Climate modulated sequence development in a tropical rift basin during the Late Palaeocene to Early Eocene super greenhouse Earth

Sequence developments in rift basins are considered to be influenced largely by tectonics and to a lesser extent by eustatic sea-level and climate. Studies indicate that in passive margin basins, climate can mask the effects of tectonics and eustasy by modulating the sediment supply. It is, however, less understood how the sedimentary sequence in rift basins might respond to strong climatic fluctuations where tectonic pulses generate rapid accommodation space. Here a case study has been provided to assess the effect of climate vis a vis sea-level and tectonics on sequence development in the Cambay rift basin, western India, during the Early Palaeogene (Late Palaeocene to Early Eocene) super greenhouse globe. Facies analysis of this shale-lignite sequence suggests deposition in a lagoon/bay, developed over the Deccan Trap basement. Detailed sequence stratigraphic analysis using basin-wide representative composite sections, marker lignite seam, event bed and high-resolution carbon isotope (delta C-13) chemostratigraphy suggest an overall transgressive motif. Among the three prominent Early Eocene eustatic highstands, only the one at ca 53.7 Ma is expressed by the thickest coal accumulation throughout the basin. Expression of the other sequence stratigraphic surfaces is subdued and can be due to the overall finer grain size of the sediment or local variation in the subsidence rate at different fault-controlled mini-basins. Enigmatic presence of a maximum flooding surface coinciding with the 53.7 Ma climate event (Eocene Thermal Maximum 2), manifested by negative carbon isotope excursion, indicates possible influence of climate over and above tectonics in developing the rift sequence. Qualitative rainfall variation assessed using the magnitude of the carbon isotope excursion and pollen abundance show that a relatively dry/low precipitation climatic phase during the Eocene Thermal Maximum 2 hindered the siliclastic supply to the basin. Thus, it has been inferred that climate-induced high siliciclastic supply possibly enhanced the autocyclic reorganization and hindered the development of the key sequence stratigraphic surfaces across the basin during climate extremes.