从底栖生物化石再释贵州独山泥盆系龙洞水段的时代

贵州独山的猴儿山组龙洞水段是一套浅海相碳酸盐沉积,盛产底栖牛物化石.通过对该段地层中所含四射珊瑚、床板珊瑚和腕足类等化石属种的详尽分析,再次证实龙洞水段的地质时代仍应属中泥盆世早期(Eifelian).而不是像某些根据痕迹化石或层序地层学的研究那样将它置于下泥盆统.

AGE RE-ASSESSMENT OF THE LONGDONGSHUI MEMBER (MIDDLE DEVONIAN)IN SOUTHERN GUIZHOU ON THE BASIS BENTHIC FOSSILS

Dushan,located in southern Guizhou,is an important area for the study of Devonian shallow marine strata.Abundant benthic fossils(corals and brachiopods)have been found in the Middle and Upper Devonian.The Longdongshui Member of the Houershan Formation was established by Wang et al.,in 1964.Based on rugose and tabulate corals and brachiopods,the Longdongshui Member has long been regarded to be of early Middle Devonian (Eifelian)age.Although no index or zonal conodont species have ever been found so far,the Eifelian age has been adopted by most geologists.However,Zhou Xiyun(1992)did not agree that the genera and species of Rugosa,Tabulata and Brachiopods from the Longdongshui Member are of Eifelian age.Afterwards,Wang et al.(1997)and Wu et al.(1997)also regarded the Longdongshui Member to be Lower Devonian in age on the basis of ichnofacies and sequence stratigraphy respectively.However,Zhu's data on the Rugosa,Tabulata and Brachiopods in 1992 are questionable.As for trace fossils and sequence stratigraphy,the formet mainly reflected marine environments as well as the sedimentary facies and the latter was prineipally related to sea level changes or transgressionregression cycles.The Longdongshui Member is characrterized by abundant rugosa,tabulata and brachiopod fossils.The studies on the Rugosa(Yu and Liao 1978),the Tabulata(Ye and Zhou 1975;Deng 1979),and the Brachiopoda(HOH and Xian 1975:Wang and Zhu 1979)afforded considerable evidence for the age of the member.The rugose corals Utaratuia sinensis and Sociophyllum minor are similar to Utaratuia laevigata and Sociophyllum glomerulatum from the Hume Formation(Eifelian)of northwestern Canada respectively.The tabulate corals Favosites goldfussi eifelensis and Pachycanalicula barrandei from Europe are of early Middle Devonian age.Beside,the brachiopods Athyrisina squamosaeformis,Eospiriferina lachrymosa,Acrospirifer fongi and Kwangsia perfecta were found not only in the Longdongshui Member of Guizhou,but also in the Eifelian Yingtang Formantion,Yarang Formation and"Baima Shale"of Guangxi as well as in the upper part of the Yangmaba Formation of Sichuan.Therefore,the Longdongshui benthic fauna strongly suggests an early Middle Devonian(Eifelian)age.