11. ORIGIN OF BASALTIC GRAVELLY SANDS, HOLES 410, 410A, 412A, AND 413

William P. Roberts, Department of Geology, James Madison University, Harrisonburg, Virginia

INTRODUCTION

Drilling into basaltic basement in Holes 410, 410A, and412A was terminated owing to jamming of the drill bit. Ineach instance a gravelly sand consisting mostly of angular tosub-angular fragments of basalt, volcanic glass, andlimestone and calcite fragments was recovered from the drillbit or bumper sub. A gravelly sand similar to thosedescribed above was also recovered from within basalticbasement in Hole 413, but did not cause the drill bit to stick.In the core-catcher sample from Hole 410, recovered at thecontact of the basement with the overylying nannofossilooze, two thin layers (5 to 10 cm thick) of well-sortedbasaltic sand were interlayered with thin layers (5 to 10 cmthick) of nannofossil ooze.

In Holes 410, 410A, and 413, pieces of basaltic brecciacemented by very pale orange (10YR 8/2) to dark yellowishorange (10YR 6/6) micrite were intercalated in the basalt.In each of these holes, the micrite-cemented basaltic brecciaor chalk occurs above the basaltic gravelly sands.

Dick et al., 1976, reported basaltic sands and gravels inthe lower 90 meters of Hole 396B (Leg 46), in basalticbasement. They examined the mineralogic and texturalcharacteristics of the basaltic sands and gravels, togetherwith the lithology, magnetic inclinations, chemicalstratigraphy (based on Tiθ2 concentrations), and dualinduction lateral (conductivity) logs of the basement rockand concluded that the basaltic sands and gravels werenatural deposits rather than drilling artifacts. Dick et al.(1976) attributed the fragmentation of the basalts to (1)fragmentation during eruption and cooling or (2) tectonicbrecciation after consolidation, or (3) erosion,transportation, and deposition to form talus slopesassociated with fault scarps. They were unable to make aclear choice among the three hypotheses. They ruled out thepossibility that the sands were formed from "shaving orreaming out" the hole by the drill bit, because no re-entryhad been made.

The purpose of this paper is to determine the origin of themicrite-cemented basaltic breccias and the associatedbasaltic gravelly sands. Special attention will be given to thehypothesis that the gravelly sands may be drilling artifactsderived from the breccias.

METHODS

Thin sections of eight representative pieces ofmicrite-cemented basaltic breccia were examined with apetrographic microscope (Table 1). Seven samples ofbasaltic gravelly sands were sieved at one-half phi intervals(Table 2). Three additional sand samples were sieved: twobasalt samples crushed in the laboratory and Sample410-36, CC.

TABLE 1Locations of Thin Sections of Micrite-Cemented Basaltic Breccias

(Hole-Core-Section, Interval in cm)

410-26-1,55-58410-38-11, 139-140410-39-1,91-94410-39-2, 5-741041-1,24-28410A-2-2,6-8413-1-2, 17-20413-2-1, 13-20413-3-1,53-60

TABLE 2Locations of Samples of Basaltic Sands

Hole

410410A412A412A413413410

Core

416

15 "A"15 " B "

11

36

Section

2Bumper subCore barrelCore barrel

11

CC

cm From Top

88-150

after jammingafter jamming

10-13100-102

The size distributions of the basaltic sands were tested fortheir similarities to the theoretical size distribution ofcrushed particles according to the Rosin-Rammler Law(Rosin et al., 1933) by plotting the sieve results on graphpaper with a scale of iterated logarithm as ordinate andlogarithm as abscissa. According the the Rosin-RammlerLaw, the iterated logarithm of the mass of crushedparticles larger than a certain size ("oversize" weight),plotted against the logarithms of the sieve openings (mm),will be a straight line (Herdan, 1960). The Rosin-RamlerLaw has been applied most often in studies of the sizedistribution of broken coals (Landers and Reid, 1946)(Figure 1), but has also been found applicable to glass,gypsum, cement, quartz, clay, and many other natural andartificial materials (Herdan, 1960).

After the basaltic sands were sieved, the followingstudies were conducted:

1) binocular estimation of the composition of coarsefractions (>177 µm);

2) petrographic microscope determination of grainmounts of fine fractions (>63 µm <177 µm). Using themechanical stage, 100 non-opaque grains were identified(Krumbein and Tukey, 1956);

3) roundness estimation of the modal size class and thetwo finest sieve fractions of each sample by comparisonwith the Powers Roundness Scale (Powers, 1953). Theresults of three independent operators were averaged.

421

W. P. ROBERTS

36 7 9 O J -

§2o

Screen Opening (mm)

.4 .51.0

.6 .7 .8 .9 5 6 7 8 9 1080 100

20 30 40 50 60 70 90 200 300

99100 80 60 50 40 30 20 18 16 14 12 10 8

U. S. STANDARD SIEVE DESIGNATION

I I I I I I I I I I I I I100 80 60 48 35 28 20 16 14 12 10 9 8

TYLER SIEVE DESIGNATION

SCREEN OPENING, INCHES

Figure 1. Size distribution of broken coal (modified after Landers and Reid, 1946, fig. 1).

RESULTS

Petrography of Micrite-Cemented Basaltic Breccias

The micrite-cemented basaltic breccias consist of roughlyequal proportions of the following two major constituents:

1) Basalt clasts — a mosaic of plagioclase spherulitesand rare olivine crystals in a black to dark brown oryellowish brown (when palagonitized) glass. The clasts areequant to elongated, angular to sub-angular, and range indiameter between 1 mm to 3 cm. The clast size is the onlymajor difference in this constituent between the samples.

2) Micrite matrix — gray to yellowish orangemicrocrystalline calcite with scattered foraminifers and rarediatom and radiolarian fragments. Thin veinlets of clearcalcite (0.3 to 0.5 mm thick) occur in intersecting patterns.Sub-angular to sub-rounded intraclasts up to 5 mm long

occur near the boundaries of the micrite and basalt clasts. Insome areas, the intraclasts are cemented by sparry calcite.Rare angular cavities about 0.5 mm in diameter, some withyellowish (palagonitized ?) rims, that may have originallybeen glass clasts, occur in the micrite.

Basaltic Sands and Gravels

Grain Size

The sieve results plotted on "Rosin-Rammler" graphpaper show that the samples plot as either nearly straightlines, two straight-line segments, or three straight-linesegments (Table 3, Figure 2). Owing to overlap of the plots,only 5 of the 10 samples are plotted (Table 4, Figure 2). Theplots that overlap very closely are: samples " A " and " B "of Core 412A-15; the two samples of basalt crushed in the

422

BASALTIC GRAVELLY SANDS

TABLE 3Shape of Size Distribution Plots of Basaltic

Gravelly Sands

Shape of Plot Samples

Nearly straight line

Two straight-linesegments

Three straight-linesegments

410-36, CC412A-15 "A"412A-15 "B"413-1-1, 10 cmCrushed basalt 1Crushed basalt 2410A-6 413-1-1, 100 cm413-1-1 412A-13-1

laboratory; and samples from Core 410A-6, Sections413-1-1 and 412A-13-1, and Sample 413-1-1, 100 cm.

Most of the basaltic sand samples are well sorted, but onesample, 412A-15 " B " is poorly sorted (Table 5). The twocrushed basalt samples are poorly sorted. The layeredbasaltic sand of Sample 410-36, CC is very well sorted. Themean grain size varies from —1.660 to 2.97Φ (Table 5).

Roundness

The basaltic sand samples consist of predominantlyangular to sub-rounded particles (Table 6). The layeredbasaltic sand (Sample 410-36, CC) is mostly sub-angular to

Screen Opening (mm).07 .09

.03 .04 .05 .06 .08 .1 5 6 7 8 9 1 0

400325

1 1400

325

270

1270

200

1200

140

U.S.I

150

100 80 60 50 40

STANDARD SIEVEI

100I

80I

60I I

48 35

30 20 18 16

DESIGNATIONI I I

28 20 161

14

14

I12

12

I10

10

I9

8

I8

6

I6

4

I4

.265

TYLER SIEVE DESIGNATION

Figure 2. Size distribution of basaltic gravelly sands. (See Table 4 for key to numbers next toplate.)

423

W. P. ROBERTS

TABLE 4Key for Sample Numbers on Graph

Numbers on SampleGraph (Interval in cm)

12345

413-1-1,10-13410-36, CC412A-13-1

Crushed basalt412A-15

TABLE 5Sorting Values of Basaltic Sands and Gravels

Sample410A-6413-1-1412A-15-A412A-15-B412A-13-1413-1-1, 100-102 cm413-1-1, 10-13 cm410-36, CCBasalt 1Basalt 2

Mz (0)a

.631.0

-1.66-1.630.870.932.972.920.670.20

σi (Φ)

0.660.720.651.50.680.800.650.171.641.50

SortingClassification0

Moderately well sortedModerately sortedModerately well sortedPoorly sortedModerately well sortedModerately sortedModerately well sortedVery well sortedPoorly sortedPoorly sorted

aGraphic mean (Folk, 1968).

Inclusive graphic standard deviation (Folk, 1968).cFolk (1968).

TABLE 6Roundness of Basaltic Sands and Gravels

SizeFraction

(µm)

>88 <125>63 <88Modal sizes

VeryAngular

(%)

1.01.32.6

Angular(%)

20.519.520.9

Sub-Angular

<%)

41.042.362.7

Sub-Rounded

(%)

31.828.613.8

Rounded(%)

4.77.0

0

Weil-Rounded

(%)

1.01.3

0

angular (Table 7). The few rounded particles are calcitespherules or foraminifers. The crushed basalt samples arevery angular to angular (Table 8).

Composition

The coarse size fraction (>177 µm) is mostly brownish,angular chips of volcanic glass and angular to sub-angularbasaltic clasts. Almost all the glass is very fresh, but someappears to be partially palagonitized. Small amounts ofcalcite spherules, yellowish micrite clasts, and abradedforaminifers also occur.

The finer size fractions (>63 µm to <177 µm) arecomposed mostly of calcite and altered grains (Table 9).The altered grains, although unidentifiable, are probablymostly basalt fragments, micrite, or possibly some alteredglass. Small amounts of broken or abraded foraminifers areincluded as "calcite." Palagonite and fresh volcanic glass,and small amounts of broken olivine and plagioclasecrystals occur (Table 9).

Origin of the Basaltic Gravelly Sands

Three major types of evidence suggest that most of thebasaltic sands recovered from Holes 410, 410A, 412A, and413 are drill cuttings:

SizeFraction

(µm)

>88 <125(Mode)>63 <88

VeryAngular

4

2

TABLE 7Roundness of Core 410-36

Sub-Angular Angular

36 55

33 59

Sub-Rounded

5

5

Rounded

0

1

Weil-Rounded

0

0

SizeFraction

(µm)

>88 <125>63 <88Modal sizes

VeryAngular

7830

100

TABLE 8Crushed Basalt Samples

Sub-Angular Angular

22 059 11

Sub-Rounded

00

Rounded

00

Well-Rounded

00

TABLE 9Averages Composition of Fine Size Fractions of

Basaltic Sands and Gravels (%)Size

Fraction(µm)

>125 <177>88 <125>63 <88

Opaque

13.311.3

8.3

Nonopaque Calcite

86.788.791.7

21.529.439.2

Altered

64.354.745.1

N<

Palagor

8.38.88.6

jnopaque

lite Glass

2.53.33.4

Olivine

2.72.52.9

Plagioclase

0.71.20.9

1) the fragments of basalt, glass, yellowish micrite,along with crystals of olivine and plagioclase that are majorconstituents of the basaltic sands, appear to be identical incomposition, color, and texture to the same constituents ofthe basaltic breccias;

2) the basaltic sands, except Sample 410-36, CC, wererecovered either stratigraphically below or mixed withpieces of basaltic breccia;

3) the size distributions of the basaltic sands approachstraight-line plots on "Rosin-Rammler" graph paper.

It seems especially significant that the sands wererecovered only after coring the basalt and that thecomposition and texture of the sands are nearly identical tothe breccias. Drilling through any type of rock mightlogically be expected to produce drill cuttings with acomposition similar to the rock being drilled. If the rockmatrix is brittle and tends to have conchoidal fracture, asmicrites do, then enough drill cuttings might be produced tofinally jam the drill bit.

Although Dick et al. (1976) mention the presence of"pieces of calcite-cemented microbreccia" in the sandsfrom Hole 396B, they seem to attach no significance to thatevidence. It does not seem logical that the fragmentation ofa cooling basalt flow or the erosion and deposition of abasaltic talus would result in the presence in the basalticsands of clasts of micrite identical in texture, color, andcomposition to a basaltic breccia found stratigraphicallyabove the sand, indicating deposition after the sand.Similarly, it would seem that tectonic brecciation wouldresult in a sand similar in composition to the rock belowrather than above.

Some of the size distribution plots of the sands approachthe straight-line plot on "Rosin-Rammler" graph paper.

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BASALTIC GRAVELLY SANDS

Two notable exceptions, however, significantly weaken thecredibility of this evidence. These exceptions are the almostperfect straight-line plot of Sample 410-36, CC and thebent-line plot of the basalts crushed in the laboratory (Figure2). The interlayering of the basaltic sands of Sample410-36, CC with thin layers of nannofossil ooze, along withthe fine grain size and very good sorting (Table 5) are strongevidence that the sands of Sample 410-36, CC are naturaldeposits. A slight grading was reported in the coredescription for the basaltic sand layers. These sands may beturbidites or accretionary sediment deposits from a current.The samples of basalt crushed in the laboratory would beexpected to have straight-line size distributions; instead theyhave two nearly straight-line segments.

SUMMARY AND CONCLUSIONS

The basaltic gravelly sands are probably drilling artifactsformed as a result of drilling into the brittlemicrite-cemented basaltic breccias above the sands. Sample410-36, CC, with its layered nature, its fine, well-sortedtexture, and its position immediately above the basement, isthe exception to the above conclusion. This fine to very finesand is apparently either an accretionary sediment deposit ora turbidite.

It should be noted that, in view of the small number ofsamples studied, and the widespread locations of the drillsites, the conclusions reached in this paper are tentative.Many more samples from several drill sites in a small areawould be needed to verify the above conclusions.

ACKNOWLEDGMENTS

We wish to thank Captain J.C. Clark and the crew of theGlomar Challenger, for supplying major equipment and services.The laboratory work of David Collins and Jay Rhoderick, of JamesMadison University, is greatly appreciated. Henry Dick and DavidPoché critically read the manuscript.

REFERENCES

Dick, H., et al., 1976. Glass-rich basaltic sand and gravel withinthe oceanic crust at 22°N, Nature, v.262, p.768-770.

Folk, R.L., 1968. Petrology of sedimentary rocks: Austin(Hemphills).

Herdan, G., 1960. Small particle statistics: London(Butterworths).

Krumbein, W.C., and Tukey, J.W., 1956. Multivariate analysisof mineralogic, lithologic and chemical composition ofrock-bodies, Journal of Sedimentary Petrology, v.26,p.322-337.

Landers, W.S., and Reid, W.T., 1946. A graphical form forapplying the Rosin and Rammler equation to the sizedistribution of broken coal, Bureau of Mines InformationCircular 7246: Washington (United States Department of theInterior).

Powers, M.C., 1953. A new roundness scale for sedimentaryparticles, Journal of Sedimentary Petrology, v. 23, p. 117-119.

Rosin, P., and Rammler, E., 1933. The laws governing thefineness of powdered coal, Journal of the Institute of Fuel, v. 7,p. 29.

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