Genetic mechanism of mud-coated intraclasts within deep-water massive sandstone in Yanchang Formation, Ordos Basin

Key words:  mud-coated intraclasts  Yanchang Formation  sedimentary environment  geochemistry  deep-water sedimentary  Ordos Basin

Abstract:

The genetic mechanism of mud-coated intraclasts within deep-water massive sandstone in Yanchang Formation of Ordos Basin was studied combining the test method of element geochemistry, XRD and rock thin section analysis. The findings of this study include the following:the Fe/Mn is low, the content of MnO₂ is high, Th/U is high, the δU is less than 1, and the Sr/Ba is low in the argillaceous cladding layer, reflecting the geochemistry characteristics of shallow water environment are nearshore, oxidation and low salinity. There is a large difference in depositional environment with the associated layered mudstone of the semi-deep lake. The main components of the core of the mud-coated intraclasts are sandy, muddy or dolomitic lump, which were formed in shore-shallow lake or delta front environment. The formation process of mud-coated intraclasts was long,in which the soft sandy or muddy sediments within the delta front were transported into deep lakes in clumps from Chang7 to Chang6₃ period through four stages:lump formation of shallow lake facies, initial formation of structure, mixed transfer of lithology and eventually deposition setting.