The Maracaibo Basin in northwestern Venezuela, one of the most prolific oil-producing areas in the world, is a composite foreland basin limited by foredeeps both to the south and west, as well as by piggy-back basins and half-grabens in the east. The western foredeep is part of a retro-foreland basin associated with the Oligocene-Miocene uplifted Sierra de Perijá. The La Rosa and the lower Lagunillas formations (lower-middle Miocene) were deposited in the distal portion of this retro-foreland basin. Cores and well logs allow the identification of tidally influenced fluvial, tidal flat, central basin and embayment, shoreface and offshore, lagoonal and organic rich deposits, as well as zones of subaerial exposure within regressive deltaic or strandplain, and transgressive barrier bar or estuarine environments. Core-log calibrations and the identification of stacking patterns and key stratigraphic surfaces allow two types of high-frequency stratigraphic sequences to be recognized. Most sequences are of a simple transgressive-regressive type, implying continuous but irregular rates of relative rise of sea level. However, some have a lowstand segment, implying a relative sea level fall associated with the regressive-to-transgressive turnaround. The repetition of high-frequency sequences, each one of duration less than 0.5 m.y., is believed to have been controlled by high subsidence rates in the basin. The basinward and landward stepping of sequences is interpreted in terms of milder and strong advance of the western thrust-belt wedge and the accompanying variable migration of a forebulge to the east.

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