SNAPSHOTS OF EARLY PALEOZOIC COLONIZATION WINDOWS: TRUE SUBSTRATES WITH TRACE FOSSILS (BALCARCE FORMATION, ARGENTINA)

HALPERN, KAREN; ARREGUI, MARIANO; OLIVO, MARIANA S.; DE LA PUENTE, G. SUSANA; MUÑOZ, DIEGO F.; MOYANO-PAZ, DAMIÁN; ISLA, MANUEL F.

La Plata

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

Asociación Argentina de Sedimentología

True substrate surfaces represent bedding planes that were sediment-water or sediment-air interfaces at the time of deposition. Despite being considered rare, they are more often found in the field than mentioned in the literature. These surfaces are genuine windows to past time lapses when biota interacted with the substrate. That is why one of the key features that provide additional information when studying these beds is the presence of trace fossils. Ichnofossils are abundant in the early Paleozoic (?Silurian) Balcarce “Quartzite” (Tandilia), where we could identify some true substrate surfaces along its outcrops. The Balcarce Formation is represented mainly by quartzites interspersed with scarce kaolinitic shales and quartzitic fine-grained conglomerate layers. Quartzites correspond to quartz arenites with low textural maturity. The grains are mainly composed of angular quartz, minor feldspar, bent muscovite flakes, and rare non-opaque heavy minerals represented by pyroxenes. Sedimentological (uniform sandy calibers and dominant multidirectional cross-bedding, mainly oriented parallel to the shoreline) and ichnological attributes (locally abundant trace fossils) suggest a tidal shallow-marine origin for the succession. In this contribution, we present an ichnoassociation characterized by a variety of behaviors occurring in high abundance at one true surface of approximately 500 m2, which is located at the base of the Los Pinos section (5 km south of the Balcarce locality, in the Buenos Aires province). The trace fossil association includes Arthrophycus isp., Cruziana isp., Diplichnites isp., Herradurichnus isp., Monomorphichnus isp., Psammichnites isp., Rusophycus isp., and Teichichnus isp., elements that correspond to the archetypal expression of the Cruziana ichnofacies. Some of them have large and small morphotypes. The differential preservation of this true surface shows the water-sediment interface represented by medium- to fine-grained sandstone beds, internally structureless and with current and wave ripples at the top; the immediate below surface dominated by arthropod epichnial trace fossils; and the underlayer with endichnial trace fossils. Moreover, the tiering of these biogenic structures evidences that the producers were filter- and detritus-feeders suggesting that the food could be taken from the water column and the substrate. We argue that the environment was stable and rich in nutrient supply to sustain the variety and abundance of the benthic fauna. The Balcarce Formation has been traditionally regarded as being deposited in a well-oxygenated tidal shallow-marine environment. The high ichnodiversity and type of feeding strategies identified from this trace fossil association are consistent with the previous environmental interpretation. Also, a halt in the erosional processes was necessary to preserve this and other substrates from the Balcarce Formation, immediately followed by the rapid burial of these layers. The interpretation of the interaction of the trace fossils with the sediment is of the utmost importance to identify true substrate surfaces and decipher their development and preservation.