Magnetostratigraphic dating of the Shixia red sediments and implications for formation of Nihewan paleo-lake, North China

Abstract

The sedimentary sequence in Nihewan Basin, northern China, is rich in mammalian fossils and Paleolithic sites. Extensive magnetostratigraphic investigations have been carried out in the Nihewan sediments, but precise age control on the earliest Nihewan red sediments, which have a fluvio-lacustrine origin in the upper part and an eolian origin in the lower part, has remained unavailable. The formation process of Nihewan paleo-lake, therefore, remains unclear. Here we contribute to understanding the age of the early Nihewan reworked and eolian red clays by presenting detailed magnetostratigraphic and rock magnetic results from the Shixia section coupled with geochemical and petrographic analyses. Magnetostratigraphic correlation to the geomagnetic polarity timescale indicates that the Shixia sedimentary sequence recorded part of the Gilbert Chron with the onset of deposition of the Nihewan Formation occurring prior to the Gilbert-Gauss geomagnetic reversal at 3.6 Ma. With tectonic development of a graben basin, early red clay was deposited first in Nihewan Basin during the Early Pliocene before giving way to lacustrine environments in Nihewan paleo-lake. Geochemical and petrographic results indicate a wind-blown origin for the lower red clay deposits, which have been reworked and mixed with conglomerates produced by local tectonic activity to produce the basal Nihewan sediments. Later formation of carbonate nodules has given rise to the sediments that are now exposed in Nihewan Basin.