长江三峡地区中、上泥盆统的分界

THE DELIMITATION BETWEEN MIDDLE DEVONIAN AND UPPER DEVONIAN IN YANGTZE GORGES

秭归周坪乡怀抱石剖面中泥盆统云台观组产吉维特期的植物化石 ,化石层离顶界仅 3.5m ;上泥盆统黄家磴组产弗拉斯期的标准分子 ,含化石的地层离其底界 1.3m ;另外 ,长阳榔坪乡三串岭剖面黄家磴组底部有一层鲕状赤铁矿 ,其上的泥岩中产腕足类Lingulasp .及上泥盆统的叶肢介化石。根据植物化石垂直分布特征 ,长江三峡地区中、上泥盆统的界线应划在Protolepido dendronscharyanum Barrandeinadusliana组合与Leptophloeumrhombicum Cyclostigmakiltorkense Archaeopteris组合之间。

Givetian and Frasnian plant fossils have been found from the Huaibaoshi section in Zhouping town,Zigui county,they locate in the bed 3.5m to the top of the Yuntaiguan Formation, Middle Devonian and 1.3m to the bottom of the Huangjiadeng Formation,Upper Devonian,respectively. Besides,an oolitic hematite bed has been found at the bottom of the Huangjiadeng Formation from the Sanchuanling section in Langping town,Changyang county,above which there occur Brachipoda and Estheriao in the mudstone beds. According to the plant fossil assemblage,the delimitation between Middle Devonian and Upper Devonian in the Yangtze Gorges area should locate beneath the oolitic hematite and between the bed of the Yuntaiguan Formation bearing Protolepidodendron scharyanum Barrandeina dusliana assemblage and the bed of Huangjiadeng Formation bearing Leptophloeum rhombicum Cyclostigma kiltorkense Archaeopteris assemblage