Cavity‐filling dolomite speleothems and submarine cements in the Ediacaran Dengying microbialites, South China: Responses to high‐frequency sea‐level fluctuations in an ‘aragonite–dolomite sea’

Abstract

Speleothems, mostly composed of calcium carbonate, are widely present in modern karst‐originated caves, but have rarely been reported in palaeokarst systems. This study presents a novel type of dolomite speleothem and subsequent submarine dolomite cement, which are widely present in the upper Ediacaran Dengying Formation in the upper Yangtze area. These precipitated materials occur in the cavity system that cuts across several peritidal cycles. The interconnected cavity networks with irregular shapes, embayed walls, internal breccias on cavity floor and their preferential development in the shallower cycle tops (for example, tepee‐deformed beds) suggest that they were initially generated by subaerial dissolution. As the earliest infills, the hemispherical protrusions, icicle‐like pendants and ground‐up columns show similar morphological features and occurrence patterns to the cave popcorn, stalactites and stalagmites, respectively. Thus, these earliest infills are speleothems resulting from associated meteoric precipitation during subaerial exposure. The isopachous growth pattern of subsequent more extensive fibrous dolomite cements points to a submarine diagenetic environment in which they were precipitated. Microscopically, the micritic to micro‐crystalline dolomite, acicular dolomite in speleothems and the subsequent fibrous dolomite share similar crystal fabrics to metastable precursors (for example, Mg‐calcite). Meanwhile, the carbon‐oxygen isotope compositions of the speleothem and fibrous dolomite, although partly altered by burial diagenesis, share a large overlap with host rock and coeval marine carbonates all over the Yangtze Platform. For these reasons, these speleothems and fibrous cements are considered to have been initially precipitated as metastable carbonate precursors in meteoric and submarine environments, respectively, and stabilized during submarine mimetic dolomitization. The cyclic occurrence of cavity systems filled with speleothems and submarine cements reflects periodic subaerial exposure and marine flooding of broad tidal flat in the upper Yangtze area, driven by high‐frequency sea‐level fluctuations. Furthermore, the Neoproterozoic seawater chemistry that favoured early dolomitization of carbonate precursor mineralogies was an advantage for the preservation of fabrics from metastable precursor minerals.

Fracture dolomite as an archive of continental palaeo-environmental conditions

“…Special features of continental-meteoric dolomite formation at Erzberg: In contrast to previously published studies on sedimentary dolomite formation in continental-meteoric or marinelacustrine environments at low temperature 20,21,23,25,26,44,47 , we found no evidence for detrital dolomite in the local carbonate host rocks and in the adjacent sedimentary rocks from the catchment areas, demonstrating that the dolomites at Erzberg are authigenic. This argues against the possibility that young faultactivity has pulverized any host rock dolomites and that such reactive (nano-scale) grains could have acted as nuclei for the later (over)growth of the fracture dolomites at a low temperature 48 .…”

Formation, diagenesis and palaeoenvironmental significance of upper Ediacaran fibrous dolomite cements

“…An alternative interpretation can be found in Ding et al . (), who suggest subaerial exposure and meteoric dissolution as a formation mechanism of sheet cavities. However, field, petrographic and geochemical data lack evidence of meteoric karst features in the study area.…”

“…Fibrous dolomites are exceptionally well‐preserved in the upper Ediacaran ( ca 551·1 to 548·0 Ma) Dengying Formation in the Sichuan Basin, China. Due to their potential significance as archives of Ediacaran palaeoenvironmental properties, these fabrics have been debated in the literature, with topics including: (i) the precursor mineralogy (Lin et al ., ; Mou et al ., ; Peng et al ., ; Qian et al ., ; Cui et al ., ; Ding et al ., ); and (ii) the parent fluid (freshwater: Shi et al ., ; Qian et al ., , mixed waters: Xiang et al ., , or seawater: Hao et al ., ; Mou et al ., ; Peng et al ., ; Cui et al ., ; Ding et al ., ). Here, a multi‐proxy, fabric‐specific data set is shown to help elucidate these controversies and provide the foundation for more sophisticated interpretations.…”

Effects of diagenesis on quality of dengying formation deep dolomite reservoir, Central Sichuan Basin, China: Insights from petrology, geochemistry and in situ U-Pb dating

“…The isopachous FD was formed in a subaerial or submarine environment, suggesting a relatively low precipitation temperature (Ding et al, 2019). Although associated fluid inclusion Th could not be determined, both Th of primary fluid inclusions within Ediacaran halites in the study area and silicon-oxygen isotopes of cherts consistently record Ediacaran seawater temperatures ranging between 13 °C and 40 °C (Meng et al, 2011).…”