The lithology and structure of these deposits show some similarities: dominant marls, dark color, wealth of organic matter, abundance of planktonic remains such as foraminifers, and occurrence of first-order cycles creating two types of alternation: marl/limestone for the La Luna-Querecual Formation and bioturbated/laminated layers for the Cariaco succession. The difference in importance of diagenetic precipitation (concretions up to several meters in size in La Luna-Querecual and millimeter-scale in the Cariaco succession) is not significant given the disparity of the two diagenetic states.
From a microfacies point of view, the two successions exhibit a laminated structure with elementary cycles formed from two layers: a light one, enriched with materials derived from surface production (planktonic foraminifers, radiolarians, and diatoms, the latter occurring only in the Cariaco deposits); and a dark one, where clay, detrital quartz, and organic matter are concentrated. In the two successions, these fundamental cycles can be regarded as varves; their genesis has been well documented in the Cariaco Basin (Peterson et al., 1991; Hughen et al., 1996). They testify to the seasonal extreme of a tropical climate with a dry season leading to upwelling conditions and high surface productivity and a wet season with increased fluvial runoff.
We used the average thickness of varves to compare the mean sedimentation rates of the La Luna Formation and the Cariaco succession. This required us to make the two deposits equally compacted and then correct them for approximately the same porosity. The chosen porosity (24%) is that of the middle Campanian at Site 1001 (southeastern edge of the Nicaraguan Rise), buried under 500 m of younger deposits (Fig. 17). This value is assumed to illustrate (1) the porosity of the Cariaco succession submitted to the same burial and (2) the porosity of the La Luna-Querecual Formation before its cementation resulting principally from a tectonic burial.
After this correction, the calculated sedimentation rates reached 13 m/m.y. for the La Luna Formation and 128 m/m.y. for the Cariaco succession. This great difference must certainly result from both surface productivity and terrigenous flux. The latter, however, could play a prominent part, given all the nutrients brought by terrestrial runoff (Haq, 1993).
The fabric of the lower order cycles (groupings of light and dark bundles) signifies the same alternation of high-productivity and high-terrigenous inputs as for elementary cycles, with a particularly important concentration of detrital quartz in the dark layers. These cycles are proxies for climatic variations affecting atmospheric and oceanic circulation. The most frequent periodicities registered in the Cariaco Basin (3-5, 20, and 40 yr), possibly related to El Niño and some solar cycles, are not so well recorded and sometimes less clearly expressed in the La Luna-Querecual Formation because of the slower sedimentation rate as well as the diagenetic transformations associated with tectonic activity.
A common occurrence in the two deposits is beige micritic ovoid patches, which may have formed from more or less continuous laminae precipitated under a bacterial control. The patches were lithified before the major compaction, and their early diagenetic formation is demonstrated by a first occurrence at 6.5 m below the top of the uppermost core from Hole 1002C.
Regarding diagenetic processes, abundant criteria reveal that the succession in the Cariaco Basin illustrates the initial state of the La Luna and Querecual Formations. This initial state is a clay and carbonate varved deposit, rich in calcareous and siliceous planktonic remains and in organic matter (the abundance of organic matter and preservation of varves indicate an anoxic environment). Between 0.10 and 1.00 m beneath the sediment/water interface, an oxidation of methane controlled by bacterial activity may be assumed (Raiswell, 1987) as well as carbonate precipitation in an environment becoming more and more alkaline.
Dissolution of calcareous planktonic remains during the decay of organic matter in the upper part of the Cariaco succession could yield part of the carbonate involved in the different types of precipitates: dolomitic layers and concretions from a depth of at least 28.0 m, discontinuous beige micritic layers from at least 6.5 m, and sparitic concretions most often initiated by foraminifer shells from at least 45.0 m. Calcareous infillings of foraminifers, which occur systematically in the La Luna-Querecual Formation, require at Cariaco a burial exceeding the 169-m depth reached in Hole 1002C.
All these precipitates occur before the major compaction. This is demonstrated especially in the La Luna-Querecual Formation by foraminifers devoid of calcite filling and crushed (Fig. 15), whereas the carbonate concretions have preserved the initial thickness of laminations (Fig. 18). Such occurrences are also present in the Cariaco succession. The richness in H2S yielded by sulfate reduction may explain the abundance of tiny pyrite grains occurring from the uppermost core of Hole 1002C and filling partly or totally organic voids like foraminifer shells.
It is not easy to verify if sparitic concretions can constitute an initial process leading to the meter-scale carbonate concretions present in the La Luna-Querecual Formation and individualized before the major compaction. Berner (1968) suggested that calcareous nodules, when rapidly formed, could originate from "adipocires": organic products generated by the decay of lipids and proteins of animal origin (e.g., fish and jellyfish). The weak representation obtained by drilling does not permit a choice of either hypothesis.
The abundance of diatoms in the Cariaco series may explain the near absence of siliceous concretions. The only concretion that includes some proportion of silica is that marking the bottom of drilling in Hole1002C at 167 mbsf. Conversely, the La Luna-Querecual Formation is relatively rich in cherty concretions, but diatoms are absent. This suggests that diatoms and radiolarians probably yielded the major part of the silica that formed some concretions in the La Luna-Querecual Formation. In addition, the siliceous concretions precipitated after the carbonate concretions against which they are deformed (Fig. 19). The recrystallization features occur mostly in the La Luna-Querecual Formation.
Despite important disparities in age, sedimentation rate, and depth of burial, the Upper Cretaceous La Luna-Querecual Formation and the Pleistocene-Holocene succession in the Cariaco Basin share a basic structure as revealed by annual cycles of deposits. The interpretation of these cycles, demonstrated as a result of upwelling in the Cariaco Basin, can be applied to the La Luna-Querecual Formation despite a major difference in sediment flux. Also, upwelling conditions have been deduced from a geochemical study of the La Luna Formation (Mongenot et al., 1996).
The grouping of varves in bundles, defining lower order subdecadal to decadal cycles, exhibits two sets of cyclic units:
These lower order cycles are themselves grouped into megacycles well represented in the La Luna-Querecual Formation. The more frequent groupings include five to seven units corresponding to the following brackets: 50-70 to 60-84 yr (for units provided with 10-12 varves each); 110-154 to 120-168 yr (for units gathering 22-24 varves). Two values are in evidence: 84 and 168 yr, close to the longest cycles of solar activity.
Finally, obliquity cycles seem have been recorded in the two successions. In the Cariaco series, they correspond to the alternation between predominantly bioturbated and predominantly laminated sections, driven seemingly by the alternation between glacial and nonglacial periods corresponding to periods of higher and lower sea-surface productivity in open basins (e.g., Bowles and Fleischer, 1985; Mortyn and Thunell, 1997; Abrantes et al. 1998). This is not necessarily coincident with the same CaCO3 percentage fluctuation, given the possible dissolution or dilution factors. In the Cariaco Basin, isolated from the open Caribbean Sea during glacial times, a reverse correlation between Quaternary climatic periods and productivity is assumed (Peterson et al., 1991). Therefore, laminated intervals of deposits have recorded the highest productivity. In the La Luna-Querecual Formation, the basic alternation between marls and limestones, or between marls and calcareous marls, seems also to be a result of an orbital control. This pattern has been recognized in all the Mesozoic successions, where it illustrates an alternation of higher (limestones) and lower (marls) sea-surface productivity (Fischer et al., 1985; Herbert and Fischer, 1986; Bottjer et al., 1986; Cotillon, 1991; Huang et al., 1993; Cotillon et al., 1994; Erba and Premoli Silva, 1994; Bellanca et al., 1996; Sageman et al., 1997) if the dilution feature is not considered. Consequently, a first attempt at correlation leads us to draw parallels between the carbonate beds of the La Luna-Querecual Formation and laminated intervals in the Cariaco series.
The striking similarities in the two series—in facies, structures, and depositional environments—despite very different ages (as much as 90 m.y.) can be justified by (1) a permanence of paleogeographic and latitudinal setting lying in a marginal position on the northern edge of the South America Craton, between 2°N and15°N latitude; and (2) common seasonal climatic controls characterized by an alternation of wet and dry conditions leading to seasonal atmospheric and marine currents. Thus, Marcellari and De Vries (1987) have demonstrated the occurrence of upwelling and anoxia in northwestern South America during the Late Cretaceous. Both events characterize a local prolonged occurrence of the Cenomanian-Turonian Oceanic Anoxic Event (Arthur et al., 1987). Beyond these regional factors, a permanence of El Niño events in the Pacific as well as rhythms of solar activity variations must be envisaged.