BAJADA DE JAGÜEL, NEUQUÉN PROVINCE, ARGENTINA: COMPLETE OR INCOMPLETE K/PLG BOUNDARY SECTION?

The Bajada de Jagüel section, located in the Province of Neuquén, central-west Argentina, was first described as a Cretaceous/Paleogene (K/Plg) boundary site on the basis of dinoflagellate cysts considered reliable biostratigraphic markers for this interval (e.g., Manumiella/Isabelidinium, Damassadinium californicum; and Senoniasphaera inornata). Detailed studies of dinoflagellate cysts show a significant diversity change consisting of species extinctions, survivals, and first appearances that is similar to dinoflagellate diversity change used to define the K/Plg boundary in other parts of the world. Dinoflagellate cysts and foraminifera indicate that the K/Plg boundary occurs at the base of a volcaniclastic layer where species characteristic of the lowermost Danian first appear. Radiometric analysis of the volcanic material gives an age of 65.97 ± 0.18 Ma. Geochemical analysis of the same sample set shows slightly elevated iridium content, but no distinct anomaly at the paleontological boundary. Study of Bajada de Jagüel terrestrial palynomorphs, which is reported here, shows that terrestrial vegetation experienced marked declines in abundance and diversity at the same stratigraphic horizon as the turnover in marine microfossils. In addition, a short term spore spike (probably bryophytes) appears above the turnover horizon. This evidence points to simultaneous disruption of both terrestrial and marine ecosystems in the Bajada de Jagüel region. These results support the interpretation that the K/Plg boundary event is represented at Bajada de Jagüel and that there is no significant hiatus present in the sequence. The data argue against the idea that the ash layer is a tsunamite or that the section is incomplete. The presence of a boundary layer composed of pyroclastic material makes this section unique. Signatures of local volcanism and global-scale impact are probably superimposed, creating a complex scenario of multiple causes and effects.