The Origin and Importance of Red Bole Horizons Within the Deccan Traps, India

The Deccan Continental Flood Basalt Province (CFBP) represents a major volcanic episode at c. 67 - 64 Ma, and the effects of these eruptions have recently been linked to the global pulse of faunal extinction at the KTB. However, since the environmental impact of volcanism is governed by the style of the eruption, it becomes necessary to first determine the precise nature of the Deccan Traps volcanism in order to evaluate any CFBP-climate-extinction link. We have discovered evidence of widespread explosive volcanism in the Deccan Traps, suggesting that fire-fountaining topped by convecting plinian or subplinian columns was characteristic of some of the most voluminous eruptive episodes. This discovery contrasts strongly with the currently accepted view that CFBP eruptions were characterised by quiescent fissure eruptions. Well-preserved boles occur throughout the Deccan succession intercalated between lava flows. These have commonly been interpreted as the result of in situ alteration of flow tops. Whilst this is often the case, petrographic analysis has identified that a significant number of red bole horizons contain angular, blocky and cuspate basaltic shards diagnostic of explosive pyroclastic origin, thus forcing a re-evaluation of the significance of Deccan boles. We interpret these type of boles as basaltic ash-fall deposits. Moreover, whilst geochemical analysis reveals that the flow-top alteration boles display chemical signatures similar to that of the underlying lavas, those of the ash-derived boles do not. Instead, many of the ash-fall boles often display a slightly more evolved character. This, together with recently published evidence indicating the high gas content of many Deccan lavas, indicates that explosive volcanism played an important role during the development of the Deccan CFBP. Most importantly, this type of explosive volcanism provides a potent mechanism by which Deccan-derived sulphur and fine ash could have reached the stratosphere, and thus influenced the global climate during Late Cretaceous times. Clearly, determining the nature of the volcanism in the Deccan CFBP is fundamental to understanding sulphur mass release budgets, and in aiding the development of robust palaeo-climate modelling.