Two outcrop sections from the Maastrichtian Colón and Mito Juan Formations were analyzed for dinoflagellate cyst biostratigraphy. Samples from the Río Molino section (Cesar-Ranchería Basin, Colombia) represent a subset of samples from the 240 m-thick hemipelagic limestones and calcareous mudstones reported in Martínez’s (1989) foraminiferal study. The upper Campanian, lower Maastrichtian and uppermost Maastrichtian intervals were identified in this section based on some significant dinocyst biostratigraphic events, which are in agreement with planktonic foraminiferal data. In ascending stratigraphic order these events include the lowest occurrence (LO) of Areoligera spp., LO of Senegalinium spp., highest occurrence (HO) of Trichodinium castanea, HO of Xenascus ceratioides, LO of Cerodiniumdiebelii, LO of Trithyrodinium evittii, HO of Odontochitina operculata, LO of Phelodinium tricuspe, HO of Palaeohystrichophora infusorioides, LO of Glaphyrocysta perforata, LO of Disphaerogena carposphaeropsis and LO of Manumiella seelandica. Relative higher abundance of terrestrial-derived organic matter in the upper Colón Formation is consistent with a marine regression and consequent shallowing of depositional environments at the end of the Cretaceous in northwestern South America.

The Río Loro section (Mérida Andes, western Venezuela) includes 980 m of Maastrichtian, open marine, marginal marine and deltaic siliciclastic and calcareous sediments. The occurrence of Phelodinium tricuspe and Paleocystodinium australinum below the highest occurrence of Hystrichodinium sp. and Odontochitina sp. indicates that the base of the studied interval in this section is early Maastrichtian in age. The K/T boundary in the Río Loro section is placed within a 11.5 m-unexposed interval, which is above the lowest occurrence of G. perforata and just below the lowest occurrence of the Danianmarker, Damassadinium californicum. Very high sedimentation rates during the K/T transition in northern Colombia and western Venezuela may have produced one of the most expanded K/T Boundary sections reported in the literature. Spiniferites–Achomosphaera Group and peridinioid cysts dominate most of the dinoflagellate cyst assemblage in both sections. The peridinioid assemblage reported here is assigned to the tropical to subtropical Malloy suite of Lentin and Williams (1980).

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