QUATERNARY

RESEARCH
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Loess deposits
<<--download PDF 70 KB: Loess deposits
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<<--download PDF 324 KB: Synthetic table of loess deposits
Karstic fillings
The Jurassic limestones of the Ajoie Tabular Jura are fractured by a network of Cenozoic faults that have been exposed to dissolution processes. These later created numerous sinkholes and as a consequence provided sediment traps. The sediments deposited in these traps during more than 100 000 years.
Schematic section showing various sinkhole (doline) situations with their sedimentary filling.
The karstic internal movements occurred during various climatic phases
(Lower Pleniglacial to Holocene).
They are related to different periods : Old Glacial, Middle Glacial, Upper Glacial and Holocene (Mesolithic; Bronze and Iron Ages, Middle Age, Subactual).

The numerous sinkholes fillings observed in Ajoie, particularly on the sites of Boncourt-Grands’Combes, Courtedoux-le Sylleux and Chevenez-Combe Ronde, consist mostly of loess (eolian sediment) and contain mammals bones. These bones belong to Pleistocene herbivorous mammals associations adapted to cold environments (open steppes): mammoths, woolly rhinoceros, buffalos, red deers, horses and reindeers.

We used the OSL method (Optically Stimulated Luminescence, University of Köln, A. Hilgers) to date the loess that contained the bones: they deposited during the Lower and Middle Pleniglacial.

An example: the sinkhole of Courtedoux-le Sylleux
The discovery in 1998 of a fragment of mammoth’s tusk in the sinkhole of Sylleux led to a palaeontological excavation on a large scale between 1999 and 2000. The depression was filled with more than 10 meters of loess and gravels of the Last Glaciation in which remains of glacial fauna were trapped. The top of the filling was made of approximately 2 meters of Holocene colluvium.
 

Mammoth tusk, sinkhole of Sylleux – Ajoie
The glacial fauna was found between six and twelve meters of depth. It consist of mammoth, horse, red deer and woolly rhinoceros.

The remains of mammoths are the best represented and are composed of cranial, basin and scapula fragments, as well as two whole tusks of three meters length and three segments of tusks. They suggest the presence of at least two individuals.

A radiocarbon dating, realized on a cranial bone of mammoth, places this individual towards 37 000 cal BP (31595 ± 1335 BP). The animal probably lived during a relatively cold phase of the end of Middle Pleniglacial, perhaps between the interstadials Hengelo – Denekamp (Heinrich H4 cold stade).
Period OSL (mean values) 14C
Upper Pleniglacial 29600 BP; 28600 BP;
26600 BP; 22300 BP		 22000cal BP Mollusc fauna
Middle Pleniglacial (the up-per part contain animals bones) 37000 BP; 32000 BP 36000 cal BP (bone);
39000 cal BP (charcoals)
Lower Pleniglacial 65000 BP  
Old Glacial 100600 BP; 91000 BP  
OSL and 14C radiocarbon datings from the sites of Boncourt Grand’Combes and Courtedoux le Sylleux – Ajoie.
Fluviatile environment
More than ten years of research in the Delémont basin have led to a new understanding of palaeoenvironments in these synclinal valley. The different quaternary units as well as their geometry are presented in a synthethic cross section.
<<--download PDF 300 KB: Cross-Section
Three alluvial units (T1, 2 and 3) delineate local geomorphology. T2 was probably deposited during the last Pleniglacial (30-15 ka) and is covered by lateglacial slope and alluvial deposits. Recent investigations (granulometry, micromorphology) on the upper terrace T3 have shown that it is partly loess-covered and OSL datation of these loess delivered stratigraphic-consistent ages going from the end of the middle Pleniglacial (60-30 ka) to the late Pleniglacial. Holocene deposits – as one would expect – are mostly present at the bottom of the valley where they cover and deeply erode the most recent alluvial sheet T1.
Closer investigations on holocene sedimentation and pedostratigraphy have led to a better understanding of past land use in the Delémont basin: indeed, archaeological surveys have shown that this area was not settled by agricultural communities before 2300 cal. BC, during the Bronze Age. This observation has been used as a working hypothesis and confronted to pedo-sedimentological as well as palynological approches. In terms of pedo-sedimentological dynamics, a buried soil dating to the Bronze Age, which overlies local quaternary units, is situated at a rupture point between different depositional regimes in a number of sequences from several different microenvironments. This key event – the beginning of agricultural activity – was documented by a palaeochannel fill sequence dating from 3500 to 2500 BP within a major archaeological site, situated in an alluvial plain. Pollen analysis of the fill sequence was able to show the increasing human impact on plant ecology during this period of time (Braillard et al. 2002).
Current research is focalised on palaeo-environment and palaeo-hydrology during the early medieval period (ca. 500-1000 cal. AD). Two different deposition environments – the alluvial plain of La Pran river (Guélat, in prep.) and a small oxbow-lake filled with organic deposits (Braillard, in prep.) – are used and combined with palynology to precise the climatic impact on medieval human communities.