THE CRETACEOUS/PALEOGENE BOUNDARY IN ARGENTINA: NEW EVIDENCE FROM DINOFLAGELLATE, FORAMINIFERAL AND RADIOMETRIC DATING

Evidence of the KPg boundary in the Neuquen Province of Argentina was first presented on the basis of dinoflagellate markers, including the first appearance of the Danian species Senoniasphaera inornata in a boundary layer within the Jaguel Formation in the Bajo de Jaguel section. New evidence is presented here, based on the reinterpretation of this layer, on additional dinoflagellate species and on foraminiferal and radiometric dating. The boundary layer, originally interpreted as a tsunami deposit on the basis of field observations, proved to be of volcanic origin dominated by pyroclastics in an ash texture. Planktonic foraminifera, other than Guembelitria cretacea, are rare in the 40cm interval directly below the 17cm-thick ash, but the occurrences of Hedbergella monmouthensis, Heterohelix globulosa, Pseudoguembelina costulosa and Rugoglobigerigina rugosa indicate a late Maastrichtian age. Corroborative dinoflagellate species include the first appearances of Cyclapophysis monmouthensis and Pierceites pentagona. The co-occurrence of G. cretacea and H. monmouthensis within the ash bed and of foraminiferal Zone P alpha (Parvularugoglobigerina eugubina) immediately above suggests that the bed lies within Zone P0. Dinoflagellate evidence includes the co-occurrence of S. inornata and Damassadinium californicum within the ash. The 50cm interval directly above the bed contains a planktonic foraminiferal stratigraphy ranging from Zones P alpha through P1c. Benthic foraminiferal analysis indicates inner neritic shallow paleodepths of ca 50m below the boundary and similar paleodepths above. Feldspar from the ash bed yielded a 40Ar/39Ar plateau age of 66.0+/-0.5 Ma, referenced to 520 Ma for the Mmhb hornblende monitor standard.