Flow stratigraphy of selected sections of the Rajmahal basalts, eastern India

Abstract

The Rajmahal basalts form a north-south trending belt extending over 4100 km² along the eastern margin of the Indian Shield. The basalts form part of a widespread magmatic episode which coincided with continental break-up between India and Australia-Antarctica during the Late Jurassic-Early Cretaceous. In the sections examined, between 2 and 15 flows have been encountered, ranging in composition from tholeiitic basalt to dacite (pitchstone). The flows vary in thickness from < 1 m to > 70 m. Significant volcanic features observed include vent sites, marked by volcanic breccia, welded agglomerate, tuff and lapilli-tuff (including volcanic bombs). The tuffs occur in widely scattered areas, and are more abundant than hitherto recognised. These rocks were probably erupted during the early stages of volcanic activity, following extensive differentiation in sub-volcanic magma chambers.

Several characteristics of sediments interbedded with the lavas suggest a humid, sub-tropical lacustrine depositional environment during eruption of the lower part of the volcanic sequence, and a temperate climate during emplacement of the upper flows. Quiescent eruption of the Rajmahal basalts took place largely through fissures sited at the faulted margin of the Indian Shield. This magmatic activity is interpreted as resulting from extension of the lithosphere above a mantle plume.