鄂尔多斯盆地延长组深水块状砂岩“泥包砾”结构成因机制

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

关键词:  “泥包砾”结构  延长组  沉积环境  地球化学  深水沉积  鄂尔多斯盆地

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

摘要:

通过元素地球化学分析、X-衍射全岩分析、岩石薄片分析等试验方法对鄂尔多斯盆地延长组块状砂岩中“泥包砾”结构成因机制进行研究。结果表明:该结构中泥质包壳层Fe/Mn值较低、MnO₂含量较高,Th/U值较高、δU小于1,Sr/Ba值较低,反映近岸、偏氧化及低古盐度的浅水地球化学特征,和与其伴生的半深湖层状泥岩的沉积介质环境存在较大差异性;结构中内核的主要成分为砂质、泥质或白云石团块,形成于滨浅湖或三角洲前缘环境;“泥包砾”结构非瞬态形成,而是贯穿长7—长6₃期三角洲前缘砂泥岩软沉积物往深湖区块体搬运的全过程,经历浅湖相团块形成、结构初始形成、岩性混合转移、最终停积定型4个阶段。

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.