新疆准噶尔盆地西北缘洪古勒楞组时代的新认识

NEW KNOWLEDGE ON THE AGE OF HONGGULELENG FORMATION IN NORTHWESTERN MARGIN OF JUNGGAR BASIN,NORTHERN XINJIANG

标准地点的洪古勒楞组长期以来一直被认为是准噶尔盆地西北缘，乃至整个新疆北部以海相为主的层位上最高的晚泥盆世晚期的沉积。然而牙形类和微体脊椎动物的研究证实，该组跨弗拉阶－法门阶（Ｆ－Ｆ）界线，它至少包括了一小部分弗拉晚期和主体部分为法门早期的沉积。两个牙形类带ＬａｔｅｒｈｅｎａｎａＺｏｎｅ和ＭｉｄｄｌｅｃｒｅｐｉｄａＺｏｎｅ被识别出，它们在Ｆ－Ｆ界线附近，并分别在大绝灭事件（在ｌｉｎｇｕｉｆｏｒｍｉｓＺｏｎｅ内）之前后。另一个层位更高的牙形类带ＬａｔｅｃｒｅｐｉｄａＺｏｎｅ也可能存在。同时还证实，长期以来一直被认为“正常层序”的标准地点的洪古勒楞组的层序是倒转的。由此，含植物大化石Ｌｅｐｔｏｐｈｌｏｅｕｍｒｈｏｂｉｃｕｍ等的陆相层在下（此层可能归于该组下伏的朱鲁木特组），而产牙形类和其它丰富的无脊椎动物的海相层在上。按牙形类年代带的年代地层时间表（Ｆｏｒｄｈａｍ，１９９２），该组距今约３６８－３６４百万年。至于它和邻近地区和布克河组的关系，由于后者发现了更高层位的牙形类，两者还不能完全对比。

At its type locality,the Hongguleleng Formation has been identified as the deposits of the latest Late Devonian and regarded for a long time as the highest unit predominated with marine beds of the Famennian age in northwestern margin of Junggar Basin,as well as in the whole northern Xinjiang.However,an analysis of the microvertebrates,and especially of the conondont faunas,indicates that the formation spanning the Frasnian-Famennian(F-F) boundary is composed partially of latest Frasnian and mainly of early Famennian deposits.Two of the global standard conodont zones,the Late rhenana Zone and the Middle crepida Zone,could exist at the type locality and at least occurred near the F-F boundary.The first zone could be below the F-F boundary and perhaps just before the global Late Frasnian mass extinction event happening in the linguiformis Zone,while the second zone is above the F-F boundary.Based on the occurrences of Palmatolepis glabra,etc.(Xu et al.,1988),the higher conodont zone,i.e.,the Late crepida Zone,is probably present in the formation.Meanwhile,the evidence of conodonts and microvertebrates confirms that the 'normal sequence' recognized for a long time for the formation at the locality should be reversed.Thus,the deposit containing the fossil plants Leptophloeum rhombicum,etc.should be lower than the conodont and microvertebrate-bearing marine beds which are characterized by the abundance of invertebrate fossils(brachiopods,bryozoans,cephalopods,corals,crinoids and trilobites)and plant microfossils(acritarchs and spores).According to the standard chronostratigraphic time-scale for the Frasnian to Famennian conodont chronozones quoted by Fordham(1992),the formation is about 368 to 364 Ma (late Frasnian-early Famennian) in age and cannot be correlated with the whole Hebukehe Formation of the adjacent areas,where some conodonts have been found at or above the Late crepida Zone.