Loess Records - Mineral Dust: A Key Player in the Earth System - page 424

fields toward the west and southwest. Thick loess deposits are located in the sub-
Andean mountainous area of northwestern Argentina (Tucumán); they also occur
in the highland plains of Sierras Pampeanas of Córdoba and San Luis. In addition,
loess has been reported in Tierra del Fuego and the eastern Andean piedmont (Zárate
2003 ), while Holocene peridesert loess has been reported in the Atacama Desert of
Peru (Eitel et al. 2005 ).
Much of the knowledge of South American loess comes from the Pampean
Plain where the deposits consist of complex stratigraphic sequences composed of
primary loess and loess-like (loessoid) deposits that are usually much more abundant
(Fig. 16.2 c; Zárate and Blasi 1993 ; Zárate et al. 2009 ). Loess deposits are commonly
interlayered with fluvial gravels and/or colluvial deposits along the piedmonts of
the southern Buenos Aires ranges (Ventania, Tandilia) and the Pampean ranges of
Córdoba and San Luis provinces. Carbonate accumulations of variable morphology
and genesis are common in the loess sequences. Fossil vertebrate remains are
very common, together with abundant bioturbation features of both vertebrates and
invertebrates (Zárate 2007 ).
The Pampean stratigraphic record dates back to the late Miocene ( 12-10 Ma).

The thickest ( 40-50 m) Quaternary loess and loess-like deposits are located

in the northern Pampas with only the uppermost 10-15 m exposed. The loess
sedimentation process during the Neogene-Quaternary was characterized by the
occurrence of several pulses of landscape reactivation marked by major erosional
episodes (Folguera and Zárate 2011 ).
The chronology of the Pampean loess sequence traditionally has been estimated
on the basis of their vertebrate fossil content, being subdivided into several
biostratigraphic units (land mammal ages; see Marshall et al. 1983 ) later redefined
as stage-ages (Cione and Tonni 1995 ). In recent years, numerous luminescence
dates were obtained from several upper Quaternary sections of the northern and
southern Pampas (e.g., Kemp et al. 2006 ; Frechen et al. 2009b ; Kruck et al. 2011 ).
The loess-paleosol sequences in the mountains of the Tucumán region (Kemp
et al. 2003 ) are situated at an altitude between 1,800 m and 2,500 m asl and west of
the western Chaco Plain (Fig. 16.4 b). This sequence, which exhibits a much higher
stratigraphic resolution than the Pampean successions, consists of a thick (

40-
50 m) record of loess and reworked loess. The deposits, rich in vertebrate fossil
remains, are characterized by the occurrence of numerous (28-32) discrete paleosols
(Zinck and Sayago 2001 ; Schellenberger and Veit 2006 ).
The Pampean loess deposits are chiefly of volcanoclastic composition, with
particles made up of volcanic lithic fragments (basaltic, andesitic, and rhyolithic
rocks), volcanic glass shards, feldspar (mainly orthoclase), plagioclase (albite to
labradorite), and quartz, usually with percentages lower than 20 %. The miner-
alogical and geochemical compositions of loess and loess-like deposits (Teruggi
1957 ; Smith et al. 2003 ) suggest that the explosive, volcanic Andes Cordillera is the
main loess source area, including the direct input of volcanic particles as well as
discrete tephra layers. The mountain loess of Tucumán is thought to be derived
from the northwestern Argentinian/southern Bolivian Andes and the Altiplano