Geochemical and geochronological investigations on miocene limestone in northwestern Sri Lanka

Abstract

The Miocene limestone of northwestern Sri Lanka, referred to as Jaffna Limestone, contains various vertebrate and invertebrate fossils. The Miocene Jaffna Limestone is sandwiched between the Precambrian basement, referred to as Wanni Complex and the Pleistocene Red Beds. The formation of the vast extent of marine carbonate deposits there might be related to a marine transgression event. The Miocene fossiliferous limestone may have recorded the isolation of Sri Lanka from India in Miocene, which resulted in the endemism of flora and fauna between south India and Sri Lanka. It is evident from the absence of large wild cats (lions and tigers) in Sri Lanka with the exception of the Sri Lankan leopard. Jaffna Limestone holds a great limestone reserve for quarrying and the production of cement. The limestone also serves as an important aquifer for groundwater resources for people living in the Jaffna Peninsula. As a result, there has been detailed studies on the mineralogy and the engineering properties of the limestone. However, detailed investigations concerning the geochronology of the entire Jaffna limestone unit are rare due to the political unrest during the Sri Lankan Civil War. Therefore, absolute geochronology would help answer certain fundamental questions related to paleoenvironment of Sri Lanka, for example, the temperature condition.

This study aims at obtaining the absolute age of the limestone. Strontium (Sr) isotope analyses and Uranium (U) – lead (Pb) dating were applied to obtain a numerical age of the hand specimen samples sampled in the limestone outcrops along the northwestern coast of Sri Lanka. The measured 87Sr/86Sr ratios (0.708721 – 0.708228), which correspond to 15.5 – 23.5 Ma, indicate that the age of Jaffna Limestone ranged from late Oligocene to Mid-Miocene. The U-Pb dating result of calcite minerals generated an age of 16.12.0 Ma.

Microfossil analysis was used as a cross-checking tool to confirm the results of absolute geochronology. The Burdigalian index fossil, Pseudotaberina malabarica, was identified, indicating the Miocene formation age of the limestone is highly probable. Foraminifera and gastropod fossils were the most common microfossils found in the Jaffna Limestone. The high abundance of carbonate-based microbiota demonstrated that the ocean temperature was warm and the sea level was much higher than nowadays. The increased sea temperature and sea level may be related to the Mid-Miocene Climatic Optimum, which occurred during 15 – 18 Ma. The warming event favored the survival of a large range of marine microbiota and the deposition of the thick carbonate unit.