洞庭盆地两护村孔重矿物特征及其对第四纪构造活动与环境演变的响应

第四纪洞庭盆地主体自西向东分为安乡凹陷、赤山隆起和沅江凹陷等3个次级构造单元。在安乡凹陷东南部两护村新施工揭穿第四系的ZKC1孔,其地层组成自下而上依次为早更新世华田组、汨罗组,中更新世早期—中期洞庭湖组,晚更新世坡头组以及全新统等。对钻孔岩芯进行了系统的重矿物分析,进而根据特征重矿物来源和含量变化,结合钻孔岩性和岩相变化以及区域地质和地理背景,探讨洞庭盆地南部早—中更新世沉积环境暨河湖变迁以及构造沉降过程。研究表明,洞庭盆地存在幕式断陷活动,早更新世早期、早更新世末期和中更新世中—后期等3个时期强烈断陷沉降,相对湖平面上升,来源于盆地南缘中段的资江河水部分向西注入安乡凹陷,导致ZKC1孔华田组下段下部、汨罗组顶部、洞庭湖组上段等相应层位中的锆石、金红石、锐钛矿和菱铁矿等(主要来源于资江流域)含量显著增高。其它时期断陷作用较弱,河湖水位低,沅水和资江分别沿其主水道于赤山隆起西侧和东侧向北汇入长江,导致ZKC1孔相应沉积层位中锆石、金红石、锐钛矿和菱铁矿的含量明显偏低。对重矿物特征指数变化与孢粉和化学蚀变指数所反映的气候变化的相关性研究,暗示上述3次相对湖平面上升的主要原因不是降水增加,而是构造沉降增强。

Heavy Minerals Characteristics of Sediments in Lianghucun Borehole and Their Responses to the Quaternary Tectonic Movement and Environmental Evolutions of the Dongting Basin

The main body of Quaternary Dongting basin, northwestern Hunan Province,is composed of Anxiang sag, Chishan uplift and Yuanjiang sag from west to east. The ZKC1 borehole was carried out at Lianghucun in southeastern Anxiang sag. The Quaternary sediments were composed of the Early Pleistocene Huatian Formation and Miluo Formation, the early-middle Middle Pleistocene Dongtinghu Formation, the Late Pleistocene Potou Formation and Holocene. The heavy minerals from the core of the borehole were analysed by the numbers. Based on the source and contents variations of diagnostic heavy minerals and combined with lithological and petrographic variations, and regional geologic and geographic backgrounds, the writers probed into the evolutions of the sedimentary environment and tectonic subsidence of the southern Dongting basin during Early and Middle Pleistocene. The studies show that there existed episodic faulted subsidences in the Dongting basin, and there existed three violent subsidences apart in early Early Pleistocene, last stage of Early Pleistocene and middle-late Middle Pleistocene; when the relative lake-level rose, and thus partial water of the Zijiang River got west toward into Anxiang sag, which caused notable raises of the contents of heavy minerals came mainly from the Zijiang valley in the low part of lower Huatian Formation, the top of Miluo Formation and the upper Dongtinghu Formation, such as zircon, rutile, anatase and siderite. The subsidences were weak and the river-lake levels were low in other epochs, which led the water of the Yuanshui River to the west of Chishan uplift and the water of th e Zijiang River to the east of Chishan uplift flow into the Yangtze River along their primary flume respectively, and thus the contents of zircon, rutile, anatase and siderite in corresponding sediments in ZKC1 borehole were lower. Studies about the relationship between characteristic indexes of heavy minerals and paleaoclimate variations indicated by pollens and chemical index of alteration show that the main cause of the three raises of relative lake-level is subsidence strengthening but not precipitation increase.