Abstract

The Cambrian Miaolingian Series strata are continuously exposed in the North China Platform and contain diverse sedimentary phenomena, including substantial with oncoid-rich units. Limestone samples from the Zhangxia Formation in the Sandaogou section were collected to characterise unique macroscale to microscale sedimentary characteristics of the marine carbonate oncoids. In the oncolitic beds (∼4.5 m thick), the individual size of oncoids gradually increases from bottom to top. The oncoid morphology and the presence of a nucleus and laminae in thin-sections are used to classify these oncoids into three types (concentric laminar, lateral growth and multicore oncoids). As evidenced by scanning electron microscopy and energy-dispersive X-ray observations, Girvanella are abundant inside these oncoids and are associated with nanospheres, framboidal pyrite, and spherical and filamentous microbial fossils, confirming the biogenicity of the studied oncoids. These results suggest that the oncolitic–oolitic limestone formed in an upward-shallowing marine environment caused by a forced regression process that resulted in a decrease in accommodation space in a fourth-order sequence. Thus, the conditions became increasingly suitable for the development of cyanobacteria-dominated microbial mats and large oncoids. The late stage of the depositional setting and elevated solar radiation resulted in the formation of the growth termination surface.

1. KEY POINTS

2. Evidence of microbial origin from the oncoids of the Cambrian Miaolingian Series in the North China Platform.

3. Vertical variation in oncoid size and distribution coincide with geochemical data.

4. A shallowing depositional environment is interpreted as the reason for variation in oncoid size and formation of oncoid growth termination surfaces.

摘要

寒武系苗岭系地层在华北地台连续出露，沉积现象多样，包括大量富含类球状体的单元。收集了三道沟剖面张夏组的石灰岩样品，以表征海相碳酸盐巨噬细胞独特的宏观到微观沉积特征。在肿瘤床（约 4.5 m 厚）中，肿瘤的个体大小从底部到顶部逐渐增加。片状瘤的形态以及细胞核和薄层中存在的薄片用于将这些瘤状瘤分为三种类型（同心层状、横向生长和多核瘤状瘤）。扫描电子显微镜和能量色散 X 射线观察证明，Girvanella在这些 oncoids 中含量丰富，并与纳米球、framboidal 黄铁矿以及球形和丝状微生物化石有关，证实了所研究的 oncoids 的生物成因。这些结果表明，在向上浅的海洋环境中形成的鲕粒灰岩是由强迫回归过程引起的，导致容纳空间以四阶序列减少。因此，条件变得越来越适合于蓝藻占主导地位的微生物垫和大型肿瘤的发展。沉积环境的后期和太阳辐射升高导致生长终止表面的形成。

1. 关键点

2. 华北地台寒武系苗岭系类瘤类微生物来源的证据。

3. oncoid 大小和分布的垂直变化与地球化学数据一致。

4. 变浅的沉积环境被解释为oncoid 大小变化和oncoid 生长终止表面形成的原因。