Contrasting characteristics observed between the early and late Cretaceous periods: Evidences from geochemical and isotopic studies of clastic rocks from the Tethys Himalaya, India

Abstract

This study uses a multi-proxy approach on Cretaceous shales (consisting of early Cretaceous Giumal shale and late Cretaceous Chikkim shale of Lagudarsi Group) from the Spiti region of the Indian Tethys Himalaya to assess paleo-weathering, redox conditions, paleoclimate, provenance characteristics and tectonic setting. Geochemical weathering indices such as the Chemical Index of Alteration (CIA), Weathering Index of Parker (WIP) and Ca-free weathering indices (CIX) suggest a shift in weathering intensity from low to high degrees during the Cretaceous period in the source area, also shown in the Al₂O₃–CaO* + Na₂O–K₂O (A–CN–K) diagram. Redox-sensitive trace element ratios like V/(V + Ni), V/Sc and Ce anomalies, Total Organic Carbon (TOC), δ¹³C and δ³⁴S values indicate an anoxic marine depositional environment, documenting an ocean anoxic event (OAE). Provenance discrimination diagrams and geochemical ratios incorporating Nd–Sr isotopes indicate a felsic-dominant source for early Cretaceous Giumal shale and a more mafic-dominant source for late Cretaceous Chikkim shale. Geochemical proxies (Rb/Sr, Mg/Ca, Al/K and CIA) record a shift from humid (non-glacial) climate during the early Cretaceous to cold (glacial) climate during the late Cretaceous. Discriminant diagrams (K₂O/Na₂O–SiO₂, La/Sc–Ti/Zr, and La–Th–Sc) suggest a passive margin tectonic setting for the early Cretaceous, whereas an active continental margin for late Cretaceous shale in the Spiti region. These findings provide insights into the climatic and tectonic evolution of the Indian Tethys Himalaya during the Cretaceous period, contributing to the understanding of global anoxic events and regional sedimentary and tectonic history.