TAPHONOMIC AND PALEOECOLOGICAL ANALYSES (MOLLUSKS AND FISHES) OF THE SUA MEMBER CONDENSED SHELLBED, UPPER ONZOLE FORMATION (EARLY PLIOCENE, ECUADOR)

Resumen

Abstract The early Pliocene sediments of the Sua Member (Onzole Formation) are spectacularly exposed along coastal cliffs in the surroundings of Sua (northwestern 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, ichnological signatures, and taphonomic features presented here, all indicate that the shellbed is a stratigraphically condensed (hiatal) skeletal concentration generated primarily by low rates of net background sedimentation. 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 mollu...