Gondwana sedimentary rocks of Andigama Basin, Sri Lanka: unraveling weathering dynamics, tectonic setting, and paleoclimate

Abstract

The Andigama Basin is a pre-rift Gondwana sedimentary basin containing Late Jurassic to Early Cretaceous calcareous sandstones and carbonaceous shales of varying thickness. This study aims to reconstruct the weathering, tectonic setting, and paleoenvironment of East Gondwanaland using a 90 m deep drill core. Whole-rock geochemistry and elemental analysis were carried out using X-ray fuorescence spectrometry and CHNS/O elemental analyzer, respectively. Lithological changes in the borehole core refect diverse hydrodynamic conditions. X-ray difraction patterns indicate a signifcantly high content of quartz and kaolinite peaks. Scanning electron microscope images suggest that quartz, carbonate, and aluminosilicate dominant detrital particles and chemical residues enhanced the cementation by reducing the porosity and permeability of sealing interfaces. Major oxide and trace element concentrations are approximately similar to the Upper Continental Crust values. High Chemical Index of Alteration, Plagioclase Index of Alteration, Index of Compositional Variability, and high content of kaolinite peaks refect intense chemical weathering, suggesting a hot and humid climate during the Late Jurassic–Early Cretaceous periods. The calculated paleo-land surface temperature (24.7±5 °C) and mean annual precipitation (1120 mm) values are also consistent with the interpretation of weathering indices and global oxygen isotopic studies. Provenance and tectonic setting discrimination diagrams suggest the deposition of quartzose and mafc igneous sources under the passive margin stage. In addition, elemental analysis indicates a nutrient-rich (average total organic carbon=4.67±1.04 wt. % and total nitrogen=3.13±3.39 wt. %) and oxic to oxygen-poor reducing (average total sulfur=2.13±1.43 wt. %) swamp environment. Consequently, the Late Jurassic–Early Cretaceous climate was simulated as a prominent deglaciation/hot and humid climate in the Gondwana supercontinent, based on calculated paleo-land surface temperatures and mean annual precipitation.