The green amphibole abundance as a stratigraphic tool in Palaeolithic sites: examples from Belgium

The green amphibole abundance as a stratigraphic tool in Palaeolithic sites: examples from Belgium
Paul Spagna1, Paul Haesaerts1, Jean-Marc Baele2 & Stéphane Pirson1,3
1) Department of Paleontology, Royal Belgian Institute of Natural Sciences, 1000 Brussels, Belgium ;
2) Department of Fundamental and Applied Geology, University of Mons, 7000 Mons, Belgium ;
3) Direction de l'Archéologie, Service public de Wallonie, 5100 Namur, Belgium.
Variations in the green amphibole abundance of the Belgian and surrounding loess deposits have been studied for more than sixty years. Different ratios were defined in the literature, and subsequently used for stratigraphic purpose to quantify the green amphibole (generally associated to green hornblende- gh) in the heavy mineral assemblage of the loess (e.g. Gullentops 1954; Juvigné 1978). Firstly confined to the last glaciation deposits (MIS 4 to 2), high MI loesses have then been recognized in the penultimate glaciation deposits (MIS 6) by Balescu & Haesaerts (1984). Now used as a parameter on its own, the gh abundance is synthesized in the reference loess sequence in Middle Belgium (Haesaerts & Pirson, in Pirson 2007).
Recently, new samples were studied in the Belgian loess belt. Our research objectives consist in completing and homogenizing the mineralogical stratigraphy of the Belgian loess reference sequence by (re)analysing sections in Romont (Eben-Emael), Remicourt, Harmignies and Maisières-Canal. Moreover, an attempt to determine the exact composition of these so-called 'green amphibole' has been made. The green minerals have long been grouped under the 'green hornblende' appellation in previous papers. However, our preliminary observations suggest a more diversified mineralogical assemblage based on optical anisotropy, which shows both B+ and B- species, had to be confirmed. The chemicals results obtained both in Mons (UMons-GFA laboratory) and Bochum (Ruhr Universitaet - Zentrale Elektronen-Mikrosonde), confirm the dominance of magnesio-hornblende in the assemblage, followed by pargasite (and ferro-pargasite), actinolite, edenite, and tschermakite.
We also applied mineralogical stratigraphy to totally different depositional environments such as cave entrances, where pedostratigraphic markers are often missing. This has been made in two sites: Scladina and Walou caves. Trends observed in the studied loess and cave sequences are consistent with the gh trend of the reference loess sequence (Pirson, 2007), even if they present lower values. One of our goals is to refine the chronostratigraphic frame of some Palaeolithic sites in Belgium. Results from Scladina and Walou caves will illustrate this approach.

References :
Balescu S. & Haesaerts P. (1984) The Sangatte raised beach and the age of the opening of the Strait of Dover. Geologie en Mijnbouw, 63, pp. 355-362.
Gullentops F. (1954) Contributions à la chronologie du Pléistocène et des formes du relief en Belgique, Louvain (Mémoires de l'Institut géologique de l'Université de Louvain, 18), pp. 125-252.
Juvigné E. (1978) Les minéraux denses transparents des loess de Belgique. Zeitschrift für Geomorphologie N. F., 22, p. 68-88.
Pirson S. (2007) Contribution à l'étude des dépôts de grotte en Belgique au Pléistocène supérieur. Stratigraphie, sédimentogenèse et paléoenvironnement. Thèse de doctorat, Université de Liège et Institut royal des Sciences naturelles de Belgique, 435 p. + annexes.