Sedimentary Environment of Ediacaran Sequences of South China: Trace Element and Sr-Nd Isotope Constraints

Abstract

Post-Marinoan sedimentary rocks from the Yangtze block in South China were investigated to seek constraints on their provenance, genetic relationship with paleoclimate, depositional environment, and changes. Sedimentary rocks from the Doushantuo Formation in the Jiulongwan section of the Yangtze block were collected for whole-rock trace elements and Sr-Nd isotopic analyses. Flat-to-slightly middle rare earth element (REE)–enriched REE-plus-yttrium patterns, negative-to-positive Ce anomalies (Ce/Ce* of 0.86–1.40), positive La anomalies (La/La* of 1.02–1.42), and suprachondritic Y/Ho ratios (36.8–49.5) obtained for the Doushantuo cap carbonates indicate their deposition in a fluctuating inner-shelf setting where freshwater and seawater mixed together and the influence of hydrothermal fluid from the seafloor is absent. All Doushantuo carbonates and shales have variable Sr isotopic compositions, with initial Sr isotopic ratios (0.704030–0.712720) that support significant river-water contribution during the deposition of the Doushantuo Formation. Samples of the Doushantuo Formation collected across sedimentary profiles show distinct variations in Nd isotopic compositions and diagnostic trace elemental ratios (Th/Sc). The cap carbonates received significant input from a broad continental area undergoing extensive weathering immediately after the cease of the glaciation era. In contrast, the upper Doushantuo Formation sediments received more mature detritus from the nearby Archean Kongling complex, reflecting receding sea levels. In contrast to the negative Ce anomalies observed in lower Doushantuo rocks, negligible Ce anomalies in upper Doushantuo rocks suggest an anoxic setting, in agreement with the growth of phosphatic concretions and the presence of framboids of pyrite and a higher concentration of organic matter.