INVESTIGADORES
PONCE Juan Jose
congresos y reuniones científicas
Título:
Protobranch trace fossils in Miocene tide-influenced deltaic deposits from the Chenque Formation: role of substrate in morphologic variations
Autor/es:
CARMONA, N.B.; MÁNGANO, M.G.; BUATOIS, L.A.; PONCE, J.J.
Lugar:
Buenos Aires
Reunión:
Congreso; XII Reunión Argentina de Sedimentología; 2008
Resumen:
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.