Logging Interpretation

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Interpretation of Well-logging Data to

Study Lateral Variations in Young Oceanic

Crust: DSDP/ODP Holes 504B and

896A, Costa Rica Rift

ABSTRACT

Deep Sea Drilling Project/Ocean Drilling Program Holes 504B and 896A were drilled into the

upper oceanic crust of the Costa Rica Rift, only 1 km apart. Thus they provide an excellent op-

portunity to study lateral variation in the volcanic-rock section of the oceanic crust. Recovery of

cores from these holes was poor. Therefore, comparison of one hole with the other is based on continuousinformation from geophysical well-logging data. Continuous lithologic profiles were created by calibration

of logging data to cores, followed by statistical analysis.

Four rock types can be distinguished as electrofacies from sets of log responses: massive units of

basalt, thin flow-basalts, pillow basalts, and fractured or brecciated basalts. Massive units, thin flow-

basalts, and pillow basalts are in both holes. These terms describe morphologies of lava flows. The mor-

phologies can be distinguished by using physical information from logs; the evidence results from charac-

teristic differences in fractured and altered rock. Fractured or brecciated basalts were classified only in

Hole 896A; they are restricted to the depth between 350 m and 380 m below seafloor (mbsf). They are re-

lated genetically to a fault zone.

Comparison of records of the two holes shows that average P-wave velocities and total gamma radia-

tion are similar. In all log responses from Hole 504B, scattering is larger. This is especially true of electri-

cal resistivities and P-wave velocities. Differences among physical properties measured in the two holes canbe related to variation in thicknesses of lava flows and variation in fracturing and alteration. Massive units

of basalt are much thicker in Hole 504B than in Hole 896A, which explains the higher electrical resistivity

and P-wave velocity recorded in this hole. Fractures are more numerous and altered rock more abundant

in Hole 896A.

INTRODUCTION

Structure of the oceanic crust can be described simply as layered. A section of eruptive and intrusive

basalts is underlain by a thick layer of gabbros, beneath which is peridotitic mantle. Evidence for this model

mainly is from seismic-refraction data (e.g., Raitt, 1963; Christensen and Salisbury, 1975); it is confirmed by

Bartetzko, A., R. Pechnig, and J. Wohlenberg, 2002, Interpretation of well-logging data to study lateral variations in young oceanic crust:DSDP/ODP Holes 504B and 896A, Costa Rica Rift, in M. Lovell

and N. Parkinson, eds., Geological applications of well logs: AAPGMethods in Exploration No. 13, p. 213228.

A. Bartetzko

Aachen University of

Technology

Aachen, Germany

R. Pechnig


Technology

Aachen, Germany

J. Wohlenberg


Technology

Aachen, Germany


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investigation of ophiolites, which are sections of ancient

oceanic crust on land (e.g., Coleman, 1977; Nicolas, 1989).

Direct observations of the seafloor by dredging and by obser-

vation during dives of submersible vehicles support the layer

model (e.g., Auzende et al., 1989; Juteau et al., 1995). Deep

drilling into oceanic crust is a necessary extension of these

observations. In the last 30 years, more than 1000 holes havebeen drilled through the seafloor as part of the Ocean Drilling

Program (ODP) and its predecessor, the Deep Sea Drilling

Project (DSDP). Some holes penetrated through the cover of

sediment and sedimentary rock into oceanic basement, and

from them complementary information about the nature of

oceanic crust was compiled (e.g., Anderson et al., 1982;

Dilek et al., 1998).

Few investigations have been made concerning the later-

al structure of the crust. To study local variations in stratigra-

phy of basement rocks (Alt et al., 1993), Holes 504B and

896A were drilled only 1 km apart. About 100 m of extrusive

basalt is available from both holes, for comparison. Both

holes were cored continuously; however, units of volcanic

rock cannot be defined precisely, because less than 30% of the

cores was recovered. Consequently, correlation based on core

data is difficult. Therefore, the continuous information from

well-logging data was used in this study for reconstruction of

the stratigraphic sections of the two boreholes. Using electri-

cal resistivity logs, Ayadi et al. (1998) presented a reconstruc-

tion of the lithology drilled in Hole 504B. Brewer et al. (1995,

1998) used FMS-images (Formation MicroScanner) to re-

construct the lithology of Hole 896A. To some extent, these

reconstructed profiles show large differences between lithos-

tratigraphy reconstructed from well logs and lithostratigraphy

established from cores. The reconstructions also differ con-

cerning types of rocks that were distinguished. The profile by

Brewer et al. (1995, 1998) is distinguished by large amounts

of breccias, which are not in large quantities in any other pro-

files. This discrepancy indicates that the lithostratigraphic

succession is still not considered in a way that permits com-

parison of the two holes. Therefore, in this study, lithology

was reconstructed by a method that enables classification of

rocks in both holes by identical criteria. For the first time, the

reconstructed profiles are a basis for comparison of lithos-

tratigraphy of the two boreholes.

DESCRIPTION OF BOREHOLES504B AND 896A

Holes 504B and 896A were drilled about 200 km south

of the Costa Rica Rift (Figure 1). The Costa Rica Rift is part

of the Galapagos Spreading Center; the rate of spreading is

approximately 7.0 cm/year (full rate). At the drill sites, the

crust is young, dated as having been emplaced 6.63 m.a. at

Hole 504B and 6.68 m.a. at Hole 896A (Allerton et al.,

1996). Hole 504B is the deepest hole drilled into oceanic

crust. Total depth of 2111 meters below seafloor (mbsf) was

reached in stages, during several expeditions (legs) between

1979 and 1993. Legs 69, 70, and 83 (Figure 2) were drilled

during the Deep Sea Drilling Program, whereas Legs 111,

137, 140, and 148 were drilled during the Ocean Drilling

Program (Figure 2) (Cann et al., 1983; Ander son et al.,

1985; Becker et al., 1988, 1992; Dick et al., 1992; Alt et al.,

1993). In 1986, during Leg 111, Hole 896A was drilled to

evaluate sediments (Figure 2); it was deepened into basement

rocks during Leg 148, in 1993 (Alt et al., 1993).

Hole 504B penetrated 274.5 m of pelagic sediments and

1836 m of basement rock (Figure 2). Of the basement rock,

the uppermost partapproximately 570 mis composed of

pillow basalts and basaltic lava flows. Between about 846 m

and 1055 m deep, the proportion of pillow basalts decreases

in transition, as the proportion of dikes increases. The strati-

graphic sequence below1056 m thickcomprises sheeted

dikes (Cann et al., 1983; Anderson et al., 1985; Becker et al.,

1988, 1992; Dick et al., 1992; Alt et al., 1993). Hole 896A

drilled through 179 m of pelagic sediments and 280 m of pil-

low basalts and basaltic lava flows (Alt et al. 1993).

Basalts were altered by contact with circulating seawater

and hydrothermal fluids. In Hole 504B, the uppermost 310

m of basement rock underwent low-temperature seafloor ox-idation. The underlying 314 m of rock was altered under re-

ducing conditions. Rocks of the transition zone and of the un-

derlying sheeted-dike complex (below 898.5 mbsf) were

altered hydrothermally; these sequences are characterized by

mineral assemblages of greenschist facies. Basement rocks of

Hole 896A were altered at low temperatures, under oxidizing

conditions, similar to alteration of the uppermost 310 m in

Hole 504B (Alt et al., 1993; Laverne et al., 1996; Teagle et al.,

1996). In general, basalts from the two holes are very similar

Bartetzko et al.214

Figure 1. Location of Holes 504B and 896A on the Nazca

Plate, south of the Costa Rica Rift (C.R.R.).


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in minerology, petrology, and geochem-

istry, and in type of alteration (Alt et al.,

1993).

In this study, comparison of the two

boreholes was restricted to the logged sec-

tion of Hole 896A (200 to 400 mbsf) and

to the volcanic sequence and transitionzone of Hole 504B. The boundary be-

tween the transition zone and the under-

lying sheeted-dike complex is 1054.3

mbsf, where the lowermost pillow basalt

in the cores can be identified on wireline

logs.

LOGGING DATA ANDQUALITY OF DATA

Boreholes 504B and 896A were

logged extensively. Downhole measure-

ments were conducted in Hole 504B dur-

ing DSDP Legs 70 and 83 (Cann et al.,

1983; Anderson et al., 1985). In 1986,

when Leg 111 was drilled, logs were re-

corded as deep as 1525 mbsf (Becker et al., 1988). During

Leg 140, Hole 504B was logged to 2000 mbsf (Dick et al.,

1992), and a set of logs was run to 2080 mbsf during Leg 148

(Alt et al., 1993). Hole 896A also was logged during Leg 148

(Alt et al., 1993). Figure 3 and Table 1 give an overview of

well logs used in this study. Log interpretation of Hole 504B

was based mainly on data from Leg 148. Data from Leg 111

were complementary. All downhole measurements were

recorded by Schlumberger Ltd.

The quality of most logs of Hole 504B was degraded by

borehole conditions, especially in the case of logs with shallow

depths of investigationfor example, neutron porosity1 and

bulk-density logs. In the volcanic-rock section of Hole 504B,

the borehole is enlarged; at some places, the diameter is al-

most 16 in. (40 cm). The hole was drilled with bits of 9.875

in. (25 cm) in diameter. Hole diameter decreases slightly

downward. Hole 896A is mostly in gauge; deviations from bit

size (9.875 in.; 25 cm) are small (Figure 3).In both holes, electrical resistivity was recorded with the

Dual Laterolog (DLL). Resistivities of formations range

from 3 to 285 ohm-m, as recorded by the Laterolog Deep

(LLD). Rock-forming minerals of basalt generally are con-

sidered to be electric insulators (Schn, 1996). Thus, the

very low values of resistivity are evidence that seawater and

conductive secondary minerals fill fractures (Pezard, 1990).

During Leg 148, the Sonic Digital Tool (SDT) was used to

determine sonic velocities of formations in both holes. P-wave

velocities range from 2.6 to 6.8 km/s, typical of rocks in the

upper oceanic crust (White, 1991). In both holes, natural ra-

dioactivity was recorded with a Natural Gamma Ray Tool.

Because radioactive elements generally are not abundant in

oceanic basalts, values are very low. Average values of the

total gamma ray are 2.9 API in Hole 504B (in the interval

274.5 to 1054.3 mbsf) and 3.8 API in Hole 896A. Contents

of thorium and uranium are less than 2 ppm and at some

places are negative, a phenomenon that is common in forma-

tions with low radioactivity. This is because of limits of thespectral-resolution capacity of the detector. Data about thori-

um and uranium were excluded from further analyses be-

cause of these problems with poor quality. Neutron porosity

and bulk density were recorded only in Hole 504B. Neutron

porosity (NPHI) is very high, ranging from 4% to 58%, with

a mean of 21%. Porosities measured from cores vary from

0.4% to 5.8% (Christensen and Salisbury, 1985; Becker et al.,

1988). In contrast to measurements of porosity from cores,

porosities recorded on well logs integrate voids and fractures

Interpretation of Well-logging Data to Study Lateral Variations in Young Oceanic Crust

Figure 2. Generalized lithostratigraphy and zones of altered basalt in Holes 504B and

896A. Figure composed after Cann et al. (1983), Anderson et al. (1985), Becker et

al. (1988, 1992), Dick et al. (1992), and Alt et al. (1993).

1Neutron logs are calibrated in units of porosity, on the assumption

that pores in rock are full of water. The logging tool measures signals that

are proportionate to the hydrogen contents of materials logged. Thus, in

rocks that contain gas, porosity commonly is underestimated, but in rocks

that contain clayin which hydrogen exists in intercrystalline formporos-

ity is overestimated. Thus, the term neutron porosity is useful but ambiguous

unless evaluated by consideration of the kinds of materials logged.


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that are filled with seawater, and particularly, they integrate

the illusory response generated by hydrous minerals. Addi-

tionally, basalts contain elements with large-capture cross

sections, such as gadolinium. These elements cause the neu-

tron-porosity tool to overestimate porosity (Lysne, 1989;

Brewer et al, 1996b). The fact that the tool was not central-

ized additionally affected the measurements (Broglia and

Ellis, 1990). For reasons described above, the neutron poros-

ity log (NPHI) can be used only qualitatively as an indicator

of fractured and altered rock. Measurements of the LithoDensity Tool in Hole 504B were influenced largely by the

state of the borehole wall but give reliable results in massive

rocks.

RECONSTRUCTION OF LITHOLOGYUSING WELL-LOGGING DATA

The purpose of reconstructing lithology from well-log

data is to translate physical measurements from logs into

lithologic terms. In this study, reconstruction was carried out

by using the concept of electrofacies. Each type of rock is

characterized by a set of log responses, which distinguishes it

from other types of rock. This set of log responses is called an

electrofacies; the prefix electro differentiates electrofacies from

geologic facies (Serra, 1986). Use of the term is not restricted

to interpretation of electrical measurements. The concept of

electrofacies was developed for interpretation of sedimentary

rocks, but was applied successfully to crystalline rocks by

Haverkamp and Wohlenberg (1991), Pechnig et al. (1997),and Dick et al. (1999).

The reconstruction described in this study was based on

calibration of electrofacies by data from cores, followed by

statistical analysis of the data. Calibration of electrofacies is

conducted within stratigraphic sections called training inter-

vals. In this paper, parts of the stratigraphic sequence that

were not training intervals are referred to as remaining

depth intervals.

The basic principle of the discriminant analysis is reduc-

Bartetzko et al.216

Table 1. Tools and logs used to record lithostratigraphy of Holes 504B and 896A.

Tool Logs 504B 504B 896ALeg 111 Leg 148 Leg 148

Electrical formation resistivity

DLLTM LLD (ohm-m) Laterolog Deep

(Dual Laterolog) LLS (ohm-m) Laterolog Shallow

Sonic velocity

SDTTM VP (km/s) compressional wave velocity

Array sonic tool

Induced radioactivity

CNT-GTM NPHI (%) neutron porosity

(Compensated Neutron Tool)

LDTTM RHOB (g/cm3) bulk density

(Litho Density Tool) PEF (barns/e-) photoelectric factor

Natural radioactivity

NGTTM CGR (API) computed gamma ray(POTA+THOR)

(Natural Gamma SGR (API) sum (total) gamma ray

Spectrometry Tool) POTA (wt.%) potassium content

THOR (ppm) thorium content

URAN (ppm) uranium content

Borehole diameter

HD or C1, C2 (in) hole diameter

TM

Trademark of Schlumberger Ltd.


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tion of the numerous logs to a few functions that are used (1)

to check the reliability of calibration and (2) to predict a clas-

sification of rocks in depth intervals not included in the train-

ing intervals. Prediction is done by application of Bayes rule,

i.e., a depth point is classified into the electrofacies on the

basis of its largest probability. The principle of discriminantanalysis for well-log interpretation is described in detail by

Doveton (1994). Discriminant analysis was performed using

the statistical software package SPSS.

Calibration of Electrofacieswithin Training Intervals

Training intervals were chosen with respect to core re-

covery, borehole conditions, data quality, and rock types. The

longer training sections were selected from logs of Hole 504B

because more logging data were

available (Table 1; Figure 4).

The training interval of Hole

896A was chosen on the basis of

studies of FMS images by Brew-

er et al. (1995, 1998), which

showed that the interval containsbrecciated rocks (which were

not recorded in Hole 504B).

The sections of rock from 280

mbsf to 587 mbsf in Hole 504B

and from 360.3 mbsf to 376

mbsf in Hole 896A were treated

as one training interval. Dis-

criminant analysis was carried

out to check the reliability of cal-

ibration and to classify the depth

interval 587 to 920 mbsf of Hole

504B and the remaining inter-

vals of Hole 896A. The depth

interval 920 mbsf to 1054.3

mbsf of Hole 504B was treated

separately because the transition

zone causes slight depth trends

in the logs (Figure 3). The sec-

tion 920 mbsf to 1000 mbsf was

chosen as the training interval.

Basic information about lithos-

tratigraphy was taken from

Adamson (1985, Table 2). Ad-

ditional information about

lithology, petrology, and alter-

ation of rock was compiled from

core descriptions in Cann et al.

(1983), Anderson et al. (1985),

and Alt et al. (1993).

The objective of calibration is to determine the principal

relations between wireline logs and core lithology, so that each

type of rock can be characterized by an electrofacies. Of rocks

studied in the investigation described here, four electrofacies

can be distinguished: massive units of basalt, thin flow-

basalts, pillow basalts, and fractured and/or brecciated rocks.Discriminant analysis was used first to check the reliability of

calibration. The result for the two training sections is shown

in Table 3. Numbers in Table 3 should be compared to the

result of a random distribution, which would be 33.3% in the

case of three electrofacies. In total, 73.0% of the depth points

of the upper training interval and 80.2% of the lower training

interval were classified as being of the same electrofacies, by

calibration and by discriminant analysis. The percentage of

consistent classification is high for massive units (69.6% and


Figure 3. Composite log of Hole 504B (volcanic-rock section and lithologic transition zone)

and Hole 896A, showing standard logs used in this study. In Hole 504B, these logs show evi-dence of difference in log signature below 920 m: HD, LLD, VP. Formation resistivities are

shown in ohm-meters, on logarithmic scale. Abbreviations for logs are as follows: HD: hole di-

ameter. LLD: Laterolog Deep. VP: compressional wave velocity. NPHI: neutron porosity.

RHOB: bulk density. SGR: total gamma ray. C1C2: average diameter of hole.


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77.0%, respectively), pillow basalts (86.2% and 92.4%, re-

spectively) and fractured and/or brecciated rocks (91.3%),

but low for thin flow-basalts (49.8% and 34.6%, respectively).

These percentages indicate that the electrofacies can be rec-

ognized within the training intervals by use of discriminant

analysis. Of course, this is a necessary condition for classifica-

tion of the remaining intervals of Holes 504B and 896A by

use of discriminant analysis.

Classification of RemainingDepth Intervals

After electrofacies were defined, discriminant analysis

was used to predict the lithic classification of the remaining

depth sections of Hole 504B and Hole 896A (Figure 4). The

result was a computed lithologic profile of each hole; the pro-

files were controlled and corrected as necessary. Control was

especially important where the borehole was enlarged and the

classification of lithology was liable to be false. Furthermore,discriminant analysis describes very thin layers, which are not

in accord with reality. Logging data are recorded every 15

cm; thus, discriminant analysis might calculate layers 15 cm

thicki.e., one depth pointbut such layers are thinner than

vertical resolution of most of the standard logging tools (0.4

m to 2.0 m). Because of the effects of smoothing of logs at the

boundaries between beds, misclassification occurred, but it

was corrected. The final result was a synthetic lithologic pro-

file that is called an electrofacies log (EFA log).

RESULTS OF LOG INTERPRETATION

Relations between Lithologyand Log Responses

The main rock types distinguished in the lithostratigra-

phy of Hole 504B were massive units of basalt, thin flow-

basalts, pillow basalts, and dikes (Adamson, 1985). In Hole

896A, the recorded core lithology discriminates among mas-

sive basalts, pillow basalts, and breccias (Alt et al., 1993;

Table 2). Three of these rock types were classified as electro-

facies in both holes: massive units of basalt, thin flow-basalts,

and pillow basalts. Additionally, fractured and/or brecciated

rocks were classified as an electrofacies. Table 4 gives the

mean value and standard deviations of log responses of

the electrofacies.

General relations between log responses of electrical re-

sistivity, sonic velocity, and total gamma ray are shown in Fig-

ure 5. The logs show evidence of variation in fracturing andalteration of the rocks. The basalt was fractured during cool-

ing and contraction of lava. Electrical-resistivity, sonic-veloc-

ity, neutron-porosity, and bulk density logs indicate fractures

filled with seawater. Electrical resistivity, neutron porosity,

and density also are sensitive to fractures filled with sec-

ondary minerals, especially clay minerals, because they are

conductive, contain hydrogen and, compared with basalts,

are of lower density. The total-gamma-ray log is an indicator

of low-temperature seafloor weathering because potassium-

Bartetzko et al.218

Table 2. Definition of rock types in lithostratigraphy of cores, Holes 504B and 896A.

Hole 504B (Adamson, 1985) Hole 896A (Alt et al., 1993)

Rock type Definition Rock type Definition

Massive basalts Massivebasalts

Thin Thinflow-basalts flow-basalts

Dikes Dikes

Pillow basalts Pillow basalts

Breccias Breccias

(a) Medium to very coarse grained;little affected by drilling. Core is

recovered in long intact pieces.

(b) Medium to coarse grained butaffected by drilling. Cores generallybreak into short pieces.

Microcrystalline to fine grained. Well-devel-oped brown oxidative alteration, generally

higher recovery and longer sections ofcontiguous core than in pillowed units.Regular fracture pattern.

Many could be classified as thin flows,following the definition of Adamson (1985).

Fine to medium grained.Homogeneous areas of core thickerthan 1 m.

Not defined (included in massiveunits).

One or two chilled margins ofintrusive rock

Chilled margins in contact withcoarser-grained rocks

Fine grained and commonly highlyfractured. Chilled, curved, or inclinedpillow margins; hyaloclastic breccias.

All material remaining after otherlithologic types are identified.

Fine grained. Curved to planar orirregular chilled and/or glassymargins, interior variolitic zone,

poorly developed oxidative alteration,abundant fractures and veins.

Not defined as a separate rock type(included mostly in pillow basalts)

Recorded as a unit where two or threepieces occur together


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rich minerals such as celadonite and smectite are formed dur-

ing this type of alteration (Alt et al., 1986; Gillis and Robin-

son, 1988).

Pillow basalts are identifiable by these attributes: low

electrical resistivity, low P-wave velocity, low density, high

neutron porosity, and high values of total gamma ray. These

log responses indicate that alteration, fractures, and interpil-low zones have strong influence on physical properties of pil-

low basalts. Seawater that fills fractures and voids and con-

ductive clay minerals in fractures or in pervasively altered

rock cause formation resistivity to be reduced. As well, low P-

wave velocity is evidence of extensive fracturing of the rocks.

High neutron porosity is a result of seawater and hydrous sec-

ondary minerals in fractures. The high natural-gamma-ray

signal is related to enrichment of potassium in basalt, during

seafloor weathering. Therefore, pillow basalts of the upper al-

teration zone in Hole 504B (274.5 mbsf to 587 mbsf) are

characterized by higher gamma-ray values than those of un-

derlying sections.

Pillow basalts commonly are penetrated by radial ther-

mal-contraction fractures that extend from the outer rim to-

ward the central part of the mass. A second set of fractures is

approximately perpendicular to the first. In outcrops of pil-

low basalt, 7% to 18% of the rock is composed of interpillow

zones (Gillis and Sapp, 1997). These interpillow zones gen-

erally are filled with rock fragments, glass particles, and hyal-

oclastic breccias and have high porosity (Gillis and Sapp,

1997). Where downhole measurements integrate signals

from comparatively large volumes, interpillow zones make

significant contribution to log responses of pillow-basalt se-

quences.

Massive units of basalt are characterized by high forma-

tion resistivity, P-wave velocity, and bulk density, and low

neutron porosity and gamma-ray intensity. These log re-

sponses show that massive units are compact rocks, which are

only slightly fractured and altered. Thermal-contraction frac-

tures produce nearly vertical polygonal columns in some lava

flows, but fractures are much less abundant in lava flows than

in pillow basalts (Gillis and Sapp, 1997). Core properties of

massive basalts are evidence of thick lava flows extruded onthe seafloor or sills emplaced into the sequence (Adamson,

1985).

Thin flow-basalts show log responses intermediate be-

tween those of massive basalts and pillow basalts. Formation

resistivities and sonic velocities are less than in massive units;

thus, the conclusion is that thin flow-basalts are more frac-


Figure 4. Training intervals and transfer sections of Holes

504B and 896A. Electrofacies were calibrated within the train-

ing intervals. Discriminant analysis was used to classify the re-

maining intervals. Because of depth trends in the data, the

lower part of the transition zone in Hole 504B was analyzed

separately.

Table 3. Result of discrimination analysis: Classification matrix for training intervals (upper table:Hole 504B: 280287 mbsf; Hole 896A: 306.3376 mbsf; lower table: Hole 504B: 9201000 mbsf).

Calculated electrofaciesElectrofacies from Fractured,

calibration Massive basalts Thin flow-basalts Pillow basalts brecciated basalt n

Massive basalts 69.6% 26.1% 4.3% 0.0% 257

Thin flow-basalts 5.6% 49.8% 43.3% 1.3% 639

Pillow basalts 0.8% 11.1% 86.2% 2.0% 1049

Fractured, brecciated basalt 0.0% 0.0% 8.7% 91.3% 104

Calculated electrofaciesElectrofacies from

calibration Massive basalts Thin flow-basalts Pillow basalts n

Massive basalts 77.0% 1.8% 21.2% 113

Thin flow-basalts 24.7% 34.6% 40.7% 81

Pillow basalts 4.5% 3.0% 92.4% 331


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tured and more altered than massive units. Average total-

gamma-ray values given for thin flow-basalts in Table 4 arehigher than those of pillow basalts in the depth interval from

274.5 to 920 mbsf in Hole 504B. Thin flow-basalts are more

numerous in the upper 300 m of the hole, where gamma-ray

signals generally are larger because of seafloor weathering.

The name thin flows is applied to this rock type and elec-

trofacies, but it is misleading to some degree. The electrofa-

cies thin flows and massive units are distinguished by

different in-situ physical properties, owing to differences in

fracturing and alteration, but no threshold for the thickness is

defined. The name fractured or altered lava flows describes

the electrofacies better, but the name thin flows is kept, fol-lowing the terminology of Adamson (1985).

In Hole 896A, fractured and brecciated basalts were

logged predominantly between 350 and 380 mbsf. The elec-

trofacies is denoted by extremely low resistivity (less than 7

ohm-m), low P-wave velocities (on average 4.4 km/s), and

high total-gamma-ray values (on average 3.9 API). In the

core from Hole 896A, most of the interval consists of micro-

crystalline to intergranular basalts, mostly assignable to mas-

sive units. The rocks were strongly affected by drilling and

Bartetzko et al.220

Figure 5. Cross-

plots of forma-

tion resistivity

(LLD) versus

P-wave velocity,

and formation

resistivity

(LLD) versus

total gamma ra-

diation for Hole

504B (274.5

1054.3 mbsf)

and Hole 896A,

showing log re-

sponses of elec-

trofacies units of

massive basalts,

thin flow-ba-

salts, pillow ba-

salts, and frac-

tured/brecciatedbasalts. The

data set was re-

duced for dis-

play by use of

every third

value.


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were broken into pieces. Many pieces are cut by networks of

thin veins filled with clay minerals, and numerous pieces are

brecciated. In FMS images, many sections of these rocks ap-

pear as breccias (Brewer et al., 1995, 1998). However, the en-

tire section consisting of breccias is not in the core; therefore,

the name fractured/brecciated rocks was assigned to the

electrofacies.

Besides the strongly fractured and brecciated basalts de-

scribed above, breccias such as hyaloclastic breccias could

not be distinguished from fractured and brecciated basalts or

pillow basalts. They are in cores from both holes, but most

are in layers too thin to be identified by use of standard logs.

For similar reasons, dikes could not be identified. In the core

lithostratigraphy described by Adamson (1985), dikes were

defined by chilled margins of intrusive-rock units, evident be-

cause near the margins, basalt is finely crystalline. The chilled

rock generally is only a few centimeters thick and thus notthick enough to be manifested by logs used in this study. In

the electrofacies logs, dikes are classified as massive units or

as thin flows.

Most electrofacies described above represent morpho-

logic and structural variation of basalts, but not mineral-

ogic or geochemical differences. Electrofacies can be distin-

guished by use of standard logs because the morphologic

types of basalt are different in fracturing and alteration, which

affect in-situ physical properties of the rocks. Differences

among the electrofacies/morphologic types of basalt are

gradational and equivocal. Therefore, the log values of elec-

trofacies overlap (Figure 5). This overlap affected the dis-

criminant analysis, as could be observed in the classification

matrices of Table 3. Pillow basalts and massive basalts are

end-members of the spectrum of lava-flow morphologies.

They are defined better by log responses than are thin flow-

basalts. Therefore, the percentage of depth points classified

as one electrofacies by calibration and discriminant analysis is

greater for massive units and pillow basalts than for thin flow-

basalts (Table 3).

Reconstructed LithologicProfiles (EFA Logs)

Hole 504B

Figure 6 shows the electrofacies log of Hole 504B and thecore lithostratigraphy side by side. The EFA log and the core

profile show good agreement. This is especially the case for

the massive units at 320, 520, 545, 580, 680, and 720 mbsf. In

most instances, massive units are slightly thicker in the core

than they are as shown on the EFA log (Figure 6). This is an

effect probably of poor core recovery. Core recovery generally

is better from massive units than from the more fractured and

altered pillow basalts. Therefore, when the stratigraphic suc-

cession is interpreted from cores, thickness of massive rocks is


Table 4. Mean values and standard deviations, wireline-log responses of electrofacies. LLD:Laterolog Deep. VP: Compressional wave velocity. SGR: Total gamma ray. NPHI: Neutron porosity.RHOB: Bulk density.

Electrofacies Log LLD VP (km/s) SGR (API) NPHI (%) RHOB n(ohm-m) (g/cm3)

s

s

s

s

sX X X X X

Hole 504B (274.5920 mbsf)

Massive units 1.54 0.27 5.6 0.5 2.3 1.0 13 6 2.7 0.2 565

Thin flows 1.20 0.24 4.9 0.6 3.7 1.3 22 7 2.5 0.3 783

Pillow basalts 0.96 0.17 4.7 0.6 3.1 1.4 29 8 2.5 0.3 2780

Hole 504B (9201054.3 mbsf)

Massive units 1.97 0.64 5.7 0.3 1.9 0.6 20 5 2.6 0.2 296

Thin flows 1.8 0.15 5.4 0.4 2.0 0.7 23 6 2.6 0.1 137

Pillow basalts 1.58 0.14 5.3 0.4 2.0 0.7 24 5 2.6 0.2 448

Hole 896A

Massive units 1.26 0.16 5.2 0.3 2.7 0.5 111

Thin flows 1.13 0.09 5.1 0.3 3.34 0.7 253

Pillow basalts 0.89 0.13 4.7 0.4 3.7 0.9 715

Fractured/brecciated basalts 0.70 0.07 4.4 0.4 3.9 1.0 243


10/16

likely to be overestimated. Between 420 and 465 mbsf and be-

tween 800 and 850 mbsf, massive units recorded in the core

lithostratigraphy are classified as thin flows, because of the log

responses (Figure 6). Between 500 and 570 mbsf, many thin

flows are interpreted in the EFA log, although they are not as

abundant in the core lithostratigraphy (Figure 6). In this depth

interval, zeolites are present as secondary minerals (Honnorez

et al., 1983). Zeolites are not as conductive as seawater or clay

minerals, which predominantly fill fractures in basalts of theother depth intervals. This circumstance may result in higher

formation resistivities and thus falsify classification in the EFA

log.

The electrofacies designated as fractured/brecciated ba-

salts is not recorded in the EFA log. Discriminant analysis

classified only a few very thin layers of only one or two depth

points (0.15 to 0.3 m) in thickness; these are below the verti-

cal resolution of standard logging tools. These thin layers are

between 347 and 396 mbsf.

Figure 7 shows a comparison of proportions of electrofa-

cies in the EFA log and of rock types in the core. Pillow

basalts make up 64.5% of the EFA log; this fraction is greater

than that recorded in the core lithostratigraphy, in which

55.5% of the rocks are classified as pillow basalts. The higher

proportion of pillow basalts in the EFA log may be explained

by the preferential recovery of massive basalts from cores, as

described above. The proportion of thin flow-basalts is slight-

ly greater in the EFA log (18.4% versus 15.3%) and the pro-portion of massive units is less: 17.1% in the EFA log and

23.6% in the core lithology. In comparing these numbers, one

should take into account the fact that dikes could not be iden-

tified by using the logs; therefore, they were classified as mas-

sive basalts or thin flow-basalts.

Reconstruction of the section from 280 to 960 mbsf in

Hole 504B was carried out by Ayadi et al. (1998) by use of

electrical resistivity logs. Resistivity logs are among the more

important logs for identification of lava-flow morphologies,

Bartetzko et al.222

Figure 6. Electrofacies logs of Hole 896A and Hole 504B in comparison to the core lithostratigraphy from Alt et al. (1993) and

Adamson (1985), respectively. The EFA logs are limited on right side by the electrical-resistivity log smoothed over a seven-data-point window, displayed on logarithmic scale.


11/16

and the EFA log and the reconstruction from Ayadi et al.

(1998) show good agreement. Both profiles indicate a larger

portion of pillow basalts than does the core lithostratigraphy.

Ayadi et al. (1998) were able to identify dikes separately by

using a method of core-log integration. However, dikes were

identified only where their locations were known from cores.

No criterion derived from the logs was found to be reliable

for identification of dikes.

Hole 896A

With reference to Hole 896A, comparison of the EFA

log with lithostratigraphy documented from the core is more

difficult at first view (Figure 6). Most massive units are clas-

sified as thin flows on the EFA log. However, this is no con-

tradiction of the core lithology; Alt et al. (1993) pointed out

that most massive units could be classified as thin flows

(Table 2). Lava flows (massive units and thin flows) are morenumerous but thinner, as classified in the EFA log (Figure 6).

Moreover, fractured/brecciated basalts shown on the EFA log

do not have an equivalent in the core lithostratigraphy (Fig-

ure 6).

Comparison of the proportions of electrofacies in EFA

logs and of rock types in core lithostratigraphy is shown in

Figure 7. Numbers in Figure 7 are difficult to compare, be-

cause fractured/brecciated basalts, which make up 18.4% of

the EFA log, are not recorded in the core lithostratigraphy.

The portion of lava flows (massive units plus thin flows) is

less in the EFA log (27.5%) than in the core lithostratigraphy

(44.7%), but percentages of pillow basalts are similar (54.1

and 53.1%, respectively) (Figure 7). However, the electrofa-

cies fractured/brecciated basalts probably include rocks

that were erupted as both lava flows and pillow basalts.

As mentioned, reconstruction of the lithology of Hole

896A was established by Brewer et al. (1995, 1998), based on

Formation MicroScanner (FMS) images. The major differ-

ence between the EFA log and the reconstruction from FMS

images concerns the interpretation of breccias. Breccias are

the most abundant constituent (46%) of the profile con-

structed by Brewer et al. (1995, 1998). Although one section

classified as breccias by Brewer et al. (1995, 1998) was used

as a basis for calibration, the amount of fractured/brecciated

rocks in the EFA log is significantly less.

COMPARISON OFHOLES 504B AND 896A

Figure 6 shows a comparison of the EFA log of Hole

896A with that of the upper part of Hole 504B. Massive units

are shown as being much thicker in Hole 504B than in Hole

896A. Average thicknesses of massive units are 8.74 m in

Hole 504B and 2.74 m in Hole 896A. Electrical resistivities


Figure 7. Proportions of elec-

trofacies in EFA logs of Holes

504B and 896A and rock types

in core lithostratigraphy

(Adamson, 1985, and Alt et

al., 1993, respectively). Per-

centages shown for core litho-

stratigraphy are valid for thelogged intervals only.


12/16

measured in Hole 504B are greatermore than 100 ohm-m;

this accounts for interpretation of the thicker massive units of

Hole 504B.

Thicknesses of pillow-basalt sections are similar in both

holes. The proportion of pillow basalts is slightly less in Hole

896A (Figure 7). However, numbers given for Hole 896A

should be interpreted carefully. Taking into account only thedepth interval above 350 mbsf in Hole 896Ai.e., above the

zone with fractured/brecciated basaltspillow basalts make

up 66.5% of the rock, which is similar to the 64.5% estimated

by the EFA log for the whole section of Hole 504B (Figure

7). In Hole 504B, pillow basalts are most abundant between

587 and 920 mbsf, where they compose 79% of the section;

above 587 mbsf, they are 54% of the section.

On EFA logs, fractured/brecciated basalts are recorded

only in the log of Hole 896A. A long depth interval was clas-

sified as fractured/brecciated basalt in Hole 896A, but only

single depth points were assigned to this electrofacies in Hole

504B. The depth interval in Hole 896A classified as frac-

tured/brecciated basalts (Figure 6) is characterized by re-

markably low and almost uniform resistivities. The same sec-

tion was classified as breccias by Brewer et al. (1995, 1998);

in their interpretation, breccias even constitute 46% of the

lithologic column. Several types of breccias, of different ori-

gins (e.g. hyaloclastic breccia, tectonic breccia, broken pil-

lows), were described as parts of the oceanic crust (Robinson

et al., 1980). If the stratigraphic positions or quantities of

breccias within a column of rock are used for a geological in-

terpretation, differentiation among various types of breccias

is necessary. Brewer et al. (1995, 1998) did not make distinc-

tions among different types of breccias. Several hints exist

that the section of rock classified as fractured/brecciated

basalts (Figure 6, Hole 896A, 350 to 380 mbsf) may be relat-

ed genetically to a fault zone. Structural investigations of

cores indicate tectonic breccias (Harper and Tartarotti, 1996;

Dilek, 1998). De Larouzire et al. (1996) interpreted the sep-

aration between two sets of fractures observed in FMS im-

ages as evidence of an active fault.

A depth interval with log characteristics similar to thoseof fractured/brecciated basalts does not exist in Hole 504B.

Pezard et al. (1997) described evidence that a fault zone was

penetrated in Hole 504B, between 800 and 1100 mbsf. With-

in this depth interval, resistivity diminished, but not to the ex-

tremely low values recorded in Hole 896A.

Wireline-log data of the two holes are compared in Fig-

ure 8. At first inspection, data from the two holes correspond

well. Averages ofP-wave velocity and total gamma radiation

are almost identical: 4.7 km/s and 3.8 API. However, the

range of formation resistivities and P-wave velocities of Hole

504B are greater than those of Hole 896A (Figure 8). In par-

ticular, formation resistivity of more than 100 ohm-m was

recorded in Hole 504B. No similar resistivity was recorded in

Hole 896A.

Comparison of electrofacies yields similar conclusions.

Scattering of the data is larger in Hole 504B (Figure 5). Dif-

ferences between electrofacies of the two holes are made es-

pecially obvious by comparison of formation resistivity and

P-wave velocity (Figure 5). The crossplot of resistivity versus

P-wave velocity for Hole 504B (Figure 5) indicates separa-

tion between pillow basalts and lava flows. Pillow basalts are

characterized mostly by strong increase in velocity, from 3 to

5.5 km/s, whereas resistivity varies over approximately one

decade, from 3 to 20 ohm-m (Figure 5, upper left crossplot).

In contrast, lava flows cover a smaller velocity range from

Bartetzko et al.224

Figure 8. Frequency distributions of electrical resistivity, P-wave velocity, and total gamma radiation, from wireline logs of Holes

504B and 896A. In this comparison, the data set of Hole 504B is restricted to the interval of oxidative alteration (274.5587 mbsf),

to avoid the effects of different kinds of alteration.


13/16

about 5 to 6.5 km/s, whereas resistivities vary over approxi-

mately one decade, from 10 to 300 ohm-m. Resistivity and P-

wave velocity are positively correlated with formation porosi-

ty (Pezard 1990, Wilkens et al., 1991). Differences between

lava flows and pillow basalts in pore-space geometry and

fracture networks are described above, in the section on rela-

tions between lithology and log responses. These differencesmay explain the relationship between P-wave velocity and re-

sistivity, which can be observed in Figure 5 (upper left cross-

plot). Similar high values of electrical resistivities and P-wave

velocities do not exist in Hole 896A; therefore, the separation

between pillow basalts and lava flows is not as distinct as in

Hole 504B (Figure 5, upper right crossplot).

Several variables may be responsible for differences

among the in-situ physical properties of the two holes. As de-

scribed above, massive units are much thicker in Hole 504B

than in Hole 896A. The highest values of electrical resistivity

and P-wave velocity observed in logs of Hole 504B are within

the inner parts of thick massive units. Log responses of mas-

sive units and of thin flows, classified in the EFA log of Hole

896A, plot generally very close to log responses of pillow

basalts and fractured/brecciated basalts (Figure 5). This indi-

cates that porosity and pore-space structure of lava flows are

more nearly similar to those of pillow basalts and fractured/

brecciated basalts than to porosity and pore-space structure

of massive units of Hole 504B. Fractures are more numerous

in Hole 896A than in Hole 504B, and in Hole 896A they tend

toward being pervasive.

This inference is in agreement with observations of ba-

salts in the cores. Wilkens and Salisbury (1996) showed that

massive basalts of Hole 896A generally are more porous than

those of Hole 504B, and that the relation between P-wave ve-

locity and bulk density is different. In basalts of similar densi-

ty, P-wave velocities of core samples from Hole 504B are

greater than those of core samples from Hole 896A. This im-

plies a difference in mechanical properties of the rocks, per-

haps because of microfissuring and/or pervasive alteration.

Laverne et al. (1996) observed that basalt altered by oxida-

tion is more widespread in Hole 896A. They concluded that

water/rock ratios, and thus primary permeability, are greater

in Hole 896A.

Holes 504B and 896A are only about 1 km apart; one ofthe purposes of drilling Hole 896A was to evaluate the lateral

extensions of lava flows (Alt et al., 1993). A study of paleo-

magnetic data proposed the correlation of sections between

the two holes (Allerton et al., 1996; Dilek, 1998). Allerton et

al. (1996) stated that the magnetic properties of rocks in Hole

896A, above the massive unit at 340 mbsf (Figure 6), corre-

late with magnetic properties of basalts above the massive

unit at 320 mbsf in Hole 504B. Correlation of lava flows at

approximately 3775 m below sea level, following the interpre-

tation by Allerton et al. (1996), is shown in Figure 9. The au-

thors explained the situation by the ponding of lava flows and

extrusion of a volcanic pile onto this ponded lava flow.

Comparison of the two EFA logs shows no indications


Figure 9. Upper parts of EFA logs of Hole 504B and Hole

896A are shown side by side, with reference to sea level.

Dashed lines: Correlation of massive units at approximately

3775 mbsl was proposed by Allerton et al. (1996), based on

paleomagnetic data. EFA logs are limited on right-hand side by

the resistivity log, smoothed over a seven-data-point window

and displayed on logarithmic scale.


14/16

for correlation of single lava flows or successions of lava flows

(Figure 9). Such correlation should be based on patterns in

the logs or characteristic successions of lava morphologies in

the EFA logs and should be confirmed by geochemical, min-

eralogic, or petrologic data. However, geochemical data do

not indicate correlation between rocks of the upper parts of

the two holes. The geochemical composition of rocks fromthe two holes is very similar, but the uppermost 150 m of rock

in Hole 896A is higher in Al2O3 and Ni and lower in P2O5,

Ti, V, Y, and Zr (Alt et al., 1993; Brewer et al., 1996a).

SUMMARY AND CONCLUSIONS

Holes 504B and 896A were drilled only 1 km apart, into

oceanic crust created at the Costa Rica Rift. They provide an

excellent opportunity to study lateral variation in the oceanic

crust. Because of poor recovery of cores from these holes,

volcanic-rock units in the holes cannot be defined precisely.

Therefore, in this study, geophysical downhole measure-

ments were used for a continuous reconstruction of lithology

of the two holes. In contrast to previous reconstructions of

lithology and to the existing lithostratigraphic descriptions

based on cores, one of the purposes of the present study was

to make a lithologic classification of rocks in both holes, based

on identical criteria. Thus, the synthetic lithologic profiles

provide a base for comparision of the two holes. Each rock

type is characterized by a set of wireline-log responses, which

is called an electrofacies; the electrofacies were calibrated by

reference to cores, in selected training intervals. Classification

of the remaining depth intervals was based on discriminant

analysis. In this way, a continuous lithologic profile of each

hole (electrofacies log, or EFA log) was established.

Four electrofacies can be distinguished: massive units of

basalt, thin flow-basalts, pillow basalts, and fractured/brec-

ciated basalts. Massive units, thin flow-basalts, and pillow

basalts were classified from logs of both holes. These three

electrofacies represent different types of lava-flow morpholo-

gy; they are differentiated by variation in fracturing and alter-

ation, which influences the in-situ physical properties of the

rocks. Standard logs used in this study were electrical forma-tion resistivity, P-wave velocity, and total-gamma-ray logs.

They are suitable to distinguish among the lava morpholo-

gies.

Wireline-log data from the two holes seem to be similar

at first view, but scattering of responses is larger in well logs of

Hole 504B. This is valid for the whole data set but also for log

responses of individual electrofacies. Clear separation can be

made between pillow basalts and lava-flow basalts (massive

units and thin flows) in logs of Hole 504B by use of electrical

resistivities and P-wave velocities; however, on logs of Hole

869A, log responses of lava flows are similar to those of pillow

basalts. This indicates stronger overall fracturing and alter-

ation of rock in Hole 896A. Moreover, massive units are three

to four times thicker in the EFA log of Hole 504B than in the

log of Hole 896A, and high values of resistivity and P-wave

velocity in logs of Hole 504B are related to the inner parts ofmassive units. Fractured/brecciated basalts are in Hole 896A

only. This electrofacies can be related to a fault zone, a con-

clusion derived from observation of the structure of basalts in

cores (Harper and Tartarotti, 1996; Dilek, 1998) and from

FMS images (De Larouzire et al., 1996).

Although paleomagnetic studies of cores indicate corre-

lation between lava flows penetrated in the two holes (Aller-

ton et al., 1996; Dilek, 1998), no indications of correlation

can be found in the EFA logs.

Comparison of logs of the two holes shows that in-situ

physical properties of the oceanic crust reflect significant

variation in fracturing, brecciation, and faulting over short

distances, which might not be visible at first view. A struc-

tural overprint, observed in logs and cores of Hole 896A,

makes identification of the original lava morphology diffi-

cult. Knowledge of the original morphology would be im-

portant to know if it were to be used for interpretation of the

processes of accretion.

ACKNOWLEDGMENTS

This study was funded by the German Science Founda-

tion (DFG Wo 159/9). We thank Baker Hughes for providing

log-interpretation software eXpress, which was essential for

data management and display. Comments of two anonymous

reviewers improved the manuscript significantly.

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