Sedimentological and ichnological signatures of changes in wave, river and tidal influence along a Neogene tropical deltaic shoreline

摘要

Analysis of Neogene cores from the Eastern Venezuela Basin along 65km of a west-east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty-three facies are defined and grouped into four categories (wave-influenced, river-influenced, tide-influenced and basinal). Wave-dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide-influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm-wave reworking and suspended sediment fallout. Delta-front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid-generated ichnogenus Scolicia. River-dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low-sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher-sinuosity rivers. The subaqueous delta plain includes distributary channels and tide-influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary-channel and distributary mouth-bar deposits, as well as by delta-front and prodelta deposits showing evidence of sediment gravity-flow and fluid-mud emplacement. Delta-front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river-dominated deltas, tide-influenced delta plains, wave-dominated deltas and wave-dominated strandplain-offshore complexes.