The Late Cenozoic impact glasses in the Argentine Pampean sediments

PETER SCHULTZ; SCOTT HARRIS; MARCELO ZÁRATE

San Salvador de Jujuy

Congreso; XVII Congreso Geológico Argentino; 2008

Asociación Geológica Argentina

The late Miocene to Recent continental record of the Argentine Pampas comprises a vast cover of relatively homogeneous sediments consisting of light reddish brown, massive sandy siltstones that are usually reported as loess and loess-like (loessoid) deposits with thicknesses up to 300m (Schultz et al., 1994). They are derived primarily from Andean and Patagonian volcanics redeposited by fluvial and eolian processes during dry periods. Much of the Pampean loessoid deposits in Buenos Aires Province were winnowed from enormous alluvial fans developed during the Andean uplift. Within these sediments, at least eight different layers of impact-generated glasses have been identified within these sediments since the late Miocene  (Schultz et al., 1998, 2004, 2006, Harris et al., 2005).  The impact materials include suevitic masses (e.g., brick-red tierra cocidas), vesicular melt-matrix breccias (with clast contents ranging from 10% to 90%), and clast-poor tektite-like glasses (Harris and Schultz, 2006). Radiometric dating methods (40Ar/39Ar) establish precise ages for their formation that are consistent with both biostratigraphic and magnetostratigraphic studies of the sequences in which they are found (Schultz, et al., 2004, 2006): 6 ± 2 ka (Rio Cuarto, upper); 114±26 ka (lower); 445 ± 21 ka (Centinela del Mar); 3.27 Ma ± 0.08 Ma (Chapadmalal); 5.28 ± 0.04 Ma (Bahía Blanca); and 9.21 ± 0.08 Ma (Chasicó).  Petrographic evidence for their impact origin includes: ultra-high temperature phases and deformation temperatures (e.g., baddeleyite, lechatelierite, molten rutile), and shocked shock-induced damage in mineral grains (e.g. PDFs, diaplectic glass, and coesite inclusions in quartz; asymmetric isotropization in feldspar, etc.) (Schultz et al., 1998, 2004; Harris and Schultz 2006, 2007; Bland and Artemieva, 2004). Their ages, extents, abundances, and geologic settings indicate that each occurrence should be associated with a crater at least 1 km in diameter.  The preserved impact record in Argentina may seem appear to represent an unexpectedly high impact flux.  While generally consistent (order of magnitude) with recent estimates of impact rates (Bland and Artemieva, 2004), the ages of several impact glass layers also coincide with the well-constrained timing of major asteroid break-up events (Schultz and Harris, 2006).  The Argentine Pampean sediments, however, provide an ideal geologic depositional environment that enhances impact-melt generation and preserves its record.