FIRST RECORD OF ROSETTE TRACE FOSSILS IN NEARSHORE ENVIRONMENTS FROM THE EARLY PALEOZOIC BALCARCE FORMATION (SOUTHEASTERN ARGENTINA).

MUÑOZ, DIEGO F.; HALPERN, KAREN; ARREGUI, MARIANO

Salta

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

Asociación Paleontológica Argentina

The widely distributed Balcarce Formation (Tandilia System, southeastern Argentina) outcrops represent a shallow epicontinental sea with tidal influence, hosting a diverse ichnofauna. Previous research recognizes an Early Paleozoic age with certainty, and so far, the presence of Cruziana bonariensis suggests that it is probably Silurian. Further surveys allowed us to identify radial to rosette trace fossils not previously mentioned for this region. These structures are feeding traces that vermiform animals can produce. The studied material consists of two negative epirelief different structures: (1) four to five non-overlapping petaloid lobes radiating from a slightly noted central knob; and (2) elliptical rays radiating from a poorly defined central area. The first resembles Gyrophyllites cristinae, a rosette trace fossil recently described in the Early Ordovician (Tremadocian) Santa Rosita Formation (NW Argentina). The second shares similarities with Volkichnium isp. Gyrophyllites have been identified in near-shore environments during the Cambrian and in younger offshore to slope settings, suggesting an expansion from shallow to deep waters. Particularly, Gyrophyllites cristinae was recorded in offshore transition Lower Ordovician deposits. The new specimens from the (?Silurian) Balcarce Formation appear on top of fine to medium tangential cross-stratified sandstone, which is interpreted as subtidal bars. Conversely, a migration to deep-water environments was proposed for Volkichnium since its earliest records belonged to shallow-marine settings (lower Cambrian) and then was restricted to deep-marine deposits (Lower Ordovician; ?Lower Carboniferous). The Balcarce Formation material contrasts with the current paleoecological assumptions showing an environmental expansion for Volkichnium and a new record of G. cristinae extends its lifespan.