Provenance study of Pliocene–Pleistocene sands based on ancient detrital zircons (Alvalade Basin, SW Iberian Atlantic coast)

Abstract

Pliocene-Pleistocene sand of the Alvalade basin was taken from the sea-cliffs of SW Iberia coast for a provenance study using radiometric dating. The U-Pb ages obtained revealed a wide interval ranging from Cretaceous to Archean, with predominance of Paleozoic, Neoproterozoic and Cretaceous zircon ages. Cretaceous ages interpreted to indicate a Sines Massif provenance are dominant in sands close to Cape Sines but are absent in sand sampled 12 km north. Carboniferous ages younger than ca. 315 Ma suggesting a possible contribution from the Central-Iberian Zone originally; however, these zircons may be multi-cyclic, having been reworked from Eocene-Miocene siliciclastic deposits related to transport from central Iberia (Lower Tagus basin drainage evolution). These signatures provide important constraints on the location and extent of the Pliocene-Pleistocene topography and drainage system that were probably controlled by: i) Miocene to Pleistocene landscape rejuvenation driven by Alpine movements along major faults; and ii) residual reliefs related to inherited Variscan structure. The U-Pb ages obtained also trace the pre-Cenozoic paleotectonic evolution of SW Iberia recorded in their sources: i) the North Gondwana accretion and breakup; ii) the Gondwana and Laurussia collision; and iii) the Pangea breakup and opening of the Atlantic Ocean.

Provenance of Holocene beach sand in the Western Iberian margin: the use of the Kolmogorov–Smirnov test for the deciphering of sediment recycling in a modern coastal system

“…). Pliocene–Pleistocene sands forming the sea cliffs to the north and south of Cape Sines were transported by a drainage system controlled by Alpine faults from a source area close to the Atlantic Ocean made up of Mesozoic sedimentary (Alentejo basin) and igneous rocks (Sines massif), an eastern source in the Palaeozoic rocks of the Ossa‐Morena and South‐Portuguese zones, and sources located to the north containing Miocene sedimentary rocks (Albardeiro et al ., ; Fig. ).…”

“…The radiometric results obtained for beach sand were compared by conducting a statistical test using the age populations of: (i) igneous zircon from the syenites of the Sines pluton (U–Pb dating using LA–ICP–MS; Miranda et al ., ; this study); (ii) igneous zircon from the syenite and gabbro of the Sines pluton [U–Pb dating using isotope dilution thermal ionization mass spectrometry (ID–TIMS) of fractions of previously abraded grains; Grange et al ., ]; and (iii) detrital zircons from Carboniferous greywacke and Pliocene–Pleistocene sand (U–Pb dating using LA–ICP–MS; Pereira et al ., ; Albardeiro et al ., ) forming the sea cliff surrounding Cape Sines.…”

“…). The Holocene beach is discontinuously limited landward by Holocene dunes and sea cliffs mainly made up of the Pliocene–Pleistocene sand of the Alvalade basin (Oliveira, ; Oliveira et al ., ; Albardeiro et al ., ) with an average height not exceeding 50 m (Gama & Jalobeanu, ). Locally, Miocene sandstones presenting marine fossils can also be found at the base of the sea cliff (Fig.…”

“…The occurrence of Pliocene–Pleistocene sediment recycling in the Western Iberian margin has recently been demonstrated using the U–Pb dating of detrital zircons (Albardeiro et al ., ). The findings presented in these studies revealed that Cretaceous zircon grains were recycled into the Pliocene–Pleistocene basin, but a significant number of older Cretaceous ages did not match the interval of zircon fertility previously established for the Sines pluton, suggesting provenance from the older Cretaceous Sintra pluton located further north.…”

“…However, in many of these studies the K–S test was not used, while in those in which it was used, its full potential was rarely taken advantage of. In this paper, a detailed report on the use of the K–S test for the deciphering of sediment recycling in a modern coastal system is presented, based on new U–Pb analyses conducted on Holocene beach sand detrital zircons, testing previously published U–Pb data indicating potential sources forming the sea cliff (Carboniferous greywacke, Pereira et al ., ; Pliocene–Pleistocene sand, Albardeiro et al ., ; Cretaceous syenite and gabbro, Miranda et al ., ; Grange et al ., ).…”