Shell concentrations as tools in characterizing sedimentary dynamics at sequence-bounding unconformities: examples from the lower unit of the Canoa Formation (Late Pliocene, Ecuador)

The Canoa Basin in Manabì, Ecuador, contains a mainly marine, clastic sedimentary succession of Late Pliocene and Pleistocene age (Canoa Formation and Tablazo Formation). A stratigraphic and sedimentologic study of the entire sedimentary succession indicated that sedimentary facies recur in consistent deepening-shallowing transgressive-regressive patterns and that on this basis it can be divided into three different informal units (Clow, Cupp, Tb). The lowest of these units (Clow) is composed by at least four depositional sequences, each bounded by a ravinement surface and containing a basal environmentally time-averaged shell bed accumulated during rapid sea-level rise under conditions of reduced terrigenous sediment supply (transgressive systems tract). Owing to intense heavy bioturbation, the internal architecture of these shell beds is not recognizable. Each of these shell beds is mantled by poorly fossiliferous sandy and silty shales accumulated during subsequent progradation (highstand and regressive systems tracts). Fossil assemblages, accounting for inner/middle shelf settings, are dominated by infaunal suspension-feeding bivalves (Linga cancellaris, Chione mariae, etc.) most of which require sandy substrate or are ubiquitous. In terms of trophic/life habit groups, the total succession shows an increase of infaunal suspension-feeder species richness from the lowest (C1) to the upper (C4) shell bed, whereas epifaunal taxa decrease in richness. Shell packing density also decreases from C1 to C4. The reconstruction of the short-term sedimentary dynamics based on taphonomic and palaeoecologic observations indicates a slightly progradational staking pattern of depositional sequences, representing high-frequency sixth-order eustatic sea-level fluctuations within a third-order tectonically-induced cycle.