Ocean Drilling Program, United States

Two geographically close hemipelagic deposits are compared: (1) the Pleistocene-Holocene part of the Cariaco Basin succession off Venezuela drilled in Hole 1002C and (2) the Upper Cretaceous (Cenomanian to Santonian) La Luna-Querecual Formation, which crops out in northern Venezuela from the Sierra de Perija, west of the Gulf of Maracaibo, to the Gulf of Paria eastward. This analysis is based on thin-section descriptions, porosity data, and examination of core photographs. Despite their very different ages, these formations are similar in facies, structure, and diagenetic behavior. Their facies are calcareous black shales rich in organic matter and planktonic remains such as foraminifers, radiolarians, and diatoms. Their structures result in a high-frequency cyclic fabric with superimposed units ranging from obliquity cycles to annual varves, as well as several types of solar and pluriannual El Nino-type cycles. In the Cariaco succession, the frequencies of pluriannual cycles have been determined by a statistical study on deposits with standardized compaction and sedimentation rates to convert the thickness of cycles to a scale proportional to their duration. Their diagenetic behavior is signaled by (1) early carbonate precipitation under bacterial control leading to beige micritic layers, (2) carbonate and siliceous concretions before major compaction, (3) recrystallization, (4) deformation from compaction, and (5) dissolution. The main observed lithologic differences between the Cariaco Basin and the La Luna-Querecual successions result from the weak diagenetic evolution for the Cariaco series. This evolution reached an ultimate state for the La Luna-Querecual Formation because of tectonic overburial, leading to recrystallization and precipitation of large carbonate and siliceous nodules, and to cementation. Thus, it is appropriate to consider the Cariaco series as an illustration of the initial state of the La Luna Formation. Major common characteristics relative to the genesis of the two deposits signify the similar anoxic depositional environment (900 and 500 m deep for the Cariaco and the La Luna-Querecual Formations, respectively), similar paleogeographic and latitude (2 degrees -15 degrees N) settings on the Guyanese Craton Margin, and similar climate and current controls leading to varves and lower order depositional cycles. Cycles are manifested as an alternation of light layers enriched with planktonic microfossils deposited during dry periods and of dark layers composed of clay-organic complexes with a maximum of detrital quartz, deposited during wet periods. The major discrepancy in the sedimentation rates (128 m/m.y. for the Cariaco series, 13 m/m.y. for the La Luna Formation) of equally compacted deposits fits principally with different tectonic and sea-level histories in the two series. Indeed, the sedimentary fluxes were low during the Late Cretaceous, before the Laramide orogeny. They were much more important in the Pleistocene-Holocene because of enhanced land erosion due to block faulting in the Venezuelan Andes and the Guyanese Craton and to a lower sea level.