Evolution Features Of Sedimentary And Structural Environment Of Representative Basins Since Late Palaeozoic In Southeast China

Southeast China as a research object and "three representative basins" as working space, Jinhua-Quzhou basin, Yong’an basin and Pingle basin were specifically studied, by means of field survey of geology and structure, thin section analysis of samples, discrimination of geochemical quantification and synthetical analysis of interior references and documents. And there were such research subjects as petrophysics, sequence strata, constituents of basin, features of major elements and REE(rare-earth elements), characteristics of organic carbon isotopes. In view of what was mentioned, and under the prerequisite of recognizing sedimentary structure, depocenter, facies change and tectonic setting evolution of every basinal region, writers had comparatively studied morphological distribution framework of regional basins and structural environment evolution from Late Paleozoic to Meso-Cenozoic in Souteast China.Jinhua-Quzhou Basin is one of the late Mesozoic rifted basins in Southeast China. It developed on the Pre-mesozoic metamorphosed fold basement, mainly including three structural units of northern shallow depression zone, central uplift belt and southern hollow zone:①Looking on material information contained in strata rocks as master line, features of the basin’s Meso-cenozoic formation-litho and structural evolution had been studied by means of geochemical analysis on rare-earth elements, trace elements and isotopic elements, being supplemented by field investigating and synthetically surveying two aspects of vertical deep position and horizontal surface level. The findings suggest as follows:a) Tectonic phenomena of unconformity, syncline et cetera in the inner Jinhua-Quzhou basin had reflected the dynamic building of the basin, with the basin’s receiving deposition accompanied by horizontal pulling; bimodal volcanic rock was displayed by the deviation of SiO2content overtaking18%between rhyolite and basalt, rare-earth element (REE) partitioning curves had exhibited not having europium depletion in basalt and obviously having europium depletion in rhyolite, and trace element cobweb figure had manifested being as trough curves of Nb and Ta, comparatively concentrating of Zr and K and relatively depleting of Ti in basalt and rhyolite, from which could be drawn a judgement that specimen studied were the products of Earth crust melted, b) During the late period of early Cretaceous, in the formations of part of the basin, there was alkaline basalt with the geochemical characteristics showing its being formed within plate environment by fast extension. Those basalts in red layers from late period of early Cretaceous to late Cretaceous in the Guangfeng basin in the western part of Jinhua-Quzhou basin fundamentally had the same features.②Making use of analytic approach of basin prototype, tectonic events, deep seated structure, provenance, subsiding history, depocenter, basin bonndary structure and cessation of deposition, etc were studied. J-Q basin had undergone several evolutionary phases of early intracontinental compressing (NW-SE), late early Crataceous extending and drafting (NW-SE), late Crataceous depression, suppression of last stage of Crataceous and reformation of later stage. From research above, basin filling material was coming from ablation due to incipient fold and uplift of pre-Mesozoic formation and weathering denudation of mountain mass with the background of late period drafting, confirming that J-Q basin had experienced uplifting and denuding events attributing to the mechanism conversion from extruding to pulling. These events had important affection on the shaping of rifting and half graben-like basin. Sedimentation and subsidence center was in the vicinity of Quzhou with more or less translational migration during the three palaeo-lithofacies stages. From the study above, we can see that J-Q basin has favorable oil and gas foreground, and these four structural traps of Qianjia, Shuiting, Yangtang and Jiangtang have major prospecting potentiality.③rom the further multidisciplinary analysis, we can see that the features of formation lithology of Meso-cenozoic Jinhua-Quzhou basin were material records of its tectonic environment evolution, totally presenting "fault structure in the northern part and overlap construction in the southern part", reflecting the basin migration under the background of extension; Based on the deducing morphological feature of the basement of Jinhua-Quzhou Basin by gravity surveying and geological drilling, using trigonometric algorithm method, we have made the extension estimation of Jinhua-Quzhou Basin during late Cretaceous. The quantity of extension is about2100m, and the amount of subsidence is around5100m.④In connection with one typical representative of Meso-cenozoic land facies basins of South-east China, the tectonic setting which J-Q basin had been situated in had complex evolution characteristics of multi-phase with the stress field superimposed by vertical and horizontal stress fields. These cognitions are helpful to provide references for the study of synchronizing continental facies basins.In the Pingle basin, the formations from Late Paleozoic to Early Triassic outcropping along the zone from Pingxiang to Leping was in-situ system. And Banxi Group of adjacent areas had both Pre-Sinian metamorphics series and probably the composition of Mesozoic tectonic melange, being external system. From synthesizing the research results of predecessors, we could see:①Hydrocarbonaceous mother rocks of oil and gas system in Pingle depression include carbonatite, mudstone, and coal of Permian and Triassic, having the features of high abundance of organic substance, vast distribution area, moderate maturity and preferable show of oil and gas.②There are two kinds of capping formations which are respectively red petrographic series from Middle Crataceous to Paleogene and overriding petrographic series from Proterozoic to Triassic.③The middle-and-western part of Pingle depression has the priority of gas location with the eastern part being preferable for oil detection. The favorable areas for prospecting are Yugan and Pingxiang.④Having the potentiality of prospecting oil and gas reservoir above the nappe.The Yong’an basin developed on the Pre-Devonian basement, locating on the southern margin of the Cathaysian block, distributing along the NE-SW and Neoproterozoic Zhenghe-Dapu fault zone which was the background of the deposition-rifted basin formed from early Cambrian Yong’an-Longyan gulf by extending action. Making use of the method of sequence stratigraphy and being supplemented by field investigation, writers studied the stratigraphic sequences and rock assemblages of the basin:①An analysis on the stratigraphic framework and geological column from Late Paleozoic to Early Triassic shows that marine deposit sequence is episodic with the fluctuation of sea level, and different sedimentary facies have disproportion in the eastern part, central part and western part of the basin. The center of deposition and subsidence is approximately in the areas of Longyan and Meixian.②A research on the cross sections traversing the basin and the intrabasinal auxiliary sections shows that the block faulting has played a main part during Meso—Cenozoic, reformed the Late Paleozoic strata and formed compound fault—fold strata.③The layers with organic carbon provide the time limits for the land—sea changes in the basin, and their locations and shapes can demonstrate the dynamic mechanism of the tectonic evolution in the basin. Based on analyzing tectonic events, paleogeography and provenances, deep seated structures, boundary structures, and so on, studying the tectonic setting evolution and its stress mechanism, we can get the conclusions as follows:①The basin is characterized by devoloping uplift—extension structures, and it has mainly undergone phases of Variscan paralic tremendous thick deposition, Indosinian transition from sustained platform to active continental margin accompanied by compressing and uplifting and Meso—Cenzoic tectonic reworking with the conversion from compressing to pulling after Indosinian.②During Late Paleozoic, the speed of incursion and regression of the sea was controlled by the strong and weak alternation of tectonic events in different time, for this reason, the ablation of coastal uplifting areas and weathering denudation of mountain mass with the drafting background provided sedimentary source materials for the basin, and there were two provenance areas in the north and south of the basin. Taking marine ingression and recession as a label, the paleo-geographic environment of late Paleozoic and the spreading and changing of sedimentary facies of every stage had their own special characteristics. The distribution of sedimentary facies belts was in repeated change. The evolutionary series was obvious. During the period of marine regression, the sequence of river-facies, delta-facies, offshore-facies,marine-facies was along NE-SW direction, with the absence of marginal facies. The basin’s depositional environment of ingression period was divided into several sedimentary facies belts of tidal flat lagoon, platform edge shallow, platform foreslope, continental shelf peripheral basin. During Meso—Cenozoic, the basin had totally the piedmont and river—lake sedimentary facies on volcanic environment. In addition, during different geologic epoch and in distinct districts of the same period, the features of similiarities and differences of facies transition of the basin itself and even the whole depositional environment of southeast China are proved by δ13Corg. value alternation of samples from these stratigraphic positions of different era in the basin and the comparative study of δ13Corg. value between the basin and its neighborhood areas. The δ13Corg. value interval of Yong-an basin’s palaeozoic sediment is mainly (-15～-25‰), relatively having similarities compared with the δ13Corg.value interval (-20～-30‰) of Jinhua-Quzhou basin’s northern margin sediment; and the organic carbon TOC value of Yong-an basin also has similar points with the synchronous Ping-Le basin; these features demonstrate that the sedimentary environment of Yong-an Basin’s forming phase had the same points as other basins to the regional extent, being the environment of land-and-sea transition;at the same time, to some extent, there are differences that stratigraphic positions containing carbon are far more Meso-Cenozoic in ZM (Zhejiang-Fujian coast) area; analyses have made it known that ZM area had been one uplifted area, constituting the main provenance of Yong-an Basin together with Wuyi Mt. upwelling area. This cognition is fundamentally unanimous with the findings of regional palaeocurrent direction. The knowledge from above could provide new fundamental referential information for the weak basic research of the basins in southeast China.On the basis of the above research content of "three representative basins", we know that, in Southeast China, sustained Marine depositional sequence from Late Devonian to Early-Middle Triassic developed above the pre-Devonian basement; and after tectonically reworked, it was complexly folded with Meso-cenozoic land depositional sequence, looking on Indosinian event as a symbol. By means of building up sequence strata framework made up of thirty representative columnar sections with nine contour maps made from them (chronologic age from Neopalaeozoic to Early Mesozoic, related areas including provinces of Guangdong, Hunan, Zhejiang, Anhui, and Fujian), to reconstitute the basin shape correspondent with the Marine depositional formation of every stage, describe the features of difference and change about the time and space evolution due to deposition, and explore new thinking on the aspect of theory and technique in the marine oil and gas prospecting of Southeast China, could be realized in the research work including such aspects as regular characteristics showing by strata correlation, distribution regularities of the environment and space developing hydrocarbon source rocks, essential characteristics of the main basins and the distribution regularities of residual outcrops, the relation between structural environment evolution history since Eopaleozoic and oil-gas, and so on. From the study we can see:①Controlled by three major fault zones of Jiangshan-Shaoxing, Ganjiang and Zhenghe-Dapu, the Southeast China Block has the basin-uplift-depression framework consisting of basin districts of south of the River, South China (Wuyi to Nanling), and southeast coast, palaeohigh of Wuyi Mountain, depression zones of Jiangshan-Shaoxing and Yong’an-Meixian.②Unit formation has regional continuance, with the high lithologic stratum depth matching deep water district, the much content of fossil organisms matching good water stability, and on the contrary, matching shallow and strongly active water.③Carboniferous System, Permian System and Triassic System have significant denotation for researching the oil-gas potentiality in Southeast China, among which Longtan Group (or Tongziyan Group, or Maokou Group) is the most optimal hydrocarbon source rock formation.From above research, valuable fruits and innovative recognitions have been acquired, including as follows:①elic basset map from Late Devonian to Paleogene have been first completed;②The structure framework of one paleohigh and four basin areas since Late Paleozoic have been built;③The transformation from Thetis structural domain to Pacific structural domain has labeling function for the changing of sedimentary and structural environment in the interest region;④Longtan Group is the most optimum formation of hydrocarbon source rock, while Bantou Group is secondary;⑤Making use of biology comparison in the sequence strata, the change of regional sedimentary features have been made known, and the relation of time correlation between rock stratigraphic units of different sections in the interest areas has been built up;⑥By means of describing the features of field primary and auxiliary cross profiles and referring to multidisciplinary analysis of predecessors’ fruits, and according to the method of outcrop sequence stratigraphy and referring to the features of biostratum, sequence carbon isotopes, et cetera, paleo-environmental change of the basin domain and regional paleo-environment and tectonic setting have been relatively and obviously combined together;⑦The sequence formation features of Carboniferous, Permian and Triassic in Southeast China has been represented in detail.⑧Jinhua-Quzhou (JQ) basin is one of the typical Meso-cenozoic land facies basins in Southeast China, Meanwhile, Yong’an (YA) basin is one of the representative composite basins not only having the features of Late Palaeozoic marine facies but also having the features of Meso-cenozoic land facies in Southeast China. During the evolutionary process, there are similarities between the two basins about the inner driving force and mechanism transition undergone by them including the conversion from Meso-cenozoic paleo-Asian kinetics framework to paleo-Pacific system, the mechanism transition from compressing to pulling at the early stage of Late Crataceous and the upwelling of mantle materials in the vertical direction. Thus the fractal assumption of energy source mass has come out, that is to say, basin’s outline is the extrincic display of propagating, advancing and folding of the energy source mass surface made up by spot energy source masses distributing with nonhomogeneity along horizontal and vertical direction. The idea has provided not only for the research on the basin structural environment evolution mechanism with new visual angle but also for the prediction of earthquake catastrophe with significant references.