The Cretaceous–Tertiary boundary (KTB) transition in NE Brazil

At 7800 km from Yucatan the Cretaceous–Tertiary boundary (KTB) transition of the Poty Quarry, NE Brazil, is the most distant locality with published accounts of Chicxulub impact–tsunami deposits, impact spherules and Ir anomaly. New investigations based on sedimentology, biostratigraphy, mineralogy and geochemistry fail to confirm these reports. Latest Maastrichtian planktic foraminiferal zones CF1 below an erosive and bioturbated disconformity and early Danian zone P1a(1) above indicate a short hiatus, with the KTB clay (zone P0), the Ir anomaly and the characteristic negative δ13C excursion missing. The disconformity coincides with the globally recognized latest Maastrichtian sea-level fall. Above the disconformity, an upward-fining micro-conglomerate with abundant reworked Cretaceous foraminifera, sub-angular phosphate clasts, calcitic and phosphatic spheroids along with an early Danian zone P1a(1) assemblage is interpreted as a gravity-flow deposit. Common spheroids throughout the late Campanian–Maastrichtian appear to be chamber infillings of the benthic foraminifer Dentalina alternata. Minor Ir anomalies in thin clay layers of zone Pla and no evidence of the Chicxulub impact reveal that the Poty Quarry section remains a very important example of the complex global environmental and sea-level changes observed in KT sequences from North America to Central America that are commonly misinterpreted as impact–tsunami events.

巴西东北部波蒂采石场的白垩纪-古近纪界线（Cretaceous–Tertiary boundary, KTB）剖面，距尤卡坦半岛7800千米，是目前已发表文献中记录的、产出希克苏鲁伯撞击事件（Chicxulub impact）海啸沉积、撞击球粒与铱异常（Ir anomaly）的最偏远位点。本次依托沉积学、生物地层学、矿物学与地球化学手段开展的全新研究，未能证实此前的相关报道。侵蚀且受生物扰动的不整合面之下，发育晚马斯特里赫特期浮游有孔虫带CF1；不整合面之上则为达宁阶早期P1a(1)带，二者指示存在短暂沉积间断，缺失了作为KTB标志的黏土（对应P0带）、铱异常与特征性δ¹³C负偏移。该不整合面与全球公认的晚马斯特里赫特期海平面下降事件相吻合。不整合面之上的向上变细型微型砾岩，赋存丰富的再沉积白垩纪有孔虫、次棱角状磷酸盐岩屑、方解石与磷酸盐球粒，且伴生达宁阶早期P1a(1)带生物组合，被解释为重力流沉积。整个晚坎潘期-马斯特里赫特期地层中广泛产出的球粒，被认为是底栖有孔虫交替瓶形虫（Dentalina alternata）的房室充填物。Pla带薄层黏土中仅存在微弱铱异常，且未发现希克苏鲁伯撞击事件的相关证据，这表明波蒂采石场剖面仍是极具价值的实例：它清晰展现了北美至中美洲KT序列中常被误判为撞击海啸事件的复杂全球环境与海平面变化过程。