第四纪洞庭盆地临澧凹陷构造-环境演化的水系和重矿物约束

临澧凹陷为第四纪洞庭盆地西部的一个南北向次级构造单元,居于武陵隆起和太阳山隆起之间。前人已通过地貌、沉积和构造特征重塑了凹陷第四纪地质演化过程。本文研究探讨了临澧凹陷水系特征以及凹陷北部ZK 257孔重矿物特征的构造-环境成因,从而进一步为第四纪地质演化过程提供了轮廓。中更新世中期和中后期临澧凹陷处于断陷阶段,凹陷北段为相对封闭的南北向小湖盆,两侧山麓形成EW向水系;南段为相对开放的河流环境,凹陷东、西两侧分别形成NE向和NW向水系,河水向中央入渐水后再向南汇入沅水。中更新世晚期临澧凹陷整体抬升并遭受剥蚀,雷公庙以北降水向北汇入澧水,凹陷西侧形成总体NE走向的次级水系;雷公庙以南继续形成NW向次级水系（渐水西侧）。构造抬升的同时产生向东的倾斜,导致凹陷西侧水系远较东侧发育。ZK 257孔内中更新世洞庭湖组中的重矿物均来源于凹陷及周缘沉积岩而未受沅水影响,表明沅水古河道未经过临澧凹陷。周缘侵蚀作用随临澧凹陷扩张而向两侧扩展,使物源岩性发生变化,从而导致洞庭湖组上部（晚期）重矿物含量高于下部（早期）。

Constrains of river system and heavy minerals to tectonic-environmental evolution in Quaternary Linli Sag of Dongting basin

Quaternary Linli sag is a SN-trending secondary tectonic unit in the western part of the Dongting basin,and is located between Wuling and Taiyangshan uplifts.Previous studies have reconstructed the Quaternary geological evolution of the sag by means of geomorphic features and sediment-tectonic characteristics.In this paper,the authors probed into the tectonic-environmental genesis of the water system in Linli sag and the heavy minerals characteristics of sediments in ZK257 borehole in north of the sag,which provided more constrains on the Quaternary geological evolutions.During middle and middle-late periods of the mid-Pleistocene epoch the Linli sag was in a faulted-subsided stage,when a SN-trending in the north segment of the sag closed-lake basin with EW-trending water system formed at the foothills in both sides was existeel and an opened fluvial environment was existed in the south segment of the sag,with NE-trending and NW-trending rivers in the east and west respectively,and the water entered into the Jianshui river and then flowed southward into the Yuanshui river.During late periods of the mid-Pleistocene epoch,the Linli sag totally rose and suffered denudation,the precipitated water in the north of the Leigongmiao flowed northward into the Lishui river with NE-trending secondary rivers formed in the west side of the sag,were continuously developed in the south of Leigongmiao and the EW-trending secondary rivers（in the west of Jianshui）.At the same time,the eastward inclination resulted from fectonic uplifting caused that the west side of the sag developed more rivers relative to the east side.All of the heavy minerals in the mid-Pleistocene Dongtinghu Formation from the ZK257 borehole derived from the pre-Quaternary sediments in the sag and its adjacent areas,indicating that the paleochannel of the Yuanshui River didn′t passed by the Linli sag.The denudation of the adjacent areas spread to lateral areac with the lithologic change of sediment source when the sag expanded,thus causing that the heavy minerals contents in the upper part of the Dongtinghu Formation are higher than those in lower part.