Age | Group | Formation | Name of the Rock | Description |
Upper Miocene Pliocene |
| Dupi Tilla (2500 to 3000 feet) |
| Coarse Ferruginous soft sand tone with numerous layers of quartz and chert pebbles; clays sometimes sandy and mottled, occur at intervals. Pebbly sandstone conglomerate beds and fossil wood common. |
Unconformity | ||||
Middle Miocene | Tipam | Tipam (About 4000 feet) | Girujan Clay | Dark grey and bluish grey clay with mottling’s; lower part consists of ferruginous sandstones. |
Tipam sandstone | Coarse and gritty ferruginous sandstone with interlayers of clays; sandstones are massive, cross bedded and yellowish brown colored; conglomerates in pockets; fossil wood and lignite (about 1600 feet thick in Sitapahar). | |||
Surma | BokaBil (4000 - 6000 feet) | Upper unit | Bluish grey shales with conchoidal fracture lenses of hard calcareous shale. | |
Middle Unit | Alterations of 75 - 350 feet bands of massive and bedded sandstone and th.in layers of shales and siltstones; lenticular and spheroidal concretions common at the top; fossiliferous conglomerates occasional. | |||
Lower Unit | Thin bedded, bluish grey shales with intercalations of shaly siltstone. | |||
Bhuban (7500 - 13,000 feet) | Upper unit | Bands of massive and sandstone with interlayers of siltstone at the top, laminated sandy shales interbedded with sandstone at the middle and alternation of sandstone and siltstone at the base. | ||
Middle Unit | Predominantly a shaly and silty unit with numerous partings of arenaceous material; upper part sandy. | |||
Lower Unit | Highly micaceous bedded to massive compact sandstones and fossil shales. |