The geochemistry of modern sediments from the Gulf of Paria—I The relationship between the mineralogy and the distribution of major elements

The geochemistry of modern sediments from the Gulf of Paris-I The relatio~~p between the mineralogy and the ~~bution of major elements Department

D. M. HIRST of Geology, University (Received 7 January

of Durham

1961)

Ah&&-The distribution of the major elements in some recent sediments from the Gulf of Park has been determined. In interpreting the variation in the major element contents across the basin of deposition, consideration is given to such factors as rate of deposition, physico-chemical environment, s&&y and f&es. The majority of the major elements are already present within the lattices of vsxious clay minerals when they enter the basin of deposition, and the variations in their contents in the different sediments reflect variation in the clay mineralogy of these deposits. This variation in the clay mineralogy is thought to be due to variation in weathering conditions in the source &pea of the sediments.

EARLY work on the geochemistry of sediments was largely confined to studies of the distribution of one or a limited number of elements through a range of samples An examination of the Manganese Shale having no close geological relationships. formation of the Harlech Dome (BOHR, 1955, 1959) indicated, however, that a geochemical study confined to a single stratigraphical horizon could be of great value. This approach has been followed by SPENCER (1957), who examined the distribution of elements in the “Greenstreak” horizon of the Lower Silurian, and CAMERON (1957), who studied the distribution of 23 elements in the Upper Lias Sands of Southwest England. In such investigations the physico-chemical oonditions of sedimentation must be deduced since, obviously, they cannot be Some value should therefore attach to a geochemi~al study directly observed. of modern sediments ~coumulating in a restricted basin where the various conditions of sedimentation can be observed or measured. The Gulf of Paria, west of Trinidad, is an example of such a basin, in which the distribution of sediment types and conditions of deposition have already been established by VA.N ANGEL and POSTMA (I!&&). Samples of the actual sediments studied by these authors were kindly made available for geochemieal studies by the Bataafsche Petroleum Maatschappij, N.V., The Hague. They were supplied in airtight polythene bags and are believed to have suffered no contamination since oolleetion. Usually between 500 and 1009 g of sample were supplied. GENERAL

GEOLOGY

OF

THE GULF

OF PARIA

The Gulf of Paria is bounded to the south by an area including the Orinoco delta, the Llanos and the igneous and high grade metamorphic Guyana Shield. The northern limit is formed by the mountainous Paria Peninsula and the Northern Range of Trinidad, composed of low and medium grade metamorphics. The gulf forms the northern part of a larger basin and is being filled by sediments derived mainly from the south. The southern part of this basin is occupied by the lower 309

D. M. HIRST

310

l 345 Coroni Platform

Trinidad

.6ll Boco

.

592

Erin

Venezuela

Morugo

Plotform

\

Fig. 1. Sample localities in the Gulf of Park

reaches of the Orinoco drainage system, where the river gradients are exceedingly low and vast expanses are covered by flood plains and swamps. Thus it is unlikely that any but the very finest suspensions of solid material will reach the gulf waters from the Orinoco system. In general the gulf bottom is flat and slopes are very gentle. The Boca Vagre delta (Fig. X) is marked by a wide zone with water depths between 0 and 2 fathoms, whilst a wide platform, bounded by the 10 fathoms line, exists off southwest Trinidad (the Soldado platform-Fig. 1). A much smaller platform, the Oropuche bank, with very steep outer slopes, lies to the northeast of South of Trinidad the bottom slopes gently southwards the Soldado platform. forming the Erin Moruga platform and then suddenly dips steeply towards the deep asymmetric Serpents Mouth Channel. (i) Sediment types The usual principles of sedimentation hold for the Gulf of Paria. An inverse relationship exists between grain size and distance from source and the more distant the sediments from their source area the more homogeneous they are. Three main types occur, viz., gravels, sands and clays. Gravels are of subordinate importance and are of local derivation. They occur around the shores of southwest Trinidad, together with ferruginous sandy aggregates and limonitic concretions, and also in the northern parts of the gulf. The sands are derived from three main source areas. Those derived from the Orinoco region are characterised by the presence of iron stained quartz and one of two heavy mineral assemblages, viz., O-hornblende (hornblende, epidote and minor chloritoid) and O-zircon (essentially zircon alone). Mica and schist fragments

!l’he geochemistry of modern sediments from the Gulf of Park-1

311

characterise the sands derived from the Paria chain, together with such heavy minerals as chloritoid, tourmaline, hornblende and epidote. Sands derived from southwest Trinidad are more variable and three different mineral assemblages can Gl auconitic sands with abundant authigenic glauconite and chert be recognised. fragments occur on the Soldado platform, the Oropuche bank and the Erin Moruga On the Oropuche bank they pass shorewards into quartz sands containplatform. ing limonitic pellets and aggregates of sand and clay (the Oropuche sands). On the high southwestern part of the Soldado platform the sands contain coarse, wellThese sands are rounded quartz and abundant rock fragments of local derivation. known as the Soldado sands. Three heavy mineral assemblages have been recognised in these sands of Trinidad provenance viz. T-epidote, T-zircon and Tgarnet (for details see VAN ANDEL and POSTMA, 1954). Two principal types of clay occur. The clay on the Boca Vagre delta (within the g-fathom limit) contains kaolinite and illite but little montmorillonite. That from the gulf proper contains a similar amount of kaolinite but considerable amounts of both montmorillonite and illite. The effects of transport and deposition have largely obscured the source of the clays but the Orinoco drainage system is probably the principal source. The generally accepted view that marine clays are dominantly illitic whereas montmorillonite is more characteristic of lagoonal and paralic environments is in direct contrast with the observed distribution in the VAN ANDEL and POSTMA (1954) suggest that the distribution of the Gulf of Paria. different clay minerals may be affected by the nature of their flocculation. WHITEHOUSE (1951, 1952) has shown that suspensions of kaolinite and illite reach maximum flocculation almost instantaneously at a chlorinity of 2-O g/l., whilst montmorillonite shows a steadily increasing flocculation with increasing chlorinity to a maximum at much higher ohlorinity values than those of kaolinite and illite, so although montmorillonite may be deposited in the estuaries it is likely that greater amounts would occur in the gulf proper, A study of sounding records indicates a discontinuity below the sea floor of probable Pleistocene (Wisconsin) age. Sub--Recent deposits have been recognised in several localities. They are bluish clay, more compact than the modern greyish olive-green surface muds. Generally this bluish clay is covered by the greenish mud, but in the northeast of the gulf it forms the surface of the sea bed, The directions of sediment transport in the gulf, deduced by VAN ANDEL and POSTMA, are shown in Fig. 2. (ii) Rates of delpasition Recent rates of deposition in different parts of the gulf are recorded in Fig. 3. Rates for the bluish clay, calculated by VAN ANDEL and POSTMA from radio carbon dating, are 18 cm/century for the Soldado area and 10 cm/century for the Caroni platform. (iii) P~~~~co-c~e~~~l

co~d~t~~n~ in the gz@

The acidity of the gulf waters ranges from pH 6.4 to pH 8-O. The pH is highest in the more oceanic regions, such as the Bocas de1 Dragon. Relatively high values are also found around the southwestern peninsula of Trinidad. In the central and

Il. M. HIEST

312

*..----.

‘\ “

\ is \,

:

Venezuela

Pig. 2, Generalized pattern of sediment bansport in the Gulf of P&a and POSTMA, 1954).

(after VAN ANDEL

30cmlcentury

1 “”

Venezuela

Fig. 3. Tentative

rates of depositian for the period covering approximately years (after VAX ANI)EL and POSTMA, 1954).

the last 70C

The geochemi&ry of modern sediments from the Gulf of Pa;ria--I

313

western parts of the gulf the pH is less, especially in the extreme west, and in those areas where CaCO, is scarce, organic matter relatively abundant and sedimentation rapid. Generally the gulf water is well oxygenated and the low values of pH found in some areas cannot be att~buted to a primary lack of oxygen. Reducing conditions are encountered in the sediments in areas where large amounts of organic matter are being rapidly buried e.g. the Boca Vagre delta and in the extreme west of the gulf. Oxidising conditions prevail in the sediments on the Oropuche bank In the centre of the gulf conditions in the and around the Bocas de1 Dragon. sediments are neither strongly oxidising nor strongly reducing. For a more detailed account of the sedimentation and geography of the Gulf of Paria the original report of VAN AWDEL and POSTMA (1954) should be consulted. LOCATIONAND DESCRIPTIONOF AWALYSED SAMPLES The positions within the Gulf of Paria, from which the samples analysed were collected, are shown in Fig. 1. The numbers assigned to the collecting stations are those given to them by VAN ANDEL and POSTMAin their original work. Except where otherwise stated samples were collected from the surface of ;the sea bed. Brief descriptions of them, based largely on the work of VAN ANDEL and POSTMA, follow. Snnds, silts and clays from the Boca Vagre 611. A fine grained, brownish sand with some clay size material. The light fraction is composed of abundant “quartz with fefspar”, common Orinoco quartz and soarce mica, and constitutes 84 per cent of the sediment. The heavy mineral association is O-hornblende forming 0.13 per cent of the sediment whilst the size grade < 0.005 mm is composed of 6-10 per cent montmorillonite, &70 per cent illite and lo-20 per cent kaolin&e. 592. A fine grained muddy silt with a considerably greater amount of clay size material than 611. This sample also belongs to the O-hornblende association, heavy minerals again constituting 0.13 per cent of the sediment. Kaolinite and illite were noted although montmorillonite was not detected. No clay fraction was, however, separated from the sediment. 139. A fine grained brownish sand containing abundant “quartz with felspar” and common Orinoco quartz; light minerals constitute S9 per cent of the sediment]. The sample, containing 0*12 per cent heavy minerals, belongs to the O-hornblende association. 587. Very similar to 611. Although no data is available, the location within the inner delta suggests that the heavy mineral association is probably O-zircon. Sands, silts and clays from the platforms surrounding S. W. Triaidad 398. A greenish brown sand with a light fraction constituting 72 per cent of the sediment and composed of abundant “quartz with felspar”, common glauconite and calcareous detritus, scarce rock fragments and rare faecal pellets and limonitic aggregates. The heavy mineral assemblage is T-epidote forming 0.21 per cent of the sediment. X-ray study indicates a mineral with ill&e xefle~tions, undoubtedly glauconite, but no kaolinite or montmorilloni~e.

324

D.

M. HIRST

395. Described as a gravel by VAN ANDEL and POSTMA (1954) this sediment has a light fraction similar to 398 constituting 82 per oent of the sediment. The heavy mineral assemblage is T-epidote. 77. A greenish grey, sandy clay with numerous soft angular pieces of green micaceous silt. The light fraction is composed of abundant “quartz with felspar”, scarce calcareous detritus and rare chert and rock fragments. The heavy mineral assemblage, 0.12 per cent of the sediment, is T-garnet. X-ray study indicates subsidiary illite and kaolinite whilst montmorillonite was not detected. 68. A greenish grey sand with clay occurring mainly as hard, consolidated, angular pieces. The light fraction is composed of abundant “quartz with felspar”, common chert and limonitic aggregates and scarce calcareous detritus, The heavy mineral assemblage is forming 46 per cent of the sediment. X-ray study revealed the presence of ill&e, kaolinite probably T-epidote. and montmorillonite. 152. A greenish grey sand. The light fraction, forming 60 per cent of the sediment, is composed of abundant “quartz with felspar” and common glauconite and calcareous detritus. The heavy minerals constitute 0.13 per cent of the sediment, the assemblage is T-epidote. Subsidiary kaolinite and an illitic mineral, probably glauconite, are indicated by X-ray diffraction. 158 (top). A mixture of soft olive mud and glauconitic sand, probably with the T-epidote assemblage. 158. A brownish-yellow sand, probably Tertiary. 214. This fine grained, olive green silt contains abundant “quartz with felspar”, common glauconite and calcareous detritus and scarce chert, the light fraction constituting 93 per cent of the sediment. The heavy mineral assemblage, 0.25 per cent, is T-epidote. 171. A fine grained green sand containing small sandy limonitic concretions. The light fraction composing 94 per cent of the sediment is reported to contain abundant “quartz with felspar”, common glauoonite and chert, scarce calcareous detritus and rare faecal pellets. The heavy mineral assemblage is probably T-epidote. 174. A fine greenish brown sand with a light fraction, 94 per cent, containing abundant “quartz with felspar”, common chert and limonitic aggregates and scarce calcareous detritus. The heavy minerals form 0.19 per cent of the sediment, the assemblage being T-epidote. 222. Similar in appearance to 174, The light fraction forms 78 per cent of the sediment and is composed of abundant “quartz with felspar”, common glauconite, scarce chert and limonitic aggregates, Probably contains a T-epidote heavy mineral assemblage. 209. A moderately coarse mottled sand containing abundant “quartz with felspar”, common limonitic aggregates and rare calcareous detritus. Probably contains a T-zircon heavy mineral assemblage. The sediment has a very low clay content. 202. A fine grained, dark brown sand with considerable clay size material. The light fraction constitutes only 42 per cent of the sediment and is composed of

The geoohemistry of modern sediments from the Gulf of Paria-I

3I5

scarce glauconite, chert and calcareous abundant “quartz with felspar”, detritus, rare mica and limonitic aggregates. The heavy mineral assembl&ge is probably T-zircon. The gulf clays 53 (20-60 cm below surface). A soft, greyish green mud in which the light fraction has dropped to O-3 per cent of the sediment and is composed of abundant “quartz with felspar”, scarce Orinoco quartz and calcareous detritus and rare glaueonite. The heavy minerals are probably exclusively zircon. X-ray study indicates considerable quartz, probably therefore of very fine grain size as also are the kaolinite, illite and montmorillonite present in the sediment. 47. Closely similar to 53. 538. Closely similar to 53. The light fraction, 0.3 per cent of the sediment, is composed of calcareous detritus. 33. Again similar to 53. No detectable light fraction. The sediment is composed entirely of fine clay size material. I 11. Very similar to 53. 554 (55-75 cm below surface). Very similar to 53. 345 (top). A stiff bluish clay containing faecal pellets and sparse caleareous macrofauna. X-ray study indicates a clay composition similar to 53. The basal 7 A peak of the kaolinite is somewhat sharper than in 53 perhaps indicative of somewhat greater crystal size. 345a (30-45 cm below surface). Similar in every respect to 345 (top). 345b (200 cm below surface). Similar to 3451top but with a more predominant calcareous mac~ofauna. 525. Similar to 345 (top). Iron rich fractions were separated from 77, 395, 158 and I58 (top), and will subsequently be referred to as 77(Fe), 395(Fe) etc. Kaolinite and illite were positively identified from these fractions. Previous work by FAVEJEE (reported in VAN ANDEL and POSTMA, 1954) established that, in the size grade less than 0*0005 mm diameter, kaolinite OCOUFS to the extent of lo-20 per cent throughout the gulf and delta, montmorillo~te varies from 5-10 per cent in the delta to 20-30 per cent in the gulf and illite varies roughly antipathetically with montmorillonite from &70 per cent in the delta to f50 per cent in the gulf. Several samples have subsequently been examined using a Phillips high-angle X-ray diffractometer instead of the photographic methods adopted by FAVEJEE. No attempt was made to separate the various size fractions. Quartz peaks are present on the diffractometer traces of all specimens and are often quite strong, even in the clays from the central parts of the gulf. Such strong peaks cannot be accounted for by quartz in the almost negligible coarse fraction of these clays (often less than I per cent) and indicate that some of the quartz must be of sub-silt size. The basal peaks of kaolinite, illite and montmorillonite, at “d” values of approximately 7 A, 10 A and, after treatment with ethylene glycol, I7 If respectively, are often low and broad in the clays. This indicates that these clay minerals are present in very small crystals, particularly montmorillonite, which may be almost para or meta colloidal. Much sharper basal peaks characterise the

316

D. M. FIRST

kaolinite and illite in the deltaic sediments, indicating that, in these sediments, the clay minerals form larger crystals, as previously suggested by VAX ANDEL and POSTMA (1954). The (h, k, o) peaks of i’tlite have pronounced “tails” towards lower “d” values, whilst the (h, k, 1) peaks are either ill defined or absent. This suggests that the illite sheets have a random layer displacement, possibly due to displacements which are simple fractions of the layer dimensions and of the type na,/3 or nb,/3 (BRIPU’DLEYet aE., 1951). It is likely that the illite in these samples is a degraded form with a deficiency of potassium in the intersheet positions. In view of the relatively high sodium contents of the analysed samples the Na illite ~brammallite) was sought. Characteristic reflections of this mineral could not be distinguished on the diffractometer traces, but some shading of the basal 10 A illite peak towards higher “d” values may reflect some substitution of K by Na together with Al by Pe3+ in this mineral. Authigenic glauconite, of Recent age, occurs in moderately to highly oxidising environments in the gulf. Structurally this glauconite resembles illite and VAN AX-DEL and POSTMA. (1954) have suggested that it is a transformation product of the latter mineral in oxidising conditions, in shallow water. Such a view, in contrast to the general hypothesis that glauconite formation requires reducing conditions, recalls the older theory of HADD~EG (1932). As shown later, support for the suggestion of VAN ANDEL and POSTMA can be derived from considerations of the chemistry of the glauconite bearing sediments. Faecal pellets, macroscopioally similar to glauconite, occur in the sediments to the north of the Oropuche bank, usually associated with shell beds. They lack the characteristic green colour of glauconite and there are no transitions, such as would have indicated glauconit,e fornlation from such pellets. METHODS OF AN_~LPSIS The majority of the samples were almost unconsolidated and could be reduced to a fine powder by grinding in an automatic agate mortar. The resulting powders were each thoroughly mixed by shaking in a large bottle to offset any selective crushing by the agate mortar. In certain cases iron nodules were separated from the samples for analysis by hand picking with fine tweezers. K,O, Na,O, total Fe as Fe,O,, TN,, MnO and P,O, were determined by methods essentially similar to those of SHAPIRO and BRAXXOCK (I 952). Carbon dioxide was determined by the method of SHAPIRO and BRAKNOCK (1955). Using techniques outlined by SHAPIRO and BEAXXOCK (1952) sufficient determinations were made of CaO, MgO and Al,O, contents to standardise a spectrographic method using Fe as a variable internal standard. Most of the determinations of these constituents were then made spectrographically. The results of the analyses are presented in Table 1. GEOCHEMISTRY OF THE ~;~AJORELE~WNTS K?icon

Although few analyses have been made of this element, those available indicate that Si varies roughly antipathetically with the clay mineral content. The five analyses of the clays show little variation with an average Si figure of 2ci.96 per

Si

H,O+ H,Oco,

1

~ ~

~ ~

’

0.29 1.01 9.52 0.76 1.53 0.07 3.01 3.52 11.5

n.d. 0.34 6.71 6.34

1 ~ ~

I__

~.

0.15 1.66 0.58 2.56 1.85 0.05 6.30 3.43 0.“3 i

md. 0.43 7.85 4.71

i

Delta

of the

n.d. 0.14 1.46 1.07 0.03 0.12 0.20 0.35 0.50 0.008 n.d. ILd. 0.08

139

0.11 I.54 0.70 1.83 2.01 0.05 6.57 4.63 0.36

0.53 8.02 4.8”

n.d. 23.16 0.48 10.07 5.43 0.2” 1.30 0.79 2.37 1.99 0.06 6.95 4.27 0.43 ’

2.50 0.03 0.54 6.53 0.81 0.87 0.04 1.69 1.41 6.20

0.25 3.27

r1.d. 0.46 8.74 4.92 0.18 1.78 1.11 1.62 1.95 0.07 6.39 4.63 0.40

1 /

i

I

//

s md. 0.32 4.39 4.27 0.03 0.40 0.80 0.89 1.30 0.05 n.d. n.d. 0.35

25.18 0.48 9.10 5.47 0.19 1.41 0.56 2.43 1.89 0.08 6.29 4.06 0.23 1

25.67 0.47 8.94 5.14 0.09 1.19 0.46 1.46 1.83 0.05 7.14 4.74 0.30

1. (Continued)

35.08 0.26 4.30 2.80 0.05 0.69 1.13 1.26 1.29 0.03 2.13 2.27 0.78

S 158

of sediments

152

I-

L

Table

1l.d. 0.30 3.80 4.88 0.11 0.55 6.43 0.74 0.78 0.17 2.74 2.12 4.20

S 77

composition

32.49

398

1L.d. 0.19 2.43 3.25 0.05 0.44 13.22 0.82 0.70 0.04 IA. n.d. 11.90

EM

EM 395

-

1. Chemical

E.M. Samples from the Erin Moruga platform S. Samples from the Soldado platform 0. Samples from the Oropuche bank

4.99 3.27 0.17

0.51 1.08

Ca. Na

~

0.04

0.69

Mn

Mg

29.00 ~ 0.52 8.16 4.21

n.d. 0.27 3.40 2.34 0.04 0.30 0.26 0.50 0.79 0.03 1.99 1.21 0.14

r1.d. 0.3‘2 3.80 2.27 0.04 0.32 0.46 0.44 0.93 0.02 2.65 1.73 0.22

Si Ti Al Fc

CO,

Ti Al Fe Mn Mg Ca NE K P H,O+ H,O-

611

587

HOCR Vagro

Dolta sands

Table

n.d. 0.55 7.41 4.83 0.09 1.22 0.82 1.37 1.88 0.08 5.96 3.29 0.31

md. 0.43 7.66 6.41 0.09 1.53 4.02 1.88 2.03 0.05 5.26 5.21 2.60

*345 (200 cm below

ad. 0.20 2.22 2.46 0.06 0.41 0.64 0.83 0.93 0.03 n.d. n.d. 0.24

0 174

L

27.24 0.47 7.96 4.74 0.08 1.20 0.59 1.28 1.83 0.05 6.88 4.42 0.23

*j.)g

md. 0.27 4.02 3.11 0.04 0.39 1.11 1.11 1.01 0.04 n.d. n.d. 0.72

I-

_

T-

-I

md. 0.24 3.75 2.44 0.05 0.63 0.86 1.15 1.09 0.03 1.91 1.92 0.47

-

n.d. 0.25 5.74 28.61 0.38 1.02 2.98 0.68 0.67 0.28 7.48 3.20 0.96

77 Fe

-

n.d. 0.23 4.04 2G.03 0.20 1.47 15.02 0.79 0.56 0.39 md. n.d. n.d.

Ft?

395

0

222

n.d. 0.27 4.26 16.11 0.12 0.26 1.43 0.73 0.86 0.20 4.70 0.99 0.46

158 Fe

n.d. 0.30 3.12 3.55 0.05 0.48 0.85 0.83 0.86 0.04 1.87 1.41 0.34

Iron rich fractions

n.d. 0.43 1.39 4.41 0.06 0.90 0.75 0.41 0 51 0.04 1.32 0.75 0.31

Clay from the Boca Vagre delta * Samplrs from t,he 01&r bluish cla:, n.d. Not determined.

n.d. 0.48 8.06 4.8% 0.09 1.32 0.78 1.38 1.99 0.04 n.d. r1.d. 0.52

I

42.37 0.14 1.85 0.98 0.002 0.34 0.43 0.73 0.91 0.04 0.91 0.83 0.16

171

0

sands

*345 30-45 cm brlow

R.V.

n.d. 0.31 4.29 4.65 0.06 0.65 0.84 0.96 1.13 0.05 3.21 2.41 0.38

S 58 (top

Platform

from the Gulf of Paria

318

D. M. HIRST

cent, which is below the average Si figure for argillaceous rocks quoted by RANKAMA and SAHAMA (1950) viz., 28.89 per cent Si. In the Paria clays Si occurs in four main positions: in kaolinite, in illite, in Earlier, evidence has been presented Ghat the montmo~llonit~ and as “free SiO,“. “free SiO,” is of very fine grain size, and thus may represent very fine detrital quartz dust or recrystallised colloidal SiO,. KRAUSKOPP (1956) states that SiO, in true solution is more common than colloidal SiO,, while RANKAMA and SAHAMA (1950) suggest that 95 per cent of the SiO, content of the parent rook remains in the solid products of weathering. Thus it appears more likely that the “free SiO,” of the Paria clays represents very fine detrital quartz dust rather than precipitated silica. Table 2. Comparison of the aluminium contents of the Prtria sediments with other sediments

-

Al -Igneous rocks (CLARKEand WASHINGTON, 1924) Composite of 253 sandstones (in CLARKE, 1924) Composite of 371 sandstones used for building (in CLARKE, 1924) Average 12 Platform sands Average 3 Delta sands Middle Cambrian Shale, Alabama (in CLARKE,1924) Average 28 shales (in CLAEKE, 1924) Average 52 terrigenous clays (in CLARKE,1924) Average 8 Mn and associated shales (Mom, 1955) Average 9 Greenstreak shales (SPENCER, 1957) Average 18 Black shales (SPENCER,1957) Average 6 Greenish muds Average 4 Bluish clays Delta clay

% 8.13 2.5 3.2 3.24 2.89

11.1 i

8.2 9.1 10.17

12.49 12.01 8.79 7.77 8.16

In the delta sands Al varies between 3.80 per cent and 1.46 per cent (average 2.89 per cent) whilst the platform sands vary between 4.39 per cent, and 1.39 per cent (average 3.24 per cent). The clays vary between 10.07 per cent and 6.71 per cent (average 8.22 per cent) with a trend to lower values in the cIays from the delta and the Soldado platform. In the iron-rich fractions the Al content is similar to that of the sands, averaging 4.68 per cent. The varying Al contents of the sands reflect the varying clay mineral content and are similar to those found by CA~WER~N (1957) in sands low in mica and clay and high in quartz and felspar. Table 2 compares the average Al contents of the different types of Paria sediment with average Al figures for other sediments. It is apparent that although the Paria clays have Al contents comparable to the average contents for argillaceous rooks quoted by CLARKE (1924) they are below the contents report&d by MOHR (1955) and SPENCER (1957). Intensive chemical weathering of source rocks, such as occurs in the Orinoco basin, might be expected to result in argillaceous rooks or sediments with a higher Al content than normal, but there is no evidence

The geochemistry of modern sediments from the Gulf of Paria-I

319

that this is so, Possibly the type and amount of clay mineral deposited, and further, the amount of substitution for Al in the octahedral positions are bigger The relatively low Al figures of the contributory factors to the final Al content. gulf clays may thus be due to the presence of considerable illite (with some substitution of Al by Fe3+) and montmorillonite, a mineral with a low AljSi ratio and The considerable substitution of Al by Fe3+ and Mg in the octahedral positions. general greenish colour of the sediments may be regarded as evidence of such substitution by Fe3+, since, according to KELLER (1953) such a green colour is indicative of ferric silicate. Table 3. Comparison of the average Ti and AlaO,/TiO, ratios of the Gulf of Paris, and other sediments Ti %

Magmatic rocks (CLAFZEand ~~ASH~~GTON, 1924) Composite of 253 sandstones (CLARKE,1924) Composite of 371 sandstones used for building (CLARKE,1924) Average 12 Platform sands Average 3 Delta sands Composite of 235 Mississippisilts (CLAREE,1924) i Composite of 78 shales (CLBRKE,1924) Average of 212 Terrigenousmuds and marine shales / / (G~LD~~Iw~DT, 1937) Average 12 claysfrom the Gulf of Paris

I

0.64 0.15 0.25 0.27 o-25 0.35 0.39 0.46 0.43

I Al,O,/TiO, Hydrolysate sediments (GOLDSCH~IDT, 1954) Average 12 Platform sands Average 3 Delta sands Average 6 Greenishmuds Average 4 Bluish clays Delta clay

25 15 13 18 21 18

The Ti contents of both sands and clays are closely similar to those found by previous workers for these types of sediment (Table 3). In the delta sands Ti varies between 0.32 per cent and 0.14 per cent (average 0.25 per cent) while the platform sands vary from 0.43 per cent to 0.14 per cent (average 0.27 per cent). On the platform Ti shows a slight increase in the near shore sands. Titanium is higher in the clays than in the sands, varying from 0.55 per cent to 0.34 per cent (average 0.47 per cent). There is no significant difference between the Ti contents of the delta clay and the gulf clays. In the iron rich fractions Ti averages 025 per cent. The ratio of Al,O,/TiO, suggests that Ti follows Al in a general way but that the geochemical coherence is not strong, (compare SPENCER, 1957). The rather higher Al,O,fTiO, ratio of t.he shales, compared to the sands suggests that in the

D. M. HIRST

320

latter detrital Ti minerals play a more prominent part. The heavy mineral fractions examined by VAN ANDEL and POSTMA (1954) however, were restricted to nonopaque varieties, amongst which Ti bearing minerals such as rutile, anatase and The increase in Ti relative to Al in the brookite form a very small percentage. sands is therefore probably related to the presence of such detrital minerals as ilmenite, the distribution of which was not reported by VAN ANDEL and POSTMA. CAMEROR (1957) has reported that the geochemistry of Ti in certain Liassic sands is dominat*ed by contributions of detrital minerals. GOLDSCHMIDT (1954) has suggested that very little Ti is chemically or physically bonded to clay minerals but that the Ti of hydrolysates is probably present as very finely crystalline TiO, (or TiO, hydrate) deposited along with the tiny flakes of The sedimentation intensity of the Ti appears to vary with the clay minerals. t,otal sedimentation intensity in the Gulf of Paris as the A1203/Ti02 ratio remains fairly constant from delta to gulf both in sands and clays. Thus it appears unlikely that much Ti was precipitated from solution and more probable that it was associated with the clay fraction during weathering and transportation. #odizcm and potassium In the delta sands K varies between

0.93 per cent and 0.50 per cent (average

0-74per cent) while the platform sands vary from f-30 per cent to O-51 per cent (average 0.95 per cent). Potassium is lowest in the sands from the open platform and highest in those from the Soldado platform, the distribution in the sands being no doubt related to their micaceous contents since K shows close coherence with Al. The K contents of the clays are higher than those of the sands, varying from 2.03 per cent to 1.53 per cent (average 1.87 per cent). Potassium is lower in the delta and platform clays than in those from the gulf, the latter exhibiting little variation in content. In the iron rich fractions the average K content is O-70 per cent. In the delta sands sodium averages 0.43 per cent while the platform sands vary from 1.15 per cent to O-41 per cent (average 0.85 per cent). Sodium varies with Al in the platform sands but the sympathetic behaviour does not extend to the delta where Na values are low. In the clays Na varies from 2.56 per cent to O-76 per cent (average 1.67 per cent). The lowest value is for the platform clay whilst low values are also shown by the delta clay and clays from the northern and north-eastern parts of the gulf, the latter including the bluish clays. Sodium contents are higher than normally found in argillaceous or pelitic sediments. In the iron rich fractions Na averages 0.73 per cent. Compared with igneous rocks the Paria sediments show the usual depletion of Na relative to K, but the extent of this is not as great as that normally found in sediments (Table 4). The deltaic sediments have the lowest Na/K ratios though not as low as those quoted for the average argillaceous sediment. Degraded illite is abundant in the clay fraction of these deltaic sediments. JACKSON et al. (1952) have suggested that degraded illite, low in K and relatively high in Na is produced during weathering by removal of K through the agencies of leaching and cropping by plants. The Orinoco basin is heavily vegetated and thus the low K contents and relatively high Na/K ratios of these sediments may be due to the removal and fixation of K by plants which have a decided affinity for this element.

The geochemistry

of modern

sediments

321

from the Gulf of Parin-

In an &tempt to account for the relatively high Na contents of these sediments several of the chemical analyses have been recalculated in terms of the probable mineral composition based on the knowledge of t.he minerals actually present. The wide 1imit.s of substitution possible in cert,ain constituents of t.hc I’aria sediments imply t,hat, such calculations will be, to some degree, hypothetical. T&lc

4. Average

h’n and R contenb

in the l’aria

and ot,hcr sedimcnt,s ’

Sn 0,

__ _ _-..

/‘0

_~_..

_.-

-I

.-

/ 2.83

Igneous rocks (I~.~NLuL~ and SAHAMA, 1950) .+gillaccous setliments (RASKAMA tend SAHA~~A, 1950) Averugc 3 Delta sands Average 12 Platform sands A\rcrage 6 Greenish muds A\:erage 4 Bluish clays Delta clay _. -_._----. _

_. ~_

Rate .--

Del& clay C;reenish muds Uluish clays

of deposition _.---

-_..-.

100 cm/century approx. 50 cm/century npprox. 10 cm/century

-

/

Ii 01

! n;a/K

:” ; .____. .._

1.09

2.59 I

; 0.97 * 0.43 0.88 2.05 148 I.08 -

2.70 0.74 0.95 1.93 1.93 1.64

..-._

/ 0.36 / 0.58 0.93 1.06 0.71 0.66

~

Clay minerals .--

1 illito dominant ’ illitc and mont~morillonito : illite am1 mont.morill0nit.e

The analysis of sample 554 (a typical gulf clay) n-as initially recalculated the following formulae: lllitc;

K,Fc,3+Al,Si,0,,(OH)4

Montmorillonite;

Pl’a,MgAl,Si,,O,,(OH),,

Kao1init.e;

Al,Si,O,(OH),

Limonitc;

Fe,O,(OH),

Quartz;

550,

Calcit,e;

CaCO,

using

The recalculated percentages of these various constituent,s were: Mite, 20.1 per cent, Montmorillonitc, 36.8 per cent, Kaolinitc, 7-O per cent, Quartz 26.4 per cent, Limonitc, 4-O per cent, Calcite, 0.7 per cent. According to this calculation the clay minerals constitute 63.9 per cent of t,he sediment, the fraction being composed of illite 31.6 per cent, montmorillonit~e 57.6 per cent and kaolinite IO+ per cent. VAN ANIIEI, and POSTMA (1954) state, however, that t,he clay fract.ion of sediments from the gulf proper is composed of illite +50 per cent, montmorillonite

D. MI.HIRST

322

20-30 per cent and kaolinite 19-20 per cent. The agreement theoretical formulae were therefore adopted: viz. Illite;

(Na,K),B’e,3fAl,Si,0zo0,

Montmorillonite;

(Na,Ca)Mg,Al,Si,,O,G(OH),,

Kaolinite;

Al~Si~O~(OH)~

Quartz ;

SiO,

Limonite;

Pe@,tOH),

Calcite;

taco,

Half of the available

is poor.

Revised

Na,O was placed in the illite, the remaining half, plus after formation of CaCO,, being located in the intersheet The latter was also made more MgO rich as considerposition of montmorillonite. able MgO was unaccounted for by the previous formula. The percentages of the various constituents now became: Illite, 33.6 per cent, ~o~tmorillonite, 20.5 per cent, Kaolinite, 12.2 per cent, Quartz, 27.8 per cent, Limonite, l-5 per cent, Calcite, O-7 per cent. The clay minerals now constitute 663 per cent of the sediment, this fraction being composed of illite, 50.7 per cent, montmorillonite, 30.9 per cent and kaolinite 18.4 per cent; a very close agreement with the estimates made by VAN ANDEL and POSTMA (1954) from X-ray studies. Although the above formulae are hypothetical this treatment of the analytical data suggests that the illite contains considerable Na+ in the intersheet positions; with an approximate K,O : Na,O molecular ratio of 3:2. The assignment of Pe,O, and MgO however, may not be correct, as there is sti1l.a small surplus of the latter constituent. Chemical analyses have also been re-calculated for 592, a clay from the outer delta, and 587, a sand from the inner delta. The revised compositions were retained and a small amount of felspar, reported to occur in these sediments, was taken account of in the recalculations. The results were: 592. Illite, 29-9 per cent, Montmorillonite, 8.2 per cent, Kaolinite, 14.9 per cent, Quartz, 39.3 per cent, (Na, Ca) Plagioclase, 3.5 per cent, Limonite, 0.8 per cent, Calcite, O-5 per cent. Clay minerals thus constitute 53 per cent of the sediment, composed of illite, 57.4 per cent, montmorillonite, 15.5 per cent and kaolinite, 28.1 per cent. 587: Illite, 16.8 per cent, Montmorillonite, 1.7 per cent, Kaolinite, 5.7 per cent, Quartz, 68.6 per cent, (Na, Ca) Plagioelase, 3.3 per cent, Calcite, O-5 per cent. Clay minerals constitute 24.2 per cent of the sediment, composed of illite, 64.4 per cent, montmorillonite, 7-O per cent and kaolinite, 23.6 per cent. This agrees well with VAN ANDEL and POSTMA (1954) who report that the clay fraction from the inner delta contains &70 per cent illite, 5-10 per cent montmorillonite and 10-20 per cent kaolinite. Thus the mineralogical composition of these sediments, derived by re-calculation of the chemical analyses, agrees quite closely with the modes, semi-quantitatively derived from clay mineral and light fraction studies, provided that the K,O : Na,O molecular ratio in the illite is approximately 3 : 2. JOHNS and GRIM (1958) have suggested that illites in the ~ssissippi delta similarly contain considerable Na, basing their assumption on the variation of Na

surplus CaO remaining

The geochemistry of modern sediments from the Gulf of P&a-I

323

with the content of illite. Probably the Na, postulated in the illite of the Paria sediments, was absorbed to compensate for the loss of K from inter-sheet positions during the degradation of the illite. Although the muds and clays of the central gulf have lower sedimentation rates than the deltaic sediments there is a notable increase in the Na/K ratios of these sediments, particularly in the greenish muds which have been shown to be much richer in the mineral montmorillonite than those of the delta. This difference in sodium content may be ascribed, in part at least, to mineralogical differences. A quite pronounced difference exists between the greenish muds and bluish clays, the latter having a lower Na/K ratio, as these sediments have a comparable clay mineralogy, this may reflect the lower sedimentation rate of the bluish clay. A separation of Na and K in the sedimentary environment may have become effective in the bluish clays, K replacing Na. The rather sharper basal reflections on the diffraction patterns for the bluish clays would lend support to this theory. The assimilation of K and loss of Na which takes place in the reconstruction of degraded illite need not take place simultaneously with deposition and it may be suggested that geochemical equilibrium cannot be attained in the sedimentary cycle unless the rate of deposition is below a certain value. The evidence from the Paria sediments suggests that this value is less than 10 cm/century. An alternative, or perhaps contributory, explanation could be that the gulf waters have an extremely low K content. The Orinoco river waters are reported to have a very low salinity (VAN ANDEL and POSTMA, 1954) and thus, not only would water entering the Boca Vagre be deficient in K, but also that entering by the Serpents Mouth, as much of the latter derives from the main Orinoco delta region. The salinity of the Amazon river water is also low (RANKAMA and SAHAMA, 1950) with Na constituting 4.24 per cent of the dissolved solids and K, 4.76 per cent, as opposed to average contents from all rivers of 5.79 per cent Na and 2.12 per cent K. Thus it appears that in the Amazon drainage basin, and also probably that of the comparable Orinoco, there It is is not the usual depletion of Na relative to K in the products of weathering. suggested that, provided the sedimentation rate is high, the Na/K ratio may be similar to that of igneous rocks for sediments containing considerable montmorillonite, the mineralogical components of which were formed under tropical or equatorial conditions such as those of the Amazon and Orinoco drainage basins. The chemical analysis of a glauconite sand, sample 222, from the Oropuche Bank was recalculated into compositions of only those minerals for which there is definite evidence. Results differing considerably from the mode were obtained unless the K,O : Na,O molecular ratio assigned to the glauconite was 3 : 2. Adopting a glauconite formula of (K, Na),O, MgO, Fe,O,, Al,O,, 6SiO,, 2H0, and placing all surplus Na,O and CaO in felspar, the sediment was then found to contain the following percentages of the major constituents: Glauconite, 14.3 per cent, Kaolinite, 4-O per cent, Quartz, 66.4 per cent, (Na,Ca) Plagioclase, 9.5 per cent, Calcite, 0.8 per cent, Limonite 2.4 per cent. The result is in agreement with the figures presented by VAN ANDEL and POSTMA (1954) who reported abundant quartz with felspar (50-90 per cent), common glauconite (lo-50 per cent) and scarce calcareous detritus and limonitic aggregates (l-10 per cent). Since glauconite appears to possess the same K,O:

D. M.HIRST

324

Na,O ratio as the illite these ~aleulations support the view of VAN ANDEL and POSTMA that this glauconite is, in fact, reconstituted illite.

No analyses were made of FeO, all distributions thus referring to total iron. Iron varies with Al throughout the gulf except in sediments in which limonitic concretions and/or glauconite are evident. Iron averages 1.90 per cent in the delta sands while the platform sands vary from 4.88 per cent to O-98 per cent (average 3.28 per cent). In the clays the average is 5.15 per cent with variation from 6.41 7 0

0

6-

/ x

0 Sediments glauconite

with

% Fe OS tdtel

Fig.

limonile

and/w

iron

% Al 4. Variation of total Fe with Al in the Gulf of Paria sediments.

per cent to 4.21 per cent. Iron averages 23.58 per cent in the iron rich fractions and varies from 28.61 per cent to 16.11 per cent. In the Paria sediments Fe may occupy two main positions. (a) As Fez+ or Fe3+ substituting for Mg or Al in the lattice of the clay minerals. (to) As Iimonitic nodules, concretions or aggregates, chiefly found in the platform sands. Examination of Fig. 4 clearly demonstrates that, while the Fe/Al ratio is reasonably constant in the sediments relatively free from glauconite or limonitic nodules, values of this ratio in limonitic or glauconitio sediments are higher and more variable. Sediment colours and EH and pH distributions (for details, see VAN AWDEL and POSTMA, 1954) suggest that the majority of Fe is present as Fe3+. The entry of such Fe 3+ into the lattice of the clay minerals could have taken place in the basin of deposition, or at the source. The constancy of Fe/Al ratios across the gulf, despite varying rates of sedimentation, suggests fixation of the Fe at the source or during transportation. ~~ont~norillonite forms under conditions in which Fe could be fixed as Fe3f, namely oxidising and alkaline. On the other hand the conditions of formation of kaolinite are such that some, at least, of the Fe may be dissolved in the source areas and carried to the gulf in solution as Fe2+. It has been suggested earlier that glauconite is formed by the reconstitution of illit?e and the

325

The geochemistry of modern sediments from the Gulf of Paria-I

necessary substitution of Fe w for A13+ implies some entry of Fe from solution. This Fe would probably form positively charged Fe(OH), sols and in this form be attracted to the clay minerals. VAN ANDEL and POSTXA (1954)have suggested that the limonite nodules are residual from weathered Pleistocene soil profiles. The low H,O contents of the limonitic nodules (rather low for Fe(OH), precipitates) support this view, as do RANKAMA. and SAHAMA (1950)have the presence of quartz and clay impurities. reported that transported laterites often contain such impurities. The conditions where such nodules ooour are oxidising and alkaline, ideal for the precipitation of Fe from solution and it is, therefore, unlikely that there would be any dissolution of residual limonite already present. Table 5. Comparison of the Fe and Mn of the Paria sediments with average figures compiled by various workers -7

Fe

1

--. Igneous rocks (GOLDSCH~DT, Sandstones (in CLARKE 1924) Shales (in CURKE 1924) Average 3 Delta sands Average 12 Platform sands Average 6 Greenish muds Average 4 Bluish clays Delta clay Limonitic concretions

1937) 1 i

* CLARKE and WASH~~GTO~(~~~~) 7 &kTAMI (1935) : CAMERON (1967)

j /

%

1 j

Mn

%

Mn/Fe

5.0

0.10”

0.99

trace

4.71

0.067

1.90

0.04 0.07 0.20

0.020 0.007 0.013 0.019 0.015 0.032

0.11 0.05 O-23

0.017 I 0.010 0.010

3.28 5.08 5.20 I 4.21 / 2558

Fe/Al

-

0.60 O-50$ 0.61 0.66 1.01 0.58 0.67 0.52 j 5.07

Manganese In the delta sands Mn averages 0.04 per cent while the platform sands vary from O-11per cent to 0.002 per cent (average O-05 per cent). Manganese follows total Fe quite closely in the sands. In the clays Mn varies from O-29 per cent to 0.04 per cent (average O-13 per cent). The gulf clays show a fairly uniform Mn content although the clays, including the bluish clays, from the north and northeastern parts of the gulf have a lower Mn content than the remaining greenish muds. In the iron rich fractions Mn averages 0.23 per oent. Some separation of Mn and Fe is evident as the Mn/Fe ratio is lower in the delta clay and limonitic nodules and higher in the greenish muds than the average for igneous rocks (Table 5). Like Fe, Mn is dissolved at low EH and pH although it is possible that much of the variation in Mn/Fe is attributable to their separation at or near the site of weathering. Such a process has previously been advocated by VOCT (1906),HANSON (1932),HEWETT (1932)and LJUNQGREN (1953). Fez+- in solution is more easily oxidised than Mn2+ and is stabilised as Fe,O, or Fe(OH), whereas Mn2+ oxidises to Mn(OH),, Mn(OH), or MnO,. At the site of weathering Mn may, like Fe, enter developing minerals such as montmorillonite or it may be co-precipitated with Fe, Fe(OH), sols being +ve charged and Mn(OH), sols -ve 18

D. M. HLRST

326

charged. The lower Mn/Fe ratio (O*OlO)of the limonitic nodules, presumably residual, compared with that of igneous rocks (O-020) does not support the latter l~ypothesis but rather suggests a preferential retention of Fe with a relative concentration of Mn in the transporting solutions. The extraotion of Mn carried to the gulf in solution appears to be highest in the area occupied by the greenish muds (average Mn/Fe 0.032). The extraction of Mn2+ from solution by precipitation as Mn(OH),, Mn(OH), or MnO, appears unlikely in the gulf, as the EH of the area, approximately 0.22-0.24 v, locally reaching 0.32 v, is probably insu~cient. KRAUSIL~PF(1957) states that in solutions containing 10-a mole/l Mn at an EH of O-2 v, Mn is not precipitated until the pH has reached 9.5. ZOBELL(1946) gives -0.5 to $0.35 v as the usual EH range of sea water and suggests that manganese oxide can only form at the extreme upper end of this range, again outside the average values for the Gulf of Paria. Whatever the agency of extraction of Mn from solution the higher contents of the greenish muds are probably related to the strong clockwise eddy current developed in the central and eastern gulf (Fig. 2). This current is believed to be a relatively recent feature not developed during the period of bluish clay deposition. An examination of Mn/Fe ratios and Mn contents of the individual greenish muds lends support to this theory. Bin/Feratio

in the greenish muds

Muds influenced by the eddy

111 &In/Fe %Mn

1 /

0,036 0.19

/ / 1

33

/

538

j

/p.....-_

O-046 i 0.22 j

Muds not influenced

__._~

0.037 0.18

/ i

47

0.031 0.15

/ ---,

/

53 0.022 0.11

554 / /

0,018

O-09

.--__592 / /

0.010

0.04

The insignificance of carbonate precipitation on the geochemistry of Mn may be illustrated by comparing Ca rich and Ca poor sands. Ca rich sands

Ca poor sands

__ I %Ca %Mn

398 6.53 0.03

/

j I

395 13.22

0.05

77 6.43 0.11

152 --

_-

/

1.13 0.05

158

158 (top)

0.80 0.03

0.84 0.06

-

/

Magnesium averages 0.24 per cent in the delta sands while the platform sands vary from 0.90 per cent to 0.34 per cent (average 0.54 per cent). There is little systematic variation on the platforms. In the clays Mg varies from 1.78 per cent to O-69 per cent (average l-32 per cent). Relatively low values are found in the delta clay, platform clay and north and north-eastern parts of the gulf. Comparing the Mg content of the argillaceous sediments with the average figures of other workers, Table 6, it will be seen that the Mg content of the gulf sediments

The geochemistry of modern sediments from the Gulf of Par&-I

327

is relatively high. The constancy of Mg/Al ratio suggests that Mg lies chiefly in The lower Mg/Al ratio of the delta sediments, relative to the the clay fraction. greenish muds and bluish clays is probably related to the higher concentration of montmorillonite in the latter sediments. Magnesium is probably included into the structure of montmorillonite at, or near, the site of weathering, otherwise variations in Mg/Al ratio might be expected between the greenish muds, bluish clays and platform sands. It is interesting to note that the Fe/Al ratio remains reasonably constant into the delta and thus, whilst Mg is probably confined exclusively to montmorillonite, Fe probably occurs structurally replacing Al in both illite and montmorillonite. It is, of course, possible for some Mg to be incorporated into the sediments from solution. Table 6. Comparison of the Mg in the Paria sediments with the average values derived by other workers

Igneous rocks (RANKAMA and SAEAMA, 1950) Argillaceous sediments (RANKAMA and SAHAMA, 1950) Soils, shales and muds (GOLDSCHMXDT,1954) Average 3 Delta sands Average 12 Platform sands Average 12 Paris clays. ll___----__~_ _ _....__~

-._-

Igneous rocks (WICKMAN, 1954) Shales (WICKMAN, 1954) Liassic sands (CAMERON, 1957) Average 3 Delta sands Average 12 Platform sands Average 6 Greenish muds Average 4 Bluish clays Delta clay

-

I I 1

MglAl

I

I

Mg/Fe

-

0.23 0.15 0.07

0.08 0.19 0.17 0.17 0.08

1.26 0.89 0.90-1.51 0.24 0.54 1‘32 __-

/

I

0.38 0.24 0.13 0.13 0.18 o-29 0.25 0.16

The sedimentation rate of elements precipitated or absorbed from solution may be expected to remain relatively constant while the total sedimentation rate varies. Thus, if much Mg was incorporated from solution, the Mg, and MgfAl ratios would be lowest in the delta where the sedimentation rate is highest. GRIM (1953) has stated that at lower pH values there is a seduction of the ion exchange capacities of clays present. The delta sediments are at a pH of 6.5-7.5 whilst those of the open gulf have pH values approximating to 8.0. Such relatively small pH changes could, though it is considered unlikely, affect the ion exchange capacities of the clays and thus also help to reduce the’Mg values of the delta sediments. On the other hand the similar Mg,/Al ratios of the greenish muds and bluish clays which accumulated at different rates (see Tables 4 and 6) suggest that relatively little of the Mg of these sediments was incorporated from solution. It is possible that ooncentration in the clockwise eddy current, as was postulated for Mn, was responsible for elevating the ratio in the greenish muds to a figure comparable to that of the

D. MI. IF_ibST

328

bluish clays. Such a hypothesis would not, however, explain the similarity in Mg/Al ratio between the latter and the platform sands, nor can this be attributed to concentration of Mg by carbonates in the platform sands. Examination of sediments notably rich in calcareous organic detritus indicates no increase in Mg or MgJAl ratio relative to other sediments, and it is evident that l%g is not concentrated in these carbonates* This is probably due to the carbonate being aragonite, since CRAVE (1954) has stated that aragonite rarely contains more than 1o/0MgCO,. Thus the balance of evidenoe suggests that little if any of the Mg in these sediments was incorporated from solution. Table 7. Average calcium figures for the gulf sediments, with the average values for igneous rocks and argillaceous sediments

I -“-

i

/ Igneous rocks (~Al?KAMA and SAHABIA, f950) Argillaceous sediments (RANKAMA and SAHAMA, 1950)I Average 3 Delta sands Average 3 Open platform sand8 Average 9 Protected platform sands Average 6 Greenish muds j Average 4 Bluish clays

Bdta. day

; /

Cs I%) 3.63 2.23 0.31 8.73 0.84

;:;; 041

Calcium, carbon dioxide alzd phosphorus The average calcium contents of the various types of sediment found in the gulf are shown in Table 7. Calcium is low in the delta sands which average only 0.31 per cent while the platform sands exhibit wide variation from 13.22 per cent to 0.43 per cent. This enormous variation of Ca reflects the varying content of calcareous organic detritus which appears to concentrate more on the open, than on the protected platform. If Ca required to satisfy CaCO, and Ga,(PO,), is subtracted from total Ca contents, the different sand types show less variation in Ca content although values are still low in t#hesands of the delta and high in those of the open platforms. That the latter &ill exhibit high Ca values may reflect some inaccuracy in CO, determinations at high concentrations. In the clays Ca varies from 9.52 per cent to 0.46 per cent (average 1.70 per cent). This average figure is somewhat misleading due to the presence of considerable organic remains in 68 and 345b_ The average figure of 0.70 per cent for the greenish muds is probably more representatise of the average total Ca content of these clays. After subtraction of Ca to satisfy CaCO, and C+(PO,), the average Ca figure of the clays is 0.39 per cent while that of the greenish muds is 0.29 per cent. In the iron rich fractions the average Ca figure, after subtraction to satisfy CaCO, and Cas(PO,), is 1.10 per cent. Calcium can probably occur in four main positions in the gulf sediments. (a) As GaCc), in she11debris or inorganic precipitates, (b) As oalcium phosphate. (of In 12 fold co-ordination in montmo~ilIoni~. (d) In felspars, mainly Na, Ca, plagioclase.

The geochemistry of modern sediments from the Gulf of Park-1

329

The possible significance of three of these different locations for the Ca will be discussed in turn. (a) The sympathetic variation of Ca with CO, indicates the importance of CaCO,. In the delta sands CO, averages 0.15 per cent, reflecting the virtual absence of calcareous organisms, whilst in the platform sands the average is 2-17 per cent and values vary from 11.9 per cent to 0.16 per cent. In the clays CO, varies from 1 I.5 per cent to 0.17 per cent. VAX ANDEL and POSTMA (1954) have drawn a distinction between organically and inorganically precipitated CaCO,; the latter being determined by analysis of the size grade less than 0.05 mm in diameter. VAN ANDEL and POSTNA found that the absolute percentages of CaCO, in this fraction are very Jaw, varying from O*O-4.2 per cent increasing towards the marine zones of the Bocas de1 Dragon and Serpents Mouth and decreasing into the estuaries of the rivers draining the Orinoco basin. The distribution pattern suggests that no detrital CaCO, is supplied by the rivers. RANKAMA and SAHAMA (1950) state that in tropical seas there is often supersaturation of CaCO, and that a lowering of salinity would result in precipitation. Evidently this may happen in the Gulf of Paria where salinities seldom exceed 20x, as compared with the 35%, of the open ocean. The bulk of the CaCO, is, however, of organic origin. ZELLER and ORGY (1956) state that more CaCO, is separated by organisms in warm seas than cold seas, and further, that the warmer waters of the tropics favour the extraction and precipitation of CaCO, as aragonite rather than calcite. Calcareous fractions and shells were examined from several samples by means of the Phillips high-angle X-ray diffractometer, and in every case shown to be aragonite. (b) The precipitation of Ca, as calcium phosphate, is controlled by pH in the same manner as the precipitation of CaCO,, increases in pH causing decreasing solubility. Phosphorus contents average 0.02 per cent in the delta sands whiIst in the platform sands contents average 0.05 per cent and vary from 0.17 per cent to 0.03 per cent. Within the platform sands there is little variation. In the clays the average P content is only slightly higher (0.06 per cent) than that of the sands. Most specimens lie within O-01 per cent of the mean. Calcium carbonate is more readily precipitated than the phosphate and thus concentrates relative to the latter in calcareous deposits. The lower P/Ca ratio of the sands (0.018) as compared to the clays (O-035) may be partly attributed to this fact. Calcium phosphate in sediments may also be of organic origin, RANKAMA and SAHAMA (1950) state that P can be concentrated by phytoplankton which form the food of many marine organisms. They further state that this P may be deposited in the sediment by excretion in faecal pellets. VAX ANGEL and POSTI\IA(1954) report such faecal pellets in many gulf clays. In core 346 they report that the faecal pellets are more concentrated towards the top where the P content is also higher as the following figures show: 345 (top). O*OS% P. 345 (30-45 cm).

0.04% P.

345 (200 cm).

0.05% P.

Although no detrital npatite was reported in these sediments by VAN ANDEL POSTMA (1954),the occurrence of this mineral, very finely divided, is possible.

and The

D. Iif. HIRST

330

amount of apatite required to utilize the reported P would only be very small; of the order of O-1 to O-2 per cent of the sediment. In most sediments however, the P contents are so low that only small proportions of the Ca can be located in calcium phosphate. The P contents of the iron rich fractions are higher, averaging 0.29 per cent, i.e. 5 to 10 times more than the amounts present in the normal sands and clays. It is probable that in the oxidation of Fe 2+ to Fe3+, P is fixed as the very insoluble basic or normal ferriphosphate, as suggested by GOLDSCHMIDT (1954).The formation of such a fe~iphosphate may be attributed to primary adsorption resulting from the attraction of negatively charged phosphate anions to slightly positively charged Fe(OH), ~01s. Thus there is no reason to believe that substantially more Ca is present in phosphate minerals in the iron rich fractions than in the remainder of the sediments. (c) Calcium probably enters the gulf, from the Orinoco drainage basin structurally held in montmorillonite, the conditions of formation of this mineral being suitable for the retention of Ca. VAN ANDEL and POSTMA (1954) state that little Ca is present in the Orinoco river system, either as Ca2+ ions or as Ca(HCO,),. Calcium therefore probably occurs in 12 fold co-ordination in the montmorillonite helping to balance charge deficiencies due to substitution of Al by Mg. The large size of the Ca2+ ion would preclude its entry into the octahedral positions. RANKAWA and SAHAMA (1950) state that the main agent for the removal of Ca in the seas is precipitation, adsorption on clays playing only a very small part, and it thus seems reasonable to assume that Ca present in the clays did not derive from the sea water but rather that it entered the structure at or near the source of these minerals. After subtraction of Ca to satisfy CO, and P,O,, the Ca content of the sediments, particularly the clays, is comparable to the average for 212 marine hydrolysates in GOLDSCHMIDT. (1954) O/oC;t. 2 12 marine hydrolysates 0.36 o-14

3 delta sands 9 protected

platform

Clays (excluding

sands

34533)

0.39 O-28

The slightly lower Ca content of the clays compared to the average for 212 marine hydrolysates may be a further indication of the Ca, deficient nature of the source area, composed mainly of igneous and high grade metamorphics, sands and shales. Water Table 8 clearly illustrates that the water content of the Paria sediments is rather higher than that of the average shale. This fact may probably be attributed to greater adsorption of water on the very small grains of the component clay minerals, whilst differences between the delta clay and gulf clays suggest that the montmorillonite has more associated water than the illite and/or that the grain size increases into the delta. The variation of H,O+ with H,O-, illustrated in Fig. 5, suggests that water is exclusively associated with the clay minerals. A similar variation was found by CAMERON (1957) in certain Liassic Sands. Such close

The geochemistry

of modern sediments from the Gulf of Paris--I

331

Table 8. Water

-

H,O+ _‘_

Composite of 78 shales Composite of 52 terrigenous muds Average 3 Delta sands Average 12 Platform sands Average 6 Greenish muds Average 4 Bluish clays Delta clay

-1

x

Limonitic

(%) ____2.32 1.97 6.61 6.03 4.99

H,O-

Total water

(%I

(%I

1.47 1.64 3.96 3.97 3.27

5.02 7.17 3.79 3.61 10.57 IO.00 8.26

concretions

Fig. 5. Variation of H,O+ with H,O-

in the Paris sediments.

of H,O+ with H,O- suggests that much of the so-called adsorbed or incidental water (H,O-) is more closely associated with the component minerals than may have previously been believed. The temperature at which H,O- is driven off (1lO’C) would probably be sufficient to drive off intersheet water from such minerals as montmorillonite. With the probable exception of H,O- recorded for two specimens lying well away from the curve (see Fig. 5), much of the water present in these clays and sands, both H,O+ and H,O-, is probably related structurally to the component minerals. The temperature of 110°C is arbitrarily chosen and thus, in sediments, the terms H,O+ and H,O- may have little significance other than the type of bonding in the mineral. H,O- would probably occupy intersheet positions whilst H,O+ would probably occur within the octahedral layer as (OH)- ions. variation

CoNCLUsIoNs

Mineralogical studies have confirmed that illite predominates over montmorillonite in the deltaic region whilst the latter becomes more predominant in the open

332

D.M. HXRST

gulf, and that the clay particles are of a very fine grain size. The large drainage basin of the Orinoco provides sufficient variation for conditions required in the formation of illite and kaolinite and aonditions required in the formation of l~ontmorillonite to occur in the source area. It seems very unlikely that these minerals formed in situ in the basin of deposition. The high rate of deposition in this basin, 100 cm/century in the delta and >50 cm/century in the greenish muds, the latter composed essentially of clay minerals are not favourable to the formation of clays by precipitation of ionic species from solution, particularly when it is considered that the Orinoco drainage syst,em contains a low content of dissolved matter. The geochemistry of the major elements is largely controlled by the clay minerals which are the hosts of the majority of these elements. Silicon is of course a notable exception since quartz is present in abundance. The main location of Al is in the clays. it is suggested that the content of Al is controlled by the type and amount of clay mineral deposited, and further, the amount of substitution in the octahedral positions rather than the type of weathering of the source rocks. The Ti/Al ratio suggests that Ti follows Al in a general way but that geochemical ooherence is not strong. It is likely that little or no Ti enters the basin of deposition in solution. The Na/K ratio is high compared to the average argillaceous sediment. This is attributed to the presence, not only of montmorilloI~ite with Na in iutersheet positions, but also to degraded illite containing Na. The presence of Na in this illite and the relatively low K content of the sediments are attributed to the degradation of illite either at the source or during transit, probably through removal of K through leaching and cropping by plants. The failure to replace the Na in the illite, by K, after deposition has been attributed to K deficiency in the gulf waters, a theory supported by the low salinity of the Orinoco river waters, and/ or the fact that the rates of sedimentation may be too great to permit geochemical equilibrium to be attained. The glauconite present in the sands surrounding S.W. Trinidad would appear to possess the same K,O/Na,O ratio as the illite and would tend to support the theory that this mineral can be a re-constituted illite. Iron would appear to either substitute for AP in the lattice of clay minerals or occur as limonite in the sands surrounding SW. Trinidad. Evidence suggests that the Fe is present as Fe3+ and that it probably entered the clay mineral lattice at or near the site of weathering. The limonitic concretions exhibit features suggestive of residual origin rather than precipitation from solution in the basin. Some separation of Mn and Fe is apparent, the evidence suggesting that this is due to the relative ease of oxidation and s~abalisation of Fe as Fe,O, or Fe(OH),. Manganese would appear to be an exception amongst the elements examined in that it is probably carried largely in solution into the basin of deposition. The geochemistry of magnesium appears to be largely controlled by the mineral montmorillonite. Calcium and carbon dioxide are largely accounted for by the calcareous organic remains, the CaCO, being extracted from the sea water with little or no contribution from the rivers of the drainage system. The CaCO, is in the form of aragonite. There is a close correlation between H,O+ and H,O- which suggests that both may be closely connected in the clay minerals. H,O- probably occupies intersheet positions and H,O+ positions in the octahedral layer,

The geochemistry

of modern

sediments

from the Gulf of Paria-I

333

Acknowledgements-The experimental work was carried out in the Department of Geology, The University of Manchester. The author is indebted to Dr. J. ZTJSSMAN for discussion of X-ray diffractometer data, to Mr. J. J. FAWCETT for analyses of SiO, and especially to Dr. G. D. NICHOLLS for constructive criticism during the experimental work and preparation of the manuscript.

REFERENCES BRINDLEY, Q. W. (1951). X-ray identification and crystal st,ruct,uros of clay minerals. -7ipiuz. 80~. Land. CAMERON, E. M. (1957). Unpublished Ph.D. Thesis. University of Manchester. Cnav~, K. E. (1954). Aspects of the biogeochomistry of Magnesium I. Calcareous marine organisms. J. Geol., 6B, 266-283. CLARKE, F. W. (1924). The data of geochemistry. U.S. Geol. Surv. Bull. 770. CLARK, F. W. and WASWINGTON, H. S. (1924). The composition of the Earth’s crust. U.S. Gco2. Sum. Prof. Paper 127. ~oI~Ds~~~~DT, V. M. (1937). Geochemische ~erteilun~gesetze der Elemente IX Die Mengenverhaltnisse der Elemente und der Atom-Arten. SIcrifter i?r’orske Videnskqws A&x&. Oslo, I Ma.t.nature, Klasse, No. 4. GOLDSCHMIDT, V. M. (1954). Geochemistry. Clarendon Press, Oxford. GELIM, R. E. (1953). Clay mineralogy. McGraw-Hill, New York. HALOING, A. (1932). The pre-quaternary sedimentary rocks of Sweden. IV. Glauconite and glauconitic rocks. Lunds Univ, Arsskn. .Nli’. 2, No. 2. Hansox, G. (1932). Manganese deposits of Cauada. Canada Dept. Mines. Geol. Surt*. Bon. Geol. Series No. 12, 120. In Treatise on ~Se~~~~e~tat~~~ (Edited by HEWETT, D. F. (1932). Manganese in sediment,s. W. H. T~~EN~oF~L) pp. 562-581. Williams and Wilkins, Baltimore. JACKSON, M. L., HSEUNC, Y., COREY, R., EVANS, E. J. and VANDEN HEUVEL, R. C. (1952). II. Chemical weathering of Weathering sequence of clay size minerals in soils and sediments. layer silicates. Proc. Soil. Sci. Sot. Am,er. 16, 3-6. JORNS, W. D. and GRIB~, R. E. (1958). Clay mineral composition of Recent Sediment’s from t,he Mississippi River Delta. J. Sed. Pet., 28, 2, 186-199. KELLE~E, W. D. (1953). Illite and Montmorillonitc in Green Sedimentary rocks. .i. Serl. /‘et., 23, 1, 3-Q. Krt~rrsso~s, K. B. (1956). Dissolution et Co~oc~,~rn. A&n, 10, l-26. K&ATiSKOPF, K. B., (1957). Separation

and precipitation

of silica at low tel~~l~erat~~r~s. Ge~~?~~rn.

of manga,nese from iron in sedimentary

processes.

Ibid.

12,61-84. LJ~NGGREN, P. (1953). Some data concerning the formation of manganiferous and ferriferous bog ores. Geol. Foren. Stockholm. Forh. 75, 277-297. MINAMT, E. (1935). Selen-Gehalte von europaisohen und japanischen Tonschief&n. Sccchr. Ges. Wiss Gottingen, TV, N.F., 1, No. 12, 143. Mann, P. A. (1955). Unpublished Ph.D. Thesis. University of Manchester. MOIIR, P. A. (1959). A geochemical study of the shales of the Lowes Cambrian 3Ianganese Shale Group of the Harlech Dome, North Wales. Geoch&ra. et ~~~rno~h~m.. Acta, 17, 186-200. RANKAMA, K. and SAHAMA, Th. G. (1950). Geockemistry. Univ. of Cbicsgo Press. SWAPIRO, L. and BRANNOCK, W. W. (1952). Rapid Analysis of Silicate Rocks. Geol. Sun?. ilrrcp,r. Circ. 165. SIKAPI~O, L. and BRANNOCK, W. W. (1955). Rapid determination of CO, in silicat,e rocks. A?&. Chem. 27, 1796-1797. SPENCER, D. W. (1957). Unpublished Ph.D. Thesis. University of Manchester. VAN ANDEL, TJ. and POSTMA, H. (1954). Recent sediments of the Gulf of Pa&a. Reports of the Orinoco Shelf Expedition, Vol. 1, North Holland Pub~shing Company, Amsterdam. Voas!, J. H. L. (1906). Uber M~g~wiesenerz. 2. prakt, Geol. 14, 217-233. W~TEEOUSE, G. V. (1951, 1952). Progress Reports for the A.P.I. Research Project 51. Quoted in VAN ANDEL and POSTMA (1954).

334

D. M. HIRST

WI~KMKN, F. E, (1954). The %otal” amount of sediments and the composition of the “average igneous rock.” Geochim. et Coemochirn. Acta, 5, 97-110. ZELLER, E. J. and WRAY, J. L. (1956). Factors influencing the precipitation of calcium carbonate. Bull. Amer. Assoc. Pet. Geol. 40, 140-152. ZOBELL, C. E. (1946). Studies on the redox potential of Marine Sediments. B&E. Amer. Aseoc. Pet. CT’eoE.30, 477-513.