Geography of Kolkata - Geology

Geology

As with most of the Indo-Gangetic Plain, the soil and water are predominantly alluvial in origin. Kolkata is located over the "Bengal basin", a pericratonic tertiary basin. Bengal basin comprises three structural unit: shelf or platform in the west; central hinge or shelf/slope break; and deep basinal part in the east and southeast. Kolkata is located atop the western part of the hinge zone which is about 25 km (16 mi) wide at a depth of about 45,000 m (148,000 ft) below the surface. The shelf and hinge zones have many faults, among them some are active. Total thickness of sediment below the Kolkata is nearly 7,500 m (24,600 ft) above the crystalline basement; of these the top 350–450 m (1,150–1,480 ft) is quaternary, followed by 4,500–5,500 m (14,760–18,040 ft) of tertiary sediments, 500–700 m (1,640–2,300 ft) trap wash of cretaceous trap and 600–800 m (1,970–2,620 ft) permian-carboniferous Gondwana rocks. The quaternary sediments consist of clay, silt, and several grades of sand and gravel. These sediments are sandwiched between two clay beds: the lower one at a depth of 250–650 m (820–2,130 ft); the upper one 10–40 m (30–130 ft) in thickness. According to the Bureau of Indian Standards, on a scale ranging from I to V in order of increasing susceptibility to earthquakes, the city lies inside seismic zone III.