Madhubani Fm

The name is proposed after district Madhubani in the state of Bihar (Shukla et al., 1993). This formation is penetrated only in Madhubani well No. 1. Holo: Madhubani -1 Latitude: 26o 35' 09" N. Longitude: 86o 06' 30" E. Drilled depth: 5957 m. The top of the section is at 3523 m and the bottom at 5954 m. Absent in the entire Ganga Basin. [Bihar state is closer to Purnea Basin, so put under that regional entry suite.]

Thickness: 2431 m Madhubani well No 1. The formation can be differentiated into three major lithological units" lower clastic unit, the middle is predominantly carbonate unit, and upper is dominantly an argillaceous unit..

Upper unit (3523-3868 m): This unit is characterized by monotonous shale with occasional occurrence of sandstone, siltstone, limestone and marl.

Middle unit (3868-5652): It is divisible further into two subunits: Lower Subunit (4730-5652): Is dominated by dolomite, dolomite limestone and minor shale. Dolomites are light grey, greenish white, hard and compact. Shales are grey, brownish, grey, grayish black, hard and compact white, hard and compact. Upper Subunit (3868-4730 m): It is characterized by shale interbedded with limestone, dolomitic limestone and dolomite. Shales are brownish grey, dark grey, grayish black brownish white, hard, compact, massive and feebly calcareous. Pyrites are occasionally present.

Lower unit (5652-5954 m): It is characterized by dominance of shale, siltstone and sandstone. Shales are brown, grey, black, very hard, compact and fractured.

The palynofloral assemblage is characterized by dominance of Acritarch genera represented by group Sphaeromorph, a characteristic from of Late Precambrian age.

The bottom clastic unit probably represent shallow marine to fluvial environment. The middle carbonate unit with shale represents deposition in supratidal, subtidal lagoon and intertidal flats. The stable isotopic ration of carbon and oxygen indicate a brackish water environment (Kamal Nain, 1990).