Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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upward. Examples of both westward--fining (normal graded, fining--upward) tuff--breccia and westward--coarsening (inversely graded, coarsening--upward) tuff--breccia and lapilli--tuff were identified in the Northeast Bay area. Reliable top indications from such types of grading require corroborating top indications from within the same outcrop of a structurally undisturbed sequence of beds. The best examples of inverse grading are from an outcrop in which other types of top indications to the west are present (e.g., faintly defined cross--laminae, open matrix--filling laminae texture), and also because in the entire Northeast Bay succession tops to the west are ubiquitous, the coarsening--west beds are considered to be inversely graded. Graded tuffs are not common, suggesting that neither turbiditic subaqueous flows nor direct deposition of air fall or waterlain graded tuffs were important processes in the deposition of the finer units of the succession. SEDIMENTARY STRUCTURES One outcrop contains thick beds of well laminated tuff/sandstone (well defined planar and parallel laminae; parallel to the strike of other local beds and thus probably orginally horizontal laminae) which may be interpreted as either airfall tuff or fluvial sheetflood deposits. A laterally discontinuous band of clasts between laminated horizons represents either volcanic bombs or a sedimentary erosional lag horizon. Only one bed containing well defined volcanic bomb--sag laminae was identified. Crossbedding is rare. Cross--lamination (ripple foresets) is uncommon, with cross--sectional views of locally very well preserved ripple features. Apparent paleocurrent directions of the cross--laminae, defined by steep foreset slopes and the preserved stoss and lee shapes of some asymmetric ripple shapes, are nearly always to north (to northwest). The ripples appear to be asymmetric current ripples; no symmetric ripples or wave ripple structures were identified. Most of the ripples are in units or stratigraphic horizons interpreted to have been deposited in a subaerial setting, and the consistent paleocurrent direction is thought to reflect fluvial transport in an environment with a high--angle paleoslope (e.g., flank of a stratovolcano reworked by braided streams). Rare examples of slump facies include contorted layering in tuff beds, and very rare soft--sediment injection fabrics which may be related to slumping. A spectacular >10 m block (the size is approximate because the block or mega--clast is larger than the outcrop) with flanking conglomerate beds, pebble bands and sandstone beds is interpreted as a slump block buried by fluvial deposits. The block is internally stratified, an erosion surface cross--cuts the intra--block layering, and the onlapping clastic beds, which are parallel to the generally north--striking beds in the area, are angularly discordant to the intra--block layering. In one relatively small area adjacent to the block, angular clasts form a small wedge of breccio--conglomerate, with some clasts identical to thinly bedded strata inside the block. The wedge is interpreted as a local colluvium deposit of block--derived talus. At one lapillistone outcrop, a minor amount of quartz is sporadically present in matrix (inter--clast) areas, suggesting the precipitation of quartz via subaerial vapour phase crystallization (McPhie et al. 1993) in inter--clast pore spaces. (Nearby outcrops contain other features suggestive of subaerial deposition.) At one of the outcrops studied by McKenzie (1995), a wide variety of pyroclastic features have been identified by the author, McKenzie, and others. Highlights of this well bedded outcrop include: 1) normal and inversely graded lapilli--tuff beds, some of which are low--angle strata (i.e., having depositional dip angles several degrees from the paleo--horizontal), interpreted as pyroclastic surge deposits, with the low--angle strata similar to that of the large--scale antidune deposits of Rowley et al. (1985); 2) unusual small clasts with round to ovoid outlines, some of which appear indented or deformed, and some of which display fine concentric laminae, interpreted as accretionary lapilli; some of these soft clasts were dented during deposition; 3) very angular and broken--looking clasts (including odd “ridgy” textured clasts), suggestive of the explosive fragmentation of solidified (preconsolidated) lava; and 4) one bed with discontinuous rims in the matrix surrounding large clasts (as noted above, interpreted as heat welding of the matrix by hot bomb clasts). 22
Part of this outcrop (about 1 m of section) contains laterally discontinuous bands of accretionary lapilli (described above) which are capped by cross--laminated (rippled) beds, and are interpreted as a genetically linked sequence of beds. In this scenario: a) within an ash cloud, some ash grains formed accretionary lapilli and other grains acted as condensation nuclei for raindrops; b) accretionary lapilli landed on the earth surface; c) during or after the rain event, ephemeral runoff deposited tuffaceous sandy laminae and cross--laminae that buried the accretionary lapilli bands; and d) following a runoff event, mud in slack water pools settled from suspension to form ripple drapes (flaser bedding). Alternative interpretations are available in McKenzie (1995), who recognized neither accretionary lapilli nor ripples, and interpreted this deposit as a “hyper--concentrated flow” (by definition, hyperconcentrated flows do not contain lower flow regime structures such as ripples: e.g., Smith and Lowe 1991). An adjacent outcrop (to the west) displays facies of a more sedimentary upper part of the local cross--section, with crossbedding, a small scour or channel, cross--laminae (ripples), and load structures. LAVA FLOW BRECCIA FACIES: SUBAERIAL DEPOSITS Lava flow facies are a minor component of the predominantly volcaniclastic western andesitic succession. Parallelism of plagioclase phenocrysts locally defines flow banding (regarding this fabric, note that no schistosity is readily observable in these rocks). Autoclastic flow breccias are composed of plagioclase--phyric porphyritic andesite, with ovoid shapes representing vaguely defined breccia clasts and curvilinear “wisps” of darker matrix outlining large “pseudo--clasts.” These subtly defined clast fabrics suggest incipient brecciation. At one unusual site, laminated sandstone is present in one triangular matrix area between clasts. This is interpreted as an open framework--filling “sieve” deposit; sand from the upper surface of the flow, and/or sand produced by the mutual abrasion of blocks within a moving flow, was funneled down into a large open matrix space between clasts to form infilling laminae. Breccias with sharply defined fragments consist of angular, blocky oligomict clasts, some of which fit together in interlocking fabric (jigsaw clasts, or crackle breccia). The degree of brecciation, clast interlocking, and packing density of clasts all vary locally (e.g., over 0.1–1.0 m scale). Such breccias are interpreted as aa flows (rubbly flows or flow tops) or flow--marginal talus deposits. Some clasts are flow banded, suggesting that in some lava flows, laminar flow formed rheomorphic flow banding, followed by cooling, brecciation, and continued flow as an aa--type aggregate of blocks. Because of the high viscosity of these types of flows, indicated by the flow banded and variably developed rubbly flow breccia fabrics, such flows are typically interpreted to have been short and thick (versus the long thin form of low viscosity flows: Cas and Wright 1987) and thus should indicate a vent--proximal setting. The lack of quench fabrics suggest deposition in a subaerial setting. DIKES Dikes are less common than flows in this area. Based on both field observations and geochemical analyses, the dikes are of the same composition as the surrounding clasts, tuffs and flows, demonstrating their comagmatic origin (see “Geochemistry”). A few gabbroic dikes are also present. Most notable are the rare examples of dikes with margins that are “squiggly” (tortuous and embayed, at a 10 cm scale), chilled and vesicular. This squiggly form shows that such dikes intruded into unconsolidated volcaniclastic sediments, with some sagging and loading of the dike margin by the surrounding sediments. Kokelaar (1982) has suggested that the presence of a vapour film at a dike margin would provide the cohesiveness necessary to prevent the peperitic mixing of dike magmas with host sediments. Other examples of unusual dike structures include an outcrop of crystal tuff showing the lateral termination of a dike and irregularly shaped dike margins (boudinaged and swirly to wispy to embayed 23
Page 1: Ontario Geological Survey Open File
Page 5: e Queen’s Printer for Ontario, 20
Page 11: Large--Scale Structures ...........
Page 15: 25. Stage 5 sedimentation and tecto
Page 19: A large volume of basaltic strata (
Page 22 and 23: Many of the Sioux Lookout orogenic
Page 24 and 25: pillowed basalt flows) are not give
Page 26 and 27: Units of chert-magnetite (BIF), up
Page 28 and 29: SANDIER FACIES At central Vermilion
Page 30 and 31: wrench--induced verticalization of
Page 32 and 33: alteration). Low--intensity mafic a
Page 34 and 35: few clues regarding the specific se
Page 36 and 37: seems to be the case here; see disc
Page 38 and 39: 1969, 1972; Trowell et al. 1977, 19
Page 40 and 41: Page and Clifford (1977) interprete
Page 44 and 45: shapes, suggesting intrusion into s
Page 46 and 47: The abundance of very coarse and an
Page 48 and 49: One small island outcrop contains t
Page 50 and 51: AGE AND UNCERTAIN STRATIGRAPHIC CON
Page 52 and 53: and sedimentation). Other good exam
Page 54 and 55: (illustrating how an increased clas
Page 56 and 57: For the present study, attention wa
Page 58 and 59: Structural Geology Because the focu
Page 60 and 61: LARGE--SCALE STRUCTURES Description
Page 62 and 63: As, Pb, Zr, Y); inductively coupled
Page 64 and 65: DISCRIMINATION PLOTS Data from the
Page 66 and 67: It is notable that the Northeast In
Page 68 and 69: (ca. 2734 Ma) 60 km to the east at
Page 70 and 71: The Stage 5 cartoon (Figure 22) is
Page 72 and 73: also during later phases of Stage 5
Page 74 and 75: aquabasinal (lacustrine?) mudstones
Page 76 and 77: Note that similar felsic volcanic r
Page 78 and 79: 22) At Southeast Bay of Minnitaki L
Page 80 and 81: Given that the adjacent Sturgeon La
Page 82 and 83: structurally--controlled Archean me
Page 84 and 85: This andesitic sequence and its com
Page 86 and 87: young as 2701 Ma. However, a few mi
Page 88 and 89: volcanism (interpreted extensional
Page 90 and 91: eyond the resolution of surface bed
Page 92 and 93:
asin would be adjacent to an orogen
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The moderately to steeply dipping,
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References Allen, R.L. 1988. False
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Ciceri, D.L., Cruden, A.R. and Robi
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Dupuy, C. and Dostal, J. 1984. Trac
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Horwood, H.C. 1938. Geology of the
Page 104 and 105:
Nilsen, T.H. and McLaughlin, R.J. 1
Page 106 and 107:
Sage, R.P., Breaks, F.W., Stott, G.
Page 108 and 109:
Trowell, N.F., Blackburn, C.E. and
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Figure 1b. Location of Sioux Lookou
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Figure 3. Roadside cross--section o
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Figure 5a. Geochemical sample locat
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Figure 6. Discrimination plots show
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Figure 8. Plots of selected high fi
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Figure 9. Plots of selected oxides
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Figure 10. Plots of selected trace
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Figure 11. Continued. --77 to --81)
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Figure 12. N--MORB normalized exten
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Figure 12. Continued. m) Northeast
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Figure 14. For all Sioux Lookout or
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Figure 15. Zr/Yb versus Nb/Yb discr
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Figure 17. Discrimination diagrams
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Figure 19. REE plots for representa
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Figure 21. Plot of N--MORB normaliz
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Figure 22b. Corresponding stages of
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Figure 24. Schematic cross--section
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Figure 26. Stage 6 Sioux Lookout or
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Figure 28. Cartoon views of potenti
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Figure 30. Theory of small--scale k
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Figure 32. Riedel flake structure;
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Figure 34. Theoretical shear fractu
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Table 1. Continued. Sample 94JRD--0
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Table 1. Continued Sample 94JRD--00
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Table 1. Continued. Sample 94JRD--0
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Table 1. Continued. Sample 94JRD--B
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Table 1. Continued Sample 94JRD--B1
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Table 1. Continued. Sample 95JRD--B
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Table 2. Continued. Northeast Bay P
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Table 4. Altered Sioux Lookout orog
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Metric Conversion Table Conversion
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Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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