John Reistroffer¹, Gerald Baum², Christopher G. Kendall³
(1) Mobil Oil, Dallas, TX
(2) Rice University
(3) University of South Carolina, Columbia, SC

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

The Rupelian aged La Pascua Formation is a basal transgressive sandstone unit that onlaps the Cretaceous Tigre Formation along the cratonic margin of the Eastern Venezuela basin. Ten fourth-order sequences were identified within the context of one third-order sequence which is constrained to a 1.2 million year period. Almost 400 wells were correlated over a 3500 square kilometer area. Two net sand isopach maps were contoured for each sequence, one for the transgressive systems tract (TST) and one for the the highstand sytems tract (HST)/ lowstand sytems tract (LST). Fourth-order incised valley systems (IVF) with very high width:thickness ratios were located in the aggradational third-order LST, with very low ratios grouped in the retrogradational TST and medium ratios in the progradational HST. Shape was an important factor also. Wide V-shaped and amalgamated incised valleys were mostly found in the third-order LST. Narrow meandering valleys in the TST and wide straight-to-slightly-meander-form in the HST. Width:thickness ratios and shape of the fourth-order IVF systems were directly influenced by relative sea level. Fluvial gradients and currents were reduced during rapid third-order relative sea level rise, depositing coarser-grained sediments upstream, and increasing suspended-load:bed-load ratios, reducing the capacity for erosion and increasing deposition of overbank facies. Rapidly falling sea level increased stream gradients and current, in addition to their load carrying capacity. More, and coarser-grained sediment was carried through the IVFs, along with greater amounts of erosion. The incised valleys are the shelfal sediment conduits for deep water systems which are expected to have similar fourth-order cyclicity and compositional changes at third-order and fourth-order frequencies

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana