An anomalous shallow-marine ichnofacies gradient from the Lower Devonian Talacasto Formation of the Argentine Precordillera

Abstract

The Lower Devonian Talacasto Formation in western Argentina records deposition in wave-dominated shallow-marine environments. This unit comprises a large-scale progradational succession, transitioning from black, parallel-laminated mudstone in the lower interval to siltstone and very fine- to medium-grained sandstone in the upper interval. The succession spans environments ranging from the shelf to the upper shoreface. Three trace-fossil assemblages have been identified: (1) Palaeophycus assemblage, including Palaeophycus heberti, P. tubularis, and Helminthopsis isp., corresponding to the shelf; (2) Phycosiphon-Zoophycos assemblage with Zoophycos isp., Phycosiphon incertum, Nereites missouriensis, and Chondrites isp., ranging from the lower offshore to the lower/middle shoreface; and (3) Rosselia assemblage containing Rosselia socialis, Skolithos isp., Arenicolites isp., Palaeophycus tubularis, and escape trace fossils, corresponding to the offshore transition and the lower/middle shoreface. Two ichnofacies have been identified: distal Cruziana and Skolithos Ichnofacies. Unlike the traditional ichnofacies model, the Talacasto Formation shows an onshore expansion of the distal Cruziana Ichnofacies, which is present not only in the lower offshore, but in the upper offshore, offshore transition, and lower shoreface as well. In contrast, the archetypal Cruziana Ichnofacies is not present. This anomalous distribution is attributed to an interplay of ecologic and taphonomic factors combined with the restricted topography of the southern portion of the Precordillera Basin. Low-energy conditions and high food supply for prolonged times allowed for the obliteration of shallow-tier trace fossils by deeper and widely distributed burrows, favoring the onshore expansion of the distal Cruziana Ichnofacies and preventing the preservation of the typical elements of the archetypal Cruziana Ichnofacies. Areas subjected to greater storm influence provided favorable conditions for the development of the Skolithos Ichnofacies.