ABSTRACT

The Precambrian Kolhan Formation of Singhbhum district, India, is a narrow elliptical epicontinental shallow marine basin. In its roughly NNE-SSW trend, the basin follows the general bearing of the "Iron-Ore Formation" synclinorium and thus appears to represent a "Basin and Swell" episode of epeirogenic phase of the Iron-Ore geological cycle.

This Formation is younger than the Iron-Ore Formation and Singhbhum Granite. It consists of a conformable sequence of basal conglomerate (impersistent) followed by clay-rich ferruginous sandstone, the recrystallized, silicified limestone and the ferruginous phyllitic shale within a total thickness of a few tens of a meter. The prevailing westerly dip of only a few degrees and the absence of folds and foliation planes in the rocks testify to their very minor tectonic and metamorphic effects.

Field data and laboratory investigations of the four different Members of the Formation suggest the following details of the basinal conditions in the overall shallow marine basin of sedimentation:

(1) The basal conglomerate was deposited in a very irregular fashion under somewhat turbulent conditions and it slopes grades imperceptibly into the overlying sandstone. (2) A shallow beach (southern) and a shelf lagoonal (northern) environment is envisaged during the time of deposition of the ferruginous clay-rich sandstone. (3) Similar conditions of depth persisted during the sedimentation of the limestone which developed as thicker beds in the north and thin layers toward the southern portion of the basin. Change in slope towards the west is evidenced by the trend of convolute laminations in the limestone. (4) The shallow sea was converted into a tidal flat (?) with more or less similar depth conditions during the deposition of the uppermost shale member. Shrinkage of basin by grad al regression is also suggested.

An attempt has been made to draw a few generalized conditions supposed to be prevalent in a Precambrian shallow epicontinental basin based on the above findings.