thick) grade downstream (south-eastwards and eastwards) into pebbly sandstones and interfinger with heterolithic deposits and dark mudstones. The eastern sector of the OB (Lago di Barbellino area), on the contrary, is characterised by thick coarsening-upward successions of dark mudstones, heterolithics and well-bedded coarse sandstones interfingering south-eastwards with massive conglomerates and amalgamated pebbly sandstones hosting isolated gravel-filled channels. In both areas, conglomerates are either massive, planar or cross-bedded and may be interpreted as mass-flows, poorly confined flows, or rarer high-energy channel deposits in a high-discharge, likely seasonal alluvial fan system. Amalgamated sandstones and thin-bedded heterolithic facies are characterised by the preservation of climbing, current and wave ripples, water-escape structures, mud-cracks, mud-chip lags, rare raindrop prints, burrows and vertebrate traces. These features suggest rapid vertical aggradation driven by sheet-flood events and the post- flood reworking in a floodplain environment and/or at the margins of shallow ephemeral lakes. Mudstone-dominated successions locally host continental carbonates (stromatolites, oncoids, groundwater calcretes), likely recording the recurring temporary development of relatively long-lasting palustrine conditions.The new data (geological maps, stratigraphic logs, thin sections) have been integrated in the existing OB’s central sector model providing additional information about the distribution, geometry and facies architecture of its fringing conglomerate wedges, as a whole. Compositions of conglomerates and coarse sandstones clearly indicate different source areas for the alluvial fan systems of the two sectors. Conglomerates dominated by volcanic lithics (Val Sanguigno Conglomerates) typically occur at the southern margin, constituting a fairly continuous, mainly alluvial fan belt connected to extensive sandflats. At the northern and north-western margin, conglomerates are generally thicker and richer in quartz and metamorphic lithics (Ponteranica, Monte Aga and Val Vedello Conglomerates). There, compositional differences among individual fan systems may be evident, indicating the presence of heterogeneous sediment source areas with similar, but different, outcrops. In addition, in the northern conglomerate belt, debris flow deposits are common, which may indicate steeper gradients or greater margin instability. Finally, the northern conglomerates interfinger with finer grained floodplain or palustrine deposits not only towards the basin centre but also laterally (in some instances), suggesting they formed a set of locally non-coalescent fans.