SEDIMENTOLOGY AND GEOCHEMISTRY OF P-T BOUNDARY STRATA IN ABADEH SECTION, IRAN: IMPLICATIONS FOR THE BOUNDARY EXTINCTION

Uppermost Permian limestones in Abadeh section in Iran consist of 18 m of red, nodular lime mudstone/wackestone containing abundant pelagic fauna. Wide distribution, combined with very slow sedimentation rates (6 m/Myr) indicate that this lithofacies was deposited in deep, oxygenated waters. Immediately overlying the Permian strata are a layer (1 m) consisting of decimeter-size mounds of marine cements and a layer (2 m) of peloid grainstone indicative of sedimentation in a shallow, oxygenated environment. The cement-grainstone lithofacies is in turn overlain by 100 m of gray, bioturbated to nodular lime mudstone suggestive of subtidal, deep-water sedimentation. The Permian to Triassic lithofacies succession indicates a rapid and major drop in sea-level at the latest Permian, followed by a sea-level rise during the earliest Triassic. Carbon and Oxygen isotopic compositions decrease toward the boundary suggesting both values are diagenetic formed as a result of meteoric diagenesis associated with the exposure. The lack of an Ir attributed to a gap at the boundary in this section. However, Au, Hf, Sn, Pd, Rh, and Ru concentrations increase or decrease toward the boundary, but their variations appear to be related to clay content of the samples.

Several Permian-Triassic boundary extinction mechanisms invoke global marine anoxia as a contributing or the sole cause of the mass mortality. To be effective, anoxia must be extended to shelf and intertidal areas. Our study suggests shallow and moderately deep waters during the latest Permian and earliest Triassic were well-oxygenated. On this basis, P-T mass extinction mechanisms that rely upon widespread marine anoxia should be viewed with caution. Furthermore, the absence Ir anomaly in the Abadeh section and other “complete” sections as well as the lack of shocked quartz, spherules, and impact ejecta suggests that bolide impact hypothesis of the extinction should also be viewed with care.