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Chemical variations in sedimentary facies of an inner continental shelf environment, Northern Gulf of Mexico
Sedimentary Geology. 9 (1973): 101-115. ~ FAsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
CHEMICAL VARIATIONS IN SEDIMENTARY FACIES OF AN INNER CONTINENTAL SHELF ENVIRONMENT, NORTHERN GULF OF MEXICO
THOMAS T. TIEtt, THOMAS E. PYLE I DAVID H. EGGLER 2 and RONALD A. NELSON
Department o f Geology, Texas A and M University, College Station, Texas (U.S.A.) (Acceptcd for publication January 12, 1973)
ABSTRACT Tieh, T.T., Pyle, T.E., Eggler, D.H. and Nelson, R.A., 1973. Chemical variations in sedimentary facies of an inner continental shelf environment, northern Gulf of Mexico, Sediment. Geol., 9: 101 - 115. Four major sedimentary provinces of the inner continental shelf off the Louisiana coast have been recognized by textural studies (Krawiec, 1966): deltaic, non-deltaic, and relict sediments; and deltadestructional sands. Samples from these provinces have been analyzed for Rb, Sr, Ni, Fe, Mn, Ti, and Zr; trend surface analysis has been used to discern regional trends of these elements. These trends substantiate the previous division of shelf provinces. Although deltaic and non-deltaic sediments cannot be distinguished by chemical criteria, they are distinctive from relict sediments, which are low in Mn, Fe, Ti, Zr, Rb, and Ni, and high in St. The delta-destructional sands are distinctive from the other three provinces by their low contents of Mn, Fe, Rb, and Ni, and high Ti, Zr, and Sr. In an energetic environment such as this, processes subsequent to deposition tend to disperse Rb, Fe, Ni, and Mn, as these elements are closely associated with the clays; these processes may give rise to enrichment of Zr due to the high stability and density of the mineral zircon, or the enrichment of Sr by accumulation of organic remains.
INTR ODUCTION
Surface sediments of the inner continental shelf of Louisiana from the Mississippi River to Sabine Pass and from the beach to a water depth o f 15 fathoms (equivalent to 27.43 m; see Fig. 1) are highly variable in physical and chemical characteristics. The sediments are products o f complex interactions between fluvial and marine systems, and their distribution pattern has been further complicated by tectonic movements and by sea-level changes. Krawiec (1966) studied 259 offshore sediment samples collected in this 1 Present address: Marine Science Institute, University of South Florida, St. Petersburg, Fla, U.S.A. 2 Present address: Geophysical Laboratory, Carnegie Institution, Washington, D.C., U.S.A.
102
T.T. TIEtt ET AL.
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DELTAIC SEDIMENTATION
Fig. 1. Sedimentary provinces in the inner continental shelf off the coast of Louisiana and sample locations. (Modified from Krawiec, 1966.)
area and, on the basis of grain size, sedimentary structures, and partial analyses of carbonate and organic matter content, distinguished four major sedimentary provinces. He also determined such environmental parameters as water salinity, temperature, dissolved oxygen, turbidity, wave direction, period and height, and sediment E h - p H . Some of Krawiec's samples were examined by Davies and Moore (1970) for heavy minerals as part of their study of the dispersal of Mississippi River sediments. This report presents results of chemical analyses of the sediment samples studied by both Krawiec (1966) and Davies and Moore (1970). Analyses were made to determine to what extent facies determined by physical parameters may be distinguished on the basis of chemical composition, and to characterize chemical variations within and among the sedimentary facies. METHODS
A total of 41 samples was selected for this study from Krawiec's original collection. Samples chosen had a distinctive grain-size distribution, because recognition of the various sedimentary facies was largely based on those data. The sample population was also required to be representative of each sedimentary facies and to be sufficiently widely-spaced so that any chemical variations o f a local nature might be reduced. All analyses were made by the X-ray spectrographic method. Samples were prepared in the form of pressed pellets. A split portion of each sample, approximately 4 g in weight, was dried at 105°C, weighed, and ground to pass a 200-mesh (74-/a) sieve. The pulverized sample was mixed with Somar Mix powder, which serves as a binder in the pressed pellet, in a 3:1 weight ratio. The sample-binder mixture was placed in an aluminum cup and pressed
CHEMISTRY OF INNER C O N T I N E N T A L SHELF SEDIMENTS
103
into a pellet suitable for analysis. Standards used were W-l, G-2, GSP-1 and AGV-I, obtained from the U.S. Geological Survey; they were prepared in the same manner. Precision of analysis by this method has been given previously (Tieh and Pyle, 1972); for all the elements analyzed the maximum coefficient of variation is less than 3%. The seven elements analyzed during this study are Mn, Fe, Ni, Ti, Zr, Rb, and Sr. PHYSICAL AND M I N E R A L O G I C A L C H A R A C T E R I S T I C S Ol: THE SEDIMENTS
Krawiec's (1966) data on the sand, silt, and clay percentages and the median diameter of the samples analyzed during this study are included in Table I. He recognized four major sedimentary provinces: (1) Deltaic sedimentation: sedimentation exceeds erosion, and the sediments are characterized by "modes in the silt and clay classes, median diameters in the silt and clay classes and less than 2 percent sand" (Krawiec, 1966, p. 45). (2) Delta-destructional sands: erosion exceeds sedimentation, and the sediments are "characterized by modes in the fine to very fine sand classes, median diameters coarser than 5.0 phi, the presence of Mulinea shells and heavy minerals of the Mississippi River suite" (p. 39). (3) Relict sediments (Pleistocene): in the slightly-uplifted region in the western part of the area, sediments are characterized by "modes coarser than fine sand; a median diameter coarser than 5.0 phi; greater than 20 percent sand..." (p. 35) and other features indicative of sub-aerial exposure and oxidation of the sediment. (4) Non-deltaic sedimentation: this province constitutes the largest of the four recognized; it consists of sediments derived from erosion along the shores, banks, and shoals; the sediments are characterized by "modes in the silt classes, median diameters in the silt classes, less than 20 percent sand" (p. 37). Distribution of the four sedimentary provinces is presented in Fig. 1, which is identical to Krawiec's (1966) fig. 13, except that near-shore physiographic details and sample locations not used in this study have been omitted. Because of the highly variable nature of size distributions of the sediment samples, the mineralogical composition of each sample cannot easily be determined with acceptable accuracy and precision. Attempts were made to determine relative abundances of feldspars and quartz in the coarser-thanclay fraction of each sample by the staining method, but were not successful due to the very poor sorting of many samples. Hence a semi-quantitative method was devised, which involves X-ray diffraction scanning of a finelyground portion of a sample. A Cu-target tube was used, and a scan was made
T.T. TIEIt ET AL
104
TABLE I Size distribution and chemical and mineralogical composition of sediment samples from the various provinces off the coast o f Louisiana Sample No.
Position
lat.
Size analyses*
long.
depth (ft.)
sand (%)
silt (%)
clay (%)
90 94 54 40 88 64 42 29 38..8_ 60
22.0 14.0 99.0 73.0 6.0 31.0 65.0 99.0 49.0 50.9
61.0 82.0 1.0 20.0 49.0 44.0 24.0 1.0 37.0 35.4
17.0 4.0
89°57'08 '' 90010'07 '' 90034'03 '' 90°39'05 '' 91012'07 '' 91°39'01 '' average=
16 15 21 26 24 49 25
82.0 93.0 79.0 99.0 97.0 72.0 87.0
18.0 7.0 27.0 1,0 3.0 27.0 13.8
89030'04 '' 89°24'08" 89°42'15 '' 89041'03" 89040'00 '' 89050'06" 91037'00 '' 92°18'07" average =
66 13 90 48 15 60 14 __14 40
1.0 0.5 3.0 3.0 14.0 2.0 6.0 1.0 4.8
90 66 66 54 84 42 17 71 33 18 48 62 44 19 16 22 47
1.0 3.0 14.0 24.0 37.0 18.0 17.0 15.0 95.0 68.0 26.0 18.0 6.0 1.0 14.0 24.0 23.8
Relict sediments H-8 28°56'09 '' 93°35'05 '' 1-1 28046'07 '' 9 3 ° 0 1 ' 0 9 " i-3 29°06'07" 93°01'05" K-6 29023'03 '' 93030'03 '' J-ll 28044'03 '' 93038'05 '' J-9 29000'00 '' 93041'06 '' J-7 29025'00 '' 93043'00" J..6 2 9 0 2 7 ' 0 4 '' 9 3 ° 4 3 ' 0 2 " J-3 29031'07 '' 93043'09 '' average= Delta-destructional sands B-1 29013'08" C-1 29005'05 '' D-10 29001'07 '' K-1 28055'03 '' K-2 28052'04 '' F-2 28058'06 '' Deltaic sediments 0-2 28059'00 '' O-1 29002'09" A-I 29004'05 '' A-3 29°09'07 ' ' ' A-6 29012'07 '' B-6 29008'06 '' F-7 29017'09" G-7 29029'07 ''
Chemistry
Non-deltaic sediments C-5 28058'04" 89059'00" C-2 29°00'05" 9 0 ° 0 2 ' 0 8 " D-4 28039'05 '' 90025'00 '' D-7 28052'09 '' 90030'05 '' E-1 28033'02" 90056'07 '' E-3 28048'09" 90055'08" E-7 29°01'01 '' 9 0 ° 5 9 ' 0 1 " G-1 29°00~00 '' 92013'03 '' G-3 29°08'10 '' 92013'05 '' G-6 29020'06 '' 92012'08 '' K-4 29010'09 '' 92025'06 '' H-7 29013'03" 92037'06 '' H-6 29025'08 '' 92038'02 '' H-I 29033'03 '' 92037'06 '' 1-7 29°42'01 '' 93000'07 '' J-1 29041'00" 93043'09 '' average =
Mn (p.p.m.)
Fe (%)
5.4 2.7 1.1 7.0 2.6 45.0 7.1 25.0 6.1 11.0 3.3 2.7 14.0 4.1 13.7 3.9
420 480 240 310 530 355 195 320 330 353
3.26 1.51 0.69 1.32 4.37 1.10 0.85 0.38 2.53 1.78
1.0 0.2
3.5 3.0 3.7 2.8 3.3 3.6 3.3
270 405 330 340 320 315 330
1.21 1.94 1.60 1.39 1.41 1.58 1.52
77.0 67.5 48.0 45.0 43.0 72.0 59.0 54.0 58.2
21.0 32.0 49.0 52.0 43.0 26.0 35.0 45.0 37.9
6.5 6.7 7.7 8.4 7.1 5.6 6.5 7.1 7.0
720 670 700 975 660 500 870 1~040 767
4.10 4.25 4.98 5.05 3.26 3.94 4.94 6.07 4.57
52.0 59.0 43.0 61.0 51.0 56.0 52.0 72.0 5.0 31.0 51.0 72.0 60.0 53.0 82.0 51.0 53.2
47.0 37.0 43.0 15.0 12.0 26.0 31.0 13.0 1.0 23.0 11.0 34.0 46.0 4.0 25.0 22.4
7.3 6.2 6.1 5.0 4.4 5.5 6.0 4.6 3.6 3.3 4.6 5.5 5.8 7.5 2.7 5.5 5.2
515 695 685 440 425 550 580 330 430 530 445 395 575 11325 1,520 660 631
4.42 3.64 4.07 2.73 3.14 2.77 3.87 1.87 1.57 1.95 3.13 3.11 3.81 6.11 5.15 3.69 3.44
548
2.98
grand average *
median dia. (phi)
Data from Krawiec (1966); ** montmorillonite, usually the predominant clay, is not included.
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
105
Mineralogy**
Ti Zr (p.p.m.) (p.p.m.)
Rb (p.p.m.)
Sr (p.p.m.)
Ni (p.p.m.)
Plag. (204)
K-Spar (002 and 220)
Qtz. (100)
Kaolin (001)
lllite (001)
3,310 2,910 890 1,870 4,320 2,460 2,360 920 2,310 2,372
404 1,408 216 546 246 1,074 713 344 483 604
150 100 100 100 185 95 150 110 140 126
270 350 310 470 190 420 300 340 290 327
40 35 35 25 55 35 30 30 40 36
27.1 27.2
16.9 27.2
45.7 36.3
5.0 0
5.0 9.0
23.3
14.9
55.1
2.8
6_5
29.8 35.4 26.5
32.9 35.4 28.5
35.0 22.5 36.7
1.0 3.2 4.0
1.0 3.2 4.0
1,930 5,060 4,340 3,560 3,080 2_600 3,428
602 3,229 3,553 1,474 1,412 735 1,834
130 95 110 105 120 130 115
390 360 350 370 390 360 370
35 40 30 35 40 40 37
43.7 41.4 43.4
28.8 24.4 22.0
25.1 31.9 31.0
0.7 1.0 1.4
1.4 1.0 2.0
20.0 26.6
60.0 40.0
20.0 28.8
0 2.2
0 2.2
3,900 4,260 3,980 3,980 3,340 3,880 4,320 4,550 4,026
304 436 224 193 407 333 255 152 289
175 160 190 180 150 170 170 205 175
240 240 210 200 300 250 230 180 231
55 55 55 60 45 55 60 70 57
16.2 32.0
22.0 17.2
52.3 23.4
3.4 9.8
5.8 17.2
20.3 31.9 24.1 33.3 20.5
17.1 34.1 14.7 16.6 12.8
38.2 29.7 43.6 39.5 43.5
10.9 2.1 6.7 4.1 10.2
13.2 2.1 10.7 6.2 12.8
4,000 3.450 3,730 3,290 3,350 3,250 3,930 2,870 2,820 2,020 3,220 3,430 3,630 4,470 3,990 3,360 3,426
250 275 367 520 418 411 526 631 1,142 267 471 500 342 152 205 449 433
185 170 160 145 145 150 165 145 125 120 155 155 155 195 185 15_._0 157
230 270 230 300 280 300 270 350 360 360 180 280 250 170 200 240 267
55 60 50 45 50 50 60 30 40 45 45 50 45 75 60 45 50
44.6 37.3 38.4 55.0 26.0
14.0 15.6 26.9 17.5 42.4
35.5 39.8 19.2 20.0 23.9
2.5 1.9 5.7 2.5 3.4
3.3 5.2 9.6 5.0 4.1
41.8 48.7 39.9 33.3 32.2 25.0 32.5 13.8 21.9 47.7
21.8 17.5 22.1 19.0 19.3 31.2 24.3 13.8 36.5 21.0
30.9 26.6 40.7 38.0 41.9 37.5 34.5 41.4 34.1 27.8
1.8 1.9 2.6 4.7 3.2 0 3.3 14.8 2.4 1.7
3.6 5.1 3.5 4.7 3.2 6.2 5.2 15.9 4.8 1.7
3,306
658
147
289
46
106
T.T. TIEH ET AL.
from 0 ° to 40 ° 20 in order to record high-intensity reflections of clays, feldspars and quartz; these minerals c o m m o n l y constitute the bulk of most samples. Reflections from calcite and aragonite were present in some sampies; they were omitted in the final tabulation, because much of the carbonate is of biogenic origin. On each diffractogram the following reflections were identified and the peak height measured : (001 ) of illite, (001 ) of koalinite (and chlorite), (100) of quartz, (002 and 220) of potash-feldspar, and (204) of plagioclase. The very strong reflections of montmorillonite (001) and quartz (101) were purposely omitted in order to emphasize the weaker reflections. Montmorillonite is known to be the most abudant clay in the Mississippi sediments (Griffin, 1962). Peak heights of the above-mentioned reflections on each diffractogram were totaled, and the relative abundance of each mineral was expressed in terms of the percentage of the total peak height. These data are also included in Table I. It should be noted that the percentage figures serve merely as indications of the relative intensity of the reflections; they should not be taken as percentages of the mineral present. Although in general montmorillonite and quartz are the most abundant mineral constituents, all samples contain feldspars and, with a few exceptions, kaolinite and illite. CHEMICAL COMPOSITION OF THE SEDIMENTS
Results of X-ray spectrographic analyses o f M n , Fe, Ti, Zr, Rb, Sr, and Ni for each sample are presented in Table I. The samples are arranged in groups according to Krawiec's classification in order to facilitate comparison between groups. These data were used to obtain the distribution trend of each element in the area of study. TREND-SURFACE ANALYSIS
Trend-surface analysis, a special application of multiple regression, is a m e t h o d o f smoothing areally-distributed geologic data. The program used in this paper was adapted from various routines of the Kansas Geological Survey, principally KWIKR8 (Esler et al., 1968). The program treats one dependent variable (Mn, Rb...) and two independent variables (geographic coordinates) with surfaces to degree 9. A principal problem in trend analysis is deciding which order of surface " b e s t " represents the data. Although statistical tests exist, geologic inference must enter into any decision (Baird et al., 1971 ). In the case of the Louisiana shelf sediment data, geologic inference indicates that a degree-4 surface for most of the variables is hopelessly complex, reflecting too much "noise". A
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
107
degree-2 or degree-3 surface is a better indicator of regional trends, which appear in either surface. Happily, two statistical tests support that conclusion: the F-ratio, which is a procedure o f analyses of variance in which "... the total variance of a variable being measured is broken down into the portions or components caused by the different factors .... it is possible ... to test each in turn for a significantly large contribution at a confidence level" (Baird et al., 1971, p. 1223), and the F-test, which compares the ratio o f the sample mean squares with the F-test table for a desired significance level. The F-ratio increases, in most cases, up to degree-3, but shows no improvement with higher degrees, indicating degree-3 is a best choice (Chayes, 1970). Degree-3 surfaces also have slightly higher significance levels when an F-test is applied (Table II); degree-4 surfaces fall o f f considerably in significance level. Table II presents percent sum of squares of variability accounted for by the surfaces, the mean-squares ratio F, and the significance level from an F-test. We consider surfaces at the 0.10 confidence level to be valid, especial-
TABLE II
Trend surface statistics Variable
Order
S.S. (%)
Signif. level
Rb
2 3
34 44
2.30 1.91
0.10 0.10
Sr
2 3
29 43
1.84 1.86
0.25 0.10
Ni
2 3
44 60
3.53 3.64
0.01 0.01
Ti
2 3
42 59
3.16 3.57
0.05 0.01
Fe
2 3
48 63
4.14 4.15
0.01 0.01
Mn
2 3
60 82
6.83 11.02
0.01 0.01
Ti/Mn
2 3
22 38
1.23 1.53
n.s. 0.25
Ti/Zr
2 3
47 65
3.99 4.53
0.01 0.01
Zr
2 3
9 25
.44 .83
n.s. n.s.
F
108
T.T. TIEH ET AL.
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Fig. 2. A - G , third-degree trend surface maps for Mn, Fe, Ti, Ni, Rb, Sr, and Zr in sediments in the inner continental shelf off the Louisiana coast; H, that of Ti/Mn ratio.
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
109
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T.T. TIEtI ET AL.
ly because Rb and Sr, at the 0.10 level, have map patterns similar to Fe and Ni, which are significant at the 0.01 level. Third-degree trend surfaces for the various elements analyzed during this study are presented as Fig 2, A - G ; a trend map of Ti/Mn is included as Fig. 2H. DISCUSSION
Table I shows that, in general, the analytical data within each sediment group are fairly consistent despite the fact that samples within a group are widely spaced and they have variable size distributions. Such consistency of data is particularly evident in the delta-destructional sands and tile clay-rich deltaic sediments. To characterize each group chemically, the group average of a given element is compared with the grand average for all samples analyzed; where the group average is lower than the grand average, it is designated as low, and where higher it is designated as high. Thus, the chemistry of the four major sediment groups is summarized as follows: Relict sediments: low = Mn, Fe, Ti, Zr, Rb, Ni high = Sr Delta-destructional sands: low = Mn, Fe, Rb, Ni high = Ti, Zr, Sr Deltaic sediments: low = Zr, Sr high = Mn, Fe, Ti, Rb, Ni Non-deltaic sediments: low = Zr, Sr high = Mn, Fe, Ti, Rb, Ni It is evident that the deltaic and non-deltaic sediments can not be distinguished on the basis of chemical data available. These sediments have high silt and clay contents and median diameters in silt and clay classes; they represent modern-day delta deposits of Mississippi and other major rivers as well as deposits from erosion of beaches and shoals. Although Krawiec (1966) recognized that there is a greater amount of sand in the non-deltaic sediments than the deltaic deposits, the very poor sorting, high variability in size distribution, and high clay content of both types of sediments make
CHEMISTRY OF INNER C O N T I N E N T A L SHELF SEDIMENTS
1 11
their distinction difficult on either physical or chemical basis. On the other hand, distinctions between relict sediments, delta-destructional sands, and deltaic sediments can be readily seen on the basis of the analytical data. Relict sediments and deltaic sediments have contrasting chemistry in that for every element analyzed, except for Zr, it is always high in one and low in the other. The very high contents of Zr, Ti and Sr in the delta-destructional sands make this group further distinctive from all the others. Differences in chemistry of the various sediment groups are further den> onstrated in the trend surface maps. A comparison of Fig. l, the sedimentary facies map drawn on the basis of physical properties, and Fig. 2, A - G , the trend maps, indicates that modern-day deltaic and non-deltaic sediments in the areas of Mississippi Delta and along the coast of Louisiana are high in the content of Rb, Ni, Fe, Mn, and Ti; decreasing trends of these elements towards the west of the area, where the relict sediments occur, are clear and systematic. The trend map of Zr, however, is exactly contrary to those mentioned above. Zr shows a trend pattern somewhat different from all the others, and is controlled to a large extent by the distribution pattern of the delta-destructiona] sands. Zr occurs principally in zircon crystals. The close association of Zr with the delta-destructional sands is due to the high stability and density of the mineral. These properties have let the mineral survive the delta-destruction 4000 O DELTAIC SEDIMENTATION
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• NON-DELTAIC SEDIMENTATION • RELICT SEDIMENTS [ ] DELTA- OESTRUCTIONAL SANDS
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O DELTAIC SEDIMENTATION • NON-DELTAIC SEDIMENTATION • RELICT SEDIMENTS D DELTA- DESTRUCTIONAL SANDS 40
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60 CLAY(%)
Fig. 3. A b u n d a n c e of Zr versus percentage of sand in all samples. Fig. 4. Relationship between clay c o n t e n t and abundance of Fe in samples from the various sedimentary provinces.
112
T.T. T I E H E T A L .
phase and become concentrated in this sediment group. However, there does not seem to be any consistent relationship between Zr and the size distribution of the sediment samples, although one would expect that Zr would be enriched in the sand and silt fractions. This lack of correlation can be seen in tile scattered pattern of Fig. 3, a plot of Zr against sand percentage of all samples studied. In Fig. 3 samples o f the delta-destructional sands can be readily distinguished from the other groups, especially those of the deltaic sedimentation group, by the high sand and Zr contents. The principal Ti-bearing minerals are ilmenite, rutile, leucoxene, and sphene. Ti-bearing pyroxenes occur in sands farther west of the present area of study (Bullard, 1942). Unlike Zr, the range of variation for Ti in the four sediment groups is relatively small, perhaps because the cryptocrystalline leucoxene may occur in particles of sand to finer-than-clay sizes. The fact that Ti is high in the clay-rich deltaic and non-deltaic sediments suggests that much of the Ti is incorporated in the clay fraction. It is interesting to note that the trend surface map of Ti/Mn ratio (Fig. 2H) is similar to that of Zr. This similarity is evidently due to the faster rate of removal of Mn than Ti subsequent to deposition. Whereas the bulk of Mn and some Ti occur as cryptocrystalline oxides and/or hydroxide particles adhered to the surfaces or cleavages of clays, there are no Mn-bearing minerals in the silt-sand fractions that are equivalent in abundance to ilmenite, rutile, and sphene. Consequently, removal of clays during delta destruction processes has given rise to an increase in Ti relative to Mn. 1600
1400
1200
0 DELTAIC SEDIMENTATION • NON-DELTAIC SEDIMENTATION • RELICT SEDIMENTS [[]DELTA-- DESTRUDTIONAL SANDS / / '
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5
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CHEMISTRY OF INNER CONTINENTAL
SHELF SEDIMENTS
11 3
8°f 70
i./.
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DELTAIC SEDIMENT AT IO N NON-DELTAIC SEDIMENTATION RELICT SEDIMENT S DELTA - DESTRUCTIONAL SANDS
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2001 175 150
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50
DELTAIC S E D I M E N T A T I O N NON-DELTAIC SEDIMENT AT IO N RELICT S E D I M E N T S DELTA - DESTRUCTIONAL SANDS
25
0
1(30
200
300
400
500
600
Sr(ppm)
Fig. 7. R b v e r s u s Sr.
The close association of Fe with tile clays (Fig. 4) indicates that the bulk of Fe also occurs as limonite (amorphous to.cryptocrystalline mixtures of oxides and/or hydroxides of iron) particles adhered to the clays. As both Mn
114
T.T. TIEIt ET AL.
IAB LE I!1 Composition of Gulf of Mexico sediments Location
No. of samples
Fc (%)
Mn (p.p.m.)
Ni (p.p.m.)
Rb (p.p.m.)
Sr (p.p.m.)
Ti (p.p.m.)
Zr Ip, p.m.)
Inner shelf
(39)
2.98
548
46
147
289
3.306
658
Slope*
(36)
Abyssal* plain
3.15
911
86
87
257
4.332
79
Abyssal* hill
(19)
3.36
1A64
91
99
194
4,538
81
(13)
2.12
2,409
75
54
596
2,208
92
* f:rom Tieh and Pyle (1972).
and Ni vary proportionally with Fe (Fig. 5,6), it may be concluded that freshly deposited clay-rich sediments, whether deltaic or non-deltaic in origin, have overall high contents of these three elements and that processes subsequent to deposition tend to redistribute these elements and cause their reduction. The inverse relationship between Rb and Sr (Fig. 7) is a c o m m o n occurrence in marine sediments (Tieh and Pyle, 1972). Sr occurs mainly as a replacement for Ca in carbonate shell fragments, which are abundant in the delta-destructional sands. Rb is present as adsorbed ions in clays or in the structure of feldspars in the silt- and clay-rich samples. Table III is a summary of the distribution of these seven elements in sediments of the various physiographic provinces of the Gulf of Mexico. The figures represent averages either of some areally distributed samples (such as those of this study) or of samples from one or more cores taken from a certain physiographic location. Tim abyssal-hill samples represent a peculiar and limited environment. If we consider only the shelf, slope, and abyssal plain, we find that the sediments are gradually enriched in Fe, Mn, Ni, and Ti, and become depleted in Zr, Sr, and Rb from the shelf to the abyssal plain. ACKNOWLEDGI';M ENTS
We thank Dr. Robert L. Lankford for making the samples available to us. A grant from the Robert A. Welch Foundation (Grant A-351 to Tieh) made this study possible.
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
115
RI~FER I'~NCES Baird, A.K., Baird, K.W. and Morton, D.M., 1971. On deciding whether trend surfaces of progressively higher order are meaningful: discussion. Geol. Soc. A m., Bull., 82: 1219-1234. Bullard, I:.M., 1942. Source of beach and river sands on Gulf Coast of Texas. Geol. Soc. Am., Bull., 53: 1021- 1(144. Chayes, 1:., 197(t. On deciding whether trend surfaces of progressively higher order arc meaningful. Geol. Soc. Am., Bull., 81: 1273- 1278. Davies, D.K. and Moore, W.R., 1970. Dispersal of Mississippi sediments in the Gull" of Mexico..I. Sediment. Petrol., 40: 339-353. Esler, J.E., Smith, P.F. and Davis, .I.C., 1968. KWIKR8, a I.'OP,TRAN IV program for multiple regression and geologic trend analysis. Kansas State (;eol. Sz4rv., Compttter Contrih., 28:31 pp. Griffin, G.M., 1962. Regional clay-mineral facics products of weathering intensity and current distribution in the northeastern Gulf of Mexico. Geol. Soc. Am., Bull., 73:737 767. Krawiec, W., 1966. Recent Sediments o f the l.ouisiana Inner Continental Shel]i Thesis Rice University, 50 pp. Tieh, T.T. and Pyle, T.E., 1972. Distribution of elements in Gulf of Mcxico sediments. In. R. Rezak and J. ttenry O'dit0rs) , Contributions on the (;eologieal and Geophysical Oceanography o f the Gulf o f Mexico, 5. Gulf Publishing Co., Houston, Texas, pp. 129 152.
CHEMICAL VARIATIONS IN SEDIMENTARY FACIES OF AN INNER CONTINENTAL SHELF ENVIRONMENT, NORTHERN GULF OF MEXICO
THOMAS T. TIEtt, THOMAS E. PYLE I DAVID H. EGGLER 2 and RONALD A. NELSON
Department o f Geology, Texas A and M University, College Station, Texas (U.S.A.) (Acceptcd for publication January 12, 1973)
ABSTRACT Tieh, T.T., Pyle, T.E., Eggler, D.H. and Nelson, R.A., 1973. Chemical variations in sedimentary facies of an inner continental shelf environment, northern Gulf of Mexico, Sediment. Geol., 9: 101 - 115. Four major sedimentary provinces of the inner continental shelf off the Louisiana coast have been recognized by textural studies (Krawiec, 1966): deltaic, non-deltaic, and relict sediments; and deltadestructional sands. Samples from these provinces have been analyzed for Rb, Sr, Ni, Fe, Mn, Ti, and Zr; trend surface analysis has been used to discern regional trends of these elements. These trends substantiate the previous division of shelf provinces. Although deltaic and non-deltaic sediments cannot be distinguished by chemical criteria, they are distinctive from relict sediments, which are low in Mn, Fe, Ti, Zr, Rb, and Ni, and high in St. The delta-destructional sands are distinctive from the other three provinces by their low contents of Mn, Fe, Rb, and Ni, and high Ti, Zr, and Sr. In an energetic environment such as this, processes subsequent to deposition tend to disperse Rb, Fe, Ni, and Mn, as these elements are closely associated with the clays; these processes may give rise to enrichment of Zr due to the high stability and density of the mineral zircon, or the enrichment of Sr by accumulation of organic remains.
INTR ODUCTION
Surface sediments of the inner continental shelf of Louisiana from the Mississippi River to Sabine Pass and from the beach to a water depth o f 15 fathoms (equivalent to 27.43 m; see Fig. 1) are highly variable in physical and chemical characteristics. The sediments are products o f complex interactions between fluvial and marine systems, and their distribution pattern has been further complicated by tectonic movements and by sea-level changes. Krawiec (1966) studied 259 offshore sediment samples collected in this 1 Present address: Marine Science Institute, University of South Florida, St. Petersburg, Fla, U.S.A. 2 Present address: Geophysical Laboratory, Carnegie Institution, Washington, D.C., U.S.A.
102
T.T. TIEtt ET AL.
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DELTAIC SEDIMENTATION
Fig. 1. Sedimentary provinces in the inner continental shelf off the coast of Louisiana and sample locations. (Modified from Krawiec, 1966.)
area and, on the basis of grain size, sedimentary structures, and partial analyses of carbonate and organic matter content, distinguished four major sedimentary provinces. He also determined such environmental parameters as water salinity, temperature, dissolved oxygen, turbidity, wave direction, period and height, and sediment E h - p H . Some of Krawiec's samples were examined by Davies and Moore (1970) for heavy minerals as part of their study of the dispersal of Mississippi River sediments. This report presents results of chemical analyses of the sediment samples studied by both Krawiec (1966) and Davies and Moore (1970). Analyses were made to determine to what extent facies determined by physical parameters may be distinguished on the basis of chemical composition, and to characterize chemical variations within and among the sedimentary facies. METHODS
A total of 41 samples was selected for this study from Krawiec's original collection. Samples chosen had a distinctive grain-size distribution, because recognition of the various sedimentary facies was largely based on those data. The sample population was also required to be representative of each sedimentary facies and to be sufficiently widely-spaced so that any chemical variations o f a local nature might be reduced. All analyses were made by the X-ray spectrographic method. Samples were prepared in the form of pressed pellets. A split portion of each sample, approximately 4 g in weight, was dried at 105°C, weighed, and ground to pass a 200-mesh (74-/a) sieve. The pulverized sample was mixed with Somar Mix powder, which serves as a binder in the pressed pellet, in a 3:1 weight ratio. The sample-binder mixture was placed in an aluminum cup and pressed
CHEMISTRY OF INNER C O N T I N E N T A L SHELF SEDIMENTS
103
into a pellet suitable for analysis. Standards used were W-l, G-2, GSP-1 and AGV-I, obtained from the U.S. Geological Survey; they were prepared in the same manner. Precision of analysis by this method has been given previously (Tieh and Pyle, 1972); for all the elements analyzed the maximum coefficient of variation is less than 3%. The seven elements analyzed during this study are Mn, Fe, Ni, Ti, Zr, Rb, and Sr. PHYSICAL AND M I N E R A L O G I C A L C H A R A C T E R I S T I C S Ol: THE SEDIMENTS
Krawiec's (1966) data on the sand, silt, and clay percentages and the median diameter of the samples analyzed during this study are included in Table I. He recognized four major sedimentary provinces: (1) Deltaic sedimentation: sedimentation exceeds erosion, and the sediments are characterized by "modes in the silt and clay classes, median diameters in the silt and clay classes and less than 2 percent sand" (Krawiec, 1966, p. 45). (2) Delta-destructional sands: erosion exceeds sedimentation, and the sediments are "characterized by modes in the fine to very fine sand classes, median diameters coarser than 5.0 phi, the presence of Mulinea shells and heavy minerals of the Mississippi River suite" (p. 39). (3) Relict sediments (Pleistocene): in the slightly-uplifted region in the western part of the area, sediments are characterized by "modes coarser than fine sand; a median diameter coarser than 5.0 phi; greater than 20 percent sand..." (p. 35) and other features indicative of sub-aerial exposure and oxidation of the sediment. (4) Non-deltaic sedimentation: this province constitutes the largest of the four recognized; it consists of sediments derived from erosion along the shores, banks, and shoals; the sediments are characterized by "modes in the silt classes, median diameters in the silt classes, less than 20 percent sand" (p. 37). Distribution of the four sedimentary provinces is presented in Fig. 1, which is identical to Krawiec's (1966) fig. 13, except that near-shore physiographic details and sample locations not used in this study have been omitted. Because of the highly variable nature of size distributions of the sediment samples, the mineralogical composition of each sample cannot easily be determined with acceptable accuracy and precision. Attempts were made to determine relative abundances of feldspars and quartz in the coarser-thanclay fraction of each sample by the staining method, but were not successful due to the very poor sorting of many samples. Hence a semi-quantitative method was devised, which involves X-ray diffraction scanning of a finelyground portion of a sample. A Cu-target tube was used, and a scan was made
T.T. TIEIt ET AL
104
TABLE I Size distribution and chemical and mineralogical composition of sediment samples from the various provinces off the coast o f Louisiana Sample No.
Position
lat.
Size analyses*
long.
depth (ft.)
sand (%)
silt (%)
clay (%)
90 94 54 40 88 64 42 29 38..8_ 60
22.0 14.0 99.0 73.0 6.0 31.0 65.0 99.0 49.0 50.9
61.0 82.0 1.0 20.0 49.0 44.0 24.0 1.0 37.0 35.4
17.0 4.0
89°57'08 '' 90010'07 '' 90034'03 '' 90°39'05 '' 91012'07 '' 91°39'01 '' average=
16 15 21 26 24 49 25
82.0 93.0 79.0 99.0 97.0 72.0 87.0
18.0 7.0 27.0 1,0 3.0 27.0 13.8
89030'04 '' 89°24'08" 89°42'15 '' 89041'03" 89040'00 '' 89050'06" 91037'00 '' 92°18'07" average =
66 13 90 48 15 60 14 __14 40
1.0 0.5 3.0 3.0 14.0 2.0 6.0 1.0 4.8
90 66 66 54 84 42 17 71 33 18 48 62 44 19 16 22 47
1.0 3.0 14.0 24.0 37.0 18.0 17.0 15.0 95.0 68.0 26.0 18.0 6.0 1.0 14.0 24.0 23.8
Relict sediments H-8 28°56'09 '' 93°35'05 '' 1-1 28046'07 '' 9 3 ° 0 1 ' 0 9 " i-3 29°06'07" 93°01'05" K-6 29023'03 '' 93030'03 '' J-ll 28044'03 '' 93038'05 '' J-9 29000'00 '' 93041'06 '' J-7 29025'00 '' 93043'00" J..6 2 9 0 2 7 ' 0 4 '' 9 3 ° 4 3 ' 0 2 " J-3 29031'07 '' 93043'09 '' average= Delta-destructional sands B-1 29013'08" C-1 29005'05 '' D-10 29001'07 '' K-1 28055'03 '' K-2 28052'04 '' F-2 28058'06 '' Deltaic sediments 0-2 28059'00 '' O-1 29002'09" A-I 29004'05 '' A-3 29°09'07 ' ' ' A-6 29012'07 '' B-6 29008'06 '' F-7 29017'09" G-7 29029'07 ''
Chemistry
Non-deltaic sediments C-5 28058'04" 89059'00" C-2 29°00'05" 9 0 ° 0 2 ' 0 8 " D-4 28039'05 '' 90025'00 '' D-7 28052'09 '' 90030'05 '' E-1 28033'02" 90056'07 '' E-3 28048'09" 90055'08" E-7 29°01'01 '' 9 0 ° 5 9 ' 0 1 " G-1 29°00~00 '' 92013'03 '' G-3 29°08'10 '' 92013'05 '' G-6 29020'06 '' 92012'08 '' K-4 29010'09 '' 92025'06 '' H-7 29013'03" 92037'06 '' H-6 29025'08 '' 92038'02 '' H-I 29033'03 '' 92037'06 '' 1-7 29°42'01 '' 93000'07 '' J-1 29041'00" 93043'09 '' average =
Mn (p.p.m.)
Fe (%)
5.4 2.7 1.1 7.0 2.6 45.0 7.1 25.0 6.1 11.0 3.3 2.7 14.0 4.1 13.7 3.9
420 480 240 310 530 355 195 320 330 353
3.26 1.51 0.69 1.32 4.37 1.10 0.85 0.38 2.53 1.78
1.0 0.2
3.5 3.0 3.7 2.8 3.3 3.6 3.3
270 405 330 340 320 315 330
1.21 1.94 1.60 1.39 1.41 1.58 1.52
77.0 67.5 48.0 45.0 43.0 72.0 59.0 54.0 58.2
21.0 32.0 49.0 52.0 43.0 26.0 35.0 45.0 37.9
6.5 6.7 7.7 8.4 7.1 5.6 6.5 7.1 7.0
720 670 700 975 660 500 870 1~040 767
4.10 4.25 4.98 5.05 3.26 3.94 4.94 6.07 4.57
52.0 59.0 43.0 61.0 51.0 56.0 52.0 72.0 5.0 31.0 51.0 72.0 60.0 53.0 82.0 51.0 53.2
47.0 37.0 43.0 15.0 12.0 26.0 31.0 13.0 1.0 23.0 11.0 34.0 46.0 4.0 25.0 22.4
7.3 6.2 6.1 5.0 4.4 5.5 6.0 4.6 3.6 3.3 4.6 5.5 5.8 7.5 2.7 5.5 5.2
515 695 685 440 425 550 580 330 430 530 445 395 575 11325 1,520 660 631
4.42 3.64 4.07 2.73 3.14 2.77 3.87 1.87 1.57 1.95 3.13 3.11 3.81 6.11 5.15 3.69 3.44
548
2.98
grand average *
median dia. (phi)
Data from Krawiec (1966); ** montmorillonite, usually the predominant clay, is not included.
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
105
Mineralogy**
Ti Zr (p.p.m.) (p.p.m.)
Rb (p.p.m.)
Sr (p.p.m.)
Ni (p.p.m.)
Plag. (204)
K-Spar (002 and 220)
Qtz. (100)
Kaolin (001)
lllite (001)
3,310 2,910 890 1,870 4,320 2,460 2,360 920 2,310 2,372
404 1,408 216 546 246 1,074 713 344 483 604
150 100 100 100 185 95 150 110 140 126
270 350 310 470 190 420 300 340 290 327
40 35 35 25 55 35 30 30 40 36
27.1 27.2
16.9 27.2
45.7 36.3
5.0 0
5.0 9.0
23.3
14.9
55.1
2.8
6_5
29.8 35.4 26.5
32.9 35.4 28.5
35.0 22.5 36.7
1.0 3.2 4.0
1.0 3.2 4.0
1,930 5,060 4,340 3,560 3,080 2_600 3,428
602 3,229 3,553 1,474 1,412 735 1,834
130 95 110 105 120 130 115
390 360 350 370 390 360 370
35 40 30 35 40 40 37
43.7 41.4 43.4
28.8 24.4 22.0
25.1 31.9 31.0
0.7 1.0 1.4
1.4 1.0 2.0
20.0 26.6
60.0 40.0
20.0 28.8
0 2.2
0 2.2
3,900 4,260 3,980 3,980 3,340 3,880 4,320 4,550 4,026
304 436 224 193 407 333 255 152 289
175 160 190 180 150 170 170 205 175
240 240 210 200 300 250 230 180 231
55 55 55 60 45 55 60 70 57
16.2 32.0
22.0 17.2
52.3 23.4
3.4 9.8
5.8 17.2
20.3 31.9 24.1 33.3 20.5
17.1 34.1 14.7 16.6 12.8
38.2 29.7 43.6 39.5 43.5
10.9 2.1 6.7 4.1 10.2
13.2 2.1 10.7 6.2 12.8
4,000 3.450 3,730 3,290 3,350 3,250 3,930 2,870 2,820 2,020 3,220 3,430 3,630 4,470 3,990 3,360 3,426
250 275 367 520 418 411 526 631 1,142 267 471 500 342 152 205 449 433
185 170 160 145 145 150 165 145 125 120 155 155 155 195 185 15_._0 157
230 270 230 300 280 300 270 350 360 360 180 280 250 170 200 240 267
55 60 50 45 50 50 60 30 40 45 45 50 45 75 60 45 50
44.6 37.3 38.4 55.0 26.0
14.0 15.6 26.9 17.5 42.4
35.5 39.8 19.2 20.0 23.9
2.5 1.9 5.7 2.5 3.4
3.3 5.2 9.6 5.0 4.1
41.8 48.7 39.9 33.3 32.2 25.0 32.5 13.8 21.9 47.7
21.8 17.5 22.1 19.0 19.3 31.2 24.3 13.8 36.5 21.0
30.9 26.6 40.7 38.0 41.9 37.5 34.5 41.4 34.1 27.8
1.8 1.9 2.6 4.7 3.2 0 3.3 14.8 2.4 1.7
3.6 5.1 3.5 4.7 3.2 6.2 5.2 15.9 4.8 1.7
3,306
658
147
289
46
106
T.T. TIEH ET AL.
from 0 ° to 40 ° 20 in order to record high-intensity reflections of clays, feldspars and quartz; these minerals c o m m o n l y constitute the bulk of most samples. Reflections from calcite and aragonite were present in some sampies; they were omitted in the final tabulation, because much of the carbonate is of biogenic origin. On each diffractogram the following reflections were identified and the peak height measured : (001 ) of illite, (001 ) of koalinite (and chlorite), (100) of quartz, (002 and 220) of potash-feldspar, and (204) of plagioclase. The very strong reflections of montmorillonite (001) and quartz (101) were purposely omitted in order to emphasize the weaker reflections. Montmorillonite is known to be the most abudant clay in the Mississippi sediments (Griffin, 1962). Peak heights of the above-mentioned reflections on each diffractogram were totaled, and the relative abundance of each mineral was expressed in terms of the percentage of the total peak height. These data are also included in Table I. It should be noted that the percentage figures serve merely as indications of the relative intensity of the reflections; they should not be taken as percentages of the mineral present. Although in general montmorillonite and quartz are the most abundant mineral constituents, all samples contain feldspars and, with a few exceptions, kaolinite and illite. CHEMICAL COMPOSITION OF THE SEDIMENTS
Results of X-ray spectrographic analyses o f M n , Fe, Ti, Zr, Rb, Sr, and Ni for each sample are presented in Table I. The samples are arranged in groups according to Krawiec's classification in order to facilitate comparison between groups. These data were used to obtain the distribution trend of each element in the area of study. TREND-SURFACE ANALYSIS
Trend-surface analysis, a special application of multiple regression, is a m e t h o d o f smoothing areally-distributed geologic data. The program used in this paper was adapted from various routines of the Kansas Geological Survey, principally KWIKR8 (Esler et al., 1968). The program treats one dependent variable (Mn, Rb...) and two independent variables (geographic coordinates) with surfaces to degree 9. A principal problem in trend analysis is deciding which order of surface " b e s t " represents the data. Although statistical tests exist, geologic inference must enter into any decision (Baird et al., 1971 ). In the case of the Louisiana shelf sediment data, geologic inference indicates that a degree-4 surface for most of the variables is hopelessly complex, reflecting too much "noise". A
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
107
degree-2 or degree-3 surface is a better indicator of regional trends, which appear in either surface. Happily, two statistical tests support that conclusion: the F-ratio, which is a procedure o f analyses of variance in which "... the total variance of a variable being measured is broken down into the portions or components caused by the different factors .... it is possible ... to test each in turn for a significantly large contribution at a confidence level" (Baird et al., 1971, p. 1223), and the F-test, which compares the ratio o f the sample mean squares with the F-test table for a desired significance level. The F-ratio increases, in most cases, up to degree-3, but shows no improvement with higher degrees, indicating degree-3 is a best choice (Chayes, 1970). Degree-3 surfaces also have slightly higher significance levels when an F-test is applied (Table II); degree-4 surfaces fall o f f considerably in significance level. Table II presents percent sum of squares of variability accounted for by the surfaces, the mean-squares ratio F, and the significance level from an F-test. We consider surfaces at the 0.10 confidence level to be valid, especial-
TABLE II
Trend surface statistics Variable
Order
S.S. (%)
Signif. level
Rb
2 3
34 44
2.30 1.91
0.10 0.10
Sr
2 3
29 43
1.84 1.86
0.25 0.10
Ni
2 3
44 60
3.53 3.64
0.01 0.01
Ti
2 3
42 59
3.16 3.57
0.05 0.01
Fe
2 3
48 63
4.14 4.15
0.01 0.01
Mn
2 3
60 82
6.83 11.02
0.01 0.01
Ti/Mn
2 3
22 38
1.23 1.53
n.s. 0.25
Ti/Zr
2 3
47 65
3.99 4.53
0.01 0.01
Zr
2 3
9 25
.44 .83
n.s. n.s.
F
108
T.T. TIEH ET AL.
A •
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-
-
~
~
-~
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~oo
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~0o
Ti
D
Fig. 2. A - G , third-degree trend surface maps for Mn, Fe, Ti, Ni, Rb, Sr, and Zr in sediments in the inner continental shelf off the Louisiana coast; H, that of Ti/Mn ratio.
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
109
*%.~
E LOUISIANA
.,,o
~
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~
~
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o
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T.T. TIEtI ET AL.
ly because Rb and Sr, at the 0.10 level, have map patterns similar to Fe and Ni, which are significant at the 0.01 level. Third-degree trend surfaces for the various elements analyzed during this study are presented as Fig 2, A - G ; a trend map of Ti/Mn is included as Fig. 2H. DISCUSSION
Table I shows that, in general, the analytical data within each sediment group are fairly consistent despite the fact that samples within a group are widely spaced and they have variable size distributions. Such consistency of data is particularly evident in the delta-destructional sands and tile clay-rich deltaic sediments. To characterize each group chemically, the group average of a given element is compared with the grand average for all samples analyzed; where the group average is lower than the grand average, it is designated as low, and where higher it is designated as high. Thus, the chemistry of the four major sediment groups is summarized as follows: Relict sediments: low = Mn, Fe, Ti, Zr, Rb, Ni high = Sr Delta-destructional sands: low = Mn, Fe, Rb, Ni high = Ti, Zr, Sr Deltaic sediments: low = Zr, Sr high = Mn, Fe, Ti, Rb, Ni Non-deltaic sediments: low = Zr, Sr high = Mn, Fe, Ti, Rb, Ni It is evident that the deltaic and non-deltaic sediments can not be distinguished on the basis of chemical data available. These sediments have high silt and clay contents and median diameters in silt and clay classes; they represent modern-day delta deposits of Mississippi and other major rivers as well as deposits from erosion of beaches and shoals. Although Krawiec (1966) recognized that there is a greater amount of sand in the non-deltaic sediments than the deltaic deposits, the very poor sorting, high variability in size distribution, and high clay content of both types of sediments make
CHEMISTRY OF INNER C O N T I N E N T A L SHELF SEDIMENTS
1 11
their distinction difficult on either physical or chemical basis. On the other hand, distinctions between relict sediments, delta-destructional sands, and deltaic sediments can be readily seen on the basis of the analytical data. Relict sediments and deltaic sediments have contrasting chemistry in that for every element analyzed, except for Zr, it is always high in one and low in the other. The very high contents of Zr, Ti and Sr in the delta-destructional sands make this group further distinctive from all the others. Differences in chemistry of the various sediment groups are further den> onstrated in the trend surface maps. A comparison of Fig. l, the sedimentary facies map drawn on the basis of physical properties, and Fig. 2, A - G , the trend maps, indicates that modern-day deltaic and non-deltaic sediments in the areas of Mississippi Delta and along the coast of Louisiana are high in the content of Rb, Ni, Fe, Mn, and Ti; decreasing trends of these elements towards the west of the area, where the relict sediments occur, are clear and systematic. The trend map of Zr, however, is exactly contrary to those mentioned above. Zr shows a trend pattern somewhat different from all the others, and is controlled to a large extent by the distribution pattern of the delta-destructiona] sands. Zr occurs principally in zircon crystals. The close association of Zr with the delta-destructional sands is due to the high stability and density of the mineral. These properties have let the mineral survive the delta-destruction 4000 O DELTAIC SEDIMENTATION
I
x
• NON-DELTAIC SEDIMENTATION • RELICT SEDIMENTS [ ] DELTA- OESTRUCTIONAL SANDS
i
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O DELTAIC SEDIMENTATION • NON-DELTAIC SEDIMENTATION • RELICT SEDIMENTS D DELTA- DESTRUCTIONAL SANDS 40
50
60 CLAY(%)
Fig. 3. A b u n d a n c e of Zr versus percentage of sand in all samples. Fig. 4. Relationship between clay c o n t e n t and abundance of Fe in samples from the various sedimentary provinces.
112
T.T. T I E H E T A L .
phase and become concentrated in this sediment group. However, there does not seem to be any consistent relationship between Zr and the size distribution of the sediment samples, although one would expect that Zr would be enriched in the sand and silt fractions. This lack of correlation can be seen in tile scattered pattern of Fig. 3, a plot of Zr against sand percentage of all samples studied. In Fig. 3 samples o f the delta-destructional sands can be readily distinguished from the other groups, especially those of the deltaic sedimentation group, by the high sand and Zr contents. The principal Ti-bearing minerals are ilmenite, rutile, leucoxene, and sphene. Ti-bearing pyroxenes occur in sands farther west of the present area of study (Bullard, 1942). Unlike Zr, the range of variation for Ti in the four sediment groups is relatively small, perhaps because the cryptocrystalline leucoxene may occur in particles of sand to finer-than-clay sizes. The fact that Ti is high in the clay-rich deltaic and non-deltaic sediments suggests that much of the Ti is incorporated in the clay fraction. It is interesting to note that the trend surface map of Ti/Mn ratio (Fig. 2H) is similar to that of Zr. This similarity is evidently due to the faster rate of removal of Mn than Ti subsequent to deposition. Whereas the bulk of Mn and some Ti occur as cryptocrystalline oxides and/or hydroxide particles adhered to the surfaces or cleavages of clays, there are no Mn-bearing minerals in the silt-sand fractions that are equivalent in abundance to ilmenite, rutile, and sphene. Consequently, removal of clays during delta destruction processes has given rise to an increase in Ti relative to Mn. 1600
1400
1200
0 DELTAIC SEDIMENTATION • NON-DELTAIC SEDIMENTATION • RELICT SEDIMENTS [[]DELTA-- DESTRUDTIONAL SANDS / / '
f
/
/ /\.O\
/'
~,
/
/
/
/
I000
/ ,/
"~ 800
.."; / ,,'" .-" /',/ o.,..... ,%//° ,"
60(
/
/ /
,,,
40¢
20(
,
0
I
2
Fig. 5. Mn v e r s u s Fe.
5
4
5-
, Fe(%) , 6 7
CHEMISTRY OF INNER CONTINENTAL
SHELF SEDIMENTS
11 3
8°f 70
i./.
60
/
.S/..
....... o ~ /
5oi o_ 40 z
5C o • • D
2C
DELTAIC SEDIMENT AT IO N NON-DELTAIC SEDIMENTATION RELICT SEDIMENT S DELTA - DESTRUCTIONAL SANDS
10,[
0
I
2
5
4
5
6
Fe(%) 7
Fig. 6. Ni v e r s u s Fe.
0
2001 175 150
\
125
\
A IOO E & 75 G • • []
50
DELTAIC S E D I M E N T A T I O N NON-DELTAIC SEDIMENT AT IO N RELICT S E D I M E N T S DELTA - DESTRUCTIONAL SANDS
25
0
1(30
200
300
400
500
600
Sr(ppm)
Fig. 7. R b v e r s u s Sr.
The close association of Fe with tile clays (Fig. 4) indicates that the bulk of Fe also occurs as limonite (amorphous to.cryptocrystalline mixtures of oxides and/or hydroxides of iron) particles adhered to the clays. As both Mn
114
T.T. TIEIt ET AL.
IAB LE I!1 Composition of Gulf of Mexico sediments Location
No. of samples
Fc (%)
Mn (p.p.m.)
Ni (p.p.m.)
Rb (p.p.m.)
Sr (p.p.m.)
Ti (p.p.m.)
Zr Ip, p.m.)
Inner shelf
(39)
2.98
548
46
147
289
3.306
658
Slope*
(36)
Abyssal* plain
3.15
911
86
87
257
4.332
79
Abyssal* hill
(19)
3.36
1A64
91
99
194
4,538
81
(13)
2.12
2,409
75
54
596
2,208
92
* f:rom Tieh and Pyle (1972).
and Ni vary proportionally with Fe (Fig. 5,6), it may be concluded that freshly deposited clay-rich sediments, whether deltaic or non-deltaic in origin, have overall high contents of these three elements and that processes subsequent to deposition tend to redistribute these elements and cause their reduction. The inverse relationship between Rb and Sr (Fig. 7) is a c o m m o n occurrence in marine sediments (Tieh and Pyle, 1972). Sr occurs mainly as a replacement for Ca in carbonate shell fragments, which are abundant in the delta-destructional sands. Rb is present as adsorbed ions in clays or in the structure of feldspars in the silt- and clay-rich samples. Table III is a summary of the distribution of these seven elements in sediments of the various physiographic provinces of the Gulf of Mexico. The figures represent averages either of some areally distributed samples (such as those of this study) or of samples from one or more cores taken from a certain physiographic location. Tim abyssal-hill samples represent a peculiar and limited environment. If we consider only the shelf, slope, and abyssal plain, we find that the sediments are gradually enriched in Fe, Mn, Ni, and Ti, and become depleted in Zr, Sr, and Rb from the shelf to the abyssal plain. ACKNOWLEDGI';M ENTS
We thank Dr. Robert L. Lankford for making the samples available to us. A grant from the Robert A. Welch Foundation (Grant A-351 to Tieh) made this study possible.
CHEMISTRY OF INNER CONTINENTAL SHELF SEDIMENTS
115
RI~FER I'~NCES Baird, A.K., Baird, K.W. and Morton, D.M., 1971. On deciding whether trend surfaces of progressively higher order are meaningful: discussion. Geol. Soc. A m., Bull., 82: 1219-1234. Bullard, I:.M., 1942. Source of beach and river sands on Gulf Coast of Texas. Geol. Soc. Am., Bull., 53: 1021- 1(144. Chayes, 1:., 197(t. On deciding whether trend surfaces of progressively higher order arc meaningful. Geol. Soc. Am., Bull., 81: 1273- 1278. Davies, D.K. and Moore, W.R., 1970. Dispersal of Mississippi sediments in the Gull" of Mexico..I. Sediment. Petrol., 40: 339-353. Esler, J.E., Smith, P.F. and Davis, .I.C., 1968. KWIKR8, a I.'OP,TRAN IV program for multiple regression and geologic trend analysis. Kansas State (;eol. Sz4rv., Compttter Contrih., 28:31 pp. Griffin, G.M., 1962. Regional clay-mineral facics products of weathering intensity and current distribution in the northeastern Gulf of Mexico. Geol. Soc. Am., Bull., 73:737 767. Krawiec, W., 1966. Recent Sediments o f the l.ouisiana Inner Continental Shel]i Thesis Rice University, 50 pp. Tieh, T.T. and Pyle, T.E., 1972. Distribution of elements in Gulf of Mcxico sediments. In. R. Rezak and J. ttenry O'dit0rs) , Contributions on the (;eologieal and Geophysical Oceanography o f the Gulf o f Mexico, 5. Gulf Publishing Co., Houston, Texas, pp. 129 152.