Protracted intercontinental aridification preserved within the early Late Cretaceous strata of the Eastern Gobi Basin, Mongolia

Abstract

Mongolia's Eastern and Western Gobi Basins preserve a globally significant record of Cretaceous terrestrial vertebrates, yet their biostratigraphic correlations are complicated by a complex geological history. The Eastern Gobi Basin, a northeast-southwest trending fault-bounded rift system, includes several minor sub-basins with distinct sedimentary sequences, notably the Upper Cretaceous Bayanshiree Formation. This formation hosts key localities of iconic taxa, including Segnosaurus, Erlikosaurus, Duonychus, Garudimimus, Amtocephale, Gobihadros and Adocus amtgai, yet previous correlations throughout the area lacked precision. Our sedimentological and stratigraphic campaigns (2022 to 2024) in the Bayanshiree Formation and overlying red-bed sequences at Baishin Tsav refined these biostratigraphic and palaeoenvironmental frameworks. Field surveys at local (Baishin Tsav) and basin-wide scales (Unegt and Zuunbayan sub-basins) revealed a significant erosional unconformity dividing the formation into two distinct palaeoenvironmental stages: lower expansive erg (aeolian dunes) and upper mature fluvial floodplains. Additionally, this study redefines the previously misidentified red-bed sequences above the Bayanshiree formation as the Javkhlant Formation (formerly Djadokhta or Baruungoyot formations). Taphonomic analyses indicate fossil assemblages were extensively reworked, demonstrating significant time-averaging. This study provides evidence for climatic shifts through three successive environmental phases (erg, floodplain, alternating palaeosols) within the Bayanshiree and Javkhlant formations. These findings suggest widespread aridification in eastern Asia initiated during the Cenomanian–Turonian transition, contemporaneous with a subtropical high-pressure shift and onset of the Cretaceous Thermal Maximum, challenging prior assumptions that placed this event later in the Late Cretaceous. This palaeoclimatic interpretation aligns with global records of past hyperthermal events and significantly refines the temporal context for interpreting regional palaeobiodiversity patterns.