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 GRAVEL Martins & Barboza 61 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