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 <em>Dentalina alternata</em>. 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.

距尤卡坦半岛7800千米的巴西东北部波蒂采石场（Poty Quarry）的白垩纪-古近纪界线（Cretaceous–Tertiary boundary, KTB）剖面，是目前已发表记录中记载有希克苏鲁伯撞击事件（Chicxulub impact）海啸沉积（tsunami deposits）、撞击球粒（impact spherules）与铱异常（Ir anomaly）的最远露头点。本次基于沉积学（sedimentology）、生物地层学（biostratigraphy）、矿物学（mineralogy）与地球化学（geochemistry）的全新研究，未能验证此前的相关报道。侵蚀且经生物扰动的不整合面（disconformity）之下发育晚马斯特里赫特期（Maastrichtian）浮游有孔虫带（planktic foraminiferal zones）CF1，之上则为达宁阶（Danian）早期P1a(1)带，二者指示存在短暂沉积间断（hiatus），缺失了对应KTB界线的黏土（P0带）、铱异常以及标志性的碳同位素δ¹³C负偏移（δ¹³C excursion）。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，一套向上变细的微砾岩（micro-conglomerate）中富集再沉积的白垩纪有孔虫、次棱角状磷酸盐岩屑、方解石质与磷酸盐质球粒，且伴生达宁阶早期P1a(1)带组合，被解释为重力流沉积（gravity-flow deposit）。晚坎潘期（late Campanian）-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（benthic foraminifer）*交替齿形虫（Dentalina alternata）*的房室充填物（chamber infillings）。在Pla带薄层黏土中仅发现微弱铱异常，且无任何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为重要的实例，展现了北美至中美洲KT序列中复杂的全球环境与海平面变化——这类变化常被误判为撞击海啸事件。