特提斯喜马拉雅白垩纪层序地层分析

Sequence stratigraphy of the Cretaceous strata in the Tethyan Himalayas

白垩纪是新特提斯演化过程中一个极其重要的阶段,其沉积蕴涵着新特提斯早期演变的丰富信息。在对典型剖面进行层序地层分析的基础上,结合前人的研究成果,笔者分别对特提斯喜马拉雅沉积带南、北两亚带白垩系进行较为详细的露头层序地层学研究,在沉积南带识别出为24个三级层序、5个层序组(亚二级层序)、2个二级层序(中层序),在北亚带识别出22个三级层序、5个层序组(亚二级层序)、2个二级层序(中层序)。特提斯喜马拉雅早白垩世层序地层总体表现为海进的退积序列,反映了特提斯洋壳的扩张阶段;晚白垩世层序地层总体表现为海退的进积序列,反映了特提斯洋盆地持续收缩和长期海平面逐步下降的过程,应是洋壳俯冲阶段的产物。整个白垩纪显示出一次极其明显的海水进退旋回,是特提斯洋从扩张到收缩这一演化过程的客观反映。由对层序特征、沉积特征及古生物特征等的分析所得出的特提斯喜马拉雅在白垩纪的海水进退规程,与同期的全球海平面的变化基本一致。

The Cretaceous period is believed to be an important stage in the evolution of the Neo-Tethys. The sediments rested on during this period recorded the early evolution of the Neo-Tethys. The detailed outcrop sequence stratigraphic analysis has been made for the Cretaceous strata in the southern and northern subzones of the Tethyan Himalayan zone on the basis of the examination of the representative sections in combination with the pre-existing data. The following sequences have been distinguished: twenty-four third-order sequences, five sequence sets (subsecond-order sequences), two second-order (mesosequences) for the southern subzone, and twenty-two third-order sequences, five sequence sets (subsecond-order sequences) and two second-order (mesosequences) for the northern subzone. In the Tethyan Himalayas, the Early Cretaceous strata display an overall transgressive retrogradational sequence, representing a spreading phase of the Tethyan oceanic crust, whereas the Late Cretaceous strata display an overall regressive progradational sequence, representing a steadily collapse of the Tethyan oceanic basin and long-period falling of sea level, and thus should be assigned to the products created during the subduction phases of the Tethyan oceanic crust. Collectively, the well-defined transgressive-regressive cycles of the Cretaceous strata recorded the transition from the spreading to the decrease of the Tethyan Ocean. These evidences are in general agreement with the coeval global sea-level changes.