Sea level oscillations, climate change and landform evolution in the western coastal lowlands of Trivandrum block in Peninsular India

摘要

The coastal areas of Kerala falling within the Trivandrum block, in the southern side of the Achankovil Shear Zone (ASZ) of the Peninsular India, host a series of coast perpendicular estuarine basins. These estuarine basins entrenched over the Neogene sediments enfold a nearly complete record of Holocene transgressive-regressive events. Borehole cores retrieved from the fluvial end of these basins show a coarsening upward sequence with sand dominant sediments at the top and clay dominant sediments at the bottom. The high terrestrial inputs resulted from torrential rains in river catchments coupled with the sea level rise during Early Middle Holocene was instrumental in the development of bay head deltas in the fluvial end and flood tide deltas/islands in the marine end of these estuaries. The faster sea-ward growth than lateral spread of these deltas was responsible for the cut-off of some of the prominent arms of the pre Holocene estuaries into separate freshwater bodies. The Sasthamkotta kayal, Chelupola and Chittumala chira in the Ashtamudi basin, Kotta kayal and Pola chira in the Paravur basin and Poovankal wetland in the Nadayara basin were evolved by this way. The Pallikkal river debouching into the Kayamkulam lagoon also responded significantly to the Early Holocene climate change and sea level oscillations. The river once debouched into the Kayamkulam estuary at its middle presumably through the Krishnapuram Ar, was later took a southerly course linking the Late Pleistocene (confirmed from C-14 age of sediments) wetland bodies like Chunakkara Punja, Komallur Punja, Vatta kayal and Valummel Punja, and finally debouched into the southern arm of the estuary near Vatta kayal. Heavy rainfall and beach barrier build up under the rising spells of the sea in the Early - Middle Holocene was responsible for the wetland capturing and diversion of river flow. The Karunagapalli borehole core gave an age of 7270 +/- 250 yrs BP for an organic rich sediment sample at a depth of 6.9 m. The Holocene sequence here is 9m thick and rests uncomfortably over the Neogene sediments. This clearly indicates that the river received the present channel configuration only in the second half of Early Holocene.