On both sides of the transcurrent Bocono Fault (Fig. 11), the La Luna Formation is characterized by marly anoxic facies, including varied amounts of carbonate and siliceous concretions. Most Venezuelan oil is derived from this formation.
North of the fault, there are outcrops of alternating marls and marly limestones that are 200 m thick, black, laminated, and rich in such organic matter and phosphatic debris as fish scales. Here the formation is Cenomanian to Santonian in age and is subdivided into three members (Fig. 11).
South of the fault, the La Luna Formation is only 60 m thick and is Coniacian-Santonian in age. It is composed of a monotonous succession of indurated, micaceous, and laminated marls, overlying an eroded surface. Its richness in organic matter (as much as 6% of the sediment) and fish scales is conspicuous. The same facies continues eastward in the Querecual Formation, which thickens up to 740 m.
As for the Pleistocene-Holocene sediments of the Cariaco Basin, the analysis of the La Luna-Querecual Formation is based on observation of thin sections from cores drilled from the La Luna Formation as well as from the Querecual Formation (near Puerto La Cruz, Bergantin, and San Francisco [Fig. 2]), which is the eastern equivalent of the La Luna. Several orders of cycles have been identified and described as well as various types of organic remains and diagenetic processes. In this study, porosity measurements made by the Paris School of Mines Laboratory have been considered.
Cyclicity is expressed again through elementary and lower order cycles of various thicknesses. Elementary cycles are best expressed in the darkest intervals, where fine and discontinuous light layers are included. The dark layers, formed of a clay-organic matter complex, may be nearly black; they are discontinuous and deformed against organic remains such as planktonic foraminifers and radiolarians and also against detrital quartz (Fig. 12). The light layers, generally discontinuous, are calcareous and formed of planktonic remains (mainly foraminifers and coccoliths).
The thickness of elementary cycles varies from 0.01 to 0.08 mm. Given the hypothesis that elementary cycles are varves, the average varve thickness over the total thickness of the La Luna Formation (200 m), implies that ~16 m.y. of time are represented by this unit, compared to 15.4 m.y. for the biostratigraphic time scale (Gradstein et al., 1994). The assumption of elementary cycles as varves therefore seems reasonable to a first approximation.
Lower order cycles are made of (1) a dark bundle of elementary cycles resulting from a clayey-organic matrix and always relatively enriched in detritic quartz; and (2) a light bundle, generally less thick and sometimes recrystallized with calcite (Fig. 12).
The number of elementary cycles in these units is so variable that the corresponding durations vary from several years to several tens of years. Also, light bundles always include fewer varves than dark bundles. Note that counting elementary cycles is difficult because of their thinness and because of deformation resulting from compaction. Nevertheless, among 25 thin sections including laminations, five exhibit one lower order cycle with 10-12 varves, two show one cycle with 22-28 varves, one contains cycles with 22-24 varves, and two display cycles with an average of six varves.
Field sections of the La Luna-Querecual Formation illustrate the occurrence of larger cycles, denoting an orbital control of sedimentation. For instance, north of the Bocono Fault, outcrops of the lower La Aguada Member exhibit an alternation of dark gray, laminated limestones and shales, with limestone beds 20-60 cm thick and thinner shale layers (Tribovillard et al., 1991b). Assuming an average thickness of nearly 50 cm for the corresponding cycles, and given the mean sedimentation rate of the La Luna Formation (200 m/15.4 m.y. = 13 m/m.y.), a duration close to 38 k.y. is obtained for these cycles. This is in the range of the 39-k.y. period of one of the obliquity cycles at 72 Ma (Berger et al., 1989).
Beige micritic layers are 0.05 to 0.25 mm thick on average, always irregular, and rarely continuous. The latter behavior is represented by ovoid, tapered heaps, parallel to stratification, with rare organic inclusions such as foraminifers (Fig. 13). Beige micritic layers or heaps are sometimes superimposed and joined together, but usually they are isolated and sparse. They may correspond to precipitates linked to bacterial activity. The division of primarily continuous or semicontinuous layers into ovoid units may result from compaction, which also generates deformation and moves disrupted units. Similar structures have been depicted in the Lower Cretaceous of Deep Sea Drilling Project Site 535 in the Gulf of Mexico (Cotillon and Rio, 1984).
Sections of the La Luna-Querecual Formation, characterized by abundant carbonate and siliceous concretions (Fig. 14), generally include many ammonite and inoceramid shells. In thin section, debris from inoceramid shells, foraminifers (Hedbergellidae and Heterohelicidae), and calcitized radiolarians, but no diatoms, is observed. Planktonic foraminifers are always bigger and more numerous in the light layers of lower order cycles, where their chambers are filled with calcite, than in the dark layers.
Indications of compaction are frequent and varied, including crushing of biogenic structures like foraminifer shells (Fig. 15), deformation of the laminae of elementary cycles against radiolarians or foraminifers (Fig. 16), and pressure-dissolution features such as stylolites and compaction splits.
Planktonic foraminifers have generally lost their shells by dissolution. Only their calcitized molds remain, although these can also be affected by dissolution—all the more marked when they are located in dark layers rich in organic matter. The decay of this material, accompanied by a release of CO2, could explain the dissolution features. Two facts may be connected: the occurrence of siliceous concretions and the lack of diatoms.
Carbonate or siliceous concretions as long as 25 mm are abundant in thin sections; they can be several meters in length in field sections. These concretions testify to diagenetic precipitation leading to the genesis of a plentiful cement. This abundance is suggested by weak porosity of the sediment (only 0.25%-4.80% in five samples), proving an important burial partly of tectonic origin (overthrusting). By comparison, sediment nearly as old (middle Campanian), drilled at Site 1001 and buried under 480 m of younger formations, exhibits a porosity of nearly 24% (Fig. 17). Concretions occur early, before a major part of compaction (Tribovillard et al., 1991a). Proof is given by the above-mentioned concretions, reaching a length of 25 mm in thin section (Fig. 18). The matrix of the concretions is recrystallized sparite tending to unify the facies; in the latter, the clay-organic matrix is relegated to dark pelletic heaps. Nevertheless, remains of light- laminae-bearing planktonic debris can be recognized; they become more tightened at one extremity of the nodule. The tightness intensity allows us in this case to define a relative compaction of ~9.
Calcitic recrystallization has occurred in some structures as the filling of foraminiferal chambers. The latter occurred early because it resisted crushing by compaction. Carbonate precipitates other than calcite are rare; some dolomite occurs as sparse rhombohedric crystals. Siliceous layers are also rarely observed. Recrystallization can also affect the light layers of varves that grow thicker and display blurred boundaries. From this state, more pronounced epigenesis can affect the sediment.