Oligocene and Lower Miocene Stratigraphy of Western and Northeastern Falcon Basin, Venezuela

ABSTRACT The Falcón Basin is in northwestern Venezuela. It formed during the late Eocene or Oligocene. During the Oligocene and early Miocene, this small, narrow, east-west trending trough covered the greater part of Falcón State and adjoining parts of Zulia, Lara, and Yaracuy. After the early Miocene, the trough disappeared, and deposition was mainly confined to northern and eastern Falcón. The first deposits in the Falcón Basin were upper Eocene conglomerates, overlain by marine shales. In the central part of the basin, Oligocene rocks lie concordantly on these shales. Toward the basin margins, the Oligocene overlaps them and rests on older Eocene beds with angular unconformity. The Oligocene to Oligo-Miocene rocks in the basin are a complex of facies deposited during a marine transgression. In most of central Falcón, this complex is represented by relatively deep-water facies. In ascending order, the section there consists of: a sandstone and shale unit, a shale, and a shale with thin limestones and sandstones interbedded. The total thickness of this section exceeds 2,000 meters in some areas. The central-basin Oligocene to Oligo-Miocene section grades to shallower-water facies toward the basin margins. These facies include: reef limestones, and backreef limestones, shales, and black chert-bearing sandstones and conglomerates near the north-central margin; thick-bedded sandstones and interbedded clays near the western margin; limestones, sandstones, and shales near the south-central margin; shales and black chert-bearing conglomerates and sandstones near the southeastern margin. In most of the basin, the Oligocene to Oligo-Miocene facies-complex is topped by a lower Miocene shale unit. Toward the basin edges, the lower part of this unit grades into the top of the underlying facies-complex. At its thickest, this shale exceeds 2,000 meters. Above it in most of the basin, lies a series of lower Miocene sandstones, siltstones, and shales. Two isopach maps included in the paper show that the Falcón Basin was an east-northeast-trending trough. It was divided into two sub-basins, one in eastern and one in western Falcón. These were separated by an area where the basin was narrower and the section thinner. Three facies maps are also included. They show that (1) mostly shales were deposited in the deeper areas of the basin during Oligocene and early Miocene time, and (2) the shales grade laterally to interbedded reefal limestones, sandstones, conglomerates, and shales near the basin rim.