Pyroclastic supply, channelization and stasis in late Cretaceous loessic plains from central Patagonia

BELLOSI, EDUARDO SERGIO; RODRÍGUEZ PETZ, AMPARO; KRAUSE, MARCELO JAVIER

La Plata

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

Asociación Argentina de Sedimentología

Continental volcaniclastic sedimentation has been mainly studied in proximal and medial fluvial settings surrounding volcanoes, called ring plains or aprons. Resulting successions include diverse type of deposits (e.g. airborne tephra, pyroclastic density currents, debris avalanches, channel or unconfined stream remobilization, floodplain deposits, etc.), which were alternately related to stages of increased eruptivity or to "normal" fluvial conditions when volcanism wanes. In comparison, distal, terrestrial volcaniclastic environments and sequences, showing higher proportion of ash-fallout deposits and paleosols, are less known. The Late Cretaceous Laguna Palacios Formation from San Jorge basin (Argentina) is an interesting study-case to assess properties and controls of this sedimentary system. This unit was previously considered a tephric loess-paleosol succession, with subordinated fluvial deposits formed in a slightly variable, temperate and seasonal-subhumid climate. In the present work, three types of features were identified in order to recognized allocyclic and autocyclic controls: 1) intervals of thick and continuous fine-grained, pyroclastic eolian deposits generally interposed with weakly-developed paleosols and minor fluvial facies, 2) erosive surfaces produced by channel deepening, and 3) intervals of stacked, moderately- to strongly-developed paleosols. In the most complete section (Cañadón Puerta del Diablo, 345 m) we identified nine intervals dominated by tephric loessites; intercalated with five episodes of fluvial incision and landscape degradation, and five intervals with andic, non-calcic Alfisols. At the remaining localities (Lag. Palacios 140 m, Ea. Helvecia 55 m) similar facies and erosive surfaces were identified. Intervals of thick subaerial deposits record periods of high aggradation, due to the sudden, direct, voluminous and intermittent input of pyroclasts resulting from eruptions and atmospheric dynamics. Abundance of loose material reduced surface infiltration, surpassed geomorphic thresholds of aggradation, promoted runoff and discharge flashiness, and destroyed vegetation. Vertical organization of the three described features suggests that high aggradation was indistinctly followed by channelization or by landscape stability and pedogenesis. Changes in subsidence-driven accommodation and in volcaniclastic input determined the nature of the next conditions. Fluvial incision occurred when accommodation could not compensate the large sediment supply which reshaped equilibrium profile of streams. On the other hand, extensive pedogenesis took place when sediment supply was null or reduced, and accommodation slowed down. This work shows that in distal reaches of continental volcaniclastic systems the proportion of siliciclastic fluvial deposits ("normal facies") can be scarce even when eruptivity decrease. Preservation of the prevailing subaerial fine-grained sediments and paleosols, along with river carving, respond to the balance between pyroclastic supply and accommodation.