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SPECIFIC GRAVITY CHARACTERISTICS OF RECENT VOLCANICLASTICSEDIMENT: IMPLICATIONS FOR SORTING AND GRAIN SIZE ANALYSIS'

GARY A. SMITH 2 AND RICHARD D. SMITH 3

Dept. of Geology, Oregon State University, Corvallis, OR 97331

ABSTRACT

Specific gravity of water-saturated, surface-dried individual grains and bulk, sieved samples of pumicelapilli, ash, and slightly vesiculated rock fragments produced by recent eruptions at Mount St. Helens,Washington, as well as the composition of sediment derived from this material, indicate two major depar-tures in the character of volcaniclastics from more familiar quartzo-feldspathic sediment. These are: (I) athree-to-five-fold variation in the specific gravity of volumetrically important detrital grains, as opposed to aless than 5% variation in 99% of the volume of average quartzo-feldspathic sandstone; and (2) significantvariation in specific gravity that is inversely related to grain size within clast populations of the somecomposition. This latter observation is attributed to the larger volume of vesicles, particularly non-interconnected vesicles, with increasing grain size. Sorting in volcaniclastic sediment, therefore, is not onlya function of depositional process, environment, and post-depositional modification but also of sedimentcomposition. Statistical analysis of sieve grain size data on weight-percent basis is inappropriate for evalua-tion of volcaniclastic sediments.

INTRODUCTION

The purpose of this paper is to evaluate thespecific gravity characteristics of water-laidvolcaniclastic sediment. The range of specificgravity exhibited in a sediment sample in-fluences hydrodynamic evaluation of itsgrain-size distribution. Interpretation of thestatistical measures obtained from sievegrain-size data (Folk 1968), particularly sort-ing, is based on study of quartzo-feldspathicsediment in which, on average, more than99% of the sediment exhibits less than a 5%variation in specific gravity (Blatt et al. 1980).The presence of vesiculated fragments involcaniclastic sediments introduces a low-density population whose influence on sort-ing is familiar, qualitatively, to anyone whohas observed sediment composed of pumicelapilli and dense lithics or crystals.

Because the average sandstone is com-posed of grains with nearly equitable specificgravity, the degree of sorting is interpretedtypically only in terms of depositional pro-

' Manuscript received December 13, 1984; re-vised March 28, 1985.

2 Current address: NORCUS, Joint Center forGraduate Study, 100 Sprout Road, Richland, WA99352.

3 Current Address: Redwood National Park, Ar-cata, CA 95521.

[JOURNAL OF GEOLOGY, 1985, vol. 93, p. 619-622]No copyright is claimed for this article.0022-1376/85/9305-004$1.00

cess and environment and post-depositionalmodifications. For instance, poorly sortedsediments are generally interpreted to be theresult of rapid deposition of a wide range ofgrain sizes with limited reworking, infiltrationof small grains into interstices between largergrains, mixing by bioturbation, or diageneticproduction of clay-size matrix aroundframework grains.

Modern sediments surrounding presentlyactive Mount St. Helens, Washington andNeogene, nonmarine volcaniclastic rocks ad-jacent to the Cascade Range in Oregon andWashington are commonly poorly to verypoorly sorted. In part, the poor sortingreflects rapid deposition of sediment duringfloods as indicated by bed thickness,sedimentary structures (Smith and Smith1983) and, in some cases at Mount St. Hel-ens, observation of the sedimentation event(Dinehart in press, Pierson and Scott inpress). But it is to be expected that sortingparameters also reflect the mixing of sedi-ment having variable specific gravity: e.g.,pumice, vesiculated and non-vesiculated vol-canic lithic fragments, feldspar and quartz,and heavy minerals.

This paper attempts to quantify the differ-ences in specific gravity between volcani-clastic and quartzo-feldspathic sediment andto evaluate an interpretation of volcaniclasticsediment texture made using the methodol-ogy standardly applied to quartzo-feldspathicmaterial.

619

620 GEOLOGICAL NOTES

SETTING AND DESCRIPTION OF SEDIMENTS

The 1980 explosive eruptions of Mount St.Helens, Washington, blanketed severaldrainage basins over a 600 km 2 area withthree types of fragmental material. These are:(1) porphyritic ash and pumice lapilli fromPlinian eruption plumes; (2) porphyritic,dense to vesiculated, "blast dacite" rockfragments, texturally gradational to thepumice lapilli, that represent a recentlysolidified intrusion disrupted by thephreatomagmatic lateral blast of May 18(Hoblitt et al. 1981); and (3) crystals ofplagioclase, pyroxene, hornblende, andmagnetite ejected as Plinian tephra derivedfrom explosive fragmentation of the dacite in-trusion or liberated from the pumice lapilliand blast dacite by water-transport abrasion.

Within the area devastated by the lateralblast, hillslope erosion has produced alluvia-tion of adjacent stream valleys by sedimentcomposed of these three juvenile componentsmixed with varying proportions of Tertiarybedrock clasts (Smith 1984). Pumice andblast dacite are present in about equal abun-dance, plagioclase and minor quartz repre-sent up to 25%, and heavy minerals (pyrox-ene, hornblende, magnetite) up to 15% of thesize fraction 1/16 mm to 16 mm (J. Carpenter,unpub. data). Sieve analysis and modal anal-ysis of grain mounts show that the overallsediment composition and texture surround-ing Mount St. Helens is similar to that ob-served in Neogene volcaniclastic sedimen-tary rocks throughout the Pacific Northwest(J. Carpenter and G. Smith unpub. data).

METHODS

An essential assumption in sieve analysis,because weight not volume is measured, isthat the sediment is of uniform specific grav-ity. Thus the influence of specific gravity onthe statistical measures of the grain size dis-tribution cannot be evaluated from sieveanalysis data, but it may be considerable ifthe sediment is composed of particles with awide range in density.

Samples of pumice lapilli and blast dacitegrains were collected within the lateral blastzone near Mount St. Helens and hand-seivedto minimize abrasion to pumice, at 1 phi sizeintervals (phi = log2d, d = grain diameter).Specific gravity characteristics were mea-

sured utilizing ASTM D854-58 Standard TestMethod for Specific Gravity of Soils (ASTM1983, p. 212-214). These data were used toevaluate the grain size characteristics of sed-iments exhibiting traction-produced sedi-mentary structures; therefore grains weresaturated with water at one atmosphere pres-sure to fill interconnected vesicles with wa-ter, thus simulating the assumed density ofthe grains during bedload transport. Pumicelapilli that continued to float after a two-weekperiod were not considered. Surface waterwas blotted from the samples during theirrapid transfer from the water-filled storagecontainers to the water-filled volumetric flaskused for specific gravity determination. Meanspecific gravity was measured for bulk sam-ples of pumice lapilli, ash, and blast dacite at1 phi size intervals and for individual pumicelapilli with a nominal diameter (Wadell 1932)larger than 5 mm. Determinations were re-producible to within 5%.

RESULTS

Specific gravity of individual water-saturated, surface-dried pumice lapilli withnominal diameters between 5 mm and 17 mmvaried widely from just above 1.00 to 1.95(fig. 1). Measurement of bulk samples av-eraged the variation illustrated in figure 1 andallowed consideration of the average specificgravity of grains too small to accurately mea-sure individually. An inverse relationship ofgrain size to mean specific gravity of bulksamples of pumice lapilli and ash is clearlyrepresented in figure 2.

The specific gravity of bulk samples ofblast dacite also exhibit a general inverse re-

2.00

1.75.

1.50.

1.25• • •.•

• •. •

1.00••

III 1 I I4 6 8 10 12 14 16 18

NOMINAL DIAMETER (mm)

FIG. 1.—Plot of specific gravity versus nominaldiameter of individual Mount St. Helens pumicelapilli.

GEOLOGICAL NOTES 621

2.00 - r = -0.92

p = < 0.01

1.75

1.50

1.25 -

1.00 I I ■ I ■>1 0 -1 -2 -3 -4

SIEVE DIAMETER (PHI)

FIG. 2.—Plot of bulk specific gravity versussieve diameter of Mount St. Helens pumice lapilliand ash sieve fractions: r = correlation coefficient;p = probability value of significance test.

lationship with grain diameter (fig. 3). How-ever, the total observed variation for the blastdacite is less than for the pumice lapilli andash.

DISCUSSION

The specific gravity characteristics of vol-caniclastic sediment depart in two fundamen-tal ways from those of quartzo-feldspathicsediment. First, and not unexpected, the den-sity of different clast types is highly variable.Unlike quartzo-feldspathic sediments, wherevirtually all grains are within the narrowspecific gravity range 2.55-2.65 with lessthan 1% heavy minerals, the volcaniclasticsediments studied here contain volumetri-cally important constituents in severalspecific gravity ranges: 1.00-2.00 (pumiceand ash), 2.20-2.40 (blast dacite), 2.53-2.65(plagioclase and quartz, Hughes 1982), 3.20-3.40 (pyroxene and hornblende, Hughes1982), and greater than 5.0 (magnetite,Hughes 1982). Second, there is a highlysignificant negative correlation of meanspecific gravity with grain size within grainpopulations of the same composition. This isthe most important difference in specificgravity characteristics of volcaniclastic mate-rial compared to quartzo-feldspathic sedi-ment. The specific gravity of single-crystaldetrital grains (e.g., quartz and feldspar) is anintrinsic property independent of the size ofthe grain.

The inverse relationship between meanspecific gravity and grain size of pumice and

2.50 .•

••

r = -0.95

p = < 0.01

5:

2.25 •

U

2.00 I I I I I1 0 - I -2 -3 -4

SIEVE DIAMETER (PHI)

FIG. 3.—Plot of bulk specific gravity versussieve diameter of Mount St. Helens blast dacitesieve fractions: r and p as in figure 2.

dacitic rock fragments (figs. 2 and 3) is causedmainly by vesicles within the grains. Ash andpumice lapilli owe their size, shape, andspecific gravity to the size and distribution ofvesicles developed during explosive disrup-tion of the parent magma. For vesicles of agiven size the ratio of solid volume to vesiclespace increases as grain size diminishes andcontributes to the observed specific gravityvariation. The specific gravity of the finestfraction probably best approximates thespecific gravity of the erupted material with-out voids (Fisher 1965).

Intra-grain permeability is also a major fac-tor determining specific gravity and may bemore important than total porosity. Whendry, Mount St. Helens pumice lapilli and ashwere put into water-filled storage containers,grains less than 1 mm in diameter sank im-mediately, 1-4 mm grains sank within a fewminutes, 4-8 mm grains within hours, andseveral grains over 8 mm in diameter did notsink in the two-week period allotted. In-creased buoyancy with increasing grain sizeis probably a result of internal vesicles notfilling with water because of impermeability.The larger the lapillus, the larger the volumeof non-interconnected vesicles.

Variation in specific gravity for the blastdacite is also a function of vesicularity. Thedacite fragments contain fewer vesicles thanpumice lapilli, and they are rarely intercon-nected. Thus, most vesicles probably re-mained air-filled during this experiment.

Other than the general inverse relationship

622 GEOLOGICAL NOTES

between grain size and mean specific gravity,it is unlikely that a rigorous mathematical re-lationship can be defined to relate these twoparameters by consideration of vesicle geom-etry. Figure 1 demonstrates that, even for agiven grain size, specific gravity is variable.This probably reflects the variable nature ofpermeability, as defined by the proportion ofinterconnected vesicles and, perhaps to alesser degree, variation in the type and pro-portion of phenocryst minerals in the pumice.The problem is further compounded by thevariable degree of vesiculation and abun-dance of phenocrysts in ejecta from differenteruptions, so that these Mount St. Helensdata are only qualitatively applicable to otherpyroclastic sediments.

CONCLUSIONSBecause of the variation in specific gravity

of volcaniclastic sediments discussed above,statistical evaluation of sieve grain size dataon a weight percent basis may not be mean-ingful. Settling tube techniques are also inap-propriate because, even in the fine sandrange, vesiculated ash behaves in a non-Stokian fashion as a result of buoyancy in-duced by air-filled vesicles (Fisher 1965).

The observations reported here indicatethat sorting in volcaniclastic sediment is notonly a function of depositional processes andpost-depositional modification but also is afunction of particle properties. Not onlyshould one expect a given pumice fragment tobe in hydraulic equilibrium with lithic frag-ments and crystals of a smaller size, but alsowith other, smaller pumice grains. Because of

the wide range in specific gravity exhibited bylithic and crystal populations in volcaniclasticsediment, the quality of sorting can be evalu-ated only after extensive study of the specificgravity of grains within the sediment samplebeing investigated. In evaluating ancient sedi-ment this laborious task is made virtually im-possible by the extensive diagenesis charac-teristic of volcaniclastic rocks, which altersthe specific gravity characteristics of thegrains, may preclude disaggregation, and of-ten produces a clay-size matrix. Sortingalone, therefore, is not as useful a sedimen-tological tool as it is in quartzo-feldspathicsandstones, and textural evaluation of vol-caniclastic sediment containing vesiculatedmaterial should not rely on the same princi-ples and procedures that have been devel-oped to describe quartzo-feldspathic sedi-ments.

ACKNOWLEDGMENTS. —The manuscript wasimproved by critical reviews from R. V. Fish-er and two anonymous reviewers. Field workwas supported by a grant from Sigma Xi.Laboratory facilities were provided by theU.S. Forest Service. Pacific Northwest For-est and Range Experiment Station, Water-shed Project, Corvallis, OR. James Carpenterprovided sediment composition data. G.Smith's research also supported by theNorthwest College and University Associa-tion for Science (University of Washington)under Contract DE-AM06-76-RL02225 withthe U.S. Dept. of Energy. Publication sup-ported by Rockwell-Hanford Operations,Basalt Waste Isolation Project.

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