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

摘要

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 intra-platform 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.