Quaternary Geological And Environmental Evolution Of The Dongting Basin

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.