Study on the Sequence of Abyssal Terrigenous Clastic Deposit of Shujiaba Formation (Devonian)in Northern Belt of Qinling Mountains

Key words: 
• bathyal to abyssal environment /
• Shujiaba Formation /
• northern belt of Qinling Devonian

Abstract: Shujiaba Formation in the northern belt of Qinling mountains belongs to the Middle-late Devonian epoch with a total thickness over 2000 m. With the exception of the upper carbonate rock section, the middle and lower part of the Formation are composed of extremely thick clastic depositional sequence, which is the host rock measure of minutely-disseminated gold ore deposits. The paper focuses on the study of depositional environment and factes characteristics of the elastic rock sequence.The sequence is lack of any indicators of shallow depositional environment,instead, some bethyal-abyssal factes are discovered, which are described as follows:1. The elastic sequence consists of proximal turbidite, thin-bedded turbidite and abyssal mudstone. And the latter two are well developed. No conglomerate, gravel grit and factes such as olistostrome etc. are found in the sequence.2. Contourite is frequently associated with the turbidite.3. On the surface of turbidite measure and abyssal mudstone, "Nereites",a trace-fossil facies relevant to abyssal turbidity, is found. The trace-fossil factes are composed of 41 ichnogenera,of which near 80% are the typical "deep water" ones,e. g.,Cosmorhaphe, Helicolithus,Helminthoida, Nereites, Palendictyon, Spirophycus, Taphrhelmiothoida,Taphrhelminthopsis. Subphylloch orda. and so on.Based on the above features, the authors believe that the Shujiaba Formation coas deposited in a bathyal-abyssal environment. Combining the distribution of water depth showed by the typical "deep water" ichnogenera,a maximum water depth of the sedimentation of the Formation is inferred to be 2000 m.In addition, on the basis of the analyses on the vertical sequence, lithofacies characteristics and paleocurrent data, we propose a non-sea-floor fan model for the Shu Jiaba Formation.