what factors control the composition of andesitic sand?people.uncw.edu/lamaskint/gly 445-545 fall...

W H A T FACTORS C O N T R O L T H E C O M P O S I T I O N OF A N D E S I T I C SAND?

GARY A. SMITH ~ JOAN E. LOTOSKY Department of Earth and Planetary Sciences, University of 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 volcan- idnstic 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 plate- tectonic 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 recon- structions, 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?

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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

JOURNAL OF SEDIMErCr/dY R~rARcH, VoL. A65, No. 1, JssvAav, 1995, P. 91-98 Copyright © 1995, SEPM (Society for Sedimentary Geology) 1073-130X/95/0A65-91/$03.00

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-man- tling deposits (Fig. 1; e.g., Hay 1959; Walker 1972). Similarly, components are fractionated during transport and settling from airborne eruptive col- umns (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 XNDESITIC VOLCXNICL~T1CS

Volcaniclastic rocks of intermediate composition offer particular chal- lenges to the petrographer attempting to determine fragment genesis. These rocks (hereafter referred to simply as andesite, but including basaltic an- desire and dacite) vary moderately in chemical composition but share highly porphyritic (30..-50% phenocrysts) textures and phenocryst assem- blages dominated by plagioclase and one or more subordinate ferromag- nesian 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 Gru- bensky 1993). Relatively viscous lavas undergo intense brrecciation when descending steep slopes to generate autoclastic fragments of sand to boul- der 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.

92 GAR Y A. S M I T H A N D J O A N E. L O T O S K Y

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FIG. 1.-- Physical fractionation of pyroclastic components during explosive volcanic eruptions. The original eruptive mixture contains loose crystals, vitric fragments containing crystals, and accessory rock fragments (not shown for simplicity). Deposition from drifting eruption plumes produces coarse, lapilli-fich deposits near the volcano, and ash is deposited far- ther away. Crystals are typically concentrated in medial deposits because most phenocrysts are I--6 mm in diameter and are coarser and denser than most of the material in the erup tion plume, which falls as fine vitfic ash at greater distances (Wolff 1985). Density stratifi- cation and elutfiation of fine-grained, low-.density ash in moving pyroclastic flows generates a vitfic-ash-rich ash-cloud deposit and a poorly sorted pyroclastic-flow deposit, sensu stricto, that has a crystal-enriched matrix rdative to the original eruptive mixture (Walker 1972). Erosion of unconsolidated pyroclastic-flow and medial ash-fall deposits can therefore produce crystal-rich sand because they were derived from crystal-enriched primary pyroclastic deposits.

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.

P R E V I O U S W O g l ~

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

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 tem- poral variations in climate.

PROBLEMS AND ALTERNATIVE INTERPRETATIONS

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 crystal- rich 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

I Washington

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0,,=,2,0 - ILI FF[G. L-Location map of samples from streams draining Mount 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 con- text, we find the data set of Mack and Jerzykiewicz (1989) to be uncon- vincing 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 ep- iclastie. 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 In- dies) have experienced explosive eruptions during this cemury that in- undated 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 glacial outwash, 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

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 pyroclastic- flow and debris-flow deposits composed of predominantly lithic-pyro-

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FIG. 3.-Variation in RFC index (rock fragments/[rock fragments+crystals]) with distance for medium sand in rivers draining pyrodastic deposits on Mount Hood (Appendix 1) and Volc~n Fuego (data from Miles 1977). Downstream decrease in RFC is interpreted to result from comminution of the glassy groundmass of the rock fragments to release sand-size phenocrysts. Mount Hood sand shows no significant change in composition as a consequence of transport, with samples collected more than 80 km from the volcano having a composition similar to the matrix of source-area pyroclastic-flow deposits. The notably anomalous Mount Hood sample with an RFC value of 0.49 was collected from a small watershed draining Pliocene lava flows rather than pyroclastic deposits. The relatively high crystal abundance of this sample may result from chemical weathering in a humid climate (el. Mack and Jerzykiewicz 1989).

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 source- area 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.

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 views of grain mounts of the medium-sand sieve fraction of pyroclasfic-flow matrix and fluvial sand. A) MatrLx of pyroclasfic-flow deposit collected on south flank of Mount Hood; plane light, field of view approximately 3.0 ram. B) Sand collected from the Hood River, approximately 80 km from the summit of Mount Hood; plane light, field of view approximately 3.5 ram. Representative grain types have Men lahelcd in each photo: plaginclas¢ (PL), pyroxene (PX), hornblende (HB), holocrystalline volcanic rock fragments (HC), and hypocrystalline oxide-rich volcanic rock fragments (G). The pyroclasfic-flow deposit lacks vesicular or bubble-wall-shaped fragments, contains numerous loose crystals, and is dominated by holocrystalline rock fragments. The fluvial sand (B) is pctrographically similar to the pyroclastic deposit (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.

Ellemsburg Formation

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 sub- Plinian 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 cate- gories interpreted to represent syneruption and inter-eruption sedimen-

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 fa- des 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 gra- dient probably existed between the sediment source and the depositional site; there are no reliable paleoclimate data for the source area.

EFFECTS OF TIIANSPORT

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 pyro- elastic-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


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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-.Oligocene Espinaso 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 pyro- clast-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 Jer- zykiewicz'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, crystal- poor pyroclastic debris and has a relatively low crystal abundance despite its humid climatic setting. The relatively high, and variable, crystal con- tents 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£Cr OF PYIIOCIASTIC CONTIRBtNIONS

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 PH2O greater 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

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o 4are. ~o¢ o u r a u ~ > 0 °700

0

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 Ellensburg Formation 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

c l l = o

=_--~ 8

EE~

,1= ,1= ~

t ! ! | conglomerate clasts

1 ~ (n = 54) 50

o ,t 0.0 0.5 1.0

Hornblende Ratio (fresh green hornblende / total hornblende)

FJc. 7.-Hornblende ratio for tufts, sandstones, and conglomerate clasts in the Espinaso Formation (data from Appendix 3 and Erskine and Smith 1993). Pu- miceous lapilli and shards in tufts are counted as rock fragments. Conglomerate clasts, interpreted by Erskine and Smith (1993) to represent autoclastic, pyroclastic, and epidastic fragments derived from volcanic domes, lack green hornblende. Sandstones contain variable amounts of green hornblende and cannot be derived only from transport breakage of conglomerate clasts. Tufts contain relatively high proportions of green hornblende and contain about 40-50% loose crystals~

P

B: T R A N S P O R T D I S T A N C E

S O U R C E I D E P O S I T I O N A L S I T E S

FIG. 8.-Factors influencing the ratio of rock fragments to crystaLs in andesitic sand. Weathering may release crystals from rock at the source. Voicaniclastic sediment eroded from penecontemporaneously active volcanoes, however, may be derived from unconsofidated pyroclasfic deposits that are enriched in crystals by volcanic processes (e.g., Fig. 1). Further enrichment in crystals may occur with a modest distance of fluvial transport in the case of relatively nondurable, typically nitric, fragments. Lithic fragments (of epiclastic, pyroclastic, or autoclastic origin) are more durable during transport, The composition of a sand is a function of three independent variables: source-area volcanic fragmentation, weathering, and transport abrasion. It is not possible, therefore, to simply relate the petrographic mode of ancient andesitic sandstone to source-area climate or transport distance.

(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; Ap- pendix 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


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. Fragmen- tation 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.

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98 GAR Y A. SMITH AND JOAN E. LOTOSKY

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Ar~olx 1.- Point-count data for Mr. Hood river sand

Dis- taoce fram Gra. Sum- Hb.

Sample mit Rock Grn. w/ Bin. Rcpl. No. (km) Frag. Opx Cpx Hb. Rim Hb. Hb, Pla& OxKle RFP RFA RFC

r~,roo~ex..m,, ~e*ca: MH5 9.7 405 3 10 0 0 I 0 79 2 0.84 0.96 0.81 MH7 7.6 402 9 14 0 0 2 0 72 I 0.85 0.94 0.80 MH27 8.1 398 7 14 2 0 1 I 75 2 0.84 0.94 0.79

Hwd Riv~ ~ laaia: MH 12 12.1 370 16 39 I 0 1 2 60 11 0.86 0.84 0.74 MH 13 11.3 391 18 24 [ 0 1 I 61 3 0.87 0.89 0.78 MHI4 10.8 399 13 20 2 0 1 I 61 3 087 0.91 0.79 MHI5 33.1 389 8 12 0 0 1 I 88 0 0.82 0.94 0.78 MH [ 6 21.7 364 I I 33 0 0 4 3 82 3 0.82 0.87 0.72 MHI9 24.1 339 18 42 0 0 0 3 92 6 0.79 0.83 0.67 MH20 20.1 320 27 55 I 0 5 2 83 7 0.79 0.77 0.63 MH21 12.9 390 3 6 I 0 I 2 92 1 0.81 0.96 0.78 MH22 9.7 387 4 10 0 0 I 0 93 0 0.81 0.96 038 MH36 41 .g 387 8 12 1 0 0 0 91 1 0.81 0.95 0.77

Sandy River dntlatge basin: MH23 13.5 275 24 57 I 0 I 3 136 3 0.62 0.71 0.49 MH24 20.9 407 6 14 I 0 0 0 72 0 0.85 0.95 0.8 I MH25 20.9 345 20 39 0 0 I 2 68 8 0.82 0.83 0.69 MH26 10.5 403 8 14 0 0 0 0 73 2 0.85 0.94 0.81 MH29 29.3 380 I I 18 2 0 0 2 83 4 0.82 0.91 0.76 MH30 59.5 350 18 31 2 0 6 2 89 2 0.80 0.85 0.70 MH31 72.4 397 I 6 2 1 4 0 85 0 0.82 0.97 0.79 MH32 72.4 389 4 I 2 0 4 0 99 I 0.80 0.97 0.78 MH33 77.7 399 4 9 0 0 5 2 79 2 0.84 0.95 0.80 MH34 833 356 16 35 1 0 13 [ 76 3 0.82 0.84 031 MH35 83.7 342 26 47 3 0 I 3 68 6 0.83 0.79 0.68

APP~DI× Z-Point-count data for Espinaso Formation sandstones and tufts

Gill. Hb.

Sample Rock Gm, wl &n. Rcpl. Ox- Hb. No. Frag*Opx Cpx Hb. Rims llb. Hb. Hag ides Blot. RFP RFA Ratio

ATI91 84 0 14 41 7 56 10 270 17 [ 0.24 0.37 0.36 ATTII 144 0 52 15 1 40 6 217 25 0 0.40 0.51 0.24 AT881 138 I 28 14 I 23 2 278 15 0 0.33 0.62 0,35 AT1243 225 0 22 7 0 7 3 224 10 l 0.50 0.82 0.41 ATI261 124 I 55 0 0 5 0 267 31 17 0.32 0.53 0.00 CC241 87 0 4 56 2 21 12 3~ 18 0 0.23 0.44 0,62 CC244 73 0 14 58 1 25 7 314 8 0 0.19 0.39 0,64 CC621 85 0 60 33 3 15 7 307 26 0 0.22 0.44 0,54 ATI001 131 0 7 71 0 58 7 211 15 0 0.38 0.45 0,52 CC422 114 0 6 71 10 13 8 260 18 0 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[041 303 0 8 2 0 3 0 137 4 42 AT[042 338 0 7 38 I 0 3 110 3 0 AT[043 315 0 7 36 I 2 0 136 3 0 ATI044 334 0 7 40 I 1 5 102 10 0 ATt05t 287 0 18 27 2 7 3 152 4 0 AT491 342 0 5 I I 0 2 0 131 5 0 AT90[ 272 0 0 40 0 1 I 182 4 0 AT94 [ 284 0 0 30 4 2 3 172 5 0 SFP8802 294 0 16 34 2 14 I 135 4 0

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~mDIX 3.--Point-count data for El]ensburg Formation sand

Sample Rock Grn. Hb. No. Facies Fral~ Opx. Cpx. Cma. Hb. w/Rim Bin. Hb. Repl. Hb. Flag. Oxides Quartz Blot. R.FP RFA Gill. lib.* Other lib.* llb. Ratio*

EBI syncrup. 49 I 0 32 0 2 2 158 54 0 2 0.21 0.32 57 6 0.90 EB2 intereup. 165 2 4 2 0 2 1 113 4 6 0 0.59 0.92 1O 15 0.40 EB3 intcreup. 166 2 3 5 0 4 5 100 2 13 0 0.62 0.90 12 21 0.36 EB4 syocmp. 170 0 I 14 I I 1 98 14 0 0 0.62 0.84 41 9 0.84 EB5 intercup. 161 0 0 19 0 12 7 91 10 0 0 0.64 0.77 28 29 0.49 E l I6 synerup. 150 0 0 22 0 3 1 110 11 2 0 0.56 0.79 47 8 0.85 EB7 syaemp, 95 I 3 32 2 I 0 160 5 0 1 0,36 0.67 61 5 0.92 EB8 synemp. 112 5 2 16 ! 2 1 ]49 10 2 0 0.42 0.75 27 6 0.82 EB9 syncmp. 39 0 I 46 0 I 0 201 10 1 0 0.17 0.41 64 2 0.97 N9 intcrcup. 155 2 3 6 0 10 5 102 10 0 0 0.61 0.82 9 21 0.30 NI5 intercup. 169 0 3 5 1 10 7 87 14 0 0 0.67 0.82 6 27 0.21

* Counts based on all loose hornblende crystals I~en t in thin soclion.

what factors control the composition of andesitic sand?people.uncw.edu/lamaskint/gly 445-545 fall...

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