Maastrichtian Microfossils of the Shallow Marine Umir Formation, Northeastern Colombia

Abstract

During the Late Cretaceous, northern South America was characterized by broad epicontinental seas, with variable surface productivity and changing bottom-water oxygenation. Global sea-level fluctuations and local tectonic shifts caused their disappearance in the latest Cretaceous. We present an integrated micropaleontological and geochemical study of a section comprising the Umir Formation and its lower stratigraphic contact with the La Luna Formation, in the Middle Magdalena Valley, northeastern Colombia. Foraminiferal assemblages were moderately diverse and mainly dominated by benthic taxa, characterizing the biozones Siphogenerinoides bramlettei and Ammobaculites colombiana (Maastrichtian). Planktonic foraminiferal assemblages were less diversified, being species assigned to Heterohelicidae and scarce keeled forms (Globotruncana spp.) the most recurrent taxa. Ostracod recovery was very scarce, and we could only identify the genus Actinocythereis. In contrast, calcareous nannofossil assemblages were moderately diversified along the section, and composed of typical Late Cretaceous low-latitude taxa such as Micula staurophora, Cribrosphaerella ehrenbergii, Gartnerago segmentatum. The identified microfossils indicate a transition from middle-inner shelf conditions, with moderately oxygenated bottom waters within the La Luna Formation, to a shallower marine setting within the Umir Formation. This interpretation is supported by Sr/Ba and log(Fe/Ca) ratios measured in bulk sediment, which indicate increased continental runoff and terrigenous input in the upper part of the section. Moreover, a significant biotic turnover was identified at the base of the section, suggesting the presence of a xenoconformity at the La Luna-Umir contact, which has been previously described and proposed as a regional stratigraphic feature.