Late Jurassic-Early Cretaceous palynostratigraphy and palaeoclimate in the Andigama Basin, Sri Lanka

Abstract

Sri Lanka consists of only ten percent of sedimentary rocks, and high-grade metamorphic rocks underlie the rest. Most of these sedimentary terrains are post-Gondwanic and such formations help to understand the geological history of the island. We report the first record of an age diagnostic (Late Jurassic-Early Cretaceous) palynological assemblage of the sedimentary rocks in the Andigama Basin (borehole AND BH01) Sri Lanka. The study was undertaken to assess the palaeoenvironmental settings in this basin based on palynological and palynofacies investigations. A palynofloral study suggests luxuriant gymnospermous forests proliferation in the Andigama Basin during this time. The palynological assemblage is characterized by the predominance of coniferous pollen grains of Araucariacites spp., Callialasporites spp. along with some stratigraphically significant taxa viz., Cicatricosisporites spp., Aequitriradites verrucosus, Triporoletes sp., Impardecispora indica, Concavissimisporites verrucatus, Distaltriangulisporites perplexus, Verrucosisporites verrucosus, and Contignisporites fornicates of Tithonian-Berriasian time. Palynofacies records suggest three distinct Palynofacies Assemblages (PF 1–3). PF 1 indicates the forest swamp; PF 2 represents the mixed assemblage of forest swamp and reed marshes, while PF 3 suggests reed marshes/lakeshore deposits under oxic-anoxic environments. These palynofacies assemblage along with variable lithological counterparts varying from carbonaceous shale laminae interlayered with calcareous sandstone to brown shale and carbonaceous shale strata reflects diverse hydrodynamic conditions. Palynofloral and palynofacies records of the present study denote the warm and humid climatic conditions, which directly corroborates with global oxygen isotopic studies of the Late Jurassic-Early Cretaceous sediments.