Coastal evolution and sediment succession of Sri Lanka: a review on quaternary sea levels, climates and sedimentation processes.

Abstract

In spite of the small size, the coastal environments of the four cardinal quadrants of Sri Lanka indicate different morphologies and evolutionary characteristics. Therefore, a review of the coastal evolution and sediment succession of Sri Lanka is presented with formal published data and multi-proxy data using modern techniques. The Sri Lankan coasts have provided insightful evidence of paleo-sea level oscillations during the last glacial period and Late-Holocene epoch. The prevailing climate since the last glacial maximum has provided excessive sediment to coastal lowlands and produced Pleistocene sedimentary signatures driven with aeolian and fluvial processes. Major coastal sediment barriers developed during late Holocene transgressions and were modified afterwards. Both northwestern (NW) and southeastern (SE) coastal zones experienced the monsoon rain-shadow effect. In the NW quadrant, the shielding effect of the Indian sub-continent preserved much of the evidence for paleo-sea level oscillations and paleo-climate, although the basement rock is Miocene limestone. In contrast, the rocky crystalline SE coast has less preservation due to its considerable exposure to the wind driven forces. The energy of the of eastward moving currents is higher due to the combined effect of westerly wind bursts (WWB) and SW monsoons, and hence, southwestern (SW) and southern coastal areas show regressive coasts due to current generated coastal processes. However, the eastern zone of the SE quadrant is progressive due to its position in the conjunction zone of the eastward moving sea currents and westward moving lowsalinity water flows of the Bay of Bengal. The pattern of sea currents and monsoon strength controlled the wind driven morphologies of the NE and SW quadrants. Though the geology is similar in the NE and SW quadrants, the general geological strike of the SW coast is parallel to the turning SW monsoon currents and most likely the rock types has supported erosion. Sediment loads generated from NE monsoon and east India coastal currents (EICC) are upwelled with the strong circulations within the Bay of Bengal and deposit on the NE coasts indicating heavy mineral beds that were sorted by swash and backwash activities. This review notes how tropical monsoon induced sea currents and other currents around the Bay of Bengal region are factors in the differences in the geology and structure of coastal morphologies in four quadrants of Sri Lanka. These factors traced/erased the global melting events on a tropical small island during the Quaternary.