Late Mesozoic Volcano-sedimentary Characteristics And Basin Evolution Of Shangyi Basin, Northwestern Hebei Province

Shangyi basin is located in the transition zone of Yingshan and Yanshan orogenic belt. Late Mesozoic geological record is well-preserved in basin. It is significant to the study of Yanshan movement, destruction of the North China Craton, tectonic domain conversion and biotic succession. Previous studies have made great progress and achievements in the stratigraphic sequence, sedimentary characteristics and source characteristics of the basin. But there is still a lack of comprehensive study of sedimentology, stratigraphy, volcanology of the basin. Few studies have been conducted on peperites in the basin. This article aims to do research on the Late Mesozoic strata filling sequence, volcanic rocks and peperites, sedimentary environment and sedimentary phase and provenance analysis of Shangyi basin, through comprehensive sedimentology, stratigraphy and volcanology means. Thus we can restore the paleogeography and paleoenvironment of the Late Mesozoic Shangyi Basin, establish evolution model of the basin and discuss the properties of the basin.It is showed that Jurassic to Early Cretaceous strata is well-developed in the basin. Apart from the omissing part of the upper Jurassic, most strata are well exposed. Lower-middle Jurassic is composed of coarse clastic rocks and coal bearing strata interbeded with volcanic rocks. The Upper Jurassic-Lower Cretaceous consisted of purple-red clastic rocks. According to the characteristics of the rock stratigraphy, paleontology and the evolution of regional basins, we redefine the Mesozoic stratigraphic sequence of the basin. From bottom up, the strata are composed of Early Jurassic Xiahuayuan Formation, Middle Jurassic Longmen formation, Middle Jurassic Jiulongshan formation, Late Jurassic-Early Cretaceous Tuchengzi Formation and Early Cretaceous Yanjiayao Formation. This scheme can be well-correlated with the Yinshan-Yanshan area.The study also found traceable surround-basin volcanic rock belt were developed in Middle Jurassic Jiulongshan Formation in Shangyi basin. Peperites are developed in the contact area of volcanic rock and sedimentary rock. Peperite is a special type of volcaniclastic rock. It is the production of magma mingling with wet unconsolidated sediments. Peperite can often be described on the basis of juvenile clast morphology as blocky or fluidal. Blocky are the peperites developed in Jiumiaogou, Beicaonian and Huangtuliang areas in Shangyi basin. Fluidal are the peperites developed in Baijiayao, Caimuliang and Touhao areas. This study further through the research of petrology and petrography characteristics of volcaniclastic rock establish the volcano-sedimentary sequences of peperite and discuss the formation mechanism of peperite. In addition, according to the previous study of tuff chronology in Jiulongshan Formation in Chengde basin, we consider that the age of the volcanic rocks developed in Jiulongshan Formation in Shangyi basin is also about 163.4±1.1Ma.Through systematic study on the lithofacies, texture of rocks and sedimentary structure in the basin,19 types of lithofacies and 5 types of sedimentary facies are recognized.5 types of sedimentary facies are desert, alluvial fan, river, delta and lake facies. Through comprehensive gravel composition, palaeocurrent and detrital zircon research, it is showed that the west of Shangyi basin is relatively high and provenance mainly comes from the west in early Jurassic. In Middle Jurassic, the north of the basin gradually uplifted and become source area. During Late Jurassic to Early Cretaceous, it is similar with the Middle Jurassic north of the basin further uplifted and be a major provenance. At the end of Early Cretaceous, west of the basin began to uplift. In the middle-late period of Cretaceous, the basin fade away and be the denudation area.We conducted systematic research on features of sedimentary environment and sedimentary facies, provenance, volcanic rocks and peperites, dinosaur footprint of Shangyi basin. Mesozoic stratigraphic framework and the comprehensive evolution model of the basin are established. The late Mesozoic paleogeography and paleoenvironmental characteristics of Shangyi basin are restored.