哈萨克斯坦东部斋桑盆地恐龙蛋壳化石与白垩系-古近系界线

DINOSAUR EGGSHELL AND THE CRETACEOUS-PALEOGENE BOUNDARY IN THE ZAYSAN BASIN,EASTERN KAZAKSTAN

在晚白垩世期间,哈萨克斯坦东部的斋桑盆地形成了一个湖盆,一直延续到现在。古斋桑湖中最老的沉积是Tayzhuzgen组,由厚达136m的泥岩、粉砂岩、页岩、石英砂岩、砾岩和淡水石灰岩红层组成。按照当地的地层名称,Tayzhuzgen组包括Manrak、Tayzhuzgen、Aulisbulak、Aktobe、Dysum bay和Kiin Kerish层。在斋桑湖以南的Tayzhuzgen河附近一个地点的Tayzhuzgen组下部,发现了恐龙蛋壳碎片,蛋壳至少包括有两个主要类型:圆形蛋类和长形蛋类。恐龙蛋壳指示了Tayzhuzgen组下部的时代为晚白垩世(可能是马斯特里赫特期)。在本组的较高层位,发现了晚?古新世的植物化石,因此斋桑盆地的白垩系-古近系界线在Tayzhuzgen组内部。在Tayzhuzgen组中,白垩纪与古新世化石层之间的地层间隔只有约50m,由此推测在Tayzhuzgen组内部可能存在一个或多个不整合,或是本组的中部为凝缩沉积,即在白垩系-古近系界线处的沉积速率非常缓慢。Tayzhuzgen组顶部有一个显著的不整合,那里中或晚始新世(Arshantan,Irdinmanhan或Egilian陆生哺乳动物"期")的地层存在于含哺乳动物化石剖面的底部。斋桑盆地的白垩系-古近系界线位于古斋桑湖沉积剖面的下部。这不是一个完整的界线剖面,可能是一个包含有一个或多个不整合和/或一个由慢速沉积造成的凝缩剖面。

During the Late Cretaceous, a lake basin developed in the Zaysan Basin of eastern Kazakstan and has been there till the present. The oldest deposits of paleo-Lake Zaysan are the Tayzhuzgen Formation, as much as 136 m of red-bed mudstone, siltstone, shale, quartzose sandstone, conglomerate and freshwater limestone. The Tayzhuzgen For- mation encompasses strata of the Manrak, Tayzhuzgen, Aulisbulak, Aktobe, Dysumbay and Kiln Kerish svitas of the local stratigraphic nomenclature. Dinosaur eggshell fragments occur in the lower part of the Tayzhuzgen Formation at one locality near the Tayzhuzgen River south of Lake Zaysan. At least two primary kinds of eggshell are present： sphe- roolithid and elongatoolithid. The dinosaur eggshells indicate a Late Cretaceous （likely Maastrichtian） age for the lower part of the Tayzhuzgen Formation. Higher in the formation, late？ Paleocenc fossil plants are present, so the Creta- ceous-Paleogene boundary in the Zaysan basin is within the Tayzhuzgen Formation. The stratigraphic interval between Cretaceous and Paleocene fossil horizons in the Tayzhuzgen Formation is only - 50 m thick, and this suggests that there may be one or more unconformities within the Tayzhuzgen Formation, or that the middle part of the formation is ＂con- densed＂, which means that sedimentation rates were very slow across the Cretaceous-Paleogene boundary. There is a substantial unconformity at the top of the Tayzhuzgen Formation, where strata of middle or late Eocene （Arshantan, Irdinmanhan or Egilian land-mammal ＂ages＂） age are present at the base of the fossil-mammal-bearing section. The Cretaceous-Paleogene boundary in the Zaysan Basin is located stratigraphically very low in the sedimentary section deposited by paleo-Lake Zaysan, and it is not a complete section of the boundary, but likely one marked by one or more unconformities and/or a condensed section produced by slow sedimentation.