Cretaceous carbonate rocks formed an extensive passive-margin section along the northern margin of the South American plate and are now found in outcrops in elevated and deformed ranges like the Merida Andes and Sierra de Perija. Regional seismic profiles correlated with well data show that a 300-m (984-ft)-thick Cretaceous carbonate platform underlies all of the Maracaibo Basin of western Venezuela. We examined the Cretaceous carbonate section beneath the southern Maracaibo Basin using a 1600-km(2) (617-mi(2)) area of three-dimensional (3-D) seismic reflection data provided by Petroleos de Venezuela, S. A., along with wells to constrain the age and environments of seismic reflectors. Well data allow the identification and correlation of the major lithologic subsurface formations with formations described from outcrop studies around the basin edges. Seismic reflection time slices at a depth range of 3.7-4.5 s (5-7 km; 3.1-4.3 mi) reveal the presence of a prominent, irregular reflection surface across the entire 3-D study area that is characterized by subcircular depressions up to about 600 m (1968 ft) wide and about 100 m (328 ft) deep. We interpret the subcircular features as sinkholes formed when the Lower Cretaceous carbonate platform was subaerially exposed to weathering in a tropical climate. The scale of the observed circular features is consistent with dimensions of limestone sinkholes described from modem karst settings. Correlation of the inferred karst horizon with well logs shows that the paleokarst horizon occurs within the shallow-water carbonate rocks of the Aptian Apon Formation. We infer that the karst formed during an Aptian eustatic sea level fall described from Aptian intervals in other parts of the world, including the Gulf of Mexico. The Aptian paleokarst zone provides a previously unrecognized zone of porosity for hydrocarbons to accumulate beneath the Maracaibo and perhaps other basins formed above the extensive passive margin of northern South America.