Cretaceous planktonic foraminifera of the Kerguelen Plateau

An almost complete Upper Cretaceous sedimentary sequence recently recovered on the Kerguelen Plateau (southern Indian Ocean) during ODP Leg 183 was analysed for planktonic foraminifera in order to refine and integrate the zonal schemes previously proposed for the Southern Ocean area. Detailed biostratigraphic analysis carried out on holes 1135A, 1136A and 1138A (poleward of 50°S palaeolatitude during Late Cretaceous time) has allowed recognition of low and mid–high latitude bioevents, useful for correlation across latitudes, in addition to known Austral bioevents. The low latitude biozonation can be applied to Turonian sediments, because of the occurrence of Helvetoglobotruncana helvetica, which marks the boundary between Whiteinella archaeocretacea and Helvetoglobotruncana helvetica zones. The base of the Whiteinella archeocretacea Zone falls within the uppermost Cenomanian–Turonian black shale level in Hole 1138A. The stratigraphic interval from upper Turonian to uppermost Santonian can be resolved using bioevents recognized in the mid–high latitude sections. They are, in stratigraphic order: the last occurrence of Falsotruncana maslakovae in the Coniacian, the first occurrence of Heterohelix papula at the Coniacian/Santonian boundary, the extinction of the marginotruncanids in the late Santonian, and the first occurrence of Globigerinelloides impensus in the latest (?) Santonian. The remainder of the Late Cretaceous fits rather well in the Austral zonal scheme, except that Globigerinelloides impensus exhibits a stratigraphic range in agreement with its record at the mid–high latitude sections and extends further downwards than previously recorded at southern sites. Therefore, despite the poor recovery in certain intervals and the presence of several hiatuses of local and regional importance as revealed by correlation among holes, a more detailed zonal scheme has been obtained (mainly for the less resolved Turonian–Santonian interval). Remarks on some species often overlooked in literature are also provided.

近期在印度洋南部凯尔盖朗高原（Kerguelen Plateau）通过大洋钻探计划（Ocean Drilling Program, ODP）第183航次（Leg 183）获取的一套近乎完整的上白垩统沉积序列，针对其中的浮游有孔虫（planktonic foraminifera）开展了分析，以期优化并整合此前针对南大洋区域提出的生物地层分带方案。对1135A、1136A及1138A三个钻孔（晚白垩世时期古纬度位于南纬50°以南）开展的详细生物地层学分析，除识别出已知的南极区生物事件（Austral bioevents）外，还确认了可用于跨纬度对比的低纬度及中-高纬度生物事件（bioevents）。由于赫氏赫氏球旋虫（Helvetoglobotruncana helvetica）的出现，该低纬度生物分带可应用于土仑阶（Turonian）沉积物，该物种标志着始祖白垩虫（Whiteinella archaeocretacea）带与赫氏赫氏球旋虫带之间的分界。1138A钻孔中，始祖白垩虫带的底界位于森诺曼阶（Cenomanian）顶部-土仑阶底部的黑色页岩（black shale）层位内。从土仑阶顶部至桑托阶（Santonian）顶部的地层间隔，可借助中-高纬度剖面中识别出的生物事件进行细分。按地层顺序依次为：康尼亚克阶（Coniacian）中马斯拉克假抱球虫（Falsotruncana maslakovae）的最后出现（绝灭事件）、康尼亚克阶/桑托阶分界处乳突异旋虫（Heterohelix papula）的首次出现、晚桑托阶边缘球旋虫类（marginotruncanids）的绝灭，以及最晚（？）桑托阶硕大球拟抱球虫（Globigerinelloides impensus）的首次出现。晚白垩世的其余地层段基本符合南极区生物分带方案，仅硕大球拟抱球虫的地层分布范围与中-高纬度剖面的记录一致，且比此前在南半球站位的记录出现层位更低。因此，尽管部分地层段的取芯效果不佳，且通过钻孔间对比揭示存在多处局域性及区域性沉积间断（hiatuses），但仍建立了更为精细的生物地层分带方案（主要针对此前分辨率较低的土仑阶-桑托阶间隔）。本文还对文献中常被忽视的部分物种进行了评述。