The Courtmacsherry raised beach exposure. Overdependence upon OSL
dates is not a good idea.
Quote:
Undoubtedly there is extensive evidence for the beach along the south coast of Ireland forming in a cold-climate environment (see above), and indeed some of the evidence which underpins this interpretation was first noted by Wright and Muff (1904). This includes the presence of erratics within the raised beach gravels and interbedding of the beach gravels with periglacial slope breccias. Previous investigations of the sand facies that overlie the raised beach at sites such as Howe's Strand and Broadstrand have interpreted it as ‘blown sand’ (e.g. Synge, 1978). This is inconsistent with the sedimentology of these sands which exhibit well defined hummocky and swaley cross-stratification consistent with a shallow marine rather than aeolian setting. Thus the facies sequence of raised beach/HCS sands/SCS sands indicates submergence following beach formation. Periglacial slope deposits and isolated angular clasts of local bedrock within these sands indicate the maintenance of a ‘cold’ depositional environment during this submergence.
Sorry chaps, but I beg to differ. There is NOT "extensive evidence"....... I have gone through the stratigraphic descrtiptions very carefully, and I see NO evidence that the climate was cold at the time of beach formation. The raised beach and associated sands do not show any evidence of a prevailing cold climate. There are no signs of a cold climate shell fauna in the beach gravels and sands, and no trace of a warm climate fauna (as in the Patella Beach) either. There seem to be no examples of interdigitated glacial materials in the raised beach or in the sands. There is either a clean break between the beach materials and the overlying slope breccia, or some interdigitations of broken bedrock materials, but no evidence is presented to demonstrate that the breccia accumulated under cold climate conditions. The breccias most likely represent local rockfalls and scree accumulations following a shift in coastline position. This could be established with reference to the details of coastal morphology and the positions of the local rock cliff.
There appear to be no glacitectonic structures or erosional contacts in the raised beach materials which might indicate the presence of a local ice front.
The fact that the overlying sands are interpretd as shallow marine beds rather than blown sands has no bearing on the "cold climate issue". And the presence of occasional angular clasts in these beds simply suggests that there were localised rockfalls nearby, or some incorporation of glaciated clasts from the disaggregation of older (pre-existing) glacial deposits on the adjacent coastline.
The junction between the raised beach and the breccia is essentially no different from that which we observe at Poppit, Ogof Golchfa, Broad Haven South and many other locations on the other side of St Georges Channel, in Pembrokeshire. It is most realistic simply to interpret this junction as representing the climatic shift from interglacial (Ipswichian) to cold climate (Early Devensian) conditions, as concluded by many researchers over many decades.
So the only evidence (as far as I can see) of the cold climate environment is that of the OSL dates. That's not a good situation. What if all of those dates are faulty, as a result of faulty calibration or a systematic under-estimation of the ages presented by the authors of this paper? We all had to learn some pretty harsh lessons from the chaos of amino acid dating a few decades ago, which was so serious that it brought much of the work of the Geological Conservation Review volumes on Wales and the Soutrh-West of England into question. We await further views on the reliability -- or otherwise -- of the dates. In the meantime, a degree of scepticism is in order.
See also:
A Marine Isotope Stage 4 age for Pleistocene raised beach deposits near Fethard, southern Ireland
Colman Gallagher, Matt W. Telfer, Colm Ó Cofaigh
Jnl of Quaternary Science, Volume 30, Issue8
November 2015
Pages 754-763
First published: 06 October 2015
https://doi.org/10.1002/jqs.2808
https://doi.org/10.1002/jqs.2808
Samples from sediments whose grains were exposed to daylight during transport or deposition date the time of transport or deposition.The most suitable sediments are those that were exposed to the most daylight during transportation, including wind-blown sands and silts.The optically stimulated luminescence (OSL) signal from minerals is more rapidly reset by daylight than is the thermoluminescence (TL) signal (Fig 8). OSL measurements have made it feasible to look at fluvial and colluvial sediments as well, but care needs to be taken in these cases to assess whether they were exposed to sufficient daylight at deposition to reset the luminescence signal being measured.
Duller, G. A. T. (2008). Luminescence Dating: Guidelines on using luminescence dating in archaeology. English Heritage.
No comments:
Post a Comment