Animal-Substrate Interactions in an Early Miocene Wave-Dominated Tropical Delta: Delineating Environmental Stresses and Depositional Dynamics (Tacata Field, Eastern Venezuela)

Abstract

Abstract The Miocene Tacata reservoirs of the Eastern Venezuela Basin contain deposits that accumulated as a result of the progradation of a wave-dominated delta complex in a tropical setting. Integration of ichnologic and sedimentologic data helps to detect different stress factors, allowing distinction between deltaic and associated along-strike strandplain shoreface successions. Fluvial discharge was repeatedly interrupted by periods of storm-wave reworking and suspension fallout, leaving diagnostic ichnologic signatures in the deposits. Repeated storms rank among the most important controlling factors on the front of the Tacata deltas. Lateral (especially on the updrift side) to the river mouths wave activity caused nearly continuous accretion of the associated strandplains. Evidence of tidal influence is essentially restricted to distributary channel and, particularly, interdistributary-bay deposits. Although elements of the Skolithos ichnofacies (e.g., Skolithos) are present, they tend to occur in subordinate numbers as a result of water turbidity. Distributary-channel deposits are sparsely bioturbated, but the presence of certain ichnotaxa (e.g., Ophiomorpha) allows distinction from freshwater fluvial channels. Ichnofabrics from interdistributary-bay deposits display all the characteristics expected from brackish-water settings (e.g., low ichnodiversity, presence of monospecific suites) and are also typically sparsely bioturbated. Teichichnus and Planolites, commonly associated with synaeresis cracks, are dominant ichnotaxa in these fine-grained, low-energy, protected brackish-water settings. In contrast, delta-front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish-water settings, such as Chondrites and Phycosiphon. Preservation of very thin storm layers is probably more common in wave-dominated deltas than in strandplain systems because deltaic stresses preclude the establishment of an abundant infauna that otherwise would have completely reworked such thin sandstone layers. Amalgamated, thick-bedded storm deposits are typically unburrowed or contain deep vertical Ophiomorpha. Scolicia ichnofabrics seem to be restricted to the fully marine parts of strandplain deposits, and are therefore useful to detect possible delta asymmetry along depositional strike or vertical changes in degree of deltaic influence.