High frequency sequence stratigraphy of the La Pascua Formation: Effect of relative sea level on the size, geometry and spacing of depositional systems.

摘要

The Rupelian aged La Pascua Formation is a basal transgressive sandstone unit that onlapped the Cretaceous Tigre Formation along the cratonic margin of the Eastern Venezuela basin. Ten fourth-order sequences were identified within the context of a 1.7 million-year third-order sequence composed of lowstand, transgressive and highstand sequence sets.; Fourth-order incised valley systems with the highest width:net-sand thickness ratios were deposited in the aggradational third-order lowstand sequence-set, the lowest ratios were found in sands of the third-order retrogradational transgressive sequence-set, while medium ratios were common to the third order progradational highstand sequence-set.; V-shaped and coalesced incised valleys dominated the third-order lowstand sequence-set. Narrow elongate-sinuous to slightly sinuous valleys the third-order transgressive sequence-set, and wide elongate-sinuous were most prevalent in the highstand sequence-set.; Much of the sediment deposited in the transgressive systems tract barrier shorefaces was reworked from the underlying highstand system tracts during wave ravinement and subsequent longshore current transport.; Elongate dip sands, unique to the transgressive systems tract; were deposited directly above the underlying incised valleys during fourth-order transgression. They dominated fourth-order transgressive systems tracts in periods of strong incision followed by strong third-order transgressions. They were formed by sand eroded along the transgressive surfaces, that was transported by longshore currents and deposited into the erosional lows of the incised valleys of the underlying lowstands, later to be reworked by fluvial and tidal processes into elongate sandbodies.; Shoreface sands of late fourth-order highstands, were often semi-detached from the younger more landward shorefaces. These intervals present excellent targets for stratigraphic trap exploration for hydrocarbons.; It was possible to predict the net-sand tendencies, sand body measurements and geometry based on their location in the depositional systems along the fourth and third order eustatic curve. Another conclusion was the hazard of comparing modern depositional systems to the ancient without first comparing their respective locations along the third and fourth-order eustatic curves.