The Gilbert Hill basalt, Deccan Traps, Bombay

The Deccan Trap geology of Bombay (Mumbai) differs from the main Deccan flood basalt province in several ways. Very few geological, geochemical and geochronological studies exist on the Deccan geology of Bombay. The basalt of Gilbert Hill, Andheri occupies a special place in Bombay geology on account of its spectacular columnar jointing, more than 50 m high, and for this has been designated a National Geological Monument by the Geological Survey of India. We have obtained an 40Ar– 39Ar plateau age of 60.5 ± 1.2 Ma (2s) for the Gilbert Hill basalt, and this basalt can be argued to have erupted considerably later (~ 6 m.y.) than the lower part of the Western Ghats lava pile, and the total duration of Deccan volcanism as a whole appears to have been at least ~ 8 m.y.