The Genetic Mechanism of Lower Cretaceous Red Beds in North Yellow Sea Basin

Abstract

Abstract

Lower Cretaceous red beds of lacustrine facies in North Yellow Sea Basin belong underwater sediments, which generate an contradiction with traditional sedimentary theory that the red mudstone is related to exposure sedimentary environment. According to the description of petrological, mineralogical and geochemical (stable carbon and oxygen isotope test, main elements test and trace elements test in 13 red beds and black mudstone samples) characteristics, together with geological analysis and igneous rock testing data, we found that the red deposits was closely related to igneous activity. This assumption can be proved by following evidences a) Granite debris, falling volcanic debris and volcanic burst-crystal can be seen under microscope in Lower Cretaceous sandstone samples. This indicates that there is a mixed provenance of terrigenous clastics and volcanic rocks. b) Coring section of sandstone from 2413-2417.12 meters in well C are characterized by expansion, texture osteoporosis, frangibility and crystallization. This may be due to the thermal influence of magma intrusion, and the high-melting-point mineral will crystallize when cooling. c) Test data shows that average ferric oxide, total iron content in Lower Cretaceous black mudstone are 2.35% and 4.45%, respectively. While in red beds are 5.66% and 6.98%, which shows abnormally high; Meanwhile, major elements analysis show that the red mudstone belong to hydrothermal origin and w (REE) -w (La) / w (Yb) graphic indicates that Lower Cretaceous mudstone are not normal lacustrine sediments. They are affected by basalt seriously. At the same time, trace elements in mudstone which are closely related to hydrothermal activity, such as Sr, Ba and Zn, have higher content than average. All above suggest hydrothermal fluid influenced the sedimentation and it offered additional ions for red beds. d) igneous rock dating results reveal that there are 4 stages of regional magmatism. However, most of the tested samples belong stage 108 ∼ 115Ma and stage 134 ∼ 145Ma which corresponding to the early Cretaceous. Although the drilled igneous rock mostly belong to instructive facies in Lower Cretaceous, but we can speculate that there must have the corresponding extrusive facies deposition of basalt under such frequently magmatic activities settings in the Early Cretaceous. The gushed out ferric ions accompanied by hydrothermal fluid are main colouring ions offered for red beds and finally led to the red deposition.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016