Ichnology of tide-dominated deltaic deposits from the lower Miocene Chenque Formation, Patagonia, Argentina

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

Calgary, Canadá

Workshop; International Ichnofabric Workshop IX; 2007

Combined ichnologic and sedimentologic analysis of the lower Miocene Chenque Formation helps to recognize a tide-dominated deltaic succession exposed along the coast of Caleta Olivia city, Patagonia, Argentina. The ichnology of these deposits is analyzed and compared with open marine deposits belonging to the same formation. Two main subenvironments are recognized within these tide-dominated deltaic deposits, prodelta and delta-front, stacked forming a progradational coarsening-upward succession. The distal prodelta is characterized by lenticular-bedded, mud-dominated heterolithic strata, with very low bioturbation intensity (BI 0-2), recording sparsely distributed trace fossils made by deposit-feeders (e.g. Planolites). Synaeresis cracks are abundant in these beds. The proximal prodelta consists of wavy-bedded heterolithic facies. These deposits display an increase in bioturbation intensity (BI 1-3), but ichnofossils are sporadically distributed throughout the proximal prodelta interval. The trace fossil assemblage is dominated by deposit feeder structures, such as Planolites, Teichichnus and Phycosiphon; subordinate and rare elements include Asterosoma, Nereites, Rosselia, Schaubcylindrichnus, Siphonichnus, Skolithos and Thalassinoides. This assemblage is considered an impoverished expression of the Cruziana ichnofacies. Heterolithic proximal prodelta facies are punctuated by thin sandstone beds with current ripples, fully reworked by Macaronichnus, recording deposition during high-energy events. The delta-front succession shows two main facies representing distal to proximal tidal deposits. The distal delta-front facies consists of flaser-bedded, heterolithic muddy sandstone, almost completely obliterated by equilibrium/adjustment trace fossils of large bivalves (Pinna) (BI 3-5). Subordinately, Teichichnus, Thalassinoides and Schaubcylindrichnus are present also. Phycosiphon is commonly reworking Thalassinoides and the Pinna casts. Proximal delta front facies are sand-dominated with thin siltstone interbeds. The trace fossil assemblage is dominated by large Rosselia socialis and Macaronichnus, which colonize the sandier beds. The intensity of bioturbation is commonly low (BI 1-2), although some intervals may show relatively higher values. The mud drapes blanketing the sandstone foresets commonly contain Nereites and Protovirgularia. The Chenque Formation also contains deposits formed in offshore transition, and lower- and middle-shoreface environments, and is therefore ideally suited for ichnologic comparisons between deltaic and non-deltaic shorelines. The main differences observed are: - The prodelta deposits show a shallow tiered community, without much vertical niche partitioning, in contrast to the highly complex tiering structure recorded in offshore transition and lower shoreface deposits; - The delta-front deposits show an impoverished trace fossil assemblage, dominated by deposit-feeder structures (e.g. Macaronichnus), as well as detritus-feeder trace fossils (e.g. Rosselia), both being tolerant to energetic conditions. In contrast, the lower-middle shoreface deposits shows a combination of trace fossils typical of the Cruziana ichnofacies with the addition of vertical structures of suspension-feeder organisms, characteristic of the Skolithos ichnofacies.