Palaeoclimate change during Glacial Periods: Evidence from Sri Lanka

摘要

In the Earth’s history, there were five major glaciations, namely, Huronian (2,300 Ma),Cryogenianor Sturtian-Varangian(850-635 Ma), Andean-Saharan (460-430 Ma, Karoo (360-260 Ma) andthe Quaternary (2.58 Ma to Present) that occurred between 2,300 Ma and 0.0114 Ma. It is revealed thatGondwanaland emerged between the Huronian glaciation (2300-2100 Ma) in the Paleoproterozoic Eraand the Andean-Saharan glaciation (460-420 Ma) in the Early Paleozoic Era. During this time, mostcontinental land masses were clustered in the southern hemisphere, and Sri Lanka was part of theGondwanaland landmass comprising present day Africa, Madagascar, India and Antarctica. Within theOrdovician (485.4-445.2Ma) to Permian Periods (299.0-254.2 Ma) there were signs of the breaking up ofGondwanaland resulting in the severing of India and Sri Lanka together and subsequently Sri Lanka fromIndia. By end of the Permian Period (260 Ma) Karoo Glaciation had ended and the present Mannar Basindeveloped within a deep canyon (about 4-7 km deep) on the Precambrian basement.Although the island of Sri Lanka presently lies in the Indian Ocean between 5o 52′N-9o 54′N and79o 30′E-81o 55′E, to the southwest of Bay of Bengal and southeast of Arabian Sea, it was positionedwithin 67oS-65oS and 34oE-43oE during the Lower and Middle Jurassic Era (201.3-166.1 Ma). Huge rockyblocks (erratic boulders) have been transported to different places by continental ice sheets due to climaticchanges in the Permian, Triassic and Jurassic Periods, but erratic pebbles (2 to 8 cm or more in size) andstreams fed deposits have been transported by glacifluvial processes. These glaciofluvial processesoccurred on four occasions during the Jurassic Period and Eocene, Miocene and Pliocene Epochs on SriLankan landmass, which fallowed the climatic changes and sea level fluctuations that broke up thesedimentary beds, initiating establishment of the present topography and structural configuration. As aresult, the earlier sedimentary deposits were obliterated from greater part of Sri Lanka. During theQuaternary Period the erosional rate increased and the resultant erratic boulders along withglaciofluvialdeposits can still be found on “Planated Surfaces”of Sri Lanka.