峡东震旦系陡山沱组燧石中的带刺微化石及其地质意义

SPINOSE ACRITARCHS FROM THE TOUSHANTOU FORMATION IN THE YANGTZE GORGES AND ITS GEOLOGICAL SIGNIFICANCE

近十年来,在我国南方震旦系陡山沱组先后发现大量生物化石,包括海绵骨针、藻类化石及疑源类等。本文报道了最近在陡山沱组发现的带刺微化石,其数量可观,刺的形态复杂多样。这些化石中,有晚前寒武纪中常见的属种,也有以往被认为属寒武纪的属种,这一组合可能代表晚前寒武纪—寒武纪过渡时期的独特生物组合。

The Toushantou Formation in the Yangtze Gorges is composed of dark brownto greyish black dolomite with chert, interbedded with shale, deposited in a sub-tidal, shallow, marine environment. It unconformably overlies the tillite of theNantou Formation and is conformably overlain by the dolomite and limestone ofthe Tongying Formation. An excellently preserved heterogenous assemblage of filamentous coccoidal andspinose microfossils has been discovered in the cherts of the Toushantou Formation. The planktonic microfossils include a variety of morphotypes. Among themost conspicuous and abundant plankters found in thin section are diverse spinosevesicles, ranging from 30 gm to 850 μm in diameter. The important ones amongthem are the spinose acritarchs resembling Baltisphaeridium, hitherto mostly dis-covered from the Early Cambrian and younger rocks. However, other previously reported acritarchs, and vendotaenids and pretrilobite small shelly fossils fromthe conformably overlying Tongying Formation indicate a Late Precambrian ageof the Toushantou Formation. This mixed benthic-planktonic assemblage preservedin the silicified nonstromatolitic cherts provides a unique sample of diverse micro-biotic elements of the initial stage of the Phanerozoic Eon. Recently, a similar microfossil assemblage has been discovered from the Tou-shantou Formation in Changyang County.In comparison with the microfossilassemblage found in the cherts of the Toushantou Formation in the Eastern YangtzeGorges (Wangfenggang and Tianjiayuanzi sections) by Dr. S. M. Awramik andthe writer, thece are obviously less filamentous fossils, with only some Salomesvalbardensis Knoll,Eomycctopsis rebusta Schopf, E. sp., Siphonophycus sp., andGunflintia minute Barhoorn; however, Obruchevella has not yet been found there. Of the coccoidal microfossils detected, the more abundant pluricellular varietiesin the Eastern Yangtze Gorges are absent, such as Myxococcoides, Palaeoanacystis,Tetraphycus and Mcrismopedia-like fossils, and only a few solitary coccoids havebeen found. The spinose planktonic microfossils are more abundant, including Tianzhushania,Comasphaeridium, Trachyhystrichosphaera, Baltisphaeridium and Sumenosphaera whichhave commonly been found in the Eastern Yangtze Gorges. in addition, some newfossils with complex spines have been discovered in Changyang County. The above-mentioned differences between the assemblage from Changyang andthat from the Eastern Yangtze Gorges may reflect differences in the depth of seawater and in sedimentary environment.