Continental red beds in the Ordos Basin (North China Block): Origins and paleoclimatic implications in the aftermath of the Jenkyns Event (Early Jurassic)

Abstract

The Jenkyns Event (also known as the Toarcian Oceanic Anoxic Event, ~183 Ma) was one of the most significant hyperthermal events of the Phanerozoic, associated with pronounced environmental and climatic perturbations. The event is characterized in the sedimentary record by a sharp negative carbon isotope excursion and widespread development of anoxic conditions in both terrestrial and marine settings. Following this negative carbon isotope excursion, red beds were deposited over a wide area in the inland Early Jurassic mega-lake systems of China (such as the Sichuan, Ordos, Qaidam, and Tarim basins). Given their widespread distribution, these red beds could hold clues for understanding the pathways by which the Earth system recovered from the Jenkyns Event, yet their origins, geological significance, and their relation to the event are largely unexplored. Here, petrographic, geochemical, mineralogical, and palynological analyses of the red beds have been conducted through the Fuxian Formation of the Anya section, Ordos Basin (North China Block). Paleoenvironmental proxies (Sr/Cu and Rb/Sr) indicate a predominantly semi-arid to semi-humid climate, albeit with oscillations, following the Jenkyns Event. The red beds exhibit characteristics of both primary and diagenetic control and were deposited in an oxidizing environment as indicated by redox indicators (U/Th, Ni/Co, and V/Cr). The sediments may have undergone secondary oxidation or reduction due to frequently fluctuating climatic conditions, resulting in the formation of variegated sediments. The lake was surrounded by a mire vegetation with ferns and lycopsids, but the riparian habitats were characterized by the presence of drier patches and pioneer plant types due to frequent fluctuations in the water level that prohibited the development of more complex lowland forests. Changes in the vegetation after the Jenkyns Event and the significant drop of regionally-sourced pollen types (produced by seed-ferns and conifers), indicate a drop in lake size and catchment area which were likely climatically driven. These climatic fluctuations may have been related to tectonism on the North China Block and the prevailing super-monsoon conditions. We further suggest that the drying climate and increased atmospheric oxygen may have collectively led to the development of red beds in the aftermath of the Jenkyns Event in major terrestrial basins in China.