PROCESSES AND EVOLUTION OF FLUVIAL SYSTEM DURING THE LATE QUATERNARY IN THE CENTRAL KERALA REGION, INDIA

Kerala, situated along southwest coast of India, is bounded by Arabian Sea on the west and western ghats on the east. Forty rivers of small and medium size drain the region. Most rivers of central Kerala drain into the Vembanad lagoon. The central Kerala is characterized by the geomorphic features of (i) the uplifted prograding shoreline associated with depositional features like spits, barrier islands and paleo-beach ridges, (ii) the zone of subsidence characterized by yazoo drainage, and (iii) inland region of uplift with laterite cappings, oxbow lakes, deserted channels, neck and chute cut-off leading to channel avulsion.

The channel avulsion is either to the north or south suggesting that the rivers are unable to cut-across the coastal alluvium to enter the sea directly and instead they deflect and debouch. A number of lineaments trending N-S, NW-SE, NNW-SSE, WNW-ESE, and E -W are observed. These lineaments active even in the Late Quaternary must have influenced the river courses and channel avulsion. The river distributory system in the coastal plains of central Kerala resembles a delta.

The low strandlines of Holocene period are indicated by the presence of submerged terraces at various levels of water depths (20-90 m) on the continental shelf. These submerged terraces suggest the debouching of rivers in the Past far away from the present day shore. The protruded extension of the 1000 m bathymetry contour with gentle slope in the offshore region supports the view that the fluvial deposition was farther away from the present coastal plains, prior to the Holocene. This further supplements the suggestion of a mighty single stream in the geological past.

The occurrence of coast parallel lagoons (called Kayals) of different sizes and plan-forms imply diverse evolutionary processes during the Late Quaternary. These lagoons act as depositional centres for the modern river discharges.

The Quaternary sediments unconformably overlie the Neogene sediments, which uncomformably overlie the Precambrian rocks. Both marine and non-marine sediments of the Neogene period include the formations of Vaikom and Quilon, and the overlying Warkali beds of Late Miocene age. The thickness of the sedimentary sequence exceeds 600 m.

In this paper, the role of surface processes and tectonics in relation to the evolution of fluvial sedimentation is discussed in detail.