Distribution Characteristics of Trace Forssil Group in Turbidite Measure(Devonian) in the Northern Belt of West Qinling Mountains

Key words: 
• characteristics of distribution /
• trace fossil group /
• devonian northern belt of west

Abstract: 43 trace fossil ichnogenera have been found in Devonian Shujiaba Formation distributed in thenorthern belt of West Qinling Mountains, and they areAcanthorhaphe Arthophcus, Belorhaphe，Bostri -cophyton, Buthotrephis, Cochilidznus, Chondrites, Cosmorhaphe, Dictyodora，Fucusopsis, Glockeria，Gordia，Gran ula ria，Helicolithus, Helminthoida，Helminthopsis, Lennea，Loph octen ium，Megagrap tort，Neonereites, Nereies, Palaeohelminthoida，Paleod ictyon，Planolites, Protopaleodictyon,,Protovirgular-is，Rhabdoglyphus, Rhobdoichnus, Rusophycus(?)，Saerichnites, Sagittichnus, Scolicia，Skol ithos,Spirophycus,Spirorhaphe,Squamodictyon,Subloren,Subularia,Subhyllochorda，Taphrhelminthoida, Taphrhelminthopsis, Tuberculichnus, Urohelminthoida.All the host rocks of trace fossils mentioned aboveare to rbidi to depo sit s, s o i t iseasy toseethere exists a close link between the formation of trace fossil and turbidity process. According tothatthetime of pre- and post-turbiditynized, and they are current event, twsharply different fossil assemblages canbe recog-1) Trace fossil assenblag a fo rmed befo re turbidi ty ev entThey are produced in a low-energy abyssal environment in the interval of tubidity process, with twotypical feature① animals live and seek for food horizontally in the surface deposits,and trace fossil sa remainly preserved in the Te mudstone surfac of the Bouma sequence;②Generally, they appear incurve,snake and network shapesof which overAnd they are the typical members of "Nereites”features.70% ichnogeneraareformed before the turbidity event.fades with high degree of differentiation and abundance2) Trace fossil assemblage formed after turbidity eventThey are of lowabundance and degreespanning fadesty peof differentiation, and are characterized by the irregular”shalloww ater”and”members, generally preserved in the sandstone parts of thesequence or piercing sandstone. They are not in the leading role among the fossils discovered.Although the abundance and degree of differentiation of trace fossils formed before the turbidityevent :ire g ready higher than that formed after the event, they are of equivalently important role in theturbidite measure for their appearrance reflects the replacement of "fastlow-fast”sedimentation rate,i. e the whole process of the formation is a complete turbidite depositional sequence. Thus we canlink the two sharply lifferent trace fossil assemblages in accordance with their origin, and use them as apaleoichnological indicator of bethyal byssal turbidite measure.