Characteristics summarized through core and microscopic thin section observations

including the organic matter content

inorganic mineral composition and sedimentary structure

were used to categorize the lithofacies types of the fine-grained sedimentary rocks in the upper section of the fourth member of Shahejie Formation(Es

4U

) and the lower section of the third member of Shahejie Formation(Es

3L

) in Dongying sag

Bohai Bay Basin.As a result

11 lithofacies types were identified.The vertical variation of the sedimentary environment was analyzed in terms of paleoclimate

palaeosource

paleo-water-depth

paleosalinity and paleo-oxidation-reduction

based on the content of major and trace elements as well as vertical variation of specific element ratios.The fine-grained sedimentary rocks formed in semi-deep to deep brackish-to-saline lacustrine environments with relatively humid climate and reduction-to-strong reduction.In addition

the evolution of sedimentary environment is obviously periodic

featuring increase in water depth

decrease in salinity

climate change from arid to humid

change from strong reduction to weak reduction then to reduction

and terrigenous sediment supply change from large to small then to large quantities.All the five elements of sedimentary environment collectively influence the vertical distribution and combination of lithofacies types.With the water depth increasing

the proportion of carbonates to lithofacies assemblages will decrease.Without considering the gravity flows

a combination of water depth

offshore distance and terrigenous debris supply will affect the contents of quartz and feldspar

which will in turn influence the development of siltstone and silty fine-grained sedimentary rocks.The setting featuring high salinity and strong reduction facilitates the preservation of organic matters

prone to produce organic-rich lithofacies types.Variation of paleoclimate makes other paleoenvironmental elements change correspondingly

then affecting the distribution and combination of lithofacies types.