扬子地块东南缘青白口系岩门寨组—南华系长安组层序地层划分及构造背景讨论

SEQUENCE STRATIGRAPHIC DIVISION AND TECTONIC SETTING OF THE QINGBAIKOUAN YANMENZHAI FORMATION AND NANHUAN CHANGAN FORMATION IN THE SOUTHEASTERN MARGIN OF THE YANGTZE BLOCK

通过对新化县厚溪村剖面的青白口纪岩门寨组—南华纪长安组露头层序和沉积相的观察、分析,确认岩门寨组上部为浅灰色条带状粉砂质板岩与凝灰质板岩及岩屑石英杂砂岩、含砾砂岩构成向上变粗的层序结构,其中多见有石英砂岩薄层滑塌变形或滑碎为砾石的现象,相环境可能处于陆棚—上斜坡过渡地带,是海平面变化最易于识别的区域。反映了海平面缓慢下降—快速下降的变化过程,属于晚期高水位体系域。南华系长安组以块状泥砾岩、含砾板岩夹含砾砂岩为主,纵向构成向上变细的主体结构,具有从退积(二元)结构—进积(块状)结构—退积(或弱加积)结构的变化,反映出低水位体系域特征,属于快速堆积的重力流沉积。由此识别出二者之间的Ⅰ型不整合界面,提出了重新划分南华系底界的方案。认为在缺少古生物依据的前寒武纪地层研究中,通过对露头层序及其变化和岩相之间关系的研究,详细划分其基本程序与堆积结构类型,归纳判别体系域,划分层序,是识别不整合类型、划分地层和对比的有效途径。通过这一不整合界面的识别,结合区域上相关沉积特征的对比分析,认为前人确立的"雪峰运动"是武陵运动后扬子东南缘裂谷作用的延续,是盆地伸展驱动海平面下降并产生相关沉积响应的过程,不是造山运动。

The sequence stratigraphy of the Qingbaikouan Yanmenzhai Formation and the Nanhuan Chang ＂an Formation at the Houxi section of Xinhua County, Hunan Province was investigated on the basis of field investigation and sedimentary facies analysis. The upper Yanmenzhai Formation is characterized by a coarsening-upward sequence deposited in the slope-shelf transitional zone, thus most sensitive to sea level changes. Sequence stratigraphic analysis suggests that it represents a late highstand system tract, recording slow and then rapid sea-level drop. The Nanhuan Chang ＂an Formation consists of massive diamictite, pebbly slate, and pebbly sandstone, with a retrogradation （binary） to progradation （massive） to retrogra- dation （or weak aggradation） structure. It is mainly a fining-upward sequence. It is a low stand system tract characterized by rapid accumulation of gravity flow deposit. Thus, a type I sequence boundary exists between the two formations and this represents the basal boundary of the Nanhuan system. In the absence of biostratigraphic data, sequence stratigraphy provides an effective way to divide and correlate Precambri- an strata on a regional scale. The recognition of an unconformity also offers insights into the tectonic histo- ry of the study area. The Xuefeng Movement, which was regarded as an orogeny by previous researchers, is re-interpreted as a continuation of the rifting process after the Wuling Movement in southeastern margin of the Yangtze Platform. Thus, it is the extensional tectonics, not orogenic movement, that drove sea lev- el changes recorded in the Yanmenzhai and Chang ＂an formations.