Relict sand wedges suggest a high altitude and cold temperature during the Early Cretaceous in the Ordos Basin, North China

Abstract

ABSTRACT The Cretaceous, with relatively high atmospheric CO2 concentrations, temperatures, and sea levels, is a typical greenhouse period in the geologic history of the Earth. Recently, gradual changes in the understanding of the Early Cretaceous climate have led to the proposal of small-scale polar glaciers or cooling events in polar regions. However, the cooling events in North China at mid-latitudes are still poorly understood. In this study, 48 wedge structures that are 35-240 cm wide and from 120 cm to more than 300 cm in depth down through the bottom of Lower Cretaceous Luohe Formation conglomerate and penetrated vertically downward into Jurassic Anding Formation mudstone in the Ordos Basin, North China. According to findings, these sand wedges are distinguished by large-scale V shapes, vertical to steeply dipping laminations and pebbles, aeolian abrasion of sand grains, ventifacts, upturned host strata, and other features that are most likely the source of thermal contraction cracking in periglacial desert region. The sand wedges formed when thermal contraction cracks were filled with wind-blown deposits and other material in dry, cold, and windy regions. This record may support the hypothesis that the Ordos Basin, western North China, may have been at a high altitude and was cold during the Early Cretaceous. During the Early Cretaceous, the complex topography and climate of North China allowed the Jehol Fauna to coexist with permafrost and glaciers.