Sedimentology and sequence stratigraphy of the mixed clastic-carbonate deposits in the Late Paleozoic icehouse period:A case study from the northern Qaidam Basin

The widely-developed,mixed clastic-carbonate succession in the northern Qaidam Basin records the paleo-environment changes under the glacial activity during the Late Paleozoic icehouse period in the context of regional tectonic stability,however,the depositional environment and sequence stratigraphy characteristics of the mixed deposits is rarely reported and still not clear.Combined the latest drilling wells data,we analyzed the sedimentary and stratigraphic characterization of the mixed strata via detailed field outcrops and core observations and thin section microscopic observations and recognized three depositional systems,including progradational coastal system,incised valley system,and carbonate-dominated marine shelf system,and identified four third-order sequences,SQ1,SQ2,SQ3 and SQ4,consisting of LST,TST,and HST.The depositional environment is overall belonged to marine-continental transition context and shifted from marine to continental environment frequently,showing an evolutionary pattern from marine towards terrestrial-marine transition and then back into the marine environment again in the long-term,which was controlled by the regional tectonic subsidence and the high-frequency and large-amplitude sea-level changes due to the Late Paleozoic glacial activity.The result is of significance in understanding the evolution of the Qinghai-Tibet Plateau and the sedimentation-climate response.

Sedimentary and evolutionary characteristics of Sinian in the Tarim Basin

Analysis of Sinian outcrops in the peripheral regions of the Tarim Basin,drilling and seismic data within the Tarim Basin,and combined with previous studies,distribution of Sinian strata,tectonic setting,sedimentary system and lithofacies palaeogeography were well discussed.Results showed that three sedimentary systems were developed in the Sinian System of the Tarim Basin,i.e.the clastic sedimentary system,the clastic-carbonate mixed sedimentary system,and the carbonate sedimentary system,vertical sedimentary characteristics evolved from the clastic sedimentary system to the carbonate sedimentary system.These three sedimentary systems were corresponded to three transgression-regression cycles in the Sinian.The marine transgression during the initial period of the Early Sinian(ZSQI)resulted in development of the clastic sedimentary system in the lower part of the Lower Sinian;the transgression during the late period of the Early Sinian(ZSQII)led to formation of the clastic-carbonate mixed sedimentary system in the upper part of the Lower Sinian;the transgression during the initial period of the Late Sinian(ZSQIII)resulted in development of carbonate sedimentary system in the Upper Sinian.Three regressions resulted in formation of three unconformities,which were on the middle part of the Lower Sinian,the top of the Lower Sinian and the top of the Upper Sinian respectively.Due to breakup of Rodinia supercontinent in the Neoproterozoic,the Tarim Basin was in the continental rifting evolutionary process.In the Sinian,the basin experienced evolutionary processes of intra-cratonic depression and passive continental margin,thus characteristic of the lithofacies palaeogeography was characterized by uplifts in the south and depressions in the north.In the Early Sinian,the Tarim Basin developed the Bachu-Tazhong-Tadongnan uplift and the Tabei residual ancient land,and the shore-shelf sedimentary environment in the north as well as the shore-shelf-bathyal sedimentary environment in the southwest.In the Late Sinian,the Tabei residual ancient land disappeared;the basin developed the tidal flat sedimentary environment in the north and the neritic sedimentary environment of the passive continental margin in the southwest.