TP01 - Text. & Disp. of sediments in Panama Basin.pdf

8/18/2019 TP01 - Text. & Disp. of sediments in Panama Basin.pdf

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TEXTURE AND DISPERSAL OF SEDIMENTS IN THE PANAMA BASIN1

TJEERD H. VAN ANDEL

School of Oceanography, Oregon State University, Corvallis, Oregon 97331

ABSTRACT

The Panama Basin in the eastern equatorial Pacific is bordered by the continental margins ofcentral and South America and by the Cocos and Carnegie Ridges. A series of banks divides it intoan eastern and a western basin. The distribution patterns of the sediments are the product of complexinteractions between biological productivity, dissolution of calcite at depth, influx of continental debris,and dispersal and reworking of sediments by deep currents. Detailed grain-size analyses provideinsight into the reworking-dispersal portion of this system. The coarsest grain-size modes in the sandand coarse-silt range are concentrated on banks and ridges by winnowing, while the chaff isdeposited on ridge slopes and in the western basin. Approximately one-half of the sediment in thedeep western basin is the product of reworking. In contrast, the finest silt and clay modes have beendispersed by near-bottom currents and show transport of continental and fine biogenous material fromthe eastern to the western basin through gaps in the dividing ranges.

INTRODUCTION

Traditionally, sediment studies on landand in shallow seas have included texturalanalysis as an essential component. Sedimenttexture has been the basis for inferencesregarding transport modes, classification,composition, and provenance. On the otherhand, textural analysis has enjoyed littlepopularity in the study of pelagic deposits. Among the many reasons for this neglect threestand out: (1) pelagic deposits are generally

fine, and proper techniques for asufficiently adetailed examination of fine size distributionshave not been available until recently; (2) theprincipal value of grain-size analysis is in thestudy of sediment transport, reworking, andmixing-processes which have generally beenassumed to be of minor importance on thedeep-sea floor; and (3) this type of studyrequires a closely spaced net of sampleswhich is rarely available. The lack of suchsample coverage has impeded, not onlytextural, but also conventional, regionalsediments-petrologic studies of pelagic

sediments, in contrast to detailedmicropaleontological and geochemical ex-aminations of isolated sets of cores.

1Manuscript received February 12, 1973;

revised March 2, 1973.

[JOURNAL OF GEOLOGY, 1973, Vol. 81, p.434-457]© 1973 by the University of Chicago. All rightsreserved.

The conviction that such generalsediment-petrological studies have consider-able potential led us to initiate in 1969 acomprehensive study of the surface sedimentsof the Panama Basin in the eastern equatorialPacific. The main objective of this study ofwhat can be properly called a miniature oceanbasin was the elucidation of deep watersedimentation processes. The Panama Basinwas selected because it is one of thebiologically most productive regions of the

Pacific, contains a broad range of waterdepths and topographic features, and receivesa modest and a really restricted supply ofcontinent-derived sediment. It has largeenvironmental gradients and represents, withina relatively small area, most of the importantdeep-sea environments so that sampling anddata analysis are manageable. A large suite ofcores collected by several oceanographicinstitutions is available. In two earlier papers(van Andel et al. 1971; Heath and van Andel1973), the morphology, tectonics, and geologichistory of the basin have been described. The

present paper, together with its companions(Moore et al. 1973; Kowsmann 1973; Heath etal., in press), focuses on the regionalsedimentation patterns and depositional pro-cesses of the latest Quaternary sediments.

THE PANAMA BASIN

The Panama Basin is bordered by thecontinental margins of southern central


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TJEERD H. VAN ANDELTEXTURE AND DISPERSAL OF SEDIMENTS IN THE PANAMA BASIN

and northwestern South America and by twosubmarine ridges, the Cocos Ridge in the westand the Carnegie Ridge in the south (Fig. 1).The Galapagos Islands are at thesouthwestern corner and the Middle Americas

Trench abuts the Cocos Ridge in thenorthwestern corner. In the southeast, thePeru trench terminates in a saddle betweenthe Carnegie Ridge and the continentalmargin. The ridges have relatively flat tops at1,400 - 1,800 m and are bordered by steep,fault-controlled slopes (van Andel et al. 1971). A saddle at about 2,220 m divides theCarnegie Ridge into two parts.

A series of banks - the Coiba Ridge atthe Panamanian continental margin, theMalpelo Ridge in the center, and the Malpelo-

Carnegie saddle in the south divides thePanama Basin into eastern and western sub-basins. A narrow, but deep (3,600 m) channelbetween the Coiba and Malpelo Ridges is theprincipal connection between the basins. In

the western basin, a young rift offset byseveral north-south trending fracture zoneslies parallel to and just north of the CarnegieRidge. In the eastern basin a number ofcurved troughs parallel the continental margin.Magnetic anomaly studies (van Andel et al.1971; Herron 1972) and deep drilling (Heathand van Andel 1973) indicate an age not inexcess of ten m.y. for the western basin; theeastern basin may be somewhat older.

The sediment cover, notwithstanding theyoung age of the basin, is quite thick (van

FIG. 1 - Bathymetry and place names in the Panama Basin. Contours in meters (not corrected forvariations in sound velocity in sea water) are derived from van Andel et al. (1971, Fig. 2) and wereselected to bring out principal topographic features. They are repeated without labels on followingfigures.

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Andel et al. 1971), but on ridge crests erosionhas locally removed the sediment covercompletely and has deeply incised the flanksdown to bedrock. The erosion processes,

which have displaced a great deal ofsediment, are still active (Lonsdale et al.1972).

Along its eastern and northern margins,the Panama Basin receives sediments ofcontinental origin. Forsbergh (1969) estimatesthe annual runoff into the basin as 350 x 10

9

m3. Using an average content of solids of 100

ppm, similar to that of the Amazon system(Gibbs 1967), the amount of solids contributedby runoff can be estimated as 35 million tonsper year. The suspended matter is distributedthroughout the eastern basin and carried by

bottom currents into the western basin throughthe Coiba gap (Moore et al. 1973; Heath et al.,in press). The surface circulation has beensummarized by Moore et al. (1973) and by -Kowsmann (1973).

The Panama Basin is located in a regionof high biological productivity which isconcentrated over the Carnegie Ridge and in aregion of domal upwelling in the Panama bight(Moore et al. 1973, Fig. 2). Productivity is alsohigh in coastal waters, but in the center of thebasin the surface waters are less productive.Thus, it might be expected that the distribution

in the sediments of the biogenic componentscalcite and opal would reflect this productivitypattern. This is not the case: the concentrationof calcite on the ridges and of opal in the deepbasins is more closely related to topographythan to productivity, (Moore et al. 1973).

From these observations, the citedauthors concluded that the distribution of thesemajor sediment components is a function, notof primary production, but of dissolution ofcalcite in deep water, of reworking, andwinnowing at intermediate depths anddeposition of chaff in deep water, and of

dilution with terrigenous material. Althoughpart of the change in composition withincreasing sea-floor depth can be explained bydissolution, it is necessary to invokeconsiderable reworking and dispersal toexplain the details. The relation betweencalcite content and water depth suggests thatnearly one-half of the biogenic sediment in thedeep basins may be on a secondary restingplace. In the eastern basin, this interplaybetween primary production, dissolution, andwinnowing and redispersal is complicated

further by the influx of fine terrigenousmaterial.

GRAIN SIZE METHODOLOGY

Traditionally, textural analysis hasemphasized the coarse grades because thebest techniques are available for this rangeand because they provide the most easilyinterpreted data for the study of transportmodes, dispersal paths, and sedimentclassification. Furthermore, in continental andshallow marine deposits, the silt fraction isunderrepresented relative to the coarser andfiner fractions (Pettijohn 1957, p. 47-51); inpelagic deposits, the reverse is true. Acomposite of about 1,000 analyses from theIndian, Atlantic, and Pacific Oceans shows

that the silt fraction is the dominant one. In thePanama Basin, the average silt content of48% exceeds that of the sand (13%) and clay(39%) fractions.

A major factor in the lack of texturalstudies of fine-grained sediments has been theinadequacy of analytical techniques.Conventional methods, such as pipette andhydrometer, require large samples and yieldwide class boundaries incapable of providingfine detail. In recent years, new techniqueshave alleviated the problem. The falling dropmethod (Moum 1967) requires small samples

and allows fairly closely spaced classboundaries. Continuously recording balances(Bascomb 1968; Oser 1972) accommodatevery small samples and yield size distributionsthat are continuous except for limitationsimposed by digitizing. Computer dataprocessing has facilitated the handling ofinformation.

The detail provided by these methodsmakes possible interpretations that differ fromthe traditional use of summary statistics orcurve shape classifications. Curray (1960) hasshown that the size distributions of many

sediments are polymodal and that individualmodes contain much information regardingsediment dispersal. Van Andel (1964) usedthis approach successfully in marinesediments of intermediate depth. Oser (1972)and Dauphin (1972) have shown that manypelagic sediments are also polymodal and thatthe individual modes can be interpreted interms of composition, dispersal, andprovenance.

The present study is based on dataobtained with the falling drop method with

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class intervals of 0.5 Φ between 4.0 and 7.5Φ, and of 0.2 Φ between 7.8 and 9.0 Φ (Φ = -log2 [diameter in mm]). The data are thus morelimited than that of Oser and Dauphin whoused a continuous recording sedimentation

balance. The samples were taken from coresin collections of the Lamont-DohertyGeological Observatory, Scripps Institution ofOceanography, and Oregon State University(Fig. 2) and represent the latest Quaternarysurface samples.

The sediments were treated with buffered(pH, 7.0) 19% hydrogen peroxide to removeorganic matter and disperse the particles,rarely dispersed by treatment with a few dropsof sodium hexametaphosphate or mildagitation in an ultrasonic bath whenflocculation occurred, sieved in distilled water

on a 4.0 Φ (0.0162 mm) sieve and washed

several times with distilled water. The sieveresidue was labeled sand; because of thegenerally small volume, its size distributionwas not determined further. A split of 1-2 g ofthe fine material, diluted to 50 ml, was placed

in a constant temperature bath at 20ºC in thefalling drop apparatus, and the size distributionbetween 4.0 and 9.0 Φ (0.062 - 0.002 mm)determined as described by Moum (1967).The remaining fraction < 9.0 Φ was labeledclay. (The terms "sand" and "clay" are used intheir textural connotation). With a computerprogram, a cubic spline curve was fitted to thecumulative data points for the 4.0 – 9.0 Φ fraction and differentiated to yield t he size-frequency distribution (Oser 1972). Asmoothing factor of 1.5 was used to reduceirregularities and peakedness. Examples of

the process are shown in figure 3.

FIG. 2 - Location of samples. Numbers are accession numbers in the OSU marine sedimentlaboratory. Large dots represent samples used in Figs. 3-5. Contours from Fig. 1.

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FIG. 3 - Derivation of component modes. Dots indicate points of cumulative curve determined byfalling drop technique. Heavy solid line is cumulative size distribution determined by fitting cubic splinecurve to data, thin solid line is size-frequency curve obtained by differentiation of cumulative curve.Dashed lines represent component modes determined by DuPont Curve Resolver (labeled as definedin text). Note tail modes on fine side of upper curve and coarse side of lower curve. Mode f in PL 031probably represents a clay mode extending into the silt domain; converted to percentage of totalsample it would add 11% to 47% clay fraction. Tail mode a of PS 131 is probably an artifact due tocurve processing; it would only add 1% to the sand fraction of 24%. Peaks of modes C (PL 031) andD and E (PS 131) are displaced with respect to apparent peak positions in respective frequencycurves indicating that visual estimation of peak positions is hazardous. Note further that a broad mode(C in PS 131) depresses peak height and decreases modal area of modes on its banks below whatvisual inspection might suggest. A double mode, E-E', is present in PS 131; the question may be

raised whether the D-E-E' cluster is real or a result of inaccurate data or of a deviation of the com-ponent modes from a normal distribution; the majority of the Panama Basin samples have wellseparated modes such as shown in Fig. 5.

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In the past, considerable difficulty hasbeen experienced in resolving the polymodalfrequency curve into individual modes (Curray1960; van Andel 1964). In principle, mostpolymodal curves can be resolved into an

infinite number of sets of modes, dependingon assumptions regarding the nature of thecomponent distributions. There has been agood deal of argument regarding the nature ofthe basic size-distribution function, but there issome concensus that it is best described by aGaussian function with a logarithmic size (orsettling velocity equivalence) scale for whichthe phi scale is appropriate. Therefore, theresolution of the polymodal curves for thisinvestigation was based on the assumption ofa Gaussian function for the component modes.Even with this assumption, the problem does

not always have a unique solution, and anumerical approach is difficult. Instead, ananalog device, the DuPont 310 CurveResolver, was used; this device can generateup to 10 individual curves of a chosen functionwith variable width, height, and position. Thesecomponents can be modified and summeduntil the summation curve matches the originalsize-frequency distribution. Althoughtheoretically such a solution need not beunique, it was generally found that only oneset of curves could be generated that fullymatched the original distribution. Most

matches are perfect within line width; the worstones are matched to better than 95% of thearea under the original curve.

The frequency curves (Fig. 3) are limitedby the 4.0 and 9.0 Φ boundaries and do nottake account of coarser or finer material thatmay be present. Consequently, edge effects atthese boundaries must be expected. Theseare of two kinds: (1) mathematical artifactsresulting from a poor fit of the cubic spline

curve at the terminals of the distribution, and(2) partial modes where a natural modestraddles the boundary. Such componentscannot be properly matched and width,position, and area will be in error; such tail

modes (Figs. 4, 5) are common. Small onesare probably artifacts of curve fitting, but thelarger ones should be included with the sandor clay fractions after converting to thepercentage of the total size distribution.

Mode studies by Curray (1960) and van Andel (1964) were based on visual inspection.This procedure has significant pitfallsillustrated by figure 4 and 5. Quite commonly,modes on the flank of a larger one have theirpeaks displaced from the apparent peak orshoulder on the original frequency distributionso that a purely visual determination of the

peak position can be quite unreliable. In suchcases - and also where a small mode occursbetween two large and wide ones - there islittle correspondence between peak height onthe composite curve and the area of thecomponent mode (e.g., Fig. 4, mode E). In themajority of the Panama Basin samples, the separation between individual modes isdistinct and an unambiguous determination ofcomponent shape and position is possible.However, in some cases the modes are closetogether or broad and overlapping and thepolymodal envelope is not easily interpreted.

Furthermore, in differentiating a cumulativecurve based on rather wide class boundaries,processing errors and deviations of the com-ponent modes from a Gaussian function mayproduce deformed modes or introduce smallartificial ones. In the following discussion,modes of more than 5% area will beconsidered as probably real and modes withmore than 10% as real.

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FIG. 4 - Tail modes. Solid line is size-frequency distribution; dashed line represents componentmodes. Small tail modes in center graph may be due to edge effects resulting from data processing ormay represent small deviations of the adjacent modes from a true normal distribution. The large tailmodes of the upper and lower graphs probably represent portions of sand and clay modes,respectively, intruding into the silt domain. Note small modes B (upper graph) and C (middle graph).These are required to fill shoulders on the C and D modes, respectively; although labeled as realmodes, they may represent deviations of the large modes from a true normal distribution. Tick marksalong base indicate class boundaries used in falling drop analysis.

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GROUPING OF MODES

Using the DuPont Curve Resolver, peakpositions on a particular size-frequency curvecan be determined to the nearest 0.05 Φ or

better. Since the class boundaries used in theanalysis are much more widely spaced thanthat and because of other errors discussedabove, the error of this value is large. Thus,the peak positions can be used only if they canbe grouped into natural categories; suchnatural categories appear to exist. A histogramof the numbers of modes having their peak

positions in successive 0.05 Φ classes (Fig. 6)consists of a series of discrete or partlyoverlapping clusters around mean values.These clusters are bound by minima in the

histogram. Eight such clusters can be dis-tinguished: two of these, a and f , are tailmodes and may be either truncated sand orclay modes or artifacts of curve processing;the others, labeled A-F, are true silt modeclusters. Each of these appears to beapproximately normally distributed and a meanpeak position can be determined. Peak

FIG. 5 - Examples of size-frequency distributions and their component modes in the Panama Basin.Letters indicate mode groups discussed in text. Note large tail modes (a) in PL 017 and PS 124; theyare probably part of a sand mode. They add 5% and 6%, respectively, to sand fractions of 87% and11%. A small mode (B) is needed in PS 130 to account for the asymmetry of mode C and a smallmode A in PL 017 to fill slight asymmetry in tail mode a. Both may be due to small deviations of themain modes from a true normal distribution. Mode D in PS 124 demonstrates the effect of broadadjacent modes in reducing peak height and area below what is visually apparent. Tick marks alongbase indicate class boundaries used in falling drop analysis.

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FIG. 6 - Histogram of number of cases in each 0.05 Φ modal class plotted against grain size. Thehistogram has been divided into clusters at minima in the number of cases and clusters have beenshown alternatingly with broken and solid bars erected in each 0.05 Φ interval. Modes assigned totwo clusters in one interval are shown with two bars of different signature. Total number of cases 103.Triangles at base indicate class boundaries used in falling drop analysis. Cluster boundaries appearnot to be directly related to these boundaries.

positions for each cluster are listed in table 1.The spread of each cluster is probably largelydue to uncertainties in the determination of thepeak of each component mode as a result ofwide spacing of size class boundaries.However, the spread may be in part a functionof a natural variation in peak position. Clearly,a technique yielding continuous sizedistributions would provide more certainly thatthe component modes do not form acontinuous series.

A few component modes occur intransition zones between clusters (table 1,

lower part); these were assigned to clusters bya somewhat subjective process. If the modewas a minor shoulder or subsidiary peak on alarger one, it was assigned to the samecluster; if it was clearly independent, it wasassigned to whichever cluster on the left orright was unoccupied in the sample. In a veryfew cases, the decision was influenced by thelogic of the resulting map pattern.

REGIONAL GRAIN-SIZE PATTERNS

The procedure used in this study dividesthe sediments into sand, silt, and clayfractions, then subdivides the silt fraction intosix mode clusters. The terms "sand," "silt," and"clay" are textural: the deposits of the PanamaBasin are dominantly to exclusively biogenicexcept for those of the northern and easternmargins where the "clay" fraction indeedconsists mainly of clay.

The distribution patterns of sand and clay(Figs. 7, 8) are nearly mutually exclusive.

Sand-consisting mainly of planktonicforaminifers (Kowsmann 1973) - isconcentrated on the ridges, while clayincreases downslope to maxima in the deepbasins and along the eastern and northernmargins of the basin. An exception is thenorthern Cocos Ridge which is covered withfine sediments. Because the sand, silt, andclay fractions are expressed in

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percentages, the silt distribution (Fig. 9) is thecomplement of the two others, showing a lowsilt content on the ridges and along theeastern and northern margins. This fraction isalso concentrated on the slopes of the ridges

and in the western, but not in the eastern.basin.

The distribution patterns of mode clusters A-F of the silt fraction (Figs: 10-15) areexpressed as a percentage of the silt fractionrather than as a- percentage of the entire sizedistribution. Otherwise, the mutually exclusivecharacter of the sand and clay fractions wouldtend to dominate the patterns. In figure 10, the

percentages of group A have been combinedwith those of coarse tail mode a, because adistinction between natural and artificial tailmodes is impossible. The artificial ones arelikely to have low values and should not distort

the pattern seriously. There are so few fine tailmodes that they have been omitted from figure15.

The distribution pattern of group A as wellas that of the next finer mode cluster (Figs. 10,11) is very similar to that of the sand fraction;both modes are nearly limited to the crests ofthe ridges. A slight difference between the twois the greater extent of B

FIG. 7 - Distribution of sand fraction (coarser than 4.0 Φ, 0.062 mm) in Panama Basin. Contours inpercentage of total sediment. Black dots are sample locations, thin depth contours refer to Fig. 1.Sample numbers and locations in Fig. 2.

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on the slopes and the existence of a smallpatch of B in the center of the western basin.The trend toward downflank displacement ismuch stronger in the distribution of group C (Fig. 12). Although also closely associated

with the ridge topography, this group is mainlyconcentrated on ridge flanks, especially on thenorthern flank of the Carnegie and thesouthern flank of the Cocos Ridge: Thesample density is not sufficient to determinewhether this is true for all ridge flanks, and thedifference between the north and south flanksof the Carnegie Ridge appears to be real. Asmall patch of group C also occurs in relatively

shallow water off the Panamanian continentalmargin.

Group D (Fig. 13) represents an evenmore extreme case of downslope shift ofprogressively finer material. On the north side

of the Carnegie Ridge, the maximumconcentration has shifted both downslope andwest of the coarser modes. In addition, groupD is also present along the continental marginin the eastern basin. As for the coarser modes,the sample distribution around the MalpeloRidge and on the Malpelo-Carnegie saddle,although suggestive of a similar relationshipbetween depth and grain size, is insufficient

FIG. 8 - Distribution of clay fraction (finer than 9.0 Φ; 0.002 mm) in the Panama Basin. Contours inpercentage of total sediment. Black dots are sample locations (Fig. 2); thin depth contours from Fig. 1.

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for a precise delineation of the pattern. It isworth mentioning that mode D is identical inposition and width to a mode found by Oser(1972) in the western Pacific which consistsnearly exclusively of a single coccolith species

(C. pelagicus).Thus, the sand fraction and modes A-D

form a series of progressive downslopedisplacement of finer fractions on the oceanicridges. The slopes of the continental marginsdo not exhibit a corresponding grain-sizepattern. The only exception is the occurrenceof a band of mode D along the lower easterncontinental margin.

To some extent, mode E (Fig. 14) repre-sents the extreme of this trend, with zones ofhigh concentration along the foot of theCarnegie and Cocos Ridge slopes. Moreimportant, however, is a new pattern not

observed in the coarser modes. This patternconsists of large concentrations in the northernparts of the eastern and western basinsconnected through the Coiba gap, and withtongues extending southwestward into thewestern basin and through the Malpelo-Carnegie saddle. Another zone ofconcentration occurs along the southeasterncontinental margin. This new pattern

FIG. 9 - Distribution of silt fraction (4.0 Φ - 9.0 Φ; 0.062 - 0.002 mm) in the Panama Basin. Contoursin percentage of total sediment. Black dots are sample locations (Fig. 2); thin depth contours from Fig.1.

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resembles that of the clay fraction but withsignificant differences, such as the absence ofgroup E on the northern Cocos Ridge.

The distribution of group F (Fig. 15) iseven more similar to that of the clay fraction. It

bears no relation to the ridges, except perhapsfor some small concentrations in the farsouthwest, but is strikingly concentrated in theeastern basin and in the northern part of thewestern basin. However, it is not directlycontiguous with the continental margin, as isthe clay fraction. A tongue extends from theeastern into the western basin through theCoiba gap and another may pass through theMalpelo-Carnegie saddle. In the western

basin, three zones of high concentrationextend southward along the foot of the CocosRidge and in areas of greatest depth. The Fgroup is also an important component south ofthe Carnegie and west of the northern Cocos

Ridge.

DISCUSSION AND CONCLUSIONS

If we accept the perhaps somewhatoptimistic assumption regarding the reliabilityof the grain-size data presented above, thedistribution patterns of the textural elementsare clear and remarkably meaningful. Twobasic patterns can be distinguished:

FIG. 10 - Distribution of modal groups A and a. Contours in percentage of total silt fraction. Black dotsare sample locations, larger if mode a is present (sample numbers and locations in Fig. 2); thin depthcontours from Fig. 1. See table 1.

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the first-represented by the clay and finest siltmode is restricted t o the deepest parts of thebasins, approximately confined by the 3,000-3,400 m contours (cf. Fig. 1), and contiguousto the northern and eastern basin margins

(Fig. 16); the other represented by the s andfraction and the coarse and medium silt modesis clearly related to the oceanic ridges andbanks, and shows concentration of coarsematerial on top and progressive finingdownslope (Fig. 17). Silt mode E istransitional; its distribution combines elementsof both patterns.

The restriction of the clay to deepestwater and its high concentration adjacent to

the continental margins suggest that this sizefraction is mainly of continental origin andbeing dispersed by bottom-water flow. This isconfirmed by a comparison of the claydistribution with bottom-water flow patterns

from the Peru trench through the eastern basinand into the western basin via the Coiba gapand the Malpelo-Carnegie saddle (Laird 1971;Kowsmann 1973). The distribution of opal andfine carbonate (Moore et al. 1973, Figs. 4 and8) indicates that the clay fraction in the easternand in the northern part of the western basin isdominantly of terrigenous origin while elsewhere it must contain a large biogenouscomponent.

FIG. 11 - Distribution of modal group B. Contours in percentage of total silt fraction. Black dots aresample locations (Fig. 2); thin depth contours from Fig. 1. See table 1.

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Mode F exhibits the same pattern as theclay fraction with two main differences: (1) it ismore important and more widespread in thewestern than in the eastern basin and (2) itsgreatest concentrations are well separated

from the continental margins. Thus, one mayconclude that this mode is being dispersed bythe same transport system, but is derived froma different source. The sharply definedtransportation paths and the restriction todeepest water suggest that this source isbasin-wide but geographically restricted.Winnowed material from the ridges is thus

more likely an important contributor thanproductivity in the surface waters. Thisexplanation also clarifies the anomaly that thedispersal of this somewhat coarser materialextends farther into the western basin than the

clay fraction. It seems probable that the F mode contains a terrigenous component aswell, but the data is inadequate to estimate itsproportion.

The second pattern shows a simplerelationship between texture and thetopography of the oceanic ridges. Essentiallyno silt fraction is found on the ridge

FIG. 12 - Distribution of modal group C. Contours in percentage of total silt fraction. Black dots aresample locations (Fig. 2); thin depth contours from Fig. 1. See table 1.

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crests and no sand in deep water; on theslopes, progressively finer silt modes overlapdownward. Moore et al. (1973) and Kowsmann(1973) have shown that in the western basinthe sand and silt fractions and a portion of the

clay fraction are of primary biogenous origin.The sand fraction and the coarsest silt modeare mainly calcareous and consist of wholeand broken foraminifers, while the finer frac-tions are progressively more siliceous.

Two explanations can be offered for thisrelation between texture and topography: (1)winnowing and dispersal of fine material onthe ridges, and (2) progressive e dissolution ofcalcareous material with depth. A third that the

size distribution of material produced insurface waters varies with the level ofproductivity can be disposed of; although thisis possible in principle (increase of diatoms),the textural gradients in the sediments do not

coincide with gradients of productivity insurface waters.

The first explanation invokes creation of acoarse lag deposit on ridge crests,accompanied perhaps by fragmentation ofskeletal material, and removal of the finematerial by currents. There is considerableevidence that this process indeed operates.Kowsmann (1973) has shown that the coarsedeposits are rich in whole foraminifers, but

FIG. 13 - Distribution of modal group D. Contours in percentage of total silt fraction. Black dots aresample locations. Thin depth contours from Fig. 1. See table 1.

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that whole radiolarians of equivalent size butsmaller density are depleted there andenriched in deeper water. Foraminiferalfragments are also enriched in slope deposits.The high concentration of coarse fraction

(more than 30%) is itself evidence for awinnowing effect; at this depth below thelysocline, the deposits should already havebeen depleted substantially in this material.Furthermore, Moore et al. (1973) have shownthat the calcium carbonate dissolution curvewith depth can best be explained by assumingthat large amounts of fine calcareous materialare added in mid-depth. Lonsdale et al. (1972)and B. T. Malfait (personal communication,

1973) have shown that large-scale erosionoccurs on the saddle of the Carnegie Ridgeand is accompanied by major transport oferosion products downslope. Truchan and Aitken (1973) have presented evidence based

on seismic reflection data that much erosionand redistribution of sediment occurs on theCoiba and Cocos Ridges.

An alternative explanation for the texturalgradients can be sought in dissolution withdepth. This process has been discussed indetail by Moore et al. (1973) and might yield adownslope decrease in size as a result ofbreakup and dissolution of foraminifers anddelicate siliceous skeletons. Intuitively,

FIG. 14 - Distribution of modal group E. Contours in percentage of total silt fraction. Thin depthcontours from Fig. 1. Black dots are sample locations. See table 1.

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it would appear improbable that such aprocess (which is unquestionably active)would be capable of producing the sequenceof well-defined textural modes, but withoutcompositional information this cannot be

decided. The strong arguments in favor ofwinnowing and dispersal, however, suggestthat dissolution is not the sole or even thedominant textural process.

It is of obvious importance for geo-chemical, micropaleontological, and sedi-mentological studies to estimate the amount ofmaterial that is reworked and ultimatelyincorporated into deep-water deposits. Suchan estimate is difficult because three sources

of sediment may contribute material to thefloor of the western basin: (1) particleswinnowed from the ridge crest (as this materialmoves downslope it is subject to dissolution ofits calcareous components while more delicate

siliceous skeletal material may be dissolved orbroken): (2) particles directly from biologicproductivity in overlying waters (also subject todissolution but less to mechanical damage);and (3) fine-grained material may be suppliedby bottom currents from outside the basin.

Moore et al. (1973), considering dissolu-tion rates with depth, have estimated thatbetween 40% and 50% of the sediment in thedeep western basin is chaff produced by

FIG. 15 - Distribution of modal group F. Contours in percentage of total silt fraction. Black dots aresample locations. Thin depth contours from Fig. 1. See table 1.

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winnowing on the ridges. If this chaff is subjectto dissolution during transport, the proportionmay be higher. An independent estimate of thequantity of reworked material would bevaluable.

Such an estimate, although crude, wasobtained by comparing the textural makeup ofthe sediments of the western Panama Basinwith that of sediments outside the basin andwell removed from input of reworked material. A set of samples from the central westernbasin where conditions are most uniform, anda set from about 200-350 km south of the

Carnegie Ridge are compared in table 2.Figure 18 shows that the averagecompositions of the two sets of samples differin significant and expected ways. The westernbasin set is depleted in coarse material and

enriched in medium-grained fractions relativeto the open ocean suite, while both are similarin the content of finest material. If we assumethat the coarse material in the western basinset represents pelagic material on a primaryresting place, the ratio between the sandcontents, or A mode percentages, in the twosets can be used to estimate the

FIG. 16 - Schematic representation of dispersal of clay and fine silt in Panama Basin (modes F, E inpart). Isopleths from Figs. 8, 14, 15. Solid arrows indicate dispersal system of land-derived materialand follow deepest part of basin below 3,200 m; dashed arrows indicate dispersal of material derivedfrom winnowing on ridge crests.

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primary deposition fraction in the other sizegrades of the western basin set. Thisprocedure yields an estimate of 60% for thecontent of primary pelagic material in thewestern basin, although our confidence in this

number is somewhat reduced by the fact thatthe open ocean set is from a somewhatgreater depth (several hundred meters) and issomewhat more calcareous (10% - 20%) thanthe western basin set. Within these limitations,the agreement with the estimate of Moore etal. (1973) is satisfactory. It is worth noting thatthe C mode is entirely absent in the pelagic

samples, suggesting that it is a product ofwinnowing.

It is obvious that this investigation has atleast established that textural studies of deep-sea sediments hold considerable promise for

evaluating depositional processes. In manyrespects, this study is only a beginning: for amore thorough and quantitative approachbetter size analysis techniques should beused, the textural information for the sandfraction should be added, and the compositionof the individual mode clusters must be deter-mined. Moreover, the sample density although

FIG. 17 - Schematic representation of dispersal of coarse and medium silt in Panama Basin. Isoplethsfrom Figs. 10-13. Distribution of mode groups E and B shown in part only. Arrows indicate dispersaldirections based on textural patterns and existing knowledge regarding current patterns and erosionchannels.

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FIG. 18 - Histogram of average textural composition of sets of samples from open Pacific Ocean(labeled "pelagic") and from western Panama Basin.

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good for a deep ocean study is inadequate inview of the apparent complexity of both thetopography and the transport systems. Thenext steps in this type of study should be theexamination of the vertical and horizontalvariation on a very small scale and a thoroughstudy in detail of a well-defined portion of thesystem, such as, for example, the northwarddispersal from the crest of the Carnegie Ridge.

ACKNOWLEDGMENTS.- Investigation wassupported by the office of Naval Research(contract N00014-67-A-039-0007 under

project NR 083-1021). The Panama Basinstudy is a team effort and many havecontributed to its overall success and to thisparticular report. The analytical methods weredeveloped and the data obtained with the aidof Ellen T. Drake, Robert M. Beer, MaryFranklin, David B. Ellis, G. Ross Heath, RobertK. Oser, and J. Paul Dauphin. Discussionsand comments by the latter and by Renato O.Kowsmann, Bruce T. Malfait, and T. C. Moore,Jr. have been extremely valuable. To all I owea large debt of thanks. The illustrations wereprepared by Natasa Sotiropoulos.

Documents

TP01 - Text. & Disp. of sediments in Panama Basin.pdf