The gravel characteristics of Pliocene conglomerates in Zhangxian area on the northern margin of the West Qinling and its geological significance
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摘要:
西秦岭漳县地区保存了2套角度不整合分隔的新生代沉积地层单元，即下部渐新统—中新统含盐沉积地层和之上的上新统粗砾岩地层。通过对该地区上新统砾岩层的沉积旋回、砾岩中砾石大小、磨圆度、排列、成分等特征及垂向变化地观测统计分析，这套砾岩及砾石主要特征可以归纳为：①这套砾岩主要由厚层中-粗砾砾岩组成，厚度达480 m，呈近水平或缓倾斜角度不整合在下伏渐新统—中新统之上，自下而上可以分为洪积砾岩、冲积砾岩、冲洪积砾岩、冲积砾岩和冲洪积砾岩5个岩性段；②砾岩的砾石大小混杂，粒径分布范围大，最大粒径达120~240 mm，但以10~80 mm粒径砾石为主，砂砾质胶结，基质支撑为主，下部发育颗粒支撑筛状砾岩，砾石以次棱角状-次圆状为主，个别层位可见滚圆状砾石；③砾石成分为复成分，主要以灰绿色变质长石砂岩、长石石英砂岩、灰色或肉红色灰岩和斑状花岗岩、花岗闪长岩或闪长玢岩为主，且不同岩性段的砾石成分差异明显，洪积砾岩段以花岗质岩石和各种砂岩为主，而冲积砾岩段以灰岩砾石为主，且砾石磨圆度较好；④砾石叠瓦斜列方向主要指向南，指示了自南向北的古流向。根据这套砾岩的特征，结合其空间分布受向南陡倾的F2逆冲断层控制，以及砾石成分与西秦岭造山带岩石组成的一致性分析，认为这套上新统粗砾岩是以西秦岭北缘断层为边界的山前近源快速堆积的冲洪积扇沉积，具有再生前陆粗磨拉石盆地性质。结合青藏高原东北缘出现的一系列时代相同、构造背景一致的基本类似的粗砾岩，如循化盆地甘家砾岩和临夏盆地的积石山砾岩等，提出西秦岭北缘漳县地区的这套上新统砾岩是印度-欧亚板块碰撞汇聚向北扩展到青藏高原东北缘西秦岭北缘构造边界，不仅造成西秦岭造山带快速隆升，且引发了北缘断裂带复活而强烈向北逆冲，从而形成的再生前陆磨拉石盆地沉积的认识。
Abstract:There are two sets of Cenozoic sedimentary stratigraphic units separated by angular unconformity in Zhangxian area on the northern margin of the West Qinling, namely, the Lower Oligocene-Miocene salt-bearing sedimentary sequence and Pliocene coarse conglomerates.Based on the observation and statistical analysis for sedimentary cycle, gravel size, roundness, arrangement, composition and its vertical changes of Pliocene coarse conglomerates in this area, the authors have summarized the main characteristics of this set of conglomerates and its gravels as follows:①This set of conglomerates is mainly composed of thick, coarse conglomerates with a thickness of 480m, which covers unconformably the underlying Oligocene-Miocene salt-bearing sedimentary sequence and, from the bottom to the top, it can be divided into five lithologic beds, i.e., alluvial, fluvial, alluvial-fluvial, fluvial, alluvial-fluvial conglomerate beds; ②the gravels of the conglomerates are characterized by poor sorting with wide grain size distribution range in which the maximum size is 120-240 mm with the major size being 10-80 mm, gritty cementation, matrix supported texture with a little grain supported texture, being mainly sub-angular and sub-roundness with a little roundness; ③the gravels of the conglomerates mainly are grayish green feldspar sandstones, feldspar quartz sandstone, gray or grayish red limestone and porphyry granite, granodiorite or diorite porphyrite.In addition, the gravel compositions in different lithologic beds are obviously different.Generally, gravels in the alluvial conglomerate section mainly are various granitic rocks and sandstones, while those in fluvial conglomerate section are dominated by limestone gravel with some roundness; ④the gravel southward imbrication orientation indicates the paleocurrent direction from south to north.Based upon the above mentioned conglomerates' and gravels' characteristics, the spatial distribution controlled by south dip F2 thrust faulting, and consistency of the gravel compositions with the rock composition of the West Qinling orogenic belt, it is suggested that the Pliocene conglomerates should be the rapid accumulation, near-source, alluvial fan deposits in front of the mountain with the boundary of the fault on the north margin of the West Qinling with the nature of regenerated foreland coarse molasse basin.Furthermore, a series of conglomerates are similar to the Pliocene Hanjiagou conglomerates in lithic characteristics, age and tectonic setting, such as Ganjia conglomerates in Xunhua basin and Jishishan conglomerates in Linxia Basin on the northeastern margin of the Tibetan Plateau, and hence it is proposed that the development of Pliocene conglomerates in Zhangxian area on the northern margin of the West Qinling was related to the northward growth and expanding of the Tibetan Plateau to the northern margin of the West Qinling Mountain under the India-Europe collision dynamic background.This northward growth and expanding not only triggered the rapid uplift of the Qinling orogenic belt but also led to the reactivation of the northern margin fault and strong northward thrusting, which resulted in the formation of the regeneration foreland molasse basin.
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