Clues to the genesis of a discontinuously distributed fragipan in the northern Apennines,Italy

At about 1000 m above sea level in the Vallombrosa Forest, northern Apennines, Central Italy, a dense pan that meets the diagnostic criteria for fragipans is discontinuously distributed. The unusual slope-parallel “digitate” distribution of the pan suggests that it could have formed on water-saturated soil material that moved as earthflows. Actually, several soils in the Vallombrosa Forest have buried or truncated horizons, indicating episodes of mass movement and deposition. However, the fragipan under study shows a homogeneous morphology throughout, hence not providing clues about the occurrence of episodes that have contributed to the emplacement of the parent material. Also, the physical, chemical, and mineralogical analyses of the fragipan do not reveal discontinuities that help to resolve distinct episodes of deposition. Nevertheless, discontinuities in features such as the degree of alteration of biotite grains and the amount of plasma, seen through the micromorphological investigation at 50–60 cm of the current depth, suggest the occurrence of at least two depositional events. Radiocarbon dating of large-size pieces of charcoal collected at three depths within the fragipan yielded three ages that may reflect different times of accumulation of the parent material during the last glacial period. Post-depositional micromorphological features, such as relatively undamaged clay coatings and a weak platy structure, indicate pedogenesis with the absence of intense pedoturbation.