Autor(es):
Martins, António 
; Cunha, Pedro ; Buylaert, Jan Pieter ; Huot, Sébastien ; Murray, Andrew ; Dinis, Pedro ; Stokes, Martin
Data: 2010
Identificador Persistente: http://hdl.handle.net/10174/4829
Origem: Repositório Científico da Universidade de Évora
Assunto(s): Optical dating IRSL K-feldspar Anomalous fading Fluvial terraces Tejo River Portugal
Descrição
We present the results of K-feldspar IRSL dating of the four lower terraces (T3–T6) of the Portuguese Tejo
River, in the Arripiado-Chamusca area. Terrace correlation was based upon: a) analysis of aerial photographs,
geomorphological mapping and field topographic survey; b) sedimentology of the deposits; and
c) luminescence dating. Sediment sampled for luminescence dating gave unusually high dose rates, of
between 3.4 and 6.2 Gy/ka and, as a result, quartz OSL was often found to be in saturation. We therefore
used the IRSL signal from K-feldspar as the principal luminescence technique. The K-feldspar age results
support sometimes complex geomorphic correlations, as fluvial terraces have been vertically displaced
by faults (known from previous studies). Integration of these new ages with those obtained previously in
the more upstream reaches of the Tejo River in Portugal indicates that the corrected K-feldspar IRSL ages
are stratigraphically and geomorphologically consistent over a distance of 120 km along the Tejo valley.
However, we are sceptical of the accuracy of the K-feldspar ages of samples from the T3 and T4 terraces
(with uncorrected De values >500 Gy). In these cases the Dose Rate Correction (DRC) model puts the
natural signals close to luminescence saturation, giving a minimum corrected De of about 1000 Gy, and
thus minimum terrace ages; this may even be true for those doses >200 Gy. Luminescence dating results
suggest that: T3 is older than 300 ka, probably ca. 420–360 ka (wMarine Isotope Stage [MIS]11); T4 is ca.
340–150 ka (wMIS9-6); T5 is 136–75 ka (wMIS5); T6 is 60–30 ka (MIS3); an aeolian sand unit that
blankets T6 and some of the older terraces is 30– 12 ka. Collectively, the luminescence ages seem to
indicate that regional river downcutting events may be coincident with periods of low sea level (associated,
respectively, with the MIS10, MIS6, MIS4 and MIS2).
