Regional Geology, Sioux Lookout Orogenic Belt - Geology Ontario

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Page and Clifford (1977) interpreted the predominantly pyroclastic succession in the upper part of the the Northeast Bay succession as proximal facies of a “composite cone” (stratovolcano) or “vent complex,” and inferred that the adjacent Northeast Bay pluton or dioritic complex (see below) was the coeval subvolcanic magma chamber source of the proximal volcanic facies. Subsequent work (cited above) and the present study confirm this interpretation. However, Page and Clifford (1977) did not explicitly describe subaerial facies. Pending the completion of a M.Sc. thesis by D. Babin. The following description is summarized from discussions with D. Babin and the author’s field observations. Eastern Predominantly Basaltic Units The eastern (lower) part of the volcanic succession in the Northeast Bay area is composed predominantly of mafic flows with well defined pillows. Flows (or flow units) with an internal east--to--west stratigraphy of massive to pillowed to pillow breccia facies (tops to the west) are common. Some units are porphyritic, with large (centimetre--scale) and abundant plagioclase phenocrysts. The pillowed flows indicate a subaqueous depositional environment (e.g., a mafic plain or shield volcano). Physical volcanological details include examples of: 1) top indications via vesicles concentrated near the arcuate, convex--to--west rims (upper parts) of pillows; 2) pillow breccia fragments near the inferred top of one flow unit that appear to have sagged down, as load structures, into the formerly soft flow below; 3) a metre--scale “infolded cusp” shape at one flow contact, suggestive of localized loading of part of an upper flow downward into a partly cooled lower flow; 4) westward (upward) increases in the abundance of vesicles/amygdules within flows; 5) scoria, highly vesicular fragments in lapilli tuff or pillow breccia; and 6), at northern Alcona Bay, pillow breccia with a matrix containing superb examples of small clasts with the form of vitric shards (e.g., triangular and cuspate shapes, and some internally jigsaw--fractured clasts). Minor amounts of interbedded felsic tuff or wacke--siltstone exhibit grading, lamination, cross--lamination (ripples), load structures, and rare scours. These units are interpreted as interflow sediments deposited in deep water (below wave base). Western Predominantly Andesitic Pyroclastic Units The mixed volcanic and sedimentary succession in the western (upper) part of the Northeast Bay area is notable for its basaltic andesite composition and the excellent preservation of a wide variety of pyroclastic and sedimentary features. Pyroclastic to variably reworked “volcaniclastic” facies are predominant, with less common flows and dikes. STRATIFICATION Bedding is crude to well defined, with crude bedding defined by subtle variations in clast size and sorting that form layers parallel to local well defined bedding. Massive horizons are more poorly sorted and lack obvious fabrics. Coarse massive to poorly internally stratified beds suggest deposition by gravity flows, either hot pyroclastic flows or cold sediment gravity flows. It is common for the coarser beds to be relatively thick and finer beds to be relatively thin. The typical proportionality of grain size to bed thickness is a primary sedimentological relationship (and is widely useful for recognizing primary layering in deformed metasedimentary rocks). PRIMARY FRAGMENTAL FABRICS Most of the tuffs and the local clasts, flows and dikes contain coarse plagioclase grains or crystals. Because the clasts and matrix may be very similar in appearance, it can be difficult to distinguish porphyritic clasts 20
(plagioclase--phyric clasts are very common) from crystal tuff matrix (similar size and appearance of plagioclase grains) in this area. Geochemical analysis shows that the local tuffs, clasts, flows, and dikes are of strongly similar andesitic composition (see Geochemistry section), reflecting their co--magmatic origin. In the coarser beds, both the clast frameworks and matrix tuff are typically poorly sorted. Clasts may be tightly packed, particularly in more well sorted lapillistone beds, or beds may appear loosely clast--supported, transitional to a more matrix--supported lapilli tuff CLAST COMPOSITION Although most or all clasts are of intermediate (andesitic) volcanic composition, and thus should be classed as oligomict in composition, some beds contain multiple types of intermediate clast lithologies and thus appear polymict. Beds described in the field as “variably polymict” or “slightly polymict” include “texturally polymict” facies (e.g., clasts are andesitic but are of different grain sizes, and some clasts may be porphyritic). Local contrasts in the composition of clasts in different beds or units (e.g., oligomict versus more polymict compositions, amygdaloidal versus porphyritic clasts) reflect compositional stratification. Leucocratic, apparently “felsic” clasts may be light--weathering intermediate composition clasts. Clusters of similar to identical clasts define compositional bedding, which may reflect individual pyroclastic flows, (e.g., a block and ash flow sourced from a lava flow during dome collapse), or local sedimentary reworking of a volcanic flow. One outcrop contains a rare example of beds of matrix--supported tuff--breccia with porphyritic clasts that have angular, fractured--looking outlines and some jigsaw (interlocking) shapes (Devaney and Babin 1996, Photo 9.2), suggesting emplacement as a pyroclastic breccia: e.g., a block and ash flow or avalanche deposit, with cooling--cracked clasts. Other notable aspects of clast composition include: 1) an unusual oligomict mega--clast band (representing some type of sediment gravity flow?); 2) very rare coated bombs, with features interpreted to be an asymmetric coating and a coating coarser than the bomb clasts; 3) clasts in one bed with discontinuous rims in the matrix surrounding the clasts, interpreted as “heat--welded” rims that developed in a hot pyroclastic flow; 4) rare clasts with spherules, suggesting a devitrification texture developed in vitric clasts; 5) clasts with highly vesicular (“bubble--supported”) texture, interpreted as pumice clasts; and 6) rare large flow--banded clasts (suggesting erosion of local proximal lava flows). CLAST SHAPE Clast outlines vary from angular to round, with increased rounding presumably reflecting an increased degree of sedimentary reworking. Angular shapes are triangular, diamond, hexagonal, and polygonal (polyhedral); the latter were likely sourced from jointed flows or blocks, with little or no sedimentary reworking. Unusual and rare clast shapes include: 1) a jigsaw micro--fractured clast rim, interpreted as a quench fabric of subaqueously erupted or peperitic lava; and 2) an outcrop horizon with large straight--edged clasts, interpreted as products of steam--fracturing (phreatic or phreato--magmatic activity) and disaggregation of solidified lava flows. CLAST SIZE Coarser lithofacies such as tuff--breccia and lapillistone are abundant; this western succession is not dominated by tuff. In the Northeast Bay area, relatively large clasts have long dimensions of 30 cm or more. The largest clast identified was a 1.8 m by 1.8 m mega--block, and one large, carefully studied outcrop contains a partly exposed rock body thought to be a >10 m mega--clast (Devaney and Babin 1996, Photo 9.1). The typically small size of outcrops and fairly continuous exposed surfaces (metres to tens of metres) limits resolution of very large clasts and very thick beds. GRADING Grading in pyroclastic rocks is problematic for field geologists in deformed successions because, particularly in coarser beds, pyroclastic flows and similar sediment gravity flows may either fine or coarsen 21
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 42 and 43: upward. Examples of both westward--
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
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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
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Nilsen, T.H. and McLaughlin, R.J. 1
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Sage, R.P., Breaks, F.W., Stott, G.
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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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