中文摘要：

位于黄土高原和西北干旱区过渡带的兰州地区，在晚新生代发育了一套风成红黏土和冲积一河湖相沉积．这套地层详细的岩石磁学研究结果表明，磁化率等一些磁学参数在细节上可能反映成壤作用强弱，但在大的趋势上大致反映干旱化，在长尺度研究中可作为干旱化的指标．S-300mT等另一些磁学参数主要反映赤铁矿质量分数，与红度值明显相关，因此同红度一样可作为温度的指标．底部冲积一河湖相沉积中黏土的主要载磁矿物是磁铁矿和赤铁矿；中下部红黏土沉积的主要载磁矿物是磁赤铁矿和赤铁矿，可能含针铁矿；中上部红黏土主要的载磁矿物是磁赤铁矿、磁铁矿和赤铁矿，可能含针铁矿．利用岩石磁学恢复的古环境表明，兰州地区在8．35-7．2Ma是一个较为温暖的以冲洪积和河湖相为主的沉积环境；7．2-3．5Ma主要为干陆地环境，同主体黄土高原一样接受粉尘沉积；6．3Ma风尘沉积在沉积物中开始占主导地位；5．2Ma兰州及其周边地区发生了急剧干旱化，可能伴随着快速降温；3．5Ma开始的青藏高原运动A幕使兰州盆地瓦解，并于1．4Ma开始接受黄土沉积，成为了黄土高原的西缘部分．

英文摘要：

Located at the transitional zone of loess plateau and northwestern arid area, Lanzhou area developed a set of aeolian red clay and alluvial-lacustrine deposits in the late cenozoic. A detailed study of rock magnetism was carried out on these strata and the results show that the magnetic parameters such as magnetic susceptibility are related to the pedogenesis in the details, but in a large trend, they can be used as an indicator of aridification because they have undergone significant changes in different sedimentary facies, which are related to the degree of the hydrodynamics and the content of eolian dust content. Like redness, other magnetic parameters such as S-300 mT mainly reflect the content of hematite, which is related to the temperature, so they can be regarded as an indicator of the temperature. The main magnetic minerals in the bottom of the section where alluvial and fiuvial-lacustrine had deposited are maghemite, magnetite and hematite. In the lower aeolian red clay the main magnetic minerals are maghemite and hematite. And in the upper aeolian red clay the main magnetic minerals are maghemite, magnetite and hematite. The paleoenvironmental record by rock magnetism shows that Lanzhou area was in a relative warm sedimentary environment at 8.35-7.2 Ma. From 7.2 to 3.5 Ma, the Lanzhou area was a dry land and accumulated aeolian deposition like the main Loess plateau. At 6.3 Ma, aeolian red clay had dominated the sediments in Lanzhou. Dramatic aridification occurred around 5.2 Ma and its adjacent regions underwent a possible cooling process. Lanzhou basin has been broken by Qingzang movement phase A at 3.5 Ma, followed by a depositional hiatus from 3.5 to 1.4 Ma, and the Lanzhou area became a part of the western margin of the loess plateau with the loess deposit from 1.4 Ma to present.