Depositional Process For The Mesozoic Epicontinental Marine Basin In Northern South China Sea

The Mesozoic epicontinental marine basin in the northern South China Seacovers the South China epicontinental region and northeast South China Sea, locatesin the overlapping intervals of Huaxia and Tethys tectonic domain and west pacifictectonic domain. Since the found of sea domain Mesozoic in1980s, to the drillingconfirmation of Mesozoic sedimentary bed series, this Mesozoic basin in South ChinaSea north sea domain is focused widely by geophysics and petroleum geologists inand aboard. Around basin tectonic evolution, magmatite activities, sedimentary fillingseries, formation distribution, hydrocarbon resource capacities and etc. predecessorshave done plenty of research. Multiple large sedimentary depression (such asChaoshan depression) and sedimentary reservoir and effective source rock are foundin South China Sea north sea domain, most scholars deem large hydrocarbonexploration capacities in this area. However, because Mesozoic drilling data is little,drilling formation is uncompleted, and the quality of seismic data is poor, the timeattribution of Mesozoic seismic reflection beds in sea domain does not confirmed,sedimentary formation system does not build completely, and the cognitionsedimentary evolution history is very rough.In this paper, firstly, according to the sequence stratigraphy contrast of6outcropprofiles in South China land domain, based on sedimentary tendency, sequence of lateTriassic to early Jurassic is divided; according to paleontology and sedimentaryenvironment data of MZ-1in sea domain, part sequence in mid-Jurassic to lateCretaceous is divided. Then, On the basis of the4principles: global development atalmost the same time of long circle sedimentary cycle (sequence), formation packingmethods same or similarity of secondary cause formation unit in different depressionof the same basin, continental facies formation paleo-climite condition similarity, andthe same volcano activity in the basin, based on the defined boundary and otherknown conditions, Mesozoic seismic reflection formation time in sea domain isinferred gradually. At last, based on the special formation development packingmethod of different stages during the basin evolution, Mesozoic formation is dividedinto4seismic contrasted formation units: low-stand filling (late Triassic),retrogradation and aggradation (early and middle Jurassic), foreset (late Jurassic toearly Cretaceous) and low-stand filling deposition (late Cretaceous). On the basis ofabove, seismic formation interpretation is launched all over the study area, regionalformation profiles and chronostratigraphy framework map across the basin arecompiled, and sedimentary evolution process of the basin is dicussed. The main results in this paper are as follow:(1) The study thought breaks the traditional constraint, using sequencestratigraphy theory; Mesozoic formation system is built in sea domain. As the lack ofcomplete continuous formation contrasted data (such as paleontology,paleomagnetism, lithofacies, zircon, or Oxygen isotopic dating and etc.), the keyformations are defined by current data. On the basis of this4formation contrastedprinciples: seafacies long circle isochronal, the piling way of factor stratigraphic unitin the stair circle is identical, the paleoclimagnetsm condition of continental faciesenvironment is identiacal roughly(such as rock facies and special mineral is similar,the time of volcanic activity is isochronal roughly. According to the feature ofdevelopment of second to third circle sedimentary cycle(sequence) at the same timeglobally, the relationship with stratigraphic age, and development orders, stratigraphicage of Mesozoic part seismic reflection beds in sea domain is inferred, andstratigraphy system is built. Although the stratigraphy system got from above willexist error, it resolves the difficulty of stratigraphic age definition which needs to besolved urgently currently. Because based on the corresponding relationships betweensecond-third sequence and stratigraphy unit(stage), the deduction of stratigraphic ageis available. Hongzhen Wang (1988) even suggested that on the basis of middle-longcircle sequence, new generation chronostratigraphy scale is built. Otherwise,Mesozoic formation and sedimentary research in the north of South China Sea maystill remain the confusion of inner formation of macroscopic distribution research; thestratigraphy system may not be built ever. Because according to the traditional methodof building the stratigraphy system, it needs to drilling couples of wells with the depthover8000m in the center of Chaoshan depression, which is unavailable.(2) Using continent domain geologic map, wild field investigation data andseismic data in sea domain for the first time, regional stratigraphy profile across SouthChina epicontinent-South China Sea north sea domain is compiled. This736kmprofile provides the base for the basin tectonic evolution since Triassic in north part ofSouth China Sea.(3) Synthesized outcrop data of South China continent domain, seismicformation interpretation results of sea domain, continent-sea formation comparison,and all data of stratigraphy and sedimentary environments in peripheral regions ofnorth South China Sea (such as Indo-China Peninsula and Philippines), late Triassic toCretaceous chronostratigraphy framework map is compiled. It displays completely theformation development distribution situation of Mesozoic basin on South Chinaepicontinent-north shelf of South China Sea, especially stratigraphy distribution andseismic reflection texture of Chaoshan depression to continental slope trough, andreveals the shoreline retreat location of the end of early and late Cretaceous; accordingto the Mesozoic formation division and sedimentary process research in the north partof South China Sea, the main geologic events happened in the Mesozoic deposition process (such as at the end of J1, seawater retreat from epicontinent; at J3, volcanoson the seabed erupt and the shelf uplift in the north part of South China Sea; at the endof K1, ocean and continent crusts crash strongly, and seawater retreat to the edge ofshelf in the north part of South China Sea, continental slope trough and transitionalcrust form; etc.) contact each other, and the time node can be defined to stage. Theseresults is significant for the geologic history of South China-north part of SouthChina Sea even the tectonic sedimentary evolution of paleo-pacific.(4) Depressions in the north part of South China Sea domain and South Chinacontinent domain are all depression filling deposition in the late Triassic, with thesame sedimentary evolution history. That makes Hanjiang and Chanshan depressiondevelop high quality source rock of semi-blocking estuarine facie in the middle of lateTriassic. Chaoshan depression and south area, early Jurassic to early Cretaceous is indeep water deposition environment, it provides a few of continental margin, thedeposition rate is low, the abundance of organic matter in Chaoshan depression andsouth area is likely many times larger than south China (organic carbon of continentdomain shale in early Jurassic is0.1~0.7%, MZ-1deep sea shale is greater than1%,Jurassic shale drilling wells in west of Taiwan is proved to be source rock. Moreover,Jurassic delta-deep water fan sandstone and mudstone(source rock) develop mutuallyin Chaoshan depression, Cretaceous forced regression sedimentary sandstone coverson the Jurassic deep water mudstone, which form good source-reservoir-cap rockassociation, making Chaoshan depression-continental slope trough area Mesozoichave large hydrocarbon exploration foreground.