SEDIMENTARY PROCESSES IN DOURO ESTUARY (PORTUGAL) A HEAVY MINERAL STUDY

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1 Thalassas, 2004, 20 (2): An International Journal of Marine Sciences SEDIMENTARY PROCESSES IN DOURO ESTUARY (PORTUGAL) A HEAVY MINERAL STUDY C. FRADIQUE (1) & J. CASCALHO (2) Keywords: Heavy minerals, grain sorting, transport mode, roundness classes, Douro estuary ABSTRACT This work focuses on the study of 90 heavy mineral samples from three different sand size classes (medium, fine and very fine sand) of three cores collected from the Douro estuary. These cores were obtained by rotary drilling. For each sample the heavy minerals were counted and twelve major non opaque species were identified. Studying the relative abundance of the heavy minerals, their roundness stage and the mean grain-size differences between sediment and heavy minerals it was possible to establish the hypothetic sediment grain sorting and transport mode. The finer sediment levels have lower mean grain size differences between sediment and heavy minerals being these minerals with predominant platy and lamellar forms. This information establishes that the sediment might have been sorted and eventually transported mainly as suspended load. Levels with coarser sediment show strong mean grain-size (1) Museu Nacional de História Natural de Lisboa, Universidade de Lisboa, Portugal, crfradique@fc.ul.pt (2) Museu Nacional de História Natural de Lisboa, Universidade de Lisboa, Portugal, jpcascalho@fc.ul.pt differences between sediment and heavy minerals, those being equant and rounded. In this case the sediment might have been sorted and eventually transported mainly as bedload. In the lower corer levels it was detected a high abundance of euhedral heavy minerals assemblage reflecting the provenance signal of the granite and gneissic bedrock. From the correlation of the heavy mineral and grain size sediment data with the information obtained from the sediment biogenic content it is possible to distinguish two fundamental situations: the sediment levels compatible with suspended load grain sorting were mainly deposited in a estuary environment; the sediment levels compatible with bedload grain sorting were likely deposited in a sandy barrier/beach face environment. INTRODUCTION Heavy minerals have long been used as a tool to interpret sedimentary processes. Generally, they are used in determining provenance, tracing transport paths, mapping dispersal patterns, depicting the action of hydraulic regimes and selection processes, locating potential economic deposits and understanding diagenetic processes (KOMAR, 1989; MORTON and HALLSWORTH, 1999). In the present work, heavy mineral analysis is applied as a contributor to the 61

2 C. Fradique & J. Cascalho deduction of the dominant modes of sediment grain sorting and transport in the recent sedimentary infilling of the Douro estuary (Portugal). For this purpose, data were obtained within the "Envi-Changes" project, whose main objective is to outline an evolutionary model for this estuary since BP (DRAGO et al., 2002). The Douro estuary is located in the northern Portuguese coast, south of Porto. This narrow funneled estuary is partly barred by a sand barrier (Cabedelo) that develops from the south and shelters the S.Paio Bay (DRAGO et al., 2002). Three cores were obtained by rotary drilling in this estuary: Core 1 (+3.5/-4.4m depth), 1B (-6.6/-16.2m depth) and 2 (-15.16/ m depth) (Fig. 1). These cores can be considered as being representative of the continuous sedimentary sequence present in the southern margin of Douro estuary (DRAGO et al., 2002). The acquisition of heavy mineral data involved the following laboratory procedure: a) the samples were washed, oven dried at 70º and sieved at 1φ interval in order to obtain the entire grain size sand distribution and its respective parameters; b) light and heavy minerals from the referred grain-size classes were separated using sodium poly-tungstate (density 2.82g/cm 3 ) (CALLAHAN, 1987); c) heavy minerals from each fraction were mounted on microscope slides in Canada balsam; d) about three hundred mineral grains were counted in each slide (FATELA & TABORDA, 2002); e) and the non-opaque species where identified (Table 1). The presence of immature together with mature grains was analyzed by grain angularity/roundness evaluation of several heavy mineral species. Three roundness classes were defined: round, subrounded/subangular and angular (adapted from roundness classes defined by POWERS, 1953) (Figure 2). RESULTS Sedimentary characterization The sedimentological, geochemical and paleoecological study of Core 1, 1B and 2 reveal the existence of four sedimentary units referred by DRAGO et al. (2002, 2003, in press) as SED 1, SED 2, SED 3 and SED 4 (Figure 3). These units were characterized following FLEMMING (2000) sediment classification. Figure 1. Core location MATERIAL AND METHODS The study of the heavy minerals was carried out in a total of 90 samples in three grain-size classes, medium (1-2φ), fine (2-3φ) and very fine (3-4φ) sand, in the sand rich levels of each of the three cores that represent the estuary sedimentary infilling. According to DRAGO et al. (2002, 2003, in press) the sedimentary sequence begins with a slightly muddy sand sequence (SED 1 unit) and it is comprised between and BP. SED 1 is followed by succession of slightly sandy mud, sandy mud and muddy sand sediments (Core 2) and an alternated succession of sandy mud and muddy sand interbedded layers (Core 1B) - SED 2 unit. This unit is comprised between and 5780 BP being present in both Cores 2 and 1B. The third unit (SED 3) was deposited from 5750 BP and prior to 1580 BP is a gravel layer with a small contribution of mud appearing in both Cores 1 and 2. The last unit (SED 4), deposited from 1580BP till present corresponds to a sandy unit capping the gravel, and also appears on the top of Core 1 and 2. In Core 1 SED 4 consists of sands with few interbedded mud and in Core 2 is essentially a sandy unit. 62

3 Sedimentary Processes in Douro Estuary (Portugal) A Heavy Mineral Study Figure 2. Roundness classes (adapted from the roundness classes defined by Powers (1953)) and euhedral forms of SED 1. Mineral legend shown in Table 1 63

4 C. Fradique & J. Cascalho Table 1. Mean content (M) and standard deviation (STD) of the most abundant heavy minerals. According to DRAGO et al. (in press) SED 1 and the lower half of SED 2 have a terrestrial signature supported by the total absence of biogenic sand components, the significant abundance of plants remains and the total absence of foraminifera and calcareous nannoplankton. This section seems to have been deposited in a changing climate that evolved from rainy temperate conditions, with some accentuated cooler periods to colder climatic conditions and finally to temperate conditions with an increase of precipitation. SED 1 sequence probably corresponds to a typical fluvial environment in the dependence of the Douro river. The lower section of SED 2 may represent a low saline estuarine environment with intensification of marine influence towards the top. The upper part of SED 2 is characterized by increasing marine influence, as suggested by the general increase of estuary mouth/marine and continental shelf foraminifera. In some levels the foraminifera abundance decreases simultaneously with the increase of terrigenous indicators. These intervals are attributed to important fluvial sediment input. The first marine signal appears between m and m and becomes persistent after -25m being characterized by the abundance and association of mollusks and foraminifera. The marine signature is also supported by the increase in C/D ratio (carbonates/detritals) and decrease in K/I ratio (kaolinite/illite), particularly after m, simultaneous with Ca and Sr increase. The climate evolutes towards more temperate conditions, with less precipitation, along Core 2, but evolutes towards a warmer and more humid climate, with seasonal contrast, along Core 1B. SED 3 was probably deposited under a climate of torrential rains favoring the coarse clastic supply. The morphometric study of the gravel components suggests intense fluvial transport and a littoral reworking, this unit being interpreted as a shingle barrier (DRAGO et al., 2002). In SED 4, micropaleontological data, obtained in the finer levels of this unit, suggests a brackish intertidal environment with low salinity that rapidly acquired fluvial and/or sub aerial characteristics towards the top (DRAGO et al., 2002). 64

5 Sedimentary Processes in Douro Estuary (Portugal) A Heavy Mineral Study Heavy Mineral Data The mean percentage of each heavy mineral species along the different sedimentary unit is represented in Table 1. In the lower levels of SED 1, the heavy mineral suite is dominated by biotite (20%), andalusite (27%) and apatite (6%), with euhedral forms (Figure 2), reflecting the presence of the bedrock (granites and gneisses). Opaque minerals correspond to 44% and all other species range between 0.1 and 3% (Table 1). In the medium and upper levels, the heavy mineral suites are essentially composed by biotite (40%), amphibole (12%) and andalusite (6%) with platy and lamellar forms. Tourmaline and garnet appear both with contents of 3%. (Table1). The other minerals occur in negligible amounts ranging between 0.2 and 1.7% except for opaque minerals (32%). The SED 2 heavy mineral assemblage is dominated by biotite (45%), amphibole (10%) and andalusite (4%). Tourmaline and garnet contents are 3% and the other minerals range between 0.2 and 1.6% (Table 1). The levels with high content of biotite (>50%) match the muddy sand/sandy mud sequences in these cores. In SED 3, heavy minerals appear only in Core 2 (less coarse). The heavy suite is mainly composed by lamellar and platy minerals such as biotite (49%) and amphibole (7%) and by rounded, spherical and ellipsoidal grains such as garnet (3%), andalusite (4%) and tourmaline (2%) (Table 1 and Figure 2). The SED 4 heavy mineral suite is composed by biotite (32%), amphibole (18%) and andalusite (5%) (Table 1). The other non-opaque minerals range between 0 and 1.7%. The distinct heavy mineral suite that characterizes the upper levels of Core 1 and 2 can be used to define a sub-unit classified as SED 4a. This sub-unit is included in the SED 4 defined by DRAGO et al. (2003) and is represented in the top of Core 2 and in a small part of Core 1 (Figure 2). The heavy mineral suite is composed by rounded grains of garnet (15%), andalusite (13%), tourmaline (8%) and staurolite (5%), and by relative small amounts of biotite (13%) and amphibole (5%) with platy and lamellar forms (Table 1 and Figure 2). These particles are included in medium to coarse sand. Heavy mineral and sediment grain-size analysis The mean grain size was computed using the moment method (FRIEDMAN & SANDERS, 1978) both for the sediment and heavy mineral fraction. In the heavy fraction the mean grain size was obtained for total heavy mineral (weight percentage) and for the species biotite, amphibole, garnet and andalusite in the 1-2φ, 2-3φ and 3-4φ grain-size classes. The values obtained were graphically represented according to the cores depth (Figure 3). In SED 1 unit the majority of the samples show small mean grain-size differences between the sediment and heavy minerals (< 0.5φ). Nevertheless the two lowermost samples show much higher differences (about 1φ), probably because they represent the bed rock weathering levels. In SED 2, from -33.5m to m (in Core 2) mean grain-sizes are generally above 2φ and the differences between sediment and heavy minerals are small (<0.5φ). In Core 1B this unity shows similar values with few exceptions that are related with the presence of some coarser sediment layers. In SED 3 (only represented in Core 2) the sediment is much coarser than in the previous units and the grain-size differences between sediment and heavy minerals are above 1. In SED 4 the heavy mineral grain size ranges between 1.5 and 3φ, and for the sediment between -0.5 and 3φ. Differences are in general about or greater than 1φ, with the exception of two samples. In SED 4a (from m to m) mean grain sizes are below 2 and the differences between the heavy minerals and the sediment mean grain sizes are about 1φ. In the SED 4a levels the heavy minerals tend to be rounder and equant in opposite to SED 1 and SED 2 where they are much more angular and subrounded/subangular. In fact, the lower part of SED 2 in Core 2, and the upper part in Core 1B reveal an increase in subrounded/subangular forms. The top of SED 4 as a light increase in rounded minerals but in the rest of the unit the angular forms are prevailing. 65

6 C. Fradique & J. Cascalho Figure 3. Variation of mean grain size and roundness classes along the three cores DISCUSSION The grain-sizes differences observed are a consequence of the grain sorting between heavy and light particles. Because heavy minerals are denser than the most abundant particles in sediment (quartz, feldspar and other particles), they behave differently during entrainment, transport and deposition. Usually the presence of smaller heavy grains together with larger and lighter particles is a result of sorting processes. According to SLINGERLAND (1977) and KOMAR & WANG (1984) the difference between heavy 66

7 Sedimentary Processes in Douro Estuary (Portugal) A Heavy Mineral Study and light mineral sizes can be significant especially if the grains are transported as bedload. In this situation these size discrepancy is normally greater than 1φ in fine sand grain-size interval being explained as effect of grain's susceptibility to be entrained by bottom stress (CASCALHO, 2000). When the particles have affinity to be transported as suspended load their hydraulic behavior can be described by settling velocity. In this case the grain-size differences between sediment and heavier particles have great proximity, normally less than 1φ in the domain of fine and very fine sands ( CASCALHO, op. cit). The results obtained in the three cores analyzed can be used as a tool to explain the most likely sorting process that had occurred during the several phases of the sedimentary estuary infilling. According to DRAGO et al. (in press) SED 2 unity represented both in Core 1B and 2 characterize a sedimentation regime of sandy mud and muddy sand layers in an estuarine environment. In this environment the deposition of the finer sand particles was controlled by their settling velocities and in consequence the grain-size differences between heavy grains and sediment is very small, less then 1φ. (Figure 3) In general the heavy minerals have angular to sub-angular forms and the occurrence of lamellar particles is frequent (Figure 2). The upper unity (SED 4) consists of sand with scarce mud layers deposited in an intertidal environment (DRAGO et al., 2002). This environment is entirely compatible with the dominance of bed load grain sorting and transport and in consequence the grain-size differences between heavy grains and sediment is normally larger than 1φ. Heavy minerals are frequently rounded to sub-rounded (Figure 2). Of particular interest is the sub-unit SED 4a whose levels are distinguished by the plentiful presence of rounded grains that resemble the heavies present in most proximal southern Douro estuary beaches (FRADIQUE & CASCALHO, 2003). The mineralogical composition of the different units seems to be mainly controlled by the sorting mechanisms being the effects of the source negligible with exception for the lower levels of SED 1 in Core 2 that reflect the in situ bed rock weathering (FRADIQUE et al., in press). The most mineralogical contrast is undoubtedly between sub-unit SED 4a and the others units (Table 1). In fact SED 4a is distinguish from the others units by an appreciable content of rounded to sub-rounded grains of garnet, andalusite, tourmaline, staurolite and epidote and a relative lower content in biotite and amphibole. This association has mineralogical and textural features extremely similar to the heavy mineral suit found in actual beaches southern of Douro estuary (FRADIQUE & CASCALHO, 2003). Thus SED 4a may likely witness a typical sandy barrier/beach face environment. CONCLUSIONS This work shows that heavy mineral analysis can be used as a tool to understand the importance of the grain sorting and transport processes in the sedimentary record of the Douro estuary. As a consequence of this sorting it is possible to observe that in units SED 1 and SED 2 the grain-size differences between sediment and heavy minerals are small, compatible with deposition in estuary environment deduced by the multidisciplinary study of the sedimentary particles. The sand heavy mineral composition, grain size, and roundness stage compared with the sediment grain size, points to a suspended load sorting and transport process. In opposite units SED 4, SED 4a and SED 3 show greater grain-size differences between sediment and heavy minerals, being these minerals rounded and equant. These observations are compatible with bed load sorting and transport processes that may likely occurred under a strong marine influence or littoral reworking of the sediment. ACKNOWLEDGEMENTS This work is a DISEPLA contribution and was undertaken as a part of the project ENVI-CHANGES- PLE/12/00 funded by Fundação para a Ciência e a Tecnologia (Portugal). REFERENCES CALLAHAN, J. (1987). A non-toxic heavy liquid and inexpensive filter for separation of mineral grains. Journal of Sedimentary Petrology, 57,

8 C. Fradique & J. Cascalho CASCALHO, J. (2000) - Mineralogia dos sedimentos arenosos da margem continental setentrional portuguesa. PhD thesis, Lisbon University (unpublished), 400pp. DRAGO, T.; NAUGHTON, F.; MORENO, J.; ROCHA, F.; CACHÃO, M.; SANCHEZ GÕNI, M. F.; OLIVEIRA, A.; CASCALHO, J.; FATELA, F.; FREITAS, C. & ANDRADE, C. (2002). Geological Record of Environmental Changes in the Douro Estuary (NW Portugal) since the Late Glacial. Proceedings of the 6th Littoral 2002 International Symposium, (Porto, Portugal), volume 3, pp DRAGO, T.; FREITAS, C.; CACHÃO, M.; MORENO, J.; CASCALHO, J.; OLIVEIRA, A.; NAUGHTON, F.; FRADIQUE, C.; SILVEIRA, T.; FATELA, F. (2003). Douro estuary sedimentary record and sea level rise evolution in the last years. Proceedings of the 4th Symposium on the Iberian Atlantic Margin, (Vigo, Spain), Thalassas 19(2a), DRAGO, T.; FREITAS, C.; ROCHA, F.; CACHÃO, M.; MORENO, J.; NAUGHTON, F.; FRADIQUE, C.; ARAÚJO, F.; SILVEIRA, T.; OLIVEIRA, A.; CASCALHO, J.; FATELA, F. (in press). Paleoenvironmental evolution of estuarine systems during the last years - the case of Douro estuary (NW Portugal). Proceedings of the International Coastal Symposium, (Santa Catarina, Brazil), Journal of Coastal Research, Special Issue nº 39 FATELA, F. and TABORDA, R. (2002). Confidence limits of species proportions in microfossil assemblages. Marine Micropaleontology, 45, FLEMMING, B.W. (2000). A revised textural classification of gravelfree muddy sediments on the basis of ternary diagrams. Cont. Shelf Research, 20, FRADIQUE, C. and CASCALHO, J. (2003). Proveniência e transporte selectivo dos minerais pesados - o exemplo do rio Douro e da plataforma continental adjacente. VI Congresso Nacional de Geologia, Ciências da Terra Special Issue V, 127 FRADIQUE, C., Cascalho, J., Drago, T., Rocha, F., Silveira, T. (in press). The meaning of heavy minerals in the recent sedimentary record of the Douro estuary (Portugal). Proceedings of the International Coastal Symposium, (Santa Catarina, Brazil), Journal of Coastal Research, Special Issue nº 39 FRIEDMAN, G. M. & SANDERS, J. E. (1978). Principles of Sedimentology. Wiley, New York. 792p. KOMAR, P.D., WANG, C. (1984). Processes of selective grain transport and the formation of placers on beaches. The Journal of Geology, vol. 92, nº 6, KOMAR, P. D. (1989). Physical processes of waves and currents and the formation of marine placers. Reviews in Aquatic Sciences, 1(3), MORTON, A. C. and HALLSWORTH, C. R. (1999). Processes controlling of heavy mineral assemblages in sandstones. Sedimentary Geology, 124, POWERS, M.C. (1953). A new roundness scale for sedimentary particles. Journal of Sedimentary Petrology, 23, SLINGERLAND, R. L. (1977). The effects of entrainment on the hydraulic equivalence relationships of light and heavy minerals in sands. Journal of Sedimentary Petrology, vol. 47, nº 2, (Received: January, 30, Accepted: September, 6, 2004) 68