Abstract: Based on the geochemical analysis of lacustrine dolomitic mudstones in the upper Bayingebi Formation of the Lower
Cretaceous in Yingejing Depression, Bayingebi Basin， Inner Mongolia, its geological significance, source rock properties, and
paleo-depositional environment have been investigated. The results show that dolomitic mudstones are characterized by the enrichment
in Ca, Na, Sr, U, V and depletion in K, P, Ba, Rb, Sc, Th, Zr. The total REEs contents vary significantly (59.29×10-6~283.80×10-6) with
differentiation of LREE and HREE, which reveals a moderately strong negative Eu anomaly (average value 0.616) and Ce normality
(average value of 1.01). Tectonic discrimination diagrams and features of Eu jointly indicate that source rocks of dolomitic mudstones
were in a continental island arc and active continental margin setting. The pattern of REE distribution and diagrammatic diagramming
of source rocks suggests that dolomitic mudstones are mainly composed of felsic rocks from upper crustal feldspar rocks. Vertically,
upper Bayingebi Formation was deposited in a salty lacustrine environment with drought, anaerobic and strong partial reduction. And
paleoclimate, paleosalinity and redox index changed obviously in the middle and lower parts of upper Bayingebi Formation. Meanwhile,
a relatively wet-cold depositional process had appeared during the warm, dry environment of K1b2-1. The paleosalinity and redox
environment were also coupled with the evolution of warm-dry and cold-wet environment, that is, a brief oxic environment existed in
the anaerobic and strongly reduced, salty lacustrine environment.

Key words: dolomitic mudstone, geochemical, paleogeography, Yingejing depression