Sedimentary response of the Shangyi basin, northwestern Hebei, to the Late Jurassic tectonism

Abstract：The Shangyi basin is located in the west segment of the frontal basin group of the Mesozoic nearly E-W-striking Yanshan thrust belt, where voluminous coarse fragments were accumulated during the deposition of the Late Jurassic Tuchengzi Formation. The Tuchengzi Formation in the basin shows a pronounced lateral sedimentary facies change from coarse gravelly deposits in the north to silty deposits in the south, displaying typical characteristics of asymmetric facies belt distribution. In the early-middle stages of deposition of the Tuchengzi Formation, there generally occurred conglomeratic alluvial fans (including mudflow deposits), braided streams and flood-plains and/or dried lakes from north to south. In the late stage of deposition of the Tuchengzi Formation, sandy and gravelly braided-river deposits were predominant, possibly intercalated with sandy dune deposits with large-scale eolian cross-bedding. Metamorphic rocks and granitoid rocks are dominant clastic components of conglomerate, indicting that the provenance was mainly the "Inner Mongolian axis" to the north of the basin. The sedimentary fillings have the nature of syntectonic conglomerate. A series of imbricate thrusts are developed at the northern margin of the basin. Preliminary analysis of the coupling relation between the faults and basin shows that the structural load of the thrust on the northern margin was an important factor for controlling the formation and evolution of the basin. The middle and lower parts of the sedimentary facies series near the side of the thrust to the north have the coarsening-upward characteristics, probably implying that the process of deposition of the Tuchengzi Formation was controlled by progressive thrust loading and lithosphere flexuring. Based on the relations between the basin and its related faults to the north, south-fining clastic size change and south-thinning, wedge-shaped section of the sedimentary basin, it is inferred that the basin formed in the continental interior in a N-S subhorizontal compressional setting.