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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
Aachen University of
Technology
Aachen, Germany
J. Wohlenberg
Aachen University of
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
Interpretation of Well-logging Data to Study Lateral Variations in Young Oceanic Crust
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-
Interpretation of Well-logging Data to Study Lateral Variations in Young Oceanic Crust
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.
7/31/2019 Logging Interpretation
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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
Interpretation of Well-logging Data to Study Lateral Variations in Young Oceanic Crust
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
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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.
7/31/2019 Logging Interpretation
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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
Interpretation of Well-logging Data to Study Lateral Variations in Young Oceanic Crust
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.
7/31/2019 Logging Interpretation
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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.
7/31/2019 Logging Interpretation
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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
Interpretation of Well-logging Data to Study Lateral Variations in Young Oceanic Crust
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.
7/31/2019 Logging Interpretation
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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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