The upper Pleistocene-Holocene of the Cariaco Basin and the Upper Cretaceous of the La Luna-Querecual Formation are marine hemipelagic deposits that accumulated under a water column several hundred meters deep. However, the latter formation includes large mollusk shells and seems to reflect closer coastal environments. These two deposits are very similar with respect to their facies, which are principally clays and marls; and to their general fabric, in which appears a fundamental cyclic pattern, particularly in its finest fabric (i.e., annual varves). These varves consist of a light layer having a planktonic character, with concentrations of foraminifers and diatoms; and a dark layer where organic and terrigenous products prevail; each couplet resulting from a seasonal sedimentation that can be observed at present in the Cariaco Basin, in the Santa Barbara Basin, and in the Gulf of California. The fundamental cyclicity of these deposits is in fact recording an alternation of dry seasons, with upwelling dynamics resulting in a spike of planktonic production; and wet seasons, with strong fluviatile inputs.
Varves are grouped in bundles that may be recording subdecadal to decadal periodicities, such as those of the El Niņo system and of solar activity. The longest cycles visible in the two formations approximately correspond in period with the obliquity periodicity. Lithologically, they appear as a decimetric marl/limestone alternation in the La Luna-Querecual outcrops and as successions of bioturbated and laminated intervals in cored series of the Cariaco Basin. This recurrence during the Pleistocene-Holocene of Upper Cretaceous structures linked to global and regional climatic forcing must be brought closer to a permanence of paleogeographic setting.
Particular structures testify to carbonate precipitation of bacterial origin, manifested as beige micritic patches that are either irregular, discontinuous, and deformed layers or ovoid heaps. A suite of early diagenetic processes are recorded in the Cariaco Basin because of the shallow depth of the burial of drilled sediments. These processes are compaction, genesis of concretions, and recrystallization.
Compaction allows us to observe the chronology of some processes like the precipitation of carbonate and the formation of siliceous concretions. Genesis of concretions is always early and before the major part of compaction. Precipitation of these concretions resulted from solutions enriched in dissolved products: (1) carbonate derived from biogenic and inorganic structures during periods of CO2 enrichment caused by bacterial decay of organic matter and (2) silica derived from diatoms and radiolarians.
Field or core observations of the La Luna-Querecual Formation reveal that the carbonate concretions precipitated before the siliceous ones. Comparison of diagenetic processes between the two successions, despite their very different stages of progress, allows us to conclude that the Cariaco succession is a good analogue of the initial state of the La Luna-Querecual Formation.
Finally, the sole significant discrepancy between the two series is that the rate of sedimentation is 10-30 and 3-8 times slower for the La Luna and the Querecual successions, respectively. This disparity may be attributed to very different terrigenous, then biogenic, flux intensities (the latter having induced a great divergence in planktonic production). In short, this discrepancy reflects two epochs of sedimentation: (1) for the La Luna-Querecual Formation, a quiet tectonic episode preceding the Laramide orogenesis and characterized by low topography (Mongenot et al., 1996) and a high sea level leading to a sequestration of detrital materials in coastal and estuarine environments; and (2) for the Cariaco succession, a sequence deposited during the Pliocene-Pleistocene, characterized by lower sea level and block faulting that affected the Venezuelan Andes and the Guyanese Craton and led to a rejuvenation of relief and to enhanced erosion.