Cretaceous calcareous nannofossils from Exmouth and Wombat Plateau, Northwest Australia

Three sites drilled during Leg 122, Site 761 on the Wombat Plateau and Sites 762 and 763 on the Exmouth Plateau, provide a composite Cretaceous section ranging in age from Berriasian to Maestrichtian. Together, these sites contain an apparently complete, expanded Aptian-Maestrichtian record. Consistently occurring and moderately well-preserved nannofossil assemblages allow reasonably high biostratigraphic resolution. Our data indicate that traditional middle and Upper Cretaceous nannofossil biozonations are not entirely applicable in this region. In this investigation, we compare in detail the relative ranges of key Cretaceous nannofossil markers in the eastern Indian Ocean and in sections from Europe and North Africa. We have determined which previously used events are applicable, and which additional markers have biostratigraphic utility in this region. Significant differences in Campanian-Maestrichtian assemblages exist between the more northern Site 761 and Sites 762 and 763. Such differences are surprising, considering that these sites are only separated by 3° of latitude. We interpret them as marking a strong thermal gradient over the Exmouth Plateau region. Other results include the recovery of an expanded Albian-Cenomanian sequence containing a mixture of Austral and Tethyan floras, which will enable correlation of biozonations established for these two realms; the recovery of two condensed but apparently complete Cenomanian-Turonian boundary sections; correlation of Upper Cretaceous calcareous nannofossil biostratigraphy with magneto- and foraminifer stratigraphy; and correlation of portions of the Barrow Group equivalents to the Berriasian and Valanginian stages.

本研究依托第122航次钻探的3个站位——袋犬高原（Wombat Plateau）上的761站位，以及埃克斯茅斯高原（Exmouth Plateau）上的762与763站位，构建了一套年代跨度覆盖贝里阿斯期至马斯特里赫特期的复合白垩系剖面。上述站位共同保存了近乎完整且展厚的阿普第期至马斯特里赫特期地层记录。持续出现且保存状态较好的超微化石组合（nannofossil assemblages）可提供较高精度的生物地层分辨率。本研究数据表明，传统白垩系中、上部超微化石生物带并不完全适用于该区域。
本次研究详细对比了东印度洋区域与欧洲、北非剖面中关键白垩系超微化石标志层的相对延限范围，明确了哪些既往使用的生物地层事件可适用于本区域，以及哪些额外的标志层具备在该区域开展生物地层学研究的应用价值。
纬度更靠北的761站位与762、763站位之间，坎潘期至马斯特里赫特期的超微化石组合存在显著差异。考虑到这些站位的纬度间距仅为3°，该差异实属意外。本研究将其解释为埃克斯茅斯高原区域存在显著的温度梯度。
本研究的其他成果还包括：采获了一套展厚的阿尔布期至森诺曼期序列，其中混合了澳大利亚域与特提斯域生物群，可支撑这两个生物地理域的生物地层对比工作；采获了两套虽凝缩但近乎完整的森诺曼期至土仑期界线剖面；实现了白垩系上部钙质超微化石生物地层学与磁地层学、有孔虫地层学的相互对比；完成了巴罗群对应层位与贝里阿斯阶、凡兰吟阶的地层对比。