Depositional history of diamictites from the Late Paleoproterozoic Dalma Formation (E India)

Abstract

The Paleoproterozoic Dalma Formation, situated on the Singhbhum craton in eastern India, consists almost entirely of mafic to ultramafic rocks that were deposited by effusive volcanic activity and as pyroclastic deposits. The main focus of the present study, however, is on diamictites, consisting – like both other rock types – exclusively of chlorite and actinolitic hornblende, that are intercalated with these volcanic rocks. These diamictites are interpreted as mass-flow deposits. The textures and mineralogical composition of the diamictites suggests a volcanic origin of the material; most probably they are reworked pyroclastic deposits with the same mineralogy as the Dalma lavas. On this basis, it is concluded that the diamictites represent the deposits of one or more volcanic mudflows. They must have formed of material that had already travelled over a considerable distance in the form of grain-by-grain transport, as indicated by the presence of both rounded and distinctly broken clasts, and by the relatively small size of the clasts. Both the grain-by-grain and the mass transport must have occurred following pathways that were covered all over with volcanic material of mafic composition. This transport must have taken place under terrestrial conditions, as no marine or lacustrine clasts or matrix is included. The results of the study suggest that the diamictites within the Dalma Formation cannot be seen as a possible equivalent of Paleoproterozoic glacigenic sediments such as known from Australia and South Africa.