前寒武纪细粒碎屑岩中纺锤状裂缝的成因分析:以燕山东部长城群为例

燕山东部天津蓟县至河北兴隆一带中元古界长城群细粒碎屑岩中,普遍发育有在层面上表现为纺锤状裂缝的沉积构造,并曾经被解释为后生动物遗迹化石或地震震荡液化脉等。这种沉积构造常与变余波痕、皱饰构造等相伴产出。根据初步观察与研究的结果推断,纺锤状裂缝可能是由于微生物席对沉积面的封闭作用,导致沉积物在成岩作用早期发生脱气与脱水等作用的产物,因此可以将其归入微生物参与形成的原生沉积构造(席底构造)的范畴。纺锤状裂缝与其他的席底构造一样,为前寒武纪的沉积环境重塑提供了一个重要的证据,同时也说明在前寒武纪的地球表层环境中,微生物以其新陈代谢的多样性及巨大的生物量,在沉积纪录中留下了重要的烙印。

Original Analysis of the Spindle-Shaped Cracks in the Fine Clastic Rocks of the Precambrian: An Example from the Changcheng Group of the Eastern Part of Yanshan Area

In the region from Jixian County of Tianjin City to Xinglong County of Hebei Province in eastern Yanshan,the sedimentary structures which present the spindle-shaped crack on the surface are widely developed in the fine clastic strata of the of the Changcheng Group of the Mesoproterozoic,and they have been explained as the metazoan trace fossils or the seismic-induced liquefied veins etc.The structures always co-exists with the palimpsest ripples,the wrinkle structures,and so on.The preliminary studies indicate that the spindle-shaped cracks might be the products of degasing and dewatering of the sediments during early diagenesis,which are caused by the seal of overlying microbial mat,thus,they can be grouped into the category of the microbially induced primary sedimentary structures(the matground structures).Spindle-shaped cracks and the other matground structures provide an important evidence to reconstruct the sedimentary environment of the Precambrian.These structures also illuminate the metabolism diversity and enormous biomass of microbes in the Precambrian on the earth surface,and thus form important marks in the sedimentary records.