| Summary: | The Late Cretaceous Umir Formation in Middle Magdalena Valley Basin (MMVB) was recently acknowledged as a new target. However, there is little known about the regional distribution of the Umir Formation since it is affected by structural complexity and a major regional unconformity. To improve biosteering of the Umir Formation and reduce exploratory risks, a detailed biostratigraphic analysis of the Umir Formation in the MMVB is presented. Eighty samples from four cores drilled in the Central Eastern MMVB were analyzed for palynological content. These cores represent 976.6 meters of the Middle to Upper Umir Formation.
The sediment yielded a good recovery of pollen, spores and dinoflagellate cysts of Maastrichtian age, typical of Northern South America. The assemblage is dominated by species such as Echimonocolpites protofranciscoi, Proteacidites dehaani, Buttinia andreevi, Spinizonocolpites baculatus, Proxapertites spp., Colombipollis tropicalis, Arecipites regio, Echitriporites trianguliformis, Echitriporites suescae, Psilatriletes spp., Scabratriletes granularis and Gabonisporis vigorouxii. Dinoflagellate cyst assemblages include abundances of Andalusiella and Palaeocystodinium genera and skolochorate cysts dominated by Achomosphaera - Spiniferites complex. Manumiella seelandica, a dinoflagellate cyst that is a latest Maastrichtian global marker is recorded for the first time in the MMVB.
Three informal zones (A, B and C) are proposed. Zone A covers the Middle Umir, and Zones B and C characterize the Upper Umir member. Alternating spikes of peridinoid (Andalusiella and Palaecosytodinium genera) and skolochorate cysts, and abundances of Echimonocolpites protofranciscoi and Proxapertites genus characterize layers close and within the Upper Umir sandstones, showing potential to assist correlations and to evaluate lateral continuity of this new reservoir.
Based on the palynological assemblages, it is suggested that the Middle Umir Formation was deposited in a lagoonal environment with coastal swamps and estuarine conditions that evolved into a semi-restricted bay with river influx for the Upper Umir formation.
Palynological record of the Umir Formation reflects both the drastic replacement of the gymnosperms by the angiosperms and the Late Cretaceous provincialism of peridinacean dinocysts.
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