Formative mechanisms. depositional processes, and geological implications of Furongian (late Cambrian) reefs in the North China Platform

Abstract

The Cambrian Series 3？Furongian successions of the North China Platform contain various microbial？metazoan and microbial reefs. This study focuses on Furongian reefs of the platform in order to understand formative processes and the evolution of the reefs during Cambrian Epoch 3 and the Furongian. Three types of Furongian reefs were differentiated in the Shandong region, China: maceriate reefs, columnar stromatolitic reefs, and small-scale microbial reefs. Maceriate reefs show dm- to m-scale domal or flat-bedded geometry, and consist of cm-scalemaze-likemaceria structures made of siliceous sponges and microbial components (microstromatolites, <em>Girvanella</em>, and <em>Tarthinia)</em>. Columnar stromatolitic reefs are characterized by stromatolite columns of 10？100 cm in height and 5？50 cm in diameter. They consist dominantly of <em>Girvanella</em>, with less conspicuous, poorly preserved sponge spicule networks. Small-scale microbial reefs commonly show cm- to dm-scale, domal macrostructures, and were constructed mainly by calcimicrobes,<em> Girvanella</em> and<em> Renalcis</em>. These three types of Furongian reefs were deposited in various shallow-marine settings in response to relative sea-level changes. The Furongian reefs are markedly different, in terms of macro- and micro-fabrics, from the Cambrian Series 3 reefs that are dominated by thrombolites and dendrolites and were constructedmainly by Epiphyton in the Shandong region. This difference is also recognized in the Beijing region, ca. 500 km away. The abrupt transition from the Cambrian Series 3-type to Furongian-type reefs, coincidently with a decrease in calcified microbe diversity, was most likely due to global euxinic oceanic conditions and a possible eustatic sea-level drop, rather than the highly diachronous, platform-wide drowning event (i.e., drowning of the Cambrian Series 3 carbonate platform). The abundant occurrence of sponge spicule networks in the Furongian reefs suggests that metazoan reef builders (i.e., sponges) resurged and became actively involved in the reefal systems prior to the Great Ordovician Biodiversification Event. This study may provide an important basis for further investigation into the evolution of reefal systems during the middle to late Cambrian when metazoan reef-builders were known to be scarce.