Ying Rena, Dakang Zhongb,∗, Chonglong Gaob, Bing Lib, Xuan Caob, Ai Wangb, Yuan Dongb, Ting Yanc
aCollege of Geosciences, Northeast Petroleum University, Daqing, Heilongjiang, 102249,China
bCollege of Geosciences,ChinaUniversity of Petroleum (Beijing), Beijing, 102249,China
cResearch Institute of Petroleum Exploration and Development, Tarim Oilfield Company, PetroChina, Korla, Xinjiang, 841000,China
ABSTRACT:During the Cambrian Period Stage 4, biodiversity and biomass declined rapidly and globally in an event known as the Botomian-Toyonian crisis. To gain a better understanding of the paleoenvironment in South China during this stage, we present and interpret complex multi-parameter carbonate analyses (petrography, carbon and oxygen isotopic composition, and major, trace, and rare earth elements) from the Toyonian Longwangmiao Formation in the Bandenggou section. The Longwangmiao Formation comprises primarily carbonate deposits, with massive detrital mineral occurrences in its lower-middle sequence. Principal component analysis of the minerals and oxides leads to the definition of two intervals, I and II. Interval I is composed mainly of dolostones interbedded with sandstones and contains exceedingly high amounts of terrestrial original minerals and oxides, as well as highΣREE + Y, Th, and Sc concentrations. Interval II, which is composed primarily of limestones, has trace amounts of terrestrial original minerals and oxides and lowΣREE + Y, Th, and Sc concentrations. Theδ13C andδ18O values show a significant positive shift from Interval I to Interval II. After eliminating the effects of diagenesis contributions through the analyses of diagenesis sensitive indexes (δ18O values, MgO/CaO, Mn/Sr and Fe/Sr ratios, and the correlations ofδ13C-δ18O,δ18O -Eu/Eu*, Eu/Eu*-Ce/Ce* and Eu/Eu*-FeO), the Sr and Na concentrations of calcites in Interval I are extremely high, exceeding even the corresponding ranges of marine aragonites, while calcites from Interval II have low Sr and Na concentrations that are within the range of marine low-Mg calcite; however, both show strong positive correlations between Ca-Sr, Ca-Na, and Sr-Na. The different calcite elemental behaviors in Intervals I and II suggest different carbonate precursors associated with seawater temperature and climate. Furthermore, the Sr/Ca ratios are negatively correlated withδ13C andδ18O values, and theδ13CC values also negatively correlated with SiO2content. These features indicate a significant change in the depositional environment during the Cambrian Toyonian Stage from an early restricted marine environment under a warm and arid climate with massive terrestrial influxes to a later relatively open marine environment under a cool and humid climate with negligible terrestrial influxes; this shift likely resulted in the observed changes in biomass and biodiversity.
Keywords:Carbonate geochemistry, Paleotemperature, Paleoclimate, Terrestrial detritus influx
The paleoenvironmental evolution of the Cambrian Longwangmiao Formation (Stage 4, Toyonian) on the Yangtze Platform, South China: Petrographic and geochemical Constrains.pdf