Nereites Trace facies(Devonian) in the Northern Bel t of West Qingl ing Mountains and Its Environment Anal ysis

Key words: 
• environment analysis /
• Nereites /
• trace facies

Abstract: Widely exposed in the northern belt of West Qingling Mountains,the Shujiaba Formation (Devonian) isa set of extremely thick terrigenous clastic rock measures. Two sharply different viewpoints on the paleogeog-raphy environment of the formation still exist,i. e. shallow sea and deep sea facies. This paper placed emphasison the 43 ichnogenera discovered by the authors to discuss the paleoenvironment information provided bythem. The description and plates of the 43 trace fossils of the Shujiaba Fm. have been reported openly,and theyare: Acanthorhaphe，Arthrophus，Belorha phe，Bostricoph yton，Buthotrephis，Chondrites，Cochilichnus，Cos-morhaphe, Dictyodora，Fucusopsis, Glockeria，Gordia，Granularia，Helicolithus，Helminthoida，Helminthopsis，Lennea，Lophoctenium，Megagra pton，Neonereites，Nereites，Paleohelminthoida，Paleod ict yon，Planolites，Pro-topaleodicyon，Protovirgularia，Rhabdoglyphus，Rhabdoichnus，Rusoph ycus(?)，Saerichnites，Sagittichnus，Scolicia，Skolithos，S pirophycus，S pirorha phe，Sguamodictyon，Subphyllochorda，Sublorenzinia，Subularia，Ta phrhelminthoida , Ta phrhelmintho psis , T uberculichnus , Urohelminthaida. Some cognition was reached according to the study on the trace fossil groups mentioned above，that is，1)Classified in terms of trace facies,only a few ichnogenera existing in the near source turbidite of trace groupin the Shujiaba Fm. belonged to the Skolithos trace facies,but a majority of trace fossil ichnogenera belongedto the Nereites facies. They were preserved on the interface between Section Tc and Te in the thin-beddedturbidite formed in the distal area of turbidity current. According to the researches at home and abroad，itcould be seen that it is the unique feature of th trace fossil assemblage in vertical distribution in turbidite mea-sures or flysch facies that the trace facies of deep and shallow water co-exist in a single depositional profile，just like the Shujiaba Fm. 2)Classified according to the morphology，the already discovered trace fossils were predominated by thecurved , spiral, regular snakelike and netlike species. Nowadays，these complex and regular geometrical patternshave already become the important sign to distinguish the deep water trace fossils. 3 ) According to the ethological classif ication，the 43 ichnogeriera bassically，belonged to the feeding trailand trail. And the latter was generally in the leading role. This kind of trace fossils was mainly distributed atthe bottom of tranquil water body in abyssal-byssal areas. 4 ) Classified according to the distribution characteristics of water depth，the trace fossil of the ShujiabaFm. was predominated by the deep water members，of which the water depth distribution ranges from thedistal one to the byssal trough one. From the discussion above，it could be seen that the trace group in the Shujiaba Fm. was formed in abathyalbyssal environment related to turbidity current , and the largest water depth in deposition period couldsometimes exceed 2000m.