Sedimentary characteristics of Pliocene Hanjiagou conglomerates in Zhangxian area on the northern margin of the western Qinling and their geological significance
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摘要:
青藏高原东北缘西秦岭北缘构造带的漳县地区出露一套具有磨拉石沉积特征的上新统韩家沟砾岩。其现今的空间分布和沉积特征对于认识青藏高原东北缘新生代构造演化和地壳隆升具有重要的科学意义。通过对上新统韩家沟砾岩层的厚度、砾石成分、形态和粒度特征、古流向特征、物源特征等研究，探讨其沉积环境和形成的构造背景。提出了西秦岭北缘上新统韩家沟砾岩代表新近纪上新世以来，西秦岭地块向北逆冲推覆构造背景下形成前陆磨拉石盆地沉积的认识。该区域上新统韩家沟砾岩现今出露最高高程与北缘断裂带之南的山顶夷平面高程相近，可能指示了其形成之后和西秦岭一起经历了长期的侵蚀夷平，最后在新近纪末期或第四纪初形成了统一夷平面。该夷平面代表了青藏高原东北缘地壳隆升的起点，新近纪末期以来，该夷平面的隆升、侵蚀和解体记录了青藏高原东北缘地壳隆升过程，即青藏高原东北缘真正隆升是新近纪末期或第四纪以来的地质事件。
Abstract:The Pliocene conglomerates, distributed in Zhangxian area of the northern margin of the western Qinling on the north-eastern margin of the Tibetan Plateau, belong to a coarse molasses suite. Their present spatial distribution and sedimentary characteris-tics can provide important constraints on understanding the Late Cenozoic uplift and tectonic evolution of the northeastern margin of the Tibetan Plateau. Based upon the detailed study of the thickness, gravel composition, gravel morphology, grain size distribution, palaeocurrent direction and provenance analyses for the conglomerates, the authors investigated their sedimentary environment and tectonic setting. it is held that the Pliocene conglomerates on the northern margin of the western Qinling represent the Pliocene mo-lasses foreland basin formed in a tectonic setting of the northward thrusting of the western Qinling since Late Cenozoic. Furthermore, the present highest elevation of the conglomerates is similar to the paleo-planation surface of western Qinling Mountain to the south of the north margin fault of western Qinling, which indicates that, after the formation of the conglomerates, they underwent longterm erosion and leveling with west Qinling Mountain and finally formed the unified planation surface at the end of Neogene or in Early Quaternary. Uplift, erosion and disintegration of the planation surface might have been the starting point of crustal uplifting of the northeastern margin of the Tibetan Plateau; in other words, the uplifting of the northeastern margin of the Tibetan Plateau was just a geological event since the end of Neogene or Early Quaternary.
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表 1 韩家沟砾岩层砾度特征
Table 1. The grain size features of Hanjiagou conglomerate
砾度参数 沟口 沟中部 沟后部 a-max/cm 25.9 40.2 22.2 a-min/cm 1.5 0.3 0.9 b-max/cm 22.8 20.1 9.3 b-min/cm 1.5 0.3 0.9 a/cm 8.2 8.0 5.7 b/cm 4.3 3.8 2.9 a50/cm 6.5 6.0 4.7 Mz 7.6 6.5 5.2 Sa 0.42 0.4 0.4 Sk 2.7 2.0 0.3 KG 1.2 1.1 0.9 δ 4.7 4.5 3.2 
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表 2 韩家沟砾岩层砾态特征
Table 2. The grain form features of Hanjiagou conglomerate
砾态 沟口 沟中部 沟后部 风化程度 2~3 1~2 0~3 扁度（F） 2.51 3.14 2.26 球度（ψ） 0.73 0.7 0.71 磨圆度(P)/% 57 62 62.7 F/ψ 3.44 4.48 3.18
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