Stratigraphic relations between lobe and volcaniclastic facies association and dome-flow Discussion

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

complex The well exposed southern lobe and volcani- clastic facies association is separated from the well exposed northern brecciated and nonbrecciated facies association, which is interpreted to be a sequence of five flows and domes, by a bay of Manistikwan Lake Fig. 3. Exposure of contacts between these two facies associations is restricted to two locations north of the bay. Flow 3 overlies a lobe and volcaniclastic sequence with a sharp contact Figs. 3 and 7, and a lobe and volcani- clastic sequence overlaps dome 2 with a sharp contact Fig. 6. Flow 4 is inferred to overlie a lobe and volcaniclastic sequence, although the actual contact was not observed Fig. 3. All other contacts are covered either by overburden or Manistikwan Lake. Where the lobe and volcaniclastic facies associ- ation overlaps dome 2, the lower 25 m of the sequence immediately above the dome is thick- bedded, lobe-free, heterolithic tuff and lapilli-tuff. The contact with crackled breccia of the dome is sharp, but, in the lower 2 – 3 m of the heterolithic unit, there are 5 – 10, 1 – 5 cm long, white-weath- ering clasts that resemble pieces of, and may have been derived from the crackled breccia. The first lobes were observed : 50 m above the dome.

7. Discussion

Except for the lowermost units, the Grassy Narrows rhyolite lava flows, lobes, and volcani- clastic units have a very low phenocryst content Fig. 13. This low phenocryst content, and, in places, lack of phenocrysts, suggests that, when erupted, much of the magma was close to liquidus temperatures, and thus had a relatively low vis- cosity. Similar interpretations have been made for other subaqueous rhyolite flows and associated lobe units Yamagishi and Dimroth, 1985. The eruptions apparently tapped a zoned chamber that had more crystal-rich, possibly cooler magma at the top. This early magma contained 8 – 15 quartz and plagioclase phenocrysts and is repre- sented by the lowermost large lobe and underlying heterolithic beds of the southern lobe-volcaniclas- tic facies association Figs. 13 and 18, and by the incomplete flow on the small island in Manistik- wan Lake east of dome 1 Fig. 3. Pillows in intercalated and overlying basaltic lava flows indi- cate that the rhyolitic units were extruded sub- aqueously, but water depth is uncertain. 7 . 1 . Origin of flow facies Although the basal contacts of domes and flows are exposed only locally, the paucity, and where present, the thinness, of basal breccia supports the concept of a relatively fluid lava that, as the flow advanced, quenched and locally brecciated on contact with the sea floor. Further advance of the still-liquid flow interior occurred above this quenched base with limited brecciation produced by flow movement. Some laterally extensive, sub- aerial and subaqueous rhyolite flows also lack basal breccia Cas, 1978; De Rosen-Spence et al., 1980; Bonnichsen and Kauffman, 1987; Green and Fitz, 1993. Combined, the upper crackled and disaggre- gated breccia subfacies are much thicker than the upper breccia found on many subaerial rhyolite flows Christiansen and Lipman, 1966; Bonnich- sen and Kauffman, 1987; Dadd, 1992; Manley, 1996 or reported from other subaqueous rhyolite flows Cas, 1978; De Rosen-Spence et al., 1980. In the Grassy Narrows rhyolite, these two subfa- cies form 40 or more of the five domes and flows Figs. 3 and 6; Table 1, all of which have roughly similar thicknesses. The breccia is just the ultimate manifestation of the dominant character- istic of the flows and domes; namely, the ubiqui- tous, closely spaced, diversely oriented fractures that occur in all subfacies but are most pro- nounced in the upper subfacies Table 1. We believe that the great thickness of the two breccia subfacies and the ubiquitous fracturing are proba- bly a result of two simultaneous processes, 1 extended interaction of the lava with the sur- rounding water column, combined with 2 dislo- cations produced by flow advance and dome growth. As domes expanded upward and outward, and lava flows advanced, cooling of the upper surface in contact with water led to an increase in viscos- ity, and ultimately to cracking and brecciation as the crust shrank in volume and the liquid interior of the flow continued to move under the quenched crust cf. subaerial rhyolite flows; Christiansen and Lipman, 1966. This initiated the develop- ment of crackled breccia. Continued ingress of water downward along cracks resulted in repeated episodes of cooling and crack development that continued long after the lava solidified. Cracks propagated downward and new generations of cracks propagated inward from the face of older cracks, resulting in progressively smaller un- cracked areas cf. Yamagishi and Dimroth, 1985. The final stage of crack development is related to water ingress along columnar joints; this pro- duced the diversely oriented fracture pattern that characterizes columns. As water moved downward along cracks into the still hot interior of the lava flows and domes, it was heated and expanded as either steam or supercritical fluid. Hydraulic action of this expan- sion would have separated the quenched lava along cracks and rotated the separated pieces Allen et al., 1996. This produced disaggregated breccia with low matrix content, which is mixed with crackled breccia in the crackled breccia sub- facies and is a less common component of the transition and columnar-jointed subfacies Table 1. The final stage of hydraulic action was the localized breccia that occurs along the margins of joint columns. Continuing dome growth or flow advance would have resulted in breaking and crumbling of crackled breccia, particularly on the steep front or sides of a flow or dome. This process produced a second type of disaggregated breccia or crumble breccia, which forms the separate, but variably developed, disaggregated breccia subfacies. Early development of this subfacies is supported by the local upward projection of spines of transition subfacies into disaggregated breccia and down- ward projection of disaggregated subfacies into crackled breccia. The relationship of the disaggre- gated subfacies to flow margins is indicated by the discordant nature of the subfacies, particularly in dome 1 Fig. 6, the spatial relationship of the subfacies to flow margins in dome 1 and flow 3, the sharp contact between this subfacies and other subfacies, and the large volume of the subfacies in flow 5. 7 . 2 . Relationship of lobe and 6olcaniclastic facies association to domes and flows The rhyolite lobe and volcaniclastic facies asso- ciation is somewhat similar to lobes that have been described from a number of subaqueous rhyolite flows elsewhere, but the facies relations are different. Dimroth et al. 1979 and De Rosen- Spence et al. 1980, for example, proposed that lobes in Archean flows at Noranda were pillows that formed at the front of advancing, thick, submarine rhyolite flows. However, unlike the Grassy Narrows rhyolite lobes, the Noranda lobes occur within a monolithic hyaloclastite produced during flow advance, not a heterolithic volcani- clastic sequence, and they typically show foreset cross-bedding De Rosen-Spence et al., 1980. They are a medial to distal facies of unbrecciated lava flows. As described above, there are both vertical and lateral facies changes within the Grassy Narrows rhyolite flows and domes. How- ever, all facies are integral parts of the flows and domes, and there is no evidence that the flows and domes grade laterally into the lobe and volcani- clastic facies association. Instead, the lobe and volcaniclastic facies association appears to be a distinct genetic entity that is interlayered with flows and domes, not a facies of the flows and domes. Where the lobe and volcaniclastic facies association immediately overlies dome 2, there is a gradational zone in which fragments apparently derived from the dome are incorporated in overly- ing heterolithic tuff; the boundary between the two facies associations is, however, sharp, and the gradation is the result of mechanical mixing, pos- sibly as a result of some downslope sliding. Furnes et al. 1980 described lobes in Quater- nary rhyolite deposits in Iceland that are not associated with lava flows. They proposed that the lobes were intruded into rhyolite pyroclastic deposits, and, during intrusion, quench fragmen- tation produced a hyaloclastite envelope that sur- rounds the lobes. This mechanism is similar to that envisaged for the Grassy Narrows monolithic unbedded tuff and lapilli-tuff facies containing small rhyolite lobes. This facies resulted from intrusion of lobes into resedimented pyroclastic deposits and concomitant development of a hyaloclastite envelope. Bailes and Syme 1989 have proposed a similar origin for all of the Grassy Narrows lobes, but we believe that the genesis of the lobe and volcaniclastic facies associ- ation is more complex. The stratigraphy of the southern lobe and vol- caniclastic facies association also precludes a di- rect facies relationship to flows and domes. The southern association comprises interlayered iso- lated lobes, close-packed lobes, lobe and hyalo- clastite units, and resedimented pyroclastic deposits. Within this sequence, there is an abrupt upward change in phenocryst population and abundance of lobes that resembles the upward change observed in the northern dome-flow se- quence. Thus the lobe and volcaniclastic facies association developed in the same time period as the domes and flows were erupted farther north. The coeval nature of the southern and northern facies association is also supported by the plagio- clase-crystal-rich unit that occurs in both associations. We believe that the southern lobe and volcani- clastic facies association represents an incomplete subaqueous tuff cone produced by phreatomag- matic eruptions; the cone was greatly modified by resedimentation and lobe intrusion. Evidence in support of the cone model includes the concomi- tant development of the two facies associations, the marked difference in thickness of the lobe and volcaniclastic facies associations beneath the pla- gioclase-crystal-rich unit in the south and north, the low aspect ratio of rhyolite lobes, which sug- gests eruption on a slope, and the more distal location of the close-packed lobe units. Stratifica- tion in the cone is variable over short distances with evidence of current activity and erosion. The inferred cone grew upward as domes and flows were erupted to the north. In this model, domes 1 and 2 were erupted close to the inner wall of the cone Fig. 3, and the domes may have prevented destruction of the cone by advancing younger flows, and also partly ponded the younger flows. The various types of lobes within the cone may represent overtopping of the cone isolated lobes and injection into the cone mono- lithic units by thin lava tongues produced from thick, partly ponded lava flows. The overtopping is a smaller-scale and subaqueous equivalent of subaerial lava tongues associated with the Bad- lands rhyolite flow of Idaho Manley, 1996.

8. Conclusions