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The magnetic signals of fluvial and loessic sediments and paleosol sequences developed in the Chacopampean plain (Buenos Aires province, Argentina) are described and interpreted. To provide an analysis of the rock magnetism, sedimentological studies were performed, which led to a stratigraphic framework of so-called 'post-Pampean' sediments deposited in different paleoenvironments. The Lujan formation, composed of fine sand and pelitic sediments, is fluvial and includes floodplain facies, whereas the La Postrera Formations is eolian but particularly loessic. Variations in the magnetic signals observed in paleosols in the units were ascribed to pedogenic processes that occurred at different stages of soil development. An extensive process of dissolution of detrital ferrimagnetic minerals was recognized. In one paleosol, dissolution was accompanied by the appearance of a superparamagnetic (SP) fraction, which may indicate that the loss is connected to subsequent formation of ferrimagnetic minerals in the same paleosol. Both processes did not act simultaneously. The abundant rainfall and absence of CO3Ca in the parent material could be necessary conditions for a defensible scenario for SP genesis. In the loessic sediments (Tapalquelocality), dissolution could be associated with the genesis of a high coercive fraction. The presence of high coercivity minerals may indicate a climate with a distinctly dry season, though their lack does not necessarily imply the absence of a dry season. |
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