浙江桐庐上奥陶统堰口组岩石特征及沉积环境分析

浙江桐庐地区上奥陶统堰口组主要为一套灰色细砂岩、粉砂岩与深灰色-灰黑色泥岩、粉砂质泥岩的韵律性薄互层, 底部发育2～3层灰色中-厚层的砾岩, 其砂泥岩薄互层中发育有典型的脉状、波状和透镜状层理及韵律层理。这种砾岩过去一直被认为是底砾岩, 其上的砂泥岩韵律性薄互层被认为是典型的古代潮坪沉积。野外调查并结合区域地质资料分析结果表明, 堰口组沉积环境应为较深水斜坡环境,而非浅水海岸环境;其底部的砾岩应为富基质的碎屑流沉积,而不应是底砾岩;中上部的砂泥岩韵律性薄互层应为较深水斜坡上的内潮汐沉积,而不是浅水潮坪沉积。这一内潮汐沉积具有典型的对偶层双向递变层序, 可能为内潮汐的大潮和小潮周期性变化的结果,这是在中国首次识别出的古代内潮汐沉积。

PETROLOGICAL CHARACTERISTICS AND SEDIMENTARY ENVIRONMENT OF THE YANKOU FORMATION IN THE UPPER ORDOVICIAN AT TONGLU AREA IN ZHEJIANG PROVINCE

The Yankou Formation in the Upper Ordovician of Tonglu County in Zhejiang Province consists mainly of rhythmically thin alternating layers of gray fine-grained sandstones, siltstones and dark-gray mudstones, silty mudstones, in which the typical flaser, wavy, lenticular bedding and rhythmic stratification are well developed, with 2-3 intercalated beds of gray medium- to thick-bedded conglomerates in the base of the formation. In the past, these conglomerates and the overlapping rhythmically thin alternating layers of sandstones and mudstones had been considered as basal conglomerates and typical ancient tide-flat deposits respectively. The results of geological work in the field and study of regional geological data show that the sedimentary environment of the Yankou Formation should be deep-water slope rather than shallow-water seastrand. The conglomerates of the formation should be matrix-rich debris-flow deposits, and the rhythmically thin alternating layers of sandstones and mudstones should be internal-tide deposits developed on a deep-water slope. These internal-tide deposits have typical and symmetrical graded-couplet succession which may be related to the periodical changes of Neap-Spring tidal dynamics. This internal-tide deposits is the first example of ancient internal-tide deposits found in China.