MAGNETOSTRATIGRAPHY OF THE UPPER FORMATIONS OF THE DECCAN TRAPS : AN ATTEMPT TO CONSTRAIN THE TIMING OF THE ERUPTIVE SEQUENCE

Close synchronism between the formation of Continental Flood Basalts and times of mass extinction throughout the Phanerozoic implies that CFBs could be involved in these environmental crises. The time required for emplacement of the bulk of large traps does not exceed 1 Myr. Traps are composed of hundreds of single flows. A lower bound of the climatic impact of a single event can be (probably strongly under-)estimated from the largest historical volcanic eruptions. Therefore, we can surmise that the climatic impact of traps should be controlled by the number, duration, and length of time between successive events. A large number of flows uniformly distributed over as much as 1 Myr might not be sufficient to significantly modify the climatic and biotic systems. The aim of the present work is to improve our estimates of the duration and pattern of trap emplacement. The Cretaceous-Tertiary (K-T) mass extinction coincides with the emplacement of the Deccan CFB. The precise time sequence of flow emplacement can be constrained using a number of approaches, among which detailed magnetostratigraphy (using geomagnetic secular variation as a century-scale clock) and soil formation (using the development of alteration). We present our first results concerning the upper part of the Deccan traps. Samples have been analyzed, using mainly thermal demagnetization, and site-mean directions of characteristic magnetizations have been obtained, allowing us to reconstruct the magnetostratigraphy of the sequence. Evolution of these directions as a function of stratigraphic position shows successive flows with well-grouped (correlated) directions, each corresponding likely to a rapidly emplaced sequence. As a result, the sampled part of the eruptive sequence can be divided into seven lava pulses. Time constraints on lava flow emplacement and intertrappean layers (red bole) lead us to suggest that the 600m-thick section could have been emplaced in less than 30 kyr, and possibly as little as 12 kyr. We next attempt to estimate the amount of volcanic gases emitted during each pulse, using data from analogous historical or geological fissure eruptions as a scaling parameter. Approximate reconstruction of the volcanic forcing function (both in amplitude and time) may allow us to constrain the possible climate change induced by eruptions (work in progress).