延安典型丹霞地貌景观特征及发育演化研究

Study on Landscape Characteristics and Development Evolution of Typical Danxia Landforms in Yan'an

本文是在延安典型丹霞地貌景观特征分析的基础上,采用宇宙成因核素^(10)Be和^(26)Al技术获得暴露年龄和侵蚀速率,研究“黄土覆盖型沟谷丹霞”地貌景观的演化历史。研究结果显示:延安典型丹霞地貌的类型有狭缝式、石蘑菇式、天生桥式,其中狭缝式丹霞的景观特征为“窄狭、多弯、韵律、多彩”,同时也是典型的“黄土覆盖型沟谷丹霞”,石蘑菇式丹霞造型奇特,天生桥式丹霞地貌整体规模宏大,这两者发育都严格受控于垂直节理和外力作用的侵蚀;运用宇宙成因核素^(10)Be和^(26)Al技术,计算获得志丹猫巷狭缝式丹霞地貌的暴露年代有两期,分别为(0.75±18)~(0.83±0.14)ka和(1.14±13)~(1.29±0.30)ka,安塞王家湾石蘑菇式丹霞地貌暴露年代为(1.76±0.45)~(1.77±0.15)ka,这两处丹霞地貌暴露地表的年代都在全新世(Qh),前者晚于后者,地貌较年青;运用宇宙成因核素^(10)Be-^(26)Al计算获得两处典型丹霞地貌侵蚀速率为(39±3.3)~(104±24.4)cm·ka~(-1),与目前国内外学者研究地表和基岩得到的侵蚀速率对比,这两处典型丹霞地貌的侵蚀速率较大,结果还有待进一步研究;延安典型丹霞地貌景观发育的演化始于白垩纪红层的堆积,大致都经历了四个演化阶段,初步揭示了地质构造、上覆黄土盖层以及流水等多种因素共同控制和影响延安典型丹霞地貌的形成,演化阶段总体都处于青年晚期。

In this study,the landscape characteristics of typical Danxia landforms in Yan'an were analyzed using cosmogenic nuclide ^(10)Be and ^(26)Al techniques to obtain exposure ages and erosion rates to investigate the evolutionary history of the"loess-covered valley Danxia"landscape.Our results showed that typical types of Danxia landform in Yan'an include slit type,stone mushroom type,and natural bridge type,and the landscape characteristics of the slit-type Danxia are"narrow,multi curved,rhythmic,and colorful".Moreover,the typical"loess-covered valley Danxia"was categorized as a stone mushroom-type Danxia landform,and was characterized by a unique and beautiful shape.The natural bridge-type Danxia landform is large in overall scale.Both of these landforms are formed by vertical joints and erosion by external forces.Using cosmogenic nuclide ^(10)Be and ^(26)Al techniques,we calculated that the exposure age of Zhidan Maoxiang slit Danxia landscape formed in two stages,(0.75±18)~(0.83±0.14)ka and(1.14±13)~(1.29±0.30)ka.The exposure age of the Wangjiawan mushroom-type Danxia landform in Ansai is(1.76±0.45)~(1.77±0.15)ka.These two Danxia landforms formed in the Holocene(Qh),the former developing after the latter,and the terrain is relatively young.Using cosmogenic nuclide ^(10)Be-^(26)Al techniques,the erosion rate of the two typical Danxia landforms was determined to be(39±3.3)~(104±24.4)cm·ka–1.Compared with the current erosion rate obtained by domestic and foreign scholars studying the surface and bedrock,the erosion rate of these two typical Danxia landforms is relatively high,and the results need further examination.The landscape evolution of Danxia landforms in Yan'an began with the accumulation of red beds during the Cretaceous.The landforms have gone through roughly four evolution stages,initially revealing that geological structure,overlying loess cover,water,and other factors jointly control and affect the formation of typical Danxia landforms in Yan'an,and the evolution stage is generally in the late early stage.