Sequence-stratigraphic analysis of Eocene clastic foreland basin deposits in central Lake Maracaibo using high-resolution well correlation and 3-D seismic data

摘要

Eocene clastic rocks of the Maracaibo Basin were deposited in an asymmetrical foreland basin formed during the oblique Paleogene collision between the Caribbean and South American plates.In this study,we use more than 300 wells and 2000 km~2 (772 mi~2) of seismic data in the central Maracaibo Basin to produce a detailed sequence-stratigraphic interpretation of the Eocene Maracaibo foreland basin. The base of the Eocene stratigraphic succession in the central Maracaibo area is characterized by an approximately 250-m (820-ft)-thick,aggradational succession of fluviodeltaic sandstone overlain by an approximately 600-m (1968-ft)-thick retrogradational succession of shallow-marine shale and sandstone containing minor progradational units.The upper part of the foreland basin sequence is marked by an approximately 100-m (328-ft)-thick aggradational succession of fluviodeltaic sandstone.In the approximately 1000-m (3280-ft)-thick Eocene section,we interpreted 17 parasequence sets,6 genetic sequences,and 1 depositional sequence.Only one classic sequence boundary was interpreted within the Eocene section that marks the boundary between the retrogradational shallow-marine section and the overlying aggradational fluviodeltaic succession.Based on the stratigraphic architecture and thickening trends of several of the parasequence sets,we conclude that the main source of clastic sedimentation was located on the South America craton south of the Maracaibo Basin,instead of along the thrusted,north-northeastern margin of the basin as proposed by previous workers. A lack of recognition of classic sequence boundaries suggests that Eocene clastic rocks of the central Maracaibo foreland basin were not subaerially exposed during most of the Eocene,and that their stratigraphic architecture was controlled by tectonic subsidence related to thrusting along the northeastern edge of the foreland basin.Eustasy was not an important control on the stratigraphic evolution of the foreland basin until its middle Eocene aggrada-tional period that marked the end of foreland basin subsidence.Well logs and three-dimensional seismic data show that depositional environments on the Eocene delta plain and shelf of the central Maracaibo foreland basin were dominated by fluvial and tidal processes that are similar to modern depositional processes of the Orinoco delta in eastern Venezuela.