摘要： 神农架群(约1400—1000 Ma)发育于扬子克拉通北缘鄂西北地区,自下而上发育有下亚群(大岩坪组、马槽园组、乱石沟组、大窝坑组和矿石山组)、中亚群(台子组、野马河组、温水河组和石槽河组)及上亚群(送子园组和瓦岗溪组)。每亚群均由相对较深水相碎屑岩和浅水碳酸盐岩组成。一系列岩石组合特征、宏微观沉积组构和沉积构造等表明,神农架群发育环潮坪相藻碳酸盐岩、浅海相碎屑岩、台缘浅滩颗粒碳酸盐岩和台缘斜坡砾屑碳酸盐岩等4类沉积组合,形成于远端变陡型缓坡型碳酸盐岩台地背景。环潮坪沉积分布最广,遍布于所有碳酸盐岩地层,不同类型叠层石发育,构成向上变浅沉积序列;进积作用强烈,干裂构造、蒸发岩等常见;沉积相带由浅潮下、潮坪及潮上带组成。浅海碎屑岩沉积组合主要见于台子组及大岩坪组,由砂岩、粉砂岩与泥岩组成,石英砂岩分选和磨圆较好,自生海绿石常见,平行层理、水平—波状层理发育,泥岩中自生黄铁矿发育,总体经历了滨岸浅滩—浅海陆棚—碎屑潮坪—局限台地等的高频变化。台缘浅滩颗粒碳酸盐岩以鲕粒白云岩、砾屑白云岩(及内碎屑白云岩)和斜歪锥、柱状叠层石为特征,主要见于乱石沟组、野马河组;大窝坑组及石槽河组以鲕粒和砂—砾屑白云岩及藻碎屑(团块或凝块石)为特征;大中型板状交错层理、递变—平行层理和冲刷—侵蚀构造普遍,表明浅潮下带强水动力条件环境。台缘斜坡砾屑碳酸盐岩发育在大岩坪组中上部及马槽园组,由滑塌堆积的透镜状、巨厚层状巨—粗—细砾岩和砂岩组成,砾岩成分以白云岩等为主,可见大型交错层理、波痕和侵蚀—冲刷等沉积构造。对神农架群沉积序列、沉积特征及沉积演化过程的研究,为扬子克拉通中元古代晚期的盆地演化与重建、沉积充填过程及地层—沉积对比研究提供了基础资料及依据。

关键词: 沉积特征, 神农架群, 中元古代, 扬子克拉通北缘

Abstract: The Shennongjia Group(ca. 1400-1000Ma)in the northern margin of the Yangtze Craton in northwestern area of Hubei Province consists of the Dayanping,Macaoyuan,Luanshigou,Dawokeng and Kuangshishan Formations in ascending order in its lower subgroup,the Taizi,Yemahe,Wenshuihe and Shicaohe Formations from the base to top in its middle subgroup and the Songziyuan and overlying Wagangxi Formations in its upper subgroup. Each subgroup was deposited as relatively deep water clastics and then as shallow water carbonates. According to a series of rock association,macroscopic and microscopic depositional fabric and structures,four types of sedimentary assemblages including peritidal algal carbonates,shallow marine clastics,platform margin shoal carbonates,and foreslope carbonates were recognized in the Shennongjia Group. They were deposited in a gentle dipping carbonate platform setting with a distally steepened slope. The peritidals are distributed most widely and are composed of peritidals carbonates,and different types of stromatolites were developed. They consitute a shallowing upward sequence with a strong progradation. Dry cracks and evaporates are common. They were deposited as shallow subtidal,tidal flat and supratidal facies. The shallow marine clastic sedimentary assemblage is mainly composed of sandstones,siltstones and mudstones and developed mainly in the Taizi and Dayanping Formations. Well-sorted and rounded quartz sandstones,common autochthonous glauconite in sandstones and autochthonous pyrites in mudstones,parallel bedding,horizontal and wavy bedding show that the Taizi Formation generally experienced a highly frequent sea level fluctuation. As a result,the sedimentary facies changed from coastal beach-platform margin shoal,to deep and shallow shelf sandy barrier-detrital tidal flat-restricted platform. The high energy shoal at the edge of the platform is mainly found in the Luanshigou and Yemahe Formations. It is characterized by oolitic dolomites,dolorudites(or intraclastic dolomite)and conical and columnar stromatolites. The Dawagou and Shicaohe Formations are characterized by oolitic and dolarenite-dolorudites and algal detritus(and lumps or phosphates). Large-medium scale cross bedding,grading bedding,and scour and erosion structures show an environment with strong hydrodynamic conditions in a shallow subtidal setting. The lenticular,thick,tremendous to coarse breccias and sandstones from collapse were developed in the Dayanping and Macaoyuan Formations. Sedimentary structures,such as cross bedding,ripple mark,scour and erosional structures,represent the foreslope and shallow water shelf environment. This study provides basic data and evidence for the sedimentary evolution and reconstruction,the depositional process of the basin and sequence correlation of the Late Mesoproterozoic in the Yangtze Craton for further studies of the sedimentary sequence,sedimentary characteristics and sedimentary evolution process of the Shennongjia Group.

Key words: sedimentary characteristics, Shennongjia Group, Mesoproterozoic, northern margin of Yangtze Craton

中图分类号: 

• P512