THE RECORD OF LATE CENOZOIC IMPACTS IN THE ARGENTINE PAMPAS: CONSEQUENCES OF HYPERVELOCITY COLLISIONS INTO SOFT SEDIMENTARY TARGETS

Over the last two decades Schultz and co-workers have identified 8 impact melt breccia deposits in the late Cenozoic strata of the Argentine Pampas. ⁴⁰Ar/³⁹Ar and fission track dating, combined with sequence and biostratigraphic controls, demonstrate that these deposits represent separate events between 9.24 Ma and 6 ka. Pampean impact breccias provide a natural laboratory for characterizing the petrography of impact ejecta formed during soft target impacts, specifically thick, very fine-grained, loosely consolidated clastic sediments. Although heterogeneously distributed and easy to overlook the first few times through a thin section, our analysis has shown that many traditional signatures of shock metamorphism do occur in these breccias. Examples include PDFs and coesite in quartz and “asymmetric isotropization” of twinned feldspar grains. Indicators of ultra-high temperatures are more common and include lechatelierite, baddeleyite, and molten rutile. Evidence of extremely high-velocity, complex particle interactions also can provide useful support for the confirmation of impact deposits formed from soft sediment targets.

Those interactions are especially significant if the particles involved have a demonstrably extraterrestrial origin. We have identified exotic clasts ranging from µ-size grains to mm-size rock fragments within several of the Pampean melts. We argue that they are surviving remains of the impactors, and we predict that such remains should be a common feature of soft sediment impacts. They have allowed us to conclude that a portion of the bolides responsible for the 3.27 Ma and 5.28 Ma impacts resembled mesosiderites and angrites, respectively.

We have found that another feature of Pampean melt breccias is their ability to trap target water in distinctive pockets of hydrous melt. Particularly abundant in 445 ka breccias, we have determined that these melts contain from ~3 to 24 wt% (OH^- + H₂O) using transmission FTIR, reflectance FTIR, and ion probe measurements.

The continued study of Pampean impacts is important for understanding the terrestrial impact record in relatively recent times and increasing our knowledge of impacts into wet, loess targets (composed largely of reworked volcanic debris). Such targets have special relevance to studying the nature of impacts on Mars.