ARGENTINE IMPACT RECORD

The Pampeano Formation (PF) of Argentina represents a widespread loess-like deposit. Within these deposits, at least five distinct horizons of impact glass (escoria) dating back to the Miocene have now been discovered. Such materials provide a unique view of the cratering process and its record in thick eolian sediments. This record also establishes critical benchmarks for refining (and redefining) the stratigraphic record through complementary paleomagnetic studies. Previous studies focused on the Holocene Rio Cuarto and the Pliocene Mar del Plata impact glasses dated to about 5Ka and 3.3Ma, respectively. Here we report three additional horizons. Glass ages have been dated using incremental heating with a CO2 laser at the Berkeley Geochronology Center.

Pleistocene impact glasses near Necochea date to 0.46 ± 0.03 Ma. based on linear regression of the isotopic data for five plateau steps, an age consistent with the magnetostratigraphy. These glasses exhibit distinctive budding, thermal cracking, and collapsed vesicles suggestive of rapid melting/quenching underwater. High phosphate and strontium levels indicate incorporation of marine sequences containing shell materials, thereby implicating a nearshore impact.

A new impact glass horizon has now been identified in early Pliocene deposits over a broad region in southern Buenos Aires Province. Radiometric dating yields an age of 5.2 Ma, generally consistent with the mammalian fossil record.

Lastly, a layer of impact glass near Chasico has been radiometrically dated to 10.1 ± 0.4 Ma. Drainage patterns delineate a concentric structure 15 km in diameter and may represent the buried impact crater. Sedimentary sequences extend to depths greater than 1 km and include shallow marine deposits. Strontium levels and unusually high levels of Ba (almost 8000 ppm) in the glasses contrast with levels in Miocene sediments containing the glasses and suggest sampling of deeply buried marine sequences.

Why Argentina? More than a dozen impact craters larger than 1 km in diameter should have occurred in the PF since the late Miocene. Consequently, the number of impacts discovered to date is not unusual. What is unusual is the effect of the loessoid sequences in enhancing melting and archiving these products.