Abstract
This study describes a sequence of microbialites and volcanics of the Lower Cretaceous Shipu Group, an example of microbialites influenced by volcanic activity. It is located at Shipu town in eastern Zhejiang Province on the coast of southeastern China. Based on macroscopic outcrop observations, microscopic examination of thin sections, electron probe microanalysis (EPMA), field emission scanning electron microscopy (FESEM) imaging analysis, and energy dispersive X-ray spectrometry (EDS) analysis, nine microbialite–tuffite assemblages have been recognized in the section. Their thickness increased gradually upwards as volcanism decreased. There are ooids, bioclastic grains, intraclasts and tuffaceous grains in the grain shoal with local dolomitization. Above the grain shoal, microbial reefs develop either individually or conjoining with adjacent ones, and consist of stromatolites and serpulid tubes with common recrystallization. Tubes of serpulids are calcified and the tube wall is micrite. The tube and intertube parts are filled by sparry calcite. Colonial serpulids are surrounded by microbes to form stromatolites. Black layers of stromatolites contain many calcite crystals with fan-shaped growth pattern and preserved organic matter. Microbes are so well preserved in crystal lattices that the original microstructure of the microbes can be clearly observed by FESEM imaging analysis. Microbial reefs develop at a local high point near or above fair-weather wave-base where waves removed fine volcanic ashes. Interreef deposits are coarse tuffite due to physical differentiation. Volcanic activity could provide rich nutrition for microbes, but too much fine volcanic ash inhibits microbial growth. As a result, a moderate supply of volcanic ash favors the development of microbialites.
Highlights
With the recent discovery of hydrocarbons in microbial carbonate reservoirs, such as the Gulf of Mexico in America (Mancini et al 2008), Santos Basin in Brazil (Rezende and Pope 2015), and the Sichuan Basin in China (Li et al 2013; Che et al 2019), microbialite (2020) 9:9Xiangshan county in eastern Zhejiang Province on the coast of southeastern China along the coastal lowlands of the western Pacific Ocean (Fig. 1)
In the Shipu section, two kinds of accretion processes generated stromatolites: (1) Fan-shaped stromatolites accreted through the growth of colonies of filamentous microbes and the early and pervasive precipitation of carbonate in the extracellular polymeric substances (EPS) sheaths of their filaments (Figs. 7c, e, 8b, c and 11c)
(2) Flat-shaped stromatolites accreted through the growth of other microbes and grains were trapped and bound in the uncalcified EPS (Fig. 7d, f)
Summary
Xiangshan county in eastern Zhejiang Province on the coast of southeastern China along the coastal lowlands of the western Pacific Ocean (Fig. 1). These well-preserved microbialites have not previously been investigated. This study provides an opportunity to investigate the sedimentary characteristics of microbialites influenced by volcanic activity. The effects of volcanic activity are made clear. Volcanic agglomerate, volcanic breccia, sedimentary volcaniclastic rocks, tuffite and some carbonates (Fig. 2). The Upper Cretaceous consists mainly of red siltstone, mudstone, and intercalated basalt. The Paleogene comprises mostly grey to purple coarsegrained clastic rocks, siltstone, mudstone and intercalated gypsum, and oil-bearing shale. The Neogene is composed of brown to yellow siltstones and is present locally (Hu et al 2012a)
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