Responses of two lithosomes of Lower Cretaceous coarse clastic rocks to tectonism in Kuqa foreland sub-basin, Northern Tarim Basin, Northwest China
Abstract Coarse clastic rocks, developed widely at foredeep adjacent to orogene, were usually regarded as the mark of renewed thrusting action. However a good example from the Lower Cretaceous in Kuqa foreland sub-basin illustrated that the coarse clastic rocks developed not only in initial thrusting stage but in early quiescence stage. The Kuqa foreland sub-basin, adjacent to the south Tianshan Orogene, lies to the north of Tarim Basin, northwest China. The Lower Cretaceous outcrop in the front of south Tianshan Orogene was identified as a filling sequence of foredeep which includes two depositional cycles of Kapushaliang Group and Bashijiqike Formation. Each of them presents fining-upwards and then coarsening-upwards sequences. Two lithosomes of coarse clastic rocks developed in the lower of Yageliemu Formation of Kapushaliang Group and Bashijiqike Formation, respectively. Both of them are mainly composed of thick-bedded, massive and chaotic conglomerate with debris flow microfacies of fan delta plain. The analyses on stratigraphic and depositional records for tectonism revealed abrupt sedimentary change from delta facies to fan delta facies, stratal geometry transformation, and reflection termination migration of truncation and top-lap between the Kapushaliang Group and the Bashijiqike Formation. These records indicated that: foredeep subsided rapidly while forebulge uplifted in the development of Kapushaliang Group, which coincided perfectly with lithosphere deformation during thrusting stage; foredeep uplifted rapidly while forebulge subsided in the later development of Bashijiqike Formation, which coincided with lithosphere deformation during quiescence stage of foreland basin development. Therefore, the Lower Cretaceous recorded a whole episode of foreland basin evolution from earlier thrusting to later quiescence. The underfilled cycle of Kapushaliang Group, corresponding to the thrusting stage, is much thick with mainly muddy and silty sediments. The overfilled cycle of Bashijiqike Formation, corresponding to the quiescence stage, is thin with mainly sandy sediments. The two lithosomes of coarse clastic rocks were very similar with each other in the lithology and sedimentology. However, the one from lower Yageliemu Formation is responded to the renewed thrusting, but the other from the lower Bashijiqike Formation is responded to the lithosphere rebounding during primary quiescence stage. Therefore, cannot only coarse clastic rocks indicated a renewed thrusting.