THE MIDDLE MIOCENE NORTH PATAGONIAN ERUPTIVE PERIOD: SYN- AND INTER-ERUPTIVE VOLCANICLASTIC SEDIMENTATION

LÓPEZ, MANUEL; D'ELIA, LEANDRO; CAROLY NÉMETH; ANDRÉS BILMES; FEO, RODRIGO; JUAN FRANZESE

Congreso; XVIII Reunión Argentina de Sedimentología ? IX Congreso Latinoamericano de Sedimentología; 2023

Large to very large (e.g., VEI6+) explosive eruptions produce high volumes of pyroclasts in extremely short times (e.g., days to months) that may feed exogenous Earth systems mainly by fall-out and pyroclastic density currents (PDC). PDC result from the collapse of discrete volcanic columns or through sustained fountaining eruptions. Discrete column collapse produces short-lived PDC with runouts up to tens of kilometres and relatively low volumes, commonly associated with fall-out deposits. Fountaining eruptions originate long-lived and quasi-stable currents with short to large runouts. In both cases, the episodic nature of explosive eruptions determines a hierarchized time scale with alternation between eruption activity with pyroclastic feed and quiescent intervals along which background sedimentation is recovered. Before attaining stable conditions, sedimentary systems mobilize volcaniclastic products through quasi-instantaneous resedimentation and post-depositional reworking. In this scenario, one of the outstanding questions is: How do large to very large explosive eruptions with different mechanisms of pyroclast delivery disrupt background sedimentary systems and their sediment transportation arteries? From the Cretaceous to the present times, Patagonia represents a natural laboratory to find answers to this enigmatic Earth system problem as a core of the continental evolution of southern South America due to the development of an active magmatic arc (i.e., North Patagonian Batholith) with volcanic delivery towards the Patagonian retro-arc. One of these cases is the Middle Miocene record of the north Patagonian region, preserved along the Pilcaniyeu, Paso Flores and Collón Cura transect (Río Negro and Neuquén, Argentina). From proximal to distal regions, volcaniclastic deposits are widely preserved in a ~150 m thick continental volcaniclastic succession that reaches >1000km3 bulk volume. The most conspicuous deposits are tuffaceous sandstones accommodated in alluvial environments with subtle or lack of paleosol development. Lapilli tuff-dominated PDC deposits and lapilli to tuffaceous fall-out layers appear intercalated. To the top, tuffaceous sandstones and para-conglomerates with increased epiclastic provenance were deposited within fluvial-alluvial to lacustrine environments. Regional distribution and facies analysis allows us to characterize syn-eruptive and inter-eruptive dynamics associated with large to very large explosive eruptions (primarily magmatic) with different eruption dynamics and pyroclast delivery. The extended distribution of tuffaceous sandstones deposited in alluvial environments through depocenters suggests the drowning of a drainage system due to the resedimentation of pyroclasts probably delivered by sudden but sustained eruptions (fountaining?). PDC deposits and fall-out layers evidence contemporaneous primary volcaniclastic products related to discrete and less frequent Plinian-style eruption phases with relatively minor environmental impact. Increased epiclastic provenance noticed the finalization of the syn-eruptive stage and the initiation of a regional inter-eruptive stage characterized by volcaniclastic reworking and final recovery of sedimentary background conditions.