Sr AND Nd ISOTOPIC SYSTEMATICS OF MID-CRETACEOUS ORGANIC-RICH ROCKS (OIL SHALES) FROM THE QIANGTANG BASIN: IMPLICATIONS FOR SOURCE REGIONS AND SEDIMENTARY PALEOENVIRONMENT

The Mid-Cretaceous oil shale from the Qiangtang Basin represents the potentially largest marine oil shale resource in China. 23 samples, including oil shale and marl, were collected from the Basin's Shengli River area to determine Sr and Nd isotope compositions, discuss sedimentary sources and paleoenvironmental changes. The Nd isotopic compositions of the Shengli River oil shale are similar to those of the Nadi Kangri Formation volcanic rocks and the underlying Suowa Formation limestone, indicating the Nadi Kangri Formation volcanic rocks and the Suowa Formation limestone origins of oil shale. The T-Dm ages of oil shale may be considered as the mean age of the upper continental crust in the Qiangtang Basin. Therefore, just like other fine-grained continental sediments, oil shale comprises sampled source rocks that were particularly well mixed through multiple stages of sedimentary recycling. The oil shale samples from the Shengli River area have higher Sr-87/Sr-86 ratios than the contemporary seawater. The high Sr-87/Sr-86 ratio reflects the Sr isotope composition of their original precipitation fluids. Marl samples from this area exhibit slightly higher Sr-87/Sr-86 ratios (mean = 0.7084) than oil shale samples (mean = 0.7076), and show a dramatic Sr isotopic shift near the boundary between the oil shale seams and marl beds. This shift is closely correlated with a rapid change in Nd isotopic compositions, indicating paleoenvironmental changes across the oil shale-marl boundary.