sand gravel marine deposit

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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 Uruguaifoi utilizada como laboratório natural para testar as relações entre propriedadesde tamanho de grão e ambiente sedimentar.

A evolução Pleistoceno/Holoceno da região foi intensamenteestudada através de um mapeamento detalhado, e de estudos sedimentológicose estratigráficos, oferecendo, dessa forma, uma excelente oportunidade paraesse tipo de trabalho.

Acumulações de areia e cascalho, vinculadas a níveis de estabilizaçãoidentificados da transgressão Holocênica, localizados nas isóbatas de 110-120e 20-30 metros, fornecem elementos confiáveis relacionados com a fonte,transporte e nível de energia de deposição e podem ser utilizados como linhasde evidencias na interpretação ambiental.

ABSTRACT

The Atlantic Rio Grande do Sul (Brazil) and Uruguay innercontinental 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 intensivelystudied through detailed mapping, sedimentological and stratigraphic researchthus offering an excellent opportunity of developing this type of work.

Sand and gravel deposits linked with identified stillstands of theHolocene 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.


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INTRODUCTION

During the last four decades,sedimentologists from of all over world have beeninterested in obtaining environmental information

from grain-size analysis of sand and gravel mainlyrelated with coastal areas. For a partial list ofreferences (see MARTINS et al., 1997 andMARTINS, 2003).

Grain-size distribution reflectsdepositional processes and combined with otherlines of evidence (roundness, sphericity, grainsurface texture, detrital light and heavy minerals,

biogenic components, syngenic minerals etc) could be used for sedimentary environmentidentification.

Erosion, progradation and reworking ofsediments are important shore and nearshore

processes, and it is necessary to understand thechanges produced near the modern shoreline inorder to interpret the origin and depositionalaspects of the preserved sediments.

The size distribution of sand or sandstoneis one of its fundamental properties, because itlargely determines porosity and permeability,

provides insight to transport processes and has been used by geologists to determine theenvironment of deposition of ancient sandstones. Itis also closely related to the geotechnical propertiesof sand.

Our interest is to test in marine modernsands how well the size distribution properties candiscriminate between old riverine, eolian, beachsands and gravels occurring along the continentalshelf, as it was studied on the actual adjacentcoastal plain (MARTINS, 1962, 1965, 1967, 2003and MARTINS et al., 1997).

The data set of CECO/IG/UFRGS isexceptionally adequate for such a study and offers promising discrimination for at least five reasons:

a)

all the analyzed modern terrigenous sandswere derived from a nearby source ofPre-Cambrian and Phanerozoic rocksrather than from multiple, distant sources;

b)

the carbonate bioclastic sand and gravelcomponents are autochtonous;

c) the inner shelf and the coastal area have been mapped in detail, so that the

environments of deposition of thesampling area are known with precisionand carefully delimited;

d)

these environments were sampled usingclosely spaced sites, through the methodof suite samples rather than only few

samples as it is typical in many paststudies;

e) the laboratory methods of analysis are allthe same, with uniform development atthe CECO/IG/UFRGS laboratories.

DISCUSSION

A large part of published papers isdevoted to understand how riverine, eolian, beach,lagoon and lake processes transform the grain sizedistribution of sand.

In the present exercise, the methodapplied to characterize coastal sands and gravels(beach, dune) are employed on sandy and gravellydeposits of the inner and outer continental shelf of

southern Rio Grande do Sul (Brazil) and Uruguayin the influence area of Rio de La Plata, in anattempt to learn how they fit in similar shallowmarine deposits.

CRONAN (1972) working on polymodalsediments from the Irish Sea consisting of varyingmixtures of gravel, sand, silt and clay, discussedthe usefulness of the grain-size parameters incharacterizing sedimentary processes.

Zones of positive and negative skewnessalternate in the basin and this behavior can belargely explained in terms of variations in the

proportions of the various grain-size populations inthe sediments. The strongly positively skewedsediments consist of sandy gravels in wich thegravel mode is predominant. As the proportion ofsand increases the sediment becomes less

positively skewed, passes through a zone of zero,where the gravel and sand modes are subequal and

become negatively skewed as the sand mode becomes predominant.

Variations in kurtosis can largely berelated to the degree of polymodality of thesediments. MARTINS (1962) discussed the severalaspects of this statistical measures and establishedthat the grade of peakness of a grain-sizedistribution reveals the relation of the sorting of thecentral part of the curve in relation with the coarseand the fine tail.

When both gravel and sand populationsare present in the sediment in more or lesssubequal proportions, the kurtosis value is low

(platicurtic). With the increase of the sand population, kurtosis value rises and then falls tonear normal as the sand approach unimodality.

According to CRONAN (1972) the inter-relations between grain-size parameters found byFOLK & WARD (1957) in fluvial and by


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MARTINS (1965) in beach/dune environmentsalso occur in the marine milieu.

In strongly polymodal sediments the prime influence on skewness is the proportion inwhich the various grain-size populations in the

sediments are mixed, and this nature and degree ofthe polymodality should be taken into account. Inother words, the alternating negative and positiveskewness reflect the proportions in which thedifferent modal populations in the sediment aremixed.

The importance of the grain-size properties in the sedimentary processes wasresponsible for the establishment, in 1964,(TANNER, 1969) of the Grain Size StudyCommittee by the Society of Economic

Paleontologists and Mineralogists-SEPM(currently Society for Sedimentary Geology), thathas been responsible, since then, for thedevelopment of a large amount of discussions andcontributions carried out so far (SIVITSKI, 1991;BASILIE et al., 2002, 2003; TIPPER, 2003 andWELLS, 2003).

Several questions arised about thevalidity of the available methods to characterize ordiscriminate sedimentary processes using grain-size and other textural properties.

SHEA (1974) discussed the allegatedgaps (deficiencies) on clastic particles distributionsand concluded after the analysis of 11.212 samplesfrom different environments (glacial, fluvial,eolian, beach, lacustrine, estuarine and marine)that: a) only a small number of data sets from arelative few environments and locations have beenconsidered, b) some data sets have beenmisinterpreted, c) a large body of contraryevidence has been ignored and d) statistically

rigorous techniques were not used.MARTINS et al. (1997) and MARTINS(2003) aggregate other field/laboratory reasons forthe failure of a successful interpretation: 1)inadequate sampling, not representative of thesedimentary body, 2) inappropriate splitting of therepresentative original sample, 3) mechanicalanalysis routine without the recommended pointsof reference, 4) technicians not well trained todevelop a sometimes “exhaustive” type of work 5)erroneous draft of the cumulative curve or other

graphic device and 6) reduced number of samplesto give an overall picture of the studiedsedimentary body.

METHODS

Graphic and moment measures (mean,standard deviation, skewness, kurtosis) were

calculated in detail for each sample, average valuesfor each environment were computed and Q or Rmode factor analysis was applied to the results.

Mc BRIDE (1971) established that grain-size analysis was developed for one or more of the

following reasons: a) to describe samples in termsof statistical measures, b) to correlate samples fromsimilar depositional environments or stratigraphicunits, c) to determine the agent (wind, river, wave,tide) of transportation and deposition, d) to studythe processes (suspension, traction) of finaldeposition and e) to characterize the environmentof deposition (channel, beach, dune, flood plain,marine).

Special attention was given to the fineand coarse tails of the size distribution, which is

widely believed to be its environmentally sensitive part.

Several questions were raised andexamples of these questions include: How welldoes the less than 62 micron fraction discriminate

between all environments? Which environmentsare most distinct and which overlap the most?What are the best bivariate combinations of

parameters to distinguish the environments? Howwell if all do the results from the studied areacompare with those of other parts of the world?

This type of work is always based on alarge volume of samples to get objective answersabout the environment.

In the region, all the previous studiesregarding grain-size parameters and environmentwere developed on modern beach, dune and riversands occurring along the coastal zone.

The present exercise was developed oncontinental shelf sediments related with anextensive old Pleistocene coastal plain (and its

associated environments) drowned by theHolocene transgression and somehow submitteduntil now to modern hydraulic conditions.

RESULTS

These successive movements wereresponsible for the migration of a high energyshallow zone over the continental shelf that allowsthe development of stillstands of the sea level andthe concentration of bioclastic carbonate (shells

and shell debris) forming linear shoals parallel tothe coastline. This situation is also favorable to thedevelopment of calciferous sandstone – beachrock

– (quartzose sand cemented by calcium carbonate)and “coquina” – calcirudite – (shell fragmentscemented by calcium carbonate) and calcarenite –(carbonate sand cemented by calcium carbonate).


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The authigenic activity in the area isrepresented by the presence of glauconite(MARTINS & MARTINS, 1985) and theoccurrence of phosphatic concretions (KLEIN etal., 1992 and HOSANG & ABREU, 2002).

To test the validity of the grain-sizetextural parameters, only psephitic and psamiticsediments were used.

The five sites chosen were submittedfrom moderate to high energy level of depositionand showed depositional clean graded beddingwhich is diagnostic of the presence of stormsurges, reflected through storm layers.

Five areas and their sedimentary facies ofthe actual marine environment were chosen toapply the methodology used on the transitionalcoastal sediments (Fig. 1):

a)

Outer continental shelf

b)

Inner continental shelfc) Carpinteiro shoald)

Albardão shoale) La Plata shoal

The main properties of the sedimentaryfacies occurring in the five places are summarizedin Table 1.

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

INNER CONTINENTAL

SHELF

(Rio Grande do Sul – Uruguay)100 m

Quartzose sand and carbonate sand and gravel of shallow waters, moderate towell sorted, negative skewed. Bioclastic and terrigenous components showinggraded bedding, indicating presence of storm action over shallow waters at thesea level 110/120 m.Relict sequence.

References: MARTINS et al. (1967, 1977, 1989); MARTINS & MARTINS(1985); DILLENBURG (1990).

LA PLATA SHOAL

(Argentina)Uruguay/Argentina


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c) La Plata shoal

One of the most important concentrationof bioclastic rich sand and bioclastic sand andgravel, related with and ancient coastal barrier,

linked with sea level fluctuations.

d) Carpinteiro shoal

Through an OSNLR-COMEMIRmission, CALLIARI et al. (1994) identified fivetextures: 1) muddy fine sand, 2) fine sand with

bioclastics, 3) shelly gravel, 4) shelly gravel andrelict sand and 5) beachrock outcrops. Thissedimentary cover represents another area of the

presence of material from different sources:

terrigenous (quartzose sand), autochtonousactivities (bioclastic sand and gravel) and syngenicevolutions (beachrock). In this exercise the textures2, 3 and 4 were used.

e) Albardão shoal

Two components occur in this facies:carbonate sand gravel (modal texture class 0 to 2.6)and carbonate and terrigenous sand (modal texturalclass 0-1).

The grain size parameters of the fivemarine facies are shown on Table 2. Histogramsshowing the number of texture classes, modalclasses and its percentages are shown on Figure 2.

Table 2. Grain-size parameters of the studied facies.

GRAIN-SIZE PARAMETERSAREA FACIES

Mz Φ SK1 K´g

QUARTZOSE SAND WITHOR WITHOUT SHELL ASH

2-3 0.35-1.00 Negative LeptocurticOUTERCONTINENTAL

SHELF BIOCLASTIC SAND ANDGRAVEL

-1.50-0.50 1.32-0.88 NegativeMeso to

LeptocurticCLEAN QUARTZOSE SAND 2-3 (2.61) 0.30-0.52 Negative LeptocurticINNER

CONTINENTALSHELF

QUARTZOSE SAND WITHBIOCLASTIC MATERIAL 2-3 (2.36) 0.98-1.73 Negative LeptocurticQUARTZOSE SAND WITHBIOCLASTIC MATERIAL

>2.0 0.35-1.00 Negative Leptocurtic

BIOCLASTIC GRAVEL 1.0-2.0 0.5-1.00 Negative MesocurticLA PLATA

SHOALBIOCLASTIC GRAVEL AND

RELICT COARSE SAND-3.0-1.0 0.5-1.50 Negative Platicurtic

QUARTZOSE SAND WITHBIOCLASTIC MATERIAL

3-4 0.35-1.00 Negative Leptocurtic

BIOCLASTIC GRAVEL -2-2 1.0-2.0 NegativeMeso to

LeptocurticCARPINTEIRO

SHOALBIOCLASTIC GRAVELWITH RELICT COARSE

SAND0-2 1.0-3.0 Negative Meso toLeptocurtic

BIOCLASTIC SANDYGRAVEL

-4-2 1.0-2.0 Negative MesocurticALBARDÃOSHOAL

BIOCLASTIC COARSE SAND -3-3 1.0-2.0 Negative Mesocurtic

The chosen area, Rio Grande do Sul,(Brazil) and Rio de La Plata (Urugay-Argentina)continental shelf is a gently inclined submerged

platform from 120 to 170 km wide, covered by

sediments composed of relict, palimpsest sands,modern and relict muds and bioclastic calcareousdebris. The shelf break ranges in depth from 60 to100 m. In Rio de La Plata the shelf-slope transitionarea presents a step or bench, as a relict of a paleo-continental shelf border of the Late Tertiary-EarlyQuaternary age, related to an offlapping paleo-deltas, built during low sea level stands and then

mantled by probably Quaternary and Holocene prograding sedimentary sequence during the lastlowermost sea level stillstand (URIEN et al.,1995).

On the modern continental shelf, relictsof beach ridges, barrier coast and river distributarychannels and deltas are found (URIEN et al.,1980a e b; MARTINS et al., 1996). Shelfsediments are predominantly sandy, product of asuccession of Holocene west shifting transgressiveshorelines that form a blanked body. Silty clay


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Figure 2. Representative histograms of the sedimentary facies.


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sediments are found, but concentrated to lagoons, bays and estuaries or inner shelf channels, close toRio de La Plata. Shelf lutites are found only on theRio Grande do Sul shelf break, as relict of Rio deLa Plata paleochannels into the shelf and

converging to the Rio Grande submarine fan. Riverdrainage from the nearby Pre-Cambrian plateau,also with widespread silty sediments onto the outercontinental shelf.

Several fluctuations of the sea level are presumed to have taken place in the area beforeand after the Holocene transgressions, the lastdated around 18,000 BP, which modifiedsediments facies distribution and particularly thetopography of the sea bottom with constructive anddestructive features.

The application of this experiment,obtained through the characterization of thesediments present in the transitional coastalenvironments, using grain-size statistical

parameters in the sedimentary cover of the adjacentcontinental shelf, represents a constant challengefor the sedimentologists devoted to textural

properties of the sedimentary rocks and sediments.With the increasing knowledge of the Quaternaryevolutive history of the area, an adequateunderstanding of the transgressive/regressiveepisodes, stillstands sea-level stabilizations and

prevalent sedimentary pattern, many questionsarose like: Which was the behavior of thetransitional sediments environments during theepisodes of subaerial exposition of the actualcontinental shelf? Which were the effects caused

by the drowning of these environments in the properties of the sediments? What fundamentalaspects that the so called relict sediments maintainin its consequent palimpsest correspondents? Is it

possible to evaluate the depositional energy levelof these deposits through the application of thetraditional statistical techniques? The obtainedresults have reliable validity for a wideapplication?

In analyzing grain-size frequencydistributions, the most common measures used arearithmetic mean (or average) and standarddeviation (or degree of scatter about the mean)while skewness and kurtosis are also used forspecial purposes and usually are diagnostic in the

comparison of sediments of different sedimentaryenvironments, or peculiarities of the sedimentsoccurring in the same environment (MARTINS,1967).

Figure 3 shows the graphic significanceof skewness (curves A and B) and kurtosis (curvesC and D) in relation to the normal distribution(curve E). A normal frequency distribution is a

continuous, bell-shaped curve that is symmetrical,and its arithmetic mean, mode and median arenumerically equivalent, and is known as aGaussian distribution. Geometrically independentof the sorting grade, it indicates clearly the

tendency of the grain-size distribution to have acoarse (negative) or a fine tail (positive).Using phi units, a positive skewness

shows a mode (Mo) smaller than median (Md) andmean (M). The relation is inverse if millimetersunits are used. In the distribution of curve A theskewness is positive (fine tail more expressive). Inthe curve B, when the relation is M>Md>Mo (inmillimeters values) or M


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Figure 3. Graphical significance of the statistical measures (according to Martins, 1966).

2) The carbonate bioclastic componentsshowed a mean between -4 to 3, they had unimodalor bimodal distributions, with a large range ofsorting values (from moderately to very poorlysorted) a predominant negative indices for theskewness and lepto (unimodal distributions) to

platicurtic (bimodal distribution).Even the sediments are coarser than the

sandy facies, these sediments are also sensibilized by the high energy present in a shallow marine area(nearshore). A comparison with the modern

bioclastic sand beaches like Albardão (southBrazil) confirms this assertion.

As much as the sandy sequences of outer& inner continental shelf, the bioclastic sequence,storm generated graded bedding were identified

produced mainly during the Wisconsin sea-levelconfirming the presence of an high energy levelenvironment. In fact, graded sequences in

quartzose sand with bioclastic material and bioclastic conglomeratic sand, both related tostorm layers, were identified along the inner andouter Rio Grande do Sul continental shelf byMARTINS et al. (1977, 1985, 1989) andMARTINS & MARTINS (1985).

3) These petrographic attributes weredeveloped through ancient shorelines(Pleistocene/Holocene) like the 110-120 and 30-20m. The so called relict sediments are representativeof shallow water energy parameters. Thereworking of these sediments through thedrowning of these environments did not changethese properties found in the palimpsest sediments.

On the other hand, the shallow modernhydrodynamic usually reinforces the acquiredattributes, as it occurs with the Albardão shoalsediments.

Outside the studied area, in the southernArgentina continental shelf, URIEN et al. (1993)identified relict glaciomarine sandy gravel linkedwith glacial sediments (moraines) that reached theold Pleistocene coastal plain. Prior to thesefindings URIEN & OTTMANN (1971) describedthe presence of relict sediments in the continental

shelf adjacent to Rio de la Plata, as MARTINS etal. (1967) did in relation to Rio Grande do Sul(Brazil).

4) These use of the grain-size statistical parameters to identify and characterize coarse(sand and gravel) marine sediments is a useful toolto be applied to other similar sequences, provided


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they are based on a trustful data, as suggested byMARTINS (1967), SYVISTKY et al. (1991) andMARTINS et al. (1997 and 2003).

5) Answering the questions that arose

during the discussion proposed in the developmentof this paper, we can summarize:

a) the grain-size smaller than 62 micron isquite important when you have a predominantcoarse distribution and a fine tail reveals itssensivity in relation with energy level of theenvironment;

b) in the studied area, the “old beaches”(Pleistocene) show usually similar properties tothose “modern beaches” (Holocene). Sometimes

beach and dune overlaps in “old” correspondents;c) the published literature confirms

similarities of the obtained data of the presentstudy when compared with other careful studies ongrain-size analysis;

d) the drowning effect of the Holocenetransgression had little effect over the grain-size

properties, except for the stillstands stops thatreworked these deposits;

e) usually, the palimpsest sequences didnot erase the relict properties (negative skewness,for instance) and sometimes the third statisticalmoment was strengthened by removal of the finecomponents of the distribution;

f) the results obtained with theapplication of the statistical devices confirm that itis a reliable technique, especially when used withother grain attributes, and it has a wide distributionwhen the field/laboratory procedures are wellconducted.

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sand gravel marine deposit

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