Abstract:

The Middle Permian Zhesi Formation comprises a suite of dark mudstone, gray sandstone, and carbonate in the Songliao Basin, of which the limestone contains marine brachiopod and bivalve fossils. Limited by drilling footage and core data, few previous studies were conducted on the upper Paleozoic strata and sedimentary environment of the basin. Compositions of trace elements (incl. rare earth elements (REEs)) and carbon-oxygen isotopes were measured on the Zhesi Formation dolomitic limestone in the well D101 and D103, and we first presented the element geochemical data of these Middle Permian limestone in the basin. The results show that the δ¹³C_PDB values are generally >0 (mean 1.68‰) and δ¹⁸O_PDB values < -15‰ (mean -19.98‰), the carbon-oxygen isotope plot indicates positive correlation between the two isotopes, which suggests that dolomitization has obvious effect on the isotopic transformation of limestone. Total $\sum$REE+Y contents of the dolomitic limestone range from 56.88×10^-6 to 143.72×10^-6 (mean 108.92×10^-6). Compared with MREE and HREE, PAAS-normalized LREE depletion was observed with no or negative δEu anomalies (mean 0.94, max 1.57). Negative Ce anomalies are common (mean δCe 0.87, max 1.06). The samples have relatively high Y/Ho ratios (mean 34.59), and the same REE distribution pattern as normal marine facies. The mean trace element contents are relatively high in general, such as Ba (281.55×10^-6), Zn (71.91×10^-6), Y (17.44×10^-6), Zr (117.75×10^-6) and Rb (66.84×10^-6), and the samples have medium mean ratios of Sr/Ba (2.91), Th/U (2.75), V/Cr (1.45), V/(V+Ni) (0.66). We concluded that the region was a carbonate platform or coastal environment affected by terrigenous debris in the Middle Permian.

Key words: geochemical feature, dolomitic limestone, carbonate platform, coastal environment, Middle Permian, Zhesi Formation, Songliao Basin