浙江桐庐晚奥陶世内潮汐沉积

Internal-tide Deposits of the Late Ordovician in Tonglu,Zhejiang

本文论述了在中国首次发现的内潮汐沉积的特征及其形成环境。该内潮汐沉积发现于浙江省桐庐县桐君山地区上奥陶统上部，具有典型的对偶层双向递交层序。据其岩类组合和层序特征，可将其定为韵律性砂泥岩薄互层相。其中发育有特征的脉状、波状和透镜状层理。其沉积特征表明形成于开阔的深水斜坡环境，这种对偶层双向递交层序可能为内潮汐的大潮和小潮周期性变化的结果。

Oceanographic studies more than 30 years revealed the existence of internal tides and internal waves in deep-water,however,internal-tide deposits were not recognized and studied systematically in the stratigraphc record until Gao Zhenzhong and K. A. Eriksson studied Ordovician deep-water sediments in the central Appalachians of the USA in the early 1990s. The term of internal-tide deposits was introduced for the first time in their paper. Two factes were recognized in the Ordovician of the Fincastle area, Appalachians: (1)bidirectional cross-laminated very fine grained sandstone factes, and (2)unidirectional cross-bedded and cross-lamimated medium-to fine-grained sandstone factes.Recently, internal-tide deposits were first found in the Upper Ordovician in the Tonglu area, Zhejiang Province,which are similar to that found in Appalachians in some sedimentary characteristics but also have some marked difference from that in Appalachians. During the late age of Late Ordovician the Tonglu area was located on the southeastern slope of a deep-water basin. The internal-tide deposits described in this paper were found within the Upper Ordovician Yankou Formation,which consists of a succession of thin alternate layers of sandstones and mudstones. This set of thin alternate layers displayed clear rhythmicity and could be classified into many sequences from fine-grained to coarse-grained,and to fine-grained again upwards. Each sequence consists of more than ten or tens of couplets of sandstones and mudstones with 0.2～1 m in thickness and could be refered to as a symmetrically graded couplet sequence. This kind of internal-tide depositswith this sequence could be called as a rhythemic thin alternate layer factes of sandstone and mudstone, which formed in deep-water slope environment and should probably be the product of internal tide sedimentation. Palaeoslope dipping towards northwest and southeast ward and northwest ward sedimentary structures indicate that the internal tide currents flew mainly alternatively up and down on the slope. Based on the study,the interanl-tide deposits were inferred to be formed in open slope where there was a longer relative static period (the period of slack water)during tidal current reversals,so mud-size materials could deposit and form muddy beds. The formation of each couplet of sandstone and mudstone might be the result of diurnal or semidiurnal tide. But the formation of symmetrically graded couplet sequence might be the result of periodical change of spring tide and neap tide.