陕西段家峡地区上奥陶统平凉组重力流沉积特征

陕西陇县段家峡剖面奥陶系平凉组以碳酸盐岩重力流沉积为主,然而部分层位发育砂岩层,长期以来人们对这套砂岩的沉积特征特别是其物质来源缺乏系统化的研究。针对这一不足,本次研究采取野外剖面实测、镜下薄片观察和地球化学测试分析等方法,取得如下成果:①识别出2种岩相类型,其均具有典型的浊流沉积特征,分别代表近端朵叶沉积和远端朵叶沉积;②垂向上变细变薄的层序特征,反映了海侵背景下水道末端迁移—消亡这一演化过程;③微量元素数据表明,研究区具有炎热干旱的气候特点,沉积时水体具有较高盐度并处于氧化-还原界面附近。在沉积学研究的基础上,结合构造背景、古流向、岩性变化趋势以及地化数据,认为研究区具有双物源的特征,其中这套陆源碎屑物质来自北祁连-北秦岭岛弧而非鄂尔多斯盆地南缘的古陆,最终建立了该地区平凉组重力流沉积模式。

Characteristics of Gravity Flow Deposits of Upper Ordovician Pingliang Formation in Duanjiaxia area, Shaanxi Province

Abstract: The Ordovician Pingliang Formation in Duanjiaxia profile, of Shaanxi Province is dominated by gravity flow deposits of carbonate rocks. However, sandstones are developed in some layers. For a long time, there has been a lack of systematic study on its sedimentary characteristics, especially its sources. In order to solve this problem, the following achievements have been achieved in this study by means of field profile measurement, microscopic observation and geochemical analysis. Results show that: (1) Two types of lithofacies have been identified, which all have typical turbidity depositional characteristics, representing the proximal lobe deposits and the distal lobe deposits respectively; (2) the sequence characteristics of vertical thinning reflect the end of the channel migration under the background of transgression. (3) Trace element data show that the study area is characterized by hot climate, and the water has high salinity and is near to the oxidation-reduction interface during deposition. Based on the study of sedimentology, combined with tectonic setting, paleocurrent , lithological change trend and geochemical data, it is considered that the terrigenous clastic material comes from the North Qilian island arc rather than from the land in the southern margin of Ordos. The study area indicates that it has the feather of double provenance supply Finally, the gravity flow depositional model of Pingliang Formation in study area is established.