| Fossil organic-walled microplankton, dinoflagellates primarily, from marine deposits of the Jaguel Formation at two locations, Lomas Coloradas and Bajada de Jaguel, Neuquen Province, Argentina, were analyzed for age assignment, paleoenvironmental interpretation and paleobiogeography of selected species.; The abundant and diverse dinoflagellate cyst assemblages show that the Jaguel Formation is of Late Maastrichtian/Danian age and contains the Cretaceous/Paleocene boundary. The precise location, at the Bajada de Jaguel section, of a biologic turnover that is interpreted to represent the Cretaceous/Paleogene boundary event occurs at the bottom of a 17cm water lain ash layer and is defined on the basis of the first occurrence of worldwide known Danian dinocyst biostratigraphic markers such as Damassadinium californicum and Senoniasphaera inornata. Detailed studies of the dinoflagellates recovered show that the group was more resistant than the calcareous microfossils to the Cretaceous/Paleogene boundary event and did not suffer mass extinction. This is probably because they are represented in the fossil record only by the encysted stage. However, dinoflagellates did undergo a sharp turnover with extinction of some species and migration of others. Locally, the last occurrence of Maastrichtian autotrophic, the gonyaulacacean affinity group, appears to be severely affected by the K/Plg boundary event. With the exception of Deflandrea galeata and the Isabelididnium-Manumiella complex, the peridinialean affinity group, considered as heterotrophic, survived the event.; Within 1m interval below the boundary, the first occurrence, peak and disappearance of the Isabelidinium/Manumiella group was documented. This group evolved through the Upper Cretaceous at high latitudes and its presence is interpreted as indicating a short-term episode of regression and/or water cooling prior to the boundary.; Within the 2m interval above the boundary, an unusual peak of acritarchs, single celled and colonial algae and small peridiniod species bloom are interpreted as the response to environmental stress in the aftermath of the boundary event.; Comparisons with studies in sections from the Southern Hemisphere suggest that temperature and/or salinity barriers controlled the paleogeography of the assemblages. The assemblages from Tierra del Fuego and Antarctica are very similar to one another, but differ from those of middle latitude analyzed in this study. |
