Chronostratigraphy of the Upper Cretaceous succession in the Middle Magdalena Valley of Colombia (Northern South America)

Abstract

The Upper Cretaceous succession in northern South America is significant because the related rock deposits are among the most prolific oil sources in the region. Although several geological studies have focussed on this period, we still have poor knowledge of the exact deposition time of the lithostratigraphic units. In this study, we constructed a chronostratigraphic framework based on U–Pb ages and time-calibrated carbon isotopic events to constrain the chronostratigraphy of the Upper Cretaceous formations. The carbon isotope curve of the La Luna-1 core allowed us to identify several oceanic anoxic events also identified in the stratotypes of the Upper Cretaceous formations. The La Luna-1 also preserves a complete record of ash beds, from which zircons were extracted for U–Pb dating. The stratigraphic distribution of the ash beds indicates a crucial influence of volcanic activity during the Late Cretaceous. Sedimentological descriptions and mineralogical analysis were used to establish the primary lithological differences between the units. The Upper Cretaceous formations can be categorised as mixed rocks comprising variable percentages of carbonates, quartz and clay minerals. They also show silicification and, to a lesser extent, dolomitisation and recrystallisation. The Salada, Pujamana, and Galembo formations range in age between the Cenomanian and Coniacian, whereas the overlying La Renta Formation spans from the Santonian to at least the middle Campanian. The top of the La Renta Formation corresponds to a regional disconformity that contacted the middle Campanian with the Maastrichtian.