Lahil Fm

The name is newly proposed to represent the Jurassic sediments (Upper Gondwana) of Purnea basin (Shukla et al., 1993). Holo: Lahil-1, Latitude 26o 02' 22" N, Longitude 87o 53'35" E, Drilled depth: 3335.2 m. The top of the formation is at 1110 m and the bottom at 2078 m.

Sandstone. In the type section two subunits can be identified. Lower = Subunit -I (1442-2078 m): This unit is predominantly made up of sandstone with interbeds of mottled claystone. Sandstone is light grey to bluish grey, coarse to fine grained and at places pebbly. Claystone is chocolate red to brown in color, mottled with greenish grey patches, massive, soft and sticky. The contact between the sandstone and claystone is, sharp and erosional. Upper = Subunit-II (1110-1442 m): This unit is made up of dominantly fine to coarse-grained sandstone interbedded with siltstone and claystone. Sandstone is light grey to grey, soft and loosely cemented. Siltstone is grey in color, while claystone is dark grey. Brown to brownish black, soft peat is also encountered. In Karandighi-1 the formation is only 50 m thick and is composed of white to light grey, medium to coarse-grained sandstone with greenish grey, soft claystone. While in Purnea-1 the section is composed of calcareous sandstone with regular interbedding of thick claystone. Thin layers of sandy limestone are developed locally. Heavy mineral assemblage is characterized by dominance of garnet. Thickness of the formation varies from 50 m to 968 m, with an increase towards north.

No diagnostic palynoflora have been recorded from the type section. In Purnea-1 a few Mesozoic polospore have been recorded in the interval of 1223 m to 1493 m. The microflora comprise at Cicatricosisporites sp. Disaccites indicum, Collialsphorites sp. and Incertaesedis sp.