EVIDENCE FOR A LARGE IMPACT AT THE MIOCENE-PLIOCENE BOUNDARY

Schultz et al. (MAPS, 2006) reported the occurrence of 5.28 (±0.04) Ma impact melt breccias distributed near the Miocene-Pliocene boundary over a broad region in the vicinity of Bahía Blanca, Argentina. Among the melt breccias are sparse examples of high-grade yellow glass that contain shocked quartz and other mineralogical evidence of ultra-high transient pressures and temperatures. They exhibit many textural and chemical (e.g., extremely low water contents) similarities to tektites, and we interpret them as being representative of melts formed closest to the interface between the asteroid and the target surface. Consequently, trace element data (LA-ICPMS) indicating elevated concentrations of strontium and uranium, together with major element data (WDS/EMP) showing more than 10 wt% Na₂O, suggest that the impact may have occurred in a marine environment, incorporating seawater into the melts.

Trace element data, most directly La/Th and La/Sc, demonstrate the plausibility that these melts share a common source with microtektites reported by Kelly and Elkins-Tanton (MAPS, 2004) close to the Miocene/Pliocene boundary on the South Tasman Rise. Those microtektites exhibit an elevated scandium concentration that can be explained by a contribution from a differentiated basaltic impactor. Inclusions in the Bahía Blanca impact melt breccias appear to be surviving fragments of a highly refractory angrite-like bolide. Elemental analyses of the merrillite and Mg, Ca-silicophosphate-rich basalts suggest that they could have been formed on a large differentiated parent body.

We propose that a fragment of that parent body collided with South America at or very close to the Miocene-Pliocene boundary distributing ejecta across a significant portion of the South Hemisphere. The impact may have had immediate regional consequences on biota and depositional systems. But if the event also delivered large concentrations of dust to Antarctic latitudes, it could have contributed to short and long-wavelength temperature declines on a global scale. It also is possible that this impact is related to a broad positive helium-3 excursion reported by Farley et al. (Nature, 2006) that spans the boundary.

Stratigraphic and paleontological work are needed across the Miocene/Pliocene boundary that takes this event into account.