唐王陵砾岩的形成时代及其构造与沉积环境探讨

唐王陵砾岩自20世纪60年代发现以来,其形成时代和沉积环境就争议不断,至今没有定论。作者根据唐王陵附近多个露头点唐王陵砾岩与上下地层的接触关系、岩性和古生物特征,结合渭北隆起西段2口油气探井的钻探结果,将其地质时代确定为晚奥陶世赫南特期;基于北秦岭岛弧在中奥陶世之后持续隆升,秦岭北坡及其以北的古生代和更老地层不断向北倾覆倒转,同时遭受不同程度的变质与剥蚀,北秦岭和华北陆块之间的弧后盆地演变为前陆盆地并逐渐闭合的共识,并结合该套砾岩地层的反旋回沉积特征和发育大量碎屑流、颗粒流及浊流等重力流沉积的事实,认为唐王陵砾岩是一套沉积在晚奥陶世末期,不断抬升并遭受剧烈崩塌剥蚀的北秦岭以北的前陆盆地陡岸背景下的前陆斜坡脚海底扇沉积体。该结论不但厘清了唐王陵砾岩与东庄页岩的时代归属,也为北秦岭和渭北地区加里东期前陆盆地的构造演化及石油地质研究奠定了基础。

Geological age and tectono-sedimentary setting of the Tang Wangling Conglomerate

Since the discovery of the Tang Wangling Conglomerate in 1960s,its depositional age and sedimentary environment have been controversial. Based on strata contact relationship,lithology and palaeontological data between its upper and lower strata on several outcrops and combined with drilling results of two wells in the west part of the Weibei Uplift,this paper defines its geological age as the Hirnantian Stage of the Late Ordovician. The Tang Wangling Conglomerate was deposited in a submarine fan at the base of the foreland slope in the northern North Qinling island arc under a steep bank background of a foreland basin. The basin was continuously uplifted and undergone collapse and erosion during the latest Ordovician. The result is consistent with the continuous uplift of North Qinling island arc after the Middle Ordovician. The northern slope of the Northern Qinling island arc and its Paleozoic and other older strata were continually overturning to the north and subjected to varying degrees of metamorphism and denudation. The back-arc basin between North Qinling Mountains and North China landmass evolved into a foreland basin and gradually closed. There are other supporting evidence such as the coarsening upward depositional cycles and a large number of gravity flow deposits in the Tang Wangling Conglomerate such as debris flows,grain flows,and turbidity currents. This paper not only clarifies the geological age of the Tang Wangling Conglomerate and the Dongzhuang Shale,but also establishes a foundation for its tectonic evolution during the Caledonian orogeny and for petroleum geology research in the foreland basin between North Qinling Mountain and the west part of the Weibei Uplift area.