Consequences of sea level and climate changes on the morphodynamics of a tropical coastal lagoon during Holocene: An evolutionary model

摘要

Vembanad Lagoon is the largest backwater system in the southwestern coast of India, and it is one of the wetlands of international importance, as defined by the Ramsar Convention. The coastal area encompassing the Vembanad Lagoon comprises a spectrum of barrier islands, ridges and swales, lagoons, and flood plains. The lagoon and its adjoining coastal lands are known for economically viable deposits of ilmenite rich beach placers, lime shells, and high purity silica sands. The sedimentological, palynological, and geochronological studies reveal that the sea level and climate changes have brought significant modifications in the geomorphic settings of the system. The depositional regimes and the evolutionary phases showed marked differences in the northern and southern reaches of the Lagoon. The geomor-phological settings, occurrence of a suite of major heavy minerals (sillimanite + opaque) and predominance of marine elements in the palynological assemblage in the southern reach of the lagoon are indicative of barrier spit development, which is further influenced by reworked Neogene sediments with characteristic sillimanite-opaque heavy mineral suite of khondalitic affinity. The northern half evolved during the transgressive-regressive events of Early-Middle Holocene from a sheltered coastal water body surrounded by thick coastal vegetation. The opaque-inosilicate dominant heavy mineral suite in the northern sector indicates contribution of sediments from charnokitic source rocks in the area north of the Achankovil Shear Zone. The proposed four stage evolutionary model of the Vembanad Lagoon indicates that it evolved from an embayment of the Arabian Sea by the progradation and development of a barrier spit during transgressive-regressive cycles under heavy rainfall events of the Holocene.