Rôle du volcanisme explosif et du vent dans la formation des précipités calcaires quaternaires («tosea») de la Pampa argentine

Abstract

Calcretes have been a traditional subject of polemics between edafologists considering them as generated by pedogenic processes, and geomorphologists who attributed the transportation of their calcium carbonate to surficial run-off. Recently, some english-speaking Authors have gathered evidence of the role of wind in dust transport and on the contribution of this dust for the formation of soils. J. Tncart, in 1968, demonstrated that the alcalis which turn rural development very difficult in the pampa Deprimida of Argentine, has been transported by westerly winds and has probably a volcanic origin. The present paper treats of the regions which form half a circle from the NW to the SW of the pampa Deprimida They are characterized by limestone deposits, indurated ("tosca", equivalent of the mexican "caliche"), or not The following results have been obtained from field observations, as unfortunately no laboratory investigations have been possible. 1) The calcium carbonate, which predominates in the area, and the alcalis of the "sahtrales" are wind transported. 2) In some places, calcium carbonate is mixed with volcanic ashes and small pyroclastic debris. It is therefore presumed of volcanic origin. 3) Rainfalls, presumably resulting for a part of the volcanic explosions themselves, have partially dissolved the limestone dust and precipitated the carbonate when water evaporated It resulted in a diagenesis of the carbonate into laminated calcrete and, underneath, when water was abundant enough, into veins and concretions along the water paths, amist the unconsohdated silty carbonate Regional differences can be noted according to the amount of precipitations, which is lower westwards, and which have changed during the pleistocene with alternating dry and wet periods, as demonstrated in the Pampa Deprimida. 4) "Tosca" does not result from pedogenic processes : it crops frequently, without any cover, on the highest points of the landscape, so that it cannot be an exhumed BCa horizon. Presently, in the wetter parts of the area investigated, i.e. around Sta Rosa, calcrete suffers a destruction by pedogenesis and rendzina soils are forming In the West of La Pampa Province, alcalis have been deposited on solid basalt, contained in aeohan sandy loams. Their lixiviation by throughflow feeds extensive "sahtrales" which seem to be the equivalent of the closed basins of the western side of the Andes, in the Chilean Pampa del Tamarugal, where nitrate has been mined and still is in decreasing quantities. 5) The volcanic dust, both calcarous and alcaline, has been transported by westerly winds, as formerly established by investigations in the Pampa Deprimida. Sedimentation decreases eastwards. Calcareous dust, probably coarser than alcaline dust, has been deposited first and has not reached the Pampa Deprimida when alcali aerosols did Alcaline dust is found along a kind of beam from Corral de Piedra to the Pampa Deprimida, passing through the extensive "sahtrales" near Puelén (Salitral de la Perra). The distribution of both calcareous and alcaline wind-born products suggests that they come from the South of Neuquén Provinc but from volcanoes, not yet precisely identified. 6) Calcareous dust have been deposited from the North of the Rio Negro Province to the South of Cordoba Province, during at least 3, more probably 4 paroxysmal periods during the Pleistocene.