Stable and ultrastable heavy minerals of alluvial to nearshore marine sediments from Central Portugal: Facies related trends (original) (raw)
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THE SOURCES AND HYDRAULIC SORTING OF HEAVY MINERALS ON THE NORTHERN PORTUGUESE CONTINENTAL MARGIN
Knowledge of the transparent heavy minerals present in the sand fraction of sediments on the northern Portuguese continental margin rests on the study of 504 samples, obtained from the shelf and upper slope (358), and from the major rivers (146). Results show the main association comprising biotite, andalusite, tourmaline, amphibole, garnet, staurolite, zircon and apatite. A second, subordinate, suite includes orthopyroxene, clinopyroxene and olivine and is present only in the outer shelf and upper slope south of Porto canyon. These two distinctive assemblages indicate derivation from different primary sources: the main heavy mineral suite is sourced from metamorphic and igneous rocks, widespread in the drainage basins of all northern Portuguese rivers, whereas the minor one is of different origin and points to a basic igneous source, located probably elsewhere in the outer shelf or upper slope, south of Porto canyon. The distribution patterns of the mineral species in the major association indicate that these terrigenous assemblages were delivered to the shelf by rivers in a highly selective manner. Only biotite and other platy grains are able to reach the middle shelf that is constrained by the analysis of the hydraulic behaviour of the different grains. Mobile grains, such as biotite and the most platy amphiboles, are excellent sedimentary tracers of the present shelf transport dynamics. Their distribution patterns suggest the influence of a downwelling oceanographic regime, which predominantly affects the inner and middle shelves. Corresponding distribution patterns between the most mobile heavy mineral grains and fine-grained sediments (muddy patches) may indicate the similarity of their hydraulic behaviour. In contrast, the less mobile grains, such as coarse garnet, andalusite, tourmaline, staurolite and zircon, reach only the inner shelf. Polycyclic, rounded grains found in the middle shelf in coarse-grained deposits represent relict and palimpsest sediments and contrast with the angular forms characteristic of the modern river and inner-shelf sediments.
Chemie der Erde - Geochemistry, 2011
Geochemical and mineralogical data from a Pliocene to Pleistocene alluvial sequence are integrated to access the factors that control rare earth elements (REE) geochemistry and the evolution and spatial differences in provenance. The studied alluvial system is situated in a tectonic active setting at the contact between the Variscan Massif, with several Paleozoic and Precambrian units that support a coastal range, and the Atlantic margin. REE and HREE abundances are generally higher in swamp-lake than in floodplain sediments. The majority of the REE in floodplain sediments is hosted by Y and Th-bearing minerals and illite; in swamp-lake sediments is also probable an association with organic matter. The high Gd/Yb, Eu/Eu* and kaolinite content in older sediments suggest that during the earlier phases provenance was mainly from the hinterland (weathered granitic rocks and its sediment cover). The subsequent illite (Mg-rich) clay assemblages and REE patterns indicate mainly lateral input from the eastern basin edge (Palaeozoic and Precambrian metapelitic rocks). These sediments tend to have lower Gd/Yb and La/Sm than their source rocks. The analysis of the chemical index of alteration (CIA) indicates that although a north-directed fluvial axis existed at that time the "mature" basin edge units found to the south (Silurian) had a limited role in supplying sediments. This shift in provenance is attributed to the uplift of the coastal range. Afterwards, the contribution of recycled Cretaceous and Cenozoic sedimentary units increased progressively. This is demonstrated by the increase is SiO 2 /TiO 2 , CIA and kaolinite/illite when the climatic conditions are expected to have become less chemically aggressive.
Journal of Coastal Research
Knowledge of the littoral Tróia-Sines (SW of Portugal) sand grain-size distribution and transparent heavy minerals rests on the study of 195 samples collected from beachface, berm, dune and sea-cliffs. The results show the dominance of coarse sand and a heavy mineral suite composed by tourmaline, muscovite, andaluzite, biotite, amphiboles, staurolite, pyroxenes, silimanite and kyanite. The textural sediment characteristics allow the definition of three different littoral long shore sectors: north, central and south. In the north sector there is a north-south increment in grain-size linked with clear evidences of a bioclastic supply from intertidal benthonic macrofauna communities. In the central sector, sea-cliffs are the source of heterometric sediments to the beach (from coarse sand to silt sized particles). In the southern sector, sea-cliffs also represent an important sediment source, nourishing the beach system with fine sands. The heavy mineral composition shows high quantitie...
Provenance of Heavy Minerals: A Case Study from the WNW Portuguese Continental Margin
Minerals, 2019
This work describes and interprets the presence of heavy minerals in the WNW Portuguese continental margin using a set of 78 bottom samples collected from three distinct areas of this margin: the Porto, Aveiro, and Nazaré canyon head areas. The main transparent heavy mineral assemblage (mineral grains with frequencies ≥1% identified under a petrographic microscope) is composed of amphibole, andalusite, tourmaline, biotite, garnet, staurolite, pyroxene, zircon, and apatite. The felsic igneous and metamorphic rock outcrops in the main Northern Portuguese river basins and the relict sedimentary continental shelf deposits explained the presence of most of these mineral grains (both considered as distal sources). However, the presence of pargasite, augite, diopside-hedenbergite, enstatite-ferrosilite, and forsterite in the Porto and Aveiro areas (minerals identified by electronic microprobe analysis) is probably related to the presence of an igneous basic source next to dolomitic limestones affected by thermal metamorphism. These geological formations are considered as local sources. The high concentration of biotite observed in the Nazaré area is the result of the selective transport of the most lamellar sand particles of this mineral.
The Meaning of Heavy Minerals in the Recent Sedimentary Record of the Douro Estuary (Portugal)
Journal of Coastal Research
The relative abundance of the different heavy mineral species may be used as a tool to deduce the provenance signal of the sediment and the dominant transport mode that occurred just prior to sedimentation. In this work, the sedimentary infilling of the Douro estuary was analysed using Principal Components Analysis (PCA) in order to characterize the sedimentary sequence over the past 14 000 years. Three cores were obtained by rotary drilling in this estuary and several samples were collected. Heavy minerals were separated using SPT (sodium poly-tungstate); different species were identified and the non-opaque minerals were counted. The mineralogical composition of the fines was analysed by X-ray diffraction (CuK radiation) on random-oriented powder samples.Asimple relation was obtained: the finer sediment levels are less mature and the heavy mineral assemblage, transported mainly as suspended load, is composed by minerals with platy and lamellar forms. Levels with coarser sediment ha...
Th is work aims to study recent climatic oscillations and their infl uence on sedimentation in the Ria de Vigo, a coastal embayment in Galicia, NW Spain. It is based on the study of clay mineral assemblages, in conjunction with other proxies (granulometric, geochemical, geochronological and microfaunal), in the core KSGX 24. A Benthic Foraminifera High Productivity (BFHP) proxy was used to determine changes in the fl ux of organic matter (OM) at the bottom of the study area. Total organic carbon (TOC) content is not a suitable proxy to estimate changes in the past supply of OM due to diagenetic processes. Th e sedimentation was fi nest in 3 sections: ~ 230-214 cm, ~ 185-73 cm and ~ 20-0 cm. Th ese muddy sections are characterised, in general, by higher proportions of detrital minerals, concentrations of several chemical elements related to lithogenic sources and BFHP values. In addition, these sections are impoverished in carbonates, Ca, Sr and La when compared with the layers with the highest sand content. Th e clay mineral assemblage of the studied site, characterised by the dominance of illite, intermediate concentrations of kaolinite and minor amounts of smectite and chlorite, reveals the prevalence of a typical temperate humid climate in the last 3 ka BP, the estimated age for the core base. However, the quantities of illite and chlorite increase in the muddy layers. Th e characteristics of these muddy layers were interpreted as representing relatively cold climatic oscillations associated with the strengthening of northerly winds and the prevalence of an upwelling regime corresponding to wellknown periods, such as the fi rst cold period of the Upper Holocene (~ 2.9 ka cal BP), the Dark Ages (between ~ 2.2-1.2 ka cal BP) and the Little Ice Age (~ 0.6 ka cal BP).
JESUS, C.C., MIRANDA, P., ROCHA, F. AND OLIVEIRA, A., 2007. Fine-fraction mineralogy and geochemistry of beach sediments of SW Portugal as markers of sedimentary dynamics. Journal of Coastal Research, SI 50 (Proceedings of the 9th International Coastal Symposium), 990 -994. Gold Coast, Australia, ISSN 0749.0208