Molecular composition and indigenity of organic matter in Late Neoproterozoic sedimentary rocks from the Yangtze region, South China

Diamictites from Late Neoproterozoic Nantuo tillites (-600 Ma), and dolomites from the overlying Dousantuo and Dengying formations in the Yangtze region, southern China, were analyzed for solvent extractable hydrocarbons. Even though all these samples have low contents of TOC and have undergone overmature thermal evolution, there has been still preserved quite a large amount of hydrocarbons. Analysis of the extracts by gas chromatography-mass spectrometry (GC-MS) revealed the presence of n-alkanes, regular acyclic isoprenoids, tricyclic terpanes, hopanes, gammacerane, steranes, and polyaromatic hydrocarbons. Strict experimental measurements were performed in the analytical procedure to prevent any potential contaminants from being introduced. All these bitumens have molecular markers of Precambrian characteristics and no external organics derived from current contamination events or migrated hydrocarbons from younger strata. The maturity parameters for bitumens indicate that the hydrocarbons are of over-maturity, which is consistent with the thermal maturity of the host rocks. Consequently, it is concluded that the Late Neoproterozoic bitumens in the Yangtze region, South China, are indigenous to their host rocks, which provides the basis for our organic geochemical research on "Snowball Earth" and "Cambrian Explosion."