AN ENVIRONMENTAL MAGNETIC ANALYSIS OF LATE MIOCENE (CHASICOAN) LOESS IN SOUTHERN BUENOS AIRES PROVINCE, ARGENTINA ASSOCIATED WITH THE 9.23 MA METEORITE IMPACT EVENT

The Late Miocene Chasicoan loess/loessoid sequence, located roughly 65km west of Bahía Blanca in southern Buenos Aires province, contains the oldest vertebrate fossil assemblage of the Argentine Pampas as well as a primary deposit of meteorite impact glass dated to 9.23±.09 Ma. The sequence accumulated between 9.07-9.43 Ma and consists of several distinct paleosols separated by episodic low-energy fluvial deposits. This study is a continuation of Zárate and co-author’s work published in 2007 and the goal is to characterize changes in depositional and post-depositional processes as they relate to the meteorite impact event and Late Miocene climate. To understand these processes, we examine the environmental magnetic properties (concentration, grain size and mineralogy) of core samples taken from outcrops exposed by the Arroyo Chasicó. Our results show that 1) the paleosol units have a distinct magnetic signature (higher magnetic concentration) compared to the fluvial deposits and 2) following the impact event, the magnetic concentration increases and the quantity of ultrafine-grained magnetic particles decreases. A relatively low frequency dependence of magnetic susceptibility (<4%) in both the paleosols and the fluvial intervals indicates that ferrimagnetic minerals in this sequence were generally depleted, likely due to reducing conditions. This finding is consistent with a similar environmental magnetic study from younger loess/loessoid deposits (< 2 Ma) in the central eastern Pampas of Buenos Aires province that show a depletion of ultrafine-grained magnetic minerals in the A horizons of recent soils and paleosols. These findings suggest that the loess/paleosol sequences of Argentina are more similar to the Alaskan loess sequences than they are to the Chinese loess sequences. In addition, the magnetic concentration of the sediment unaltered by pedogenesis is significantly higher than the equivalent sediment found in the younger sequences. This suggests that the concentration of magnetic minerals in the parent loess has decreased through time. The intercalation of paleosols and fluvial deposits suggest a strong climatic forcing on deposition/post-depositional processes at this locality, but it is evident that the meteorite impact event played a significant, discreet role as well.