Sand-Gravel Marine Deposits and Grain-Size Properties
GRAVEL ISSN 1678-5975 Novembro - 2005 Nº 3 59-70 Porto Alegre
Sand-Gravel Marine Deposits and Grain-Size Properties
L. R. Martins1,2 & E. G. Barboza2
1 COMAR- South West Atlantic Coastal and Marine Geology Group; 2 Centro de Estudos de Geologia Costeira e Oceânica – CECO/IG/UFRGS.
RESUMO
A plataforma continental Atlântica do Rio Grande do Sul e Uruguai foi utilizada como laboratório natural para testar as relações entre propriedades de tamanho de grão e ambiente sedimentar. A evolução Pleistoceno/Holoceno da região foi intensamente estudada através de um mapeamento detalhado, e de estudos sedimentológicos e estratigráficos, oferecendo, dessa forma, uma excelente oportunidade para esse tipo de trabalho. Acumulações de areia e cascalho, vinculadas a níveis de estabilização identificados da transgressão Holocênica, localizados nas isóbatas de 110-120 e 20-30 metros, fornecem elementos confiáveis relacionados com a fonte, transporte e nível de energia de deposição e podem ser utilizados como linhas de evidencias na interpretação ambiental.
ABSTRACT
The Atlantic Rio Grande do Sul (Brazil) and Uruguay inner continental shelf was used as a natural laboratory to test the relationship between grain-size properties and sedimentary environment. The Pleistocene/Holocene evolution of the region was intensively studied through detailed mapping, sedimentological and stratigraphic research thus offering an excellent opportunity of developing this type of work. Sand and gravel deposits linked with identified stillstands of the Holocene transgression located at 110-120 and 20-30 meters isobath provided elements related to the source, transport and depositional energy level and can be used as a tool for environmental interpretation.
Keywords: marine deposits, grain-size, sand-gravel, Holocene. 60 Sand-Gravel Marine Deposits and Grain-Size Properties
INTRODUCTION samples as it is typical in many past studies; During the last four decades, e) the laboratory methods of analysis are all sedimentologists from of all over world have been the same, with uniform development at interested in obtaining environmental information the CECO/IG/UFRGS laboratories. from grain-size analysis of sand and gravel mainly related with coastal areas. For a partial list of DISCUSSION references (see MARTINS et al., 1997 and MARTINS, 2003). A large part of published papers is Grain-size distribution reflects devoted to understand how riverine, eolian, beach, depositional processes and combined with other lagoon and lake processes transform the grain size lines of evidence (roundness, sphericity, grain distribution of sand. surface texture, detrital light and heavy minerals, In the present exercise, the method biogenic components, syngenic minerals etc) could applied to characterize coastal sands and gravels be used for sedimentary environment (beach, dune) are employed on sandy and gravelly identification. deposits of the inner and outer continental shelf of Erosion, progradation and reworking of southern Rio Grande do Sul (Brazil) and Uruguay sediments are important shore and nearshore in the influence area of Rio de La Plata, in an processes, and it is necessary to understand the attempt to learn how they fit in similar shallow changes produced near the modern shoreline in marine deposits. order to interpret the origin and depositional CRONAN (1972) working on polymodal aspects of the preserved sediments. sediments from the Irish Sea consisting of varying The size distribution of sand or sandstone mixtures of gravel, sand, silt and clay, discussed is one of its fundamental properties, because it the usefulness of the grain-size parameters in largely determines porosity and permeability, characterizing sedimentary processes. provides insight to transport processes and has Zones of positive and negative skewness been used by geologists to determine the alternate in the basin and this behavior can be environment of deposition of ancient sandstones. It largely explained in terms of variations in the is also closely related to the geotechnical properties proportions of the various grain-size populations in of sand. the sediments. The strongly positively skewed Our interest is to test in marine modern sediments consist of sandy gravels in wich the sands how well the size distribution properties can gravel mode is predominant. As the proportion of discriminate between old riverine, eolian, beach sand increases the sediment becomes less sands and gravels occurring along the continental positively skewed, passes through a zone of zero, shelf, as it was studied on the actual adjacent where the gravel and sand modes are subequal and coastal plain (MARTINS, 1962, 1965, 1967, 2003 become negatively skewed as the sand mode and MARTINS et al., 1997). becomes predominant. The data set of CECO/IG/UFRGS is Variations in kurtosis can largely be exceptionally adequate for such a study and offers related to the degree of polymodality of the promising discrimination for at least five reasons: sediments. MARTINS (1962) discussed the several a) all the analyzed modern terrigenous sands aspects of this statistical measures and established were derived from a nearby source of that the grade of peakness of a grain-size Pre-Cambrian and Phanerozoic rocks distribution reveals the relation of the sorting of the rather than from multiple, distant sources; central part of the curve in relation with the coarse b) the carbonate bioclastic sand and gravel and the fine tail. components are autochtonous; When both gravel and sand populations c) the inner shelf and the coastal area have are present in the sediment in more or less been mapped in detail, so that the subequal proportions, the kurtosis value is low environments of deposition of the (platicurtic). With the increase of the sand sampling area are known with precision population, kurtosis value rises and then falls to and carefully delimited; near normal as the sand approach unimodality. d) these environments were sampled using According to CRONAN (1972) the inter- closely spaced sites, through the method relations between grain-size parameters found by of suite samples rather than only few FOLK & WARD (1957) in fluvial and by
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MARTINS (1965) in beach/dune environments calculated in detail for each sample, average values also occur in the marine milieu. for each environment were computed and Q or R In strongly polymodal sediments the mode factor analysis was applied to the results. prime influence on skewness is the proportion in Mc BRIDE (1971) established that grain- which the various grain-size populations in the size analysis was developed for one or more of the sediments are mixed, and this nature and degree of following reasons: a) to describe samples in terms the polymodality should be taken into account. In of statistical measures, b) to correlate samples from other words, the alternating negative and positive similar depositional environments or stratigraphic skewness reflect the proportions in which the units, c) to determine the agent (wind, river, wave, different modal populations in the sediment are tide) of transportation and deposition, d) to study mixed. the processes (suspension, traction) of final The importance of the grain-size deposition and e) to characterize the environment properties in the sedimentary processes was of deposition (channel, beach, dune, flood plain, responsible for the establishment, in 1964, marine). (TANNER, 1969) of the Grain Size Study Special attention was given to the fine Committee by the Society of Economic and coarse tails of the size distribution, which is Paleontologists and Mineralogists-SEPM widely believed to be its environmentally sensitive (currently Society for Sedimentary Geology), that part. has been responsible, since then, for the Several questions were raised and development of a large amount of discussions and examples of these questions include: How well contributions carried out so far (SIVITSKI, 1991; does the less than 62 micron fraction discriminate BASILIE et al., 2002, 2003; TIPPER, 2003 and between all environments? Which environments WELLS, 2003). are most distinct and which overlap the most? Several questions arised about the What are the best bivariate combinations of validity of the available methods to characterize or parameters to distinguish the environments? How discriminate sedimentary processes using grain- well if all do the results from the studied area size and other textural properties. compare with those of other parts of the world? SHEA (1974) discussed the allegated This type of work is always based on a gaps (deficiencies) on clastic particles distributions large volume of samples to get objective answers and concluded after the analysis of 11.212 samples about the environment. from different environments (glacial, fluvial, In the region, all the previous studies eolian, beach, lacustrine, estuarine and marine) regarding grain-size parameters and environment that: a) only a small number of data sets from a were developed on modern beach, dune and river relative few environments and locations have been sands occurring along the coastal zone. considered, b) some data sets have been The present exercise was developed on misinterpreted, c) a large body of contrary continental shelf sediments related with an evidence has been ignored and d) statistically extensive old Pleistocene coastal plain (and its rigorous techniques were not used. associated environments) drowned by the MARTINS et al. (1997) and MARTINS Holocene transgression and somehow submitted (2003) aggregate other field/laboratory reasons for until now to modern hydraulic conditions. the failure of a successful interpretation: 1) inadequate sampling, not representative of the RESULTS sedimentary body, 2) inappropriate splitting of the representative original sample, 3) mechanical These successive movements were analysis routine without the recommended points responsible for the migration of a high energy of reference, 4) technicians not well trained to shallow zone over the continental shelf that allows develop a sometimes “exhaustive” type of work 5) the development of stillstands of the sea level and erroneous draft of the cumulative curve or other the concentration of bioclastic carbonate (shells graphic device and 6) reduced number of samples and shell debris) forming linear shoals parallel to to give an overall picture of the studied the coastline. This situation is also favorable to the sedimentary body. development of calciferous sandstone – beachrock – (quartzose sand cemented by calcium carbonate) METHODS and “coquina” – calcirudite – (shell fragments cemented by calcium carbonate) and calcarenite – Graphic and moment measures (mean, (carbonate sand cemented by calcium carbonate). standard deviation, skewness, kurtosis) were
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The authigenic activity in the area is Five areas and their sedimentary facies of represented by the presence of glauconite the actual marine environment were chosen to (MARTINS & MARTINS, 1985) and the apply the methodology used on the transitional occurrence of phosphatic concretions (KLEIN et coastal sediments (Fig. 1): al., 1992 and HOSANG & ABREU, 2002). a) Outer continental shelf To test the validity of the grain-size b) Inner continental shelf textural parameters, only psephitic and psamitic c) Carpinteiro shoal sediments were used. d) Albardão shoal The five sites chosen were submitted e) La Plata shoal from moderate to high energy level of deposition The main properties of the sedimentary and showed depositional clean graded bedding facies occurring in the five places are summarized which is diagnostic of the presence of storm in Table 1. surges, reflected through storm layers.
Figure 1. Studied areas.
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Table 1. Main Characteristics of the five facies chosen to test grain-size parameters and environment. FACIES CHARACTERISTICS Quartzose sand (unimodal) with shell debris (bimodal), moderate to well sorted, negatively skewed, meso to leptocurtic. Described as sand facies (MARTINS, 1972) or Atlantic facies (LABORDE, 1999), cover old INNER CONTINENTAL Pleistocene coastal features (beaches, dunes, barriers, beachridges and SHELF erosional steps) as a transgressive Holocene sand blanket related to migratory
coastal zones, of high energy level of sedimentation. (Rio Grande do Sul – Uruguay) Relict/palimpsest sequence. <50 m
References: MARTINS et al. (1967, 1972, 1978, 1997, 1999, 2003); LABORDE (1999); MARTINS &URIEN (1979). Quartzose sand and carbonate sand and gravel of shallow waters, moderate to OUTER CONTINENTAL well sorted, negative skewed. Bioclastic and terrigenous components showing SHELF graded bedding, indicating presence of storm action over shallow waters at the sea level 110/120 m. (Rio Grande-Torres) Relict sequence. Rio Grande do Sul >100 m References: MARTINS et al. (1967, 1977, 1989); MARTINS & MARTINS (1985); DILLENBURG (1990). Quartzose sand and bioclastic sand and gravel. Bioclastic material formed by entire and fragmented shells and slabs of beachrock. Occurs as elongated LA PLATA SHOAL shoals parallel to de bathymetric contours. Relict feature of an old barrier
island, that closes partially the estuary when the sea level was at 20/30 meters. (Argentina)
Uruguay/Argentina References: URIEN & MOUZO (1968); URIEN & OTTMANN (1971); <50 m. MARTINS & URIEN (1979); URIEN et al. (1980a e b, 1995); MARTINS et al. (2003). Quartzose sand and bioclastic sand and gravel. Poor to well sorted, negatively ALBARDÃO SHOAL skewed. Occurs through elongated shoals parallel to subparallel to the Pleistocene (relict) and its sedimentary material reworked through Holocene (Rio Grande do Sul) and modern dynamics. Brazil <50 m References: MARTINS et al. (1972); CORRÊA & PONZI (1978); CALLIARI et al. (1999). Calcareous shell fragments, sand and gravel of high calcium carbonate content CARPINTEIRO SHOAL associated with terrigenous coarse to fine quartzose relict sand. Quartzose fine sand with bioclastic ash is well sorted. Medium sand with higher content of (Rio Grande do Sul) bioclastic gravel is moderately sorted. Beachrock pavement is an important Brazil bottom component. <50 m References: CALLIARI et al. (1994); BUCHMANN & TOMAZELLI (2003). a) Outer continental shelf direction to the continent and subsequent mud accumulation. Two coarse textures, quartzose sand and The sometimes intricate pattern of bioclastic sand and gravel occurs with different occurrence is due to the mixture and reworking of proportions of mud. The cores obtained in the area Pleistocene sediments, through the Holocene (MARTINS et al., 1985) show repeated small transgression. graded bedding of the coarse components resembling storm layers of high energy shallow b) Inner continental shelf coastal areas. The absence of fines as a matrix is one of the main difference with the graded layers In this sandy facies it is possible to found in the continental slope produced by identify two main textures: clean quartzose sand gravitational flows (MARTINS et al., 1989). and quartzose sand with bioclastic carbonate. The This very interesting sequence is usually modal texture class is sand (>50% of the total covered by terrigenous mud that was deposited sample) but showing sometimes a fine tail (mud), a during the gradual displacement of the coastline in coarse tail (bioclastic) or both (mud and bioclastic).
GRAVEL 64 Sand-Gravel Marine Deposits and Grain-Size Properties
c) La Plata shoal terrigenous (quartzose sand), autochtonous activities (bioclastic sand and gravel) and syngenic One of the most important concentration evolutions (beachrock). In this exercise the textures of bioclastic rich sand and bioclastic sand and 2, 3 and 4 were used. gravel, related with and ancient coastal barrier, linked with sea level fluctuations. e) Albardão shoal
d) Carpinteiro shoal Two components occur in this facies: carbonate sand gravel (modal texture class 0 to 2.6) Through an OSNLR-COMEMIR and carbonate and terrigenous sand (modal textural mission, CALLIARI et al. (1994) identified five class 0-1). textures: 1) muddy fine sand, 2) fine sand with The grain size parameters of the five bioclastics, 3) shelly gravel, 4) shelly gravel and marine facies are shown on Table 2. Histograms relict sand and 5) beachrock outcrops. This showing the number of texture classes, modal sedimentary cover represents another area of the classes and its percentages are shown on Figure 2. presence of material from different sources:
Table 2. Grain-size parameters of the studied facies. GRAIN-SIZE PARAMETERS AREA FACIES Mz Φ SK1 K´g QUARTZOSE SAND WITH OUTER 2-3 0.35-1.00 Negative Leptocurtic OR WITHOUT SHELL ASH CONTINENTAL BIOCLASTIC SAND AND Meso to SHELF -1.50-0.50 1.32-0.88 Negative GRAVEL Leptocurtic INNER CLEAN QUARTZOSE SAND 2-3 (2.61) 0.30-0.52 Negative Leptocurtic CONTINENTAL QUARTZOSE SAND WITH 2-3 (2.36) 0.98-1.73 Negative Leptocurtic SHELF BIOCLASTIC MATERIAL QUARTZOSE SAND WITH >2.0 0.35-1.00 Negative Leptocurtic BIOCLASTIC MATERIAL LA PLATA BIOCLASTIC GRAVEL 1.0-2.0 0.5-1.00 Negative Mesocurtic SHOAL BIOCLASTIC GRAVEL AND -3.0-1.0 0.5-1.50 Negative Platicurtic RELICT COARSE SAND QUARTZOSE SAND WITH 3-4 0.35-1.00 Negative Leptocurtic BIOCLASTIC MATERIAL Meso to CARPINTEIRO BIOCLASTIC GRAVEL -2-2 1.0-2.0 Negative Leptocurtic SHOAL BIOCLASTIC GRAVEL Meso to WITH RELICT COARSE 0-2 1.0-3.0 Negative Leptocurtic SAND BIOCLASTIC SANDY -4-2 1.0-2.0 Negative Mesocurtic ALBARDÃO GRAVEL SHOAL BIOCLASTIC COARSE SAND -3-3 1.0-2.0 Negative Mesocurtic
The chosen area, Rio Grande do Sul, mantled by probably Quaternary and Holocene (Brazil) and Rio de La Plata (Urugay-Argentina) prograding sedimentary sequence during the last continental shelf is a gently inclined submerged lowermost sea level stillstand (URIEN et al., platform from 120 to 170 km wide, covered by 1995). sediments composed of relict, palimpsest sands, On the modern continental shelf, relicts modern and relict muds and bioclastic calcareous of beach ridges, barrier coast and river distributary debris. The shelf break ranges in depth from 60 to channels and deltas are found (URIEN et al., 100 m. In Rio de La Plata the shelf-slope transition 1980a e b; MARTINS et al., 1996). Shelf area presents a step or bench, as a relict of a paleo- sediments are predominantly sandy, product of a continental shelf border of the Late Tertiary-Early succession of Holocene west shifting transgressive Quaternary age, related to an offlapping paleo- shorelines that form a blanked body. Silty clay deltas, built during low sea level stands and then
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Figure 2. Representative histograms of the sedimentary facies.
GRAVEL 66 Sand-Gravel Marine Deposits and Grain-Size Properties sediments are found, but concentrated to lagoons, continuous, bell-shaped curve that is symmetrical, bays and estuaries or inner shelf channels, close to and its arithmetic mean, mode and median are Rio de La Plata. Shelf lutites are found only on the numerically equivalent, and is known as a Rio Grande do Sul shelf break, as relict of Rio de Gaussian distribution. Geometrically independent La Plata paleochannels into the shelf and of the sorting grade, it indicates clearly the converging to the Rio Grande submarine fan. River tendency of the grain-size distribution to have a drainage from the nearby Pre-Cambrian plateau, coarse (negative) or a fine tail (positive). also with widespread silty sediments onto the outer Using phi units, a positive skewness continental shelf. shows a mode (Mo) smaller than median (Md) and Several fluctuations of the sea level are mean (M). The relation is inverse if millimeters presumed to have taken place in the area before units are used. In the distribution of curve A the and after the Holocene transgressions, the last skewness is positive (fine tail more expressive). In dated around 18,000 BP, which modified the curve B, when the relation is M>Md>Mo (in sediments facies distribution and particularly the millimeters values) or M
GRAVEL Martins & Barboza 67
Figure 3. Graphical significance of the statistical measures (according to Martins, 1966).
2) The carbonate bioclastic components 3) These petrographic attributes were showed a mean between -4 to 3, they had unimodal developed through ancient shorelines or bimodal distributions, with a large range of (Pleistocene/Holocene) like the 110-120 and 30-20 sorting values (from moderately to very poorly m. The so called relict sediments are representative sorted) a predominant negative indices for the of shallow water energy parameters. The skewness and lepto (unimodal distributions) to reworking of these sediments through the platicurtic (bimodal distribution). drowning of these environments did not change Even the sediments are coarser than the these properties found in the palimpsest sediments. sandy facies, these sediments are also sensibilized On the other hand, the shallow modern by the high energy present in a shallow marine area hydrodynamic usually reinforces the acquired (nearshore). A comparison with the modern attributes, as it occurs with the Albardão shoal bioclastic sand beaches like Albardão (south sediments. Brazil) confirms this assertion. Outside the studied area, in the southern As much as the sandy sequences of outer Argentina continental shelf, URIEN et al. (1993) & inner continental shelf, the bioclastic sequence, identified relict glaciomarine sandy gravel linked storm generated graded bedding were identified with glacial sediments (moraines) that reached the produced mainly during the Wisconsin sea-level old Pleistocene coastal plain. Prior to these confirming the presence of an high energy level findings URIEN & OTTMANN (1971) described environment. In fact, graded sequences in the presence of relict sediments in the continental quartzose sand with bioclastic material and shelf adjacent to Rio de la Plata, as MARTINS et bioclastic conglomeratic sand, both related to al. (1967) did in relation to Rio Grande do Sul storm layers, were identified along the inner and (Brazil). outer Rio Grande do Sul continental shelf by 4) These use of the grain-size statistical MARTINS et al. (1977, 1985, 1989) and parameters to identify and characterize coarse MARTINS & MARTINS (1985). (sand and gravel) marine sediments is a useful tool to be applied to other similar sequences, provided
GRAVEL 68 Sand-Gravel Marine Deposits and Grain-Size Properties
they are based on a trustful data, as suggested by Research, Special Issue 35: 318-322, Itajaí, MARTINS (1967), SYVISTKY et al. (1991) and Brasil. MARTINS et al. (1997 and 2003). CALLIARI, L. J.; ESTEVES, L. S.; OLIVEIRA, C. P. L.; TOZZI, H. A. M.; SILVA, R. P. & 5) Answering the questions that arose CARDOSO, J. N. 1994. Padrões Sonográficos during the discussion proposed in the development e Sedimentológicos de um afloramento de of this paper, we can summarize: “Beachrock” na Plataforma Interna do Rio Grande do Sul (COMEMIR/OSNLR). a) the grain-size smaller than 62 micron is UFRGS, Notas Técnicas 7: 27-32, Porto quite important when you have a predominant Alegre, Brasil. coarse distribution and a fine tail reveals its CALLIARI, L. J.; CORRÊA, I. C. S. & ASP, N. E. sensivity in relation with energy level of the 1999. Inner Shelf and Beach Seashell environment; Resources in Southern Brazil. In: L.R. Martins b) in the studied area, the “old beaches” & C.I. Santana (eds.) Non Living Resources of (Pleistocene) show usually similar properties to the Southern Brazilian Coastal Zone and those “modern beaches” (Holocene). Sometimes Continental Margin. OAS/IOC- beach and dune overlaps in “old” correspondents; UNESCO/MCT. Publication: 39-49, Porto c) the published literature confirms Alegre, Brasil. similarities of the obtained data of the present CORRÊA, I. C. S. & PONZI, V. R. 1978. study when compared with other careful studies on Depósitos de calcário Biodetrítico das Regiões grain-size analysis; de Albardão e Mostardas na Plataforma d) the drowning effect of the Holocene Interna do Rio Grande do Sul. Congresso transgression had little effect over the grain-size Brasileiro de Geologia, 30º, Anais..., vol. 2: properties, except for the stillstands stops that 851-866, Recife, Brasil. reworked these deposits; CRONAN, D. S. 1972. Skewness and Kurtosis in e) usually, the palimpsest sequences did Polymodal Sediments from the Irish Sea. not erase the relict properties (negative skewness, Journal of Sedimentary petrology vol. 42 (1): for instance) and sometimes the third statistical 102-106, Tulsa, USA. moment was strengthened by removal of the fine DILLENBURG, S. R. 1990. Atributos components of the distribution; Petrográficos e Proveniência dos Sedimentos f) the results obtained with the Superficiais da Plataforma Continental application of the statistical devices confirm that it Externa do Rio Grande do Sul. Trecho Rio is a reliable technique, especially when used with Grande – Chuí. Congresso Brasileiro de other grain attributes, and it has a wide distribution Geologia, 36º, Anais..., vol. 2, p. 250-587, when the field/laboratory procedures are well Natal, Brasil. conducted. FOLK, R. L. & WARD, W. C. 1957. Brazos River bar: a study on the significance of grain size REFERENCES parameters. Journal of Sedimentary Petrology 27 (1): 3-26, Tulsa, USA. BALSILLIE, J. H.; DONOGHUE, J. F.; BUTLER, HOSANG, D. & ABREU, J. G. N. 2002. K. M. & KOCH, J. 2002. Plotting Equation for Concreções Fosfáticas na Plataforma Externa e Gaussian Percentiles and Spreadsheet Program Talude Superior da Margem Continental Sul for generating probability plots. Journal of Brasileira. Congresso Brasileiro de Geologia, Sedimentary Research, vol. 72 (6): 929-943, 41º, Anais..., João Pessoa, Brasil. Tulsa, USA. KLEIN, A. H. F; GRIEP, G. H.; CALLIARI, L. J BALSILLIE, J. H.; DONOGHUE, J. F.; BUTLER, & VILLWOCK, J. A. 1992. Ocorrência de K. M. & KOCH, J. 2003. Plotting Equation for Concreções Fosfáticas no Terraço de Rio Gaussian Percentiles and Spreadsheet Program Grande. Congresso Brasileiro de Geologia, for generating probability plots – Reply. 38º, Resumos, p. 202-204, São Paulo, Brasil. Journal of Sedimentary Geology, vol. 73(6): LABORDE, J. L. 1999. Sand Deposits of the outer 1086-87, Tulsa, USA. Rio de la Plata and adjacent Continental Shelf. BUCHMANN, F. S. C. & TOMAZELLI, L. J. In: L.R. Martins & C.I. Santana (eds.). Non 2003. Relict Nearshore Shoals of Rio Grande Living Resources of the Southern Brazilian do Sul, Southern Brazil. Origin and Effects on Coastal Zone and continental Margin Nearby Modern Beaches. Journal of Coastal OAS/IOC-UNESCO/MCT, Special Publication: p. 80-91, Porto Alegre, Brasil.
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