The structural and sedimentological evolution of the Somali Basin : paleoceanographic interpretations

Abstract

The paleooceanographic development of the Somali Basin in the Northwestern Indian Ocean is determined through considering the structural, evolution of the area; its influence on Neogene atmospheric and oceanic circulation as indicated in the sediment record; and variation in these circulation systems as seen in late Quaternary sediments. Chain Ridge forms the main structural element of the Somali Basin. Correlation of geophysical measurements along its topographically elevated portion with geophysical transects in the southern Somali Basin indicates that a buried portion of the Chain Ridge sector of Owen Fracture Zone extends at least to 7.5°S. In areas where crustal age may be estimated on both sides of the fracture zone either by elevation or by other methods, the sea floor to the west is at least 30 m.y. older than that to the east and may be considerably older. The trend of the fracture zone and the age of its segments control the time and direction of movement for India relative to Africa and Madagascar. In addition, they limit the number of proposed reconstructions and indicate that the Western Somali Basin is a small remaining part of the original Tethys. These constraints result in a model for the area which considers Gondwanaland dispersal in the Western Indian Ocean. The final stages of development of the Somali Basin respond with the destruction of the Tethys with the regression in northern India during Middle Miocene. Neogene sedimentation rate determinations for the Somali Basin show considerable increases between the Miocene and Recent, in spite of the fact that the basin is becoming larger and deeper. It is proposed that the dessication and subsequent elevation of the area in northern India caused by structural evolution resulted in the development of a land-sea heating contrast which drives the southwest. monsoon, and due to increased upwelling and productivity it is recorded by increased sedimentation rates in the Somali Basin. The late Quaternary sediment record of the area indicates maintenance of these large scale circulations with little change in sea surface temperature as determined from nannofossil assemblage variation, and occurrence of upwelling over a broader area as indicated by the distribution of coarse carbonate. Several significant bottom water incursions into the basin are interpreted from analyzing the total carbonate curves but their effects do not extend to the area of maximum upwelling due to depression of the CCD by high productivity.