LOESSFEST'09 | Aug. 31st – Sept. 3rd, 2009 |Novi Sad-Serbia
Origin and Emplacement of the Loess Deposited Under the English Channel
Lefort J.P.1, Danukalova G.A.2
1Université de Rennes 1, Campus de Beaulieu, Laboratoire d’Archéosciences (bât. 24- 25),74205 CS, 35042 Rennes cedex. France
2Institution Russian Academy of Sciences Institute of geology of the Ufimian scientific centre.450077, Ufa. K. Marx Str. 16/2. Russia
There are no more loess deposits in the English Channel, they have been completely
removed during the last Holocene transgression. But its former extension
and origin can be understood when looking at the submerged Pleistocene conglomerate
and at the loess of northern Brittany. The submerged Pleistocene conglomerate
of the English Channel is a shelly and cobble-rich formation which
has been only found on the southern flanks of the Channel. It results from the
cementing of beach deposits overlain by loessic silts at a time when the seawater
was at a lower level (Lefort, 1969). Study of the fauna included in this conglomerate
(Danukalova and Lefort, 2009) allows the separation of three shelly belts located
between –24 and –54 m around the Channel Islands of the Normano-Breton
Gulf, -40 and –65m off Tregor and – 52 and - 80m off Leon. A fourth belt
but without fauna may exist at –93m. These belts correspond with four regressive
Pleistocene episodes. Comparison with one of the many published sea level
curves (Shackleton, 1987), slightly smoothed by Buchdahl (1999), may help to
date the three different beach deposits containing fauna at around 105/110 Ka,
73/80 Ka and 30/57 Ka. These beaches were sealed by the three main loess deposits
already studied onshore (Loyer, 1993) and dated at around 82 Ka, 67 Ka
and 18 Ka. We assume that there are at least three different Pleistocene conglomerates
in the English Channel. The northern extension of the conglomerates
probably corresponded with the northern limit of the “thick” loess formations.
These formations which may reach 4 metres in Northern Brittany must be of a
minimum thickness to develop the calcareous cement we sampled offshore. The
northern limit of the submerged conglomerates also corresponds with the linear
mid-Channel trough which represents a remnant of the old Seine river valley
(Lautridou, 1999). This valley developed at a time when the English Channel was
free of any water. North of it, no Pleistocene conglomerate has ever been sampled.
This does not mean that loess never deposited off southern England but we
think that its thickness was probably too limited to generate the cement found
in the south. Such a thin loess cover is known in the Silly Islands and South
of Wales where it is never thicker than 40 cm (Van Vliet et al., 1997). Onshore
studies carried out in Normandy (Lautridou, 1967) have demonstrated that the
loess deposited in this area resulted from the eolian sweeping of the muddy sediments
outcropping at the mouth of the Seine River. Because the heavy mineral
association recognized in the coastal cliffs of northern Brittany is characterized
by the same assemblage (epidote + amphibole + garnet +”ubiquist”minerals)
as that found in Normandy (Monnier, 1973), we think that part of the loess deposited
off Brittany was generated by the now submerged section of the Seine
river valley. This is supported by the study of the direction of the Pleistocene
paleo-winds undertaken in this area. This study evidences that the eolian sediments were transported from a source located north or
Northwest off St Brieuc Bay (Bigot, 1986). Taken as a
whole, we consider that the terraces and the pocket-like
depressions observed in seismic reflection at the level of
the mid-Channel trough (Lefort, 1975) were at the origin
of the offshore and onshore loess of Northern Brittany.
Rare earth elements and various results obtained
by geochemistry on recent loess of northern Brittany
and Normandy (Gallet, 1997) confirm these views. Major
elements show that, like many other loess deposited
elsewhere, those of northern Brittany experienced at
least one cycle (and probably more than one cycle) of
aquatic sedimentary differentiation process. They also
show that if glacial grinding was really an important
mechanism for the production of silt-size particles, the
nature of the swept bedrock cannot be of igneous or
meta-igneous composition (Gallet et al, 1998).
Taken all together these informations suggest that part of the loess deposited south of the axis of the English Channel originated in the sediments deposited in the valley of the old Seine River and that an other part was extracted from the Mesozoic and Cenozoic formations which form the actual bottom of the English Channel and southern England (Lefort, 1975). Grinding of the ophiolites, granites and metamorphic rocks outcropping south of England can be ruled out since the ice sheet has never being reaching this area (West, 1963). If we take only account of the surfaces swept by the Upper Pleistocene winds, we can speculate that the contribution of the Mesozoic and Cenozoic calcareous formations dominated the younger Seine valley sources. This reasoning cannot be supported if we admit that the sediments deposited by the Seine River were easier to remobilize than the indurate limestones of Mesozoic and Cenozoic age.