Abstract

The biostratigraphy of the La Luna Formation has long been in dispute, despite the importance of this unit as the most important source rock for hydrocarbons in Venezuela. In this paper, calcareous microfossil biostratigraphy combined with carbon-isotope stratigraphy provides a stratigraphic framework for the formation, permitting revision of temporal and spatial patterns of deposition of organic-rich sediments. Detailed studies were conducted on a cored borehole and five outcrop sections distributed across the Maracaibo Basin of western Venezuela.

Planktic foraminifera have fair to good preservation, and nannofossils are poorly preserved. Many of the Cenomanian to Campanian planktic foraminiferal marker species are present, permitting the application of a traditional zonal scheme. An informal nannofossil biostratigraphic zonal scheme, based primarily on dissolution-resistant species, has been developed. Integration of these zonal schemes has enabled the correlation of changes in carbon-isotope ratios to the global C-isotope stratigraphy. The results have been used to estimate temporal variation in sedimentation rates as well as to reconstruct depositional patterns across the Maracaibo Basin. Deposition began in the eastern and northwestern part of the Maracaibo Basin in the middle Cenomanian and progressed towards the south and west, reaching the southwestern corner by the middle Coniacian. Although the uppermost part of the formation could not be dated, deposition in the eastern basin continued at least into the Coniacian and in the western basin at least until the middle–late Santonian. Sedimentation rates were highly variable with a period of condensed sedimentation at the Cenomanian–Turonian boundary.

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