Ichnology and sedimentology of a tide-influenced delta, Lower Miocene Chenque Formation, Patagonia, Argentina: Trace-fossil distribution and response to environmental stresses
Abstract Combined sedimentologic and ichnologic analysis of the Lower Miocene Chenque Formation allows recognition of a tide-influenced deltaic succession exposed along the coast of Caleta Olivia city, Patagonia, Argentina. Two main subenvironments were identified, prodelta and delta front, stacked forming a progradational coarsening-upward succession, up to 10 m thick. The prodelta deposits are mainly characterized by heterolithic strata (lenticular and wavy bedding), with low bioturbation intensity and sporadic distribution of trace fossils. The trace-fossil assemblage is dominated by deposit-feeder structures (e.g. Planolites montanus, Protovirgularia isp., and Teichichnus rectus), constituting an impoverished expression of the Cruziana ichnofacies, with respect to their fully marine counterparts. A transition zone between the prodelta and the delta front is discontinuously distributed along this outcrop. This interval consists mainly of flaser-bedded sandstone, almost completely obliterated by equilibrium/adjustment trace fossils of large bivalves (Atrina), and subordinately, by the trace fossils Nereites missouriensis, Phycosiphon incertum, T. rectus, Thalassinoides isp., and Schaubcylindrichnus freyi. The delta-front deposits consist of sigmoidal cross-stratified sandstone with mud drapes. The trace-fossil assemblage is dominated by large Rosselia socialis and Macaronichnus segregatis in the sandier beds, whereas the mud drapes blanketing the sandstone foresets commonly contain N. missouriensis and Protovirgularia isp. Ichnologic characteristics (e.g. shallow-tiered communities, impoverished trace-fossil assemblages, dominance of deposit-feeder structures, and inhibition of suspension-feeder elements) suggest that different paleoenvironmental stresses, such as changes in salinity, water turbidity, and fluctuations in energy and in sedimentation rates, affected the infaunal communities of these tide-influenced delta settings.