乌鲁木齐后峡地区侏罗系沉积特征、剥露过程及中新生代盆山关系讨论

沉积学研究和古流向统计分析表明,侏罗纪时期后峡坳陷不是一个独立的盆地,而是与准噶尔南缘相连的同一盆地。后峡南缘侏罗系底部发育边缘相的冲积扇,表明当时的侏罗纪盆地范围至少达到后峡一带,比现今山前侏罗系分布范围大很多。煤岩镜质体反射率(Ro)分析表明,中侏罗统西山窑组(J2x)的埋深至少在3 km以上。根据磷灰石裂变径迹年龄并结合构造分析资料,提出是晚新生代以来的天山快速隆升过程和前陆冲断推覆构造,分隔了后峡坳陷和天山北缘侏罗系。

Mesozoic-Cenozoic Relationships between Tianshan Mountain and Peripheral Basins: Evidences from Sedimentology and Exhumation of Jurassic in Houxia Area, Urumchi

The Houxia depression is now an intermountain basin near Urumchi, Xinjiang. In the southern Houxia section were developed the alluvial fans in Badaowan Formation (J1b) of the earlest Jurassic, while braided river was developed in the northern section. Therefore, the Houxia area was close to the south edge of the basin in the beginning of Jurassic. The ancient current directions of early to middle Jurassic in the south and north sides of Houxia depression were all northward. We propose that Houxia and southern Junggar were joined together and constituted a unifeied basin in Jurassic, while the border between basin and mountain was close to Houxia area and differed largely from present time. There exist a few intermountain depressions within Tianshan Mountain and they all have Jurassic sediments with coal-containing strata of lower to middle Jurassic. These indicate that their sedimentary environment was very similar and swamps were developed. Based on these facts, we may conclude that there were no high mountains in Tianshan area in Jurassic and it just was hilly. To constrain the sedimentation and exhumation history of Jurassic in Houxia area, we conducted integrated research on apatite fission track dating and vitrinite reflectance (Ro) of coals. The results show that the middle Jurassic coals in Houxia area were buried at least 3 km in depth, and the apatite fission track dating records a rapid cooling event beginning at about 10 Ma, which is interpreted as exhumation of rocks because of the uplift of the mountains and foreland thrusting northward in Cenozoic caused by the India-Asia collision.