Protobranch trace fossils in Miocene tide-influenced deltaic deposits from the Chenque Formation: role of substrate in morphologic variations

CARMONA, N.B.; MÁNGANO, M.G.; BUATOIS, L.A.; PONCE, J.J.

Buenos Aires, Argentina

Congreso; XII Reunión Argentina de Sedimentología; 2008

Lower Miocene, tide-influenced deltaic deposits from the Chenque Formation, exposed along the coast of Caleta Olivia city, Patagonia, Argentina, contain well-preserved biogenic structures attributed to locomotion of deposit-feeder protobranch bivalves. The analyzed trace fossils, assigned to the ichnogenus Protovirgularia, consist of delicate, dominantly horizontal, chevronate structures, symmetrical with respect to a median axis. This morphology is essentially understood in terms of a push-and-pull mechanism: the penetration and the terminal anchors working sequentially. The separation between successive chevrons indicates the distance that the shell moved towards the foot during protraction of the pedal retractors. The chevrons record the split-foot of protobranch bivalves, and the open side indicates the direction of locomotion. In the studied outcrop, Protovirgularia specimens are abundant, being recorded in three of the four recognized facies (distal and proximal prodelta and proximal delta-front). Interestingly, the studied specimens show remarkable morphologic variations, which can be related to type and consistency of the sediment and toponomy (i.e. stratal position in relation to the casting medium). Specimens with sharp, closely-spaced chevrons tend to occur in sand-dominated units (e.g. proximal prodelta heterolithics and delta-front sandstone). These specimens are preserved as full relief and constitute relatively long, straight to curved structures. This morphology indicates efficient penetration and stable anchorage in more or less stiff sediment. In contrast, examples of Protovirgularia in mud-dominated deposits (e.g. distal prodelta or mud drapes of the delta-front), commonly show poorly defined outlines and loosely spaced chevrons. These trace fossils are mainly preserved as negative epirelief and full relief. This morphology suggests that the sediment was quite fluid during locomotion and anchorage was not optimal, preventing preservation of sharp details. This interpretation agrees with studies on modern bivalves, which indicate that when the substrate is fluid, these organisms can move easily because friction is minimal, although anchorage is more difficult to achieve. The specimens from the proximal prodelta facies are more abundant, commonly longer, and more continuous than those from the distal prodelta and proximal delta front. These characteristics most likely reflect optimal conditions of food availability and proper type and consistency of substrate in the proximal prodelta. In addition, the occurrence of Protovirgularia in these Miocene deltaic deposits, suggests tolerance of the tracemaker to fluctuations in salinity conditions.