川中地区中下寒武统风暴岩特征及沉积地质意义

Characteristics and sedimentary geological significances of Lower-Middle Cambrian tempestites in central Sichuan Basin

川中地区下寒武统沧浪铺组上部、龙王庙组和中寒武统高台组下部发育风暴岩。风暴沉积构造主要有冲刷-充填构造、菊花状构造、风暴撕扯构造和丘状交错层理等。风暴沉积序列发育均不完整,以A＋B＋C、A＋B、A＋C和A＋D为主,厚5~12cm;风暴期次共发育5大期,且风暴岩与碎屑石英伴生,风暴期碎屑石英含量普遍高于风暴平息期;风暴岩横向连续性较好,自下而上风暴活动中心呈现出从MX202井区→GS10井区→MX21井区→MX202井区—NCH1井区的迁移特征,平面上形成了NE-SW向展布的3个厚度中心：MX202井—MX12井—MX17井区、GS10井—GS23井—GS26井区和NCH1井区。川中地区中下寒武统风暴岩的发现,指示着该时期研究区为风暴混积潮坪,在NE向的风暴潮流作用下,形成风暴潮汐砂脊,而后在砂脊障壁下形成潟湖;同时指示着沉积古地貌以磨溪地区为高地,分别向SW和NE缓斜,说明绵阳—长宁拉张槽的北段在沧浪铺组沉积晚期至龙王庙组沉积期已经关闭。

The tempestites are developed in the upper Member of Lower Cambrian Canglangpu Formation and low Member of Longwangmiao Formation and Middle Cambrian Gaotai Formation in Central Sichuan Basin.Storm-generated sedimentary structures mainly include scouring-filling structure,chrysanthemum-like structure,storm tearing structure and hummocky cross stratifications,etc.The storm sedimentary sequences show incomplete development,dominated by A＋B＋C,A＋B,A＋C and A＋D with a thickness of 5-12 cm.There are five stages of storm-generated sedimentary structures,and tempestites are associated by clastic quartz.The content of clastic quartz is generally higher in storm stage than that in storm ceasing stage.The tempestites present better lateral extension,and storm centers show a migration from Well Block MX202,GS10,MX21 to MX202-NCH1 from bottom to top.Meanwhile,three thickness centers are formed with planar NE-SW distribution,i.e.,Well Block MX202-MX12-MX17,GS10-GS23-GS26 and NCH1.The discovery of Lower-Middle Cambrian tempestites in Central Sichuan Basin indicates that the depositional environment of this study area was storm mixed tidal flat,and storm tidal sand ridges were formed under the effect of NE-trending storm tides,leading to lagoons under sand ridge barrier.Meanwhile,it is also demonstrated that Moxi area was taken as a highland in sedimentary paleo-terrain,presenting agentle dipping in SW-NE directions,and the north part of Mianyang-Changning intracratonic sag might be closed during the late sedimentary stage of Canglangpu Formation to that of Longwangmiao Formation.