Soils in the Holocene alluvial sediments of the Rio Fraja Valley, Spain: in situ or soil-sediments?

In the Holocene alluvial deposits of the Fraja Valley (Cadiz Province, SW S pain) we have used geomorphological, mineralogical, sedimentological and pa leomagnetic data with dating based on radiocarbon and pottery to distinguis h episodes of erosion, sedimentation and pedogenesis during the last 4000 y ears. The data sets allow us to distinguish soils formed in situ from trans ported soil-sediments. Three depositional episodes have been distinguished, two of them resulting in accretion of river alluvium and the third in coll uvium that covers the floodplain sequence. Between 3000 and 1000 BP, there were three periods of soil formation resulting in humic horizons; these in situ soils are separated by four humic soil sediments. All are similar in m ineralogical composition and organic matter content, but the magnetic inten sity changes abruptly from < 7 mA m(-1) in the in situ soils to > 10 mA m(- 1) in the transported soil layers. Changes in declination are related to th e continuing sedimentation, and a brief paleomagnetic excursion is recognis ed at approximately 2000 BP. (C) 2000 Elsevier Science B.V. All rights rese rved.