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<p>An exceptionally thick coal seam in the Middle Jurassic Xishanyao Fm. (NW China) has been investigated for numerous characteristics. The coal exists of several immediately superimposed individual coal layers that originated by coalification of peat that accumulated within a swamp along a lake margin. An astronomical time scale has been established by assuming that the 22.4&#xa0;m cycles represent ~ 405&#xa0;ka eccentricity; this resulted in 11 drying- and wetting-upward cycles. Moreover, an unconventional ~ 173&#xa0;ka obliquity-modulated cycle associated with the hydrological cycle was recognized in a 20.5-m section of the investigated coal unit; it is reflected by humid/arid climate oscillations. The overlying sediment consists of lacustrine mudstones, indicating that the top of the peat was affected by true or relative fluctuations of the lake level. Little is still known, however, about the conditions under which such a thick coal seam could form. Since the thickness of the peat was reduced by 90% (possibly more) during coalification and compaction, at least some 1350&#xa0;m of peat had accumulated. Tectonic activity, though present, cannot explain in itself the ongoing peat formation. The swamp area underwent subsidence, whereas lake area was hardly affected by tectonics; the peat accumulation kept pace with the subsidence, and the increase in the weight of the peat succession caused a tectonically instable situation that resulted in repeated faulting. This caused fluctuations of the groundwater level in the swamp, which explains the alternations of drying and wetting conditions. In conclusion, the thickness of the peat succession can be explained satisfactorily only by ongoing faulting at the boundary between the lake and the swamp areas, and the drying/wetting cycles in the coal should be ascribed to climate fluctuations that were astronomically induced.</p>

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Development of the super-thick (135 m) coal layer in the Jurassic Xishanyao Fm. (NW China)

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  • 发表日期：

    2025年10月1日

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  • Yangyang Huang,
  • Longyi Shao,
  • A. J. Tom van Loon

摘要

An exceptionally thick coal seam in the Middle Jurassic Xishanyao Fm. (NW China) has been investigated for numerous characteristics. The coal exists of several immediately superimposed individual coal layers that originated by coalification of peat that accumulated within a swamp along a lake margin. An astronomical time scale has been established by assuming that the 22.4 m cycles represent ~ 405 ka eccentricity; this resulted in 11 drying- and wetting-upward cycles. Moreover, an unconventional ~ 173 ka obliquity-modulated cycle associated with the hydrological cycle was recognized in a 20.5-m section of the investigated coal unit; it is reflected by humid/arid climate oscillations. The overlying sediment consists of lacustrine mudstones, indicating that the top of the peat was affected by true or relative fluctuations of the lake level. Little is still known, however, about the conditions under which such a thick coal seam could form. Since the thickness of the peat was reduced by 90% (possibly more) during coalification and compaction, at least some 1350 m of peat had accumulated. Tectonic activity, though present, cannot explain in itself the ongoing peat formation. The swamp area underwent subsidence, whereas lake area was hardly affected by tectonics; the peat accumulation kept pace with the subsidence, and the increase in the weight of the peat succession caused a tectonically instable situation that resulted in repeated faulting. This caused fluctuations of the groundwater level in the swamp, which explains the alternations of drying and wetting conditions. In conclusion, the thickness of the peat succession can be explained satisfactorily only by ongoing faulting at the boundary between the lake and the swamp areas, and the drying/wetting cycles in the coal should be ascribed to climate fluctuations that were astronomically induced.