Evidence of a Tsunami at the Permian-Triassic Boundary From Kashmir, India

SHAKEN AND STIRRED: SEISMITES AND TSUNAMITES AT THE PERMIAN-TRIASSIC BOUNDARY, GURYUL RAVINE, KASHMIR, INDIA

Authors:

M. E. BROOKFIELD, T. J. ALGEO, R. HANNIGAN, J. WILLIAMS and G. M. BHAT

Abstract:

The famous Permian-Triassic boundary section at Guryul Ravine in Kashmir shows repeated strong disturbances in the uppermost 3 m of the section below the main end-Permian mass extinction horizon. Two one-meter-thick disturbed beds, with convoluted bedding and fluid escape structures, are interpreted as seismites. Immediately above, three lenticular, fining-upward, bioclastic grainstone beds, interbedded with argillites, are interpreted as tsunamites. In these beds, hummocky cross-stratification and grading indicate deposition by waning irregular waves at a minimum water depth of 100 m based on physical processes and faunas. Bed grain sizes indicate that the waves needed to move even the coarse sand of the matrix, let alone associated large pebbles up to 20 cm in diameter, range from amplitudes of ∼40 m for wave periods of 10 s (the upper limit for storm waves) to amplitudes of ∼3 m for wave periods of 50 to 1000 s (typical of large open-ocean tsunamis). Fossil and sedimentary evidence suggests lengthy intervals between successive tsunami events, which, together with a lack of geochemical evidence for impact, favors terrestrial causes. Geochemical proxies show that the Guryul Ravine environment remained oxic or suboxic throughout the P–Tr transition, but that anoxia developed regionally at the time of the boundary crisis. This paper is the first to propose seismites and tsunamites at the P-Tr boundary, so the geographic extent of these deposits is unknown, although analogous deposits occur in many sections worldwide from published reports.