Sedimentary Filling And Tectonic Thermal Events Since Late Cretaceous In The Songliao Basin

Volcanic sedimentary formations in Songliao Basin since late cretaceous?K₂?have been relatively well preserved and the tectonic evolution of the basin has been comprehensively recorded.This study focuses on two scientific issues in Songliao Basin,such as the time frame of large-scale tectonic inversion and the uncertainty of the geodynamic background of the basin,as well as the correlation between tectonic inversion and uranium mineralization of sandstone type within the basin.A total of 74zircon and apatite fission trace samples were collected from 26 oil fields in different tectonic units in the south of the basin.The samples were mainly distributed in graben and horst,and the depth of the samples was 1500-4000 m,according to the dating of zircon and apatite fission tracks by low-temperature thermochronology,there have been two tectonic evolution stages in Songliao Basin since late cretaceous.The first stage was overall tectonic uplift during late Cretaceous-Paleocene?80-50Ma?.The main performance is that?1?The sedimentary facies changed from lake facies to fluvial facies.Upper Cretaceous:Including the Sifangtai Formation—Minshui Formation began to be sedimentary and diagenetic,the particle size of the clastic material is gradually coarser and shows reverse grain sequence sedimentary characteristics,It indicates that the basin uplifted and the water became shallower during this period.By the time the paleocene basins were not receiving deposits,the late cretaceous to paleogene strata were missing,and a small amount of paleogene Yi'an Formation(E_2-3y)was deposited in some areas,it shows that the period is always in the uplift and denudation stage.?2?Basic magmatic activity is relatively intense.Alkaline magmatic activities such as Shuangliao olivine basalt,Daqing northwestern mugearite,Tongliao Qianjiadian and Siping Tuanshanzi diabase developed along the east-northeast central fault zone.Intermittent continuous activity since 88-50 Ma indicates a continuous extensional tectonic background.?3?Fission track data show that tectonic uplift events occurred in different tectonic units throughout the basin in this period,which was universal.There was no significant difference between the south and the north of the basin,and the tectonic uplift events experienced on both sides of the typical ne trending fault were the same.The second stage was tectonic inversion during oligocene-miocene?40-10Ma?.The main performance is that?1?In this period,there are few sediments in Songliao Basin.the Da'an Formation and Taikang Formation of Miocene are deposited sporadically in some areas,angular unconformity is above the upper cretaceous Sifangtai Formation,Mingshui Formation and Nenjiang Formation.?2?Basic magmatic activity is weak.Alkaline Basaltic magmatism develops sporadically along the central fault zone in Songliao Basin.For example,alkaline magmatic activities such as the Shuangliao olivine basalt and Siping Tuanshanzi diabase were carried out in periods of 37-29.5Ma and 5.8Ma,and the magmatic activity intensity and time were significantly reduced compared with the previous stage,which was a reflection of stress relaxation in the process of tectonic extrusion and uplift.?3?Fold structure and thrust fault structure are strongly developed.The most obvious structural feature of this stage is that the large-scale ne fold and thrust fault structure are very developed,forming a series of ne fold uplifts and depressions in the basin,which are covered by angular unconformity of the Da'an Formation and taikang formation.The left-lateral reverse Strike-slip fault of 15.6-0.7Ma in the southern margin of the basin indicates the compressive tectonic background of this period.?4?According to the results of zircon and apatite fission track chronology,the uplift time of different tectonic units in this period was significantly different,the uplift time was earlier in the south of the basin and later in the northwest,tectonic uplift events of the typical ne upper wall are mainly in the stages of 80-50Ma and 40-10Ma,however,no 40-10Ma event was recorded in the footwall of the fault,indicating that the uplift speed of the two plates was different in the later tectonic stage,which resulted in the change of the Songliao Basin from the early normal fault to the reverse fault.Meanwhile,the rifting area and the fault ridge area are also obviously different.The former experienced the aforementioned two tectonic events,while the latter only experienced the early tectonic events,indicating that the Songliao Basin did not experience the latter at the same time.The two stages of Pacific plate activity are obviously different.In the first stage,the subduction direction of the Pacific plate is NNW with a relatively large subduction Angle,and the subducted oceanic crust enters the mantle.In the second stage,the subduction direction of the plate changed from nnw trending to NNW-trending,which started from 50±0.9Ma and lasted to 41.5±0.3Ma and lasted for about 10Ma,which was consistent with the fission track dating data in the 50-40 Ma tectonic thermal event interval.It is important to understand that structure evolution and regional dynamics of the tectonic evolution and regional dynamics of the Oligocene-Miocene tectonic inversion in the Songliao Basin.Considering that the tectonic inversion stage coincides with the metallogenic age of leaching sandstone-type uranium in the Songliao Basin,as well as the sandstone-type uranium deposit has the characteristics of hydrogenesis,dynamics and openness,It is proposed for the first time that the last uplift structure restricts the formation of in-basin leached sandstone-type uranium deposit,which is of great significance to explain the age continuity and rejuvenation of uranium mineralization in international and domestic fields.