西秦岭北带泥盆系舒家坝组深海陆源碎屑沉积序列的研究

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

西秦岭北带泥盆系舒家坝组属中晚泥盆世，地层总厚超过２０００ｍ。除最上部碳酸盐岩段外，主要是细、粉砂岩与页（泥）岩不等厚五层组成的沉积序列。本文从以下三个方面论证该序列形成于半深海－深海环境：１）时空上主要是由分布稳定的细粒浊积岩组成；２）等深积岩与浊积岩相伴而生；３）在浊积岩系和深海泥页岩中发现了高分异度、高丰度的遗迹化石共４１个属，其中占遗迹总属的８０％为深水型分子。此外，从岩相、垂直序列和古水流三方面的特征论证了舒家坝组属非扇沉积模式。

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.