鄂尔多斯盆地拉什仲组遗迹化石组合与古环境

首次对鄂尔多斯盆地西北缘桌子山地区中奥陶世拉什仲组的遗迹化石进行了系统研究。该组遗迹化石共有6属8种，建立了Paleodictyon遗迹化石组合。此组合以被侵蚀、改造呈斑状分布的遗迹大量存在与Paleodictyon的普遍出现为最显著的特征。遗迹化石的形态、行为习性、保存方式等分类特征、埋藏学特征等表明该组合相当于Nereites遗迹相，代表了深海远源浊流沉积环境；而在本组沉积晚期，水体开始变浅，大致为大陆斜坡－陆隆半深海浊流沉积环境。其沉积层序、沉积构造及岩性等特征说明拉什仲组为典型的浊流相沉积，并以远源相浊流沉积为主，其下部近相浊流沉积较发育。据古流向测定，自中奥陶世拉什仲组至公乌素组沉积期，重力流运动方向的变化大致反映了贺兰拗拉槽张裂的幅度在变小并渐呈收敛隆起的趋势。

Study of Trace Fossil Assemblage and Paleoenvironment of Middle Ordovician Lashizhong Formation, Ordos Basin

Trace fossils from the Middle Ordovician Lashizhong Formation in Zhuozishan,the northwest margin of Ordos Basin are studied systematically. The Paleodictyon assemblage, including Paleodictyon cf. majus , P. sp., Megagrapton irregulare , Neonereites biserialia, Planolites vulgaris, Chondrites filiformis, Ch.furcatus and Zoophycos cf. brianteus , has been recognized for the first time in the studied area. The trace fossil assemblage is characterized by the abundant and spotted trace fossils and the widespread occurrence of Paleodictyon . The classificatory characteristics of the trace fossils, such as morphology, behavior, preservation, etc., indicate that the Paleodictyon assemblage corresponds to the Nereites ichnofacies representing depositional environment of far turbidity current in deep sea. During the late period of sedimentation, however, the water depth became shallower as the environment of turbidity current in continental slope rising. The characteristics of sedimentary sequence, sedimentary structure and lithology in the Lashizhong Formation indicate a typical turbidite facies deposits, especially far turbites facies.According to the palaeocurrent measurement, it is inferred that the directional change of the gravity currents roughly reprepsents the trends of decrease of extensional faulting and of uplift in the Helan aulacogen during the Middle Ordovician period.