| Sequence stratigraphy and its sedimentary filling are sedimentary responses to the tectonic evolution of basins,in which the sedimentary records related to volcanic activities are important material basis for understanding the basin filling and tectonicvolcanism.The Shahezi Formation,as the major rift tectonostratigraphic unit of the Songliao Basin,its tectonic-volcanic-sedimentary study is an effective way to explain the sedimentary filling and tectonic evolution,and is also of great significance to explore the formation mechanism of Songliao rift basin.Well SK-2,a continental scientific drilling well in the Xujiaweizi fault depression of the Songliao Basin,has drilled through ca 2359 m of the Shahezi Formation,contacting directly with the Middle Triassic basement.In this thesis,we focus on the Shahezi Formation and take sequence groups as stratigraphic units.From perspective of volcanism-induced sedimentation and taking syn-eruptive pyroclastic deposits as probes,we study sequence stratigraphy and sedimentary filling based on the data of drilling,cores,seismic profile,and well logging.We identified and described cores,identified and counted syn-eruptive pyroclastic deposits,described mixed sequence filling architecture,analyzed and interpreted stratigraphy of the seismic profile,and investigated the geochemical and detrital zircon U-Pb chronology characteristics of rocks from well SK-2.Based on these data,we evaluated the controls of tectonic activities on sequence stratigraphy,determined the source of intermediateacidic volcanic materials,traced the tectonic-volcanism events and their fine processes under the background of rift development,determined the formation mechanism and evolution of the major rift stage.The following results and realizations were obtained from this research:(1)The Shahezi Formation includes characteristic mixed sediments and weathering sediments.We identified 17 types of lithofacies in the Shahezi Formation from well SK-2.There are five lithofacies closely related to volcanism,including volcanic breccia,tuff,and tuff sandstone-conglomerate with scour surface and massive-graded bedding;breccia tuff,tuff,and tuff sandstone with parallel-wavy-cross bedding;tuff breccia and glutenite with massive-graded bedding;tuff,tuff sandstone,and mudstone with scour surface and wavy-deformed bedding;and tuff breccia,breccia tuff,and tuff conglomerate with scour surface and massive-graded-deformed cross-bedding.There are 12 lithofacies closely related to weathering,including four related to tractive flow,six related to gravity flow,and two related to background deposition(deep water mud-bed and peat swamp).(2)There are 11 lithofacies associations made up with the 17 kinds of lithofacies mentioned above.We identified six lithofacies associations related to volcanism,including(1)superimposition of pyroclastic flow and mud-bed,(2)superimposition of base surge and pyroclastic flow,(3)pyroclastic flow,mud flow and turbidity flow,(4)base surgeturbidity flow,(5)superimposition of pyroclastic flow-gravity flow channel,and(6)the superimposition of pyroclastic flow and peat swamp.The six volcanic lithofacies associations reflect a transitional process from a volcanic environment to a lacustrine environment.We also determined that five lithofacies associations are related to weathering:(7)subaqueous channel,mouth bar,distal bar,mud-bed,and occasionally liquefied flow,(8)gravitational channel,gravitational bar,mud-bed,and occasionally slump-induced gravity flow,(9)turbidity flow,turbidity channel,and mud-bed,(10)gravitational channel and mouth bar,and(11)peat swamp and abandoned channel/oxbow lake.These associations represent a variety of source to sink depositional environments.With these lithofacies associations,we establish a volcanic provenance-depositional system model.Using direct observations from well SK-2,we also develop an integrated geological model that is evolved by a mixed depositional system.The integrated geological model shows both vertical evolution and horizontal changes in the depositional environments and volcanic activities.(3)According to the triangular plots of detrital composition statistics,the provenance supply has the characteristics of both volcanic and weathering provenances in the Shahezi Formation.The lower Shahezi Formation was characterized by the volcanic materials derived from the syn-volcanic activity,and consisted two evolutions from the dominated volcanic provenance to the dominated weathering provenance.The upper Shahezi Formation was only affected by the weathering provenance,and was characterized by a transition from quartz-rich to lithic-rich.Whole-rock geochemistry of rocks from the Shahezi Formation revealed that the source rocks are felsic,andesite,and sedimentary rocks from the upper crust.The depositional age of Shahezi Formation is further constrained by detrital zircon U-Pb chronology.The deposition age of the lower Shahezi Formation is about 118～115Ma,and that of the upper part is about 115～112Ma.The cumulative probability plot indicates that the detrital zircons in the Shahezi Formation are mainly zircons of similar formation age or contemporaneous with sedimentation,and the basin is under the tectonic background of convergent plate.(4)Based on the characteristics of seismic reflection,core,and logging,we divided the Shahezi Formation and determined the sequence filling architecture.The Shahezi Formation is divided into upper and lower parts.The lower part is composed of sequence groups SSI(Sequence 1,2,and 3)and SSII(Sequence 4 and 5),while the upper part is composed of sequence groups SSIII(Sequence 6,7,8,and 9)and SSIV(Sequence 10 and 11).The two parts show obviously different filling characteristics.The lower part includes characteristic volcanic-induced sedimentation,which is composed of both pyroclastic and weathering-derived clastic deposits.The sequence groups SSI and SSII both have the sequence architecture that varies from dominantly pyroclastic fill to dominantly weathering-derived clastic fill.The upper part no longer developed syneruptive pyroclastic deposits.The sequence groups SSIII and SSIV are both dominated by weathered-derived clastic deposits,with an overall inverse grading succession,characterized by the development of coarse deposits and tractive flow.(5)According to the geological interpretation of basin tectonic pattern and stratigraphic structure in seismic profile,the thinning direction switches back and forth during the formation of the Shahezi Formation’s four sequence groups;that is,there is turnover superimposition in vertical stratigraphic subsidence,demonstrating strikeslipping during deposition of the Shahezi Formation.There are three kinds of pyroclastic deposits in the Shahezi Formation: rhyolitic,andesitic,and tuff fragments.Under the strike-slip system,the contemporaneous rhyolitic-andesitic pyroclastic deposits in the Shahezi Formation show intermediate-acid volcanic activity,which is related to the activity of the local fault—branch of the Xudong strike-slip fault.The integration of geochemical data and tectonic mechanism analysis suggests the intermediate-acidic volcanic materials originated from partial melting crustal materials and are hybridized with input from the mantle.(6)Through the sequence filling architecture,volcanic activity evolution,fault tectonic activity,and basin structure style,it is synthesized that the major rift stagedeposition of the Shahezi Formation was formed under the strike-slip mechanism.Combined with the geometry of stratigraphic turnover superimposition in the Shahezi Formation,the abundant dark mudstone and coarse clastic progradation in the transgressive stage,as well as the periodicity of volcanism in the lower Shahezi Formation are the results of strike-slip extension;the abundant coarse deposits and tractive flow,coarse clastic progradation in the transgressive stage,as well as the structural inversion with stratigraphical loss in the upper Shahezi Formation are the result of strike-slip compression.Under the influenced of the Paleo-Pacific plate’s subduction,within the large-scale strike-slip the system,Xuzhong strike-slip fault and the two segments of Xuxi fault are the results of strike-slipping and associated wrench mechanism.The Xuzhong fault’s strike-slip activity also causes a regional unconformity interface in the middle Shahezi Formation.(7)Considering the tectonic-volcanism and sedimentary filling characteristics of the Shahezi Formation,we analyzed the hydrocarbon exploration potential.The volcanic activity during deposition of the Shahezi Formation not only provided an important material foundation for the formation of high-quality hydrocarbon source rocks,but also promoted the formation and maturation of deep-lake oil shale and swamp coal seams.Meanwhile,the regional unconformity in the middle part of Shahezi Formation caused by tectonic movement can be used as a good migration channel for lacustrine source rocks in the lower Shahezi Formation and glutenite in the upper Shahezi Formation.Combined with the gas logging data,it is concluded that the Shahezi Formation,especially the upper part,has a good potential for unconventional oil and gas resources. |
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