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

Pseudo-desiccation Crack:Special Network Structure from the Thin-Bedded Limestones of the Lower Triassic Feixianguan Formation in Upper Yangtze Region

【目的】上扬子地区西北缘下三叠统飞仙关组二段薄层灰岩中发育类似泥裂的沉积构造,由于前人对其成因和沉积环境存在误解,因此订正该沉积构造的特征及其环境意义很有必要。【方法和结果】通过野外调查及显微薄片观察发现该套灰岩单层厚度一般介于2~10 cm,呈薄板状或者渠槽状,显微薄片显示其为微晶颗粒灰岩,含大量腹足类、介形类、双壳类化石和粪球粒。槽模及渠模构造显示灰岩为频繁的风暴所致,薄板状灰岩中发育的假泥裂沉积构造在岩层顶底面呈不规则四边形或五边形,实为压溶作用形成的方解石脉分割所致,成岩期压实作用及构造作用是假泥裂构造形成的动力学诱因。【结论】飞仙关组中网纹构造的普遍存在预示着灰岩沉积于较深水环境,假泥裂的识别对奥陶系宝塔组网纹灰岩的成因具有借鉴意义。

[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.Pseudodesiccation 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 diageneticepigenetic 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.