SEDIMENTARY FACIES AND TRACE FOSSILS FROM THE LOWER DEVONIAN SHALLOW MARINE TALACASTO FORMATION OF PRECORDILLERA

WENGER, FEDERICO D.; BUATOIS, LUIS A.; MÁNGANO, M. GABRIELA; MUÑOZ, DIEGO F.; RUSTÁN, JUAN JOSÉ

La Plata

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

Asociación Argentina de Sedimentología

The Precordillera is a peripheral foreland basin included within the Cuyania composite terrane of western Argentina. The Lower Devonian (Lochkovian – Emsian) Talacasto Formation is present in the Central Precordillera and is dominated by a succession of dark mudstone in its lower interval, passing upwards into greenish gray highly bioturbated mudstone with very thin intercalations of very fine- to fine-grained sandstone. We integrated sedimentologic and ichnologic datasets with the aim of providing a refined depositional model for this unit. Seven sedimentary facies were recognized in the Loma de Los Piojos (LLP), Rio Las Casitas (RLC), and Quebrada de Talacasto (QT) sections from San Juan Province. Facies A (Parallel-laminated mudstone; shelf) only shows undifferentiated burrow mottling. Facies B (Siltstone and very fine-grained sandstone; lower offshore) is the most abundant facies in all sections, covering 80-90% of the unit. It contains Chondrites isp., Zoophycos isp., Phycosiphon incertum, and Nereites missouriensis. The intense bioturbation (Bioturbation Index or BI 5-6) obliterated the primary bedding. Facies C (Siltstone and very fine- and fine-grained sandstone; upper offshore) is similar to Facies B in being intensely bioturbated (BI 5-6), but the fine-grained sandstone is unbioturbated and preserves primary sedimentary structures, such as wave ripple cross lamination and parallel stratification. Facies D (Siltstone and very fine-grained sandstone interbedded with hummocky cross stratified very fine-grained sandstone; offshore transition) is characterized by unbioturbated, discrete hummocky cross-stratified sandstone separated by mudstone intervals. Facies E (Amalgamated hummocky cross stratified very fine- to fine-grained sandstone; lower/middle shoreface) is sparsely bioturbated (BI 2-3) and contains vertical burrows, namely Skolithos isp. and Arenicolites isp. Facies F (Trough cross bedded medium-grained sandstone; upper shoreface) has very low degree of bioturbation (BI 1) with Teichichnus isp. and Palaeophycus heberti?. Facies G (Conglomerate; transgressive lag) is present in the lowermost intervals of the LLP and RLC sections. It consists of unbioturbated, matrix-supported, pebble conglomerate having well-rounded quartz and sedimentary lithic fragments. It is present as tabular beds or as isolated patches. Two ichnofacies have been identified, distal Cruziana Ichnofacies (facies B, C, and D), and Skolithos Ichnofacies (facies E, F, and G). The former records persistent fair-weather, low-energy conditions allowing for long colonization windows punctuated by rare, distal storm events, whereas the latter suggests high energy due to a strongly storm-dominated regime in the lower/middle shoreface and dune migration in the upper shoreface. The absence of the archetypal Cruziana Ichnofacies represents an anomaly and may be explained in terms of an interplay of taphonomic and ecologic conditions. In LLP, high-frequency sequences comprise deposits from lower offshore to offshore transition, in RLC from lower offshore to lower/middle shoreface, and in QT from shelf to upper shoreface. this stratal pattern confirms the northward deepening of the Devonian basin.