Volcanic architecture of the Deccan Traps, western Maharashtra, India : an integrated chemostratigraphic and paleomagnetic study
Detailed volcanostratigraphic logs of seven traverses up the lava sequence in the Western Ghats, Deccan Traps, India, are presented. The main study area, the Mahabaleshwar Plateau, was chosen because the lavas were emplaced around the time of the Cretaceous-Tertiary Boundary and because there is access to exposed lavas on three of its four sides, permitting investigation of the volcanic architecture in 3-D. Besides characteristics of the lava units, the logs include integrated geochemical and palaeomagnetic samples. The lava pile is dominated by pthoehoe sheet lobes and smaller lobes and toes. It can be divided into flow-fields, the products of one eruption, by the occurrence of weathering horizons. Palaeomagnetic results demonstrate that the chron 29R/29N reversal boundary horizon occurs in all four of the traverses around the Plateau and nearby Khumbarli Ghat. The elevation of the reversal horizon on each traverse varies between 897-945 m and 982 m, a value greater than that predicted by the small regional dip. Statistical analysis of geochemical data from samples taken between the reversal horizon and the base of the Mahabaleshwar Formation do not show any apparent correlation around the Mahabaleshwar Plateau, indicating that individual sheet lobes are less than 20 km wide. Determining the lateral extent of flow-fields is not possible using this method but from the occurrence of a similar number of flow-fields in three traverses of similar length round the Plateau, it is probable that most flow fields are at least as wide as the Mahabaleshwar Plateau (more than 20 km). Comparing the thickness of the lava pile between the base of the Mahabaleshwar Formation, the palaeomagnetic reversal horizon and the laterite cap, shows that as much as 95m of topography occurred on the surface of the active Deccan lavas over a distance of approximately 20 km. The volcanic architecture is controlled by the morphology of small sheet lobes, large sheet lobes, and, on a larger scale, flow-fields. These observations, and the varying number of individual sheet lobes making up flow-fields, demonstrates that the structure of the Deccan lava province at the level of eruptive units is extremely complex.