Conglomerate characteristics and alluvial fan evolution of the Triassic Lower Karamay Formation in Karamay Oilfield, Junggar Basin

Based on core observation, five types of conglomerates are divided in the Triassic Lower Karamay Formation, Karamay Oilfield, i.e., the matrix-supported breccias, weak-cemented conglomerates (strictly speaking, this type is still uncemented), mud-supported conglomerates, sand-supported conglomerates and well sorted granule-conglomerates. Compared to the sediments of modern alluvial fans, the depositional environment of each conglomerate in the Lower Karamay Formation can be determined as following ; in the early stage, due to all arid climate, the conglomerates have similar characteristics to that of modern arid alluvial fans in Zaier Mountain area. However, in the late stage a humid climate prevailed, and the fluvial process become dominant, with the deposited conglomerates showing similar characteristics to that of Baiyanghe alluvial fans in Fukang. So, vertically, the alluvial fans showed a retrogradational sequence towards the ancient Zaier Mountain.