Karandighi Fm

This name is newly proposed by Shukla et al (1993) to represent the lower most unit of the lower Gondwana subgroup of Purnea basin. Holo: Karandighi-1, Latitude 25o 40' 53" N, Longitude: 87o 55' 15" E. Drilled depth: 3143 m. The top of the formation is at 2800 m. The base was not penetrated as the well was bottomed in this formation after drilling 343 m

Fine-grained sandstone. The type section is made up of very fine-grained sandstone, siltstone with subordinate shale. The sandstone is light greenish grey to grey in color, dominantly quartzose with angular to subangular grains, mostly arkose sandstone with 30% feldspars, 65% quartz and 5% heavy minerals. The feldspars are slightly altered and constitute microcline, orthosclase with minor fractions of perthitic plagioclase with minor fractions of perthitic plagioclase. The matrix is mostly silty with little micaceous minerals. Sandstones are interbedded with dark grey siltstone, which is highly indurated and greenish grey shale, which is splintery at places. Towards north, in the Lahil depression (Purnea-1 and Lahil-1), the section is essentially made up of greyish to white colored, hard, dense quartzitic sandstone without any primary sedimentary structures. Relict of feldspathic sandstone is visible. It is considered that due to emplacement of igneous intrusive bodies into the lower part of Lower Gondwana, the sediments have lost their original character and attained the character of quartzite. The heavy mineral assemblages are predominantly garnet (85%) with minor amount of staurolite, chlorite, hornblende, epidote and zoisite. The presence of small but significant amount of hornblende and the angular nature of heavy mineral grains are in favor of an acid igneous rock provenance. Thickness of this formation varies from 10 m to more than 343 m, the thickest section is preserved in the southern part of the basin

The microfossil assemblage as recorded from the study of core and cutting sample in the interval of 2979-3111 m in the type area is characterized by; a) predominance of radial monosaccate genera Parasaccites and Plicatipollenites, b) presence of Callumnispora and Potonieisporites and c) rare occurrence of Crucisaccites and Sahnites. Towards north, this unit is totally barren of palynomorph and other fossils.