The four holes at Ocean Drilling Program (ODP) Site1002 are located in the central part of the Cariaco Basin, at a water depth of 892-893 m (Fig. 1, Fig. 2). From these holes was recovered a Quaternary succession containing cycles having various millimeter to decimeter thicknesses and a dominant black color caused by high concentrations of organic matter. A detailed observation of cores has revealed the presence of some lithologic concretions formed by early diagenetic carbonate precipitation.
The Late Cretaceous of northern Venezuela (Fig. 2) includes a black shale formation, which was the primary source rock of hydrocarbons exploited in this country (Tribovillard et al., 1991a, 1991b). This anoxic facies is called the La Luna Formation in western Venezuela and particularly in the Transversale de Barquisimeto (Aubouin, 1975) (Fig. 1, Fig. 2) and Querecual in eastern Venezuela (Fig. 2, Bergantin area). It is composed of laminated black sediments enriched with carbonate and siliceous concretions. This facies is included in a transgressive unit of Cenomanian to Senonian age, deposited under a water column <500 m deep (Tribovillard and Stephan, 1989).
These two series are very similar with respect to depositional settling and lithology. We wished, therefore, to begin an investigation to compare microscopic and macroscopic cycles to more precisely determine their periodicities and depositional controls and to ascertain the origin of their diagenetic concretions. In particular, we asked whether the late Quaternary deposits of the Cariaco Basin, for which the environmental interpretation is already well understood (Peterson et al., 1991, Lin et al., 1997), can be considered an analogue for the Cretaceous black shales of northern Venezuela in their initial diagenetic state.