EFFECTS IN THE SEDIMENTARY RECORD OF AN IMPACT-TRIGGERED TSUNAMI EVENT IN THE EARLY DANIAN: THE POTY SECTION, NE BRAZIL

The Poty quarry near Recife, in Pernambuco, northeastern Brazil, is currently the best exposed marine Cretaceous-Tertiary (K-T) boundary section known in southern low-latitude regions. The section provides evidence for an extraterrestrial bolide impact which occurred in the earliest Danian, corresponding to near or at the boundary between the P-α and P-1a foraminiferal zones (c. 100,000 to 200,000 years after the K-T transition). The dynamics of a bolide-impact-triggered tsunami and its imprint in the sedimentary record can be clearly inferred from the boundary beds. A sharp erosional surface marks the base of a marly limestone breccia, about 5.5 cm thick, resting upon the K-T boundary, and is overlain by a 50 cm thick graded bioclastic packstone and 15-20 cm of alternating beds of fine-grained limestones and marlstones. These beds, interpreted to be the record of an impact-triggered tsunami event, yield abundant reworked Cretaceous microfossils and the first occurrences of very rare lowermost Paleocene (lower Danian) planktonic foraminifera, ostracodes and dinoflagellates. These lowermost Danian strata also yield shattered fragments of quartz grains, which are commonly shock-metamorphosed with multiple intersecting sets of continuous and straight planar lamellae, as well as common microspherules (100-300 μm in diameter), the latter interpreted as impact-derived melt droplets, analogous to microtektites, which were altered authigenically shortly after deposition. Iridium analyses show maximum concentrations in the lowermost part of a thin hemipelagic claystone layer, about 75 cm above the K-T boundary. The dating of the Poty section suggests a secondary K-T boundary event. Chicxulub and the lowermost Danian impact-triggered tsunami beds at Poty seem to be distinct impact events, perhaps part of multiple cometary impacts during a few hundred thousand years around the K-T transition.