GEOLOGÍA Y PALEONTOLOGÍA DE LOS SEDIMENTOS ENCAUZADOS DEL PLEISTOCENO TARDÍO Y HOLOCENO EN PUNTA HERMENGO Y ARROYO LAS BRUSQUITAS (PARTIDOS DE GENERAL ALVARADO Y GENERAL PUEYRREDÓN, PROVINCIA DE BUENOS AIRES)

Abstract

GEOLOGY AND PALAEONTOLOGY 0F THE LATE PLEISTOCENE ANO HOLOCENE VALLEY-FILL DEPOSITS AT PUNTA HERMENGO AND ARROYO LAS BRUSQUITAS (GENERAL ALVARADO AND GENERAL PUEYRREDÓN CoUNTIES, BUENOS AIRES PROVINCE). - Two profiles of the late Pleistocene and Holocene va lley-fill deposits are described. Both sections contain sediments of continental aquatíc origin; referred to the Guerrero and Rio Salado Members of the Luján Forrnation, and are related to estuarine sediments or to edaphostratigraphic units (Puesto Callejón Viejo Soil). At Punta Hermengo a holocene marine ingression is represented by estuarine sediments containing three kinds of organisms, as follows: very fragmented typical marine anímals such as Brachydontes rodriguezi (d'Orbigny), Tagelus plebeius (Lightfoot), Turbonilla uruguayensis Pilsbry and Arctocephalus australis (Zimmermann); continental aquatic species such as Littoridina parchappei (d'Orbigny) and Lymnaea columella Say; and strictly continental organisms such as Nothura maculosa Salvadori and Ctenomys sp. At Arroyo Las Brusquitas, the marine ingression is represented by an assemblage consisting of Mytilus platensis d'Orbígny, Brachydontes rodriguezi, Mytella falcata (d'Orbigny), Macoma uruguayensis (Smith), Littoridina australis (d'Orbigny) and Tagelus plebeius. Individuals of T. plebeius, euryhaline species, inhabiting silty-sandy substrates at river mouths, are found with both valves articulated and in life position. The estuarine sediments inclicate that the coast was located farther east than it is today. Different conditions during the late Pleistocene - Holocene are inferred for the Punta Piedras - Mar Chiquita and Mar Chiquita - Monte Hermoso sectors.