Depositional facies and stratal cyclicity of dolomites in the Lower Qiulitag Group (Upper Cambrian) in northwestern Tarim Basin, NW China

Abstract

and accommodation variations revealed by Fischer plots, six larger-scale third-order depositional sequences (Sq1-Sq6) are recognized. These sequences generally consist of a lower transgressive and an upper regressive systems tract. The transgressive tracts are dominated by thickerthan-average cycles, indicating an overall accommodation increase, whereas the regressive tracts are characterized by thinner-than-average peritidal cycles, indicating an overall accommodation decrease. The sequence boundaries are characterized by transitional zones of stacked thinner-than-average cycles, rather than by a single surface. These sequences can further be grouped into lowerorder sequence sets: the lower and upper sequence sets. The lower sequence set, including Sq1-Sq3, is characterized by peritidal facies-dominated sequences and a progressive decrease in accommodation space, indicating a longer-term fall in sea level. In contrast, the upper sequence set (Sq4-Sq6) is characterized by subtidal facies-dominated sequences and a progressive increase in accommodation space, indicating a longer-term rise in sea level.

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’

“…The domal to wavy [or laterally‐linked hemispheric (LLH)] stromatolite morphologies (Fig. E, F and H) are generally considered to be formed in the shallow subtidal to intertidal zone, where the current energy is higher than the shallower intertidal flat colonized by microbial mats (Flügel, ; Harwood & Sumner, ; Zhang et al ., ). The strata occurrence below planar dololaminite in shallowing‐upward cycles further supports this interpretation.…”

“…In the intraclastic dolograinstone, the dominance of microbial fragments and presence of grain‐supported fabrics and its occurrence as intercalated lens with erosive surfaces (Fig. B, D and E) suggest deposition in a relatively high energy environment, probably tidal creeks surrounded by restricted low‐energy microbial deposits (Cloyd et al ., ; Grotzinger & Al‐Rawahi, ; Zhang et al ., ). The basal erosive surfaces with lag pebbles cutting into thick‐bedded dolomudstone/dolowackestone and various microbialites (Fig.…”

“…fifth‐order) sea‐level fluctuations could have driven the frequent flooding and exposure of the inner carbonate shelf, forming the widespread repetitive metre‐scale, upward‐shallowing cycles (Fig. ), similar to those extensively documented in carbonate platform successions of different ages worldwide (Goldhammer et al ., ; Mutti, ; Zhang et al ., ). Vertically, the lower‐middle Algal Dolomite Member at Yankong is dominated by peritidal lithofacies with subaerial exposure‐induced features atop cycles, while the upper Algal Dolomite Member is composed of more lagoonal lithofacies (Fig.…”

“…Thrombolite is generally considered as deposition in the subtidal and even deeper environment (Fl€ ugel, 2010;Harwood & Sumner, 2011;Grotzinger & Al-Rawahi, 2014;Zhang et al, 2015b). The stratal occurrence under the dolostromatolite as described above further confirms deposition in a shallow subtidal environment linked to the tidal flat landward.…”

Multiphase dolomitization of deeply buried Cambrian petroleum reservoirs, Tarim Basin, north‐west China

“…There are abundant dolomitized burrows (size from 0·5 to 2·0 cm) (Baniak et al ., ) presented in restricted lagoon facies from the top of the Lower (Є 1w ) to the uppermost of the Middle (Є 2a ) Cambrian strata. Moreover, microbial rock (Zhang et al ., ) and probably extracellular polymeric substances (EPS) are also present in the Upper Cambrian stromatolites (You et al ., ). The above findings suggest that this dolomite type had a microbial origin.…”