Thalassas, 2004, 20 (2): 61-68 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, e-mail:
[email protected] (2) Museu Nacional de História Natural de Lisboa, Universidade de Lisboa, Portugal,
[email protected]
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
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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 14000 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/ -39.66m 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/cm3) (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. 62
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 13750 and 10310 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 10310 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.
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
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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 -28.11m and -27.45m and becomes persistent 64
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 -22.74m, 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).
Sedimentary Processes in Douro Estuary (Portugal) A Heavy Mineral Study
Heavy Mineral Data
Heavy mineral and sediment grain-size analysis
The mean percentage of each heavy mineral species along the different sedimentary unit is represented in Table 1.
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 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%).
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.
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 2, from -33.5m to -20.53m (in Core 2) mean grain-sizes are generally above 2φ and the differences between sediment and heavy minerals are small (
