Magnetostratigraphy and grain size record of the Xijiadian fluviolacustrine sediments in East China and its implied stepwise enhancement of the westerly circulation during the Eocene period

Abstract

In order to reveal the Eocene westerlies' evolution history, 1053 paleomagnetic samples were collected and thermally demagnetized for the fluviolacustrine sediments at Xijiadian, Hubei Province, East China. Fourteen normal and 14 reversed polarity zones are recognized for the section. Correlation of the resultant magnetic polarity column to the Geomagnetic Polarity Time Scale suggests that the section was generally continuous and spanned 56.4-37.1 Ma. With respect to the stable Eurasian continent, variations in magnetic declination of the section indicate no significant rotations, supporting the earlier view that the South China Block has experienced no significant rotation relative to Eurasia since the Late Cretaceous. Provenance analysis suggests that the dominant fine sediments (red beds) of the Xijiadian section were transported by wind into the study area. Grain size measurements indicate that the 20 μm fraction and clay in the study area decreased from 56.4 to 50.7 Ma and stepwise increased during 50.7-37.3 Ma, implying that the westerly circulation in the Northern Hemisphere weakened during 56.4-50.7 Ma and stepwise strengthened during 50.7-37.3 Ma. In contrast, the 20-63 μmfraction, mainly from nearby dust sources of the study area, generally increased during 56.4-50.7 Ma and stepwise decreased from 50.7 to 37.3 Ma, implying aridity in the study area strengthened during 56.4-50.7 Ma and stepwise mitigated from 50.7 to 37.3 Ma. Changes in global temperature during the Eocene period probably played the primary role on evolutions of the westerly circulation in the Northern Hemisphere and of the arid conditions in the study area.