前寒武纪海侵砂岩中的微生物砂质碎片：以北京南口虎峪剖面大红峪组为例

碎屑岩中微生物形成的原生沉积构造,由于缺乏直接的微生物证据,其识别常常比较困难。位于北京西北郊的南口虎峪一带,古元古代晚期的大红峪组(1650～1600Ma),海侵体系域砂岩地层与高水位体系域砂泥质白云岩和叠层石白云岩组成了较为有规律的沉积相序特征,成为将大红峪组划分为三个三级层序的主要标志。在大洪峪组的海侵体系域砂岩层中,一种类似于碳酸盐岩中的“内碎屑”的奇特的砂质碎片,代表了前寒武纪海侵砂岩沉积面上曾经发育过微生物席;该砂质碎片与变余波痕和皱饰构造组成系列席底构造,特殊的形态和产出特点成为窥视前寒武纪砂岩中微生物席发育的一种典型标志。因此,南口虎峪剖面大红峪组海侵砂岩中的微生物砂质碎片及其相关的席底构造的发现,有益于今后对类似沉积构造的阐释与研究,特别是那些常常被厘定为前寒武纪古老的后生动物遗迹化石的沉积构造;同时也表明,在前寒武纪地层之中,除了要关注叠层石之类的微生物沉积构造以外,碎屑岩中也存在微生物活动的若干证据即席底构造(第五类原生沉积构造),这些特别的沉积构造对前寒武纪沉积环境重塑具有重要意义。

Microbial sand chips in transgressive sandstones of the Precambrian: an example from the Dahongyu Formation at the Huyu Section of the Nankou Town in Beijing

The identification of microbial induced primary structures in terrigenous clastic rocks is a challenge because direct evidence of microbial activity is commonly absent. For the Paleoproterozoic Dahongyu Formation at the Huyu section of the Nankou Town in the northwestern suburb of Beijing, both sandstone strata of the transgressive system tract and sandy to muddy dolomite strata with stromatolite of the high-stand system tract make up the succession of sedimentary facies, which represents the main criterion for subdividing the Dahongyu Formation into three third-order sequences. In the sandstone beds of the transgressive system tract of the Dahongyu Formation, an odd type of sand chip that is similar to the carbonate intraclast may reflect the development of microbial mats in the depositional surface of the Precambrian terrigenous clastic deposits. Together with palimpsest ripples and with wrinkle structures, this particular type of sand chip makes up a series of matground structures in the sandstone beds of the transgressive system tract of the Dahongyu Formation. Both the special configuration and surface features of these matground structures provide an insight into the development of the microbial mat in Precambrian sandstones. Therefore, the observation of the microbial sand chip and associated matground structures in the Dahongyu Formation at the Huyu section of the Nankou Town is very useful for the understanding of similar sedimentary structures, especially for those that are frequently thought as the oldest trace fossils of metazoa in the Precambrian. Importantly, same to the microbial sedimentary structures in carbonate rock such as the stromatolite, the matground structure in terrigenous clastic rocks is clear evidence of microbial activity and is also meaningful to the reconstruction of sedimentary environment of the Precambrian.