广西凤山、西林等地中三叠统上部双壳类分带研究

A STUDY OF BIVALVE ZONAL SUCCESSION FROM UPPER PART OF MIDDLE TRIASSIC IN NORTHWEST GUANGXI, S. CHINA

讨论广西西部中三叠统上部的双壳类生物地层序列,建立5个化石带;并附述黔南和滇东南晚三叠世1个双壳类带.这些带自下而上为:(1) Daonella (Longidaonella) producta-D. americana 带,属晚安尼早中期;(2) D. moussoni 高峰带,属晚安尼晚期;(3) D. rieberi-D. indica 带,属早拉丁期;(4) D. kotoi-D. jilongensis 带,属晚拉丁早期;(5) Halobia subcomata-D. varifurcata带,属晚拉丁晚期;(6) H. rugosoides-H. bifurcata 带,属早卡尼期.文内简要描述了各带的主要标志化石23种.

Especially abundant bivalves occur at many levels in the upper part of the Middle Triassic stratigraphic sections in northwest Guangxi, south China; there the fossiliferous strata assigned to the 'Hekou Formation' and Member 4 of the Baifeng Formation are usually recognized as formed under a turbidite environment on basin slope, containing the faunas monotonously grouped only by such pelagic forms as the bivalve genera Daonella, Posidonia and Halobia together with some ammonites. Based on faunal analyses the levels ranged from Upper Anisian to Upper Ladinian in age. Recently the bivalve fauna has been divided into five zones according to vertical occurrence and comparison with those of European, Asian and American ammonite standards, with discussion on the age of each zone. Incidentally, this paper records an Upper Triassic (Lower Carnian) bivalve zone from the 'Falang Formation' of the adjacent region (eastern Yunnan and southern Guizhou) which belongs to the same basin as northwest Guangxi in Triassic and bears Middle Triassic strata very similar to that of the present region with the Upper Triassic more developed. A bivalve (daonellan-halobian) succession in the Upper Anisian-Lower Carnian stratigraphic sequence is suggested as indicated in the following table