NEW OBSERVATIONS OF BAHIA BLANCA MELT BRECCIAS: NARROWING THE SEARCH FOR A LARGE MIOCENE-PLIOCENE BOUNDARY IMPACT CRATER IN ARGENTINA

Of the seven Neogene impact melt deposits reported by Schultz et al. (Geology, 1998; Science, 1998; EPSL, 2004; MAPS, 2006), source craters have been identified only for the 6 ka Rio Cuarto and 445 ka Centinela del Mar materials. Of the remaining events, the locations of structures associated with the 3.3 Ma and 9.2 Ma events are constrained by the occurrence of meter-scale melt blocks in proximity to Chapadmalal and Chasíco. The largest masses are amalgamations of ultra-high temperature scoriaceous glass and oxidized masses of welded loess (tierras cocidas). 5.3 Ma impact glasses found in southern Buenos Aires Providence probably formed during a large impact as evidenced by inclusions of excavated basement rocks, a diverse suite of mixed melt compositions, and a geochemical link to Miocene-Pliocene microtektites identified by Kelly and Elkins-Tanton (MAPS, 2004) in the Southern Ocean. However, these melts typically are centimeter-scale and widely distributed. Recently recovered decimeter-size samples, some of which include large blocks of tierras cocidas, provide new reason to suspect the crater is hidden offshore near Bahía Blanca.

Some of the glasses contain high concentrations of sodium, even quenched halite crystals. Trace element data from tektite-like samples (as well as the Tasman Rise microtektites) also suggest the assimilation of seawater during a marine impact. However, many of the melts appear to have fallen on land. They incorporated fragments of grass, preserved in a different manner than the ultra-high temperature, flash quench encapsulation described by Schultz et al. (Geology, 2014) for the Chapadmalal, Chasíco, and Centinela impacts. In this case, the plant remains occur primarily as carbonized fragments wrapped into the margins of the melt bodies, consistent with our experimental results from relatively low temperature (<1300°C), low oxygen fugacity entrainment of grasses in molten loess. Again, a marine collision close enough to throw melt onshore is indicated.

The new samples continue to exhibit a hallmark of Bahía Blanca glasses: the survival of microscopic bolide fragments. They consistently coat the surfaces of melt bodies suggesting that they were ejected through a plume rich in fine impactor debris. The mineralogy of these fragments continue to demonstrate an affinity with angrites.