Shiyou shiyan dizhi (Jan 2021)
Discovery and significance of Cretaceous-Paleogene freshwater foraminifera in Subei-South Yellow Sea Basin
Abstract
In previous studies, the "marine facies" fossils in the second member of Taizhou Formation and the second and fourth members of Funing Formation of the Subei-South Yellow Sea Basin were generally considered to be organisms associated with transgression and organisms that lived in saline lakes. However, after reviewing the data, this theory proved to be wrong: most of the foraminifera do not exist in the above mentioned members. The distributions of "marine facies" fossils are not concentrated, and the surrounding rocks do not contain gypsum-salt. The evidence of paleoclimate and paleohydrology is also contrary to lake salinization theory. Formation comparison results show that 97 foraminifera are from 4 new strata, and the remaining 3 are in the second member of Funing Formation. The source of transgression, the ancient East China Sea, does not exist, and there are no marine fossils here. The "marine facies" fossils found here are all euryhaline organisms that can also live in fresh water. During the Late Cretaceous-Paleocene lake invasion period, the ancient climate was humid and the ancient lake was deep, and the water could not be salinized. Glauconite, apatite and trace elements cannot be used as a criterion for distinguishing between marine and terrestrial facies. Different types of euryhaline organisms are usually flourishing separately, and co-exist with a large number of freshwater organisms; meanwhile, the surrounding rocks do not contain gypsum-salt. According to the evidence above, the euryhaline organisms in this area are interpreted to live in freshwater, since foraminifera can live in a freshwater environment, and there was no water salinization in the three stages, of regional lake invasion. The large scale, high quality source rocks in Mesozoic-Cenozoic terrestrial basins in China are mainly freshwater lacustrine sediments, which can provide rich oil and gas resources.
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