This study analyzes the variations in trace element
contents and ratios, the distribution patterns of rare earth elements, and the
sedimentary environment and its evolution in the Early Ordovician. In
particular, the analysis is based on measured section of the Qingjiang Village
located in Nangao Town, Danzhai County, Guizhou Province. The analysis also
considers thin sections and the characteristics of C, O isotopes.
The lower part of the Tongzi Formation has a simple lithology and mainly
consists of light gray to gray thin-medium
bedded muddy dolomite and dolomicrite with a few dolarenites and dolorudites.
The upper part of this formation includes gray thin-medium bedded fine
crystalline dolomite and gray massive bioclastic limestone. Only a fraction of
gray massive fine crystalline limestone, sparry calcarenite, and calcirudite
are on top of the upper part. The Honghuayuan Formation is generally composed
of light gray to gray medium-thick bedded or massive bioclastic limestone, reef
limestone, and calcarenite with a few sandstones at the bottom. The δ18OPDB values of 18
samples are less than -11‰; in addition, the Pr/Pr* and Ce/Ce* ratios
are greater and less than 1, respectively. These data reflect a sedimentary
environment. The lower part of the Tongzi Formation might have been formed in an
evaporite platform, which was then transformed into a restricted one, which
featured a dry climate, a shallow water depth, and an insufficient amount of oxygen.
By contrast, the upper part of the formation was deposited in an open platform,
which featured a humid climate and relatively increased water depth and reducibility.
During the sedimentary period of the Honghuayuan Formation, the water energy
further increased, the climate became humid, the water depth increased, and the
reducibility increased. Correspondingly, the sedimentary environment, which
originally comprised littoral facies that slowly developed into an open
platform and into a platform marginal bank, gradually evolved into a platform
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