Determination of Flow Direction of Rhyolitic Ash-Flow ...

WOLFGANG E. ELSTON EUGENE I. SMITH

Department of Geology, University of New Mexico, Albuquerque, New Mexico 87106

Determination of Flow Direction of Rhyolitic Ash-Flow Tuffs from Fluidal Textures ABSTRACT

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Although mid-Tertiary calc-alkalic volcanics equal in volume and composition to major batholiths played a crucial role in the evolution of western North America, interpretations are hampered by Basin and Range faulting and subsequent erosion and sedimentation which obscure primary volcano-tec tonic features. To reconstruct them, criteria for tracing volcanic formations to their source are needed. Seventy-one oriented samples were collected from the Pleistocene Bandelier Rhyolite ashflow tuff and Battleship Rock welded tuff, which erupted from known centers in the Jemez Mountains, New Mexico. In each sample, orientation of elongated shards, pumice fragments and crystals was measured in thin sections cut parallelto primary layering. Lineation was pronounced and indicates a preferred orientation of microscopic or megascopic components, resulting from primary flowage. The statistical significance of results is documented by the Tukey Chi-square test and the vector method. Statistical parameters for these two techniques were calculated by Fortran IV program. Only seven samples (9 percent) indicated Chi-square values below the 90 percent confidence limit. Flow azimuth, which indicates the absolute direction of movement at any point on a flow, is determined by observing objective textural criteria in thin sections cut parallel to the dip, or in vertical sections cut parallel to the predetermined flow-lineation direction. The orientations of both equidimensional and non-equidimensional fork-shaped shards and penetration effects were found to be reliable objective criteria for determination of flow azimuth in dip-parallel sections. Imbrication, blocking effects, and orientation of spindleshap^djobjects_werejiound to be reliable objec-

tive criteria for flow azimuth determination in vertical sections. The plot of flow lineations and flow azimuths of the Bandelier Rhyolite tuff indicate flowage radially away from the Valles Caldera. Deviations from this pattern probably can be explained by influence of preflow topography. When applying these techniques to a region where the source of ash-flow tuffs is unknown, sampling should be carried out over a large area. More than one sample should be collected at each station to assure a flow pattern relatively free from small-scale variation.

INTRODUCTION This article deals with techniques for determining the direction of movement of ash-flow tuffs (ignimbrites) by means of microscopic fluidal textures in oriented thin sections. The techniques were developed on samples of Pleistocene Bandelier Rhyolite tuff and Battleship Rock welded tuff which erupted from known centers in the Jemez (Valles) Mountains of north-central New Mexico (Ross and others, 1961; Smith and Bailey, 1966). The techniques described here are now being successfully applied to unravelling the volcanotectonic structure of a much larger and complex area in which the source vents were not previously known, the mid-Tertiary Datil and Mogollon Plateau volcanic provinces of southwestern New Mexico. They supplement conventional field mapping in (1) locating source cauldrons of ash-flow tuffs, and (2) distinguishing between coeval and similar rocks from different vents (Elston and others, 1968; Rhodes, 1970; Krimsky, 1970). Experience has shown that most of the criteria useful in determining flow direction in the Bandelier Rhyolite ash-flow tuff and Battleship Rock welded tuff can be applied to other ash-flow tuffs, but that the petrographer must be selec-

(N ASA-CB-131 772n~DETEfiHINATl6N OF FLOS DIBECTION OF BHYOLITIC ASH FLOW TUFFS PBOH FLUIDAL TEXTUBES (New :Mexico Univ.), U p 00/99

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ELSTON AND SMITH—FLOW DIRECTION OF ASH-FLOW TUFFS

live in choosing the ones that apply best to the texture of the particular rock he is examining. New criteria, not described in this article, are being developed. We hope that our methods will find general application in other complex areas. In the Basin and Range province of western North America, for example, valley fill covers much of the bedrock, and the number of known vents is scanty compared to the volume of rhyolite, estimated at 104 mi3 (105km3) for the Great Basin alone (Mackin, 1960; Cook, 1963). With modifications, the techniques developed for rhyolite ash-flow tuffs can be applied to other types of volcanic rocks. In fact, lava flows of andesite, latite, and alkali-rich basalt tend to show much stronger lineation and much more clear-cut textural evidence for azimuth than do ash-flow tuffs. We concentrated on ashflow tuffs precisely because of their relatively low degree of lineation. Turbulent flow is generally considered to be the method of transportation, and most investigators have attributed their textural anisotropy entirely to in situ compression during and after deposition. Since lineation does, in fact, exist in the rocks studied, we attribute it to primary late-stage laminar flow over a short distance. It is most conspicuous in the more welded parts of cooling units. In extreme cases, complete welding of incandescent shards will result in a viscous fluid resembling a true lava, which moves by laminar flow. Its flow direction will be down local slopes, not necessarily parallel to the direction of turbulent flow of the original ash flow. The statistical and textural techniques described here have the virtue of being simple in principle. Only about half an hour is needed to complete the lineation and azimuth determinations of one thin section. All statistical calculations are made by computer. PREVIOUS WORK Previous literature on the determination of flow direction of true ash-flow tuffs is meager. Flow lineation has been mentioned before, but few authors have attempted to use it to determine the source of the tuffs. Hoover (1964) recognized flow structures in a welded tuff in the northern portion of the Nevada Test Site. Schmincke and Swanson (1967) described laminar viscous flowage features from the Canary Islands and suggested several megascopic criteria for determining

flow direction in ash-flow tuffs. Walker and Swanson (1968) described laminar-flow features in an ash-flow tuff from Oregon. Pinnell (1969) attempted to use differential X-ray absorption for determining directional fabric of ash-flow tuffs. Some of the directional criteria developed by Waters (1960) and Schmincke (1967) for basalts, and Cummings (1964) and Christiansen and Lipman (1966) for rhyolite lavas could be applied to ash-flow tuffs. DEFINITIONS Flow lineation indicates a preferred orientation of microscopic or megascopic components in volcanic rocks resulting from primary flow. In this study, flow lineation was determined by cutting oriented thin sections parallel to the primary dips of the rocks and plotting the long axes of phenocrysts, pumice fragments, and glass shards in 10° intervals on a 180° scale. The results were treated statistically to determine the principal flow-lineation direction. Plow azimuth indicates the absolute direction of movement at any point on a flow. For each previously determined flow lineation there are two possible flow azimuths, that is, a northsouth flow lineation is compatible with flow azimuths of 0° and 180°. The flow azimuth may be objectively determined by textural criteria in thin sections cut parallel to the primary dip, or vertical thin sections cut parallel to the previously determined flow lineation. The term is synonymous with local flow direction. Flow direction is the over-all pattern of movement of an entire flow or a large segment of a flow. Flow azimuth, in contrast, indicates direction of movement at one point only. On a map, the total pattern of all plots of flow lineations and flow azimuths indicates flow direction. In the upper member of the Bandelier Rhyolite ash-flow tuff, for example, the flow direction is roughly radial to the VallesCaldera, except where diverted by irregularities in the underlying topography. METHOD OF INVESTIGATION Rock Types Sampled Seventy-one oriented samples were collected from the Bandelier Rhyolite ash-flow tuff and the Battleship Rock welded-tuff in the Jemez Mountains, New Mexico. The samples were oriented by marking them with a line parallel

METHOD OF INVESTIGATION to the primary dip and an arrow pointing to true north. To correct for small local variations in flow azimuth, several samples were taken at each station. The particular units were chosen because the sources are known and because they have been cited as classic examples of two different types of ash-flow tuff (Ross and others, 1961). The Bandelier Rhyolite ash-flow tuff is composed of two members which spread radially in great sheets from their source calderas. The present diameter is 75 km and the volume 200 km3. The lower member originated in the Toledo Caldera, the upper member in the Valles Caldera, 18 by 23 km in diameter. All samples for this study were collected from the upper member of the Bandelier Rhyolite ash-flow tuff, except the southernmost three stations on the west side of the Valles Caldera (R. L. Smith, 1968, written commun.). The younger Battleship Rock tuff flowed from El Cajete Crater out of the Valles Caldera through a valley in the southwest caldera rim. In its middle part, pumice fragments are fused to glass spindles with an average length : width ratio of about 5:1 in the horizontal plane, indicating late-stage post-depositional laminar flow. Ross and Smith (1961) briefly described the exposures- of the western end of this ashflow tuff at Battleship Rock. Both units consist of phenocrysts and glassfroth fragments in a matrix of glass shards. To varying degrees, pumice fragments and shards were flattened and welded at the time of deposition by the weight of overlying tuff. Recrystallization induced by reaction of a magmatic vapor phase or by spontaneous devitrification may modify the unstable glass of the pumice up to the point at which it is beyond recognition. Lineation in both units is interpreted as the result of late-stage primary laminar flow. Measurement of Flow Lineation For the determination of the flow lineation, thin sections were cut parallel to the plane, indicating primary dip on the oriented samples, and mounted so that north is at one end of each slide. The dip of the volcanic rocks in the Jemez Mountains is low, and it can be assumed that the thin sections were also cut parallel to the base of the flows. At first glance, the horizontal orientation of the fragments may look random. Measurements of long axes of about 200 phenocrysts, glass shards, and pumice frag-

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ments on any one slide generally show statistically significant lineation. If possible, the longest axes of phenocrysts, pumice fragments, and glass shards were measured independently in each thin section. Crystals in which length/breadth < 2 were neglected. The relative reliability of the three sets of data obtained in this way varied from slide to slide but tended to follow the order of (1) pumice, (2) crystal, (3) shard. The three sets of flow-lineation measurements per slide usually agreed within ±30°. Orientation of elongated shards was measured with a petrographic microscope and point counter. For measuring long axes of elongated crystals and pumice fragments, thin sections were projected on a screen with a 35-mm projector. All data were plotted in 10-degree intervals. Statistical Treatment of Flow Lineation The significance of flow-lineation data of each sample was determined by the Tukey Chisquare test (Tukey, 1954; Middleton, 1965) and the vector method. Geologic applications of the Tukey Chi-square test were discussed by Harrison (1957) and Rusnak (1957), and the vector method by Krumbein (1939), Curray (1956), and Pincus (1956). The Tukey Chi-square test is based on departures of observed data from a distribution which is completely random, that is, a distribution having an equal number of observations in each interval (Rusnak, 1957). The test determines the direction of flow lineation (Chisquare orientation) and the level at which there is significant evidence against isotropy (Chisquare value). A 90 percent probability level (Chi-square for 2 degrees of freedom equals 4.61) is considered significant in this paper. Only seven samples (9 percent) indicated lineation below the 90 percent probability level. The vector method determines vector mean (flow lineation) and vector magnitude. Vector magnitude is a measure of dispersion about the vector mean placed on a scale from 0 to 1.0. A value of 1.0 indicates perfect orientation, while a value of 0 indicates random distribution. The vector magnitude thus indicates whether the degree of the preferred orientation indicated by the vector mean is strong or weak. This contrasts to the Chi-square value which indicates a certain percentage probability level that preferred orientation exists. Both techniques

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were used as checks against each other. The results were in excellent agreement. The statistical computations were made by an IBM 360 computer with Fortran IV program (in depository)1. In all, about 25,000 bits of data were reduced. Determination of Flow Azimuth Flow azimuth, the absolute direction of movement at any point on a flow, is determined for a particular sample by microscopic textural criteria. Flow lineation is determined first so that there are only two possible flow azimuths. In this study, the correct flow azimuth was already known, and the reliability of textural criteria could be tested statistically. No criteria are infallible, but all the criteria cited here have proved to be statistically significant. The properties of a good criterion are: (1) it can be applied more than once in a thin section; (2) it can be applied to the majority of thin sections; (3) it can be applied to more than one rock type; (4) it is easy to interpret; and (5) it indicates the direction of flow azimuth with a reliability of over 50 percent. Many textures were examined and discarded as unsuitable. One can be reasonablyconfident of results if several good criteria occur in a single thin section and are in agreement. For the sake of simplifying the model, the discussion that follows assumes that local flow direction is away from a vent zone. In practice, there are variations due to pre-flow topography and eddies, but the general pattern justifies the model. RESULTS Criteria for Determining Flow Azimuth Non-equidimensional Fork-Shaped Glass Shards. The term fork-shaped glass shard is synonymous with the term Y-shaped shard. Fork shaped glass shards in which the prongs are of unequal length (Figs. 1 and 2) tend to have the longest prong in the direction of flow lineation, pointing away from the source. This criterion has a reliability of 62.9 percent. Using this criterion, the direction of flow 'This material may be ordered by writing to CCM Information Corp.—NAPS, 909 Third Ave., New York, New York 10022, and requesting Document No. 01145. Enclose check payable to NAPS; $2 for microfiche, $5 for photocopies.

DIRECTION OF FLOW

Figure 1. Criteria used for flow azimuth determination. (1) Fork-shaped glass shard with a long prong. (2) Equidimensional glass shard. (3) Spindle-shaped pumice fragment. (4) Blocking effect. (5) Penetration effect. (6) Imbrication.

azimuth may be determined in dip-parallel thin section even if the direction of flow lineation is not known. The longest prong will probably lie in this direction and point away from the source. The number of non-equidimensional fork-shaped shards in each thin section ranged from none to 20. The mean was between 4 and 5. Equidimensional Forked Shards. Forked shards with three prongs of about equal length will probably have one of the prongs in the direction of flow lineation, pointing away from the source. The reliability of this criterion is 66.3 percent (Figs. 1 and 3). About eight equidimensional shards were found in each thin section. It is necessary to know the direction of flow lineation beforehand in order to determine which prong to measure. Penetration Effects. Crystals and xenoliths sometimes penetrate pumice fragments which were viscous at the time of deposition. In all samples, penetration was in the direction of flow azimuth. Examples are shown in Figures 1 and 4. Penetration was not the result of draping of pumice over the crystals during compaction because it occurs consistently on the end of the crystal pointing away from the source. The reliability of this criterion is about 100 percent, but unfortunately it is fairly rare in the Bandelier and Battleship Rock tuffs. It is more common in the mid-Tertiary rocks of southwestern New Mexico. Imbrication. Imbrication (shingle effect of pumice and crystal fragments) seen in vertical thin sections cut in the direction of flew linea-

RESULTS

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Figure 3. Photomicrograph of a fork-shaped glass shard without a distinguishable long prong indicating its use as a criterion for the determination of the direction of flow azimuth. Flow lineation direction AA' is superimposed on fork-shaped glass shard (B). Prong (C) indicates the direction of flow azimuth (with 66.3 percent reliability).

Figure 2. Photomicrograph of a fork-shaped glass shard with a long prong.

tion is shown in Figures 1 and 5. The imbricated fragments dip toward the source with a reliability of 80 percent. Blocking Effects. Faster moving smaller fragments tend to pile up on the side of larger and slower moving fragments pointing toward the vent. Empirically, it was found that the reliability is 90 percent in vertical thin sections cut parallel to the flow direction, and 50 percent in dip-parallel thin section. Spindle-Shaped Objects. The blunt end of a spindle-shaped pumice fragment will point toward the source, the tapered end away from the source, with a reliability of 68.2 percent (Figs. 1 and 7). This criterion is also useful in the field. If spindles are visible in an outcrop that can be seen in three dimensions, an estimate of flow azimuth can be made. Again, it was found that empirically this criterion is more common in vertical thin sections than in sections cut parallel to the dip. Plots of Flow Lineation and Flow Azimuth The location of the sampling stations is shown in Figure 8. Significant flow lineations

Figure 4. Photomicrograph of the penetration effect. Feldspar fragment (A) is deforming glass shard (B) as it collides with pumice (C).

of pumice fragments, shards, and crystals are plotted in Figures 9, 10, and 11, respectively. The length of flow-lineation lines is proportional to vector magnitude, and the particular

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Figure 5. Photomicrograph of the imbrication of pumice and plagioclase fragments in the Battleship Rock welded tuff.

Figure 6. Photomicrograph of the block effect in the Battleship Rock welded tuff. Pumice fragment (A) and plagioclase fragment (B) are being blocked by plagioclase fragments (C) and (D).

criterion used to determine flow azimuth is shown by a code letter. In Figure 12, the sum of all flow-lineation and flow-azimuth values scaled for the number of samples at each station is presented.

vector magnitude confirms a relatively high degree of late-stage laminar flow.

DISCUSSION

Deviations from the general radial pattern in the Bandelier Rhyolite tuff can be explained

Flow Pattern The pattern of flow azimuth and flow lineation of the upper member of the Bandelier Rhyolite ash-flow tuff indicates flow direction radially away from a common source, the Valles Caldera. Flow lineation and flow azimuth plotted for a few samples of the lower member of the Bandelier Rhyolite ash-flow tuff indicate flow direction away from the Toledo Caldera. This substantiates the theory that the Valles Caldera was the source of the upper member of the Bandelier Rhyolite, and the Toledo Caldera the source of the lower member. Flow direction of the Battleship Rock welded ashflow tuft at Battleship Rock was down a canyon, away from El Cajete center. Its high

Figure 7. Photomicrograph of a "bomb" (spindle) shaped pumice fragment in the Battleship Rock welded tuff.

Deviations from General Flow Patterns

36° 00'

EXPLANATION Battleship Rock Welded Tufl

Bandelier Rhyolite Ash-Flow Tuff 35° 40'

Limit of The Voiles Caldera (Smith and Bailey, 1966) •i Limit of The Toledo Colderc ' (Smith and B a i l e y , 1 9 6 6 ) Figure 8. Locations of Bandelier Rhyolite ash-flow tuff and Battleship Rock welded tuff samples collected in the Jemez Mountains, New Mexico. Between El Cajete crater and its outcrop area, the Battleship Rock welded tuff is covered by the younger Banco Bonito Rhyolite flow.

106° 40'

20

30

10

10

36° 00'

VAL ES CALDERA'v

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Battleship Rock Welded Tuff

35 40

10 MILES

Bandelier Rhyolite Ash-Flow Tuff Sample Location Direction of Flow Lineation o.q p.5 1.0 Vector Magnitude Scale Direction of Flow Azimuth Letter Indicates Criterion Used to Determine Flow Azimuth F-Fork-Shaped Shard B-Block Effect P- Penetration I- Imbrication S-Spindle

Figure 9. Direction of flow azimuth and flow lineation computed for pumice in the Bandelier Rhyolite ash-flow tuff and Battleship Rock welded tuff. In this Figure, and Figures 10, 11, and 12, only samples with Chi-square confidence values greater than 90 percent are plotted.

106*40

10' -

CAJET ATER

EXPLANATION Battleship Rock Welded Tuff Bandelier Rhyolite Ash- Flow Tuff Sample Location Direction of Flow Lineation 0.0 0.5 l.O 35" 40' ->

Vector Magnitude Scale Direction of Flow Azimuth Letter Indicates Criterion Used to Determine Flow Azimuth F-Fork-Shaped Shard B-Block Effect P- Penetration I - Imbrication S-Spindle

Figure 10. Direction of flow azimuth and flow lineation computed for glass shards in the Bandelier Rhyolite tuff.

10 ~~l

106° 40' I

10'

50

EXPLANATION Battleship Rock Welded Tuff Bandelier Rhyolite Ash-Flow Tuff Sample Location Direction of Flow Lineation 0.0 0.5 \.Q Vector Magnitude Scale

35° 4 0'

10 MILES

Direction of Flow Azimuth Letter Indicates Criterion Used to Determine Flow Azimuth F- Fork- Shaped Shard B-Block Effect P-Penetration I- Imbrication S-Spindle

Figure 11. Direction of flow azimuth and flow lineation computed for crystals in the Bandelier Rhyolite ash-flow tuff and the Battleship Rock welded tuff.

106*40

36"00'

\TOLEDOS ! V6ALDERA

LCAJETE CRATER

EXPLANATION Battleship Rock Welded Tuff Bondelier Rhyohte Ash-Flow Tuff Sample Location Direction of Flow Lineation 0.0 0.5 1.0 35°40'

Vector Magnitude Scale Direction of Flow Azimuth Letter Indicates Criterion Used to Determine Flow Azimuth F-Fork-Shaped Shard B-Block Effect P- Penetration I - Imbrication S-Spindle

Figure 12. Vector sum of all directions of flow measurements computed for the Bandelier Rhyolite ash-flow tuff and the Battleship Rock welded tuff.

30!

20'

10'

,0'

36° 00'

THE VALLES CALDERA

50

35 40

,»- Direction of Flow Lineation

10 MILES

-~»- Direction of Flow Azimuth

0.0

0.5

1.0

Vector Magnitude I

Figure 13. Vector sum of all directions of flow azimuth and flow lineation computed for the Bandelier Rhyolite tuff plotted on an isopach map of the upper member of the Bandelier Rhyolite ash-flow tuff.

ACKNOWLEDGMENTS by (1) influence of pre-Bandelier topographic irregularities on primary flowage; (2) the presence of thin overlapping ash-flows, each coming from a different direction; and (3) secondary movements and turbulence within the flow. In Figure 13, all flow lineations are plotted on an isopach map of the upper member of the Bandelier Rhyolite ash-flow tuff (Smith and Bailey, 1966, p. 7). Pre-flow relief, reflected by abrupt local changes in thickness, seems to have affected flow lineation in several places. For instance, samples 147 to 153 (Fig. 8), show a strong southeast-northwest lineation. The isopachs show that they came from a thickened section of Bandelier Rhyolite ash-flow tuff that apparently filled a deep and narrow southeasttrending valley. By way of contrast, directly to the northeast, samples 30-33 (Fig. 8) came from a basin of irregular outline and have an irregular flow pattern. The application of the techniques described in this article to the reconstruction of pre-flow topography may turn out to be as valuable as their application in determining locations of eruptive vents. Post-depositional reorientation of components could be brought about by slippages during welding and compaction, because of instability of the ash flow on the ground (Smith, 1960, p. 807). Slippages may or may not be in the direction of primary flow. If preferred orientation in ash flows were caused entirely by such secondary movements, it would be unlikely that flow lineation and flow azimuth of most samples radiate from a common center. It is concluded that secondary movement is not the cause of the preferred orientation but may complicate or accentuate the primary flow pattern. Turbulence may complicate or obliterate the primary flow pattern but, by the evidence presented here, is not the cause of preferred orientation seen in thin section. The Mechanism of Ash-Flow Emplacement A turbulent mechanism has been suggested by most authors as the most important mechanism for the primary movement of ash-flows. Ross and Smith (1961) and Fisher (1966) include the word turbulent in their definition of ash flow. However, if the movement of an ash flow were entirely turbulent, there would be no statistically significant flow lineation. \Ve suggest that laminar flow becomes important during the last stages of primary movement. The ash flow gradually loses volatile components, cools, and increases in density and vis-

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cosity. Eventually, the velocity falls below the critical Reynolds velocity, allowing laminar flow to occur. During late-stage laminar flow, orientation of elongated particles becomes possible. Similar conclusions were reached by Schmincke and Swanson (1967).

SUGGESTIONS FOR SAMPLING AND MEASUREMENT Sampling should be carried out over a large area. Even if no criteria for determining flow azimuth were found, flow lineations would converge toward a possible source. Several samples should be collected at each station because flow direction may vary as a result of underlying irregular topography, turbulence, or the presence of thin overlapping flow units. If possible, more than one parameter should be used on each of several samples for measuring direction of flow lineation and/or flow azimuth. A mean direction should then be calculated to minimize the effects of deviations due to local causes. In the Bandelier Rhyolite ash-flow tuff, pumice is a better indicator of flow direction than either crystal fragments or glass shards. Time might be saved by measuring orientation on pumice only. For determination of flow lineation, an oriented thin section parallel to the dip must be cut from the sample. If suitable fluidal textures are available, flow azimuth may be determined from the same section. It may help to cut a vertical thin section parallel to the direction of flow lineation in order to find additional criteria (for example, imbrication) for determining flow azimuth.

ACKNOWLEDGMENTS This paper is an outgrowth of a long-term study of volcanic rocks and volcano-tectonic structures of New Mexico carried out by one of us (Elston) with support from the New Mexico Bureau of Mines and Mineral Resources from 1950 to 1964, and from NASA Grant NGL (formerly NGR)-32-004-011 from 1964 through the present. Field work was done in 1966 by Smith while employed as Geological Field Assistant by the U. S. Geological Survey, Center of Astrogeology. The results reported here were included in a M.S. thesis by Smith (1968), which also covered fluidal textures of late Tertiary andesite from Mount Taylor, and Holocene basalt from the McCartys flow, both

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in northwestern New Mexico (Smith and Elston, 1968). We are indebted to Dr. Eugene M. Shoemaker for making the necessary arrangements with the U. S. Geological Survey, Center of Astrogeology. Dr. Albert M. Kudo, Department of Geology, University of New Mexico, made many useful suggestions. Dr. Robert L. Smith and Roy A. Bailey, U. S. Geological Survey, supplied an isopach map of the Bandelier Rhyolite tuff and offered suggestions regarding the effect of pre-Bandelier topography on flow direction of the Bandelier Rhyolite tuff. Sam Grey, formerly at the Research Center, University of New Mexico, assisted in writing the computer program.

REFERENCES CITED Christiansen, R. L., and Lipman, P. W., 1966, Emplacement and thermal history of a rhyolite lava flow near Fortymile Canyon, Southern Nevada: Geol. Soc. America Bull., v. 77, no. 7, p. 671-684. Cook, E. F., 1963, Ignimbrites of the Great Basin, USA: Bull. Volcanol., v. 25, p. 89-96. Cummings, David, 1964, Eddies as indicators of local flow direction in rhyolite: U.S. Geol. Survey Prof. Paper 475-D, p. D70-D72. Curray, J. R., 1956, The analysis of two-dimensional orientation data: Jour. Geology, v. 64, no. 2, p. 117-131. Elston, W. E., Coney, P. J., and Rhodes, R. C.; 1968, A progress report on the Mogollon Plateau volcanic province, southwestern New Mexico: Colorado School Mines Quart., v. 63, no. 3, p. 261-287. Fisher, R. V., 1966, Mechanism of deposition from pyroclastic flows: Am. Jour. Sci., v. 264, p. 350-363. Harrison, P. W., 1957, New technique for the dimensional fabric analysis of till and englacial debris containing particles from 3 to 40 mm in size: Jour. Geology, v. 65, no. 1, p. 98-105. Hoover, D. L., 1964, Flow structures m a welded tuff, Nye County, Nevada (abs.): Geol. Soc. America Spec. Paper 76, p. 83. Krimsky, G. A., 1970, Flow direction of volcanic rocks in the northern part of the MogollonDatil province, New Mexico: M.S. thesis, New Mexico Univ., Albuquerque, 41 p. Krunibcin, W. C., 1939, Preferred orientation of pebbles in sedimentary deposits: Jour. Geology, v. 47, no. 7, p. 673-706. Mackin, J. H., 1960, Eruptive tectonic hypothesis for the origin of Basin-Range structure (abs.). Geol. Soc. America Bull., v. 71, no. 12, p. 1921. Middleton, G. V., 1965, The Tukey Chi-square test: Jour. Geology, v. 73, no. 3, p. 547-549. Pinnell, Michael, 1969, Directional fabric of ash-

flow tuffs studied by X-ray absorption (abs.): Geol. Soc. America, Abstracts with programs for 1969, Pt. 5, p. 65. Pincus, H. J., 1956, Some vector and arithmetic operations on two-dimensional orientation variates, with applications to geological data: Jour. Geology, v. 64, no. 6, p. 533-557. Rhodes, Rodney C., 1970, Volcanic rocks associated with the western part of the Mogollon Plateau volcano-tec tonic complex, southwestern New Mexico: Ph.D. dissert., New Mexico Univ., Albuquerque, 145 p. Ross, C. S., and Smith, R. L., 1961, Ash-flow tuffs: Their origin, geologic relations and identification: U.S. Geol. Survey Prof. Paper 366, 81 p. Ross, C. S., Smith, R. L., and Bailey, R. A., 1961, Outline of the geology of the Jemez Mountains, New Mexico: New Mexico Geol. Soc., Guidebook of the Albuquerque Country, 12th Field Conf., p. 139-143. Rusnak, G. A., 1957, A fabric and petrologic study of the Pleasantville Sandstone: Jour. Sed. Petrology, v. 27, no. 1, p. 41-55. Schmincke, Hans-Ulrich, 1967, Flow directions in Columbia River basalt flows and paleocurrents of interbedded sedimentary rocks, southcentral Washington: Geol. Rundschau, v. 56, no. 3, p. 992-1020. Schmincke, Hans-Ulrich, and Swanson, D. A., 1967, Laminar viscous flowage structures in ash-flow tuffs from Gran Canaria, Canary Islands: Jour. Geology, v. 75, no. 6, p. 641-664. Smith, E. I., 1968, Criteria for the determination of flow direction in volcanic rocks: M.S. thesis, New Mexico Univ., Albuquerque, 112 p. Smith, E. I., and Elston, W. E., 1968, Determination of flow direction of rhyolitic ash-flow tuffs and andesitic lavas from fluidal textures (abs.): Geol. Soc. America Spec. Paper 115, p. 207. Smith, R. L., 1960, Ash flows: Geol. Soc. America Bull., v. 71, p. 795-842. Smith, R. L., and Bailey, R. A., 1966, The Bandelier Tuff, a study of ash-flow eruption cycles from zoned magma chambers: Bull. Volcanol., v. 29, p. 83-104. Tukey, J. W., 1954, Comments and suggestion on note 1 by Chayes, comment no. A: Earth Sciences Panel Review, mimeo. rept., 5 p. Walker, G. W., and Swanson, D. A., 1968, Laminar flowage in a Pleistocene soda rhyolite ashflow tuff, Lake and Harney Counties, Oregon: U.S. Geol. Survey Prof. Paper 600-B, p. B37B47. Waters, A. C., 1960, Determining direction of flow in basalts: Am. Jour. Sci., v. 258-A, p. 350-366. MANUSCRIPT RECEIVED BY THE SOCIETY DECEMBER 29, 1969 REVISED MANUSCRIPT RECEIVED JUNE 9, 1970 UNIVERSITY OF NEW MEXICO CONTRIBUTION TO PLANETARY GEOLOGY No. 10 „.,.».