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 Spain) we have used geomorphological, mineralogical, sedimentological and paleomagnetic data with dating based on radiocarbon and pottery to distinguish episodes of erosion, sedimentation and pedogenesis during the last 4000 years. The data sets allow us to distinguish soils formed in situ from transported soil-sediments. Three depositional episodes have been distinguished, two of them resulting in accretion of river alluvium and the third in colluvium 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 mineralogical composition and organic matter content, but the magnetic intensity 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 the continuing sedimentation, and a brief paleomagnetic excursion is recognised at approximately 2000 BP.