Late-orogenic basins in the Archaean Superior Province, Canada: characteristics and inferences

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Sedimentary Geology 120 (1998) 177–203

Late-orogenic basins in the Archaean Superior Province, Canada: characteristics and inferences W.U. Mueller Ł , P.L. Corcoran Sciences de la Terre, Universite´ du Que´bec a` Chicoutimi, Chicoutimi, Que´. G7H 2B1, Canada Received 22 April 1997; accepted 22 September 1997

Abstract The late-orogenic Archaean Duparquet, Kirkland and Stormy basins of the Canadian Superior Province are characterized by bounding crustal-scale faults and abundant porphyry stock emplacement. Lava flows and pyroclastic deposits are restricted to the Kirkland and Stormy basins, and coarse clastic detritus characterizes the Duparquet basin. Seven distinct lithofacies are identified: (1) mafic volcanic, (2) felsic volcanic, (3) pyroclastic, (4) volcaniclastic, (5) conglomerate-sandstone, (6) sandstone-argillite .š conglomerate), and (7) argillite-sandstone .š tuffaceous sandstone). The mafic and felsic volcanic lithofacies represent effusive lava flows, the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris. The conglomerate-sandstone lithofacies is interpreted as alluvial fan, fan delta or proximal braided stream deposits, whereas the sandstone-argillite lithofacies is consistent with sandy-dominated flood- or braidplain deposits. A dominantly shallow-water lacustrine setting is inferred for the argillite-sandstone lithofacies. These different lithofacies record the basin history and can be used to identify basin-forming processes. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected by volcanic input. Catastrophic influx of pyroclastic material facilitated mass-wasting processes and formation of non-confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns. Confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift. In addition, climate which controls the weathering processes, and vegetation which stabilizes unconsolidated material, affects the transport and depositional process. A CO2 -rich aggressive weathering, humid Archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation. Although tectonism is the prevalent long-term controlling factor in restricted basins, the effects of volcanism, climate and lack of vegetation can also be detected.  1998 Elsevier Science B.V. All rights reserved. Keywords: Archaean; volcanism; sedimentology; climate; depositional setting; lithofacies; late-orogenic basins

1. Introduction Archaean supracrustal sequences reflecting the early evolution of the earth, are complex volcanoŁ Corresponding

author. E-mail: [email protected]; Fax: C1 418 545 5012.

sedimentary successions that formed between 4.0 and 2.5 Ga. Sedimentary strata are of special interest during the incipient stages of pre-vegetational earth because sedimentary structures (Miall, 1978, 1992; Pettijohn et al., 1987), the composition of sandstones (Dickinson et al., 1983; Eriksson et al., 1994) and conglomerates (Mueller and Donaldson,

0037-0738/98/$ – see front matter  1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 3 2 - 3

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1992; Corcoran et al., 1998), in addition to lateral and vertical sedimentary facies stacking (Walker and James, 1992) define the Archaean environment. First-order controls such as tectonism associated with allocyclic factors such as climate and volcanism determine the depositional setting and affect the topography. These factors control erosion and sedimentation rates, dispersal patterns, bedforms and their stacking as well as the composition of sedimentary rocks. Detailed sedimentary studies have enabled the recognition of different tectonic settings in the Archaean. Active (Dimroth et al., 1982; Rice and Donaldson, 1992) and passive margin turbidites (Eriksson, 1980) represent the dominant sedimentary lithology in the Archaean. Complex clastic to volcaniclastic shoreface sequences (Mueller and Dimroth, 1987; Nocita and Lowe, 1990; Mueller, 1991) interstratified with calc-alkaline to shoshonitic lava flows (Dostal and Mueller, 1992) or basalts (Lambert, 1988) that formed adjacent to dissected marginal arcs have been recognized. Quartz-rich sequences (Eriksson, 1977; Fedo and Eriksson, 1996) that developed adjacent to protocontinents (Tankard et al., 1982) are considered representative of stable Archaean conditions (Thurston and Chivers, 1990; Eriksson et al., 1994). In contrast, small, late-orogenic, sedimentary basins have only recently been identified (Krapez and Barley, 1987; Mueller and Donaldson, 1992). Late-orogenic or strike-slip basins that developed adjacent to major crustal-scale structures, display tectonic influence on sedimentation (Crowell, 1974a; Steel et al., 1977; Nilsen and McLaughlin, 1985). Although tectonism may be the dominant, long-term controlling factor, sedimentary dispersal patterns can be significantly influenced by short-terms effects such as volcanism (Mueller et al., 1994). Rerouting and damming of fluvial systems is the result (Kuenzi et al., 1979). Subtle controls such as climate or a lack of vegetation affect the sedimentary facies configuration but they are difficult to detect. This paper attempts to assess the influence of volcanism, climate and lack of vegetation on transport processes in tectonically controlled basins. Three late-orogenic Archaean sedimentary basins in the Canadian Superior Province are evaluated (Fig. 1). These basins were chosen because the depositional context is well defined and modern volcano-sedimentary facies analy-

ses have been conducted. Coarse clastic sedimentary units are prevalent in the Duparquet basin (Mueller et al., 1991), volcano-sedimentary units with abundant alkaline volcanism characterize the Kirkland basin (Cooke and Moorhouse, 1969), and coarse clastic deposits interstratified with bimodal volcanic lava flows typify the Stormy basin (Kresz, 1984). 2. Geological setting 2.1. Depositional history and characteristics Conglomerate in unconformable contact with pre-2.7 Ma volcanic successions in the Superior Province, commonly referred to as Timiskaming-type deposits (Thomson, 1946; Thurston and Chivers, 1990), are interpreted to have formed in late unconformable (Thurston, 1994), or late-orogenic, molasse-type basins (Mueller and Donaldson, 1992). Facies studies of Timiskaming-type deposits were initiated by Hyde (1980) and Rocheleau (1980) for the Abitibi Subprovince (Figs. 1 and 2), and by Teal (1979) and Kresz (1984) for the Wabigoon Subprovince (Figs. 1 and 3). The coarse clastic alluvial deposits were envisaged to be the proximal equivalent of deep-water turbidite deposits (Teal, 1979; Hyde, 1980; Rocheleau, 1980; Dimroth et al., 1982; Kresz, 1984). The contrasting depositional environments were perceived to be penecontemporaneous (Dimroth, 1980) although early mapping noted the presence of intervening faults (Graham, 1954) or an unconformity (Ferguson, 1968) between these two lithological units. Recent mapping in the Stormy Lake area (Corcoran et al., 1996), and Kirkland Lake (Mueller et al., 1994) and Duparquet areas (Mueller et al., 1991) revealed that the deep-water turbidites and subaerial conglomerates represent two separate basin-forming events. Mueller and Donaldson (1992) postulated an early flysch-forming event dominated by sandy turbidites, and a late molasse-forming event characterized by conglomerate interstratified with alkaline volcanic rocks. The Duparquet and Kirkland basins can be linked to transcurrent motion attributed to subduction-related processes during the terminal stages of arc–arc collision (Mueller et al., 1996). Both basins, best described as successor (Ingersoll, 1988) or pull-apart (Crowell, 1974a,b), are the result of strike-slip tectonics. The volcano-sedimentary

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179

Fig. 1. General geology and subdivisions of the Archaean Superior Province with study areas indicated (stars). Modified from Mueller et al. (1997).

rocks in the Stormy Lake area are related to faultcontrol (Corcoran et al., 1996), but further work is required to constrain the possible strike-slip and large-scale geodynamic context. Intrabasinal folding patterns (R. Daigneault, unpubl. data) and stratigraphic relationships of the Stormy basin (Corcoran et al., 1996) support a depositional setting analogous to that of the Abitibi strike-slip basins. Late-orogenic successions are important because they may indicate terrane docking, linking arcs within subprovinces or stitching large-scale subprovince boundaries. Numerous features such as bounding crustal-scale faults, basin margin unconformities, an extensive mafic volcanic hinterland, and calc-alkaline to alkaline porphyry stocks, dykes and sills are common to the Duparquet, Kirkland and Stormy basins. A similar evolutionary history is therefore postulated. Unconformable contacts between porphyry stocks and clastic sedimentary rocks are prominent at the remnant basin margins (Fig. 4A,B). A minor but important feature is the ubiquitous presence of banded

iron-formation (BIF) clasts in conglomerates of all basins. The BIF clasts, derived from the early flysch sedimentary deposits, attest to the diachronous basin evolution from flysch to molasse. Coeval evolution between intrabasinal volcanism, porphyry emplacement, and sedimentation is displayed by an abundance of porphyry (Fig. 4A,B) and lava clasts in conglomerate beds (Mueller et al., 1994, 1996). In all three cases, faults played a significant role in basin evolution. The Duparquet and Kirkland basins of the Abitibi greenstone belt developed between 2685 and 2675 Ma as indicated by U–Pb zircon age determinations of porphyry stocks (Corfu et al., 1989; Mueller et al., 1996) and detrital zircons in sandstone beds (Corfu et al., 1989). These sedimentary basins occur at the interface between older 2700 Ma flysch deposits and subaqueous basalts. The Stormy basin in the western Wabigoon Subprovince formed between 2703 and 2696 Ma (Corcoran et al., 1996) based on recent age determinations of the 2696š2 Ma Taylor Lake intru-

180 W.U. Mueller, P.L. Corcoran / Sedimentary Geology 120 (1998) 177–203 Fig. 2. Location of Duparquet and Kirkland Lake areas in the Abitibi greenstone belt. Note location of these sedimentary units, referred to as sedimentary cycle 4 (Mueller and Donaldson, 1992), along the major E-trending faults. Sedimentary cycles 1 and 3 are flysch-type deposits whereas sedimentary cycles 2 and 4 are interpreted as molasse-type deposits. Modified from Mueller and Donaldson (1992).

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Fig. 3. Regional geology of the Stormy Lake area with study area indicated. Modified from Blackburn et al. (1991) and Corcoran et al. (1996).

182 W.U. Mueller, P.L. Corcoran / Sedimentary Geology 120 (1998) 177–203 Fig. 4. Characteristics of strike-slip basins. Large arrow with black tip indicates tops in all photographs. (A) Unconformity .U/ between lath porphyry . p/ and boulder conglomerate dominated by porphyry clasts . pc/ in Duparquet basin. Arrows indicate contact. Scale pen, 15 cm. (B) Unconformity .U/ between quartz-feldspar porphyry . p/ and conglomerate with identical porphyry clasts . pc/ in Stormy basin. Marker 15 cm long; arrows indicate contact. (C) Unconformity .U and arrows) between massive lava flow of 2700 Ma Kinojevis Group .K G/ and mafic volcanic clast-dominated conglomerate .MC/ in Kirkland basin. Pen (15 cm) and arrows display contact. (D) Massive amygdule-rich lava flow in Stormy basin. A, amygdule. Pen, scale 4 cm.

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Fig. 4 (continued). (E) Flow breccia composed of angular clasts with locally highly vesicular and amygdule-rich clasts in Stormy basin. Pen, scale 15 cm. (F) Pillowed flow unit in Stormy basin with vesicles .V / and amygdules (calcite-filled). PR, pillow rim. Pen, scale 15 cm. (G) Lapilli tuff showing an irregular contact with fine-grained tuff (indicated by arrows). The lapilli tuffs of the Kirkland basin represent airfall deposits .AF/ and small bombs (arrow) indicate ballistic ejecta disrupting beds. Laminated fine-grained tuff is interpreted as a surge deposit .S/: (H) Rim-type accretionary lapilli in the Kirkland basin. ALR, accretionary lapilli rim.

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sion and the 2699 š 2 Ma Sunshine Lake tuff (Davis, 1996). The adjacent volcanic rocks are represented by the 2732 Ma Wapageisi Group (Blackburn et al., 1991) to the south and the 2722 š 5 Ma Boyer Lake Group (Davis, 1996) to the north. 2.2. Duparquet basin The 1–2-km thick, coarse clastic Duparquet basin (Figs. 2 and 5) is bounded on the south by the major E-trending, Destor Porcupine Manneville fault, and on the north by the Donchester fault splay (Fig. 5; Graham, 1954; Mueller et al., 1991). Sedimentary strata young to the north and south at the basin margins indicating a synformal structure. A series of Eto ENE-trending en-echelon folds is suggestive of a dextral strike-slip component during basin evolution (Mueller et al., 1996). The geometry of the 15-km long basin corresponds to that of a divergent faultwedge basin (Crowell, 1974b), in which a subsidiary fault splays off the master fault commonly where a change in strike occurs. Several porphyry stocks lie along the bounding faults (Fig. 5) and occur within the Duparquet basin parallel to the minor E-trending faults. Unconformable contacts between porphyry stocks and sedimentary units occur at the northern and southern margins, as well as within the basin (Rocheleau, 1980; Mueller et al., 1991). Conglomerate constitutes the principal basin fill deposit (estimate 70– 80%), and sandstone and argillite are subordinate. The conglomerates which are prominent along the margin adjacent to the fault and porphyry stocks, are the result of erosion from high-relief zones. 2.3. Kirkland basin The Kirkland basin (Figs. 2 and 6) is a complex 3–5-km thick volcano-sedimentary sequence (Fig. 7) traceable for 50 km along strike. The northern margin of the Kirkland basin is characterized by massive and pillowed basalt flows of the Kinojevis Group, unconformably overlain by clastic sedimentary and volcanic rocks of the Timiskaming Group (Fig. 4C; Thomson, 1946; Hewitt, 1963; Jackson and Fyon, 1991; Mueller et al., 1994). The southern margin is delineated by the major E-trending Cadillac–Larder Lake fault. The basin is a S-facing homoclinal unit

with local metre-scale E–W striking folds. Complex interference folding patterns in the flysch deposits, dominant in the eastern part of the Kirkland basin (Fig. 6; older sedimentary rocks) indicates that a folding phase occurred prior to the deposition of the clastic Kirkland sedimentary rocks. Subaerial alkaline lava flows of generally andesitic composition with high (5–10%) Na2 O and K2 O contents (Cooke and Moorhouse, 1969; Ujike, 1985), and primary pyroclastic surge and airfall deposits (Hyde, 1978; Mueller and Donaldson, 1992) characterize the sequence. Potassium feldspar and pseudoleucite are both abundant in lava flows and pyroclastic deposits. Elongate stocks intruding the Kirkland basin (Fig. 6) are deformed parallel to regional or local structural trends (Thomson, 1950; Levesque et al., 1991). The intrusive suite includes alkali-feldspar syenites to melasyenites, lamprophyre sills and=or dykes, and feldspar- to quartz-feldsparphyric .š hornblende š biotite) stocks (Levesque et al., 1991). Porphyry stocks and their extrusive equivalents are prominent along the crustal-scale Cadillac–Larder Lake fault. Locally, detritus from the stocks constitutes the dominant clast composition in the conglomerates (Fig. 7). Sequences of up to 700 m of conglomerate-dominated units necessitates topographic relief commensurate with rapid basin subsidence. 2.4. Stormy basin The 1.5–2-km thick Stormy basin, traceable for approximately 15 km along strike, is the eastern extension of the Manitou Group (Fig. 3). The sequence is composed of mafic and felsic lava flows of alkaline to calc-alkaline character (Teal, 1979; Blackburn, 1982; Blackburn et al., 1991; J. Dostal and W.U. Mueller, unpubl. data), as well as coarse clastic sedimentary rocks (Fig. 8). The sedimentary rocks at the southern margin are inferred to overlie the older massive to pillowed volcanic and felsic volcaniclastic rocks of the Wapageisi Group unconformably (Kresz et al., 1982). An unconformity between the Thundercloud porphyry and sedimentary rocks in the Stormy basin, as initially suggested by Bertholf (1946, cited in Blackburn, 1982), is locally exposed (Fig. 4B). The Thundercloud porphyry is quartz-feldspar-rich with up to 1-cm large quartz phenocrysts; subor-

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Fig. 5. General geology of the Duparquet area including fault-bounded Duparquet basin and dated porphyry stocks. Modified from Mueller et al. (1996) and based on original mapping of Graham (1954) and Rocheleau (1980). I and II indicate representative stratigraphic sections in basin. DPMFZ, Destor–Porcupine–Mannerille fault zone; DoF, Douchester fault.

dinate finer-grained feldspar-hornblende-phyric and aphanitic phases are also present. The E-trending Mosher Bay–Washeibemaga fault separates the remnant basin from massive and pillowed basalts and rhyolites of the Boyer Lake Group to the north (Fig. 3). The steeply dipping, generally E–W-striking strata are affected by an ESE-trending syncline which has an axial planar schistosity. Prominent intrabasinal aphanitic and feldsparhornblende-phyric lava flows of andesitic to basaltic composition (Kresz, 1984; Corcoran et al., 1996) display a calc-alkaline signature (Kresz, 1984; J. Dostal and W.U. Mueller, unpubl. data). Erosion and local reworking of these flows is reflected in the overlying mafic volcanic clast-dominated conglomerate. Felsic flows of limited extent representing a minor part of the volcano-sedimentary succession have been interpreted as the extrusive counterpart of the Thundercloud quartz-feldspar porphyry (Kresz, 1984). Thick amalgamated conglomerate beds (20–30 m thick) with cobble- to boulder-sized clasts are commonly composed exclusively of volcanic detritus derived from the adjacent lava flows. This dominance of conglomerates in the volcano-sedimentary sequence is consistent with significant topographic relief.

3. Basin lithofacies and architecture The studied basins contain distinct volcano-sedimentary units. Pyroclastic rocks and their reworked equivalents, as well as volcanic lava flows are present in the remnant Kirkland and Stormy basins and are defined by the mafic volcanic, felsic volcanic, pyroclastic, and volcaniclastic lithofacies. The sedimentary rocks are represented by the conglomeratesandstone, sandstone-argillite (š conglomerate), and argillite-sandstone (štuffaceous sandstone) lithofacies. The seven lithofacies (Table 1) in these basins are described in this section so that possible climatic and volcanic influence in association with tectonism can be assessed. The facies code for alluvial–fluvial deposits as defined by Miall (1978, 1992) with modifications for volcanic input (Smith, 1987; Mueller et al., 1994), is employed. 3.1. Mafic volcanic lithofacies The 5–150-m thick mafic volcanic lithofacies with an alkaline signature in the Kirkland basin (Figs. 6 and 7) and a calc-alkaline signature in the Stormy basin (Fig. 8), constitute 10–20% and locally

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Fig. 6. General geology of the Kirkland lake area. I to IV indicate representative stratigraphic sections in basin (see Fig. 7). Modified from Mueller et al. (1994).

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Fig. 7. Composite stratigraphy of the Kirkland basin with variation of clast population in clast composition diagrams. See Fig. 6 for location and Fig. 12 for details of I in Section III.

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Fig. 8. General composite stratigraphy of the remnant Stormy basin. Clast composition diagrams illustrate a prominent up-section change in clast composition in conglomerate from extra- to intrabasinal sources. I and II represent detailed stratigraphic sections (see Fig. 9).

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Fig. 9. Stratigraphic section of the stream-flow dominated conglomerate-sandstone lithofacies interstratified with viscous, quartz-feldsparphyric lava flows in the Stormy basin. Sedimentary facies abbreviations described in text and Table 1.

up to 40% of the stratigraphic sequence. Lava flows are aphanitic and porphyritic; the latter is composed of 0.2–3-cm large phenocrysts constituting locally 30–50% of the flow. Pseudoleucite, K-feldspar, plagioclase, hornblende or pyroxene phenocrysts are identified in the porphyritic flows (Cooke and Moorhouse, 1969; Kresz, 1984; Mueller et al., 1994). Individual flow units, 5–30 m thick, are massive (Fig. 4D) and grade into flow top breccias (Fig. 4E). Amygdules and vesicles, 0.2–3 cm large, are concentrated at the margins of massive flows (Fig. 4D), common in pillowed units (Fig. 4F) and present in the breccias. Brecciated lavas display blocky or aa flow forms. Lavas composed of angular to sub-

rounded clasts rigidly or loosely connected to the flow are considered blocky flows (Macdonald, 1972; Mueller, 1991). A subaerial setting is favoured for the massive flows and block-type breccias based on the bounding, stream-dominated, conglomeratesandstone lithofacies and absence of hyaloclastites. Aa flow breccia, characterized by spinose clasts with irregular-shaped vesicles, is suggestive of subaerial flow (Macdonald, 1972). Intrabasinal pillowed flows (Fig. 4F), locally observed in the Stormy basin, are consistent with a subaqueous setting.

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Table 1 Lithofacies of late-orogenic Archaean volcano-sedimentary basins Lithofacies

Facies, characteristics

Process

Setting

Volcanic and volcaniclastic deposits in Archaean strike-slip basins of the Superior Province Massive, brecciated and minor pillowed flow forms; vesicle content variable, but concentrated at flow margins, especially in pillowed flows; brecciated flow forms similar to clinkery aa-flows or blocky flows; porphyritic flows common

Calm effusive lava flow with brecciation caused by flow velocity exceeding internal yield strength of laminar viscous flow

Subaerial flows on flanks of volcanic edifices locally entering lakes or ponds

Felsic volcanic

Massive and brecciated flows with local flow banding; absence of hyaloclastites

Viscous effusive lava flow with brecciation caused by flow velocity exceeding internal yield strength of laminar viscous flow

Small subaerial flows forming domes (?)

Pyroclastic

Plane- to wavy-bedded alkaline tuffs or lapilli tuffs with breccia-size pyroclasts; U-shaped channel deposits; accretionary lapilli Normal and inverse graded beds, accretionary lapilli, small bombs

Pyroclastic surges (wet surges)

Subaerial deposits, possibly entering lakes, ponds or rivers

Volcaniclastic

Facies assemblages:šGms– Gma šSma –Sha šSla –Fl; and Gma šSma C Sha C Sla C Sr C Fl Matrix-supported conglomerates (Gms) Pebbly massive conglomerate (Gma ); massive (Sma) , planar (Sha ) to low-angle (Sla ) bedded sandstone; Laminated fine-grained tuff (Fl); Isolated or cosets of trough cross-beds (St); Small ripples (Sr); laminated siltstone–tuffaceous sandstone (Fl) Massive to crudely stratified volcanic clast dominated conglomerate (Gm); locally angular clasts (Gbx)

Pyroclastic fallout Volcanic fans fringing flanks of volcanic edifice Debris flows, lahars Hyperconcentrated flows, sheet flows in broad shallow channels Waning floods Confined local fluvial runoff Low-energy fluvial runoff and suspension fall-out Fluvial bedload transport; reworked mafic flow breccia; rock fall or rock avalanche

Floodplain downstream from alluvial volcanic fan Floodplain, (proximal) with non-confined deposits

Floodplain (distal)

Stream-dominated alluvial (volcanic) fan or braidplain; local talus scree adjacent to lava flows

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Mafic volcanic

Clastic-dominated sedimentary lithofacies in strike-slip basins of the Superior Province Gbx, rock fall or rock avalanche. Fluvial traction-currents: Gm, sheet gravel or longitudinal gravel bar; Gt, channel fill; St, sinuous-crested in-channel dunes; Sh tp Sl, upper flow regime bar top sand; Sp, laterally accreted sand. Facies assemblage: confined flows in broad channels with exception of rockfall deposits

Talus scree. Proximal reaches of stream-dominated alluvial fan or proximal braided stream adjacent to fault escarpments and porphyry stocks

>50% conglomerate: facies assemblage Gm–Gt–St–Sh–Sr–Fl: Gm, see above; St, see above; Sh to Sl, planar to low-angle beds; Sr, small-scale trough cross-beds; Fl, laminated mudstone, siltstone or fine-grained sandstone

Fluvial traction- currents with fining-upward sequences resulting from dissipating floods and channel migration; Sh, upper flow regime sand; Sr, lower flow regime ripples or dunes; Fl, suspended fines settling through water column during terminal flood stages

Medial to distal reaches of stream-dominated alluvial fan or proximal braided stream adjacent to fault escarpments and porphyry stocks. Fine-grained sediments deposited in overbank or ephemeral pond settings

Sandstone-Argillite .šconglomerate)

Facies assemblage: šGm–St–Sh(Sl)–Sr–Fl;

Fluvial traction currents with suspended fines

Sandy braid delta or braidplain with local ponds

Sandstone: medium- to very coarse-grained

Trough cross-beds, planar beds, low- angle planar beds with argillite drapes on foresets or between bedforms

Wave- induced structures with highly fluctuating wave energy conditions. Fluvial-induced structures during high discharge?

Transition zone: subaerial to sub-aqueous setting

Argillite: siltstone, mudstone, very fine-grained sandstone

Argillite with ripple sets in beds up to 1 m-thick

Settling of suspended fines with weak wave action

Upper to lower shoreface

Pebble trains with rip-ups (Gm)

Storm (rip currents?) or major flooding

Argillite-Sandstone .štuffaceous sandstone)

Graded and laminated siltstone=mudstone couplets

Settling of suspended fines

Shallow water: pond or lake

Sandstone: medium- to coarse-grained

Planar-laminated beds with rippled tops and fine-grained, graded bedded tuffaceous sandstone Clay-draped trough cross-beds Local conglomerate filled channels, graded and massive sandstone beds

Low wave energy and settling of suspended fines

Shallow water: pond or lake

Wave-induced structures Storm activity (rip current deposition?)

Lower shoreface Lower shoreface to proximal offshore

conglomerate: boulderto cobble-size; sandstone: coarse to very coarse-grained

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>75% conglomerate: facies assemblage šGbx C Gm–Gt–St(Sl)–ShšSp: Gbx, massive, angular clast-dominated conglomerate; Gm, massive to crudely stratified clast-supported conglomerate; Gt, massive to stratified conglomerate-filled troughs; St, isolated or cosets of trough cross-bedded sandstone; Sh, planar bedded sandstone; Sp, lower flow regime

Conglomerate-Sandstone

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3.2. Felsic volcanic lithofacies A 4–30-m thick massive to brecciated felsic volcanic unit, traceable intermittently for 500 m along strike is restricted to the Stormy basin (Fig. 8). The felsic flows are quartz-feldspar-phyric with up to 1-cm large quartz phenocrysts. The basal part of the flow is massive whereas the upper part is an in situ flow breccia composed of angular clasts between contorted flow-bands (Fig. 9). The basal contact with the conglomerates is sharp and mixing of the flow with sediments producing peperites was not observed. A high viscosity can be inferred for these rhyolitic lava flows because of the abundance of quartz and feldspar phenocrysts, and the presence of flow breccia. Viscous felsic lavas commonly form short, thick, stubby flows or domes (Yamagishi and Dimroth, 1985) and their flow length rarely exceeds 1–2 km (Williams and McBirney, 1979; Kano et al., 1991). Bounding fluvial conglomerate-sandstone lithofacies and lack of hyaloclastites suggest subaerial emplacement (Fig. 9). The petrographically similar Thundercloud quartz-feldspar porphyry, less than 2 km from this flow unit, is the probable source. 3.3. Pyroclastic lithofacies Well-defined sequences of tuff, lapilli tuff and lapilli tuff breccia (Fig. 7), 5–40 m thick, constitute the Kirkland basin (Mueller et al., 1994), whereas felsic deposits of inferred pyroclastic origin have been suggested for the Stormy basin (Blackburn, 1982; Kresz et al., 1982; Kresz, 1984). Recent mapping by Corcoran et al. (1996) in the Stormy basin favours fluvial reworking of flow breccia rather than a pyroclastic origin, especially due to the proximity of felsic flows. Felsic debris adjacent to the flows is composed of angular blocks and cobbles: massive talus scree deposits are similar. The 10–100-cm thick pyroclastic deposits in the Kirkland basin exhibit planar low-angle to wavy bedforms and erosive-based channels filled with asymmetric dunes that laterally overlap channel margins and grade into planar beds. These features are typical of surge deposits (Fisher, 1979; Fisher, 1982; Cole, 1991) with local U-channel formation (Mueller et al., 1994). Airfall deposits are loosely packed, dis-

play normal and inverse graded beds, and show disruption of bedding planes attributed to ballistically emplaced ejecta (Fig. 4G). Textural evidence for a pyroclastic origin include pumice, accretionary lapilli (Fig. 4H), shards, and abundant euhedral and broken crystals of pseudoleucite, pyroxene and hornblende (Fisher and Schmincke, 1984). The accretionary lapilli are rim-type varieties (Fig. 4H; Schumacher and Schmincke, 1991), commonly associated with the proximal reaches of pyroclastic surges or are related to airfall (Bertagnini and Landi, 1996). 3.4. Volcaniclastic lithofacies Volcaniclastic lithofacies (Table 1), 10–100 m thick, are well exposed in the Kirkland (Fig. 7) and Stormy basins (Fig. 8). Massive, volcanic, clastdominated, breccia in the Stormy basin is either a talus scree deposit based on dominance of angular clasts (facies Gbx) or a fluvial gravel bar (facies Gm) in which subangular to subrounded clasts are prominently (Fig. 10A). These two type of deposits develop preferentially adjacent to the aa- or block lava flows in the Kirkland basin. Massive matrixto clast-supported conglomerates (Fig. 10B,C) are interpreted as debris flow deposits (facies Gms) or lahars (White and Robinson, 1992) which commonly develop on proximal reaches of alluvial fans (Mack and Rasmussen, 1984). The Kirkland basin displays a complex interaction between pyroclastic surge and fall deposits and their reworked counterparts. Isolated sets and cosets of high-angle trough cross-beds (facies St) incised in massive to laminated, or low-angle planar sheetlike deposits can be explained by fluvial channel reworking. This is consistent with low-energy fluvial runoff after major non-confined flooding (Smith, 1991). Massive and plane-bedded (Fig. 10C) to low-angle cross-bedded sheet-like sandstones, represented by facies Sma , Sha and Sla , respectively, are considered to be hyperconcentrated flood flow deposits derived from penecontemporaneous volcanism (Smith, 1987, 1991; White and Robinson, 1992). Laminated, tuffaceous siltstone=mudstone and finegrained sandstone (referred to as argillite), locally up to 2 m thick, are interpreted as settled fines (facies Fl) with low-energy flow ripples (facies Sr) from flood-runoff. Local formation of desiccation cracks

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(Fig. 10D) indicate dried-up ponds or sandflats. Mudstone drapes between bedforms, indicating rapid suspension deposition from high sediment concentrations in water during the waning flood stages, have been described in volcanic-induced intra-arc basins (Mathisen and Vondra, 1983) and observed in the White River braided outwash plain, Yukon (Mueller, pers. observ.). Thin pebble lags or matrix-supported conglomerates are interpreted as hyperconcentrated, runout, flood deposits (facies Gma ; White and Robinson, 1992), and overlying planar-bedded sandstone is consistent with more diluted sheetflood phases (facies Sha ) in broad channels. Collectively, these characteristics argue for low-gradient distal reaches of volcaniclastic-dominated, non-constrained fans grading onto a floodplain (Waresback and Turbeville, 1990). An ideal proximal alluvial facies sequence in the volcaniclastic lithofacies would be š Gbx š Gms C Gm C Gma C Sha , whereas the distal floodplain facies is characterized by the assemblage Gma š Sma C Sha C Sla C Sr C Fl with local channel incision featured by facies St. 3.5. Conglomerate-sandstone lithofacies The 10–700-m thick conglomerate-sandstone lithofacies (Figs. 8, 9, 11 and 12), constituting over 50% of the stratigraphic sequence in all studied basins, is characterized by a prominent conglomerate component (Fig. 10E) which in some areas represents over 75% of the lithofacies. Individual clast-supported beds (Fig. 10F) ranging between 0.5 and 4 m occur in 2–30-m thick amalgamated beds. Bed amalgamation can be discerned by clastsize variations (Fig. 10G), truncation of sandstone interbeds by conglomerate, and a change in sedimentary structure. Beds are massive to stratified and locally display moderate- to low-angle channel scours filled with conglomerate. The amalgamated massive to stratified conglomerates (facies Gm) represent coalesced gravel sheets and=or longitudinal gravel bars (Eriksson, 1978; Wells, 1984). Conglomerate-filled channels with scour surfaces are evidence for confined high-energy deposits (facies Gt). The conglomerates are the product of high-energy traction currents, dominant in fluvial systems. Clast size varies considerably from large boulders to smaller

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cobbles (Figs. 11 and 12) further supporting deposition on alluvial fans or in proximal reaches of braided streams (Steel et al., 1977). Metre-thick, matrix-supported, mono- to oligomictic conglomerate composed of cobble- to boulder-size angular clasts represent talus scree deposits, a type of sedimentary breccia (facies Gbx), that developed adjacent to the porphyry stocks. The generally lenticular, coarse-grained, sandstone interbeds, 5–50 cm thick, are mainly planar-bedded to slightly low-angle bedded, and trough cross-bedded. Planar cross-beds are subordinate and difficult to identify. Contacts between bedforms are erosive. The planar interbeds are considered to be upper flow regime, bar top sands (facies Sh; Eriksson, 1978) formed during diminishing flood stages. Cosets of truncating sets of trough cross-beds represent migrating in-channel dunes (facies St). Planar cross-beds (facies Sp) represent lower flow regime, linguoid sandbars or straight-crested dunes (Reineck and Singh, 1980) that commonly accrete along the sides of gravel bars. In sequences where the sandstone component increased, rippled beds (facies Sr) and argillite beds (facies Fl) developed indicating overbank deposition. The abundance of traction current structures in the conglomerate and sandstone strata are consistent with a high-energy flow regime generally forming fluvial dispersal systems on alluvial fans, fan deltas, and proximal braided streams (Rust and Koster, 1984). The facies assemblages are š Gbx C Gm– Gt–St(Sl)–Sh š Sp and Gm–Gt–St–Sh–Sr–Fl (Table 1). The abrupt change in grain size, lithology, and sedimentary structures is typical of coarse clastic alluvial fans or fan deltas. 3.6. Sandstone-argillite (š conglomerate) lithofacies The 1–35-m thick sandstone-argillite lithofacies (Figs. 8 and 12), contains medium- to very coarsegrained sandstone and argillite beds composed of mudstone–siltstone and fine-grained sandstone. Local 10–20-cm thick pebble- to cobble-size conglomerate in the form of discontinuous pebble trains (facies Gm) or channel fills (facies Gt) occur at the base of fining-upward sequences. The sandstone-argillite lithofacies occurs in two distinct sedimentary as-

194 W.U. Mueller, P.L. Corcoran / Sedimentary Geology 120 (1998) 177–203 Fig. 10. Characteristics of strike-slip basins. Large arrow with black tip indicates younging direction in all photographs. (A) Massive clast-supported volcanogenic conglomerate composed of subangular to angular mafic (hornblende-feldspar-phyric) clasts (facies Gm) in Stormy basin. Tip of pen points to finely laminated clast (flow banding?). Scale, top of pen 6 cm. (B) Massive matrix- to clast-supported conglomerate composed of volcanic clasts in Kirkland basin (facies Gms). Scale, coin 2.5 cm in diameter. (C) Massive matrix- to clast-supported conglomerate composed of volcanic clasts in Kirkland basin (facies Gms) eroding massive to stratified hyperconcentrated flood flow deposits (facies Sma –Sha ). Scale, pen (small arrow) 15 cm. (D) Desiccation cracks in tuffaceous facies Fl (Kirkland basin) indicating exposure of sheetflood deposits. Scale, coin 2.2 cm in diameter.

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Fig. 10 (continued). (E) Conglomerate-sandstone lithofacies with well-defined conglomerates (facies Gm indicated by A/ and interstratified sandstone beds .B/ in Kirkland basin. Scale, book 20 cm. (F) Clast-supported massive, amalgamated conglomerate (facies Gm) in Stormy basin. Scale, book 20 cm. (G) Amalgamated conglomerate beds indicated by pebble–cobble conglomerate .B/ between two cobble–boulder-dominated conglomerate beds .A/ in Stormy basin. Scale, pen 15 cm. (H) Well-defined mediumto large-scale trough cross-beds .T B/ with local mudstone–siltstone drapes .D; indicated by small arrows) on and between bedforms in Duparquet basin. Scale, pen 17 cm (arrow).

196 W.U. Mueller, P.L. Corcoran / Sedimentary Geology 120 (1998) 177–203 Fig. 11. Two representative sections of the northern margin of the Duparquet basin comparable to that of the south. Note local coarsening-upward .CU/ then fining-upward sequence .FU/ in Section I suggestive of tectonic influence on sedimentation. Clast composition diagrams indicate are prevalence of mafic volcanic and porphyritic clasts.

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Fig. 12. Representative stratigraphic section of the stream-flow-dominated conglomerate-sandstone lithofacies and sandstone-argillite lithofacies in the Kirkland basin. Modified from Mueller et al. (1994). Note fluvial fining-upward sequences in sandstone-argillite lithofacies (680–697 m).

semblages. The Kirkland and Stormy basins contain 0.50–2-m thick cosets of trough cross-beds (facies St) with individual sets 10–30 cm thick representing migrating in-channel dunes (Eriksson, 1978; Miall, 1992) and 10–20-cm thick planar to low-angle beds in 1–2-m thick cosets, indicating upper flow regime conditions (Best and Bridge, 1992). Finer-grained,

small-scale trough cross-beds (facies Sr), 1–5 cm thick cap both of these bedforms and are indicative of waning flood stages. The finely laminated argillite (facies Fl) is interpreted as overbank deposits (Horton and Schmitt, 1996). Thicker argillite beds are suggestive of small abandoned ponds adjacent to streams. The facies assemblage š Gm–St–

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Sh(Sl)–Sr–Fl forms 1–4-m thick, well-defined fining-upward cycles that are diagnostic of fluvial flood episodes (Fig. 12; 680–697 m). The sandstone-argillite lithofacies in the Duparquet basin (Fig. 11) has been interpreted as a sandy subaqueous braid delta (Mueller et al., 1991) based on 20–300-cm thick trough cross-beds and composite cross strata with local clay drapes on foresets (Fig. 10H) and stratigraphic position. Reactivation surfaces indicate lateral accretion of beds with changing flow velocities (Richards, 1986). Although clay drapes on bedforms are usually interpreted as the result of tide-influence (Johnson, 1978; Dalrymple, 1992), Mueller et al. (1991) argued that the overall basin geometry, facies stacking, and cyclicity is more consistent with a transition zone where fluvial processes dominated a shoreface setting. The abundant clay resulted from the rapid breakdown of basalt in the adjacent uplifted volcanic hinterland. 3.7. Argillite-sandstone (š tuffaceous sandstone) lithofacies Laterally continuous graded and laminated mudstone=siltstone couplets (argillite) in addition to massive to graded sandstone beds characterize the argillite-sandstone (š tuffaceous sandstone) lithofacies which has a maximum exposed thickness of 5–6 m in the Kirkland basin and 15 m in the Duparquet basin (Fig. 11). A similar lithofacies was not recorded in the Stormy basin. Large shallow scours, 10–100 cm thick, composed of laminated and graded-bedded coarse- to fine-grained sandstone, rippled horizons and local conglomerate filled channels are preserved in the Duparquet basin (Mueller et al., 1991). The argillite units are indicative of either terminal flood stages (Miall, 1978; Flint et al., 1986) or suspension fallout below normal wave base in shallow water (Hyde, 1980; Reinson, 1984). Because the argillite beds are relatively thick, a shallow-water setting rather than overbank deposition is favoured. Graded beds in shallow scours resulting from storms (Squires, 1981) or large coastal floods (Davis, 1985; Mueller and Dimroth, 1987) support this interpretation. Laminated sandstone in flat scours is common to the storm-dominated nearshore (Tunbridge, 1983) and the presence of channelized conglomerate in these deposits can be explained by the formation

of rip channels during major storm activity (Davis, 1985, p. 418). Laminated and rippled fine-grained tuffaceous units interstratified with mm- to cm-scale graded beds are limited to the Kirkland basin (Mueller et al., 1994). The planar and ripple-dominated beds represent lower flow regime conditions that can be produced by waves (Reineck and Singh, 1980). The association of these bedforms with small-scale graded tuff beds supports a subaqueous setting as described by Branney (1991). Metre-thick argillite units argue for a calm water environment. The restricted nature and geometric form of all the basins favour a lacustrine environment. 4. Discussion The Duparquet, Kirkland and Stormy basins are the products of large- and small-scale tectonic events which have been affected by volcanic activity, climate and lack of vegetation. Tectonic influence on sedimentation has been well documented on modern (e.g. Ridge basin, Crowell, 1974a,b; Hazar Lake basin, Hempton et al., 1983) and ancient (Hornelen basin, Steel et al., 1977; Steel and Aasheim, 1978) systems. Characteristics such as elongate basin geometry (Figs. 3, 5 and 6), fold patterns (Fig. 5; Mueller et al., 1996), and bounding faults (Figs. 3, 5 and 6) in all study areas are consistent with strike slip, and enable classification as fault bend (Nilsen and Sylvester, 1995) or fault-wedge basins (Crowell, 1974b). The studied Archaean, late-orogenic basins compare favourably with strike-slip basins formed along the arc system of the western Pacific. Structural style and overall setting of the strike-slip basins along the Philippine fault system on Luzon (Ringenbach et al., 1993), and sedimentology of the Nadai strike-slip basin in Fiji (Hathway, 1993) are strikingly similar. Tectonic influence on sedimentation patterns is difficult to prove in Archaean sedimentary basins, and demonstrating that intrabasinal folds are contemporaneous with basin evolution is problematic due to the remnant nature of the basin and superposed deformation events (Nilsen and Sylvester, 1995). The basin geometry and sedimentary characteristics in association with the structural configuration are sound indicators. Several lines of evidence found

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in these basins support tectonic influence on sedimentation in Archaean sequences: (1) rapid vertical and lateral lithofacies changes over tens of metres; (2) cyclic repetition of lithological units (Figs. 11 and 12); (3) fining-upward and=or coarsening- then fining-upward sequences in conglomerate-dominated units (Fig. 11, Section I); (4) high sediment accumulation rates implying rapid subsidence; (5) the presence of basin margin (Fig. 4A–C) and intrabasinal unconformities; (6) coarse clastic detritus at the basin margin passing laterally to central shallow-water deposits (Figs. 11 and 12); (7) a thick stratigraphic sequence compared to basin size; and (8) most important, basin margin faults. Furthermore, porphyry stocks and penecontemporaneous mafic to felsic lava flows as well as pyroclastic deposits attest to local extension along the crustal-scale structures. Volcanism is locally significant in modern basins along the North Anatolian fault, Turkey (Yoshioka, 1996) and in restricted Archaean strike-slip basins (Mueller et al., 1994; Corcoran et al., 1996). Active, explosive or effusive volcanism in small basins causes congestion or rerouting of fluvial dispersal patterns attributed to the catastrophic influx of volcanic material (e.g. Smith and Swanson, 1987). Smith (1988, 1991) suggested that short-term episodic volcanic eruptions will swamp the local sedimentary budget so that sedimentation rates will be magnitudes higher over an extended period of time. The short-term influx of abundant pyroclastic debris or effusive lava flows restructured the palaeolandscape: this history is recorded in the studied sedimentary rocks. Clast composition diagrams in the stratigraphic columns from the Kirkland (Fig. 7) and Stormy basins (Fig. 8) display an abundance of intrabasinal clasts, whereas prominent extrabasinal sources are temporarily masked (Fig. 8). The rapid influx of pyroclastic debris can account for a change in transport mechanism from that of confined stream flow to non-confined mass flow. Landslides due to volcanic tremors help form talus scree deposits (facies Gbx) and torrential rain in the absence of vegetation facilitate the formation of debris flows (facies Gms), hyperconcentrated flood flows (facies Gma , Sma ) and sheetfloods (facies Sha , Sla ). Hyperconcentrated flood flows are the dominant transport medium after volcanic aggradation. The volcaniclastic lithofacies characterized by proximal facies assemblages

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š Gms–Gma š Sma –Sha š Sla –Fl and a more distal counterpart Gma š Sma C Sha C Sla C Sr C Fl, support this contention. Non-confined flows forming after significant pyroclastic activity should be the rule rather than the exception during the Archaean, especially considering the lack of vegetation. Active volcanism adds supplementary CO2 - and SO2 -rich gases to an already weathering-intensive atmosphere. As convincingly argued by White and Robinson (1992), low-density fallout or surge deposits eroded after explosive events, are easily entrained to produce high-density mass flows. Preservation of subaerial pyroclastic debris in these basins suggests that basin subsidence and tectonic uplift was significant, permitting rapid burial. Additionally, resistant lava capping pyroclastic debris limited the erosion of tephra. Abundant volcanic debris in the form of volcanic talus scree (facies Gbx) developed adjacent to the flows. Reworking and down-cutting in the form of stream incision and fluvial denudation processes occur during stages of volcanic quiescence. In the studied basins, fluvial bedload processes are subtly introduced by a change in the clast composition of conglomerate beds from intrabasinal volcanic rocks (Fig. 10A–C) to extrabasinal sources (clast composition diagrams; Figs. 7 and 8). Traction current processes in confined streams are prevalent as indicated by proximal š Gbx C Gm–Gt– St(Sl)–Sh š Sp, medial Gm–Gt–St–Sh–Sr–Fl and distal š Gm–St–Sh(Sl)–Sr–Fl facies assemblages of the conglomerate-sandstone and sandstone-argillite (š conglomerate) lithofacies, respectively (see Table 1). The up-section change of lithofacies suggests alluvial fans or fan deltas grading into braid- or floodplains with local small ponds or a central lake. Mass flow and talus scree deposits were restricted to the area adjacent to the porphyry stocks during this stage. Stream flow processes are also dependent on vegetation and climatic conditions. Lack of vegetation in the Archaean would have enhanced flash flooding, rapid erosion, and low-sinuosity braided fluvial systems (e.g. Long, 1978) as well as mass wasting of pyroclastic debris. Evidently, the CO2 -rich Archaean atmosphere, envisaged to be characterized by warm (mean surface temperature, 85ºC; Kasting and Ackerman, 1986; Kasting, 1993) and moist conditions (Des Marais, 1994), would be a

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highly efficient weathering parameter (Corcoran et al., 1998; Des Marais, 1994). Despite rapid erosion, transport and elevated uplift rates, Des Marais (1994) concluded that the CO2 -rich Archaean atmosphere was highly efficient in degrading non-resistant components. Rapid and intense weathering of the mafic-dominated volcanic rocks surrounding the basins could explain the local abundance of clay on and between bedforms and thick occurrences of the argillite-sandstone lithofacies. A warm, humid, weathering-intensive climate favours stream-dominated processes (Galloway and Hobday, 1983) and is reflected by the facies assemblages in the Duparquet, Kirkland and Stormy basins. 5. Conclusions The Kirkland, Duparquet and Stormy basins are late-orogenic, molasse-type basins bordered by major crustal-scale structures and classify as strike-slip basins. Strike-slip basins are the result of largescale horizontal tectonics and rigid crustal plate motions which, in modern settings, are generally associated with plate tectonics. Similar plate motions must have been operative in order to produce the Archaean basins in this study. It is often difficult to prove whether folding patterns are penecontemporaneous with basin formation or result from subsequent deformation, but the overall basin geometry with bounding faults, presence of unconformities and sedimentology are strong arguments favouring a strike-slip influence. Even though tectonic activity prevailed, more subtle allocyclic factors were recorded in the studied basins. The intrabasinal lithofacies and sedimentary facies assemblages showed that volcanism, climate and lack of vegetation were important factors in the history of these tectonically controlled basins. Volcanism probably congested local fluvial dispersal patterns and caused rerouting in these small restricted basins. Bimodal volcanism in the form of effusive rhyolitic and basaltic flows controlled the palaeotopography of the Stormy basin. A principal characteristic of the Kirkland basin was a change in the transport process from confined stream flow to non-confined mass and hyperconcentrated flood flow induced by the catastrophic influx of volcanic material, as suggested by abundant surge and airfall

deposits. Similar observations were made after the Mount St. Helen’s eruption in 1980. As the effects of volcanism subsided, reinstatement of confined flow processes was restored in the Kirkland basin. The aggressive weathering CO2 -rich, and probably warm and moist, Archaean atmosphere contributed to a rapid denudation process. Dominance of bedload deposits confined to broad shallow streams are in support of a humid climate. As inferred by Corcoran et al. (1998), first-cycle quartz-rich sediments and local quartz arenites sensu stricto can be generated in high-relief areas due to intensive Archaean weathering. Abundance of active volcanoes emitting SO2 -rich gases intensified the weathering process. Schumm (1968) suggested that the lack of vegetation would enhance flash flooding and rapid fluvial runoff during the Precambrian, and Long (1978) argued that low-sinuosity braided stream settings with limited preservation of overbank fines would be favoured. The inferred alluvial fan and stream-dominated settings based on the facies assemblages indicate a predominance of high-energy runoff products. Debris flow or talus scree deposits were restricted to the immediate area adjacent to the synbasinal porphyry stocks. The study shows that a combination of factors control sedimentary processes in Archaean strikeslip basins and that detailed studies permit recognition of more subtle allocyclic factors. Remnant basins older than 2.6 Ga, if studied properly, can elucidate how the early earth evolved. Acknowledgements The authors thank the numerous assistants and students that helped map the various basins. NSERC and LITHOPROBE (Lithoprobe Contribution No. 893), as well as the Universite´ du Que´bec at Chicoutimi supported this research project. The incisive comments of E.H. Chown, P. Eriksson and W. Fyson, and C. Blackburn significantly improved the quality of the manuscript. References Bertagnini, A., Landi, P., 1996. The Secche di Lazarro pyroclastics of the Stromboli volcano: a phreatomagmatic eruption

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