TAPHONOMIC AND PALEOECOLOGICAL ANALYSES (MOLLUSKS AND FISHES) OF THE SÚA MEMBER ONLAP SHELLBED (EARLY PLIOCENE, ONZOLE FORMATION, ECUADOR)

Transgressive shallow-marine strata of the Súa Member are well exposed along the sea-cliffs of Súa (northwest Ecuador) and include a basal shellbed bracketed by a Glossifungites-demarcated ravinement surface produced in the upper shoreface by wave processes below and bioturbated lower shoreface deposits above. The shellbed, up to 70 cm thick, is largely dominated by mollusk remains but also includes crab pincers, solitary corals, bryozoans, fragments of echinoids and barnacles, selachian teeth and abundant fish otoliths. In cross-sectional view the bioclastic components are densely- or loosely-packed in the lower part and pass rapidly upward into a dispersed biofabric. Poorly-preserved specimens largely dominate the mollusk assemblage making it difficult taxa identification. Bivalves are mostly disarticulated and the rare conjoined specimens are preserved out of life position. Valves are randomly oriented even if the concave-up position seems to prevail in some places. Most specimens are fragmented and display signs of intense abrasion and bioerosion, whereas encrustation is very scarce. Visually, the fossil concentration appears to be dominated by Atrina cf. maura, Oliva cf. splendidula, Turritella abrupta and T. altilira, some of which are not present in the bulk samples within which the most abundant taxa include Anadara cf. reinharti, Ostrea iridescens, Undulostrea megodon, Polinices spp. and Turritella altilira. Some groups of species contrast for substrate preference so that taxa requiring hard substrata for support (O. iridescens, U. megodon, Chama pellucida, Crucibulum scutellatum, Serpulorbis sp.) coexist with soft-sediment loving taxa including seminfauna (A. cf. maura, scaphopods) and shallow infauna (Chione mariae, C. squamosa, Polinices hepaticus) whereas deep-infaunal elements are lacking. Known depth ranges of extant species lead us to infer a shoreface setting no more than 20-30 m in water depth. The fish otoliths are relatively well-preserved. Slightly less than 2000 specimens have been examined, among which representatives of 31 taxa belonging to 16 families have been recognized. The assemblage is dominated by six families (Batrachoididae, Congridae, Engraulidae, Ophidiidae, Paralichthyidae, Pleuronectidae), which include more than 95% of the examined material. The cusk-eel Otophidium indefatigabile is by far the most abundant taxon, followed by anchovies of the genus Anchoa, the garden-eel Paraconger californiensis, and many others, including sand flounders (Citharichthys sp., Syacium ovale), ophidiids (Chilara taylori, Lepophidium microlepis), toadfishes of the genus Porichthys, gobies, tonguefishes, righteye flounders, and grunts (Haemulon sp., Haemulopsis sp., Orthopristis cf. cantharinus). From a paleoecological point of view, the fish assemblage primarily consists of demersal and benthic taxa, typical of soft sandy substrate. However, approximately 25% of the specimens consists of shallow-water epipelagic planktivores. The presence of selected taxa characterized by a restricted bathymetric range (Larimus cf. pacificus, Orthopristis cf. cantharinus, Syacium ovale), unequivocally indicates a depositional environment located in close proximity to the coast at water depth comprised between 20 and 40 m. Based on stratigraphic, taphonomic and paleoecological attributes, the Súa Member basal shellbed is interpreted to represent an autochthonous-parautochthonous hiatal concentration. It does record stratal convergence and attenuation at the costal onlap during a period of marine transgression and formed at the shoreface through sedimentologic processes represented by gradual accumulation of locally produced hardparts and low net background terrigenous sedimentation.