CRITICAL EVALUATION OF LAVA EXTRUSION RATES IN THE DECCAN TRAPS, INDIA, AND THE CHIXCULUB IMPACT EVENT

U-Pb ages (1) for the Composite Western Ghats Section (CWGS) of the Deccan Traps, suggest extrusion of a ~3.0 km thick lava pile in ~750 ka, encompassing the time of the Chixculub Impact (Cretaceous-Paleogene boundary (KPB)) at 66.04 Ma. A previous estimate, based on ⁴⁰Ar/³⁹Ar data of lower precision (2), indicated ~3.2 km of the same lava pile was formed in ~900 ka. The (three sets) of U-Pb dates (1) for the uppermost Mahabaleshwar Formation (MbF), are out of stratigraphic order. The statistical model used (1) to iron out problems, is flawed; the single reliable age from this set, is 65.51 Ma for red bole (RBE) at the base of the MbF. U-Pb ages indicate extrusion of the bottom ~2.8 km of the CWGS in ~630 ka, and a lower rate above that elevation (~300 m in ~120 ka). Numerous red bole horizons in the upper reaches of the CWGS – commonly interpreted to be paleosols – suggest lower eruption frequency. Other efforts (3, 4) look to the estimated volumes of lava extruded in the Formations of the CWGS, rather than their measured thickness; these authors argue the lava extrusion rate in the Deccan Traps was measurably affected by the antipodal effects of the Chixculub Impact. ⁴⁰Ar/³⁹Ar ages (4) were utilized to argue for a twofold increase in extrusion rate, in moving from the lowest horizons, to the uppermost reaches of the CWGS. Critical appraisal of the relevant “ages” shows the three lowest samples dated - Jawhar through Neral Formations - contain excess argon; their (isochron) ages are less (younger) than the 66.38 – 66.28 Ma (plateau) ages, utilized earlier (4).

The best ages indicate the lava extrusion rate for the pre and post KPB horizons differ by a smaller amount than reported earlier (4). Further refinements of these estimates call for (a) precise ages for lavas in the Poladpur through Mahabaleshwar Formations and (b) accurate estimates of the volume of lava in each Formation of the CWGS.

References Cited:

(1) Schoene et al., Science 347 (2015), 182-184. (2) Baksi, Jour. Asian Earth Sci. 84 (2014), 9-23. (3) Richards et al., Geol. Soc. Am. Bull. 127 (2015), 1507-1520. (4) Renne et al., Science 350 (2015), 76-78.