滇西保山地区晚泥盆世至早石炭世早期牙形石生物相

Late Devonian to Early Early Carboniferous Conodont Biofaeies in the Baoshan Region,Western Yunnan

一、绪言保山地区是滇西泥盆系和下石炭统出露的主要地区之一,以前已有不少报道。近年来,由于该地区特殊的大地构造位置,引起了国内外广大地质工作者的密切注意,因而对泥盆石炭纪地层及其古生物的研究也更加深入。保山地区的上泥盆统目前暂分为何元寨组和整合其上的大寨门组,前者底部尚属中泥盆统(龚大明等,待刊)。下石炭统下部称鱼洞组。何元寨组在其命名剖面施甸何元寨主要是一套中层灰岩和泥灰岩,厚约220m,腕足、珊瑚、层孔及苔藓动物丰富,向东北至大寨门。

Nearly 20 pieces of conodont samples,in each of which 20 Pa or I elements have been deter- mined,were collected from two sections in the Baoshan region,western Yunnan and checked by the percentages of nearshore and offshore elements.On the basis of conodont biofacies analysis,in combination with the data of macrofossils,lithology and sedimentology,some conclusions may be drawn as follows. (1)Polygnathus predominated in the Baoshan region during the Frasnian,then came second the nearshore elements such as Icriodus and Ozarkodina.Ancyrodella,a genus which used to occur in signif- icant quantities in the fore-reef environments,occasionally became the main element in the commu- nity.The offshore element Palmatolepis which seems less important may be ascribed to the Polygnathus biofacies on the whole.Nevertheless,it is different from that proposed by Klapper and Lane (1985)due to the lack of the endemic elements and the abundance of the most cosmopolitan species of Polygnathus,suggesting shelf to upper slope environments.The Heyuanzhai region is thought to be located on the nearshore side because of the higher percentage of shallow-water conodonts.The eu- static sea level fluctuated frequently,but within a narrow range during this period.The Dazhaimen. region was confined to a moderate water depth condition during the Famennian.Till the late Tour- naisian,both Dazhaimen and Heyuanzhai were in a relative offshore environment. (2)The data of macrofossils,lithology and sedimentology have revealed that Dazhaimen and Heyuanzhai were in low-(x zone)and high-energy environments,respectively during the Frasni- an,whereas during the Tournaisian,the similarity between lithology and faunas in the two regions outlined above pointed to a deep-water environment. (3)Conodont biofacies analysis has also been supported by field observations.During the Late Devonian,the Baoshan region was in shelf to slope environments,Sea water became deeper and deeper from Heyuanzhal in the southwest to Dazhaimen in the northeast.Transgression began from the lower asymmetricus subzone,then had the highest sea level at the beginning of An.triangularis zone,and lastly regression followed on by the end of the Frasnian,up to now no zone below Siphon- odella has been distinguished.This implies the missing of the lower Tournaisian.Transgression took place again during the middle Tournaisian,and it may be compared in scale with the Frasnian one. By the end of the Tournaisian,the whole area of Baoshan was located in the outer shelf environ- ments of the offshore,or even in deeper ones.