ARCHEAN CONGLOMERATES IN JHARKHAND (E INDIA) OPEN THE GATE TO TERRA INCOGNITA

Several cratons with an igneous basement complex are present in India. Among them is the Singhbhum craton in E India, which is covered by Archean (and younger) sediments in many places. Unlike other – more common - Archean rocks on this craton, the conglomerates have received little attention and their sedimentology has been studied even much less. This is due partly to a lack of continuous sections, partly to inaccessible occurrences because of terrain conditions and a cover of younger sediments. Moreover, at many places the primary characteristics of the Archean sediments have been lost through erosion and tectonics, which hampers unraveling their depositional processes, and consequently also their depositional environments.

The Archean conglomerates of the Singhbhum craton seem to form isolated patches rather than extensive bodies. The stratigraphic studies devoted to them in the past have neither resulted in clear interrelationships, nor in a clear stratigraphic framework. In the past few years, several of these conglomerates were investigated sedimentologically. Even though the stratigraphy of the areas under study in the state of Jharkhand is far from well established, it is remarkable that approximately a dozen Archean conglomerate units are situated in a relatively small, more or less E-W trending zone. It must be excluded, however, that they form (or have formed) a single unit, as the preliminary sedimentological analyses indicate different types of genesis. Fluvial conglomerates form the largest bodies; the characteristics suggest that both braided and meandering streams existed. The smaller bodies commonly are built by mass-flow deposits, which are also of different types. The recognition of the various depositional conditions provides insight in the paleogeography that is thus far almost terra incognita.