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Chronology for mountainous river terraces: OSL/IRSL and rock dating techniques applied to carbonate-rich terraces in the Atlas Mountains

Authors:

Abstract

Improvements in numerical dating methods continues to open new possibilities for understanding sedimentary archives and thus unravelling Earth surface processes (e.g. Rixhon et al, 2017). In general, the Optically Stimulated Luminescence (OSL) dating is successful for deposits spanning last 100 ka, while Infra-Red Luminescence (IRSL) dating of feldspar can be applied on longer timescales (Buylaert et al, 2012). The Atlas Mountains in Morocco contain an abundance of carbonate-rich river terraces recording glacial-interglacial river evolution (Stokes et al., 2017) and are an ideal place to apply OSL techniques on mountainous river sediments deposited at distinct time scales. River strath terraces are formed by transition between valley widening and downcutting of terraces in response to local divergence of sediment-transport capacity (Hancock; and Anderson, 2002). The formation of terraces in response to a change in climate can be distinguished from a response to a change in local uplift rates (Hancock; and Anderson, 2002). While separating climatic from tectonic signals in the geomorphic record remains a challenges, it is possible using the records of erosional surfaces and sediments of river terraces. Where river strath terraces and their sediments are preserved in mountainous settings they form the ideal opportunity to test the timescales and responses of surface process to climate and tectonic histories. This requires high resolution dating of river terraces and their coarse-grained sediments. IRSL dating has the potential to provide insight into glacial-interglacial erosional and depositional processes over the last few cycles. An experimental method of bedrock exposure (Sohbati et al, 2012) has the potential to unlock insight into erosional processes on the timescale of river terrace formation. Analysis of sediments, bedrock and pebbles was undertaken in the summer of 2018 after a terrace mapping and sampling campaign in the Atlas. OSL and IRSL analysis of the material resulted in age estimates and has established the sensitivities of various rock material to Luminescence signals. Further work will include extensive sampling and dating of terrace conglomerates, as well as targeted sampling for rock exposure dating.
UK Luminescence and ESR Dating Meeting Sheffield, 11th-12th September 2018
50
Chronology for mountainous river terraces: OSL/IRSL and rock
dating techniques applied to carbonate-rich terraces in the Atlas
Mountains
J.R. Zondervan1,*, M. Stokes1, M. Jain2, J.P. Buylaert2,3, M.W. Telfer1, A.S. Murray3
1School of Geography, Earth and Environmental Sciences, Plymouth University, Plymouth
PL4 8AA, United Kingdom
2Center for Nuclear Technologies, Technical University of Denmark, DTU Risø Campus, DK-4000 Roskilde, Denmark
3Nordic Laboratory for Luminescence Dating, Department of Geoscience, University of Aarhus, Risø Campus,
Frederiksborgvej 399, 4000 Roskilde, Denmark
*Corresponding author; Email: jesse.zondervan@plymouth.ac.uk
Improvements in numerical dating methods continues to open new possibilities for understanding sedimentary
archives and thus unravelling Earth surface processes (e.g. Rixhon et al, 2017). In general, the Optically
Stimulated Luminescence (OSL) dating is successful for deposits spanning last 100 ka, while Infra-Red
Luminescence (IRSL) dating of feldspar can be applied on longer timescales (Buylaert et al, 2012). The Atlas
Mountains in Morocco contain an abundance of carbonate-rich river terraces recording glacial-interglacial
river evolution (Stokes et al., 2017) and are an ideal place to apply OSL techniques on mountainous river
sediments deposited at distinct time scales.
River strath terraces are formed by transition between valley widening and downcutting of terraces in response
to local divergence of sediment-transport capacity (Hancock; and Anderson, 2002). The formation of terraces
in response to a change in climate can be distinguished from a response to a change in local uplift rates
(Hancock; and Anderson, 2002). While separating climatic from tectonic signals in the geomorphic record
remains a challenges, it is possible using the records of erosional surfaces and sediments of river terraces.
Where river strath terraces and their sediments are preserved in mountainous settings they form the ideal
opportunity to test the timescales and responses of surface process to climate and tectonic histories. This
requires high resolution dating of river terraces and their coarse-grained sediments. IRSL dating has the
potential to provide insight into glacial-interglacial erosional and depositional processes over the last few
cycles. An experimental method of bedrock exposure (Sohbati et al, 2012) has the potential to unlock insight
into erosional processes on the timescale of river terrace formation.
Analysis of sediments, bedrock and pebbles was undertaken in the summer of 2018 after a terrace mapping
and sampling campaign in the Atlas. OSL and IRSL analysis of the material resulted in age estimates and has
established the sensitivities of various rock material to Luminescence signals. Further work will include
extensive sampling and dating of terrace conglomerates, as well as targeted sampling for rock exposure dating.
References
Buylaert, J. P., Jain, M., Murray, A. S., Thomsen, K. J., Thiel, C., and Sohbati, R., 2012, A robust feldspar luminescence
dating method for Middle and Late Pleistocene sediments: Boreas, v. 41, no. 3, p. 435-451.
Hancock;, G. S., and Anderson, R. S., 2002, Numerical modeling of fluvial strath-terrace formation in response to
oscillating climate: GSA Bulletin, v. 114, no. 9, p. 1131-1142.
Rixhon, G., Briant, R. M., Cordier, S., Duval, M., Jones, A., and Scholz, D., 2017, Revealing the pace of river landscape
evolution during the Quaternary: recent developments in numerical dating methods: Quaternary Science Reviews, v. 166,
p. 91-113.
Sohbati, R., Murray, A. S., Chapot, M. S., Jain, M., and Pederson, J., 2012, Optically stimulated luminescence (OSL) as
a chronometer for surface exposure dating: Journal of Geophysical Research: Solid Earth, v. 117, no. B9.
Stokes, M., Mather, A. E., Belfoul, M., Faik, F., Bouzid, S., Geach, M. R., Cunha, P. P., Boulton, S. J., and Thiel, C.,
2017, Controls on dryland mountain landscape development along the NW Saharan desert margin: Insights from
Quaternary river terrace sequences (Dadès River, south-central High Atlas, Morocco): Quaternary Science Reviews, v.
166, p. 363-379.
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Article
Luminescence dating is used extensively to provide absolute chronologies for Late Pleistocene sediments. Nowadays, most optical dates are based on quartz optically stimulated luminescence (OSL). However, the application of this signal is usually limited to the last ∼100 ka because of saturation of the quartz luminescence signal with dose. In contrast, the feldspar infrared stimulated luminescence (IRSL) dose–response curve grows to much higher doses; this has the potential to extend the datable age range by a factor of 4–5 compared with quartz OSL. However, it has been known for several decades that this IRSL signal is unstable, and this instability often gives rise to significant age underestimation. Here we test against independent age control the recently developed feldspar post‐IR IRSL approach to the dating of sediments, which appears to avoid signal instability. A physical model explaining our observations is discussed, and the method is shown to be accurate back to 600 ka. The post‐IR IRSL signal is reduced by exposure to daylight more slowly than that from quartz and low‐temperature IRSL, preventing its general application to young (e.g. Holocene) sediments. Nevertheless, this new approach is widely applicable (feldspar of appropriate luminescence behaviour is even more ubiquitous than quartz). These characteristics make this a method of great importance for the dating of Middle and Late Pleistocene deposits.
Article
Many river systems in western North America retain a fluvial strath-terrace rec ord of discontinuous downcutting into bedrock through the Quaternary. Their importance lies in their use to interpret climatic events in the headwaters and to determine long-term incision rates. Terrace formation has been ascribed to changes in sediment supply and/or water discharge produced by late Quaternary climatic fluctuations. We use a one-dimensional channel- evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences. The model includes sediment transport, vertical bedrock erosion limited by alluvial cover, and lateral valley-wall erosion. We set limits on our modeling by using data collected from the terraced Wind River basin. Two types of experiments were performed: constant- period sinusoidal input histories and variable-period inputs scaled by the marine δ18O rec ord. Our simulations indicate that strath-terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates. Straths are cut when the channel floor is protected from erosion by sediment and are abandoned—and terraces formed—when incision can resume following sediment-cover thinning. High sediment supply promotes wide valley floors that are abandoned as sediment supply decreases. In contrast, wide valleys are promoted by low effective water discharge and are abandoned as discharge increases. Widening of the valley floors that become terraces occurs over many thousands of years. The transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment-transport capacity. Formation of terraces lags by several thousand years the input changes that cause their formation. Our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation. The incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces, because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the Last Glacial Maximum. This apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock-uplift rate.