华北板块南缘寒武系苗岭统碳酸盐岩硬底：缺乏生物扰动背景下的早期海底胶结作用

华北地台南缘寒武系苗岭统碳酸盐岩硬底发育在核形石灰岩和鲕粒灰岩之上,并明显截断下伏的碳酸盐沉积物。本文通过对硬底界面之下颗粒间的放射纤维状方解石胶结物和等厚环边的微晶胶结物分析,表明其形成于早期海底胶结作用,该时期早期胶结区靠近沉积物—水界面,易受潮汐和波浪冲刷而暴露海底,并在持续搅动的高能环境中经历磨蚀和平整,形成硬底。由于硬底形成后几乎没有受到强烈改造和持续生物侵蚀的影响,通常表现出简单、平坦的表面形态。研究区馒头组二段含硬底层段形成于低、高能交替的潮间—潮下水道环境,以微晶方解石为主要的胶结剂沉淀;张夏组含硬底层段形成于台内鲕粒滩高速建造期,以物理化学沉淀和早期海底胶结作用为主。研究区出现的硬底表明,该时期海水化学条件和海底生态环境利于碳酸盐沉积物的早期胶结。硬底作为早期海底胶结作用的突出证据,对于研究碳酸盐沉积物的早期成岩作用及岩化特征具有重要意义,其形成过程及成因也为古海洋化学条件和底栖生态系统的变化提供了主要依据。

Marine carbonate hardgrounds from the Cambrian Miaolingian in south of North China plateform: Early seafloor cementation in the absence of bioturbation

Carbonate hardgrounds, developed upon oncolitic wackestone and oolitic grainstone in the Cambrian Miaolingian of the southern North China platform, exhibit a sharp truncation of underlying carbonate deposits. This study demonstrates that the presence of radial fibrous calcite cements and microcrystalline cements with a thin isopachous rim, observed between carbonate grains beneath the hardground surfaces, indicates early marine cementation.During the Cambrian, the early cementation zone was sufficiently close to the sediment water interface to be susceptible to erosional reworking caused by tidal currents and wavescour, producing carbonate hardgrounds. Simple, planar hardgrounds represent early cemented surfaces that were subsequently exhumed during a periodof submarine erosion. These surfaces exhibit minimal modification and bioturbation post exposure. In the study area, the hardground interval from the second member of the Mantou Formation (Cambrian Miaolingian) developed within an intertidal subtidal channel characterized by alternation low and high water energy. Microcrystalline calcite precipitation was the main cementing agent in this environment. Conversely, another hardground interval, from the Zhangxia Formation (Cambrian Miaolingian), formed during the construction of high energy oolitic shoals along the platform margins. In this setting, physicochemical precipitation and early seafloor cementation were more active. The occurrence of hardgrounds in the study area provides evidence suggesting that seawater chemistry and benthic ecology were conducive to the early cementation of carbonate sediments during this period. As a prominent manifestation of early seafloor cementation, hardgrounds play a pivotal role in understanding the early diagenesis and lithification characteristics of carbonate sediments. Moreover, their formation process and genesis provide significant insights into shifts in palaeoceanographic conditions and benthic ecosystems.