ABSTRACT

Soil samples from 21 localities of the Pampa region of eastern Argentina were studied by X-ray and other methods to determine their clay mineralogy. The principal parent material of the soils of the major part of the Pampa is a loesslike sediment consisting of wind-blown arkosic and pyroclastic silt (Pampeano Formation, Pleistocene); the main soil type is a Prairie, with planosolic modifications near the coastal plain. The predominant clay mineral of the Pampa soils is detrital illite, derived from weathered acid volcanics (Triassic?) of northern Patagonia and to a lesser extent from crystalline basement rocks of central Argentina, and transported by fluvial and wind action mostly in the form of argillized lithic and feldspar silt-sized fragments. In the Rio de la Plata coastal plain he parent material is essentially an estuarine deposit (Pospampeano Formation, Querandino facies; Holocene); the clay is a mixture of approximately equal parts of montmorillonite, kaolinite, and illite; the former two were brought in from the Parana River basin, whereas the latter was inherited from the reworked Pampeano sediments. The illite content relative to the other two clay minerals increases inwards to the coastal plain embayments filled by a lagoonal-like facies (Lujanense) of the Pospampeano. The soil clay in these areas shows no difference from that of their respective parent materials. In southern Mesopotamia, the uplifted dissected "pampa" region east of the Parana River, the soil parent material consists mainly of limnic clayey silts corresponding to the lower part of the P mpeano formation and is rich in montmorillonite. No clay mineral neoformation was detected m any of the soils studied.