下扬子地区早古生代晚期前陆盆地沉积特征与盆山过程

Sedimentary Characteristics and Basin-Orogen Processes of the Late Early Paleozoic Foreland Basins in the Lower Yangtze Region

下扬子地区从晚奥陶世开始沉积特征发生了明显转变,从浅海相转变为三角洲相沉积.这一沉积特征转变与早古生代晚期经历的强烈造山事件密切相关.通过下扬子地区晚奥陶世到志留纪沉积序列的沉积学和碎屑锆石年代学研究,揭示沉积盆地的性质及其时空演化过程,探讨沉积盆地发育与造山带隆升剥蚀之间的关系.下扬子地区早古生代晚期沉积学特征从东南向西北岩性由岩屑砂岩变为石英砂岩,粒度由粗粒变为细粒;沉积厚度等值线具有明显的不对称性,靠近东南等值线密,且沉积厚度大;往西北等值线稀疏,且沉积厚度小;沉积中心呈狭长带状分布,并从东南向西北方向迁移;具有前陆盆地的沉积特征.上奥陶统到中志留统的碎屑锆石以900～720Ma的年龄为主,指示物源以下伏新元古代晚期裂谷层序为主;从早志留世高家边组开始,450～420Ma碎屑锆石年龄出现并逐渐增多,表明同造山岩浆岩被剥露地表并开始提供物源;碎屑锆石中没有出现明显的代表华夏地块基底1.9～1.7Ga的特征年龄峰值,表明华夏地块不是下扬子地区早古生代晚期前陆盆地的主要物源区.下扬子地区前陆盆地从晚奥陶世开始沉降,晚奥陶世的构造沉降速率超过了沉积物的供给速率,前渊沉积了巨厚的浅海相泥岩夹粉砂岩和砂岩;晚奥陶世末造山带持续隆升并向西北方向扩展,沉积速率加快,沉积物粒度明显变粗,沉积相也由浅海相转变成三角洲前缘相;早志留世开始埋深较大的同造山岩浆岩开始遭受剥蚀,导致前陆盆地中450～420 Ma的碎屑锆石含量明显增加.

The sedimentary facies in the lower Yangtze area changed from the neritic facies to the delta facies in the Late Ordovician. This change of sedimentary facies is related to the orogenic event in the late Early Paleozoic. Based on the comprehensive study of sedimentology and detrital zircon geochronology from the Late Ordovician to the Silurian in the lower Yangtze area, this paper presents the nature and spatial-temporal evolution of the sedimentary basin and discusses the relationship between the basin evolution and the orogenic uplift and denudation. The lithology changed from lithic sandstone to quartz sandstone and grain size changed from coarse to fine from southeast to northwest in the lower Yangtze area. The basin morphology shows asymmetrical wedge shape in transverse cross-section. The sediments are thicker and the slope is steeper in the southeastern part than those in the northwestern part. The narrow subsidence center of the basin parallels to the orogenic belt and migrates from southeast to northwest. All these features indicate the characteristics of foreland basin for the late Early Paleozoic sedimentary basin in the lower Yangtze area. The 900-720 Ma detrital zircons from the Upper Ordovician to the Middle Silurian indicate that the provenance of these strata is the sediments deposited in the rift basin in the Late Neoproterozoic. Occurrence and gradual increase of the 450-420 Ma detrital zircons in the Early Silurian Gaojiabian Formation indicate that the synorogenic magmatic rocks were exposed to provide materials. The absence of 1.9-1.7 Ga detrital zircons which are widespread in the Cathaysia Block might imply that the Cathaysia Block was not the sediment provenance of the late Early Paleozoic basins in the lower Yangtze area. The foreland basin in the lower Yangtze area began to develop in the Late Ordovician. The tectonic subsidence rate exceeded the rate of sediment supply, so thick neritic mudstone interbedded with siltstone and sandstone deposited in the foredeep. The sediments in the foreland basin in the Yangtze area became coarser and the sedimentary facies changed from the neritic facies to the delta front facies with the increasing uplift and northwestward propagation of the orogenic belt in the end of the Late Ordovician. In the Early Silurian, more synorogenic magmatic rocks were eroded, resulting in the increase of the 450-420 Ma detrital zircons in the foreland basin.