浙江中西部永康盆地及金衢盆地白垩系冲积扇特征

浙江众多中小型白垩纪盆地中,分布着河湖相红色沉积岩夹火山岩的白垩系上部地层。作者对位于浙江省金华地区永康市和义乌市境内的2条剖面进行了研究,即金衢盆地早白垩世晚期至晚白垩世早期的铜山岩中戴组剖面和永康盆地早白垩世晚期的方岩景区方岩组剖面,认为其主要为冲积扇沉积,具有多套沉积旋回的陆相碎屑岩准层序组,沉积特征表现为:下部紫红色块状砾岩、砂砾岩,夹泥质粉砂岩,局部有紫红色凝灰岩及深灰色玄武岩;上部棕褐色钙质粉砂岩与泥质粉砂岩组成不等厚互层,产恐龙化石Chilantaisaurus zhejiangensis;具有大型交错层理等沉积构造。其中扇根亚相的砂砾岩为槽流沉积和辫流水道沉积,扇中亚相的片状或席状砂岩为漫流沉积,漫流成因的片状或席状砂岩可能成为良好的油气储集体。此研究成果对中国南方小盆地白垩系扇根—扇中碎屑岩的沉积特征及其成因分析具有重要意义。

Characteristics of the Cretaceous alluvial fans in Yongkang and Jinqu Basins,central and western Zhejiang Province

In Zhejiang Province,southeastern China,there are lots of small Cretaceous sedimentary basins,featured with fluvial and lacustrine red sediments. In this paper,the Tongshanyan outcrop and the Fangyan outcrop,respectively in Yongkang and Yiwu,Jinhua City of Zhejiang Province,were studied. The Zhongdai Formation from Tongshanyan outcrop,in Jinqu Basin, was deposited during the later Early Cretaceous to the earlier Late Cretaceous,while the Fangyan Formation from Fangyan outcrop,in Yongkang Basin,was deposited during the later Early Cretaceous. The continental clastic rocks,in both outcrops,are mainly characterized by alluvial fan and have multiple sets of sedimentary cycle. The main sedimentary characteristics are: the lower part is composed of purplish red massive conglomerate and sandy conglomerate,intercalated with argillaceous siltstone,and locally has purplish red tuff and dark gray basalt;the upper part is composed of brown calcareous siltstone interbedded with argillaceous siltstone,which contains dinosaur fossils “Chilantaisaurus zhejiangensis”. The alluvial fan sedimentary structures are developed,such as large-scale cross beddings. It deposits the slot-flow and braided-channel facies in fan root, while the flake or sheet sandstone facies in middle fan. The latter may be interpreted as sheet flood deposits, and would have the potential to be good oil or gas reservoirs. This paper will be helpful to study the sedimentary characteristics and formation of the root to middle alluvial fan deposits in the small sedimentary basins in the southern China.