deep sea drilling project initial reports volume 37crust on the western flank of the famous...
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1. INTRODUCTION
Fabrizio Aumento, Department of Geology, Dalhousie University, Halifax, Nova Scotia, CanadaWilliam G. Melson, Department of Mineral Sciences, National Museum of Natural History,
Smithsonian Institution, Washington, D.C.and
Paul T. Robinson, Department of Earth Sciences, University of California, Riverside, California
BACKGROUND
The second planned attempt at deep penetration ofthe igneous section of the oceanic crust took place dur-ing Leg 37 of the Deep Sea Drilling Project. A previousunsuccessful attempt occurred on Leg 34. On Leg 37 wepenetrated successfully the upper part of the oceaniccrust on the western flank of the FAMOUS (Franco-American Mid-Ocean Undersea Study) area of theMid-Atlantic Ridge at 36°N. Detailed investigations ofthe Median Valley some 30 km to the east were carriedout concurrently by surface ships and three mannedsubmersibles (Alvin, Archimede, and Cyand).
The main purpose of Leg 37 was to investigate thepetrological and geophysical nature of oceanic layer 2.This 1.5-km thick layer, which underlies the floor ofmost of the world's oceans, is thought to be generatedat crustal spreading centers, and to hold evidence forthe origin of oceanic magnetic anomalies, for crustaland mantle evolution, and perhaps, for the origin ofsome types of ore deposits.
Leg 37 grew out of interest expressed by the Igneousand Metamorphic Petrology Panel of JOIDES fordeeper penetration into the oceanic crust, and out of aspecific proposal to the JOIDES Atlantic AdvisoryPanel by the Dalhousie University/Bedford Institutegroup to drill a deep hole in the area of the HUDSONGeotraverse, on the Mid-Atlantic Ridge at 45°N. Thisproposal was accepted in principle, and Leg 37 wasassigned to the deep penetration experiment. On sub-sequent evaluation of the climatic expectations at45 °N, the Atlantic Advisory Panel proposed to changethe drilling site to the more favorable weather zone ofthe Mid-Atlantic Ridge at 36°N, where FAMOUS sub-mersible and surface ship investigations were well un-derway, anticipating that a considerable body ofknowledge, comparable to that at 45°N, would beavailable by the summer of 1974. The latter is crucial toany deep penetration experiment, since it is of the ut-most importance to be able to correlate deep drillingdata with that obtained by detailed, but more conven-tional techniques.
Thanks to the concentrated efforts of the researchvessels Hudson, Discovery II, and Jean Charcot, whichtogether extended the FAMOUS survey area westwardto cover the prime drilling site with detailedbathymetric and magnetic surveys, seismic reflectionand refraction profiles and dredging, coring, and heat-flow measurements, Glomar Challenger sailed from Rio
de Janeiro with sufficiently detailed information on thedrilling site and its setting to permit a direct approachto a specific site within a 3-km-wide valley, calledDEEP DRILL VALLEY, at 36°52'N, 33°39'W, about30 km west of the Median Valley, that is, in a geo-logically very young part of the American plate.
CRUISE OBJECTIVES
The prime objective for Leg 37 was the deep penetra-tion of at least the upper part of the igneous section ofthe oceanic crust. With such penetration and itsassociated core recovery, we hoped to answer many ofthe problems associated with mid-oceanic ridges whichare still outstanding, including the following:
1) The problem of the separation of the materialnow constituting layer 2, from layer 3 and, indirectly,the nature of the upper mantle beneath oceanic risesand other spreading centers (Green, 1971). Thisproblem can be approached when a detailed knowledgeof the petrology and major, minor, and trace elementgeochemistry of the layer is available. Before the cruisewe knew some of the rock types contributing to thelayers but not their relative proportions, representativeaverage compositions, or within-layer variation ranges.A single section through the layers has provided suf-ficient information to start on this problem.
2) The construction of layer 2. This problem can betackled from the results of careful macroscopic studiesof the cores. It was hoped to determine the relative con-tributions of sediments and igneous materials, andmore difficultly, of intrusives and extrusives. Paleon-tological, paleomagnetic, and geochronological dataprovide constraints on the time required to build uplayer 2, and hence whether it forms entirely within theMedian Valley, or whether off-ridge volcanism plays asubstantial role.
3) Changes during the evolutionary history of layer2. A complete attack on this problem needs informa-tion from several drill holes and from the submersiblework on the axis of the ridge. Dredging and heat-flowmeasurements have already provided hints thattemperature gradients in excess of 100°C/km common-ly exist in layer 2 close to the ridge axes, with concom-mitant extensive early metamorphism of the con-stituents of that layer (Melson et al., 1968; Aumento etal., 1971; Aumento et al., 1974).
4) The existence, nature, and thickness of thepostulated highly magnetized layer 2A. This informa-tion can be obtained from a combined magnetic and
F. AUMENTO, W. G. MELSON, P. T. ROBINSON
petrological study of a number of sections throughlayer 2.
5) Constitution of submarine basalt lava flows. Theproportion of pillows to massive basalt can be obtainedfrom a macroscopic study of flows. The internal,chemical, mineralogical, and magnetic variationswithin flows can be obtained by the means alreadydescribed.
6) Heat flow and heat production in layer 2. This canbe determined by in-hole temperature measurementsand laboratory thermal conductivity measurements ofthe core. It was thought possible to determine whetheror not the observed low heat-flow values arise fromhydrothermal circulation. Heat production in layer 2can be estimated from laboratory measurements on thecore.
7) The possibility that processes occur in layer 2 thatresult in the concentration of economic minerals suchas sulfides. This can be tested by macroscopic examina-tion of the core, ore mineral determinations, andgeochemical investigations.
8) The nature, provenance, and state of alteration ofany sediment intercalated in the igneous rocks of layer2.
The prime goal of Leg 37 was achieved at Site 332with a penetration of 583 meters into acoustic base-ment. Site 333 was a second, but unsuccessful, attemptat deep penetration in the same sediment pond in whichSite 332 is located. Sites 334 and 335 were single .bitholes drilled along a sea-floor spreading flow line pass-ing through Sites 332 and 333 (Figure 1).
GEOLOGY OF THE FAMOUS AREA
Regional StudiesA number of research vessels, including the Knorr,
Atlantis II, Discovery II, Hayes, Jean Charcot, and Hud-son, have contributed to our knowledge of the areatermed FAMOUS between 36° and 37°N straddlingthe axis of the Mid-Atlantic Ridge. Information hascome from conventional survey and sampling tech-niques, as well as from more sophisticated techniquesinvolving acoustic navigation, submersibles, deeptowed bodies, long-range side scanning sonar (GLORIA),bottom seismographs, multiple narrow beam echosounders, and LIBEC photomosaics. Most of the in-vestigations were concentrated within the MedianValley itself, but the surrounding ocean floor has alsoundergone considerable study.
The whole area is one of intense fracturing, both byblock faults parallel to the spreading axis, andtransverse to it along both major and minor fracturezones (see Figure 1). The Median Valley is offset justsouth of 37°N by a major fracture (Fracture Zone A)and by Fracture Zone B at 36°30'N. Both fractureshave westward offsets to the south on the order of tensof kilometers, and detailed investigations show themnot to be simple lines of movement, but rather complexzones of fractures, scarps, and depressions. The MedianValley blends into these fracture zones by increasing itsoverall width between confining walls, and in one case,south of 36°30'N, there appear to be two Median
Valleys parallel to one another, possibly one of thembeing an older, extinct spreading center (Figure 2).
The Median ValleyNeedham and Francheteau (1974) describe the Me-
dian Valley in detail. Their description is of interestbecause we drilled sections of the oceanic crustgenerated within the confines of the Median Valley dur-ing the interval from 3.5 to 13 m.y.B.P.
They summarize that: "The Rift Valley between36°42'N and 36°55'N is 31 km wide, with halfwidths of12 and 19 km for the western and eastern side, respec-tively. Both outer edges of the Rift Valley stand about1500 meters above an Inner Floor where very freshpillow lavas occur. The Inner Floor probably includesthe locus of new crust; and its bordering slopes, whichare particularly well-defined on the western side, limitto less than about 2.5 km the width of the zone overwhich new crust may have evolved with little or no ver-tical displacement. Near 36°50'N, the Inner Floor ac-commodates an approximately 1-km wide, 4-km longCentral High, with a height of up to 250 meters. In thisarea, the locus of new crust may also occupy a verynarrow zone; it may lie either along the Central High oralong a trough flanking the Central High. The magneticanomaly pattern indicates that, since the beginning ofthe Brunhes epoch (6.9 × 105 yrB.P.), the eastern limbhas grown approximately twice as fast as the westernlimb. Using extrapolated spreading rates, the ages ofthe outer edges of the Rift Valley are 1.3 and 1.7 m.y.for the eastern and western side, respectively."
More recent investigations of the Median Valleyhave detected other features of interest: (a) The centralhigh itself contains a deep longitudinal axial fracture inplaces, and in others symmetrical cones with definitedimples at their apices lie over the axial fracture, (b)Recent volcanism has been found not only on the cen-tral high, but also along the foot of the eastern faultscarps, obviously overlying somewhat older sediment-covered and ferromanganese-coated material. Some ofthe lava flows showing pillowed surfaces and tubularpillowed fronts may be as much as 50 meters thick, (c)Between the central high and the eastern and westernfault scarps there are parallel zones of uplifted blocksand deep, narrow fractures. Indeed, at 45°N one suchfeature was discovered accidentally by dredging, andfound to be 400 meters deep, and sufficiently narrowfor it not to register with a standard PDR. (d) Specificrock types have been found to occur within restrictedenvironments; olivine basalts grading to picrites appearto be restricted to the central high, whereas plagioclase-phyric basalts are found primarily on the fault scarps.Complete major and minor element chemical analysesby H. Bougault of these basalts dredged from the Me-dian Valley by Jean Charcot are given in Table 1A, andelectron probe analyses by W.G. Melson on glassesrecovered from the Median Valley by Woods HoleOceanographic Institution are given in Table IB.
Geology of Deep Drill ValleyThe following criteria were used in the original selec-
tion of the prime drilling sites:
INTRODUCTION
Figure 1. Location of Leg 37 drill sites and Glomar Challenger track in relation to the FAMOUS area.
a) Sediment thickness should exceed 100 msec (two-way time equivalent to 80 m at 1.6 km~1 velocity) andthe area of the sediment pond should exceed one mile.
b) Fracture zones should be avoided.c) The magnetic anomaly pattern over the area
should be linear and undisturbed.d) The sites should lie over a magnetized block of a
single polarity, preferably negative.e) The site should be well within a block of sea floor
between fracture zones where the topographic trendsare continuous and parallel to the Median Valley.
f) The site should be as near to the Median Valley aspossible, and not further than 25-30 miles, in order toprovide the best possible correlation with FAMOUSarea work, and the freshest possible basalt forgeochemical studies.
DEEP DRILL VALLEY, with its longitudinal axisat approximately 33°38'W, and lying between 36°50'Nand 36°57'N was found to fit all the above re-quirements. The detailed geology of the valley is dis-cussed in the site report for Site 332.
SHIPBOARD LABORATORY EXPERIMENTSGlomar Challenger on Leg 37 of the Deep Sea Drill-
ing Project carried extensive laboratory facilities forshipboard investigations of igneous rocks. These in-cluded: (a) equipment for the preparation of polishedthin sections of high quality along with excellenttransmission and reflection microscopes with cameraattachments, (b) ceramic and tungsten carbide mills for
the pulverization of specimens and a precision elec-tromagnetic balance for accurate weighing of powders,(c) a gas chromatograph for rapid determination ofH2O and CO2, (d) a high temperature resistance furnacefor fusion of rock powders with preweighed mixtures oflithium tetraborate and lanthanum oxide fluxes, (e) atube-excited dispersive X-ray fluorescence unit for theroutine determination of major and minor elementcompositions, (f) a spinner magnetometer and acdemagnetization unit for determination of NRMcharacteristics (polarity and remanent intensity) ofboth igneous and sedimentary rocks, (g) equipment formeasuring sonic velocities of specimens, both at at-mospheric pressure and at elevated pressures, and (h)equipment for measurement of porosity, density, andelectrical and thermal conductivities.
The routine use of the support facilities mentionedabove permitted shipboard scientists to evaluate thedrill core soon after its recovery and to modify both thedrilling and sampling program accordingly.
SHORE-BASED LABORATORY EXPERIMENTSThe unique nature of a deep core to be recovered
from the oceanic crust spurred an unprecedentednumber of laboratories and individuals to submitresearch proposals for the study of core material wellbefore it was obtained. Over 50 laboratories were in-volved in the most exhaustive study ever undertaken ofany suite of terrestrial rocks. Laboratories from theUSA, Canada, USSR, France, Germany, Great
F. AUMENTO, W. G. MELSON, P. T. ROBINSON
100 FATHOM CONTOUR INTERVAL OVERALL
Figure 2. Bathymetric map of the Mid-Atlantic Ridge in the vicinity of the FAMOUS area. Contour interval is100 fathoms.
INTRODUCTION
TABLE 1AChemical Analyses of Basalts Dredged from the Median Valley of the Mid-Atlantic Ridge in the FAMOUS Area
by Jean Charcot (Analyst H. Bougault)
Oxides
SiO2
A12O3
Fe2O3
FeOMnOMgOCaONa2OK2OTiO2P2<>5H2OH2O+Ignition
Loss
Total
Picrite
DR1231647.4513.49
1.047.720.16
16.6511.42
1.370.080.590.090.070.46
-0.39
100.60
DR1232046.7714.20
1.207.690.16
16.0611.89
1.980.090.620.070.110.37
-0.47
101.22
DR112250.4515.40
1.377.410.159.33
12.262.230.160.900.110.090.42
-0.39
100.31
DR1124
50.0516.19
1.477.480.16
10.5312.18
2.510.150.910.120.050.46
-0.36
101.22
DR4303
49.3414.750.917.420.14
11.0011.48
1.550.160.860.100.090.75
-0.06
98.59
DR9309
49.2815.30
1.687.440.169.20
12.702.550.171.020.130.170.62
-0.20
100.44
DR5304
50.3314.92
1.437.710.15
10.1211.89
1.790.171.070.100.110.58
-0.27
100.40
Olivine Basalts
DR9322
49.9215.29
1.668.030.179.12
12.552.230.131.040.100.120.51
-0.37
100.40
DR9322
49.9615,04
1.348.240.179.02
12.592.080.131.040.100.090.53
-0.37
100.32
DR9308
49.9614.83
1.468.060.179.02
12.432.010.121.020.120.110.54
-0.34
100.66
DR8314
49.7914.86
1.398.320.179.54
12.512.180.221.090.120.110.41
-0.50
100.72
DR5100
49.9314,53
1.459.370.179.22
12.082.340.231.270.180.130.59
-0.36
100.76
DR1231949.8314,82
1.359.370.188.44
11.822.280.231.440.180.070.50
-0.53
100.51
DR11315X50.2014,86
1.659.280.188.23
11.902.280.221.450.180.070.55
-0.47
100.87
DR11315Y50.2414,720.989.500.188.24
11.822.140.231.440.180.050.50
-0.54
100.22
Oxides
SiO2
A1 2O 3
F e 2 O 3
FeOMnOMgOCaONa 2 OK2OT i O 2
P2°5H2O
H2O+Ignition
Loss
Total
DR3123
50.4614.81
1.926.710.158.07
13.262.34
0.230.90
0.110.370.64
-0.10
100.00
DR2301
50.3215.03
2.356.450.158.26
13.681.840.321.02
0.16
0.260.89
0.18
100.76
DR6306
49.27
14.571.767.900.178.44
12.502.650.250.980.120.180.55
-0.32
99.35
DP. 10310
50.19
14.272.22
7.610.178.18
12.431.740.311.000.12
0.310.74
-0.10
99.30
TABLE 1A -
DR10105
50.5814.54
1.787.850.178.35
12.671.910.301.03
0.11
0.160.74
-0.12
100.20
DR6305
50.3214.69
2.586.860.168.00
12.892.420.401.09
0.130.290.99
0.24
100.84
- Continued
DR10311
49.51
15.772.326.540.158.78
12.812.280.230.99
0.140.250.65
-0.07
100.45
DR10100
49.3117.04
2.046.150.158.17
13.421.610.270.970.150.22
0.83
0.16
100.33
DR10314
49.50
15.892.736.600.168.67
13.101.980.281.15
0.130.230.89
0.17
101.30
DR3131
49.24
18.302.005.240.127.96
14.321.600.150.730.080.190.90
0.33
100.83
DR2132
49.6519.25
1.245.510.126.90
14.601.800.12
0.750.110.090.52
-0.09
100.64
DR7318
46.7619.462.995.580.146.75
14.102.230.120.980.19
0.350.97
0.36
100.66
DR3321
48.52
20.99
1.214.670.106.59
15.031.880.120.590.070.150.52
0.01
100.46
Britain, and Australia undertook the following in-vestigations.
1) Petrography, including fabric analysis.2) Major, minor, and trace element geochemistry.3) Mineralogy, using optical, X-ray diffraction, elec-
tron microprobe and neutron activation deter-minations.
4) Isotopic studies of argon, oxygen, sulfur, stron-tium, uranium, thorium, and lead.
5) Paleomagnetism, rock magnetism, and opaquemineralogy.
6) Thermal conductivity and heat production.7) Seismic velocities and their anisotropy at ele-
vated temperatures and pressures.8) Studies of geothermal waters, mineral deposits,
and fluid motion.
9) Electrical conductivities at elevated temperaturesand pressures.
10) Age determinations by paleontological, isotopic,and fission track methods.
The results of these investigations are reported in thefollowing chapters.
EXPLANATORY NOTES
Organization of This VolumePart I of this volume consists of an introductory
chapter outlining the cruise objectives and givingbackground information on the geology of theFAMOUS area and the prime deep drill site. Part IIcontains the Site Reports which basically are detailedsummaries of the geological data obtained at each site.
F. AUMENTO, W. G. MELSON, P. T. ROBINSON
TABLE IBElectron Microprobe Analyses of Basalt Glasses Dredged
from the Median Valley of the Mid-Atlantic Ridge(Analyst W. G. Melson)
SiO2
A 12°3TiO2
FeO*a
MgO
CaONa2OK2OP2°5Total
aFeO*
36°75'N33°28'W
6
50.7015.28
1.119.208.05
12.452.210.140.14
99.28
total Fe as FeO
36°51'N33°66'W
8
51.0215.14
1.119.307.74
11.872.220.200.13
98.73
36°33°
9
50.3015.18
1.309.737.78
11.732.230.180.15
98.58
71'N29'W
10
50.4214.95
1.309.827.76
11.992.310.180.17
98.89
36°77'N33°31'W
11
50.5914.91
1.219.537.95
12.042.190.170.17
98.76
The text in the Site Reports has been kept brief with themajor emphasis on presentation of data on the coresummary forms and in data tables. The core summaryforms contain data generated by the shipboard partyduring the cruise and updated where necessary. Thetables contain prime data generated by both shipboardand shore-based investigators. The site reports areorganized as follows:
Site dataSummaryBackground and ObjectivesOperationsLithology and GeochemistryPaleomagnetismPhysical PropertiesPaleontologySpecial studies of varied nature are reported in Part
III. Part IV contains the results of physical propertiesmeasurements on both igneous rocks and sediments,and Part V deals with paleomagnetic and rock magneticstudies. Geochemical studies and petrologic studies arecontained in Parts VI and VII, respectively. Paleon-tological investigations and a biostratigraphic summaryare given in Part IX and a cruise synthesis is given inPart X.
Responsibility for Authorship
The site reports are co-authored by the entire scien-tific party. In general, summaries and background werewritten by F. Aumento and W.G. Melson; operationsby D. Edmiston; lithology and geochemistry by H.Bougault, L. Dmitriev, J. Fischer, M. Flower, P.Robinson, and T. Wright; physical properties by R.Hyndman; paleomagnetism by J. Hall; and paleon-tology by R. Howe and G. Miles.
Numbering and Depth ConventionDrill site numbers run consecutively from the first
site drilled by Glomar Challenger in 1968. The sitenumber is unique; thus, use of a leg number is optional.A site refers to the hole or holes drilled from oneacoustic positioning beacon. Several holes may be
drilled at a single locality by pulling the drill stringabove the sea floor ("mud line") and offsetting the shipsome distance (usually 100 m or more) from the previ-ous hole.
Holes drilled at a site take the site number, and aredistinguished by a letter suffix. The first hole has onlythe site number; the second has the site number withsuffix A; the third has the site number with suffix B;and so forth. It is important, for sampling purposes, todistinguish the holes drilled at a site because recoveredsediments or rocks usually do not come fromequivalent positions in the stratigraphic column atdifferent holes.
Cores are numbered sequentially from the top down.In the ideal case, they consist of 9 meters of sediment orrock in a plastic liner of 6.6 cm diameter. In addition, ashort sample is obtained from the core catcher (a mul-tifingered device at the bottom of the core barrel whichprevents cored materials from sliding out during core-barrel recovery). This usually amounts to about 20 cmof sediment which is stored separately. Basaltfragments contained in the core catcher are usually lessthan 20 cm in length and are normally added to theregular plastic liner because full recovery is rare in ig-neous rocks. The core-catcher sample represents thelowest stratum recovered in the particular cored inter-val and is designated by CC (e.g., 333-4, CC = core-catcher sample of the fourth core taken in the first holeat Site 333.
The cored interval is the interval in meters below thesea floor, measured from the point at which coring for aparticular core was begun to the point at which it wasterminated. This interval is generally 9.5 meters(nominal length of a core barrel), but may be shorter orlonger if conditions dictate. For example, at Site 333many cores are 19 or even 38 meters long. Cores andcored intervals need not be contiguous. In softsediments, the drill string can be "washed ahead"without recovering core by applying sufficiently highpump pressure to wash sediment out of the way of thebit. In a similar way, in hard rocks a center bit, whichfills the opening of the bit face, can replace the corebarrel if drilling ahead without coring is necessary.
When a core is brought aboard Glomar Challenger itis labeled and the plastic liner and core cut into 1.5-meter sections. A full 9-meter core thus consists of sixsections, numbered from the top down, 1 to 6. General-ly something less than 9 meters is recovered. In thiscase, the sections are still numbered starting with one atthe top, but the number of sections is the number of1.5-meter intervals needed to accommodate the lengthof core recovered, as illustrated below:
CORE CATCHER
SECTION 3 SECTION 2 SECTION 1
SEDIMENT RECOVERED IN LINER
INTRODUCTION
Thus, as shown, recovery of 3.6 meters of sedimentresults in a core with three sections, with a void of 0.9meter at the top of the first section. By convention, andfor convenience in routine data handling at the DeepSea Drilling Project, if a core contains a length ofmaterial less than the length of the cored interval, therecovered material is placed at the top of the cored in-terval, with the top of Section 1, rather than the top ofthe sediment, equal to the top of the cored interval.Thus, the depth below the sea floor of the top of therecovered material of this hypothetical core lies at 150.9meters (not 150.0 m) and the bottom at 154.5 meters(the core catcher is regarded as being dimensionless).
A discrepancy exists between the usual coring inter-val of 9.5 meters and the 9-meter length of corerecovered. The core liners used are actually 9.28 metersin length, and the core catcher accounts for another0.20 meter. In cases where the core liner is recoveredfull, the core is still cut into six 1.5-meter sections,measured from the bottom of the liner, and the extra0.28-meter section at the top is designated Section 0, orthe "zero section." The zero section is ignored incalculations of depth below the sea floor of cores orlevels within cores.
Sedimentary Cores
In the core laboratory on Glomar Challenger, aftersome routine processing, the sediment cores are split inhalf lengthwise. One half is designated the "archive"half, which is described by the shipboard geologists,and photographed. The other half is the "working"half, which is sampled by the shipboard sedimen-tologists and paleontologists for further shipboard andshore-based studies.
Samples taken from core sections are designated bythe interval in centimeters from the top of the core sec-tion from which the sample was taken; sample size, incm3, is also given. Thus, a full sample designationwould consist of the following information:
Leg (optional)Site (Hole, if other than first hole)Core NumberSection NumberInterval (in centimeters from the top of the section)Thus, 332B-4-3, 122-124 cm (10 cm3) designates a 10
cm3 sample taken from Section 3 of Core 4 from thethird hole drilled at Site 332. The depth below the seafloor for this sample would then be the depth to the topof the cored interval plus 3 meters for Sections 1 and 2,plus 122 cm (depth below the top of Section 3).
Core Disturbance
The rotary drill coring technique quite often resultsin a high degree of disturbance of the cored sediments.This is especially true of the softer unconsolidatedsediments. Core disturbance has been treated at lengthin other volumes of the Initial Reports of the Deep SeaDrilling Project and is not elaborated upon here. Aqualitative estimate of the degree of deformation isgiven on the sediment core logs. Four degrees of drill-ing deformation were recognized as follows:
Slightly deformedmoderately deformed
-intensely deformed (soupy)Δ Δ Δ Δ brecciated
Carbon-Carbonate Data
Sediment samples are analyzed on a Leco 70-SecondAnalyzer following procedures outlined in Volumes 9and 18 of the Initial Reports of the Deep Sea DrillingProject. Accuracy and precision of the results are asfollows:
Total carbon: ±0.3% (absolute)Organic carbon: ±0.06% (absolute)CaCOi: ±3% (absolute)
X-Ray Mineralogy
Semiquantitative determinations of the mineral com-position of bulk sediment samples are tabulated on thecore logs. In each listing the percentage of "amorphousscattering" (noncrystalline, unidentifiable material) isshown along with the crystalline, identified fraction.The percentages of the identified minerals sum up to100%. The analytical methods used are described inVolumes 1 and 2 of the Initial Reports of the Deep SeaDrilling Project and in Appendix III of Volume 4.
Grain Size Analyses
The grain size analyses presented on the sedimentcore logs are performed by standard sieve and pipettetechniques, described in detail in Appendix III ofVolume 4 of the Initial Reports, with modified settlingtimes as in Volume 9.
Sediment Classification
The sediment classification used here is similar to theone used in Volume 18 of the Initial Reports which wasdevised by O.E. Weser. A set of lithologic symbols usedon Leg 37 are given in Figure 3.
Smear slides are the basic means of mineral iden-tification for sediments on shipboard. Smear-slide es-timates of mineral abundances were based on area ofthe smear slide covered by each component. Past ex-perience has shown that accuracy may approach a per-cent or so for very distinctive minor constituents but formajor constituents accuracy of ±10% to 20% is con-sidered very good.
The results of several random sieve analyses ofsamples from Leg 37 for which smear-slide percentageswere estimated indicate that the percentage of nan-nofossils was frequently overestimated. This is at-tributed to the extreme thinness of many smears. Thisthinness made it appear that the fine-grained nan-nofossils made up a very high percentage when in facttheir percentage was 10%-30% lower; consequently,reported percentages of foraminifera, volcanic glass,and other constituents are correspondingly low. For ex-ample, volcanic glass percentages in Cores 5-10, Site334, actually run as high as 20% of the sediment. Somesamples may have even higher percentages.
F. AUMENTO, W. G. MELSON, P. T. ROBINSON
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Nanno Ooze Limestone PorphyriticBasalt
Vesicular Zones
G G G G G
G G G G G
G G G G G
G G G G G
G G G G G
ΔΔ
Δ
Δ
Δ
Δ
Δ
Δ
Δ
ΔΔ
Δ
Δ
Δ
ΔΔ
Δ
Δ
Δ
ForaminiferalOoze
Glass-RichSediment
Breccia Vugs
112 12Z Z Z Z Z
Z Z Z Z Z2 2 1 1 2
1 1 1 1 1
Nanno-Foram or ZeoliteCalcareous Ooze
Glassy Breccia Pumice Fragments
1
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1
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11
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1
11
1
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NannofossilChalk
Aphyric Basalt Glass Rindsand Fragments
Nodules;py-Pyrite
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til
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Sparsely-Phyric VeinletsBasalt
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Figure 3. Lithologic symbols used on Volume 37 core summary forms.
10
INTRODUCTION
Physical Properties of Sediments
The physical properties of sediments measured onLeg 37 were bulk density, water content, porosity, sonicvelocity, and thermal conductivity. Densities andporosities were determined from the total weight andvolume of each core section, by the syringe techniqueinvolving weighing and oven drying 0.5-1.0 cm3 of sedi-ment, and by the gamma ray attenuation (GRAPE)method. The section weight method gives values thatare of poor reliability being generally too low, becauseof incomplete filling of the liner and mixing and distur-bance of the sediment. Even well-preserved cores have athick layer of slurry between the core and liner so thedensities determined in this way are a lower limit. Den-sities by the syringe method have less bias, but theamount of material is so small that the results are oflow accuracy (Bennett and Keller, 1973, have given acritical discussion of these methods).
The GRAPE technique utilizes the attenuation ofgamma ray intensity in a beam passing through a sedi-ment sample. For the 0.30-0.36 mev Barium133 source,Compton electron scattering is the dominant attenua-tion process. The attenuation thus depends on the elec-tron density in the material, which in turn is ap-proximately proportional to the bulk density for com-mon geological materials (e.g., calcite, quartz,dolomite, and some clays). Aluminum cylinders ofdifferent diameters are used for calibration.
A shore-based program is used to correct forvariations in electron densities of different corematerial, particularly for the seawater component. Themethod is described by Evans (1965) and Harms andChoquette (1965), and as used on Glomar Challenger,by Boyce (1973, 1974). Cores are passed continuouslythrough the gamma beam on a carriage so that a nearlycontinuous profile of counts per unit time is obtained.The data are averaged over 3-cm-long core length inter-vals. Cores with significant gaps or voids give anirregular trace.
GRAPE densities are computed for 150 points alongeach core section or every centimeter. These data in-clude many low density points associated with voids,breaks, or disturbances in the core. In order to obtainestimates of the average in situ density in the sea floor,these points must be removed. An example of such aprofile is shown on page 17, Volume 26 of the InitialReports. Particularly at the ends of the core sectionsthere are points of apparent low density that representsimply unfilled core liner. A simple truncation pro-cedure is used to eliminate the spurious points. Theprocedure must be simple and universally applicablebecause the large amount of data precludes subjectiveevaluation.
The program starts by computing the average of the150 densities in a core section. It then truncates orremoves all points that are outside prescribed limitsabove and below this average. Out limits are +10% and-5%. The smaller lower limit is an attempt to compen-sate for the bias toward too low densities (gaps andvoids) rather than too high densities. The remaining
points are averaged again. If the new average differsfrom the first by more than a prescribed factor (0.5%here), a second truncation with the same window isapplied and so on until a stable average is reached. Twoiterations are usually sufficient. If more than 20% of thedata points have been truncated for the final average,we consider the section average to be unreliable and thevalue is not plotted.
Estimates of average density for each core arepresented in this volume. The complete data areavailable from the Deep Sea Drilling Project. The coresgiving irregular GRAPE traces probably have manygaps or voids and the computed densities have not beenpresented. Unfortunately, some data on cores with real,large density variations also are discarded.
Sonic (acoustic, seismic) velocities on cores are need-ed for the interpretation of seismic reflection andrefraction data, particularly for converting seismicreflection times to depths in the sedimentary column.Acoustic impedance, given by the product of the sonicvelocity and bulk density, is closely related to reflectioncoefficients. Thus, rapid changes in acoustic impedancemay be associated with seismic reflectors. The velocitieswere measured by determining the time delay of a high-frequency pulse transmitted through sediment or rocksamples using a Hamilton frame (Hamilton, 1965; Cer-nock, 1970). The resolution is better than 0.1 km/secand accuracy about ±0.02. Velocity was measured onat least one sample from each core. In the uncon-solidated uppermost cores measurements were madethrough the split core and liner, a correction beingmade for the thickness and travel time of the liner(0.295 cm and 1.36 µsec for the liner used on this leg).Variations in liner thickness were monitored, butsignificant changes were not detected. Semiconsol-idated sediments were measured in and out of the liner.There was a negligible difference in the velocities ob-tained by the two methods (less than 0.02 km/sec).
The sonic velocities were all measured at roomtemperature (20°-25°C) and 1 atm pressure. The in situtemperatures range from 0° to 15°C with most below10°C and pressures between 0.3 and 0.5 kbar. The dataof Schreiber et al. (1972) suggest that the velocities willbe 5% to 10% higher at 0.5 kbar. Wilson (1969) finds anincrease in velocity for seawater of 5.4% from 1 bar to0.5 kbar and a decrease of 5.1% from 20° to 0°C. Thus,the effects of temperature and pressure approximatelycancel for seawater. The effects are likely similar for un-consolidated sediment.
Thermal conductivity is an important physicalproperty of sediments. Its relationship to composition,other physical properties, and depth is needed tofacilitate conductivity estimates required for thedownward extrapolation of temperatures from near-surface heat-flow determinations. Conductivities arerequired to be combined with downhole temperaturesfor geothermal heat flux (see Chapter 8, this volume).The thermal conductivity was measured at least onceon each core of the unconsolidated sediments using theneedle-probe technique (Von Herzen and Maxwell,1959).
11
F. AUMENTO, W. G. MELSON. P. T. ROBINSON
Core Forms
The basic lithologic data are contained on the coresummary forms. As far as possible the following dataare presented for sediment cores:
Sediment or rock nameDeformationColor name and Munsell or GSA number
The reader is advised that colors recorded in corebarrel summaries were determined during ship-board examination immediately after splitting coresections. Experience with carbonate sedimentsshows that many of the colors will fade or dis-appear with time after opening and storage. Colorsparticularly susceptible to rapid fading are purple,light and medium tints of blue, light bluish-gray,dark greenish-black, light tints of green, pale tintsof orange. These colors change to white oryellowish-white or pale tan.
Composition
Grain Size, Carbon-Carbonate, and X-Ray Data
Analyzed samples are identified by section and depth(cm). Carbon-carbonate data are reported as % totalcarbon, % organic carbon, and % CaCOs. X-ray dataare given for both bulk samples and for 2-20 µmseparates. The following abbreviations are used on thecore forms: Amor.—Amorphous percentage, Quar.—Quartz, K-Fe.—Potassium feldspar, Plag.—Plagio-clase, Kaol.—Kaolinite, Mica—Mica, Chlo.—Chlorite, Mont.—Montmorillonite, Paly.—Paly-gorskite, Phil.—Phillipsite, Anal.—Analcime, Pyri.—Pyrite, Amph.—Amphibole, Augi.—Augite, Cris.—Cristobalite, Tr—Trace, Pres—Present.
Biostratigraphy
The following zonations were used in this report:Planktonic foraminifera, Blow (1969); calcareous nan-nofossils, Martini and Worsley (1970); and Radiolaria,Riedel and Sanfilippo (1970, 1971).
Igneous Rock Cores
Igneous rock cores are treated quite differently fromsediment cores. After the core is numbered and dividedinto 1.5-meter sections in the normal way, the core lineris cut, and the individual pieces of rock are washed withdistilled water, dried with a blower, and placed in aclean, presplit core liner. The pieces are then numberedwith india ink starting with 1 at the top of the core andmarked with an arrow pointing up. When two or moreadjacent pieces can be clearly fitted together, they aregiven the same number and designated A, B, C, etc,starting at the top. When adjacent pieces cannot befitted together, a styrofoam spacer is inserted betweenthe pieces and taped into place.
Most igneous rock cores were not split on Leg 37.Instead small minicores were taken for shipboard andshore-based investigations using a drill press and asmall diamond-studded coring device. The minicoreswere oriented, labeled, and divided for specific sam-pling needs. Samples taken from igneous rock cores aredesignated in the same way as samples from sedimentcores except that the piece number is also included.
Measurement of Physical Properties of Basement Rocks
The physical properties measured on basement rocksof Leg 37 are: seismic compressional and shear wavevelocity, bulk and grain density, porosity, water con-tent, electrical resistivity, and thermal conductivity.(The magnetic properties are treated separately.) Themeasurement techniques and interpretations are given(Hyndman and Drury; Hyndman, both this volume).The samples all are cylinders 2.5 cm in diameter and 2.5to 5.0 cm in length. It should be emphasized that thecore recovery in the basement parts of Leg 37 holes wasonly about 20%, and, although we have attempted toinclude all rock types and states of alteration andweathering, only parts of the recovered core is suf-ficiently unfractured to be suitable for measurement.Much of the section not recovered probably is brokenand fractured volcanic material with extensive voidsand some sediment. Thus, the mean physical propertiesgiven may be a poor approximation of the average bulkvalues for the section drilled. This problem is discussedparticularly in Hyndman (this volume). A summary ofthe measurement techniques and estimated accuraciesis given below.
1) Bulk Density—Samples are seawater saturated.The sample volume is obtained from the difference inweight in air and suspended in distilled water.Estimated accuracy ±0.005 g/cm3 (±0.15%).
2) Grain Density — Density of solid minerals ob-tained by subtracting weight and volume of the porefluid (from porosity estimate). Estimated accuracy-0.007 and +0.014 g/cm3 (-0.2% and +0.5%).
3) Porosity (percent volume that is pore spaces)—Obtained from the loss in weight by heating at 60° to70°C in vacuum for at least 72 hr. A small correction ismade for residual salt. Estimated accuracy -0.2 and+0.4 (-5% and +10% of value).
4) Water Content (percent of weight of saturatedsample that is water)—Obtained as for porosity, with-out correction for salt that remains after drying.Estimated accuracy -0.1 and +0.2 (-5% and +10% ofvalue.
5) Electrical Resistivity—Obtained from the elec-trical resistance of cylindrical samples with ends paint-ed with conducting epoxy resin, approximately 0.5 v, 50Hz, 1 atm, 23°-25°C, seawater saturated. Estimated ac-curacy ±10%.
6) Seismic Velocity (compressed wave)—At 0.5-kbarpressure, with hydraulic fluid excluded by a jacket soconfining pressure is much greater than internal or porewater pressure. Seawater saturated. Estimated accuracy±1%.
Core Summary FormsA new set of core summary forms was developed on
Leg 37 in order to portray the igneous rock cores. Inaddition to basic lithologic data, the core forms includeshipboard magnetic, chemical, and physical propertiesdata updated where appropriate.
Lithologic Data
Lithologies are based on macroscopic descriptions ofthe core supplemented by thin section data. Symbols
12
INTRODUCTION
used to designate the different lithologies are given inFigure 3.
Samples
A and B represent minicores taken for shipboardstudies. Magnetic, chemical, and petrographic analyseswere conducted on "A" cores and physical propertiesdeterminations and petrographic analyses were carriedout on " B " cores. " T " represents a thin section pre-pared onboard ship in addition to those prepared fromthe "A" and " B " cores. " C " represents a chemicalanalysis completed onboard ship in addition to thoseconducted on "A" cores.
Magnetic Data
NRM intensity is in units of 10~4 emu/cm3. NRM in-tensity is the value for the undemagnetized moment,i.e., probably the value the sample had in situ. Polarityis shown before and after demagnetization and is givenin four categories as follows:
n N Normal (/ +20°)shn SHN Shallow normal (/ = 0° to +20°)shr SHR Shallow reverse (/ = 0° to -20°)r R Reverse (/ -20°)
Lower-case letters give the polarity of undemagnetizedNRM, i.e., the probable magnetic polarity of the in siturocks. Capital letters give the polarity of the cleanedNRM, i.e., the magnetic polarity of the earth'smagnetic field during the initial cooling of the rock.The demagnetized values date from the time of initialcooling of the unit and should be used in temperaturestudies. Polarities marked " ? " are likely but not certainto remain as designated after further demagnetization.
Chemical Data
Values given are the results of shipboard deter-minations by X-ray fluorescence for AI2O3, Fe2θ3 (totaliron expressed as Fe2U3), MgO, and K2O. Values forH2O and CO2 are the result of shipboard deter-minations with a CHN analyzer.
Physical Properties
All v a l u e s r e p o r t e d a r e from s h i p b o a r dmeasurements. D = Bulk density (g/cm3); V = Com-pressional velocity measured at laboratory temperature(20°-25°C) and 0.5 kb pressure.
REFERENCES
Aumento, F., Loncarevic, B.D., and Ross, D.I., 1971. Hud-son Geotraverse: Geology of the Mid-Atlantic Ridge at45°N: Phil. Trans. Roy. Soc. London, Series A., v. 268,p. 623-650.
Aumento, F., et al., 1974. Cruise Report: HUDSON 74-003,C.S.S. HUDSON Site Survey for Deep Drill 1974, Mid-Atlantic Ridge at 36°N.
Bennett, R.H. and Keller, G.H., 1973. Physical propertiesevaluation. In van Andel, J.H., Heath, G.R., et al., InitialReports of the Deep Sea Drilling Project, Volume 16:
Washington (U.S. Government Printing Office), p. 513-519.
Blow, W.H., 1969. Late middle Eocene to Recent planktonicforaminiferal biostratigraphy: Intern. Conf. Plankt.Microfossils, Geneva.
Boyce, R.E., 1973. Porosity, wet bulk density, water content.In Edgar, N.T., Saunders, J.B., et al., Initial Reports of theDeep Sea Drilling Project, Volume 15: Washington (U.S.Government Printing Office), p. 1067.
, 1974. Instructions for grain density, wet-bulk den-sity, water content, and porosity determinations by in-dividual samples and gamma ray attentuation porosityevaluator: Unpublished manuscript.
Cernock, P.J., 1970. Sound velocities in Gulf of Mexicosediments as related to physical properties and simulatedoverburden pressures: Texas A & M Univ. Tech. Rept. 70-5-T, Office of Naval Research, Contract N00014-68-A-0308 (0002 Oceanography).
Evans, H.B., 1965. GRAPE—A device for continuous deter-mination of material density and porosity, Volume 2:Logging Symp. 6th Trans. Dallas, Texas (Soc. Prof. WellLog Analysts).
Green, D.H., 1971. Composition of basaltic magmas as in-dicator of conditions of origin: Application to oceanicvolcanism: Phil. Trans. Roy. Soc. London, Series A,v. 268, p. 707-725.
Hamilton, E.L., 1965. Sound speed and related physicalproperties of sediments from Experimental Mohole(Guadalupe Site): Geophysics, v. 30, p. 257-261.
Harms, J.C. and Choquette, P.W., 1965. Geologic evaluationof a gamma-ray porosity device, Volume 2: Logging Symp.Dallas, Texas (Soc. Prof. Well Log Analysts).
Martini, E. and Worsley, T., 1970. Standard Neogenecalcareous nannoplankton zonation: Nature, v. 225,p. 289-290.
Melson, W.G., Thompson, G., and van Andel, J.H., 1968.Volcanism and metamorphism in the Mid-Atlantic Ridge,22°N latitude: J. Geophys. Res., v. 73, p. 5925-5941.
Needham, H.D. and Francheteau, J., 1974. Some character-istics of the rift valley in the Atlantic Ocean near 36°48'north: Earth Planet. Sci. Lett., v. 22, p. 29-43.
Riedel, W.R. and Sanfilippo, A., 1970. Radiolaria, Leg 4,Deep Sea Drilling Project. In Bader, R.G., et al., 1970,Initial Reports of the Deep Sea Drilling Project, Volume 4:Washington (U.S. Government Printing Office), p. 503-575.
, 1971. Cenozoic Radiolaria from the westerntropical Pacific, Leg 7. In Winterer, E.L., Ewing, J.I., etal., Initial Reports of the Deep Sea Drilling Project,Volume 7: Washington (U.S. Government Printing Of-fice), p. 1529-1672.
Schreiber, E., Fox, P.J., and Peterson, J., 1972. Com-pressional sound velocities in semi-indurated sedimentsand basalts from DSDP Leg 11. In Hollister, CD., Ewing,J.I., et al., Initial Reports of the Deep Sea Drilling Proj-ect, Volume 11: Washington (U.S. Government PrintingOffice), p. 723-727.
/on Herzen, R.P. and Maxwell, A.E., 1959. The measure-ment of thermal conductivity of deep-sea sediments by aneedle probe method: J. Geophys. Res., v. 64, p. 1557-1563.
Wilson, W.D., 1969. Ultrasonic measurement of the velocityof sound in distilled and seawater: J. Acoust. Soc. Am.,v. 32, p. 641.
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