Volcaniclastic short-term arrangement in an Argentinian retroarc basin: conservative-uniformitarian vs. difficult-to validate catastrophic processes

Abstract

Explosive volcanic eruptions produce large volumes of volcaniclastic materials that are deposited as Pyroclastic Density Currents (PDC) and fallouts. These primary deposits are commonly removilizated and resedimentated to adjacent basins generating different sedimentary mass-flow deposits. At the geological record, the short-term outstanding of primary or reworked volcaniclastic successions constitutes a real challenge, which depends on our capacity and technical approach to perform time-process schemes. There are two end members of hypothetical assessments for this type of processes, i. e. uniformitarian vs. catastrophic processes. In order to test them, we analyze a Miocene volcaniclastic succession of a North Patagonian retroarc basin: The Collón Cura Basin, whose depositional time-process scheme is controversial and its implications for the basin evolution are obscured. The volcaniclastic succession is characterized by 150 m-thick of tuffs and lapilli tuffs that reach a volume of 1800 km3 and is limited below by a lower discontinuity with on lap relationand above by an upper regional erosive discontinuity. Internally, it was subdivided into three sections. The lower and upper sections are ~50 m-thick each and are composed of massive and moderate-sorted succession of vitric-rich tuff that is roughly stratified in 1 to 2 m thick tabular strata. Strata surfaces are marked by alpha-type carbonate concretions without evidences ofsubaerial exposition. Both sections present 93% of volcaniclastic materials (cuspate well preserved ashes, moderate rounded pumice, crystal fragments (Plg + Qz + Feldk) and fine-ash matrix moderately altered to clays) and 7% of lithoclasts. The middle section is composed by two different levels. The lower level is characterized by a tabular bed of 2 to 5 m thick with a wavy base surface composed of massive, crystal-rich and moderately sorted lapilli-tuff. Above a planar and non-erosive surface, lenticular bodies of 5 to 20 m thick and tens of meters width characterized the upper level. It is composed of reddish, massive, vitric-rich and poorly-sorted lapilli-tuff that are silicified due to vapor-phase alteration. Preliminary geochronological and magnetostratigraphic analyses indicate that the whole volcaniclastic succession was constrained between 15.160 and 12.049 Ma and the depositional processes occurred during a normal polarity paleopole between 15.160 and 15.032 Ma. Although the accumulation of the middle section was clearly interpreted as derived fromshort-live processes related to PDC deposition, the lower and upper sections were historically interpreted as resedimentated volcaniclastic deposits. Our results indicate that the lower and upper sections are mainly related to short term processes, probably associated with PDC or (hot) lahar deposits. This introduce a controversy and challenge to take decisions between conservative uniformitarian and difficult-validation catastrophic processes taking into account their different basin model implications. Even though the volcaniclastic succession is limited by two discontinuities that implied a time spam of 3 Myrs, the re-interpretation here presented indicates that the whole succession was deposited in a maximum lapse of time of 128.000 years. The time-gap may be included in the upper erosive discontinuity, which represents the propagation of the volcanic signal to other basins of the North Patagonian retroarc.