WHAT FACTORS CONTROL THE COMPOSITION OF ANDESITIC SAND?
GARY A. SMITH ~ JOAN E. LOTOSKY
Department of Earth and PlanetarySciences, Universityof New Mexico, Albuquerque, New Mexico 87131 USA
AmTlWCT: The modal composition of ondesitlc sand and sandstone is not
only a function of source-area dimate and transport processes typically
considered for nonvoleanic sediment but is also strongly controlled by
volcanic fragmentation and pyroclastic-tronsport processes. Most volcanidnstic sediment deposited penecantemporoneonsly with active volcanism
is not epicinstic, and therefore its composition is not dependent on climate.
Crystal-rich andesite sand cannot simply be regarded as the product of
weathering in a humid climate. In fact, there is no relationship between
precipitation and the ratio of crystals to rock fragments. Fluvial-transport
abrasion demonstrably generates cerystal-fieh sand only in the case of porphyritic glassy roek fragments that are not durable during transport; holocrystalline pyroclnstie fragments apparently do not disintegrate during
transport to yield crystal-rich sand. Many sond-size primary volcanic deposits are crystal-rich as a result of eruptive processes that physically
fractionate particles of different sizes and densities. Reworldng of these
deposits results in crystal-rich sand that is not a product of weathering or
Wansport abrasion. The abundance of unaltered green hornblende is one
measure of the importance of pyroclastic material in a volcanic sand because this mineral is not found in lava flows. Interpretation of volcuniclnstic
sandstone requires consideration of volcanic processes not typically considered by sedimentolngists.
INTRODUCTION
The petrographic features of volcano-derived detritus have been clearly
defined in empirical studies of sandstone composition relative to platetectonic setting (e.g., Dickinson and Suczek 1979; Marsaglia and Ingersoll
1992). Subsequent petrographic and geochemical investigations of volcaniclastic sand and sandstone have distinguished between volcanogenic
rock fragments and crystals of different provenance and have elucidated
volcanic evolution of source areas (e.g., Ingersoll and Cavazza 1991; Parker
et al. 1988; Cather and Folk 1991; Larue and Sampayo 1990; Erskine and
Smith 1993). In most of these studies, volcaniclastic particles have been
treated with the same methodology and assumptions as are typically applied to nonvolcaniclastic sediment. To increase the reliability of sandstone-petrography studies for paleogeographic and paleotectonic reconstructions, we address the following question-other than composition of
the original magma and postdepositional diagenesis, what aspects of volcaniclastic-fragrnent generation and transport control the composition,
and hence influence interpretation, of andesitic sand?
tJmQOgxspEcrs oF VOLCXi~aciasncsED~rrs
Volcaniclastic sediment results from a wide variety of fragment-generating processes other than weathering. Nonvolcaniclastic terrigenous detritus is epiclastic; i.e., derived by weathering and erosion of preexisting
rocks (Bates and Jackson 1987; Pettijohn et al. 1987). Unconsolidated
debris eroded from active volcanoes, however, is largely the consequence
of fragmentation by volcanic processes unaided by weathering (Fisher
1961; Smith 1991). Principal among these fragment types are pyroclasts
(formed by explosive disruption of rising magma) and autoclasts (generated
by brecciation and comminution of flowing lava as it solidifies).
The distinction ofepiclastic, pyroclastic, and autoclastic fragments within volcanogenic debris is rarely obvious, especially after redistribution by
sedimentary processes. For this reason, some volcanologists have chosen
to apply the term epiclastic not only to fragments derived from wastage
of older rocks but also to facies reflecting transport and deposition by
typical surficial sedimentary processes irrespective of fragment origin (e.g.,
Cas and Wright 1987).
Regardless of how terms are used, it is critical to remember that most
sediment deposited contemporaneously with source-area volcanism is not
the product of weathering. This distinction from more commonly encountered sedimentary deposits is obviously essential when evaluating
sediment toads, sedimentation rates, and depositional processes in volcanic areas (e.g., Vessell and Davies 1981; Smith 1987, 1991) but has
received little attention with regard to petrographic interpretation of volcanidastic sandstone. To emphasize this distinction from nonvolcaniclastic sediments, and in deference to the etymology of terms derived from
the Greek dastos, the terms epiclastic, pyroclastic, and autoclastic are used
here to refer to particle-generating processes rather than transport and
depositional processes (sensu Fisher 1961; Schmid 1981; Fisher and
Schmincke 1984).
Volcanic processes play an important role in determining the composition of primary pyroclastic deposits that are, in turn, a source of voluminous volcaniclastic sediments (Fig. 1). Pyroclastic ejecta contain pyrogenic crystals, ash and lapilli (often, but not always, glassy and vesicular),
and accessory rock fragments derived from conduit and vent wills. In
pyroclastic-flow and surge deposits, the pyrogenic crystals are concentrated
in proximal deposits when fine, usually vitric, ash is carri~ aloft by
fluidization or turbulence to be deposited distally as thin, landscape-mantling deposits (Fig. 1; e.g., Hay 1959; Walker 1972). Similarly, components
are fractionated during transport and settling from airborne eruptive columns (Fig. I; Wolff 1985). This effect of crystal concentration in some
sand-size primary pyroclastic deposits is well-known to volcanologists who
know not to use bulk analyses of pyroclastic deposits to characterize the
original magma (Walker 1972; Wolff 1985). This crystal enrichment as a
consequence of primary eruptive processes may be inherited by derivative
sandstone (e.g., Roobol 1976; Cas 1983).
THE PROBLEM OF
XNDESITICVOLCXNICL~T1CS
Volcaniclastic rocks of intermediate composition offer particular challenges to the petrographer attempting to determine fragment genesis. These
rocks (hereafter referred to simply as andesite, but including basaltic andesire and dacite) vary moderately in chemical composition but share
highly porphyritic (30..-50% phenocrysts) textures and phenocryst assemblages dominated by plagioclase and one or more subordinate ferromagnesian silicates and virtually no quartz. Andesite composite volcanoes
typically contain a large percentage of fragmental material (Lipman 1968;
Vessell and Davies 1981; Hackett and Houghton 1989; Smith and Grubensky 1993). Relatively viscous lavas undergo intense brrecciation when
descending steep slopes to generate autoclastic fragments of sand to boulder size. Vulcanian-, Pelean-, and Merapian-type eruptive activity (see
Fisher and Schmincke 1984, Cus and Wright 1987, or Francis 1993 for
descriptions of eruption types) typical ofandesite volcanoes generates lithic
pyroclasts, i.e., hypocrystalline to holocrystalline, poorly vesicular tephra
lacking the distinctive pumiceous and bubble-wall textures of more silicic
pyroclasts (e.g., Fisher el at. 1980; Fisher and Schmincke 1984; Heiken
and Wohletz 1985). Therefore, when redistributed in sedimentary deposits, autoclasts and pyrodasts eroded from unconsolidated volcanics are
usually indistinguishable from epiclasts eroded or weathered from lithified
lava flows.
JOURNALOFSEDIMErCr/dYR~rARcH,VoL.A65, No. 1, JssvAav, 1995,P. 91-98
Copyright© 1995,SEPM(SocietyforSedimentaryGeology) 1073-130X/95/0A65-91/$03.00
92
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DEPOSIT
Resulting Deposits
I
In this paper, we reexamine the results of recent studies of andesitic
volcaniclastics and present new data relevant to understanding how processes known to affect nonvolcaniclastic sandstone composition account
for features ofandesitic detritus. This effort is undmaken by consideration
of volcanic processes that affect fragment composition and grain size that
are typically not considered by sedimentologists.
PREVIOUS
(Pumice lapilli & ash)
WOgl~
Thr~ studies of volcaniclastic sediment composition serve as a foundation for sedimentological interpretation of andesitic sand and sandstones. Each of these papers focuses on the significance of crystal abundance in sandstones; none of these investigations considered volcanic
processes controlling crystal abundance in the starting material.
Miles (1977) and Davies et al. (1978b) examined downstream modification in sediment texture and composition resulting from transport of
hypocrystalline basaltic-andesite pyroclastic debris eroded from Volcfin
Fuego, Guatemala, following its 1974 eruption. Among their conclusions
the most important to our thesis is that an approximately twofold increase
in the abundance of crystals (as a fraction of crystals and rock fragments)
developed in the medium sand fraction with approximately 70 km of
fluvial transport. Davies et al. (1978b) attributed this change in sediment
composition to reflect mechanical abrasion and disintegration of rock
fragments, to ultimately be deposited as mud, and concomitant release of
sand-size phenocrysts. Thus, besides primary crystal enrichment by volcanic processes, crystals may also be enriched in sand by transport abrasion
of volcanic-rock fragments.
Transport abrasion was similarly called upon by Cather and Folk (1991)
to explain grain-size-dependent variation in the composition of sediments
believed to be derived from the same upper Eocene to lower Oiigocene
sources in southwestern New Mexico. Conglomerate clasts were assumed
to represent source-area lava flows. Sandstones were found to be enriched
in crystals, relative to the phenocryst mode of conglomerate clasts, and
mudstones were composed mostly of mineral phases recognized in conglomerate-clast groundmasses. Weathering and physical abrasion during
transport liberated phenocryst minerals to the sand-size fraction of the
sediment, whereas groundmass constituents were concentrated in finer
sediment.
Mack and Jerzykiewicz (1989) tested the effect of climate and chemical
,
PYROCLASTIC
FLOW DEPOSIT
I
FIG. 1.--Physicalfractionationof pyroclastic
componentsduringexplosivevolcaniceruptions. The originaleruptivemixturecontains
loose crystals,vitric fragmentscontainingcrystals, and accessory rock fragments(not shown
for simplicity).Depositionfrom driftingeruption plumesproducescoarse, lapilli-fichdeposits near the volcano,and ash is deposited farther away. Crystalsare typicallyconcentrated
in medialdepositsbecausemost phenocrysts
are I--6mm in diameterand are coarser and
denser than most of the materialin the erup
tion plume,which fallsas finevitfic ash at
greater distances(Wolff1985). Densitystratification and elutfiationof fine-grained,low-.density ash in movingpyroclasticflowsgeneratesa
vitfic-ash-richash-clouddepositand a poorly
sorted pyroclastic-flowdeposit,sensu stricto,
that has a crystal-enrichedmatrix rdative to
the originaleruptivemixture(Walker 1972).
Erosion of unconsolidatedpyroclastic-flowand
medial ash-falldepositscan therefore produce
crystal-richsand becausethey were derived
from crystal-enrichedprimarypyroclasticdeposits.
weathering on the composition of andesitic sand by determining detrital
modes of modem stream sands derived from high-reliefandesitic volcanic
terranes in areas of contrasting humid and semiarid climates. They reported a higher ratio of crystals to rock fragments in medium sand from
humid regions than from arid regions. By analogy to similar relationships
between crystal content of sand and climate recognized for feldspathic and
metamo~hic-lithic sandstone (e.g., Basu 1976; Suttner et al. 1981), Mack
and Jerzykiewicz (1989) interpreted their data to indicate the importance
of chemical weathering in releasing crystals from volcanic source rocks.
They further argued that stratigraphic variation in crystal content among
Cretaceous volcaniclastic sandstones in southwestern Alberta reflect temporal variations in climate.
PROBLEMSANDALTERNATIVEINTERPRETATIONS
The results of the three previous investigations raise questions from a
volcanological perspective despite their sound sedimentological reasoning.
Because study of volcaniclastic sediment must give attention to volcanic
processes, as well as those typically operative in the derivation and transport of epiclastic sediment, we use the questions outlined below as a
foundation for our own research that is reported in later sections.
We endorse the interpretations of Davies et al. (1978b) that emphasize
downstream modification of glassy pyroclastic sediment. Are lithic pyroclasts with intergrown crystalline groundmasses as susceptible, however,
to transport disintegration? To what extent is crystal content in sand a
function of transport abrasion as opposed to being inherited from crystalrich pyroclastic deposits?
Cather and Folk (1991) emphasized transport abrasion to fractionate
components of a texturally heterogeneous source rock into sediments of
different grain size. We expect such effects to result from the normal
processes of sediment transport. What is left unaddressed, however, is
whether the source for sediment ofall size ranges is the same, as is implicit
in the Cather and Folk study, or whether gravel, sand, and mud are derived
from erosion of volcanic deposits of comparable size range but different
initial compositions. Could conglomerate clasts be largely epiclasts and
autoclasts derived from lava flows, whereas sand and mud have a largely
pyroclastic origin?
The conclusions of Mack and Jerzykiewicz (1989) can be applicable
only to epiclastic sediments. Most voluminous volcaniclastic sequences
COMPOSITION OF ANDESITIC SAND
93
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FF[G.L-Location map of samples from streamsdrainingMount Hood, Oregon.
record the erosion and deposition of unconsolidated pyroclastic and autoclastic debris during and immediately following eruptive activity and
before any significant weathering has occurred (Smith 1991). In this context, we find the data set of Mack and Jerzykiewicz (1989) to be unconvincing as a test of the influences of chemical weathering on the composition of andesitie sand. Their three relatively crystal-poor samples
representing arid climates were collected from streams draining lithilied
Upper Cretaceous and mid-Tertiary volcanic bedrock; these are truly epiclastie. The three relatively crystal-rich samples representing humid sites
were collected from streams draining Quaternary volcanoes. Two of these
volcanoes, Volcfin Fuego and Mont PelEe (Martinique, French West Indies) have experienced explosive eruptions during this cemury that inundated drainages with unconsolidated, and unweathered, pyroclastic debris (Perret 1937; Davies et al. 1978a; Fisher et al. 1980;Smith and Roobol
1990). The other site, Mount Rainier (Washington, U.S.A.), has not experienced significant ~cent eruptive activity, but the sampled sediment
is glacialoutwash, for which the importance of chemical weathering, rather
than high-alpine frost action and glacial erosion, is difficult to deduce. The
choice of data sets, therefore, leaves us uncertain on the effects of climate
to determine the composition of most andesitic sandstones encountered
in the rock record, i.e., those derived from erosion of penecontemporaneously erupted material.
Our study was designed to examine the relative influences of climate,
transport, and pyroclastic sediment sources on the ultimate composition
of andesitie sand. This was accomplished through reevaluation of data
reported in Davies et al. (1978b), Mack and Jerzykiewicz (1989), and
Cather and Folk (1991) and by collection of data from three additional
study sites.
STUDY SITES AND METIIOBS
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Q [] Volcan Fuegosarnpk~s
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Distance from Volcano Summit (km)
FIG. 3.-Variation in RFC index (rock fragments/[rockfragments+crystals])
with distancefor mediumsand in riversdrainingpyrodasticdepositson Mount
Hood (Appendix 1) and Volc~nFuego (data from Miles 1977). Downstream
decrease in RFC is interpretedto resultfrom comminutionof the glassygroundmass of the rock fragmentsto releasesand-sizephenocrysts.Mount Hood sand
shows no significantchangein compositionas a consequenceof transport, with
samplescollectedmorethan 80 km fromthe volcanohavinga compositionsimilar
to the matrix of source-area pyroclastic-flowdeposits. The notablyanomalous
Mount Hood sample with an RFC value of 0.49 was collected from a small
watershed drainingPliocenelava flowsrather than pyroclasticdeposits. The relativelyhighcrystalabundanceof this samplemay resultfromchemicalweathering
in a humid climate(el. Mackand Jerzykiewicz1989).
clastic material (Crandell 1980; Cameron and Pringle 1986, 1987). These
pyroclaslic materials are found on all sides of the volcano but are most
conspicuous in the construction of a broad apron of unconsolidated debris,
exceeding 300 m thick, that covers an approximately 90° sector of the
south flank of the volcano to a distance of 7-10 km fi'om the summit. Fill
terraces of~worked pyroclastic debris extend down all major rivers draining the volcano.
Mount Hood represents a climatic setting similar to Mount Rainier
(Steinhauser 1979) but is less extensively glaciated and includes a large
volume of unconsolidated pyroclastic sediment. Mount Hood therefore
provides an opportunity to compare sediment composition with sourcearea pyroclastie deposits in a humid climate. The question of durability
of lithic pyroclasts to transport abrasion can be addressed and compared
to the results of Miles (1977) and Davies et al. (1978b) at Volt,in Fuego.
Twenty-four samples of unconsolidated sand were gathered from banks
and bar tops of gravel-bed-load rivers draining Mount Hood and from
the pyrodastic apron (Fig. 2). The samples were collected from headwaters
to sites 80 km from the summit. The sands were sieved to recover the
medium-sand size fraction (0.25-0.5 mm) and then impregnated in epoxy
and thin sectioned. Rock fragments (defined as polymineralic or vitfic
grains with no single crystal constituting over 90% of that grain) and
crystals (single grains of plagioclase, pyroxene, amphibole, or oxide with
tittle or no adhering groundmass or glass) were point counted using 500
grains per slide. This methodology facilitates comparison with the data
reported by Miles (1977) and Mack and Jerzykiewicz (1989), who used
the same methods.
Mount Hooa~ Oregon
Mount Hood is an andesite composite volcano in the northern Oregon
Cascade Range. The cone consists of upper Pleistocene and Holocene
andesitic lava and pyroclastic debris built above a platform of Pliocene
andesitic and basaltic rocks that, in turn, rest on middle Miocene Columbia
River Basalt Group (Wise 1969; White 1980; Priest and Vogt 1982). Four
eruptive episodes occurred over the last 18 k.y., and as recently as about
200 yr B.P., each producing volcanic domes and associated pyroclasticflow and debris-flow deposits composed of predominantly lithic-pyro-
Espiaaso Formation, New M~ico
The upper Eocene and Oligncene Espinaso Formation represents the
erosional remnants of ~despread, coarse-pained volcaniclastie aprons
that accumulated adjacent to a 40-kin-long volcanic chain in central New
Mexico (Kautz et al. 1981; Smith et al. 1991). The Espinaso Formation
is similar in age to the Datil Group of southern New Mexico, which was
studied by Other and Folk (1991). Vertebrate fossils, paleosol characteristics, and regional considerations of paleoclimate suggest a semiarid to
94
GARY A, SMITH AND JOAN E. LOTOSKY
Fit. 4.-Comparative viewsof grain mounts of the medium-sand sieve fraction of pyroclasfic-flowmatrix and fluvial sand. A) MatrLxof pyroclasfic-flowdeposit
collectedon south flank of Mount Hood; plane light, field of view approximately3.0 ram. B) Sand collectedfrom the Hood River, approximately80 km from the
summit of Mount Hood; plane light, field of view approximately3.5 ram. Representativegrain types have Men lahelcd in each photo: plaginclas¢(PL), pyroxene
(PX), hornblende (HB), holocrystallinevolcanicrock fragments(HC), and hypocrystallineoxide-richvolcanicrock fragments(G). The pyroclasfic-flowdeposit lacks
vesicularor bubble-wall-shapedfragments,containsnumerous loosecrystals,and is dominatedby holocrystallinerockfragments.The fluvialsand (B) is pctrographically
similar to the pyroclasticdeposit (A).
arid climate during deposition of both the Espinaso Formation and the
Datil Group (Cather and Folk 1991).
Although the proximal volcanic rocks of the Espinaso Formation have
largely been eroded, the petrography and chemistry of volcanic-conglomerate clasts and sandstones have been used to reconstruct the petrologic
evolution of the volcanic centers (Erskine and Smith 1993). Only two lava
flows are known within the voleaniclastic-apron sediments, despite the
proximity (4-15 km) of outcrops to the source vents marked by plutons.
The paucity of lava flows and the high phenocryst content (45.-50%) of
andesite and latite clasts suggest viscous extrusion of volcanic domes that
were, in turn, sources for lithic-rich pyroclastic flows and debris flows that
dominate the volcaniclastic aprons (Smith et al. 1991; Erskine and Smith
1993).
Espinaso sandstones are lithified and not amenable to sieving prior to
thin-section preparation. Data presented here were obtained by 500-point
counts on twelve well-sorted medium sandstones collected 8-12 km from
source vents. In addition, thin sections were examined from nine deposits
of ash falls and pyroclastic-flow matrix in order to compare the sand-size
fraction of primary pyroclastic materials with the composition of the sandstones. Although sandstones are cemented by calcite, framework grains
are fresh except for clay and carbonate replacement of vitric fragments.
In the latter case, grain texture is well preserved and grain identification
is not compromised.
tation (Smith 1988, 1991). Syneruption strata are laterally extensive pumiceous sheet-flood-deposited sandstone and debris-flow conglomerate.
Inter-eruption facies are mostly well-sorted fluvial conglomerate and associated sandstone and mudstone that occupy paleochannels incised into
the synernptive sheets. The distinctive facies characteristics and geometries of these two groups of strata suggest widespread aggradation during
and shortly after eruption of easily eroded, mostly sand-size, pyroclastic
debris that overwhelmed fluvial systems, followed by degradation and
reestablishment of gravel-bed-load streams typical of inter-eruption conditions (Vessell and Davies 1981; Smith 1987, 1988, 1991).
Unconsolidated sand samples from inter-eruption and synernption fades were collected 15 km west of Yakima (N'acbes-Wenas Grade section
of Smith 1988). These were sieved and the medium-sand fraction was
mounted and point counted (300 points per slide) in the same manner as
the sands from Mount Hood. Ellenshurg Formation sands and tufts examined this study are unaltered by diagenesis. The objective of examining
Ellensburg Formation sand was to compare predominantly pyroclastic
syneruption sand and mixed pyroclastic-epiclastic inter-eruption sand. A
semiarid climate for the Eflensburg Formation depositional sites is indicated by paleobotanical evidence (Smiley 1963). By analogy to modem
Cascade Range geography, however, a substantial orographic climatic gradient probably existed between the sediment source and the depositional
site; there are no reliable paleoclimate data for the source area.
EllemsburgFormation
EFFECTSOF TIIANSPORT
Late Miocene voleaniclastic strata of the Ellensburg Formation near
Yakima, central Washington, were derived from the erosion of ancestral
Cascade Range volcanoes about 50 km to the west (Smith 1988; Smith et
at. 1988). Smith et al. (1988) interpret Ellensburg source-area volcanism
to be characterized by episodes of explosive eruptions, of Plinian or subPlinian character, followed by extrusion of andesitic-dacitic domes. Both
explosive and dome-building eruptions are thought to have contributed
large volumes of debris onto adjacent volcaniclastic fans and into valley
systems draining the Cascades, ultimately extending 120 km to the east
(Smith 1988).
Ellensburg Formation strata near Yakima are grouped into two categories interpreted to represent syneruption and inter-eruption sedimen-
Transport of porphyritic-volcanic rock fragments may cause disintegration into constituent sand-size phenocrysts and finer-grained groundmass grains', thus, derivative sands may become enriched in phenocryst
minerals relative to the source rock (Davies ct al. 1978b; Cather and Folk
1991). This effect is illustrated at Volcfin Fuego by a downstream increase
in the RFC index, (rock fragments)/(rock fragments + crystals), in medium
sand collected from the modern Rio Pantelc6n (Fig. 3; Miles 1977). The
medium-sand fraction in alluvium 80 km from source is enriched approximately twofold in crystals compared to the recently erupted pyroelastic-flow deposits in the river headwaters.
Sand from streams draining Mount Hood, in contrast, shows no significant downstream compositional variations (Fig. 3; Appendix 1). The
95
COMPOSITION OF ANDESITIC SAND
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FiG.5.--Variation in RFP and RFA indices (means and standard deviations) for medium sand as a function of annual precipitation. Accessory minerals are pyroxene,
amphibole, olivine, and opaque oxides. Solid symbols represent data from modern streams draining the indicated volcanic sources that Mack and JearzTkiewicz(I 989)
interpret as illustrating the influence of chemical weathering on sand composition. This relationship is not supported by the addition of data from Mount Hood
(Appendix l), Volc~n Fuego (Miles 1977), the Eocene Datil Group (Other and Folk 1991), and the Eocene-.OligoceneEspinaso Formation (Appendix 2). Range of
annual preopitation values for the Datil Group and Espinaso Formation are those expected for a mid-latitude, semiarid climate (MeKnight 1987).
RFC index changes very little over a transport distance of 85 kin, and
sand from the most distal sites is petrographically similar to the matrix
of pyroclastic-flow deposits (Fig. 4). One sample forms a notable outlier
on the RFC/distanee plot (Fig. 3), at a distance of 13.5 km from the volcano
summit. This stream-sediment sample is from a headwater tributary to
the Sandy River, which drains Pliocene lava flows rather than the pyroclast-mantled Mount Hood.
The comparison of data from Volc,qn Fuego and Mount Hood suggests
that no generalizations can be made about the effects of fluvial transport
on andesite sand composition or the fractionation of original phenocryst
and groundmass minerals into different-size sediment. We hypothesize
that the disparity in data from Volcfin Fuego and Mount Hood is a consequence of the greater groundmass crystallization ofpyroclastic fragments
from Mount Hood. Volc~n Fuego ejecta fragments are glassy (Davies el
al. 1978b), whereas most Mount Hood pyroclasts are holocrystalline or
nearly so (Fig. 4A). The interlocking framework of groundmass feldspar
and pyroxene laths possibly make Mount Hood pyroclastic fragments
stronger and more resistant to breakage during transport than the glassy
pyroclasts from Volc~n Fuego.
gwEcrs OF CLIMATE
The inverse relationship between precipitation and petrographic indices
measuring rock-fragment abundance that was presented by Mack and
Jerzykiewicz (1989) is not supported by the addition of data from Volcfin
Fuego (Miles 1977), the Datil Group (Other and Folk 1991), Mount Hood,
and the Esoinaso Formation obtained by using the same point-counting
method (Fig. 5). Mount Hood sand is as rich in rock fragments as the
most arid sites considered by Mack and Jerzykiewiez (1989). Datil Group
and Espinaso Formation sands are as rich in crystals as Mack's and Jerzykiewicz's most humid sites. Although the exact climatic conditions during deposition of the mid-Tertiary volcaniclastics are not known, it is
unreasonable that they experienced subtropical weathering (Cather and
Folk 1991), and mean annual precipitation in the range of 25-..64 cm is
reasonable for a mid-latitude, semiarid climate (McKnight 1987). The
addition of VolcAn Fuego data from Miles (1977) also shows that most
of the difference in crystal content between subtropical and semiarid sites
can be accounted for by downstream abrasion and crystal liberation from
glassy rock fragments, a variable not considered by Mack and Jerzykiewicz
(1989).
Additional data refute a simple relationship between mean annual precipitation and the abundance of rock fragments versus crystals in andesitic
sand. Mount Hood sand is derived from erosion ofunconsolidated, crystalpoor pyroclastic debris and has a relatively low crystal abundance despite
its humid climatic setting. The relatively high, and variable, crystal contents in the Datil Group and Espinaso Formation sands may reflect derivation from crystal-rich pyroclastic materials; this explanation is considered further in the next section.
EI~T£CrOF PYIIOCIASTICCONTIRBtNIONS
Are crystal-rich sands derived from crystal-rich pyroclastic deposits,
and if so, how can one distinguish p~odastic crystals from epiclastic
crystals weathered from lavas and other lithiflcd volcanic materials? We
evaluate this problem for the Ellensburg and Espinaso Formations by
taking advantage of the magmatie stability conditions for hornblende.
Hornblende is stable only at PH2Ogreater than 2 kb for typical andesite
compositions and temperatures (Eggler and Burnham 1973). Therefore,
hornblende in lava flows shows petrographic evidence of reaction with the
melt to produce oxide-rich rims, oxyhornblende, or both. Some pyroclastic
deposits, however, contain fresh, unaltered green hornblende because explosive eruptions tap the most volatile-rich part of the magma, erupt it
rapidly from depths of several kilometers, and quench the melt to glass
before early-formed crystals can react with the melt under conditions of
lower P,T, and PH2O"Fresh (i.e., lacking reaction rims) green hornblende
is therefore usually restricted to pyroclastic deposits, whereas cogenetic
lavas contain brown oxyhornblende or green hornblende with reaction
rims (Kuno 1950; Rose 1973; Ujike 1974; Rutherford 1993). We therefore
96
GARY A. SMITH AND JOAN E. LOTOSKY
1,0
,
0.8tD
0.6"O
~.
o
" Inter-eruption facies
4are. ~o¢ o u r a u ~ >
°700
0
0.4"
~0.2-
• RFP [rockfrags, l (rockfrags. + plagioclase)]
- [] RFA [rockflags. / (rockfrags, + accessories)]
0.0 0.0
0.2
0,4
Synerupfion/
facies
0.6
1.0
0.8
Hornblende Ratio
(fresh green hornblende / total hornblende)
l:hc.6.-RFP and RFA for medium sand from the EllensburgFormationand
hornblenderatio calculatedfromcountingall hornblendecrystalsm the samethin
sections(Appendix3). Greaterabundanceof freshgreenhornblendein syneruption
facies relativeto inter-eruptionfacies(as definedby Smith 1988) reflectsgreater
abundanceof pyrodastic debris in syneruptiondeposiB. Most syneruptiondeposits also arc more feldsparrich and contain fewer rock fiagmcntsthan intereruption sand.
evaluate pyroclastic contributions to the sediment by using a hornblende
ratio of fresh green hornblende as a proportion of total hornblende in the
sample. The hornblende ratio for EllensburgFormation syneruption facies
is distinctly higher than for inter-eruption facies (Fig. 6; Appendix 3). This
is consistent with the common presence of green hornblende in Ellensburg
Formation tufts (90-100% of total hornblende) and its absence in conglomerate clasts and lithic fragments in sandstone. Syneruption facies have
a more variable, but generally lower, abundance of rock fragments than
do the inter-eruption facies sands (Fig. 6). These data, although limited
in number, suggest that crystal content is related to the derivation of
syneruption facies primarily from unconsolidated pyroclastic debris,
whereas inter-eruption sand is a mixture of pyroclastic and epiclastic
material with lower crystal content and lower hornblende ratio.
The importance of pyroclastic deposits as a source for sand is also
apparent for the Espinaso Formation. No conglomerate dasts, presumably
epiclasts or autoclasts derived from lava flows, contain green hornblende
.~100
t
cll=
!
!
conglomerate
clasts
1 ~ (n = 54)
o
|
=_--~ 8 50
EE~
,1= ,1= ~
o
0.0
,t
0.5
Hornblende Ratio
(fresh green hornblende / total hornblende)
1.0
FJc. 7.-Hornblende ratio for tufts, sandstones,and conglomerateclastsin the
Espinaso Formation(data from Appendix 3 and Erskineand Smith 1993). Pumiceouslapilliand shards in tufts are countedas rock fragments.Conglomerate
clasts,interpretedbyErskineand Smith(1993)to representautoclastic,pyroclastic,
and epidastic fragmentsderived from volcanicdomes, lack green hornblende.
Sandstonescontainvariableamountsof green hornblendeand cannotbe derived
onlyfromtransportbreakageof conglomerateclasts.Tuftscontainrelativelyhigh
proportionsof green hornblendeand contain about 40-50%loosecrystals~
0
P
B:
TRANSPORT
SOURCE
I
DISTANCE
DEPOSITIONAL
SITES
FIG.8.-Factors influencingthe ratio of rock fragmentsto crystaLsin andesitic
sand. Weatheringmay release crystals from rock at the source. Voicaniclastic
sedimenteroded from penecontemporaneouslyactive volcanoes,however,may
be derivedfrom unconsofidatedpyroclasficdepositsthat are enrichedin crystals
by volcanicprocesses(e.g., Fig. 1). Furtherenrichmentin crystalsmay occurwith
a modestdistanceoffluvialtransportin the caseof relativelynondurable,typically
nitric,fragments.Lithicfragments(of epiclastic,pyroclastic,or autoclasticorigin)
are more durable during transport, The compositionof a sand is a function of
three independent variables:source-areavolcanicfragmentation,weathering,and
transport abrasion.It is not possible,therefore,to simplyrelatethe petrographic
mode of ancientandesiticsandstoneto source-areaclimateor transportdistance.
(Erskine and Smith 1993). Most Espinaso sandstones examined, however,
have hornblende ratios ranging from 0.2 to 0.7, overlapping considerably
with the values measured petrographically for Espinaso tufts (Fig. 7; Appendix 2). These data show that Espinaso Formation sand does not have
the same source as the conglomerate clasts. We hypothesize that the sand
was largely derived from erosion of crystal-rich pyroclastic deposits, thus
accounting for their low RFP and RFA indices despite being formed under
semiarid climatic conditions (Fig. 5). Also notable is the abundance of
pyroclastic material in the Espinaso Formation sandstones, implied by
the presence of green hornblende despite the paucity of ash shards or
pumice lapilli in these samples. We speculate that the crystal-rich Datii
Group sands are also a consequence of derivation from pyroclastic materials. Cather and Folk (1991) noted the abundance of unaltered green
hornblende in Datil sand but preferred an interpretation attributing this
feature to abrasion of reaction rims during transport. This explanation is
unlikely in the Espinaso Formation because conglomerate clasts lack any
green hornblende, with or without reaction rims. Although we cannot
discount Cather's and Folk's interpretation for the DatiI Group, we do
not endorse the validity of the assumption that sand is derived from the
same source as conglomerate clasts in comparing modes of volcanidastic
sediment of varying grain size.
CONCLUSIONS
Volcanological studies offer perspectives on voicaniclastic sedimentology that should not be overlooked by sedimentologists. Andesitic sediment
deposited penecontemporaneously with active volcanism contains a high
proportion of crystals reworked from unconsolidated pyroclastic deposits,
independently of weathering or transport abrasion. Therefore, crystal
COMPOSITION OF ANDESITIC SAND
abundance in andesitic sandstone is not a reliable climate indicator. Fresh
green hornblende can be used as a measure ofpyroclastic input even when
obvious pyroclastic fragments cannot be identified. In the case of the
Espinaso and Ellensburg Formations it is clear that sand was derived from
different starting material than conglomerate clasts and that diminution
during transport does not accurately account for differen~s in the modal
composition of sandstone and conglomerate clasts.
The degree of crystal enrichment as a consequence of fluvial transport
abrasion is dependent on durability of material and cannot be generalized.
Glassy pyroclasts are probably less durable to transport breakage than are
hypocrystalline and holocrystalline pyroclasts, autoclasts, and epiclasts.
Andesitic sand composition represented as a ratio of rock fragments to
crystals is therefore dependent on two parameters (Fig. 8). The first control
is the loose crystal abundance in the parent material. We suspect by analogy
to nonvolcaniclastic sands (e.g., Suttner et al. 1981) that the Mack and
Jerzyldewicz (1989) hypothesis is true for epiclastic andesitic sand but
remains to be viably demonstrated and separated from controls excited
by textural variability of the source rocks (Heins 1993). Humid climates
may therefore produce sand with a higher ratio of crystals to rock fragments. This may account, for example, for the anomalously crystal-rich
sand derived from weathering of Pliocene lava flows near Mount Hood
(Fig. 3). In areas of active volcanism, however, the sand-size parent material is overwhelmingly of pyroclastic, not epiclastic, origin. Fragmentation processes during eruption and fractionation of crystals from vitfic
particles by pyroclastic flows, falls, and surges determine the crystal content
of the parent material for fluvial sediment. The second control is transport
distance and rock-fragment durability (Fig. $). Glassy pyroclasts may undergo greater transport disintegration with distance, lending an important
dependence of crystal content on transport abrasion. Crystal content of
starting material (whether a function of volcanic or weathering processes)
and degree of transport modification are independent variables in determining andesitic sand composition. Therefore, modal analyses of ancient
volcaniclastic sandstones cannot easily provide information on climate or
transport distance.
ACIINOWLgDGMENTS
This research was supported by a grant from the Donors to the Petroleum
Research Fund of the American Chemical Society. Lotosky's contribution included
her Honors Thesis at the University of New Mexico. David Romero assisted in
the collection ofsamples at Mount Hood. Beneficial comments were provided by
reviewer Kathleen Marsaglia.
itlgEFIFNCES
B~sc, A., 1976, Petrology of Holocene fluvial sand derived from platonic source rocks: implications to paleoclimafic inteqaretafion: Journal of Sodimentary Petrology, v. 46, p. 694-709.
B^w~s, R.L, ANDJACKSON,D.G., 1987, Glossary of Geology, 3rd Edition: Wa*hin~tun, D.C.,
AmericanGeologicalInstitute, 788 p.
CAU~, ]CA., ^SD Pnlsot~, P.T., 1986, Pnst-glacial lahars of the Sandy Rivet basin, Mount
Hood, Oregon: Northwest G~ulngy, v. 60, p. 225--237.
CAuEtos, ICA., ASPPRINGLLP.T., 1987, A detaikxl chronology of the most recent major eruptive
period at Mount Hood, Oregon: Geological Society of America Bulletin, v. 99, p. 84.5.-851.
C.~s,R.A.F, 1983, Submarine 'crystal tufts': their Prism usmg a Lower Devonian example from
southeast~ro Australia: Geological Magazine, v. 120, p. 471..-486.
Cm, R.A.F., ~s~ Wstcar, J.L., 1987, Volcanic Successions, Modem and Ancient: London, Allen
& Unwni, 528 p.
C^zatR, S.M., Asu FuLL R.L., 1991, Pn~..diaganeticsedimentary fracfiotatian of andesitic detritus in a semiarid climate: an example from the Eocene Datil Group, New Mexico,/n Fisher,
R.V., and Smith, G.A., eds., Sodimentalion in Volcanic Settings: SEPM Sgocial Publication
45, p. 211-226.
C~mO~LL,D.R., 1980, Recent eruptive history of Mount Hood, Oregon, and potential hazards
from future eruptions: United States Geological Survey Bulletin 1492, 81 p.
D^vl~s, D.K., Qt~ARRV,M.W., ASP Bouts, S.B., 1978a, Glowmg avalanches from Ihe 1974
eruption of the volcano Fnego, Guatemala: Geological Society of Ametica Bulletin, v. 89, p.
369-384.
DAv~s, D.K., Vg~S~LL,R.K., Mu~, R.C., Fot~v, M.G., Asp Bums, S.B., 1978b, Fluvial transport
and downstream sediment modifications in an active volcamc region, in Miafi, A.D., ed.,
Fluvial Sedimentology: Canadian Society of Petroleum Geologists Memoir 5, p. 61-84.
97
D,cg,~so,, W.R., ASPSUCZEg,C.A., 1979, Plate tectonics and sandstone compositions: American
Association of Petroleum Geologists Bulletin, v. 63, p. 2164-2182.
Ec~ LE~.,D.H., ANDBU~NHAU,C.W., 1973, Crystallization and fractinnation trends in the system
andesite-H,O-CO:O2 at pressures to 10 kb: Geological Society of America Bulletin, v. 84,
p. 2517-2532.
ERS~INLD.W., ASPSMITH,G.A., 1993, Compositional characterization of volcanic products from
a primarily sedimentary record: the Oligocene Espinaso Formation, north-central New Mexico: Geological Society of America Bulletin, v. 105, p. 1214-1222.
FiSaER, R.V., 1961, Proposod classification of volcanidastic sediments and rocks: Geolngical
Society of Amecica Bulletin, v. 72, p. 1409-1414.
FluEs, R.V., Asp Somtscr~, H.-U., 1984, Pyroclasfic Rocks: Berlin, Springes-Veriag 472 p.
FJS,ER,R.V., SMrm,A.L., Asp RooaoL, M.J., 1980, Destruction of St. Pierre, Martinique, by ashcloud surges, May 8 and 20, 1902: Geology, v. 8, p. 472-.-476.
F~uos, P., 1993, Volcanoes, a Planetary Perslxctive: Oxford, Oxford University Press, 443 0.
HAcgrrr, W.R., ASPHOUGHTON,B.F., 1989, A facies model for a Quaternary andesitie composite
volcano: Ruapehu, New Zealand: Bulletin of Volcanolngy, v. 51, p. 51--68.
HAy,R.L., 1959, Formation of the crystal-rich glowing.avalanche deposits of St. Vincent, B.W.I.:
Journal of Geology, v. 67, p. 540-562.
H o ~ , G., ANDWomErz, K.H., 1985, Volcanic Ash: Berkeley, University of California Press,
246 p.
H~lNs, W.A., 1993, Source rock texture versus climate and topography as controls on the
composition of modern, plutoniclastic sand, in Johnssun, M.J., and Baan, A., ¢ds., Processes
Controlling the Composition of Clasfic Sediments: Geological Society of America Si~cial
Paper 284, p. 135-145.
IN~EgSOLLR.V, ANDC^VAZZA,W., 1991, Reconstruction of Oligo-Mincene volcaniclastic dis~rsal ~Ueerns in north-central New Mexico using sandstone ~trofacies, in Fisher, R.V., and
Smith, G.A., Ms., Sedimentation in Volcanic Settings: SEPM Special Publication 45, p. 227236.
KAtrrz, P.E, I~ERSOLL,R.V., BALDRID~LW.S., D~ON, P.E., ~ a SHAn~LLAH,M., 1981, Geology
of the Espinaso Formation (Oligocene), north-central New Mexico: Geological Society of
America Bulletin, v. 92, Part I, p. 980-983; Part II, p. 2318-2400.
Ku~o, H., 1950, Petrology of Hakone volcano and the adjacent anus, Japan: Geological Society
of America Bulletin, v. 61, p. 957-1020.
LARULD.K., ANDS&MPAYO,M.M., 1990, Lithic-volcanic sandstones deciveci from oceamc crust
in the Franciscan Complex of Cafifomia: Sediment memories of source rock geochemistry:
Sedimantulngy, v. 37, p. 879--889.
LtPuAr~,P.W., 1968, Geology of Summer Coon volcanic center, eastern San Juan Mountains,
Colorado: Colorado School of Mines Quartedy, v. 63, p. 211-226.
M^CK, G.H., ANDJEP.ZvraE~acz,T., 1989, Detrital modes of sand and sandstone derived from
andesitic rocks as a pal¢odimatic indicator. Sedimentary Geology, v. 65, p. 35.-.44.
MARS~UA,K.M., ANDINGERSOtL,R.V., 1992, Compositional trends in arc-related, d~p-murine
sand and sandstone: a reussassment of mngmatic-atc provenance: Geolngical Society of America Bulletin, v. 104, p. 1637-1649.
McKsicHr, T.L., 1987, Physical Geography, A Landscal3e Appr¢ctation: Princeton, New Jersey,
Prenti~-Hall, 539 p.
MILE R.C., 1977, Modifications of sediment composition and ~xture in two fluvial systems,
Guatemala [unpublished M.A. thesis]: Columbia, University of Missouri, 91 p.
PARrgL~,D.P., Kavsnmt, J.G., ANDMCI~ Bj., 1988, Provenance of the Gu~ydan Formation,
south Texas: implications for the late Oligocene.-earty Miocene tectonic control of the TrimsPeeps volcanic field: Geology, v. 16, p. 1085.-1088.
l:~snEL F.A., 1937, The eruption ofMt. PelE¢¢, [929-1932: Washington, Caro~gie Institution
Publication 458, 126 p.
PErr~o,~, FJ., PoTr~R,P.E., ASPSt~v~R,R., 1987, Sand and Sandstone, 2nd Edition: New York,
Springet.Verlag 553 p.
PROWLG.R., ~ Vc~L B.F. ~os., 1982, Geology and geothermal lesources of the Mount Hood
area, Oregon: Oregon Department of Geology and Mineral Industries Special Paper 14, 100 p.
RooaoL, MJ., 1976, Post-etuptive mechanical sorting ofpyroclastic material-an example from
Jamaica: CG¢olngicalMagazine, v. 113, p. 429..-440.
ROSE,W.I., 1973, Pattern and mechanism of volcanic activity at the Santiaginto volcanic dome,
Guatemala: Bulletin Vokanologique, v. 37, p. 73-94.
Rtyme~rogo, M.J., 1993, Experimental petlolngy applied to volcanic processes: EOS (l'nmsactions of the American Geophysical Union), v. 74, p. 49-55.
SCHMID,R., 198 I, Descriptive nomenclature and classification of pyrociasfic deposits and fragmaltS: rocommendations of the IUGS Subcommissiun on the Systematics of Igneous Rocks:
Geology, v. 9, p. 41-43.
SmtLtV, CJ., 1963, The Ellensburg flora of Washington: University of California Publications
in the Geological Sciences, v. 35, p. 159-276.
SM[+a, A.L., ASP Roo~oL, MJ., 1990, Mr. Pel6e, Martinique: A study of an active island-arc
volcano: Geological Society of America Memoir 175, 105 p.
SMtia, G.A., 1987, The influence of explosive volcanism on fluvialsedimcotation: the Deschutes
Formation (Neogene) in central Oregon: Junraal of Sedimentary PetrOlogy, v. 57, p. 613629.
SMIYa, G.A., 1988, Sedimantology of proximal to distal volcaniclasti~s dispersed across an
active foldlzlt: EBensburg Formation (late Miocene), central Washington: SedimentologL v.
35, p. 953--977.
SMtt,, G.A., 1991, Facies sequ~noes and geometries in continental volcaniclastic sedirnents, in
Fisher, R.V., and Smith, G.A., eds., Sedimentatian in Volcanic Settings: SEPM Special
Publication 45, p. 109-122.
Smlta, G.A., AnDG~usensr~, MJ., 1993, Towards a facies model for breccia-dominated composite volcanoes: The Oligocene Summer Coon Volcano, Colorado (abstract): EOS ffransactions of the American Geophysical Union), Fall Meeting Supplement, p. 644.
SmiTH, G.A., CAmelEEr,N.P., IYr.,~con,M.W., ~ o SHAr~OJL~8,M, 1988, Eruptive style and
GAR Y A. SMITH AND JOAN E. LOTOSKY
98
location of volcanic centers in the Miocene Washington Cascade Range: Reconstruction from
the sedimentary record: Geology, v. 16, p. 337-340.
Sa~r,, G.A., ~RSES, D., Haatar~, S.S., MclsToSH, W_C., EtsJoNL D.W., ANDTA~LOLS., 1991, A
talc of two volcaniel~tic aprons: fidd guide to the sedimentology and physical vokanology
of the Oligocene Espinaso Formation and Miocene Peralta Tuff, north-central New Mexico,
in Julian, B., and Zidek, J., eds., Geologic excursions in New Mexico and Adjacent Parts of
Colorado and Texas: New Mexico Bureau of Mines and Mineral Resources Bulletin 137, p.
87-103.
S~NHAUSER,F., 1979, Climatic Arias of North and Central America: New York, United Nations
Educational Scientific and Cultural Organization (UNESCO), 28 p.
Sl;rrsEe~LJ.~B^s~A.~DMAcK~G.H.~98~C~imateandthe~ri~(inofquartzaremtes:J~umal
of Sedimentary Petrology, v. 51, p. 1235--1246.
UJIKE,O., 1974, Change in optical properties of hornblende phenocrysts after the eruption: an
example in hornblende andcsite from Kaitaku, lkedo-cho, Kagawa Prefecture: Chishitsu
Chosajo, v. 25, p. 1%25 (in Japanese with English abstract).
VI::SSELt,R.K., AND DAVIES,D.K., ] 98 I, Nonmarinc sedimentation in an active fore arc basin,
in Ethridge, F.G., and Hores, R.M., eds., Recent and Andent Nonmarine Depositional
Environments: Models for Exploration: SEPM Special Pablicarion 31, p. 31...-45.
WAL~R, G.P.L., 1972, Crystal concentration in ignimbrites: Contributions to Mineralogy and
Petrology, v. 36, p. 135-146.
WHI~, C., 1980, Geology and geochemistry of Mt. Hood volcano: Oregon Department of
Geology and Mineral Induslries Special Paper 8, 26 p.
WisE, W.S., 1969, Geology and petrology of the Mr. Hood area: a study of High Cascade
volcanism: Geological Society of Amcfica Bulletin, v. 80, p. 96%1006.
Wotw, J.A., 1985, The effect of explosive eruption processes on geochemical ~tterns within
pyroclastic deposits: Jottmal of Volcaaology and Geothermal Research, v. 26, p. 18%201.
Ar~olx 1.- Point-count datafor Mr. Hood river sand
APP~DI×Z-Point-count data for Espinaso Formation sandstones and tufts
Sample
Distaoce
fram
Summit
Rock
No.
(km)
Frag. Opx
Gill.
Gra.
Hb.
Grn. w/ Bin. Rcpl.
Cpx Hb. Rim Hb. Hb, Pla& OxKle RFP
RFA
RFC
0.96
0.94
0.94
0.81
0.80
0.79
9.7
7.6
8.1
405
402
398
MH 12
MH 13
MHI4
MHI5
12.1
11.3
10.8
33.1
laaia:
370
391
399
389
16
18
13
8
39
24
20
12
I
[
2
0
0
0
0
0
1
1
1
1
2
I
I
I
60
61
61
88
11
3
3
0
0.86
0.87
087
0.82
0.84
0.89
0.91
0.94
0.74
0.78
0.79
0.78
M H [6
21.7
364
II
33
0
0
4
3
82
3
0.82
0.87
0.72
MHI9
MH20
MH21
MH22
MH36
24.1
20.1
12.9
9.7
41 .g
339
320
390
387
387
18
27
3
4
8
42
55
6
10
12
0
I
I
0
1
0
0
0
0
0
0
5
I
I
0
3
2
2
0
0
92
83
92
93
91
6
7
1
0
1
0.79
0.79
0.81
0.81
0.81
0.83
0.77
0.96
0.96
0.95
0.67
0.63
0.78
038
0.77
24
6
20
8
57
14
39
14
I
I
0
0
0
0
0
0
I
0
I
0
3
0
2
0
136
72
68
73
3
0
8
2
0.62
0.85
0.82
0.85
0.71
0.95
0.83
0.94
0.49
0.8 I
0.69
0.81
II
18
18
31
2
2
0
0
0
6
2
2
83
89
4
2
0.82
0.80
0.91
0.85
0.76
0.70
I
6
2
1
4
0
85
0.82
0.97
4
I
2
0
4
0
99
0.80
0.97
4
16
9
35
0
1
0
0
5
13
2
[
79
76
0
I
2
3
0.84
0.82
0.95
0.84
0.79
0.78
0.80
031
26
47
3
0
I
3
68
6
0.83
0.79
0.68
H w d Riv~ ~
Sandy River dntlatge
MH23
13.5
MH24
20.9
MH25
20.9
MH26
10.5
MH29
MH30
29.3
59.5
MH31
72.4
MH32
72.4
MH33
MH34
77.7
833
MH35
83.7
basin:
275
407
345
403
380
350
397
389
399
356
342
3
9
7
10
14
14
0
0
2
0
0
0
I
2
1
0
0
I
79
72
75
2
I
2
0.84
0.85
0.84
Hb.
Rock
Gm, wl &n. Rcpl.
Frag*Opx Cpx Hb. Rims llb. Hb. Hag
Sample
No.
r~,roo~ex..m,,~e*ca:
MH5
MH7
MH27
Received 6 April 1994; aecepled 30 June 1994.
ATI91
ATTII
84
144
0
0
14
52
41
15
7
1
56
40
AT881
AT1243
138
225
I
0
28
22
14
7
I
0
ATI261
124
I
55
0
0
CC241
87
0
4
56
CC244
CC621
73
85
0
0
14
60
58
33
Oxides Blot. RFP
17
25
RFA
Hb.
Ratio
10
6
270
217
[
0
0.24
0.40
0.37
0.51
0.36
0.24
23
7
2
3
278
224
5
0
267
15
10
0
l
0.33
0.50
0.62
0.82
0,35
0.41
31
17
0.32
0.53
2
21
12
3~
18
0
0.23
0.44
0,62
1
3
25
15
7
7
314
307
8
26
0
0
0.39
0.44
0.00
0,64
0,54
0.45
0,52
ATI001
131
0
71
0
7
211
15
0
CC422
114
0
6
71
10
13
8
260
18
0
0.19
0.22
0.38
0.31
0.48
030
WMV631
128
3
62
32
I
45
12
198
19
0
0.26
0.42
0,35
MU5971
125
2
52
34
2
54
19
163
49
0
0.43
0.37
0.31
Tds:
AT[041
AT[042
AT[043
ATI044
ATt05t
AT491
AT90[
AT94 [
SFP8802
303
338
315
334
287
342
272
284
294
0
0
0
0
0
0
0
0
0
8
7
7
7
18
5
0
0
16
2
38
36
40
27
II
40
30
34
0
I
I
I
2
0
0
4
2
3
0
2
1
7
2
1
2
14
0
3
0
5
3
0
I
3
I
137
110
136
102
152
131
182
172
135
4
3
3
10
4
5
4
5
4
42
0
0
0
0
0
0
0
0
7
58
0.40
0,90
0.92
0.85
0.61
0.85
0.95
0.83
0,67
* Includes pumice shards and lapi0i in tugs.
AP~mDIX3.--Point-count data for El]ensburg Formation sand
Sample
No.
Facies
Rock
Fral~
Opx.
Cpx.
Cma. Hb.
EBI
EB2
EB3
EB4
EB5
ElI6
EB7
EB8
EB9
N9
NI5
syncrup.
intereup.
intcreup.
syocmp.
intercup.
synerup.
syaemp,
synemp.
syncmp.
intcrcup.
intercup.
49
165
166
170
161
150
95
112
39
155
169
I
2
2
0
0
0
I
5
0
2
0
0
4
3
I
0
0
3
2
I
3
3
32
2
5
14
19
22
32
16
46
6
5
* Counts based on all loose hornblende crystals I ~ e n t in thin soclion.
Grn. Hb.
w/Rim Bin. Hb. Repl. Hb.
0
0
0
I
0
0
2
!
0
0
1
2
2
4
I
12
3
I
2
I
10
10
2
1
5
1
7
1
0
1
0
5
7
Flag.
Oxides
Quartz
Blot.
R.FP
RFA
158
113
100
98
91
110
160
]49
201
102
87
54
4
2
14
10
11
5
10
10
10
14
0
6
13
0
0
2
0
2
1
0
0
2
0
0
0
0
0
1
0
0
0
0
0.21
0.59
0.62
0.62
0.64
0.56
0,36
0.42
0.17
0.61
0.67
0.32
0.92
0.90
0.84
0.77
0.79
0.67
0.75
0.41
0.82
0.82
Gill. lib.* Other lib.* llb. Ratio*
57
1O
12
41
28
47
61
27
64
9
6
6
15
21
9
29
8
5
6
2
21
27
0.90
0.40
0.36
0.84
0.49
0.85
0.92
0.82
0.97
0.30
0.21