Ichnology of Wave-Dominated Deltas from Miocene Reservoirs in the Carapita and Capaya Formations, Tacata Field, Eastern Venezuela: Using Trace Fossils to Reveal Fluvial Input in Shallow-Marine Successions

Manuel Delgado¹, Luis A. Buatois², Solange Angulo¹, and Dennis Sánchez^3
1 PDVSA División Oriente, Guanta, Venezuela
² University of Saskatchewan, Saskatoon, Saskatchewan
³ PDVSA E y P, Puerto La Cruz, Venezuela

There is an increased interest in the ichnology of deltaic systems and in how biogenic structures may act as a signature to distinguish deltaic deposits from estuarine and strandplain shoreface successions. Cores from the Lower to Middle Miocene Carapita Formation and the Middle Miocene Capaya Formation (Tacata Field, eastern Venezuela) provide an instructive opportunity to explore the potential of ichnology in the understanding of deltaic deposits. Both units record progradation of a wave-dominated deltaic complex where periodic fluvial discharges alternated with storm events and suspension fallout. Degree of bioturbation ranges from low to moderate. Phycosiphon and Chondrites are the dominant ichnotaxa in prodelta and distal delta front deposits, commonly occurring as monospecific suites. Other components include Terebellina, Arenicolites, Palaeophycus, Teichichnus, Planolites and escape traces. Delta front deposits are dominated by vertical Ophiomorpha, Thalassinoides and Planolites; Terebellina, Phycosiphon, Palaeophycus, Teichichnus and Planolites are subordinate components. Distributary channel deposits contain Ophiomorpha and Skolithos. Planolites, Palaeophycus and root traces are the most common forms in interdistributary bay deposits. Large and deep synaeresis cracks are pervasive. Low ichnodiversity of individual suites reveals a stress factor due to reduced salinity and allows distinction from nondeltaic strandplain shoreface successions. However, the dominance of Phycosiphon, Terebellina and Chondrites suggests periods of normal marine salinity that alternated with dilution due to fluvial discharge. Integration of detailed ichnologic observations with sedimentologic and biostratigraphic data in a sequence stratigraphic framework is shown to be a powerful tool in the interpretation of this nearshore reservoir-bearing clastic succession.