Lower Devonian ichnofacies in the Argentinean Precordillera: Distribution and paleoenvironmental implications

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

Simposio; V Simposio Latinoamericano de Icnología; 2023

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 is dominated by siltstone and very fine-grained sandstone. This formation records a thickening and deepening trend from south (shelf to upper shoreface facies) to north (lower offshore to offshore-transition facies). Our study aims to integrate sedimentological and ichnological datasets to characterize the depositional environment and gain insights into the middle Paleozoic benthic ecology of shallow-marine settings in Southwest Gondwana. Two main associations of trace fossils were identified based on fieldwork and laboratory work (i.e., observation of polished hand samples and thin sections under microscope). The first and most prevalent association is characterized by complex feeding structures, such as Zoophycos isp., Phycosiphon incertum, Nereites missouriensis, and Chondrites isp. This association is present in biogenically mixed siltstone and very fine-grained sandstone, recording deposition in lower to upper offshore and offshore transition settings. Bioturbation Index is high (BI=5-6). This association illustrates the distal Cruziana Ichnofacies. However, the archetypal Cruziana Ichnofacies would have been expected according to currently accepted ichnofacies models. The most common ichnotaxa observed throughout the basin are Phycosiphon incertum and Zoophycos isp., which show their highest abundance in the northern area. For the Devonian, Zoophycos exhibits a thin planar spreiten with single whorls, developed in shallow-marine to shelf environments. The second recorded association comprises Skolithos isp. and Arenicolites isp. This association exhibits low ichnodiversity and is present in deposits of lower bioturbation intensity (BI=0-2). It is found in fine- to medium-grained sandstone formed in lower to middle shoreface environments and illustrates the Skolithos Ichnofacies. The Talacasto Formation contains a rich epibenthic biota all along the column including brachiopods, trilobites, echinoderms, bivalves, ostracods, cephalopods, corals, and conulariids. Echinoderms, brachiopods, corals and cephalopods suggest normal salinity levels. The abundance of macrofauna and the high degree of bioturbation in the finer-grained facies may suggest oxic conditions at the sediment-water interface. In turn, the distal Cruziana Ichnofacies is traditionally linked to poorly oxygenated conditions within the sediment, but not necessarily at the sediment-water interface. The high degree of bioturbation indicates low-energy conditions, high food availability, and long-term colonization windows. However, recurrent fine-grained sandstone layers recording abundant articulated multielement skeletal organisms (typically echinoderms), indicate episodic very-rapid obruption deposits. As for the Skolithos Ichnofacies, the low ichnodiversity may be attributed to the dominance of storm events. Notably, the Talacasto Formation exhibits a transition from the distal Cruziana Ichnofacies to the Skolithos Ichnofacies, without the presence of the archetypal Cruziana Ichnofacies. Colonization of deeper-tier deposit-feeding organisms may have obliterated the shallower-tier trace fossils typical of the Cruziana Ichnofacies. Alternatively, the absence of the archetypal Cruziana Ichnofacies can be attributed to limited oxygenation within the sediment.