Title: Chapter 3 The Sources and Hydraulic Sorting of Heavy Minerals on the Northern Portuguese Continental Margin
Abstract: 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.
Publication Year: 2007
Publication Date: 2007-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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Cited By Count: 23
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