Paleoambientes de la cuenca media del río Luján (Buenos Aires, Argentina) durante el último período glacial (EIO 4-2)

the sedimentary sequences exposed in cutbanks of the middle course of the luján river preserve important paleoenvironmental information on the late quaternary climatic evolution of the north-eastern pampean region. the objective of this paper is to define depositional units and infer the paleoenvironmental conditions from the analysis of the sedimentary facies and the paleobiological content (mollusks, phytoliths and diatoms) of the sequences dated between ca. 70,000 and 11,000 yr bp, oxygen isotopic states (ois) 4 to 2, of the middle region of the luján river basin. this paper follows on from previous studies about the reconstructions of the paleoenvironmental and climatic changes in the northern region of the buenos aires province presented by dangavs and blasi (1995), prieto et al. (2004) fucks et al. (2005), fucks and deschamps (2008) and blasi et al. (2008, 2009a, b). a late pleistocene-holocene cutbank (pt-h) is preserved along the banks of the luján river where three sections were analyzed (table 1). the lower exposures, found from the present river level to halfway up the bank show late pleistocene materials, while the upper portion shows holocene fluvio-lacustrine and aeolian deposits. the holocene deposits overlie paraconformably and start with a high organic matter concentration level which was deposited ca. 11,000 14c yr bp in a lentic environment of meso-eutrophical characteristic (prieto et al., 2004). between the jáuregui and manzanares cities the pt-h cut bank is not continuous but alternates with deposits from a previous sedimentary cycle that we informally named "pampeano" (ameghino, 1884; dangavs and blasi, 1995). this last unit constitutes the present fluvial channel floor and interfluvial deposits of this region. the development of the cutbanks studied here is likely to be related to a late holocene fluvial incision of the late pleistocene-holocene sediments and aeolian, fluvial and lacustrine deposits that infill blowout depressions (ameghino,