PLEISTOCENE AND HOLOCENE BEACHES AND ESTUARIES ALONG THE SOUTHERN BARRIER OF BUENOS-AIRES, ARGENTINA

The Buenos Aires aggradation plain has a good record of Quaternary sea -level fluctuations. To the east of the Tandilia Range, the elevations of the Pleistocene beaches respond to the tectonic behaviour of the S alado Basin. Holocene beaches indicate a maximum transgression higher than 2 m. The low relief permitted an extended horizontal record of be ach/chenier plains interfingered with estuarine environments (coastal lagoons, marshes) covered by a sandy (Eastern) barrier. Between the Ta ndilia and Ventania ranges, the location of Pleistocene and Holocene b eaches are related to a former higher relief; i.e. they are attached t o low-altitude cliffs and underneath cliff-top dunes composing the Sou thern Barrier. At Claromeco, Pleistocene gravel beaches, mostly compos ed of caliche pebbles, occur at heights between 4 and 7 m, and are ove rlying estuarine Pleistocene environments. Beaches of the same age are at a level of 10 m at Mar del Plata Harbour and Arroyo Sotelo (west o f Mar Chiquita Lagoon). Holocene beaches found at Punta Mogotes and Co sta Bonita are at higher altitudes than on the Eastern Barrier (ca. 2- 4 m). The Holocene estuarine sequences are related laterally to presen t operating inlets (Las Brusquitas, La Ballenera, Quequen Grande, Clar omeco, Quequen Salado). They are seldom thicker than 2.4 m, and compri se basal layers of black muds; towards the top, the layers are thinner , of coarser grain size and white colours. Grain-size analyses were pe rformed comparatively on Pleistocene, mid-Holocene and present beaches . Sangamonian beaches are gravelly or coarser than medium sand (mean). Holocene beaches are usually coarser than medium sand, but dominantly shelly to the north of Mar del Plata, and composed of volcanic clasts to the south of this city. Modern beaches are dominated by fine sand, except at some erosive beaches between the Mar del Plata capes. They have a lesser content of shells than those of mid-Holocene. Copyright (C) 1996 Elsevier Science Ltd