假泥裂:上扬子地区下三叠统飞仙关组灰岩薄层中的网纹沉积构造OA北大核心CSTPCD

[Objective]The deposits of the Lower Triassic Feixianguan Formation are widespread in the Sichuan Basin.In this formation,the Second member is dominated by brown purple mud shale.Several researchers have main-tained that it was deposited in tidal flat environment,and'mud crack'is one of the distinguished bases of its environ-ment.In this study,we find that the so-called mud cracks are developed in lamellar microcrystalline granular lime-stone in the Second member of the Feixianguan Formation.Although they show an overall grid shape in the bedding plane,they are not'V'or'U'shaped in cross section.In fact,they are split grid cut by sparry calcite veins pene-trating the rock,significantly different from real mud cracks.As depositional structures,they are different but have a similar morphology.We call them'pseudo-desiccation cracks'to distinguish from'mud cracks'in this study.Pseudo-desiccation cracks do not represent the exposed paleoenvironment,and they cannot be used to infer that the Second member of the Feixianguan Formation was deposited in a tidal flat environment.Therefore,the study regarding the features of pseudo-desiccation cracks and their forming environment will be helpful in correcting misunderstanding and recognizing a depositional environment.[Methods and Results]The structure of the studied pseudo-desiccation cracks is developed in the surface of lamellar microcrystalline granular limestones in the Second member of the Feixianguan Formation,and the study area is located in the Dagouli profile,Qingchuan county,in the northwestern Upper Yangtze region.Limestone beds are interbedded with brown purple mudstones and occur frequently as thin lay-ers in the sequence.Field survey determined that the thickness of the limestones is 2-10 cm,and they are sheet or channel shaped.Microsection observation indicate that they are microcrystalline granular limestones and contain a large number of gastropods,ostracods,bivalves,and coprolite pellets.Flute cast and channel mold structures are de-veloped in the limestone,which show the influence of frequent storms during deposition.Irregular quadrilaterals or pentagons are developed on the top and bottom surfaces of the laminated limestones,the result of segmentation by cal-cite veins under compaction and dissolution.The diagenetic compaction and tectonics is likely the dynamic induce-ment for the formation of pseudo-desiccation cracks.[Conclusions]This study has clarified that the so called'mud cracks'in the Second member of the Feixianguan Formation in the northwestern Sichuan Basin are diagenetic-epigenetic products and do not represent the exposed environment.Therefore,the Second member of the Feixianguan Formation was deposited in a shallow shelf environment under normal wave base and above storm wave base in the northwestern Sichuan Basin.The identification of pseudo-desiccation cracks is helpful for the study on the formation of reticulated limestone in the Ordovician Baota Formation.Strong compaction in the early diagenetic period indicates fast deposition rate for the overlying beds(Feixianguan Formation)or special tectonic environment(Baota Forma-tion).Relatively active tectonic movements in the Ordovician could promote the formation of reticulated structure in the Baota Formation.