【作者】 柏道远；

【导师】 李长安；

【作者基本信息】 中国地质大学 ， 第四纪地质学， 2010， 博士

【摘要】 第四纪洞庭盆地西、南、东三面分别为武陵隆起、雪峰隆起和幕阜山隆起；北为江汉盆地,二者在西段相接,东段以华容(次级)隆起相隔。洞庭盆地内部由若干次级构造单元组成,西北部为澧县凹陷；南面大部为安乡凹陷和沅江凹陷,其间为赤山次级隆起；西部发育临澧凹陷；临澧凹陷和安乡凹陷之间为太阳山次级隆起。本文在详细的地表地质调查,大量前人钻孔和少量新施工钻孔资料的利用,以及新施工安乡凹陷两护村ZKC1孔第四系的ESR和OSL年龄、孢粉、粒度、磁化率、主量与微量元素地球化学、重矿物、砾石特征等分析测试基础上,对洞庭盆地第四纪地质环境进行了较系统、深入研究,取得以下主要进展和认识：1.以大量钻孔资料和详细的地表地质调查为基础,分别对澧县凹陷、临澧凹陷、安乡凹陷、赤山隆起、沅江凹陷(东部)、华容隆起等次级构造单元的第四纪构造活动、沉积特征及环境演化等进行详细解剖,揭示出洞庭盆地第四纪构造活动与沉积作用的横向差异。2.早更新世—中更新世中期洞庭盆地处于断陷阶段。盆地及周缘地区构造活动主要表现在以下几方面：①根据大量钻孔资料所编制的第四纪沉积等厚图及第四纪地质剖面图,清楚显示第四纪洞庭盆地及其次级凹陷受NNE、NW、EW和SN向等4组正断裂控制。②各次级凹陷强烈沉降并接受沉积,沉降中心位于凹陷内部或中央而远离边界断裂,如澧县凹陷、安乡凹陷和沅江凹陷等；断裂下盘局部可具明显沉降。③随着盆地逐渐扩张,断陷活动向盆地东、西边缘(沅江凹陷东缘、安乡凹陷西缘、澧县凹陷西缘等)迁移。④盆地断陷活动具幕式特征,总体可分为早更新世早期、早更新世晚期和中更新世早—中期等3个裂陷幕。⑤盆地北面的华容隆起具明显构造沉降,盆地周缘其它隆起及盆地内部的赤山隆起具脉动式抬升。⑥赤山隆起构造较稳定期和构造抬升期分别对应于安乡凹陷缓慢沉降期和快速沉降期。中更新世晚期以来洞庭盆地处于坳陷阶段。其中中更新世晚期洞庭盆地整体抬升并遭受剥蚀；在盆地东缘和西缘产生倾向盆地的构造掀斜,局部第四系变形形成褶皱。晚更新世—全新世洞庭盆地主体产生坳陷沉降并接受沉积,盆地周缘部分地区存在小幅抬升。3.从构造活动特征出发,提出洞庭盆地及周缘地区第四纪构造活动新的动力机制模式。第四纪早期(早更新世—中更新世中期)洞庭盆地断陷活动的动力机制：洞庭盆地区首先深部地幔上隆对地壳加热,使上部中地壳韧塑性物质(低速层)膨胀而向周边侧向迁移,尔后在冷却收缩条件下物质迁出部位产生虚脱空间,上层地壳因此“塌陷”而产生整体性下拗沉降,下沉块体的侧翼发育断裂而形成控盆控凹正断裂。与之相耦合,盆地或凹陷周缘隆起带因中地壳物质补充而抬升。不同级次和不同规模范围的深部物质迁移运动,造成了第四纪洞庭盆地与其内部次级构造单元之间的叠加关系。上述机制可以很好地解释洞庭盆地早期断陷具有的以下现象：一是沉降幅度最大处并非紧邻断裂；二是边界伸展断裂下盘局部也可存在较大幅度的沉降；三是控凹正断裂存在EW向、NNE向、NW向等多组方向,而不是一般断陷盆地所具有的单走向正断裂。中更新世晚期盆地构造抬升、掀斜和褶皱变形与深部迁出物质的回返以及板块尺度的物质运动和挤压作用有关。晚更新世和全新世盆地坳陷沉降与区域挤压应力下的坳陷或拱坳变形有关,可能同时伴有NE向的深部物质流动与拉张。4.两护村ZKC1孔第四系自下而上可划分出16个孢粉组合带。ESR年龄和孢粉组合及其反映的气候特征,指示华田组下段可能形成于上新世末。根据孢粉组合特征,结合构造-沉积演化和区域气候背景,重塑洞庭盆地上新世末以来的气候演化过程：上新世末期由孢粉带Ⅰ和Ⅱ指示具暖干气候。早更新世经历了凉干(孢粉带Ⅲ、Ⅳ)→暖湿间凉干(孢粉带Ⅴ-Ⅶ)→冷干间温湿(孢粉带Ⅷ-Ⅹ)→暖较湿(孢粉带Ⅺ,Ⅻ)的气候演变过程。中更新世早期无孢粉样品(洞庭湖组下部砾石层),其沉积环境暗示冷干气候条件；中期由孢粉带ⅩⅢ反映出暖稍湿的气候特征；晚期因构造抬升缺失沉积,同期湿热化事件指示暖湿气候。晚更新世早期缺乏沉积,据区域对比应为寒冷气候；中期由孢粉带ⅩⅣ指示温较湿的气候特征；晚期缺失沉积,系寒冷气候下区域海平面下降所致。全新世经历了暖稍湿(孢粉带ⅩⅤ)→暖稍干(孢粉带ⅩⅥ)的演变。上述气候演变过程与ZKC1孔化学蚀变指数曲线反映的气候演变过程以及中国东部第四纪气候演化基本吻合。以孔深140m为界,上部孢粉数量显著高于下部,种属也更为丰富。分析提出其成因可能与早更新世早期洞庭盆地内发育多个孤立的小盆地或沉积区,早更新世晚期开始各沉积区扩展并连成一体的构造古地理差异有关。5.对ZKC1孔第四纪沉积物磁化率及有关地球化学特征的气候意义进行了探讨。根据化学蚀变指数(CIA)理论上与温度和湿度呈正相关,ZKC1孔第四纪沉积物的CIA变化曲线大致反映自下而上地层对应气候信息为：早更新世华田组下段对应气候温湿,华田组上段下部对应气候冷干,华田组上段上部对应气候暖湿,泪罗组下部对应气候冷干,泪罗组上部对应气候暖湿；中更新世洞庭湖组中部对应气候暖湿(下部砂砾层无样品),上部对应气候冷干—温湿;晚更新世坡头组对应气候温湿；全新世湖冲积对应气候变化较复杂,总体温湿—暖湿。此与上述孢粉组合指示的古气候信息大体一致。变化曲线的协变性及相关性分析表明ZKC1孔磁化率与CIA呈负相关,此与气候越干燥则陆相盆地沉积物磁化率值越高的理论相符,同时表明磁化率可作为洞庭盆地第四纪气候代用指标。Ga、Nb、Sr、Th、Rb、Li等微量元素含量与CIA、Al2O3含量暨泥质含量呈正相关,反映出粘土对微量元素的吸附作用。6.对ZKC1孔第四系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。重矿物ZTR指数和Gzi指数曲线的起伏未象孢粉和CIA曲线一样反映出干湿变化,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。7.对ZKC1孔中更新世洞庭湖组砾石层进行的统计表明,砾石的粒度变化反映出2个较大尺度的由大→小的旋回,早旋回由洞庭湖组下段砂砾层组成,晚旋回由洞庭湖组中段上部的砂砾层组成,反映出中更新世早期—中期安乡凹陷的2次由慢→快的幕式沉降过程。在上述2个大的粒度旋回之上,叠加有多个更小尺度的砾石粗、细变化,主要与气候干湿的频繁波动有关。洞庭湖组中段顶部砾石的磨圆度明显偏低,反映其沉积时期盆地沉降和周缘隆起区抬升活动的增强。8.对曾被认为属冰川成因的津市黄牯山泥砾混杂堆积进行了详细的野外考察。泥砾混杂堆积中下细上粗的层序特征,砾石扁平面的优选定向,沿层理经差异风化和磨蚀所形成的假“冰川擦痕”,由壳状裂口经磨蚀、圆化所形成凹面石,沉积体短距离内的相变,以及周边仅有低矮丘陵的地貌背景等,表明津市黄牯山泥砾混杂堆积为泥石流产物,而不是冰川成因。这一认识为湖南及中国东部第四纪冰川问题研究补充了新的资料。更多还原

【Abstract】 Quaternary Dongting basin is adjacent to west Wuling uplift, south Xiefeng uplift and east Mofushan uplift, and is adjacent to north Jianghan basin with Hurong uplift interposing at east segment. The basin possesses uplift-depressional tectonic framwork with being composed of Lixian sag, Linlin sag, Taiyangshan uplift, Anxiang sag, Chishan uplift and Yuanjiang sag. The Lixian sag is in the northwest of the basin; the Anxiang sag and the Yuanjiang sag composed of the main body of the basin with Chisha uplift interposing; the Linli sag is in the west of the basin and is adjacent to Anxiang sag with Taiyangshan uplift interposing. On the bases of detailed geologic mapping, a great deal former bore date and a few new bore data, and systemic analysises of ESR and OSL ages, sporopollen, grain size, magnetic susceptibility, main and trace elements, heavy minerals, and gravel statistics of the Quaternary deposits from the core of ZKC1 borehole in Lianghucun in Anxiang sag, the writer studied the Quaternary geological and environmental characteristics of the Dongting basin. The main progress and ideas are put forward as follows.Based on great deal borehole data and detailed geologic mapping, the writer studied amply the Quaternary tectonic and sedimentary characteristics and environmental evolution of every secondary tectonic unit of the Dongting basin such as Lixian sag, Linli sag, Anxiang sag, Chishan uplift, Yuanjiang sag(east edge) and Huarong uplift, and revealed the lateral variation of Quaternary tectonic activities and sedimentations of Dongting basin.Early Pleistocene-late Middle Pleistocene is a faulting-subsiding period for Dongting basin, when there were tectonic movements as follows:①Maps of contour line of Quaternary deposits thickness and Quaternary geological sections based on a great lot borehole datas show that the basin and its secondary sags were controlled by four groups of normal faults such as NNE-, NW-, EW- and SN-trending faults.②Every secondary sag such as Lixian sag, Anxiang sag and Yuanjiang sag subsided violently and received depostis with interior subsiding center far from boundary faults, while part of footwall of faults subsided evidently.③The faulting subsidence moved toward east and west margin of Dongting basin such as east edge of the Yuanjiang sag, west periphery of Anxiang sag and west periphery of Lixian sag for gradual expanding of the basin.④The faulting subsidence of the basin possessed episodic characteristics and can be divided into three faulting-subsiding episodes such as early Early Pleistocene, late Early Pleistocene and early-middle Middle Pleistocene.⑤Huarong uplift subsided observably, while other uplifts around Dongting basin experienced pulsative rises.⑥Steady and rising period of Chishan uplift corresponded with slow and rapid subsiding period of Anxiang sag, respectively.Dongting basin entered into a depressive phase since late Middle Pleistocene. The basin rose and were denuded in late Middle Pleistocene, while there occurred tectonic tilt on the east and west margine of the basin, and folds in Quaternary deposits formed in part areas. Main body of the basin subsided for depression and received deposits during Late Pleistoncene-Holocene, while there existed clear rise in part areas around the basin.According to the tectonic characteristics, the writer bring forward new modes about the dynamic mechanisms of Quaternary tectonic activities of Dongting basin.The dynamic mechanism of the faulted-subsidence of Dongting basin during Early Pleistocene-middle Middle Pleistocene was as follow:as the deep uplifting mantle heated the crust in Dongting basin, the ductile matter of the middle curst expanded and moved toward the periphery and then shrank by cooling, which brought about collapse and subsidence of the upper crust, and hence normal faults were formed on the margin of the subsided block. Responding to the matter movement, the uplifts around the basin or sags rose for supplement of middle curst matter. Deep matter movements of different grades and different scales caused the superposition of Dongting basin and its secondary tectonic units. this mechanism can explain satisfactorily following appearances in the faulted-subsidence of Dongting basin in early stage:Place with greatest subsiding extent isn’t adjacent to fault; part of footwall of boundary normal faults subsided evidently; there existed four groups of normal faults such as NNE-, NW-, EW- and SN-trending faults, which is different with single-direction normal fault of general down-faulted basin.The rise, tilt and fold deformations of Dongting basin in late Middle Pleistocene were possibly related with return of the earlier transported matter, the movement of the plate-scale material and compression.The depressive subsidence of Dongting basin during Late Pleistocene-Holocene was probably related with depression or dome-col deformation under regional compression, and with NE-directed deep matter movement and tectonic extension.Detailed sporopollen analysis on the Quaternary deposits from ZKC1 borehole was conducted and 16 sporopollen-zones were identified. ESR ages, sporopollen assemblages and corresponding climatic feature indicate that the lower part of Huatian Formation was formed at the end of Pliocene. According to the features of sporopollen assemblages and combined with tectonic-sedimentary evolution and regional climatic settings, the climatic evolution of Dongting basin since the end of Pliocene was revealed as follows:Sporopollen-zoneⅠandⅡindicate that the climate at the end of Pliocene was tropical and dry. From early to late, The climatic variations during Early Pleistocene are cool and dry (sporopollen-zoneⅢandⅣ), tropical and humid interposed with cool and dry (sporopollen-zoneⅤ-Ⅶ), cold and dry interposed with warm and humid (sporopollen-zoneⅧ-Ⅹ), tropic and weakly humid (sporopollen-zoneⅪandⅫ). During Early Middle Pleistocene with no sporopollen sample the climate was cold and dry, which is suggested by sedimentary environment of gravels from lower part of Dongtinghu Formation; during middle Middle Pleistocene the climate was tropical and weakly humid which is indicated by sporopollen-zoneⅩⅢ; during late Middle Pleistocene with no deposits formed for rise, the climate was tropical and humid which is indicated by tropical-humid event. According to regional climatic feature, the climate was dry in early Late Pleistocene with no deposits formed; during middle Late Pleistocene the climate was warm and weak humid which is indicated by sporopollen-zoneⅩⅣ; there were no deposits formed for cold climate and sea-level dropped. From early to late, The climatic variations during Holocene are tropical and weak humid (sporopollen-zoneⅩⅤ), and tropical and weak dry (sporopollen-zoneⅩⅥ). This climatic evolution process coincide basically with climatic change indicated by CIA (chemical index of alteration), and with Quaternary climatic variations of eastern China. There exist a sporopollen boundary at depth of 140m over which there were more pollen and spore then under, which is related with tectono-paleogeographic difference that there occurred several isolated small sags or sedimentary areas in Dongting basin during early Earlly Pleistocene, while all sedimentary areas spreaded and connected with each other to a integer since lata Early Pleistocene.The climatic implications of magnetic susceptibility and Geochemical characteristics of the Quaternary deposits from ZKC1 borehole was approahed.According to the idea that chemical alteration index (CIA) has a positive correlation with temperature and humidity, the climatic informations of CIA curve of the Quaternary deposits from ZKC1 borehole was defined as follows:For Early Pleistocene, the lower Huatian Formation correspond to warm and humid climate, the lower part of upper Huatian Formation correspond to cold and dry and the upper part to tropical and humid climate; the lower part of Miluo Formation correspond to cold and dry and the upper part to tropical and humid climate. The middle part of Middle Pleistocene Dongtinghu Formation correspond to tropical and humid, and the upper to cold and dry-warm and humid climate. The Late pleistocene Potou Formation correspond to warm and humid climate. The Holocene lacustrine-alluvial deposits correspond to warm and humid-tropical humid climate as a whole. Above informations is roughly in accordance with that indicated by sporopollen assemblages.Covariability of curves and relativity analysis show that the magnetic susceptibility of the deposits from ZKC1 borehole has a negative correlation with CIA, which coincide with the idea that drier the climate is, greater the magnetic susceptibility of deposits in continental basin is, and indicate that magnetic susceptibility can be taken for substitutive index of the Quaternary climate in Dongting basin. Contents of some trace elements such as Ga, Nb, Sr, Th, Rb and Li have a positive correlation with CIA and content of Al2O3, which hints adsorption property of clay for trace elements.The heavy minerals from the core of the ZKC1 borehole in Lianghucun 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 writer 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. Curves of heavy mineral ZTR and Gzi indexes don’t show dry and wet changes as Sporopollen assemblages and Curves of CIA, which suggest that the main cause of the three raises of relative lake-level is subsidence strengthening but not precipitation increase.The statistics and analysis of gravel particle size and shape features of Middle Pleistocene Dongtinghu Formation from Lianghucun ZKC1 borehole were taken. The variations of gravel particle size show two big-to-small cycles whose early cycle corresponded with the lower member of Dongtinghu Formation and late cycle corresponded with upper part of the middle member of Dongtinghu Formation. The two particle size cycle indicate two slow-to-rapid episodic subsidence of Anxiang sag during early-middle Middle Pleistocene. Superposed on the two large cycle there existed a lot of small scale variations of gravel particle size which were mainly related with aridity-humidity changes. The roundness of gravels from top of middle member of Dongtinghu Formation is evidently low, which indicated strengthen of tectonic movement such as subsidence of the basin and rise of around uplifts. There is a sequence of muddy gravel deposits at Huanggushan, Jinshi City. This sequence was once considered as moraine deposits. The writer of this paper re-surveyed the lithology, compositions and lateral change of the deposits, and studied the fabric characteristics and surface textures of the gravels. On the outcrop of the muddy gravel deposits, the gravels from the lower section are smaller, which suggest that the lower section is probably the main body of debris flow deposits, while the upper section represents surface float-gravel deposit. The gravels show certain degrees of orientation. There isn’t typical moraine gravel with scrape surface in the deposits. The so-called "glacial striae" on some gravel were formed under differential weathering and abrasion along stratification. The concave-surface gravels show abrasion and fracture appearance. Some gravels have cracks but are not broken, which is caused probably by strike, rotation and staggering in debris flow. From north to south, the muddy gravel deposits (viscous debris flow deposits) change into non-viscous debris flow deposits within short distance with grain size become less and roundness higher. There are small low-rise hills to the west of the deposits but there aren’t high-elevation mountains around. All evidences indicate that the muddy gravel deposits in Huanggushan, Jinshi Hunan are of debris flow deposits and not moraines. This cognition provides new information for the studies of Quaternary glaciations in Hunan and east China.更多还原