微生物席成因构造形态组合的古环境意义: 以华北南缘中-新元古代为例

微生物席成因构造(microbially induced sedimentary structures, MISS)是由微生物与沉积物相互作用形成的生物-沉积构造, 可作为早期微生物群活动的重要标识, 但对其指示古环境的研究尚显不足.华北地台南部中-新元古代汝阳群(Pt₂)和洛峪群(Pt₃)以近岸浅水陆源碎屑沉积为主, 其中发育大量MISS, 包括多向波痕、微生物席稳化波痕、微生物席碎片、微生物席平滑波痕、多种微生物席脱水形成的砂裂及不规则网状生长脊等.研究表明, 在潮坪环境中MISS最为发育.地势差异对水动力、基底暴露、水分补给和沉积条件等环境因素具有显著控制作用, 影响微生物席的发育与结构, 因此, MISS的形态组合特征能够反映沉积微相变化.研究发现, 在潮上带以微生物席脱水形成的砂裂构造为主, 潮间带上部以破坏-改造型构造为主, 潮间带下部-潮下带一般少见原位MISS, 但可见再沉积微生物席碎片.据潮间带下部至潮上带上部MISS产出类型及其形态组合分析, 识别了4个MISS形态组合带, MISS形态组合由潮间带下部至潮上带上部的变化反映了古地形由低到高的明显变化. 

Morphologic Association of Microbially Induced Sedimentary Structures As Paleoenvironment Indicator: An Example from Meso- to Neo-Proterozoic Siliciclastics of Southern North China Platform

Microbially induced sedimentary structures (MISS) are derived from the interaction of microbes and sedimentation by various geological processes, and have been studied more often as biosignature for early life, while their significance in paleoenvironmental analysis has not been adequately studied yet. Our study shows that the Ruyang (Pt₂) and Luoyu Groups (Pt₃) in the southern North China Platform are dominated by peritidal silisiclastics with abundant MISS, especially those related with mat destruction, and indicates that the morphologic variation and association of MISS are largely influenced by topography. The topography, to some extent, determines the hydrodynamics, substrate exposure duration and water supplement, and exerts influence on mat growth and their destruction, and in turn influence the morphology of MISS, especially in peritidal environments. From the subtidal to supratidal, four zones have been recognized, each of them with its own distinctiveness in MISS morphological association. The subtidal to lower intertidal zone is short of in situ MISS but has some redeposited mat chips, while the upper intertidal is featured by mat protected ripple marks and chips. The lower supertidal is rich in various MISS, especially sand cracks, for its low hydrodynamics, sufficient water supplement and frequently exposed environment, with thick mats, while the upper supertidal abounds smaller sand cracks than those in lower supertidal zone due to relatively thin microbial mats. Thus, Meso- to Neo-proterozoic MISS from the southern North China Platform show that the morphologic association of MISS can be used as good indicators for paleoenvironmental reconstruction.