Abstract:

Coralomorph-lithistid demosponge-microbial reefs are developed in deep subtidal settings of the lower part of the Zhangxia Formation (Crepicephalina Zone: Drumian of Cambrian Series 3) in the Jiulongshan section of the Laiwu area of Shandong Province, North China. These reefs are several tens of centimeters thick and formed upon stabilized substrates of microbial origin. The coralomorph-bearing reefs are clearly distinguishable from demosponge-calcimicrobial reefs and Epiphyton bioherms by the development of dendritic thrombolites and the immediately overlying columnar-layered stromatolites. The microbial degradation of sponges facilitated the precipitation of micritic, peloidal, and clotted carbonates, leading to the formation of coralomorph-bearing thrombolitic cores. In contrast, Epiphyton preferentially grew upward, and the accumulation of Epiphyton clumps formed the dendritic thrombolites and the overlying columnar frameworks of the stromatolites. Marked changes in the microbialite texture are considered to reflect some kind of ecological succession, possibly changing environmental conditions. Therefore, the reefs originated from both heterotrophic and photoautotrophic microbial activities. Concave-upward infillings and lateral bridges are common in the interthrombolite and intercolumnar spaces, suggesting episodic sedimentation on the lateral outgrowths of biofilms and highlighting the columnar-layered structures. The coralomorph-lithistid demosponge-microbial reefs are characteristic of the pioneering, stabilizing, and climax stages of a microbial succession. They are age-specific features of microbial-dominated reefs from deep subtidal settings in the Drumian Stage, when skeletal-dominated reefs were inhibited globally, long before the regime shift in reef construction around the Early Ordovician.

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