THE K/T BOUNDARY AT POTY QUARRY AND PUNTE DO FUNIL, BRAZIL

Sedimentological and biostratigraphic data from a new core drilled at the Poty Quarry and outcrop sections at Poty and Punte do Funil, provide critical new information on the Cretaceous-Tertiary (K/T) transition in Brazil. During the last 500 ky of the Maastrichian (zones CF1-CF2) marlstones were deposited in a middle neritic environment with low oxygen bottom conditions as indicated by benthic foraminifera. Planktic foraminiferal assemblages are of low diversity, dwarfed, dominated by biserials and sporadic blooms of guembelitrids. Globotruncanids and other large planktic foraminifera that thrived in stable and well oxygenated waters are absent in some levels, and dwarfed when present. These assemblages indicate high-stress conditions.

An 80 cm interval below a 50-80 cm thick conglomerate bed contains an extreme abundance of large burrows in random directions that suggest failed escape traces of invertebrates trapped beneath the sudden influx of the conglomerate. The conglomerate contains phosphatic pebbles, benthic and planktic Cretaceous and early Danian foraminifera. The Cretaceous species are reworked from the late Maastrichian. The early Danian assemblages are typical of the P. eugubina zone P1a and indicate that the conglomerate was generated in the early Danian zone P1a and is thus not of K/T age. A hiatus marks the K/T boundary. Thus, the conglomerate bed, which was previously interpreted as tsunami deposit generated by the Chicxulub impact, appears to be a typical mass flow deposit correlative with the early Danian P1a sea-level regression. Ir peaks of 0.5 and 0.6 ppb are present at the base and top of the conglomerate bed and may originate from the eroded K/T clay layer. A minor (0.3 ppb) Ir enrichment and 10-12 ppb in Pt in an overlying thin clay layer likely represents concentration in a condensed interval. No evidence of Chicxulub ejecta, such as impact spherules or their alteration product cheto smectite, was found and therefore earlier reports in this regard cannot be confirmed. Abundant calcite spherules, representing infillings of algal cysts, are common throughout the Maastrichtian interval and in the conglomerate deposit and may have been mistaken as impact spherules. These data indicate that the K/T boundary is missing in the Brazil sections due to a hiatus.