Depositional dynamics of a bituminous carbonate facies in a tectonically induced intra-platform basin: the Shibantan Member (Dengying Formation, Ediacaran Period)

Abstract

The Ediacaran Shibantan Member (Dengying Formation, South China) is characterised by a black, laminated bituminous limestone facies with diverse Ediacara-type organisms and trace fossils. However, still little is known about the evolution of the Shibantan basin and its sedimentary dynamics. A detailed sedimentological characterisation revealed that the Shibantan Member was deposited on a carbonate ramp system linked to an intraplatform basin. Evaporitic dolomites of the Hamajing Member (representing sub-to supratidal inner ramp environments above fair weather wave base) are sharply overlain by black, laminated limestones of the lower Shibantan Member, representing a subtidal lower-to middle ramp environment close to the storm wave base. This facies-shift implies a sudden local deepening event, probably due to a tectonically induced increase in subsidence. The Shibantan basin was subsequently filled as evidenced by the gradual transition into dark wavy dolomites deposited in a subtidal middle-ramp environment (i.e. between storm-and fair weather wave bases) and, eventually, the upper Dengying Formation (i.e. the Baimatuo Member), which represents the same depositional environments as the Hamajing Member. Sedimentation in the Shibantan basin was highly dynamic as evidenced by a distinct slumping horizon and mass-flow deposits, partly possibly due to synsedimentary tectonic processes. A microbial mat associated biota which includes Ediacara-type fossils is restricted to the lower Shibantan Member. The close spatial relationship between allochthonous event deposits and autochthonous fossil associations in this setting implies that the event deposits were not only important for the preservation of fossils, but probably also for the supply of nutrients.

Environmental context for the terminal Ediacaran biomineralization of animals

“…Consistent with this view, psuedomorphs of gypsum are found in the Shuram equivalent Wonoka Formation in South Australia (Calver, 2000) and in the Doushantuo (Lu et al, 2013). Above the Shuram, an increasing number of bedded evaporite horizons have been discovered in terminal Ediacaran successions, including the Dengying (Siegmund & Erdtmann, 1994;Duda et al, 2015; and this study) and Ara formations (Fike & Grotzinger, 2010), as well as the Hanseran Evaporite Group in northwestern India (Strauss et al, 2001;Mazumdar & Strauss, 2006).…”

“…635-551 Ma) (Jiang et al, 2011) and the early Cambrian Kuanchuanpu Formation (Steiner et al, 2004) in the southern Shaanxi region (Fig. 1C), which are generally correlated with Hamajing, Shibantan, and Baimatuo members, respectively, in the Yangtze Gorges area (Zhou & Xiao, 2007;Zhu et al, 2007;Duda et al, 2015). The Dengying Formation at Gaojiashan is subdivided into three intervals, including the Algal Dolomite, Gaojiashan, and Beiwan members (Fig.…”

“…Sulfur isotope profiles in both regions reveal a parallel decrease in both d 34 S pyrite and d 34 S CAS that likely reflect the significant growth of the Ediacaran sulfate pool (Fike et al, 2006;Halverson & Hurtgen, 2007;Kaufman et al, 2007;McFadden et al, 2008). Above the Shuram, an increasing number of bedded evaporite horizons have been discovered in terminal Ediacaran successions, including the Dengying (Siegmund & Erdtmann, 1994;Duda et al, 2015; and this study) and Ara formations (Fike & Grotzinger, 2010), as well as the Hanseran Evaporite Group in northwestern India (Strauss et al, 2001;Mazumdar & Strauss, 2006). Above the Shuram, an increasing number of bedded evaporite horizons have been discovered in terminal Ediacaran successions, including the Dengying (Siegmund & Erdtmann, 1994;Duda et al, 2015; and this study) and Ara formations (Fike & Grotzinger, 2010), as well as the Hanseran Evaporite Group in northwestern India (Strauss et al, 2001;Mazumdar & Strauss, 2006).…”

“…The Dengying Formation at Gaojiashan is subdivided into three intervals, including the Algal Dolomite, Gaojiashan, and Beiwan members (Fig. 1C), which are generally correlated with Hamajing, Shibantan, and Baimatuo members, respectively, in the Yangtze Gorges area (Zhou & Xiao, 2007;Zhu et al, 2007;Duda et al, 2015). Based on the 551 Ma U-Pb zircon depositional age of a volcanic ash at the top of the Miaohe Member, which has historically been correlated with Doushantuo Member IV (Condon et al, 2005), and an estimated 541 Ma age for the Ediacaran-Cambrian boundary (Amthor et al, 2003;Chen et al, 2015), the >650 m thick Dengying Formation represents the last 10 million years of the Ediacaran Period.…”

Sequence stratigraphy and facies architecture of a mound‐shoal‐dominated dolomite reservoir in the late Ediacaran Dengying Formation, central Sichuan Basin, SW China

“…After the transgressive phase that deposited the D3 member (TST of SQ2), the central part of the Sichuan Basin was dominated by a carbonate platform which shows a shallowing‐upward trend (HST of SQ2). The depositional dynamic features of SQ2 (D3 and D4 members) in the Sichuan Basin and the Shibantan and Baimatuo Members in the Yichang area are very similar (Duda et al, ; Duda, Zhu, & Reitner, ). The D4 member in the central Sichuan Basin consists of shoals and mound‐shoal complex on the platform margin, and lagoon, tidal flat, and intraplatform mound‐shoal complex in the platform interior.…”