Sedimentology and basin analysis of the Early Cretaceous Hekou Group in Lanzhou-Minhe basin

Lanzhou-Minhe basin is a typical intracontinental rift basin in northwest Gansu formed in the Early Cretaceous.The Hekou Group in Lanzhou-Minhe basin is characterized by purplish red clastic sediments with a variety of grain sizes.On the basis of previous researches,the authors conducted field sedimentary investigations and studies of sedimentary facies,provenance and palaeocurrent of the Hekou Group along different stratigraphic sections such as Hekou,Yanguoxia and Zhongpu.The results indicate that the Hekou Group experienced an evolution of sedimentary faices from alluvial fan through fan delta and lacustrine/fluvial facies to deltaic sedimentary facies.Provenance and palaeocurrents suggest that the uplifted Qilian Mountain around the basin was the main material source of sediments during the Early Cretaceous.It is also shown that the previous Datonghe Formation belonged to the Late Jurassic-Early Cretaceous period and the Heko Group was deposited since the earlier Early Cretaceous.In addition,dinosaur fossils and pollen assemblages in the Heko Group and their environmental implications were described as well.Finally,a reconstruction model for palaeogeography of Lanzhou-Minhe basin in the Early Cretaceous was established.