TAPHONOMY AND PALEOECOLOGY OF THE BIVALVE TRACE FOSSIL PROTOVIRGULARIA IN DELTAIC HETEROLITHIC FACIES OF THE MIOCENE CHENQUE FORMATION, PATAGONIA, ARGENTINA

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

JOURNAL OF PALEONTOLOGY

PALEONTOLOGICAL SOC INC

Año: 2010

0022-3360

ABSTRACT–Lower Miocene, tide-influenced deltaic deposits from the ChenqueFormation, Patagonia, Argentina, contain abundant and well-preserved biogenic structuresattributed to locomotion of deposit-feeder protobranch bivalves. These trace fossils, assignedto the ichnogenus Protovirgularia, consist of delicate, inclined to horizontal, chevronatestructures, mostly symmetrical with respect to a median axis. Identification of Protovirgulariaat sandstone sole beds (hypichnion) is quite straightforward. Endichnial, exichnial andepichnial preservation in heterolithic facies, however, provides a wide variety of forms thatdepart from the archetypal Protovirgularia, and challenges ichnotaxonomic classification.Specimens in prodelta and delta-front facies display morphologic features controlled bysubstrate fluidity, toponomy, and sedimentation rate. Most specimens show sharp, closelyspaced chevrons, and occur along sandstone/mudstone interfaces of the proximal prodelta anddistal delta-front deposits. These forms reflect how tracemakers experienced significantfriction while advancing through the sediment, which resulted in relatively smaller incrementsof movements. In contrast, variants of Protovirgularia formed in muddier beds, such as inprodeltaic facies, show irregular, poorly defined and unevenly spaced chevrons, and arelocally asymmetric in relation to the axis, reflecting softer, water-rich, and plastic substrates.This sediment offered relatively low friction, but poor anchorage for the foot. Theseoccurrences of Protovirgularia in tide-influenced, marginal-marine deposits suggests thatprotobranchs were tolerant to fluctuations in salinity, sedimentation rates, turbidity, andoxygen depletion, displaying opportunistic strategies in stressed nearshore environments. Ourevaluation of taphonomic controls and appropriate identification of Protovirgularia canprovide valuable information for expanding our knowledge of the ethology and paleoecologyof protobranch bivalves.