鄂尔多斯盆地东北部下侏罗统富县组底部石英砂岩成因与物源

Petrogenesis and provenance of the quartz sandstone from the bottom of Lower Jurassic Fuxian Formation, northeastern Ordos Basin

鄂尔多斯盆地东北部富县组底部发育一套石英砂岩,对其形成原因存在不同的认识。为了查明石英砂岩的成因及其物质来源,利用沉积学、岩石学、岩石地球化学等方法进行了研究。结果表明,富县组石英砂岩具有吉尔伯特型三角洲典型的“三层结构”,属湖泊扇三角洲相。石英砂岩的矿物组成以石英和高岭石占绝对优势,石英多为单晶石英,高岭石呈碎片状和折扇状,可能为原始沉积时带入。碎屑组分与岩石地球化学结果显示,其物源为盆地内部隆起提供的富石英质沉积岩。结合区域地质背景,推断印支期延长组顶部长石砂岩风化蚀变形成的富含石英与高岭石的源区物质是富县组底部石英砂岩形成的关键,此类型石英砂岩的形成与不整合面上强烈的风化剥蚀作用有关,且与高岭石土矿床紧密伴生,有可能作为识别不整合面存在的新证据。

A suite of quartz sandstones was deposited in the bottom of the Fuxian Formation in the northeast of Ordos Basin.There are different interpretations about its petrogenesis.The research of sedimentology,petrology and rock geochemistry was carried out on the quartz sandstones to reveal their petrogenesis and provenance.The study show that the quartz sandstones has the characteristic of“three-layer structure”as the Gilbert-type delta which belongs to the lake-fan delta facies.The quartz sandstones are mainly consisted of quartz and kaolinite.The Quartz is mostly single-crystal quartz,the kaolinite appeared to be fragmentary and pliciform,they may show characteristic of the provenance.The results of petrology and geochemistry indicate that the uplifted terrain in the interior of the basin provided large amount of sandstones which rich in quartz as the provenance.Combined with the regional geologic data,the survey results conclude that the arkose on the top of Yanchang Formation had formed the quartz-rich and kaolinite-rich provenance by alteration in Indochina period,and it's the key to form the quartz sandstones in Fuxian formation.This type of quartz sandstone relate with strong weathering on the unconformity,and it closely associated with kaolin deposit.So it may be a new evidence to prove the exist of unconformity.