Eolian origin of the Miocene loess-soil sequence at Qin’an, China: Evidence of quartz morphology and quartz grain-size

Abstract

Recent study revealed that the so-called Neogene Gansu System in the western Loess Plateau contains loess-soil sequences of Miocene age. A most complete sequence (QA-I) covers a time interval from ~22 to ~6.2 Ma BP, consisting of more than 230 visually definable paleosols interbedded with loess layers. A loess-soil sequence of late Miocene-Pliocene age, synchronous with the Hipparian Red Clay in the eastern Loess Plateau, was also recognized, and extends the Miocene sequences into the Pliocene, ~3.5 Ma BP. Recently, Miocene eolian deposits were also reported from the high terraces near Xining. Eolian origin of the Miocene sequences near Qin’an is supported by several lines of evidence, including micromorphological, sedimentological and geochemical data. Here, we further examine the morphology and sedimentological characteristics of more samples covering the entire QA-I sequence, to provide additional evidence of the eolian origin. Quartz is a dominant mineral component of eolian dust deposits . Because of its stable physical and chemical properties under surface conditions, grain morphology of the quartz is usually used to determine the sources, transportation dynamics and postdepositional modifications of the sediments . Grain-size of the quartz fraction is also better reflective to that of the original eolian dust as quartz is resistant to postdepositional chemical weathering in the semi-arid/ semi-humid loess regions. In this study, grainmorphology and grain-size distributions of the quartz fraction from the QA-I sequence are examined, and compared with those of the typical eolian deposits of Quaternary age from the Xifeng site.