Abstract: Abstract: 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.