Loess facies analysis and chronology to reconstruct morpho-sedimentary and palaeoclimatic evolution: A case study from the Belgian-Dutch Maas valley

Through sedimentary and micromorphologic analyses of four Late Pleniglacial loess sections in NE Belgium and the SE Netherlands we demonstrate that stratigraphic subdivision and correlation should not be based on facies. Three facies are identified which are not linked to specific lithostratigraphic units but are a function of local conditions and geomorphologic processes: a homogeneous aeolian facies, an eolian loess redeposited by afterflow, and a more inhomogeneous loess affected by overland flow. Correlations between the aeolian periglacial depositional environments in the studied loess region and the neighbouring type region of coversand deposition are upgraded using both available and new sedimentological descriptions, detailed analyses of grain-size properties and OSL chronologies. Both regions show a general tendency from wet to dry climate with equivalent changes from energetic to moderate overland flow and finally wind deposition. Two main phases of continuous permafrost formation have been identified by the presence of ice-wedge casts, an older one before 25 ka and a younger one around 18–21 ka followed by permafrost degradation with presence of cryoturbation. The presence of residual frozen ground between these two phases is suggested. Continuous permafrost disappeared after 17 ka. The new luminescence dates enable a better understanding of the regional evolution of processes and environmental conditions in the eastern and southern Netherlands at the end of the last glacial. They show that in-situ loess deposition was absent between c. 17 and 14.7 ka in the Maas valley of the southern Netherlands although in-situ coversand deposition was typical during this interval in the adjacent central and eastern type region of coversands, and that the characteristic tongued Nagelbeek palaeosol, an important lithostratigraphic marker in loess series, has an age of c. 24.5 ka. • Three facies of loess deposition are based on micro-morphology and grain-size (end of last glacial, Maas valley). • Facies depend highly on local geomorphological conditions and thus are unsuitable for lithostratigraphic correlations. • A similar palaeo-environmental evolution from humid to dry conditions appears in the loess and coversand region. • Two periods with continuous permafrost are recognized: one ending up at c. 24.5–25 ka, and one from c. 21 ka to c. 17 ka. • New OSL-dates estimate the characteristic Nagelbeek palaeosol at c. 24.5 ka in the loess of the Maas valley (type region).