The role of diagenesis in the development of physical properties of deep-sea carbonate sediments

Marine Geology, 69 (1985) 69--91

69

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THE ROLE OF DIAGENESIS IN THE DEVELOPMENT OF PHYSICAL PROPERTIES OF DEEP-SEA CARBONATE SEDIMENTS

D A E ~ H O U L KIM j, MURLI H. MANGHNANI l and SEYMOUR O. SCHLANGER 2

'Hawaii Institute of Geophysics, University of Hawaii, 2525 Correa Road, Honolulu, HI 96822 (U.S.A.) 2Northwestern University, Evanston, IL 60201 (U.S.A.) (Received November 26, 1984; revised and accepted February 8, 1985)

ABSTRACT Kim, D.-C., Manghnani, M.H. and Schlanger, S.O., 1985. The role of diagenesis in the development of physical properties of deep-sea carbonate sediments. Mar. Geol., 69: 69--91. Laboratory measurements of ultrasonic velocity (Vp, VS) and attenuation (Q~J, Q~) in deep-sea carbonate sequences at DSDP Sites 2 8 8 , 2 8 9 and 316 in the equatorial Pacific were made in conjunction with studies of sediment density, porosity and pore geometry in order to investigate the role of diagenesis in the development of physical properties. Bulk porosity decrease appears to be related more significantly to depth of burial than to age of strata. Both depth of burial and age, however, are important factors controlling the modal pore diameter. In deep-burial diagenesis the modification of pore geometry is influenced by the presence of silica during diagenesis. In carbonate sequences at the three DSDP sites studied, shear wave attenuation anisotropy (Q~HH/Q~Hv) correlates with the shear wave velocity anisotropy. Pore orientation, resulting from overburden pressure and other deep-burial diagenetic processes, is an important factor controlling the increase of Vp anisotropy with age and depth of burial. On the basis of observed minor changes in anisotropy values with increasing pressure for some samples, other contributions to Vp anisotropy such as grain orientation and bedding lamination cannot be ruled out. INTRODUCTION

Several laboratory studies have described the physical and acoustic properties of deep-sea sediments, such as porosity (~), density (p), and compressional (Vp) and shear (Vs) velocities in terms of age and depth of burial (Boyce, 1976; Hamilton, 1976, 1980; Milholland et al., 1980). In general, with depth of burial, density, velocity and velocity anisotropy of deep-sea sediments increase concomitantly with decreasing porosity. This relationship is due to the combined effect of simple mechanical compaction by overburden pressure at shallow depth and cementation and/or recrystallization processes at depth (Schlanger and Douglas, 1974; Van der Lingen and Packham, 1975; Hamilton, 1976). The number of studies of the diagenesis of deep-sea carbonate sediments have increased with the availability of cores recovered by the Deep Sea 0025-3227/85/$03.30

© 1985 Elsevier Science Publishers B.V.

70 Drilling Project (DSDP). Progressive changes in geochemical parameters such as magnesium (Mg2+) and strontium (Sr 2÷) content and stable isotope ratios ('60/lsO, I~C/~3C) with increasing depth and age have been d o c u m e n t e d (Matter et al., 1975; Manghnani et al., 1980; Garrison, 1981}. Numerous studies have emphasized interpretation of the changes in physical properties with depth of burial and age o f sediment sequences with diagenetic parameters (Packham and Van der Lingen, 1973; Schlanger and Douglas, 1974; Matter et al., 1975; Van der Lingen and Packham, 1975; G a r d n e r e t al., 1977; Carlson and Christensen, 1979; Mayer, 1979; Hamilton, 1980; Manghnani et al., 1980). This paper correlates laboratory data on acoustic and related physical properties, such as velocity, attenuation, density, porosity, and pore geometry, of deep-sea carbonate sequences to investigate the relationship between physical properties and diagenetic parameters. STRATIGRAPHY OF THE DSDP SITES STUDIED Of the three DSDP sites chosen for this study, two (288 and 289) are on the Ontong~lava Plateau in the western equatorial Pacific and the third (316) is in the central equatorial Pacific (Fig.l). At all of these sites (Fig.2) the sediments drilled are pelagic carbonate sequences that record the ooze-chalk--limestone transition (Andrews, Packham et al., 1975; Schlanger, Jackson et al., 1976; Berger et al., 1977). The calcium carbonate content of the section at Sites 288 and 289 is high (90--100% CaCO3 between 100 and 700 m subbottom at Site 288, and over most of the sequence at Site 289). Major components o f the sediment are biogenic calcite produced by coccoliths, discoasters, and foraminifera with some interbedding o f biogenic silica and minerals o f volcanic origin that increase with increasing depth (Andrews, Packham et al., 1975). Hole 288 was drilled on the eastern flank of the Ontong~ava Plateau at water depth of 3000 m (Andrews, Packham et al., 1975). The oldest sediments recovered are Lower Cretaceous (Aptian) limestones (Fig.2). The stratigraphic column is classified into two lithologic units by a boundary between ooze and chalk at around 500 m subbottom depth. The calcium carbonate content generally decreases with depth of burial because of increasing amounts o f chert and volcanic material (Zemmels et al., 1975). In subunits 1A and 1B of Site 288, both velocity and density increase gradually with depth (Fig.3). The abrupt changes in physical properties such as porosity, bulk density, and velocity at levels below Unit 2 are believed to be related to compositional variation and induration of the sediments. For example, the occurrence of dolomite at about 650 m subbottom depth coincides with the beginning of sharp increase in velocity from 1.6 to 2.4 km s -1 down to 740 m subbottom depth. An appreciable degree of velocity anisotropy is observed at this level, which also corresponds to a seismic reflector at the top o f subunit 2B (Andrews, Packham et al., 1975). At greater depths,

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74 in the siliceous limestone zone (subunits 2D and 2E), several jumps in velocity (2.5--3.5 km s -1) were reported (Figs.2 and 3). These variations in velocity match well with the fluctuating bulk density (2.1--2.3 g cm-~) and porosity (25--16%; Fig.3). Hole 289 (water depth 2206 m) is one of the thickest and most complete ooze--chalk--limestone sequences drilled in the Pacific Basin. The basal Aptian limestone and tuff lies on basaltic basement at 1271 m s u b b o t t o m depth (Andrews, Packham et al., 1975; Fig.2). The sediment column is divided into t w o lithologic units. The boundary o f Unit 1 (969 m), which coincides with the E o c e n e - O l i g o c e n e unconformity, is the b o t t o m of a continuous sequence of nanno-foram ooze and chalk with a high carbonate content (90--100%). As can be seen, the physical properties (~, p and Vp) change gradually with increasing depth (Fig.3). Subunit 2A (969--1231 m) is dominated by nanno-foram chalk and limestone compared with the relatively more siliceous and lithified subunit 2B (1231--1262 m). Subunit 2B consists mainly of nanno-foram and siliceous limestone, and chert interbedded with volcanic ash and tuff. The abrupt changes in physical properties below 1000 m reflect the occurrence of chert, which correlates with an observed seismic reflector at the t o p of Unit 2 (Andrews et al., 1975). Hole 316 (water depth 4465 m) was drilled south of Christmas Island in the Line Island chain (Schlanger, Jackson et al., 1976). The oldest sediment recovered is Upper Cretaceous (Fig.2). The acoustic basement consists of high-velocity limestone rather than chert. The stratigraphic sequence at Site 316 is divided into four lithologic units (Fig.2). Unit 1 (0--2 m) is a cyclic unit that consists of white and pale orange calcareous ooze interbedded with yellowish-gray siliceous ooze. Unit 2 (2--380 m) consists of thick, varicolored calcareous and siliceous ooze, and chalk. A 200 m-thick limestone, chert and dolomite sequence represents Unit 3 (380--580 m). Unit 4 (580--837 m) consists of volcanoclastic breccia, graded sandstone, and calcareous chalk and limestone. The upper part of the unit is dominated by chalk, limestone and chert whereas the lower part is dominated by volcanogenic rock fragments. EXPERIMENTAL PROCEDURE Ultrasonic m e a s u r e m e n t s

For ultrasonic velocity and attenuation measurements, samples were cut and ground into cuboids of about 1.5--2 cm on a side. The techniques for measuring compressional velocity under independently controlled confining and pore pressures and ultrasonic attenuation for both compressional (Q~I) and shear (Q~I) waves under atmospheric pressure have been previously described (Kim et al., 1983). The measurements were made in two mutually perpendicular directions, i.e., parallel and perpendicular to the drill hole orientation. For Vp measurements, the pulse transmission technique was used (Birch, 1960). Attenuation was measured by means of the spectral ratio technique

75 (Toksoz et al., 1979; Johnston and Toksoz, 1980; Sears and Bonnet, 1981). The technique involves comparison of the ratio of spectral amplitudes of pulses transmitted through the reference with those transmitted through the sample, which gives a measure of the relative attenuation. Lead zirconium titanate (PZT 4) transducers having natural resonance frequency of 1 MHz were used for velocity measurements in order to keep the ratio of the wavelength of the ultrasonic pulse to the pulse travel length (~/L) less than 1 (Kolsky, 1953). At 1 MHz, the wavelength (X) of the pulse (2--5 mm) in a sample is much larger than the maximum grain size in the specimen. In order to improve the reproducibility of the attenuation measurements and to provide high damping, aluminum buffer rods and backing pieces of nearly the same impedance as the transducers were used (Sears and Bonner, 1981; Kim et al., 1983).

Porosity and pore-size measurements Bulk porosity and pore-size distribution in the samples from Sites 288 and 289 were simultaneously measured with a mercury injection porosimeter (Micromeritics 9300). The principle is based on the capillary law governing penetration of non-wetting liquids such as mercury into a porous medium. The volume of mercury injected into the evacuated pores is a function of increasing pressure and is related to pore size, shape and connectivity. The resultant pressure--volume curve enables determination of the pore-size distribution (Fig.4). For a cylindrical pore or pore throat the size is expressed by: D = (47 cos O)/P

(1)

where D is pore diameter, P is the applied pressure, 7 is the surface tension of liquid, and 0 is the contact angle for liquid on solid. Equation (1) applies to situations in which the pores and throats are cylindrical. Wardlaw (1976) has modified eqn. (1} to a parallel sheet-like plate model, which is more applicable to dolomite and limestone. In such a case the diameter of a pore in eqn. (1)is reduced to a half at the same applied pressure. Thus (1) becomes: D = (27 cos O)/P

(2)

where D denotes the distance separating two parallel surfaces. In this study, the pressure-'volume relations were determined for a number of samples, and the pore-to-throat-size ratios were evaluated from the slopes of curves for b o t h injecting and ejecting mercury. The penetration of mercury into the pore spaces would be inhibited, especially in situations where the pores are connected to smaller than larger throats, bulk porosity being the same (Wardlaw and Cassan, 1979; Wardlaw and McKellar ~, 1981). Thus, the distribution of pore diameter, determined from the pressure versus volume of injected mercury data, m a y be related more closely to the size o f throats than of pores. The difference in cumulative pore volume curves for the same hydrostatic pressure between injected and ejected mercury is a function of pore-to-throat-size ratio (see Fig.4).

76

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SEM study T h e p o r e g e o m e t r y and o r i e n t a t i o n in h o r i z o n t a l and vertical d i r e c t i o n s in s e d i m e n t samples were investigated with Scanning E l e c t r o n M i c r o s c o p e (SEM) t e c h n i q u e s . Care was t a k e n in selecting t h e p o r t i o n s o f t h e original core samples t h a t have significant velocity a n i s o t r o p y . T h e sample surface was polished a b o u t six t o eight h o u r s b y an ion milling machine. RESULTS AND DISCUSSION

Porosity--pore geometry--acoustic property relations T h e t r e n d and m o d e o f the p o r o s i t y change in pelagic c a r b o n a t e sediments are c o n s i d e r e d t o be m o r e p r e d i c t a b l e t h a n in shallow-water c a r b o n a t e sedim e n t s (Scholle, 1 9 7 9 ; Garrison, 1981). This p r e d i c t a b i l i t y o f the pelagic c a r b o n a t e is p r o b a b l y d u e t o c o m p a r a t i v e l y u n i f o r m d e p o s i t i o n a l facies, p r e d i c t a b l e p r i m a r y chemical c o m p o s i t i o n (e.g. a l m o s t entirely low Mg calcite), absence o f interstitial m e t e o r i c w a t e r invasion, and u n i f o r m a n d fine grain size c o m p a r e d t o their shallow-water equivalents. Thus, the increase in degree o f lithification with d e p t h o f burial in t h e core samples is related t o p o r e size and shape.

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Fig.5. Porosity versus age (a) and burial depth (b) for D S D P Sites 288 (solid triangles and squares) and 289 (open triangles and squares). Triangles represent chalk and squares indicate limestone. Note the similarity in the porosity versus depth of burial patterns compared to those for the porosity versus age. Solid lines indicate the least-squares fits.

Figure 5 compares the porosity change with age (a) and depth of burial (b) at the two DSDP sites from the Ontong~Java Plateau. Porosity decreases with increasing age and depth of burial; however, the porosity--age patterns are different for the t w o different sites. In contrast, the porosity--depth relationship for both sites falls more or less on the same line. Thus, it appears that depth of burial is a more significant variable than age in determining the bulk porosity of deep-sea carbonate sediments. The depth of burial is the dominant factor controlling the porosity change in pelagic carbonate sequences (Scholle, 1979; Garrison, 1981). Moore (1979) suggested that mechanical compaction resulting from overburden pressure is the dominant mechanism for porosity reduction in a fine-grained sediment only during the water expulsion stage, that is, before the individual grains form a framework. Further porosity reduction is, however, influenced by chemical or pressure solution processes. As a result, the porosity decreases with increasing density and velocity, paralleling the increase in degree of lithification with depth of burial (Fig.3). The complete model for pelagic carbonate sediments proposed by Schlanger and Douglas {1974) is useful in explaining the porosity changes in view of the diagenetic stages from ooze to chalk to limestone. According to their model, porosity decreases rapidly from 80 to 65% in the first 200 m and then more gradually to 40% by 1000 m. This decrease is a result of a transformation from ooze to chalk to limestone through pressure solution and reprecipitation. Schlanger and

78 Douglas (1974) also introduced a concept of "diagenetic potential" to interpret the observed local deviations in progressive diagenesis such as lithification reversals from chalk back to ooze or from limestone back to ooze or chalk. At Site 289, for example, a soft ooze of Maestrichtian age lies within the limestone zone several hundred meters below the top of the chalk zone (see Fig.2). This ooze is time-correlative with a reversal in the chalk interval at nearby Site 288 (Andrews, Packham et al., 1975). Garrison (1981) reviewed the diagenetic data for twelve DSDP sites with thick carbonate sediments in several oceanic regions to study the progressive changes in lithification with age and depth. He found that the diagenetic transformations {ooze--chalk--limestone) are not a simple function of age and depth of burial. Rather, as proposed by Schlanger and Douglas (1974), other subparameters such as sedimentation rate, water depth, surface productivity, temperature, CaCO3 content, and original composition of calcareous ooze would affect the "diagenetic potential". In general, the mechanical compaction, which includes dewatering, grain reorientation, and some grain deformation, is dominant in the early stage of burial diagenesis. The fairly rapid porosity decrease from about 70% at the sea-floor boundary to 50% at 210 m seems mainly due to this mechanical compaction. Below this depth, the major mechanisms for porosity loss are pressure solution at grain contacts and reprecipitation of calcite cement (Garrison, 1981). Decreasing porosity is closely related to compressional velocity at Site 288 when data are grouped according to lithology of the samples (Fig.6). This grouping seems reasonable because the boundaries between ooze, chalk, and limestone at the site are well defined (Andrews, Packham et al., 1975). The slopes become steeper with progressive lithification. The steepest slope in the limestone group in Fig.6 can be explained b y an increase in sediment rigidity; hence there is an increase in velocity (Mayer, 1979). Secondary calcite overgrowth continues to fill up pore space (e.g., on the coccolith placoliths and inside foraminiferal chambers). Simultaneously, the particles are welded together; as a result, the porosity decreases to less than 30%, and the density and velocity increase (Van der Lingen and Packham, 1975). The chalk--limestone boundary corresponds to the deepest seismic reflector at 847 m s u b b o t t o m depth (Andrews, Packham et al., 1975). Figure 7 shows changes in the modal pore diameter as a function of age (a) and depth of burial (b). Compared to the plots of porosity versus age or depth of burial (Figs.5a and b), these plots show larger scatter, and neither age nor burial depth appears to control the modal pore size. It appears, however, that age correlates slightly better with the modal pore size than depth of burial. Figures 5 and 7 also indicate that the porosity and modal pore size decrease more rapidly in older and deeper sediments. Poor correlations between modal pore diameter and age, and modal pore diameter and depth of burial are expected with progressive diagenesis. The major diagenetic processes in the deep-burial region, i.e., below 200 m subb o t t o m , are characterized by development of calcite cement and overgrowth

79

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80 (Fischer et al., 1967; Schlanger and Douglas, 1974; Scholle, 1979). These processes reduce the polyhedral pores to tetrahedral pores with sheet-like pore connections rather than tubular connections as in the case of dolomite (Wardlaw, 1976). During the process of porosity reduction, however, especially for the uniformly cemented grains, the pore throat size would be reduced more than the pore size because throats are initially smaller than pores (see fig.6 in Wardlaw and Cassan, 1979). Thus, deep-burial diagenesis plays a greater part in reduction of modal pore size than in bulk porosity because the modal pore size is about the same as the pore throat size and is highly sensitive to cementation and recrystallization processes. The influence of silica diagenesis on carbonate diagenesis may also contribute to the scatter in Fig.7. Van der Lingen and Packham {1975) suggested that surplus calcium carbonate outside the chert nodules, which is released from the replacement of calcite by silica, can be precipitated in adjacent inter- and intraparticle pore spaces. For example, for some depth intervals below 750 m s u b b o t t o m depth at Site 288, porosity is low (<25%) and silica content is high (35 to ~65%; Andrews, Packham et al., 1975). Thus, addition of even a small amount o f excess calcium carbonate from outside these small pores will cause the pore connections to decrease faster than the pore spaces. This effect m a y result in a significant change in pore throat size without a big change in bulk porosity. The fluctuations of velocity and velocity anisotropy below s u b b o t t o m depth o f 750 m at Site 288 (see Fig.3) may be attributed to the presence o f silica and its effect on carbonate diagenesis. In such cases the interbedded silica dominates the observed acoustic property. Nonetheless, the role of porosity and pore geometry changes related to this silica diagenesis cannot be ruled out. Thus, the corresponding fluctuations of high-velocity anisotropy and velocity with a rather smooth porosity profile along this interval should be considered as possibly being influenced by silica diagenesis. Consequently, the data points in the velocity--modal pore-size plot for Site 288 (Fig.8) are more scattered than in the velocity--porosity plot (Fig.6). The modal pore size for ooze was unavailable. This trend seems reasonable if we consider that modal pore size is more sensitive to the diagenetic environment than porosity is. Moreover, some highly scattered points in Fig.8, i.e., less than 0.7 ~m size range, coincide with the depth o f high silica content. There is, however, a general increase in velocity with decreasing modal pore size.

Velocity and attenuation anisotropy Figure 9 shows an example of changes in Vp in both horizontal (VpH) and vertical (V~v) directions as a function of increasing differential pressure (Pd) for a nannofossil chalk sample. Here, the differential pressure is the difference between the hydrostatic confining pressure (Pc) and the pore pressure (Pp) in the pore system of a sample. The velocity anisotropy is defined as the difference in velocity between horizontal and vertical directions divided by mean velocity (Kim et al., 1983). The steeper slope for the Vpv compared to

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Fig.8. Modal pore diameter versus m e a n compressional velocity at Site 288. S y m b o l s are the same as in Fig.6. Solid lines indicate the least-squares fits for chalk and limestone, respectively, and the dotted line is the least-squares fit for all the data.

the VpH with increasing Pd indicates that fiat pores that are dominantly aligned in the horizontal direction (perpendicular to the core axis) can be closed with increasing Pd. This closure results in a faster increase for Vpv than for VpH. Thus, the compressional wave velocity anisotropy (Ap) for this sample decreases from 11.4 to 3.3% for Pd from 0 to 4 0 0 bar. This decrease is a result of the decrease in velocity difference (i.e., VpH -- VpV) from 0.25 (Pd = 0 bar) to 0.09 km s- ' (Pd = 4 0 0 bar). The result also confirms our earlier conclusion that the pore geometry contributes, to some extent, to the observed velocity anisotropy (Kim et al., 1983). The rapid velocity increase (~ 14%) at low Pd (up to 80 bar) suggests that the pores of low-aspect ratio (~ = minor axis/major axis) tend to close at low pressures. Despite its relatively high porosity (34%) the extremely low modal pore size (0.03 pm) in this sample represents the effects of pore geometry on the velocity and velocity anisotropy. In this case the very thin pore throats as well as some fiat pores that are developed by deep-burial diagenesis are closed at lqw Pd. This closure results in rapid velocity build-up for low Pd (Walsh, 1965; Toksoz et al., 1976). The nannofossil chalk discussed above was studied with the SEM (Fig.10). The upper photograph shows a surface perpendicular to the core axis (parallel to the bedding). In this section pores o f relatively high aspect ratio and more

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HORIZONTAL

~%

o

>t

(VpH)

0

o

zx &

2.6

[]

z~

zx

f,

C3 -J UJ >

VERTICAL

£

(Vpv)

J

g

2.4

/x

¢

U~ u.l n," m (23 U

2.2 ~

2.0

I

l

21~0

i

I

400

DIFFERENTIAL PRESSURE, bar8

Fig.9. An example of compressional velocity in both horizontal and vertical directions versus differential pressure for an Upper Cretaceous nannofossil chalk from DSDP Site 288 (subbottom depth 745 m). Porosity is 33.9% and the modal pore diameter is 0.03 #m. Note the sharp increase in velocities between 0--80 bar Pd due to closing of pores of very low-aspect ratio. Also note the steeper gradient for vertical than horizontal direction. The points marked with " # " represent the estimated in-situ pressure conditions.

or less random orientation dominate. The lower photograph is of a section cut parallel to the core axis; thinner and horizontally elongated pores (loweraspect ratio) dominate. Figure 11 shows th.e pore geometry of a siliceous limestone ( s u b b o t t o m depth 851 m) at the same site. As seen in the figure in the limestone sample, the thinner pores dominate both in the vertical and horizontal sections in contrast to sections in the nannofossil chalk sample in Fig.10. Also note that the horizontally aligned pores o f low aspect ratio dominate in the section parallel to the core axis (lower photograph) b u t n o t in the section perpendicular to the core axis (upper photograph). These SEM photographs clearly show the development of preferred orientation of horizontally elongated pores with increasing depth of burial by diagenetic processes, which supports the idea of pore geometry as a cause of velocity anisotropy. It is of interest to compare the results of the acoustic and SEM studies. Figure 12 shows measured velocity anisotropy change versus pressure for the samples recovered from Sites 288 and 289. The physical and acoustic properties of the samples discussed in Fig.12 are listed in Table 1. In general, the anisotropy is found to be decreased with increasing Pd; however, the rate of

83

I

Fig.10. S E M photographs of the nannofossil chalk from which the data in Fig.9 were obtained. The upper photograph shows a surface perpendicular to the core axis and the lower photograph shows a surface parallel to the core axis. The arcuate structures in the horizontal direction are coccoliths. In contrast to the randomly shaped and oriented pores in the horizontal direction, some pores in the vertical direction are elongated and oriented horizontally. Scale bars indicate 5 urn. The sample surfaces were prepared by ion milling.

change in velocity anisotropy with pressure varies from sample to sample. The data point plotted in asterisks (subbottom depth 745 m) corresponds to the nannofossil chalk sample shown in Fig.9, where anisotropy decreases from 12 to 7% up to Pd of 80 bar. The slopes for most of the samples in high Pd regions are comparatively flat, suggesting that the anisotropy is caused by

84

Fig.ll. SEM photographs for an Upper Cretaceous limestone from the DSDP Site 288 (burial depth 851 m). Orientation of the photograph is the same as in Fig.10. Note the development of a greater number of thin, horizontal pores in the vertical section (lower photograph) than the horizontal section (upper photograph). The sample has 22.7% porosity with 0.02 ~m modal pore diameter. The velocity anisotropy is 7.6%. Scale bars indicate 5 urn. s o m e o t h e r factors. A l i m e s t o n e s a m p l e ( s u b b o t t o m d e p t h 1 1 3 2 m ) w i t h i n t e r b e d d e d c h e r t ( o p e n squares) s h o w s little a n i s o t r o p y c h a n g e w i t h increasing Pd, indicating t h a t s o m e degree o f p r e f e r r e d o r i e n t a t i o n in p o r e s c o u l d exist in this s a m p l e . On t h e o t h e r h a n d , t h e a n s i o t r o p y values f o r a n a n n o fossil l i m e s t o n e s a m p l e ( s u b b o t t o m d e p t h 1 0 5 0 m ) d e c r e a s e t o zero a t

85

15 O,O,

10

A

~

30-289

k-

N

O*

>: 0,. C) rY I-CI U~

-~

+ (gTl m) +

;:/

O*

~s

<745 m)

+

+

% (1132 m>

~A A A

0

100

2 0

,~

(1050 m)

300

400

590

DIFFERENTIAL PRESSURE. bare

Fig.12. Velocity anisotropy versus differential pressure. The sample descriptions are listed in Table 1. Numbers in parentheses indicate depth of burial. The numbers in the legend (upper right) represent the DSDP legs and sites. The symbol " # " represents the estimated in-situ conditions.

TABLE 1 Lithology, depth, age, and physical and acoustic properties of the samples measured under pressure at DSDP Sites 288 and 289 DSDP Site

288 288 289 289 289

Lithology

nannofossil chalk limestone nannofossil limestone nannofossil limestone limestone

Burial depth (m)

Age (m.y.)

p (g cm -3)

¢ (%)

Modal pore diameter (urn)

Vp* (kin s -1) VpH

Vpv

Ap

(%)

745

83

2.01

33.9

0.03

2.63

2.46

6.9

971 1009

105 43

2.31 1.90

23.2 41.0

0.28 0.88

3.35 2.97

3.10 2.79

7.8 6.1

1050

46

2.44

20.4

0.33

3.10

3.08

0.7

1132

58

2.59

7.3

0.23

5.65

5.43

3.9

*Estimated in-situ pressure conditions.

86 Pd ~ 120 bar (open triangles) and become slightly negative at higher Pd. In this case, the pore orientation is the main cause of the observed anisotropy. Toksoz et al. (1976) studied the effect of pore aspect ratio on the seismic velocity in some sandstones under pressure conditions such that the cracks with aspect ratio smaller than 10 -4 are closed under 200 bar Pa. At Pd = 500 bar, all the fine pores with aspect ratio smaller than 10 `2 are closed. On the other hand, under compression the spherical pores will retain their spherical shape while decreasing in volume. Thus the flatter pores contribute more to velocity than the rounder or more spherical pores for samples with the same porosity. In some cases, in order to explain the small changes of anisotropy with increasing Pd, other causes of anisotropy should be considered, such as preferred orientation of calcite c-axis perpendicular to the bedding plane due to compaction and other various diagenetic processes (Bachman, 1979; Carlson and Christensen, 1979; Milholland et al., 1980). The velocity parallel to the c-axis of a calcite crystal is the slowest (Peselnick and Robie, 1963) and since the grain orientations do not change appreciably during high-pressure runs, the velocity anisotropy due to preferred orientation of grains should be independent of pore-pressure measurements. Bedding lamination can be another candidate for anisotropy because the VpH will be the fastest layer velocity whereas the Vpv will be the average velocity of all layers in a layered medium where PH denotes horizontal compressional waves and PV denotes vertical compressional waves. Recently, using an X-ray pole figure device, Schaftenaar and Carlson (1984) carried out X-ray petrofabric studies on some selected deep-sea carbonate sediments and found that the grain orientation was weak. They pointed out that the bedding lamination is a major cause of anisotropy in carbonate-bearing deep-sea sediments. The results of the ultrasonic attenuation measurements (Figs.13 and 14), however, show good correlation between velocity and attenuation anisotropy, particularly for the shear mode, supporting the idea that the horizontal orientation of grain boundaries and flat pores is a significant cause of the velocity anisotropy. A list of the physical and acoustic properties, lithology, age and depth of burial is given in Table 2. Values for Qs were unavailable for some chalks and limestones due to their excessive attenuation. The values of In (QPH/QPv) are close to zero or slightly positive regardless of the values of Ap and location of DSDP sites (Fig.13). In the case of shear waves correlation is good between In (QsHH/QsHv) and shear wave anisotropy (As; Fig.14). Except for one sample, QSHH is larger than QsHv where SHH is defined as shear waves propagating horizontally and polarized horizontally, and SHV is defined as shear waves propagating horizontally but polarized vertically. The observed attenuation anisotropy patterns (i.e. QPH ~ QPv and QSHH > QSHV), regardless of the DSDP sites, are expected because Sites 288 and 289 have the same tectonic history and Site 316 is located on a sedimentary environment similar to that of Sites 288 and 289 despite the geographical separation of the sites. In general, attenuation as well as velocity depend on pore fluid content and pore geometry of the medium through which a seismic

87

0

%

e~ (3

• OI

• 00

•

* *

~

I I

gk C

-i

COMPRESSIONAL WAVE ANISOTROPY.

%

Fig.13. The compressional wave quality factor ratios, In (QM-I]QP¢), versus anisotropy (Ap) for D S D P Sites 288 (solid squares), 289 (open diamonds) and 316 (asterisks). The values of In (QM-I]QPv) do not change with Ap. Points in Figs.13 and 14 are listed in Table 2.

wave propagates (Walsh, 1969; Nut, 1971; Mavko and Nur, 1979). In attenuation, however, the effect of pore aspect ratio is much more important than the liquid content because flatter pores generate high attenuation by their higher energy dissipation of shear stress. Because all the samples have been kept fully saturated during the measurements, the influence of pore fluids on attenuation is considered to be about the same for each sample. Consequently, the differences in the preferred orientations of the flat pores are probably the main cause for the observed difference in shear wave attenuation for the samples studied (Fig.14). CONCLUSIONS

The progressive diagenetic changes in deep-sea carbonate sediments are reflected in the variations of physical and acoustic properties with increasing depth of burial and age. With increase in burial depth, bulk porosity decreases with parallel increase in bulk density. The porosity change is almost entirely due to the effects of compaction and other later-stage diagenesis. The increase in velocity and velocity anisotropy with increasing depth of burial is explained not only by this simple porosity--depth relationship but also by changes in pore geometry. Possibly the pore throat size, which is represented

//mestone

siliceous l i m e s t o n e nannoforam limestone naunoforam limestone n a n n o f o n u n limestone nannoforam limestone nannoforam limestone nannoforam l i m e s t o n e

289

289

316 316 316 316 316

316

nannofoss/l limestone

933

1167 597 612 632 640 691 728

1132

1050

31

68 71 72 73 74 75 76

58

46 24.7 20.8 17.3 20,3 23.0 19,2 20.6

7.3

--

--

0.50 0.19 0.10 0.12 0.19 0.13 0.22

0.33

--

--

0.38 0.02 0.33 0.29 -0.47 0.20

1.93 2.22 2.37 2.29 2.28 2.26 2.35

2.60

2,34

1.91

2.16 2.19 2.30 2.32 -2.16 2.13

n a n n o f o r a m chalk

33.7 22.7 23.3 24.9 -31.1 33.1 2.74 3.36 3.36 3.77 2.83 3.04 2.88 2.58 3,74 4.66 3.70 3.04 3.00 2.69 3.04 3.10 3.68

2.59 3.07 2.89 3.62 2.54 2.62 2.43 2.50 3.48 4.55 3.41 2.68 2.69 2.39 2.65 2.74 3.22

5.6 8.7 15,0 4.0 10.8 14.3 16.5 3.3 7.3 2.4 7.9 12.6 10.9 11.8 13.7 12.3 13.3

Ap (%) 1.40 2.10 1.83 2.20 1.66 1.69 1.54 1.39 2.02 2.71 2.05 1.71 1.71 1.59 1.76 1.75 1.93

1.33 1.90 1,61 2.14 1.52 1.51 1.30 1.37 1.89 2.60 1.92 1.65 1.69 1.56 1.68 1.70 1.76

V

H

V

H

5.1 10.0 12.8 2.9 8.8 11.3 17.6 0.9 7.1 4,1 6.7 3.6 1.2 2.0 4.8 2.8 9.5

As (%) 18.0 37.7 15.8 41.6 23.6 14.2 16.6 14.8 21.0 19.4 29.7 15.2 11.6 14.9 25.3 16.8 34.9

H

Qp

pressure, of the specimens

Vs (km s -I)

at ambient

Vp (kins -1)

289

87 88 89 89 89 96 98

~)

289

849 850 859 860 860 886 914

0Jm)

Modal pore diameter

f o r a m n a n n o chalk sil/ceous l i m e s t o n e limestone fol'~rnini fel.al Limestone nannofossil limestone nannotossi] limestone nannofoss/l limestone

qb (%)

288 288 288 ~8 288 288 288

Age (m.y.)

p (gcm

(m)

Lithology

DSDP Site

Burial depth

and attenuation,

2

Sample descriptions and va]ues of physical and acoustic properties DSDP Sites 288, 289, and 316

TABLE

13.9 35.0 14.1 34.7 17.1 12.9 11.4 13.1 18.9 14.4 24.6 13.0 11.9 13.6 16.8 14.5 25.8

V

20.7 19.8 13.5 58.0 9.5 24.2 15.2 9.1 15.1 14.2 10.3 11.6 5.7 12.8 14.7 12.5 19.9

SHH

Qs

recovered

18,8 16.2 4.2 48.6 7.1 7.5 5.7 7.5 8.0 7.6 11.7 8.2 4.9 10.9 13.9 11.0 9.8

8HV

from

0o 0o

89

o

<>

*

c~

0 (-

-1

SHEAR WAVE ANISOTROPY.

Fig.14. The shear wave quality factor ratio, In (QSHH/QSHV), versus anisotropy (A s) for the same DSDP sites. Symbols are the same as in Fig.13. Note the positive correlation of the quality factor ratio to A S regardless of sites. See the text for details.

as the modal pore size here, is an important variable in determining the acoustic properties because of the very thin connections in throat and rapid size change compared to the size of pore. Apart from porosity, the pore throat-size change is not a simple function of depth of burial or age, as it is controlled largely by secondary porosity reduction. Silica content is an important factor affecting the pore throat through the process of intensifying carbonate diagenesis. SEM photographs are interpreted as showing that horizontally elongated pores contribute to velocity anisotropy. This idea is supported by the decrease in anisotropy values with increasing Pd and positive values of shear wave attenuation ratio, In (QsHH/QSHV), with increasing As. For a few samples, however, there is only slight anisotropy change with increasing P d, suggesting some other causes of anisotropy, for example, calcite grain orientation or bedding lamination. ACKNOWLEDGMENTS

This research, supported by National Science Foundation grant OCE 8310605 and by the Office of Naval Research, constitutes a part of the Ph.D. dissertation of the senior author (DCK) at the University of Hawaii. We especially thank K.W. Katahara for valuable advice in experimental work

90 during the initial phase o f this investigation, S.V. Margolis f o r advice o n SEM studies, J. Balogh f o r technical s u p p o r t t h r o u g h o u t this project, R. Pujalet for editorial assistance in p r e p a r a t i o n o f the m a n u s c r i p t , and E.L. H a m i l t o n and N.I. Christensen for reviewing it. T h a n k s are also d u e DSDP p e r s o n n e l David Allard a n d A m y A l t m a n for o b t a i n i n g samples and N a n c y Freeland f o r providing physical p r o p e r t y data. Hawaii I n s t i t u t e o f G e o p h y s i c s c o n t r i b u t i o n 1569. REFERENCES Andrews, J.E., Packham, G. et al., 1975. InitialReports of the Deep Sea DrillingProject, Vol. 30. U.S. Govt. Printing Office, Washington, D.C., 753 pp. Bachman, R.T., 1979. Acoustic anisotropy in marine sediments and sedimentary rocks. J. Geophys. Res., 84: 7661--7663. Berger, W.H., Johnson, T.C. and Hamilton, E.L., 1977. Sedimentation on Ontong-Java Plateau: observations on a classic "carbonate monitor". In: N.R. Anderson and A. Malahoff (Editors), The Fate of Fossil Fuel CO~ in the Oceans. Plenum, N e w York, N.Y., pp.543--567. Birch, F., 1960. The velocity of compressional waves in rocks up to 10 kilobars. J. Geophys. Res., 65: 1083--1102. Boyce, R.E., 1976. Sound velocity--density parameters of sediment and rock from DSDP drill sites 315--318 on the Line Islands Chain, Manihiki Plateau, and Tuamotu Ridge in the Pacific Ocean. In: S.O. Schlanger, E,D. Jackson et al., Initial Reports of the Deep Sea Drilling Project, Vol. 33. U.S. Govt. Printing Office, Washington, D.C., pp.695-728. Carlson, R.L. and Christensen, N.I., 1979. Velocity anisotropy in semi-indurated calcareous deep-sea sediments. J. Geophys. Res., 84: 205--211. Fischer, A.G., Honjo, S. and Garrison, R.W., 1967. Electron micrographs of limestones and their nannofossils. Princeton Univ. Press, Princeton, N.J., 137 pp. Gardner, J.V., Dean, W.E. and Jansa, L., 1977. Sediments recovered from the northwest African continental margin. In: Y. Lancelot, E. Seibold et al., Initial Reports of the Deep Sea Drilling Project, 41. U.S. Govt. Printing Office, Washington, D.C., pp.1121-1134. Garrison, R.E., 1981. Diagenesis of oceanic carbonate sediments: A review of the DSDP perspective. Soc. Econ. Paleontot. Mineral., Spec. Publ., 32: 181--207. Hamilton, E.L., 1976. Variations of density and porosity with depth in deep-sea sediments. J. Sediment. Petrol., 46: 280--300. Hamilton, E.L., 1980. Geoacoustie modeling of the sea floor. J. Acoust. Soc. Am., 68: 1313--1340. Johnston, D.H. and Toksoz, M.N., 1980. Ultrasonic P and S wave attenuation in dry and saturated rocks under pressure. J. Geophys. Res., 85: 925--936. Kim, D.-C., Katahara, K.W., Manghnani, M.H. and Schlanger, S.O., 1983. Velocity and attenuation anisotropy in deep-sea carbonate sediments. J. Geophys. Res., 88: 2337-2343. Kolsky, H., 1953. Stress Waves in Solids. Clarendon, Oxford. Manghnani, M.H., Schlanger,-S.O. and Milholland, P.D., 1980. Elastic properties related to depth of burial, strontium content and age, and diagenetic stage in pelagic carbonate sediments. In: W.A. Kuperman and F.B. Jensen (Editors), Bottom-Interacting Ocean Acoustics. Plenum, New York, N.Y., pp.41--51. Matter, A., Douglas, R.G. and Perch-Nielson, K., 1975. Fossil preservation, geochemistry and diagenesis of pelagic carbonates from Shatsky Rise, northwest Pacific. In: R.L. Larson, R. Moberly et al., Initial Reports of the Deep Sea Drilling Project, 32. U.S. Govt. Printing Office, Washington, D.C., pp.891--922.

91

Mavko, G.M. and Nur, A., 1979. Wave attenuation in partially saturated rocks. Geophysics, 44: 161--178. Mayer, L.A., 1979. Deep sea carbonates: acoustic, physical, and stratigraphic properties. J. Sediment. Petrol., 49: 819--836. Milholland, P.D., Manghnani, M.H., Schlanger, S.O. and Sutton, G., 1980. Geoacoustic modeling of deep-sea carbonate sediments. J. Acoust. Soc. Am., 68: 1351--1360. Moore, C.H., 1979. Porosity in carbonate rock sequences. In: Geology of Carbonate Porosity. A.A.P.G. Continuing Education Course Note Series, 11: A1--A124. Nur, A., 1971. Effects of stress on velocity anisotropy in rocks with cracks. J. Geophys. Res., 76: 2022--2034. Packham, G.H. and Van der Lingen, G.J., 1973. Progressive carbonate diagenesis at Deep Sea Drilling Sites 206, 207, 208 and 210 in the southwest Pacific and its relationship to sediment physical properties and seismic reflections. In: R.E. Burns, J.E. Andrews et al., Initial Reports o f the Deep Sea Drilling Project, Vol. 21. U.S. Govt. Printing Office, Washington, D.C., pp.495---521. Peselnick, L. and Robie, R.A., 1963. Elastic constants of calcite. J. Appl. Phys., 34: 2494--2495. Schaftenaar, C.H. and Carlson, R.L., 1984. Calcite fabric and acoustic anisotropy in deepsea carbonates. J. Geophys. Res., 89: 503--510. Schlanger, S.O. and Douglas, R.G., 1974. The pelagic ooze-chalk--limestone transition and its implication for marine stratigraphy. Spec. Publ. Int. Assoc. Sedimentol., 1: 117--148. Schlanger, S.O. et al., 1976. Initial Reports o f the Deep Sea Drilling Project, Vol. 33. U.S. Govt. Printing Office, Washington, D.C., 973 pp. Scholle, P.A., 1979. Porosity prediction in shallow versus deep water limestones - - primary porosity preservation under burial conditions. In: Geology of Carbonate Porosity. A.A.P.G. Continuing Education Course Note Series, 11: D1--D12. Sears, F.M. and Bonnet, B.P., 1981. Ultrasonic attenuation measurements by spectral ratios utilizing signal processing techniques. IEEE Trans. Geosci., Remote Sensing, GE-19: 95--99. Toksoz, M.N., Cheng, C.H. and Timur, A., 1976. Velocities of seismic waves in porous rocks. Geophysics, 41: 621---645. Toksoz, M.N., Johnston, D.H. and Timur, A., 1979. Attenuation of seismic waves in dry and saturated rocks, 1, Laboratory measurements. Geophysics, 44: 671--690. Van der Lingen, G.J. and Packham, G.H., 1975. Relationships between diagenesis and physical properties of biogenic sediments of the Ontong-Java Plateau (Sites 288 and 289, DSDP). In: J.E. Andrews, G. Packham et al., Initial Reports o f the Deep Sea Drilling Project, Vol. 30. U.S. Govt. Printing Office, Washington, D.C., pp.443--481. Walsh, J.B., 1965. The effect of cracks in rocks on Poisson's ratio. J. Geophys. Res., 70: 5249--5257. Walsh, J.B., 1969. New analysis of attenuation in partially melted rocks. J. Geophys. Res., 74: 4333--4337. Wardlaw, N.C., 1976. Pore geometry of carbonate rocks as revealed by pore casts and capillary pressure. Bull. Am. Assoc. Pet. Geol., 60: 245--257. Wardlaw, N.C. and Cassan,J.P., 1979. Oil recovery efficiency and the rock-pore properties o f some sandstone reservoirs. Bull. Can. Pet. Geol., 27: 117--138. Wardlaw, N.C. and McKellar, M., 1981. Mercury porosimetry and the interpretation of pore geometry in sedimentary rocks and artificial models. Powder Technol., 29: 127-143. Zemmels, I., Cook, H.E. and Matti, J.C., 1975. X-ray mineralogy data, Tasman Sea and far western Pacific, Leg 30, Deep Sea Drilling Project. In: J.E. Andrews, G. Packham et al., Initial Reports o f the Deep Sea Drilling Project, Vol. 30. U.S. Govt. Printing Office, Washington, D.C., pp.603--616.