天水-秦安一带中新世黄土堆积区沉积-地貌演化

CENOZOIC EVOLUTION OF GEOMORPHIC AND SEDIMENTARY ENVIRONMENTS IN THE TIANSHUI-QIN′AN REGIONS

前人的构造地质学研究,将天水-秦安一带的中新世黄土分布区划归两个不同的构造单元。文章基于野外调查和已有年代地层学工作,结合前人成果,对该区新生代沉积-地貌演化历史进行研究,并划分为以下主要阶段:1)古近纪初南部秦岭山地的剥蚀,使本区在原有基岩准平原地形的基础上,形成以冲洪积平原为主的地形。古近纪末-新近纪初的构造活动使冲洪积平原解体,在秦安地区形成基岩台地与沉陷盆地相间、天水-西和地区形成拉分盆地与隆起山地交错的地貌景观,这些高地为中新世黄土堆积提供了地形基础。2)中新世从22Ma到11Ma,基岩台地和相对平缓的高地上堆积典型黄土-古土壤序列,盆地内则主要发育次生黄土等洼地沉积,表明研究区类似于今天的黄土高原。3)中新世晚期约11Ma起发生的一次侵蚀事件,使研究区的一些小盆地内发育河流相和间歇性浅湖相沉积,秦安一带的黄土堆积也遭到侵蚀,形成的洼地内发育黄土状土或洼地静水沉积,其中包含较多哺乳动物化石,而大范围的相对平坦高地上一直继续发育黄土-古土壤序列。这次侵蚀对本区内甘肃群的沉积多样性有重要贡献,但一直没有深水湖泊发育的条件。4)发生于3.5Ma以后的另一次重大侵蚀,奠定了该区今天狭窄长墚地形的基础,是第四纪黄土堆积在本区保存较差的主要原因。

In the western Loess Plateau in northern China, the Miocene eolian sequences reported in the recent years from different sites have extended to about 22Ma ago. The eolian origin of these sequences are evidenced by （ 1 ） their wide distribution mantling the broad highlands; （2） the spatially correlative magneto- and litho-stratigraphy, magnetic susceptibility and grain-size records; （3）the presence of several hundreds of paleosols with the interbedded loess layers; （4）the fine silty textures with the maximum grain-size mostly 〈 120μm throughout the ca. 16Ma sequence; （5）the similarity of quartz grain morphology and geochemical properties to Quaternary loess; （6） the well-preserved land snails and lack of aquatic species throughout the sequences; and （7）the cyclical changes of various climate proxies along the sequences, similar to those in Quaternary loess-soil sequences in China. These Miocene eolian sequences, combined with the well-known loess-soil sequences of the last 2.6Ma and the eolian Red-Clay （8.0 -2.6Ma） ,provide a nearly continuous terrestrial climate record since the Early Neogene. In this study, we address the evolution of geomorphic and sedimentary environments in the Tianshui-Qin＇an regions where Miocene loess deposits widely spread. Earlier geological studies showed that the Tianshui and Qin＇an regions are within two distinct tectonic units, which have different tectonic histories. During the Paleogene, both regions are dominated by pluvial-plain landscapes as evidenced by the widely spread pluvial sandy-gravel deposits. A tectonic event occurred near the Oligocene-Miocene boundary, leading to the formation of plateau and basin settings in these regions. Since the early Miocene, typical loess-soil sequences started to form on the plateaus while secondary loess （loess-like deposits） were formed in the basins. During Late Miocene times, the Tianshui region experienced an intense erosion leading to the formations of some small shallow lakes. Their deposits are characterized by interbedded grey-green silty-clays with coarser fluvial sandy or gravel layers. Some of them are affected by pedogenesis. These indicate that these small lakes, were frequently dried up along with the fluctuations of paleoclimates. Loess deposits in the Qin＇an region have also experienced erosions, resulting in the formations of some shallow depressions in which water-reworked loess or shallow-water deposits were formed. They sometimes contained large mammalian fossils. On the contrary, typical loess-soil sequences continued to form in fiat areas, that contained only micromammalian and landsnail fossils. This Late Miocene erosion period,probably characterized by several erosion events,was started ca. 11Ma ago and lasted until 7Ma. It may correspond to the Late Miocene growth of the Himalayan-Tibetan complex. The basic patterns of the hilly landscape in these regions were formed in Late Pliocene or Early Pleistocene times due to another intense erosion. The severely fragmented topography of the regions since then has not been favorable for latter loess deposition, thus the Pleistocene loess is purely preserved in these regions. This Late Pliocene or Early Pleistocene erosion may be linked to another tectonic event or to the climate changes relative to the northern hemispheric cooling.