川北震旦系灯影组软沉积物变形构造及成因分析

Characteristics and origins of soft-sediment deformation structures in the Sinian Dengying Formation,northern Sichuan Basin

摘要

四川盆地北缘震旦系灯影组发育软沉积变形构造,其成因机制尚不明确。结合野外露头对川北广元地区灯影组软沉积变形构造发育特征、分布及成因进行系统分析。研究表明,广元地区震旦系灯影组三段软沉积变形构造主要分布于中下部灰白色薄层灰岩及灰绿色—紫红色薄层泥岩中,可见液化脉、液化卷曲变形、液化角砾岩及层内断层4种典型构造。灯三段早期浅水、较安静水体沉积物受同期循环剪切应力及地层应力复合作用,为持续性古地震诱发的液化变形所致。垂向上可识别出2期不完整振动液化序列,表明灯三段至少存在2次持续的古地震活动。综合分析指示上扬子克拉通北部灯三段沉积期构造活动强烈,地震活动应与桐湾运动Ⅰ幕晚期裂陷槽的拉张活动相关。

Abstract

The mechanisms of soft-sediment deformation structures(SSDS)in the Precambrian Dengying Formation in the northern Sichuan Basin remain poorly understood. This study presents a comprehensive analysis of these structures in the Guangyuan area,examining their characteristics,distribution,and origins through field outcrop studies. The results indicate that these deformation structures,which are predominantly located in the middle to lower strata of the third member of the Dengying Formation,are composed of grey-white thin-layered limestone and purplish-red to grey-green thin-layered mudstone. The identified structures include liquefaction veins,convolute folds,breccias,and intrastratal faults,which are primarily observed in sediments from shallow,low-energy,aquatic environments. These deformations are closely linked to a combination of cyclic shear stress and stratigraphic pressure,suggesting recurrent ancient seismic activity as a key contributor. The finding of two distinct seismic liquefaction sequences suggests a minimum of two significant seismic events within the Member 3 of the Dengying Formation. It is further inferred that there was a period of intensive tectonic activity during the deposition of the member 3 of the Dengying Formation in the northern Yangtze Craton,and the seismic activity seemed to be related to the extension of the Late-Tongwan Movement in the rift.

关键词

四川盆地 / 灯影组 / 软沉积变形 / 触发机制 / 古地震

Key words

Sichuan Basin / Dengying Formation / soft-sediment deformation structure / trigger mechanism / paleoearthquake