he Yanchang Formation was deposited during the Middle Triassic in a vast lacustrine basin in the modern Ordos
Basin and is a main target for hydrocarbon exploration in Central China. It is divided, based on sedimentary
cycles and lithology, into the Chang 10 (the oldest) to Chang 1 (the youngest) members. During the deposition of
the Chang 7 Member, the Ordos lake system reached its maximum depth and large volumes of organic-rich
sediments were deposited. The evolution of the Ordos Basin sedimentary system during this phase is, howev-
er, not completely understood, and uncertainty still exists as for the chronostratigraphy of Chang 7 Member. We
acquired palynological markers and palynofacies and a high-resolution δ13Corg record through the entire Chang 7
Member, and a ID-TIMS 206Pb/238U date of 240.95 ± 0.033 Ma from a volcanic ash bed in the middle of this
Member. These imply that the maximum deepening phase of the lacustrine system was during the earliest
Ladinian. Evidence of marine influence in the Ordos Basin at that time and comparison to the sea-level oscil-
lations observed in Western Tethys suggest that a global eustatic rise and highstand may have played a role in
determining lake-level variations.
