Sylhet Traps

Sylhet Traps the andesitic Sylhet Trap unconformably underlies the Upper Cretaceous which is considered as the probable continuation of the Rajmahal Traps. The geological and mineral map of northeast India published by the Director General of Geological Survey of India in 1973 show 4 areas of Sylhet Traps in the southern part of Khasi Hills, their age being Lower Cretaceous. In southwest of Shillong Plateau, Sylhet Traps (Upper Jurassic/Lower Cretaceous) overlies the granitic/ gneissic Archean basement unconformably which is overlain by the Mesozoic Tertiary sedimentary sequence. It can be best studied in Jadukata River gorge. In the south about 150 m of Sylhet Trap was encountered in a well below 2,390m. It is dark green to greenish-grey, frequently vesicular and amygdoidal. These are microporphoric with phenocrysts of labradorites and olivine. A trapwash (3-m) bed overlies the top most flow of trap non-conformably and consist of highly fissile brownish-black shales. In South Shillong Shelf Zone Sylhet Traps is directly in contact with the Miocene formations along Dauki Fault for about 10 km where the entire Cretaceous- Palaeocene- Eocene-Oligocene sequence of about 2,000 m thickness is missing. As there is no continuity of Rajmahal Traps with the Sylhet Traps, so it is logical to conclude that the two traps were extruded separately along differently fissures in their present sites. [DK Guha] [Guha, DK former General Manager, Petrobangla]