Distribution pattern and geological evolution of combustion metamorphic rocks in the northeast margin of Ordos Basin

Combustion metamorphic (CM) rocks are the main primary product of coal seam fires, commonly found worldwide. Therefore, studying combustion metamorphic rocks is helpful to infer the tectonic movements, paleoclimate, and paleogeography since the formation of coal seams. This study conducts a field geological investigation of the northeast margin of the Ordos Basin using tracing, traversing, and unmanned aerial photography. The northeast margin of the Ordos Basin was divided into the Yuyang District-Shenmu-Dongsheng Jurassic combustion metamorphic area and the Jin-Shaan Valley Carboniferous-Permian combustion metamorphic area. The Jurassic combustion metamorphic rocks in the Yuyang-Shenmu-Dongsheng area are primarily in the upper part of the Jurassic coal measures, while Carboniferous combustion metamorphic rocks in Jinshan Valley are mainly in the upper part of Carboniferous Permian strata along the Yellow River in the study area. The study's findings show the influence of multi-phase tectonic movements on the combustion metamorphic rocks of the study area. Due the tectonic events during Indo-Chinese, Yanshan, and Himalayan orogenies, the Jinxi flexural fold belt and the Yimeng uplift formed in the study area's eastern and northern parts. Jurassic and Carboniferous-Permian coal seams were also exposed on the surface due to weathering and denudational-erosional activities of the Yellow River and its tributaries. The multi-phase tectonic movements developed conditions necessary for their combustion, resulting in the formation of two combustion metamorphic rock zones: Yuyang District-Shenmu-Dongsheng Jurassic and Jin-Shaan Valley Carboniferous-Permian.