Southern rhyolite lobe and volcaniclastic facies association — a partial cone?

5. Southern rhyolite lobe and volcaniclastic facies association — a partial cone? This association is well exposed only on the south side of the dome-flow complex although similar facies occur between the domes and over- lying flows on the north Fig. 3. Because all sequences have the same defining characteristic, namely the ubiquitous presence of cogenetic rhyo- lite lobes and clasts within rhyolitic volcaniclastic material of ash to lapilli size, our description will focus on the well-exposed southern association. In the following sections we use the terms tuff, lapilli-tuff, and tuff-breccia in the context of Fisher and Schmincke 1984 to denote only parti- cle size, and not genetic processes. The southern association is at least 60 m thick and 500 m long, but the original thickness is unknown because the base is covered by Manis- tikwan Lake and the top has been intruded by a gabbro sill Figs. 3 and 13. The lobe and volcani- clastic facies associations vary from stratified to chaotic. Stratification, which is typically lenticu- lar, is defined by interlayering of four, partly intergradational facies, and by internal structures within facies; the facies include 1 isolated rhyo- lite lobes that are as much as 50 m long and 5 m thick and occur mostly within the bedded het- erolithic facies, 2 close-packed lobes, some of which appear to be well-formed pillows, 3 monolithic, unbedded tuff and lapilli-tuff contain- ing smaller, randomly dispersed rhyolite lobes; and 4 bedded heterolithic tuff, lapilli-tuff, and minor tuff-breccia Fig. 13. Facies are described in Table 2. In the southern facies association, the strike of stratification is variable over short dis- tances, both vertically and laterally Fig. 13, and this variation appears to be a primary feature of the deposit. The strike ranges from 145 to 180°, but the average strike is about the same as that of flows in the northern association. 5 . 1 . Rhyolite lobes — general characteristics The defining parameter of the lobe and volcani- clastic facies association is the presence of white- to yellow-weathering, rhyolite lobes. Lobes range in thickness from 0.5 to 5 m, and in length from 0.5 to \ 50 m. Some lobes, particularly the smaller ones, are relatively equidimensional in cross-sectional outcrop exposures Figs. 14 and 15, but larger lobes are lenticular, and many have an aspect ratio thickness over length of B 0.1 Figs. 13 and 15 – 17. Lobe terminations are Fig. 13. Simplified map of the southern lobe and volcaniclastic facies association; units young to west-southwest. L .D . Ayres , A .S . Peloquin Precambrian Research 101 2000 211 – 235 Table 2 Facies of the southern lobe and volcaniclastic facies association a Comments Facies Shape and dimensions Description Internal features White-weathering with most lobes having distinctly pointed Low aspect ratio; 0.5–5 m thick, Vary from being iso- Highly fractured, with fractures best de- Isolated rhyolite lobes within and 0.5–\50 m long; concordant heterolithic volcaniclastic fa- terminations; many lobes are in sharp contact with the host lated lobes, at least on veloped at nonbrecciated lobe margins; the present outcrop sur- volcaniclastic facies, which contains only rare clasts or small with enclosing heterolithic facies cies fractures generally healed by narrow face, to being in contact quartz and carbonate veins; lobe margins lobes that could have been derived from the larger lobes by with adjacent lobes typically nonbrecciated, but there is local breakage, spalling, or injection; other lobes, however, have Fig. 15 to being spa- a marginal zone that is 10 cm to several metres thick and matrix-poor, marginal breccia that rapidly decreases in intensity inward, and tially associated with either surrounds the lobe or is confined to the upper part of forms discontinuous zones that are gener- the lobe Fig. 18; this marginal zone, which has sharp to other lobes in units 1–2 ally B10 cm, but locally as much as 30 m thick Fig. 16; best gradational contacts with both the lobe and the host het- erolithic volcaniclastic facies, is composed of smaller lobes cm thick Fig. 21; sharp contact between developed in north and marginal breccia and host heterolithic central part of southern and blocks within a matrix of monolithic tuff and lapilli- tuff, the particles of which have the same phenocryst popu- unit facies association Fig. 13 lation as the lobe Fig. 18; many of the smaller lobes are irregular in shape, possibly as a result of plastic deforma- tion; in large lobes, the marginal zones appear to represent injection of small lava lobes upward from the main lobe into fragmental material produced by simultaneous quench brecciation Close-packed rhyolite lobes Lobe margins vary from nonbrecciated to Most lobes are lenticular with pointed ends, but some have Lenticular lobe units are as much Concentrated in central distinctly rounded ends and appear to be pillows Fig. 22; and south part of as 35 m thick and \100 m long; brecciated, with the amount of breccia- lobes vary from touching, with overlying lobes moulded tion variable among lobes, and around southern facies associa- individual lobes range in length against lower lobes, to separated by tuff-breccia, lapilli-tuff, the circumference of single lobes; where tion Fig. 13 away from 0.5 to 6 m, but most are and tuff screens containing variable amounts of angular to several metres long from the flows and present, the zone of marginal brecciation is generally B10 cm wide and it typically domes of the northern irregular blocks and locally small rounded lobes Figs. 22 and 23; many touching lobes have yellow-weathering mar- has a sharp contact with adjacent lobes facies association or interlobe tuff; where well exposed, ginal zones that are 2–30 cm wide and have a sharp contact lobes have an apparent selvage defined by with lobes; these touching lobes are partly separated by dis- continuous tuff and lapilli-tuff seams an inner, 1–2-cm-thick nonfoliated zone that varies from slightly browner to slightly whiter than the typical white- weathering lobe interior and an outer yel- lowish, schistose zone several centimetres thick L .D . Ayres , A .S . Peloquin Precambrian Research 101 2000 211 – 235 223 Table 2 Continued Comments Facies Shape and dimensions Description Internal features Monolithic, unbedded tuff and Thin lenses to almost equidimen- No internal bedding although clast size is Best developed in the Mostly tuff and less abundant lapilli-tuff containing sparse, variable across some units; lobes highly upper part of the facies sional areas that range in thick- lapilli-tuff containing small large, white-weathering, isolated lobes that only rarely ex- association; gradational, tend beyond the facies boundary Figs. 13, 15 and 17, and fractured, with fractures marked by nar- ness from 1 to 20 m Figs. 13 rhyolite lobes interfingering 25–60, smaller, rounded to lenticular, white- and yellow- row zones of very fine-grained quartzo- and 15; includes both mappable boundaries with adja- lobes as much as 2 m thick and feldspathic recrystallization rather than weathering lobes that vary considerably in trend over dis- 15 m long, and smaller unmap- veins; lobe margins vary from brecciated tances of 5–10 m within units, and vary in distribution from cent heterolithic tuff to nonbrecciated, the degree of breccia- pable lobes 0.5–2 m long and lapilli-tuff contain- closely packed to isolated; where the matrix is lapilli-tuff, there is commonly a complete range in size from 2-m-long tion is more intense in yellow-weathering ing isolated lobes that lobes to 1-cm clasts, with the decimeter-size population in- only rarely extend than in white-weathering lobes and over- all it is greater than in isolated lobes; cluding both small, round to lenticular lobes and angular across the boundary Figs. 13 and 15; sharp many of the small, yellow-weathering blocks apparently broken from lobes; textures in the tuff matrix are poorly preserved, and the tuff is a very fine- lobes are surrounded by breccia zones as contacts with isolated much as 30 cm thick, and these lobes lobes and closely grained quartz+plagioclase+sericite aggregate in which packed lobe units variations in sericite content outline ash- to small lapilli- grade outward into, and are almost indis- tinguishable from, the adjacent yellow- size, lenticular to locally equant areas that appear to repre- weathering monolithic matrix sent original clasts 0–40, white- to yellow-weathering, aphyric to porphyritic, Bedded, heterolithic tuff, Unbedded to internally bedded with Discontinuous and lenticular to Irregular map distribu- lapilli-tuff, and minor tuff- locally flow-layered, generally nonamygdaloidal rhyolite sharply to gradationally bounded, thick tion Fig. 13 that is a irregular units that range in function of the spatial to very thick, mostly ungraded beds lapilli; lapilli are mostly 0.2–5 cm, but locally up to 20 cm breccia thickness from several metres to defined by variations in crystal content, relationship with the \ 25 m long, and are angular to rounded to oval and lenticular in clast size, abundance of lapilli, and abun- shape; clast elongation parallels both bedding and a ubiqui- two other facies; in places, this facies partly tous metamorphic foliation in the matrix; many lapilli are dance of intermediate to mafic clasts; rare spherulitic and were probably originally vitric, and they are normal grading, reverse grading, and to completely envelops texturally and compositionally identical to the larger lobes, the other two facies cross-bedding; beds vary from relatively from which they were probably derived; matrix composed continuous and \50 m in strike length to lenticular and only several metres long; largely of somewhat elongated, felsic ash and small lapilli that vary from nonamygdaloidal to pumiceous Fig. 4; in some lenticular beds probably occupy scours Fig. 16. In places, bed strike can matrix, primary clast shapes largely masked by metamor- phic recrystallization, but better preserved, small lapilli were change by 10–25° over distances of only several tens of metres with lower beds originally angular; in any given bed, these felsic clasts are texturally variable and range from aphyric to porphyritic truncated by overlying beds along ero- sional surfaces; local interbeds of other with as much as 15 phenocrysts; plagioclase phenocrysts are generally more abundant than quartz phenocrysts; most facies of these clasts lack the spherulitic textures characteristic of the larger lapilli; other components include trace to 10 plagioclase and quartz pyrogenic crystals, and sparse inter- mediate to mafic volcanic clasts; some intermediate to mafic clasts are tourmaline rich and were possibly derived from a source that was altered prior to brecciation and incorpora- tion in the heterolithic unit a Similar facies also occur in more poorly exposed parts of the association. Fig. 14. Small, equidimensional, rounded, rhyolite lobe within monolithic unbedded tuff and lapilli-tuff facies of southern lobe and volcaniclastic facies association. Lobe has sharp boundaries with, and is distinguished from, enclosing foliated lapilli-tuff by slightly whiter colour and better developed frac- tures; adjacent to lobe, foliation in lapilli-tuff parallels lobe boundary. This is a possible cross-section through a tubular lobe. Drawn from a photograph. Many lobes that are \ 1 m thick have discon- tinuous, thin internal domains, defined by a higher degree of fracturing, incipient brecciation, and a more yellowish weathering that typically accompanies a higher degree of sericitic alteration Figs. 16 – 18. These domains, which pinch and swell, are generally only several centimetres thick, but locally they swell into pockets, as much as 50 cm thick. These pockets contain 0.5 – 20-cm-long, round to angular clasts of rhyolite, apparently broken from adjacent lobes, in a schistose tuffa- ceous matrix Fig. 18. These domains, which are most common in the upper part of lobes, outline lenticular to ovoid to irregular areas of massive rhyolite, generally B 1 m across Figs. 16 – 18, that appear to be closely packed, small lobes within the larger lobe. These small internal lobes differ from lobes of close-packed lobe subfacies Table 2; Figs. 13 and 15 in being smaller and generally less well defined. The lobes vary from pumiceous Fig. 5 to nonvesicular; in some lobes amygdules are re- stricted to flow margins Bailes and Syme, 1989, but in other lobes they appear to occur through- out the lobes. Lobes vary from aphyric to plagio- clase-phyric to quartz 9 plagioclase-phyric; the groundmass is uniformly spherulitic throughout the lobes, attesting to its original vitric nature, and there is only limited evidence of marginal quenching. Lobes in the lower 15 m of the south- ern facies association contain 10 – 15 quartz + plagioclase phenocrysts Fig. 13, and many clasts in spatially associated heterolithic tuff, lapilli-tuff and minor tuff-breccia facies have similar phe- nocryst contents. Most lobes higher in the associ- ation, but below the plagioclase-crystal-rich unit, and clasts in spatially associated heterolithic tuff, lapilli-tuff and minor tuff-breccia facies are aphyric or contain sparse quartz 9 plagioclase phenocrysts, although a few lobes and lapilli-tuff beds here have higher phenocryst contents. The upward change in phenocryst population is com- parable to that in the northern facies association. The phenocryst-rich lobes and volcaniclastic rocks are comparable to the nonbrecciated facies ex- posed on the small island in Manistikwan Lake and the aphyric to phenocryst-poor lobes and volcaniclastic rocks are comparable to flows and domes of sequences 1 – 5. mostly pointed but some are rounded, lobate, or blunt Figs. 14 – 20. Upper and lower margins of lobes vary from relatively smooth and curving to undulating to lobate. Generally the lower margins of lobes are smoother than the upper margins. Lobe margins also vary from sharp and nonbrec- ciated, to sharp and brecciated with breccia confined within the lobe Fig. 21, to gradational and brecciated with the broken pieces intimately mixed with enveloping finer volcaniclastic material. Because of the generally flat nature of the expo- sure, the three-dimensional shape of the lobes is uncertain, but at least some of the lobes appear to be tubular. Evidence for this interpretation comes from a single cross-section through a sequence of close-packed lobes Fig. 20. In this exposure, lobes that are several metres long on the outcrop surface terminate 50 – 75 cm below the outcrop surface in a perpendicular cross-section Fig. 20. Thus, variation in shape among lobes is probably a function of differences in both lobe volume and the orientation of lobes relative to the outcrop surface. For example, the small equidimensional, typically rounded lobes could also be cross-sec- tions through tubes Figs. 14 and 15. Many lobes contain sparse, centimetre-size pla- gioclase-phyric xenoliths that have a very fine- grained groundmass containing felted plagioclase laths. Texturally the xenoliths do not resemble any of the lobes or flows and the more crystalline nature of the xenoliths suggests slower cooling at depth. The xenoliths were probably derived from an earlier magma that was emplaced and crystal- lized at shallow crustal depths. 5 . 2 . Isolated rhyolite lobes within a heterolithic 6 olcaniclastic host 5 . 2 . 1 . Description These typically white-weathering rhyolite lobes form : 15 of the southern facies association, and they are concordant with, and readily distin- guished from enclosing yellowish heterolithic vol- caniclastic facies. Characteristic features include low aspect ratio Figs. 13 and 15, wide range in size, distinctly pointed terminations, generally sharp, nonbrecciated contacts with heterolithic tuff and lapilli-tuff, and, in some lobes, a mar- ginal zone of smaller lobes and blocks, some of which are irregular in shape as if plastically de- formed while still hot Fig. 18; Table 2. In places, several lobes of varying sizes occur at the same horizon, where they are partly enveloped by tuff- breccia containing both small lobes, some of which are internally brecciated, and blocks and lapilli Fig. 16. These spatially associated lobes and associated tuff-breccia form concordant stratigraphic units 1 – 2 m thick that are in sharp contact with both underlying and overlying het- Fig. 15. Detail of parts of Fig. 13 showing shapes of larger rhyolite lobes within both heterolithic lapilli-tuff and monolithic tuff and lapilli-tuff. Note the wide variation in length to width ratio of lobes. This figure also illustrates the lenticular nature of close-packed lobe units and the rapid lateral termination of monolithic, unbedded tuff and lapilli-tuff facies that contains small rhyolite lobes. Fig. 16. A large aphyric rhyolite lobe and numerous smaller lobes occur at the same horizon within massive, crystal-poor heterolithic tuff; younging direction is towards top of figure. White lines represent thin domains that appear to define internal lobe boundaries. The lobes are connected, and partly completely enveloped, by monolithic, apparently cogenetic tuff-breccia that contains angular to irregular blocks, some of which appear to have been plastically deformed while hot. The tuff-breccia is in sharp contact with the enclosing heterolithic tuff, which contains only rare small lobes and lapilli. This figure also shows the lenticular nature of bedding in the bedded heterolithic tuff, lapilli-tuff, and minor tuff-breccia facies. The variable thickness of the heterolithic tuff immediately beneath the lobes suggests that the largest lobe either depressed the tuff, or ploughed into and removed some of the tuff. From mapping by L. Slezak. Fig. 17. Several spatially associated aphyric rhyolite lobes are enveloped in unbedded, crystal-free, monolithic rhyolite tuff and lapilli-tuff; younging direction is towards top of figure. White lines represent thin domains within lobes that appear to define internal lobe boundaries. Lobe terminations vary from smoothly pointed to complexly lobate with inderdigitating lenses of apparently cogenetic tuff-breccia. The largest lobe is overlain by apparently cogenetic tuff-breccia that contains small lobes and angular to irregular broken pieces of rhyolite. The contact between tuff-breccia and the enveloping tuff and lapilli-tuff is sharp. From mapping by L. Slezak. erolithic tuff and lapilli-tuff Fig. 16. Some iso- lated lobes have tails, as much as 10 m long, composed of small lobes and angular to irregular blocks within a tuff matrix Fig. 19. The irregular blocks also appear to have been plastically de- formed. These lobes with tails are also concordant stratigraphic units. 5 . 2 . 2 . Interpretation Although the shape of many isolated lobes is compatible with an intrusive origin Figs. 13 and 15, we believe that these lobes are miniflows that flowed over a surface composed of heterolithic volcaniclastic deposits rather than intrusions. However, we cannot preclude an intrusive origin Fig. 18. The well exposed, northern end of the largest isolated rhyolite lobe in the southern lobe and volcaniclastic facies association see Fig. 13 for location shows the complexity of external and internal contact relations; younging direction is towards top of figure. The rhyolite lobe contains quartz and less abundant plagioclase phenocrysts. The lower boundary of the large lobe varies from smooth and gently curving to lobate with pillow-like projections downward into the underlying, very thick bed of crystal-rich heterolithic tuff; there is only local marginal brecciation and only rare, apparently isolated small lobes below the large lobe. Within the large lobe, discontinuous, thin domains or boundaries defined by increased fracture intensity, increased alteration, and incipient brecciation locally merge with pockets of monolithic tuff and lapilli-tuff containing pieces broken from adjacent lobes; these boundaries, only the most obvious of which are shown on the figure, outline smaller closely packed lobes. The upper boundary of the large lobe also varies from smooth and curving to lobate, but it is overlain by a discontinuous zone, as much as several metres thick, of small, rounded to irregular, isolated lobes that are separated by apparently monolithic tuff that contains angular clasts broken from the adjacent lobes. Some of the irregular lobes appear to have been plastically deformed while hot. This zone is sharply overlain, in turn, by heterolithic lapilli-tuff that lacks lobes not shown. From mapping by C. Nikols. for some lobes, particularly the few lobes that cross the boundary between heterolithic and monolithic tuff and lapilli-tuff facies e.g. Fig. 15. The flow interpretation is supported by the con- cordant attitude of both single lobes and lobe plus tuff-breccia units, the concordant tails of small lobes and blocks that occur with some lobes, paucity of cross-cutting relations Fig. 15, and sharp, commonly smooth basal contacts of many of the isolated lobes with enclosing heterolithic tuff and lapilli-tuff. Unlike many of the isolated lobes in the Grassy Narrows rhyolite, intrusive rhyolite lobes described in subglacial deposits in Iceland are discordant Furnes et al., 1980. Flow may have been by a single pulse, in which case, considering the thinness of many lobes, the lava must have had a low viscosity and deposition was probably on a slope, or by pillow-like lava fingers erupted through fractures in the chilled Fig. 19. Pointed end of a lenticular rhyolite lobe within heterolithic tuff and lapilli-tuff has a tail of broken pieces to right. Upper surface is a relatively smooth curve but lower contact is undulating to almost lobate. Note paucity of obvi- ous marginal brecciation and of broken pieces either above or below lobe. Pencil for scale is 8 mm wide. Fig. 20. Exposure of close-packed rhyolite lobes on a vertical face, oriented at right angles to the outcrop strike of lobes, shows that these lobes rapidly terminate downward into the outcrop; younging direction is to right. This suggests that, in three dimensions, the lobes are tubular. Note the variation in lobe terminations, which range from pointed to rounded to undulating. lobes. Pillow-like morphology is also found in some close-packed lobes Figs. 22 and 23. In some of the larger lobes, there are spatially associated smaller, typically irregular lobes and blocks, most of which occur above the main lobe. These small lobes and blocks occur in a matrix that is monolithic rather than heterolithic and has the same crystal content as the lobe Fig. 18; Table 2, and they probably represent upward injection of flow lobes from the main lobe into a thin hyaloclastite blanket that developed by quenching as the main lobe advanced. The blocks may have been derived by quench fragmentation of the small lobes. The irregular shape of many of these lobes and blocks may be a result of plastic deformation within a weak, but insulating hyalo- clastite blanket. Thus, there is some evidence of upward intrusion, but only into a cogenetic hyalo- clastite, and not into heterolithic tuff and lapilli- tuff. A puzzling aspect of many lobes is the distinctly pointed terminations. Similar-sized lobes de- scribed in other subaqueous rhyolite flows gener- ally have rounded terminations De Rosen-Spence et al., 1980; Furnes et al., 1980, although many lobes depicted by Yamagishi and Dimroth 1985, Fig. 2 have pointed or both pointed and Fig. 21. Brecciation at lower margin of a rhyolite lobe. Pen : 13 cm long is close to margin of lobe; the 8-cm-wide clast immediately below pen is in the enclosing lapilli-tuff and is not part of the lobe. Note variable nature and degree of develop- ment of breccia along margin. Breccia grades upward into nonbrecciated rhyolite at top of photograph. Fig. 22. Left side of photograph is rounded end of a 5.5-m- long rhyolite pillow lowermost large pillow of Fig. 23; younging direction is towards top of photograph. The chilled margin is indicated by the slightly paler colour of the pillow edge, which is in contact with interpillow tuff. Small rounded rhyolite areas below, above, and to right of pillow are either small pillows or deformed blocks in which deformation must have occurred while the block was still hot. Pencil for scale is 8 mm wide. crust of a progressively advancing, relatively fluid flow. The latter alternative is probably more likely, and it is supported by the internal domains found in many of the larger lobes Figs. 16 and 18, the inferred tube-like shape of some small lobes Figs. 14 and 20, and the generally non- brecciated margins and sharp contacts of many Fig. 23. Rhyolite containing sparse quartz and plagioclase microphenocrysts forms close-packed pillow-like lobes within the southern lobe and volcaniclastic facies association location is on Fig. 13; younging direction is towards top of figure. Lobes vary from touching and moulded against each other to separated by apparently cogenetic tuff-breccia containing 40 – 50 angular to irregular blocks and local small rounded lobes. East-west dimension slightly distorted because outcrop face slopes 45 – 60° towards lake. From mapping by C. Nikols. rounded ends. Superficially, the pointed termina- tions look like deformational flattening, but the good preservation of textures in the lobes and in adjacent volcaniclastic beds suggests that tectonic deformation had a minimal effect on the se- quence. We believe that the lobe shapes are largely primary, and any flattening must be the result of extrusion processes possibly combined with load generated by rapidly deposited overly- ing units; flattening must have occurred before devitrification because spherulitic devitrification textures are well preserved. The pointed termina- tions may be the result of necking and severance of lobes, enclosed within a plastic skin, from a main feeder lobe advancing down a relatively steep slope; narrow necks connecting rhyolite ‘blocks’ have been described by Kano et al. 1991. Such severed lobes could then slide farther down the slope producing trains of lobes and associated broken material Fig. 16. In Fig. 16, the change in thickness of the underlying het- erolithic tuff as a direct function of thickness of overlying lobes and the slight depression of the contact between this tuff and underlying lapilli- tuff could be the result of lobes sliding down a slope. This process would be analogous to some of the stone streams produced at the front of pillowed basaltic flows on the flanks of seamounts Lonsdale and Batiza, 1980. 5 . 3 . Close-packed rhyolite lobes 5 . 3 . 1 . Description These units, which form : 25 – 35 of the southern facies association, differ from the iso- lated lobes in the more obvious composite nature Fig. 23 and the higher aspect ratio of both lobes and lobe units Fig. 13; Table 2. Most units occur in the central and south part of the southern facies association where there is a crude inverse relationship between the distribu- tion of lobe units and of large isolated lobes Fig. 13. Relative to the flows and domes of the northern facies association, the lobe units would be more distal than many of the large isolated lobes. 5 . 3 . 2 . Interpretation On the basis of lobe shape, moulding of overly- ing lobes against lower lobes, and the relative paucity of intervening volcaniclastic material Table 2, we infer that the close-packed lobe units of the Grassy Narrows rhyolite are small pillowed lava flows produced by more restricted advance of lava fingers down a slope. These char- acteristics are not compatible with an intrusive origin. The lenticular shape of the close-packed lobe units suggests that the axis of flow advance was at some angle to the present erosion surface, and the more distal location could represent accu- mulation in topographically lower areas, possibly base of slope. The close-packed lobes, particularly those in the 4-m-thick unit shown in Fig. 23, resemble rhyodacite pillows described by Bevins and Roach 1979. 5 . 4 . Monolithic unbedded tuff and lapilli-tuff containing small rhyolite lobes 5 . 4 . 1 . Description This facies occurs throughout and forms : 25 – 35 of the southern facies association, although it is best developed in the upper part. The facies occurs as thin lenses to almost equidimensional units interlayered and interfingered with other facies Figs. 13 and 15; Table 2. Isolated lobes that occur within this facies only rarely cross the gradational boundary with heterolithic tuff and lapilli-tuff facies. The facies differs from other units in lack of bedding, abundance of small, unmappable lobes, many of which grade into the tuff matrix across a wide zone of brecciation, and poorly preserved but relatively uniform textures in the tuff matrix Table 2. The lobes are typically aphyric and the tuff matrix generally lacks crystals. 5 . 4 . 2 . Interpretation The poor preservation of textures in the rhy- olitic tuff and lapilli-tuff matrix of this facies precludes a specific genetic interpretation of the matrix component. However, this poor preserva- tion may also be a clue to the genesis. The much better textural preservation in adjacent rhyolitic heterolithic tuff and lapilli-tuff Table 2 suggests that the difference in textural preservation be- tween the two types of tuff and lapilli-tuff is a reflection of primary differences in rhyolitic clasts with the better preservation in heterolithic units being a function of textural variability among clasts. Accordingly, we infer that the poor preser- vation, yet relatively consistent textures of the matrix of this facies, reflects an original mono- lithic, vitric tuff. Within the facies, the lack of bedding, abun- dance of small lobes, variable distribution of these lobes, apparently monolithic character of the en- closing tuff and lapilli-tuff, presence of lapilli and blocks that are texturally similar to the lobes, and overall shape of units Table 2 all support a cogenetic relationship of the lobes and enclosing tuff and lapilli-tuff. From the preceding character- istics, we also infer that the tuff and lapilli-tuff are a hyaloclastite produced by extensive quench frag- mentation from advancing lobes. Unlike the minor hyaloclastite blankets associ- ated with isolated lobes interlayered with het- erolithic volcaniclastic units Fig. 18, hyaloclastite is the dominant component of the monolithic units. Furthermore, the lenticular to almost equidimensional, cross-sectional shape of the monolithic units, some of which have thick, blunt interfingering relations with adjacent bed- ded heterolithic tuff and lapilli-tuff Figs. 13 and 15, is not compatible with extrusion on a deposi- tional surface of volcaniclastic deposits. The shapes and relations with adjacent units, however, are compatible with near-surface intrusive com- plexes where intrusion of lobes into preexisting water-saturated, heterolithic volcaniclastic units resulted in concomitant development of the sur- rounding hyaloclastite by quench brecciation. This intrusion model is similar to that proposed by Furnes et al. 1980 for subaqueous rhyolite units in Iceland where the intrusive lobes and hyaloclastite envelopes are interlayered with ex- plosively generated tephra units. Intrusion is also supported by the variable attitude of lobes within individual monolithic units and the irregular shape of some lobes Fig. 17. 5 . 5 . Bedded heterolithic tuff, lapilli-tuff and minor tuff-breccia 5 . 5 . 1 . Description This facies forms : 25 – 35 of the southern association, and it is interlayered with, and partly encloses the other facies; the high aspect ratio of both close-packed lobe and monolithic facies re- sults in an irregular distribution of the heterolithic volcaniclastic facies Figs. 13 and 15B. Although clasts in the tuff, lapilli-tuff and tuff-breccia are dominantly felsic, there is a wide variation in internal textures. The larger clasts are spherulitic and are texturally very similar to the lobes, but the smaller clasts lack spherulitic textures and clasts within individual beds have a wide variation in phenocryst populations Table 2. The het- erolithic character of the beds is shown by both textural variation of the felsic clasts and the sparse mafic and intermediate clasts Table 2. In the lower 15 m of the association, felsic clasts are mostly porphyritic with many contain- ing 10 – 15 phenocrysts. These beds also contain 2 – 10 pyrogenic quartz and plagioclase crystals. Higher in the association, felsic clasts in most lapilli-tuff beds have a much lower phenocryst content, and many clasts are aphyric; there are only rare pyrogenic crystals. There are, however, isolated beds that are more comparable to those in the lower 15 m of the association. The boundary between the lower phenocryst-rich and upper phenocryst-poor parts of the association is relatively sharp Fig. 13. 5 . 5 . 2 . Interpretation The textural variation in felsic clasts of this facies indicates that the clasts were derived from a variety of sources and were mixed together either during eruptive or transportation processes. The larger felsic lapilli, which are texturally similar to the lobes, flows and domes, were probably derived from brecciated portions of lobes. However, the texturally dissimilar and texturally variable, smaller felsic clasts must have been derived from a different source, possibly by phreatomagmatic eruptions from a vent that was outside the present plane of exposure. An eruptive origin is supported by the felsic composition of most clasts, the angu- larity of some clasts, the presence of some pumiceous clasts, and the general similarity of crystal contents of lobes and spatially associated tuff and lapilli-tuff. Phreatomagmatic eruptions are supported by the mixture of various textural types of felsic clasts, by the low vesicularity of many clasts Houghton and Wilson, 1989, and the relative paucity of pyrogenic crystals. Once erupted, tephra fell through the water column to form pyroclastic deposits, which then were transported and resedimented, at least partly, by water currents or other downslope transport. Such resedimentation is indicated by the lenticularity of some beds, by rapid changes in bed attitude with truncation of lower beds by overlying beds, by the widespread distribution of spherulitic clasts derived from lobes, and by the mixing of textural types. The mixing of clast types is also compatible with clast derivation from ero- sion of texturally different felsic flows outside the present plane of exposure. However, a strictly erosional origin of clasts is unlikely considering the similar upward change in crystal content of both lobes and heterolithic tuff and lapilli-tuff, the widespread basaltic units elsewhere in the sequence but paucity of basaltic clasts, and the submarine setting of the rhyolite flows in the exposed sequence. The wide distribution of the heterolithic units, both laterally and within the stratigraphic sequence Fig. 13, combined with the upward change in crystal content, suggests that explosive activity persisted for a relatively long period of time, although it was periodically interrupted by more rapid lobe and flow emplace- ment events.

6. Stratigraphic relations between lobe and volcaniclastic facies association and dome-flow