ECOLOGICAL INSIGHTS FROM TRACE FOSSILS OF THE LOWER DEVONIAN TALACASTO FORMATION, ARGENTINEAN PRECORDILLERA

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

Florianópolis

Congreso; ICHNIA 2024 - The 5th International Congress on Ichnology; 2024

The Precordillera is a peripheral foreland basin included within the Cuyania composite terrane of western Argentina. The Lower Devonian (Lochkovian–Emsian) Talacasto Formation in the Central Precordillera consists primarily of siltstone and very fine-grained sandstone formed in a wave-dominated shallow-marine setting. This unit provides paleoecologic insights at a period characterized by repeated biotic crises. This formation shows a thickening and deepening trend from approximately 100 m (shelf to upper shoreface facies) in the south to over 1000 m thick (shelf to lower shoreface facies) in the northern area. This contribution aims to decipher the benthic ecology of shallow-marine settings in Southwest Gondwana through the analysis of trace-fossil assemblages, integrating sedimentologic and paleontologic datasets. Four different sections, covering 78 km along dip (S-N), were logged, namely Quebrada Poblete Sur (QPS), Quebrada de Talacasto (QT), Río Las Casitas (RLC), and Loma de Los Piojos (LLP). Three trace-fossil assemblages are identified in the Talacasto Formation. In ascending stratigraphic order, these are: (1) Palaeophycus assemblage (PA), including Palaeophycus tubularis, Palaeophycus heberti, and Helminthopsis isp., (2) Phycosiphon-Zoophycos assemblage (PZA), comprising Phycosiphon incertum, Zoophycos isp., Nereites missouriensis, Chondrites isp., Palaeophycus striatus, Palaeophycus tubularis, and Palaeophycus heberti?, and (3) Rosselia assemblage (RA), containing Rosselia socialis, Skolithos isp, Arenicolites isp., and Palaeophycus isp. The first assemblage (PA), recognized in the lowermost interval of the QPS section, is preserved in sparsely bioturbated (BI=2), black, parallel-laminated mudstone representing a shelf subenvironment. The tracemakers are most likely worm-like organisms, probably polychaetes. This assemblage reflects overall low hydrodynamic energy and a relatively shallow redox discontinuity surface. The PZA is typically found in thoroughly bioturbated (BI=5-6) mixed siltstone and very fine-grained sandstone (lower offshore) and wave-ripple cross-laminated very fine-grained sandstone (upper offshore) that characterize the northern sections (RLC and LLP). However, it is also present in shallower-water deposits, represented by very fine- to fine-grained sandstone (lower/middle shoreface) in the southern sections (QT and QPS). Chondrites and Zoophycos are typical of dysoxic/anoxic sediment, but the presence of Phycosiphon incertum and Nereites missouriensis requires oxygenated interstitial waters. Although this trace-fossil assemblage is present in the same sedimentary facies, not all ichnotaxa are found in the same beds. Chondrites isp. typically forms monospecific suites and more rarely is accompanied by Zoophycos isp. Phycosiphon incertum may be present either forming monospecific suites or commonly accompanied by Zoophycos isp. and Nereites missouriensis. This association may be linked to a shallow redox discontinuity surface, as well. The diverse record of brachiopods, trilobites, and crinoids indicates well-oxygenated bottom waters. The extremely high degree of bioturbation (BI=5-6) in the finer-grained deposits indicates a very low sedimentation rate that allowed long colonization windows. This is supported by the presence of the bivalve Ptychopteria sp. and the preservation of semi-articulated trilobites and crinoids. Additionally, the dominance of Pascichnia and Fodinichnia may indicate high food supply. Furthermore, the hydrodynamic conditions would have been calm between storm events. Finally, the RA includes mid-tier detritus and suspension-feeding trace fossils and is dominated by Rosselia socialis. This trace-fossil assemblage is present in amalgamated hummocky cross-stratified very fine-grained sandstone indicative of a lower/middle shoreface both in LLP and RLC. The tracemakers inhabited permanent dwelling structures that were re-established after successive event of erosion and sedimentation above the fair-weather wave base. RA is associated with high hydrodynamic energy and high sedimentation rate. The distal Cruziana Ichnofacies is typically present in offshore deposits (such as LLP and RLC sections). However, its presence in the sandstone deposits of the lower shoreface (QT and QPS sections) may imply an anomalous onshore expansion of this ichnofacies.