Taphonomic and paleoecological analyses (mollusks and fishes) of the Sua Member condensed shellbed, Upper Onzole Formation (Early Pliocene, Ecuador)

The early Pliocene sediments of the Súa Member (Onzole Formation) are spectacularly exposed along coastal cliffs in the surroundings of Súa (northwest Ecuador). The shellbed at the base of these strata contains rich mollusk and fish assemblages and provides a rare opportunity to document the virtually unknown Pliocene shallow-water faunas of Ecuador. Stratigraphic context, faunal composition (both mollusks and fishes), biofabric trend, and taphonomic features presented here, all point to indicate that the shellbed is a stratigraphically condensed (hiatal) skeletal concentration recording a prolonged period of reduced net sedimentation and formed through dominantly biological processes of concentration. Shellbeds of this type, immediately overlying a Glossifungites-demarcated ravinement surface, are regarded as onlap shellbeds and record landward stratal convergence and attenuation during the earliest phases of marine transgression. Although composed of largely autochthonous-parautochthonous specimens, the fossil assemblage contains mollusk species from a range of water depths and from both soft- and hard-bottom habitats, implying in situ time-averaging and admixture of different assemblages as transgression proceeded on a sediment-starved shoreline. The mollusk assemblage is composed of 54 taxa (26 bivalves, 26 gastropods, and 2 scaphopods), whereas the fish assemblage consists of 31 taxa belonging to 16 families. Both assemblages are indicative of a well-oxygenated marine biotope swept by currents and waves and devoid of a benthic macrophyte cover. From a zoogeographic point of view, the largest part of the mollusk and fish taxa are presently distributed throughout the Tropical Eastern Pacific Biogeographic Region, with the exception of a few fish species that are interpreted as northern guests today resident in the Californian Province and a gastropod species nowadays living in the West Atlantic.