Ages of Jurassic volcano-sedimentary strata in the Yanshan Fold-and-Thrust Belt and their implications for the coal-bearing strata of northern China

<p>Jurassic coal-bearing strata are widely distributed in the North China Craton (NCC) and other areas of northern China. These coal-bearing strata were previously considered to be Early–Middle Jurassic in age based on plant fossils, particularly the fossil assemblage of <i>Coniopteris</i>–<i>Phoenicopsis</i>. Since coal-bearing strata are interbedded with volcanic units in the basins of the Yanshan Fold-and-Thrust Belt (YFTB), northern NCC, isotopic dating of the volcanic units can therefore provide age constraints on the coal-bearing strata and the <i>Coniopteris</i>‒<i>Phoenicopsis</i> assemblage. In this paper, we performed a systematic geological survey and present the results of zircon U–Pb dating of the volcanic units and a pluton in typical basins of the YFTB. These data, combined with the results of previous studies, indicate that the ages of the Nandaling/Xinglonggou, Haifanggou, Jiulongshan, and Tiaojishan/Lanqi formations are 180–168, 169–161, 161–157, and 161–153 Ma, respectively. The ages of the interbedded coal-bearing Yaopo and Beipiao formations are constrained to be 169–161 and 177–169 Ma, respectively. Our results demonstrate that both the coal-bearing strata and the <i>Coniopteris</i>‒<i>Phoenicopsis</i> assemblage are Middle Jurassic in age, which is younger than that previously considered. This fossil assemblage plays a critical role in age constraints on the Jurassic coal-bearing strata. The refinement of its age permits a more precise dating of the coal-bearing strata, especially in northwestern China, where datable interbedded volcanic units are lacking. </p><p>Stratigraphical framework for the Jurassic strata of Yanshan region is established.</p><p>The fossil assemblage of <i>Coniopteris</i>‒<i>Phoenicopsis</i> occurred in Middle Jurassic.</p><p>The Jurassic coal-bearing strata in northern China are mainly Middle Jurassic.</p><p></p> <p>Stratigraphical framework for the Jurassic strata of Yanshan region is established.</p> <p>The fossil assemblage of <i>Coniopteris</i>‒<i>Phoenicopsis</i> occurred in Middle Jurassic.</p> <p>The Jurassic coal-bearing strata in northern China are mainly Middle Jurassic.</p>