Relative sea level change along the Slave craton coastline: Characteristics of Archean continental rifting

Sedimentary Geology 176 (2005) 97 – 119 www.elsevier.com/locate/sedgeo

Research paper

Relative sea level change along the Slave craton coastline: Characteristics of Archean continental rifting W.U. MuellerT, C. Pickett Sciences de la terre, Universite´ du Que´bec a` Chicoutimi, Que´bec, Canada G7H 2B1 Received 5 November 2004; received in revised form 9 December 2004; accepted 20 December 2004

Abstract The N2.8 Ga siliciclastic-dominated, craton-cover succession with subordinate mafic volcanic rocks at Beniah Lake in the Slave craton has been informally referred to as the Bell Lake group. The Beniah Lake succession, up to 1 km thick, is composed of 7–120 m thick coarsening-upward (CU) sequences arranged with the following lithofacies architecture: (1a) argillite–sandstone, (1b) mafic volcanic lithofacies, (2) sandstone–argillite, (3a) quartz arenite and (3b) conglomerate lithofacies. A pan-Slave unconformity, indicating an autochthonous 2.8 to 4.013 Ga basement–cover relationship is present and locally well exposed. The lithofacies and their arrangement are consistent with a tide-influenced coastal setting, whereby the change from argillite–sandstone to quartz arenite lithofacies represents an initial estuary system, defining transgression with a maximum flooding surface, evolving into a fan-delta, defining progradation with a maximum regressive surface. The mafic volcaniclastic lithofacies and associated pillowed flow units, suggesting drowning of the siliciclastic succession, coincide with a change from rift to drift tectonics, as crustal attenuation facilitated magma ascent. The Slave-wide unconformities and lithofacies contacts permit a sequence stratigraphic approach, with the major Slave basement–quartz arenite unconformity suggestive of a 1st order sea level change coinciding with the proposed (super) continent break-up or rifting. The consistent stacking of CU-sequences with capping subaqueous volcaniclastic-volcanic units are considered the expression of 2nd order sea level cycles probably caused by changes of spreading rates at oceanic ridges. The CU-sequences and their thicknesses constitute 3rd order sea level changes and reflect the immediate response to crustal thinning. Time constraints for all these orders remain elusive but the hierarchy is consistent with sequence stratigraphic events. The observed relative sea level change is attributed primarily to tectono-eustasy where mantle dynamics caused rifting, basin subsidence and mafic volcanism. The latter occurred as the crust was sufficiently attenuated. D 2005 Elsevier B.V. All rights reserved. Keywords: Archean; Slave craton; Quartz arenite; Sequence stratigraphy; Estuary; Fan-delta; Volcanism; Rifting

T Corresponding author. E-mail address: [email protected] (W.U. Mueller). 0037-0738/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2004.12.015

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1. Introduction Documenting relative sea level change, which includes the concepts of sequence stratigraphy (Vail et al., 1977; Van Wagoner et al., 1988; Haq, 1991), and its relationship to glaciations, eustasy, extensive volcanism, and tectonism (Plint et al., 1992; Eriksson, 1999; Catuneau, 2002, 2003), has recently been expanded to the Precambrian (e.g., Krapez, 1993; Catuneau, 2001). Establishing high- and low-order sequence concepts in the Archean successions is commonly inhibited by outcrop paucity and lack of radiometric age determinations (Catuneanu and Eriksson, 1999). In contrast to well-documented tectonic and volcanic influence on sedimentation patterns in Archean strike-slip basins (e.g., Eriksson et al., 1994; Mueller et al., 1994), Archean tectonic-influenced shoreline successions with relative changes in sea level remain elusive. As sequence stratigraphic concepts are related to changing accommodation at the shoreline, preserved Archean shallow-marine successions would be prime candidates. Previous work conducted on relatively flat-lying strata of the Kaapvaal craton (2.1–3.0 Ga; Beukes and Cairncross, 1991; Catuneanu and Eriksson, 1999; Catuneau and Biddulph, 2001) and intra-cratonic Proterozoic (1.6– 2.0 Ga) successions of Canada (Ramaekers and Catuneanu, 2004) indicates that a sequence stratigraphic approach with viable high- and low-order correlations may be applicable if sound sedimentary facies analyses are conducted in areas of sufficient outcrop and/or drill core availability. The Slave siliciclastic-dominated succession contributes to understanding relative sea level change on Mesoarchean continents in the northern hemisphere, for which the only comparable analogue would be the South African (Kaapvaal) Witwatersrand basin (Catuneau, 2001). The N2.8 Ga Bell Lake group at Beniah Lake in the Slave craton (Isachsen and Bowring, 1997) shows a systematic change in accommodation at the Mesoarchean coast, attributed by Mueller et al. (2005) to result from continental rifting and crustal attenuation. This study focuses on the physical expression of relative sea level change at Beniah Lake, based on the well-exposed sedimentary structures and lithofacies, coarsening-upward sequences, volcaniclastic sedimentation, and volcanic flows. Even though these strata are Archean, careful map-

ping and determination of volcano-sedimentary facies permit paleogeographic reconstruction.

2. Slave geology and the Bell Lake group The Slave craton is a combination of basement, late tectonic plutons and supracrustal rocks (Fig. 1A), that can be divided during the Neoarchean into a 2725– 2700 Ma basement-dominated western segment comparable to a continental rift, superposed by 2700–2610 Ma continental margin basins and arcs, and an eastern juvenile oceanic arc (Kusky, 1989). The N-trending Beniah Lake fault is roughly the line of demarcation between these two segments. Prior to this arc collisional event, the proto-Slave craton (Fig. 1A) displays an unequivocal unconformable relationship between 4.0–2.8 Ga gneisso-plutonic basement and continental N2.8 Ga quartz arenite-dominated successions, traceable at the craton-wide scale (Fig. 1A; Bleeker et al., 1999; Sircombe et al., 2001). The oldest basement segment is the 4.013 Ga Acasta gneiss (Bowring and Williams, 1998) located in the northwest, but the largest aerial extent of basement is between the Point Lake volcanic belt and the Sleepy Dragon complex (Fig. 1A). In areas protected from extensive deformation, well-defined unconformities (Fig. 2A; Pickett, 2002; Mueller et al., 2005) indicate an autochthonous basement–cover relationship prior to 2.8 Ga. The established Slave stratigraphy, excluding 4.01–2.9 Ga basement, is a succession of events straddling ca. 250 My (2850–2600 Ma), whereby Fig. 1B only exhibits the evolution between 2850 and 2700 Ma. This craton-cover succession overlies basement unconformably. A major unconformity delineates this succession from an extensive 2725– 2610 Ma greenstone belt-turbiditic flysch association, referred to as the Yellowknife Supergroup (Henderson, 1981, 1998; Helmstaedt and Padgham, 1986). These volcano-sedimentary supracrustal rocks are divided into a 2725–2700 Ma volcanic rift (Fig. 1B) and b2700 Ma continental arc-marginal basin sequence (Mueller and Corcoran, 2001), with the volcanic rift phase overlain unconformably by the latter (Helmstaedt and Padgham, 1986). The 20–1000 m thick, quartz arenite succession (Fig. 3A, B), initially recognized by Covello et al. (1988) at Beniah Lake and defined informally by

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Fig. 1. (A) General geology of the Slave craton with quartz arenites (Bell Lake group) adjacent to the margin of the basement rocks. Beaulieu River volcanic belt (BRVB), Cameron River (CRVB) volcanic belt and Point Lake volcanic (PLVB) belt are continental arc sequences (ca. 2700–2660 Ma) that rest unconformably on basement and Bell Lake group. (B) General Slave stratigraphy with N2.8 Ga continental cratoncover and N2.7 Ga volcanic rift succession.

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Fig. 2. Characteristic Slave craton-cover lithofacies. Large yellow arrow indicates tops. (A) Unconformity (small yellow arrows) at Bell Lake between tonalitic-granodioritic basement and quartz arenite. Tabular strata with sharp bounding argillite drapes composed of Cr-rich mudstone laminae, now fuchsitic mudstone due to metamorphism. Scale, pen=14 cm. (B) Pillowed flow units of the Bell Lake group at Beniah Lake located at the top of the stratigraphy in contact with the mafic volcaniclastic lithofacies. Scale, pen=12 cm. (C) Laminated and rippled beds of the argillite–sandstone lithofacies. Scale, coin=2.5 cm in diameter. (D) Black mudstone and planar bedded sandstone of the argillite–sandstone lithofacies. Small yellow arrows indicate sharp erosive contact with sandstone–argillite lithofacies. Scale, pen=12 cm. (E) Sharp contact between argillite–sandstone and sandstone–argillite lithofacies. Note the ball-and-pillow structure due to synsedimentary deformation. Scale, pen=15 cm.

Isachsen and Bowring (1997) as the Bell Lake group, is a clastic-dominated succession with quartz arenite, quartz-pebble conglomerate, argillite, iron formation

and minor felsic volcaniclastic deposits (Roach, 1990; Isachsen and Bowring, 1997). Pickett (2002) identified coeval pillowed flows (Fig. 2B) and mafic volcani-

W.U. Mueller, C. Pickett / Sedimentary Geology 176 (2005) 97–119 Fig. 3. (A) Geology of Beniah Lake area with studied locations. Note that layered mafic–ultramafic igneous complex with chromite bands is part of the basement at Beniah Lake. (B) General Bell Lake group stratigraphy with coarsening-upward sequences representative of the Beniah Lake area. Basal fining-upward sequence located at Bell and Dwyer Lake sections (see Fig. 1; Mueller et al., 2005).

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clastic deposits that represent the top of Beniah Lake stratigraphy. The felsic volcaniclastic facies at Dwyer Lake with 2834.5F3.5 Ma (Isachsen and Bowring, 1997) and 2853+2/ 1 Ma ages (Sircombe et al., 2001), and at Bell Lake with a 2826F1.5 Ma age (Sircombe et al., 2001) are the best approximations of the Bell Lake group depositional age (Fig. 1A).

3. Bell Lake group at Beniah Lake The Bell Lake group of the Slave craton is best exposed at Beniah Lake. Based on mapping by Covello et al. (1988) and Roach (1990), Pickett (2002) and Mueller et al. (2005) conducted detailed facies mapping to identify the various lithofacies, defined by prominent grain-size and composition attributes, which include: (1) quartz arenite (58% of succession), (2) conglomerate (5%), (3) sandstone– argillite (20%), (4) argillite–sandstone (15%), and (5) mafic volcaniclastic lithofacies (2%). To simplify description, the prefix bmetaQ has been omitted for these upper greenschist–amphibolite facies metamorphic rocks. Even though strata are folded and dip vertically, sedimentary structures readily indicate younging, and hence permit stratigraphic reconstruction. Vertically dipping strata yield a cross-section of a basin segment. Argillite is defined as having mudstone–siltstone and very fine-grained sandstone. The conglomerate lithofacies is divided into sedimentary breccia and quartz-pebble conglomerate facies. Mafic volcaniclastic deposits, closely associated with pillowed units, are contemporaneous with siliciclastic deposits and indicate the volcano-sedimentary nature of the succession. Lithofacies description is conducted in stratigraphic order (argillite–sandstone to conglomerate lithofacies) as depositional contacts between lithofacies, instrumental in defining sedimentary architecture, indicating a series of coarsening-upward (CU) sequences. 3.1. Argillite–sandstone lithofacies The argillite–sandstone lithofacies (Fig. 4), with N50% argillite component, is composed of a laminated cross-bedded (LCF) and an iron-formation (IFF) facies (Fig. 5A). The 5–13 m thick LCF is a combination of laminated argillite beds interstratified

with mm- to cm-scale rippled horizons (Fig. 2C), 1–4 cm thick graded, and 0.2–3 cm thick wavy- to planarsandstone beds (Fig. 2D). Argillite is black (Fig. 2D) to light green-weathering (Fig. 2E). Local 1–40 cm thick sandstone displays tangential cross-beds with argillite-rich foresets, 0.5–1.5 cm-scale ripples, and massive to graded beds. Discontinuous, 5–15 cm thick, pebbly-granule beds in shallow scours contain quartz grains, plutonic granules and argillite rip-up clasts. The 1–28 m thick IFF (Fig. 4A), which represents the prominent argillite portion of the lithofacies, contains alternating 0.1–1 cm thick light grey to white siltstone and dark mudstone laminae that form a graded couplet. The siltstone is thinly bedded and contains minor muscovite, whereas the mudstone, both magnetic and non-magnetic, contains hornblende and/or magnetite. Locally the buff white-weathering quartz-rich layers are replaced by magnetite (Fig. 4A). This unit is common, and laterally continuous at the outcrop scale. The upper and lower contacts of argillite–sandstone lithofacies crop out at several localities (Fig. 4). The basal contact is depositional with the underlying quartz arenite or conglomerate lithofacies, whereas the upper lithofacies contact varies from sharp depositional and erosional (Fig. 2D), to sharp depositional with the formation of ball-and-pillow structures (Fig. 2E), or with slumping into finer grained material. Although bedding contacts are sharp, changes from argillite–sandstone to sandstone–argillite are transitional over 1–2 m. 3.1.1. Interpretation The argillite–sandstone lithofacies is characterized by the continuous overlap of two distinct depositional regimes, whereby the laminated argillite represents low-energy suspension sedimentation under calm water conditions, and the sandstone reflects highand low-energy traction current structures. Highly fluctuating energy conditions are common to deepwater turbidites (Stow et al., 1996) or shallow-water clastic deposits (Reading and Collinson, 1996), but the bounding lithofacies and stratigraphy help constrain a setting. The presence of 10–120 m thick quartz arenites with abundant cross-bedding and an absence of thick turbiditic and pelagic deposits discount a deep-water submarine fan or deep sea setting (e.g., Stow et al., 1996).

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Fig. 4. Local stratigraphy with sedimentary lithofacies, lithofacies architecture forming coarsening-upward sequences, and repetition of coarsening-upward sequences at Beniah Lake. Modified after Mueller et al. (2005).

Lithofacies thickness and sedimentary structures favour a coastal current-affected area, in which argillite-draped cross-beds and planar beds, interpreted as sinuous-crested dunes and lower flow regime sheet sands, respectively, were subjected to tidal current influence (Houthuys and Gullentops,

1988; Nio and Yang, 1991; George, 1994). In detail, argillite is composed of siltstone–mudstone couplets derived from suspension fallout in a subtidal setting (Deynoux et al., 1993; Ehlers and Chan, 1999). These mm- to cm-scale couplets compare favourably with and in a qualitative fashion with Miocene

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Fig. 5. Characteristic Slave craton-cover lithofacies. Large yellow arrow indicates tops. (A) Iron-formation facies of the argillite–sandstone lithofacies exhibiting mm-scale mudstone–siltstone couplets (small yellow arrows) and alteration to iron formation. Scale, pen=14 cm. (B) Sharp depositional contact between quartz pebble conglomerate facies of the conglomerate lithofacies and overlying mafic volcaniclastic lithofacies. Scale, pen=7 cm. (C) Massive tuffaceous units of the mafic volcaniclastic lithofacies with quartz clasts. Scale, pen=15 cm. (D) Small arrows indicate sharp erosive contact between argillite–sandstone and sandstone–argillite lithofacies. Note argillite drapes (AD) between sandstone beds and 15–20 cm thick argillite (Ar) capping composite cross-strata. (E) Planar (laminated) beds and dunes draped by argillite (siltstone). Scale, pen=14 cm. (F) Tangential cross-bed exhibiting reactivation surfaces (RS) and an irregular basal bedding contact (BBC). The basal and upper bed contacts of the sandstone–argillite lithofacies, as displayed here, feature argillite laminae that constrain bedforms.

vertical tidal bundles (e.g., Tessier and Gigot, 1989) or Archean tidal rhythmites (Eriksson and Simpson, 2000). The graded quartz-rich siltstone–Fe-rich mudstone couplets were partially replaced and transformed into iron formation during amphibolite-grade

metamorphism and associated fluid migration. Interstratified ripples result from weak current action, and graded bedded sandstone and pebble lags were either flood- (Mueller and Dimroth, 1987) or stormgenerated (Kreisa, 1981).

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3.2. Mafic volcaniclastic lithofacies The 1–3 m thick mafic volcaniclastic lithofacies (Fig. 5B, C) occupies the same stratigraphic position as argillite–sandstone lithofacies, and at one locality shows a transition from buff-weathering laminated siltstone into green-weathering planar bedded to laminated tuff or a tuffaceous unit. The tuff-tuffaceous deposits are in constant proximity to mafic pillowed flows, and display a sharp depositional contact with pillowed flows. Fine mm-scale laminated to rarely rippled tuff has at one locality a sharp basal depositional contact with the quartz-pebble conglomerate facies (Fig. 5B). Up-section the tuff grades into a massive tuffaceous unit containing quartz pebbles (Fig. 5C). 3.2.1. Interpretation The mafic volcaniclastic deposits originated from reworking of the adjacent autoclastic flow breccias and thermal granulation of pillowed flows (Dimroth et al., 1978). Parallel lamination of the fine-grained tuff reflects suspension sedimentation with weak current action producing ripples, whereas the massive tuffaceous beds suggest deposition under mass flow conditions. A subtidal low-energy volcaniclastic setting can be inferred (e.g., Mueller, 1991). 3.3. Sandstone–argillite lithofacies The 1–16 m thick sandstone–argillite lithofacies (Fig. 4), with 70–90% sandstone and 10–30% argillite, displays argillite drapes on foresets and between medium- and small-scale bedforms (Fig. 5D, E). Individual bedforms in the 1–50 cm thick medium- to very coarse-grained sandstones are cross-beds with tangential foresets (Fig. 5F), planar to slightly wavy beds, and ripples or dunes. Reactivation surfaces are common in thicker crossbeds (Fig. 5F). Composite cross-strata, 0.5–1 m thick, are wedge-shaped and characterized by omnipresent argillite drapes (Fig. 5D, E), and within these structures planar beds and lens-shaped cross-beds (10–20 cm thick) are locally superposed with smallscale cross-beds. The 0.2–1 m thick argillite component contains prominent parallel laminated beds, ripples, and interstratified 1–8 cm thick graded sandstone beds. Minor pebble trains in 2–5 cm thick

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flat scours contain quartz, argillite rip-ups and plutonic clasts. The contact with the underlying argillite–sandstone lithofacies is sharp (Fig. 5D) and exhibits synsedimentary deformation (Fig. 2E), but field observations indicate a transition over 1–2 m, as suggested in Fig. 5B. The upper contact with the quartz arenite is preserved in a 0.5–2 m thick transition zone (Fig. 4). 3.3.1. Interpretation The increase in sandstone suggests more high- to low-energy bedload transport processes rather than suspension sedimentation, as indicated by argillite. Accordingly, sandstone bedforms become more composite in nature but still have an important argillite component that corroborates contrasting depositional processes. Such a combination favours tidal current influence in a shallow-water setting (Dalrymple, 1992). The wedge-shaped cross-strata represent inchannel, sinuous-crested sandwaves (George, 1994) and ubiquitous laminated argillite drapes are interpreted as reflecting pause periods during tidal activity (Dalrymple, 1992; Davies et al., 2003). As tidal currents have rapidly changing current velocities, the observed reactivation surfaces in tangential cross-beds support tidal activity (e.g., Richards, 1994). The capping argillite units (Fig. 5D), consistent with channel abandonment or migration, reflect a lower energy regime, probably subtidal shoals (Simpson and Eriksson, 1989; Richards, 1994; Johnson and Levell, 1995), because desiccation cracks, mud curls and rill marks, supportive of local emergence, are absent. As described for the argillite–sandstone lithofacies, graded beds and pebble trains in argillite units are consistent with a sudden sedimentary influx due to either flash flooding or storm activity (Kreisa, 1981; Mueller and Dimroth, 1987). 3.4. Quartz arenite lithofacies The 10–120 m thick quartz arenite (Figs. 4, 6A, B), the prominent lithofacies of the Bell Lake group, is composed of 0.15–1 m thick tabular to wedge-shaped composite cross-strata (Fig. 6C). The cross-strata contain minor planar (Fig. 6A) and tangential crossbeds with reactivation surfaces (Fig. 6B), and in both cases are usually capped by small-scale cross-beds

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Fig. 6. Characteristics of the quartz arenite and conglomerate lithofacies. Large arrow indicates top. (A) Planar cross-bed in composite crossstrata with 2 cm thick basal argillite drape. Scale, pen=14 cm. Upper parts of cross-strata are planar beds and small-scale tangential cross-beds. (B) Argillite drapes delineate upper and lower bounding surfaces of tangential cross-beds that form composite cross-strata. (C) Sets of composite cross-strata in the quartz arenite lithofacies. Note planar-tabular boundaries accentuated by argillite drapes. (D) Subrounded to well-rounded quartz pebbles in a clast-supported conglomerate. Scale, pen=8 cm. (E) Sandstone interbeds with wavy argillite drapes and argillite rip-up clasts. Note continuous cm thick argillite draping conglomerate. Scale, pen=15 cm. (F) Sedimentary breccia with prominent angular mudstone rip-up clasts. Scale, coin=1.3 cm in diameter.

(Fig. 6A, B). Argillite drapes, 0.2–2 cm thick, are located at the upper and lower bounding surfaces, but are also deposited on foresets. The argillite in the

quartz arenite is Cr-rich mudstone, and has transformed into green mica (fuchsite) during metamorphism. In addition to fuchsite, detrital chromite grains

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constitute part of the argillite component. Minor cmscale quartz-pebble trains erode the quartz arenite locally. The contacts of the quartz arenite lithofacies are commonly transitional, but can be abrupt to the underlying sandstone–siltstone lithofacies, and nonerosive, sharp with the overlying argillite–sandstone lithofacies. The conglomerate lithofacies is the onstrike equivalent of this lithofacies and locally, quartz-pebble conglomerate scours the quartz arenite. 3.4.1. Interpretation The quartz arenites reflect a high energy bedload transport regime, yet the fuchsite- and detrital chromite-rich argillite drapes at all sedimentary scales infer a significant drop in transport energy that requires specific hydrodynamic conditions, which are generally accommodated by tides (Houthuys and Gullentops, 1988; Nio and Yang, 1991; George, 1994). The stacked sets of tabular-planar and wedge-shaped composite cross-strata represent straight- to sinuous-crested sandwave migration in possibly primary tidal channels (Houthuys and Gullentops, 1988; George, 1994; Johnson and Levell, 1995). Waning or modified tidal current activity causes bed top reworking in the form of dunes-ripples (Kreisa and Moiola, 1986), or the formation of reactivation surfaces, respectively. The Cr-rich mudstone and chromite grains in thin laminae (see figure in Mueller et al., 2005) represent erosion of a mafic– ultramafic source, as identified at Beniah Lake (Covello et al., 1988), but also indicate slack water periods (Nio and Yang, 1991; Dalrymple, 1992) possibly following subordinate flood current stages (Eriksson and Simpson, 2000). The scours rich in quartz pebbles support terrestrial flooding periods. 3.5. Conglomerate lithofacies The conglomerate lithofacies contains a 1–13 m thick quartz-pebble conglomerate (Fig. 6D, E) and a 3–8 m thick sedimentary breccia facies (Fig. 6F). Subrounded to well-rounded quartz pebbles (Fig. 6D) comprise the clast- and matrix-supported conglomerate. Mudstone and mafic volcanic clasts are minor. The 0.15–2.2 m thick conglomerate beds are massive (Fig. 6D), stratified (Fig. 6E) and poorly graded with thicker beds being amalgamated units. The 5–30 cm

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thick planar to cross-bedded sandstone interbeds have wavy argillite rip-up clasts, argillite-draped reactivation surfaces, and mm–cm thick argillite capping sandstone beds (Fig. 6D, E). Argillite may also be deposited on conglomerate directly (Fig. 6D, E). Basal conglomerate contacts are low-angle, channeled scours and planar erosive surfaces. The massive to diffuse bedded sedimentary breccia (Fig. 6F) features subangular to angular sedimentary clasts of quartz arenite, mediumgrained sandstone, massive to laminated argillite, iron formation, and aphanitic volcanic fragments. The sedimentary breccia is matrix-supported and has local formation of diffuse bedding. This facies has a sharp depositional contact with the overlying iron formation and erodes the sandstone–argillite lithofacies. 3.5.1. Interpretation The two facies of the conglomerate lithofacies can be accommodated readily by short-lived, catastrophic events, such as large terrestrial floods, prograding onto the shelf (Kuenzi et al., 1979; Browne, 2002) and/or estuaries (George, 1994). The conglomerates with basal scours are high-energy bedload, sheetflood deposits (Colquhoun, 1995; Rasmussen, 2000) in contrast to the sedimentary breccia that can be explained by deposition under mass flow conditions with a hyperconcentrated flood flow component (Rasmussen, 2000). As pointed out by Orton and Reading (1993), traction current and debris flow deposits are commonly introduced by floods to the coastline and subaqueous portion of the shelf. Browne (2002) recently documented a modern analogue on the Canterbury Plains, South Island, New Zealand, where a large flood debouched into the sea. Stratified and cross-bedded conglomerates are considered subaqueous gravel (mouth) bar deposits (Rasmussen, 2000) and argillite drapes support this inference. The cross-bedded and planar bedded pebbly sandstone interbeds are interpreted as sandwaves (Richards, 1994), probably tide-influenced as suggested by wavy argillite drapes on reactivation surfaces (Ehlers and Chan, 1999) and on sandstone boundaries. Subaqueous hyperconcentrated flood flow or sheetflood processes best explain the massive pebbly sandstone (Simpson and Eriksson, 1989; Mueller and Corcoran, 2001).

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4. Sedimentary architecture, coarsening-upward sequences and volcanism The prominent siliciclastic deposits in association with minor mafic volcaniclastic and pillowed flow units exhibit a well defined lithofacies architecture. Important contact relationships at Beniah Lake exhibit an up-section sequential evolution of lithofacies (Fig. 7A–D), in which the ordered sequence of lithological events is, from base to top: (1) the argillite–sandstone, (2) the sandstone–argillite, and (3) the quartz arenite or conglomerate lithofacies (Fig. 4). The mafic volcaniclastic lithofacies, locally evolving from the argillite–sandstone lithofacies (Fig. 7A), forms at the top of Beniah Lake stratigraphy (Figs. 3B, 7A, D). Lithofacies stacking at Beniah Lake exhibits 7–120 m thick CU-sequences whereby six distinct CU-sequences, 7–85 m thick, were documented at one large outcrop zone. Another outcrop zone displays a preserved 120 m thick sequence of sandstone–argillite overlain by conglomerate and quartz arenite lithofacies, suggesting that thicker CU-sequences (possibly up to 150 m thick) are present (Fig. 8). In folded Archean sequences, observation of the lateral extension of contacts is limited and discounts basin-wide scale correlations, but the observed recurring lithofacies contacts and lithofacies successions support a hierarchy of sequence stratigraphic events. The contacts, (1) a sharp depositional one between quartz arenite and argillite–sandstone (mafic volcaniclastic) lithofacies (Figs. 4, 5B), (2) a sharp, erosive but also transitional one over 1–2 m between quartz arenite and sandstone–argillite lithofacies (Figs. 2D, E, 5D), (3) a sharp, erosive and a transitional contact over 1–2 m between sandstone–argillite and quartz arenite lithofacies, express a continuum of stratigraphic events (Fig. 4). The upper part of the stratigraphy at Beniah Lake features contemporaneous volcanism in which mafic volcaniclastic rocks (Figs. 3B, 7A, D) and a 20 m thick pillowed flow unit overlie the siliciclastic succession conformably. The mafic flows generally comprise 0.2–1 m large pillows with local brecciation and massive to stratified hyaloclastites. Subaqueous volcanism is inferred. Pickett (2002) documented that numerous mafic dykes cross-cut the sedimentary rocks at high angles (Fig. 8), but dykes grading into flows were not observed.

5. Discussion Excellent outcrop zones at Beniah Lake offer the unique opportunity of discussing relative sea level change in northern hemisphere 2.8 Ga Mesoarchean successions. A sequence stratigraphy model is discussed but in order to address this in a coherent manner several pertinent aspects require consideration, including (i) the pan-Slave basin evolution, (ii) the Beniah Lake depositional setting, (iii) Slave rift volcanism, and (iv) coarsening-upward sequences. 5.1. Pan-Slave basin evolution Detrital zircon ages for the siliciclastic rocks and adjacent basement (Isachsen and Bowring, 1997; Sircombe et al., 2001) indicate an extensive 2.8–4.0 Ga Slave continent that includes the Acasta gneiss, Augustus granite, central basement high at Beniah, Brown and Patterson Lakes and the Anton gneiss (Figs. 1, 9A). The Slave basement covered at least 80,000 km2, within which rare remnant 3.1 Ga supracrustal slivers (e.g., Winter Lake greenstone belt; Hrabi et al., 1995), and minor mafic–ultramafic igneous complexes are contained; both formed prior to deposition of the siliciclastic deposits (Mueller et al., 2005; Fig. 9B). From overall distribution of Bell Lake group sites (Fig. 1), preserved lithofacies in decametric CU-sequences (Figs. 4, 7), unconformitybounded fining-upward sequences (Mueller et al., 2005), and abundant quartz arenite, a platformal setting with marginal marine sedimentation must have been well established. Of the large-scale geodynamic analogues possible, a divergent plate margin setting with a clastic passive margin (Potter, 1986; Strand and Laajoki, 1999; Miall, 2002), a rifted continental margin with volcanism (Simpson and Eriksson, 1989; Ojakangas et al., 2001; Ziegler and Cloetingh, 2004), and a continental rift (Frostick and Steel, 1993; Miall, 2002) are possible, but a convergent foreland basin setting (Allen et al., 1986; Marzo and Steel, 2000; Gani and Alam, 2003) is also appropriate. Fore- and back-arc or arc-rift sequences do not compare favourably, as they lack thick quartz arenite successions, and display complex interactions with thick turbiditic flysch deposits and abundant volcanic and volcaniclastic rocks (Orton, 1996). Comparable Archean analogues

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Fig. 7. Beniah Lake CU-sequences A-D, cross-cut by mafic dykes.

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Fig. 8. Detailed lithofacies map with mafic dykes cross-cutting siliciclastic sedimentary rocks.

stem from the southern hemisphere Kaapvaal craton, namely the 3.0–2.8 Ga Witwatersrand foreland system (Catuneau, 2001; Catuneau and Biddulph,

2001), and the 3.3–3.2 Ga Fig Tree and Moodies Groups (Barberton greenstone belt) foredeep–foreland system (Jackson et al., 1987), as both have

W.U. Mueller, C. Pickett / Sedimentary Geology 176 (2005) 97–119 Fig. 9. Interpretation of the Bell Lake group in the Slave craton with exposed basement and sub-basins. The Anton Sea represents an epeiric sea protected to the east by a continental high, and the Augustus Ocean is the open ocean segment of the Slave craton to the east.

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abundant quartz arenite and iron formation, and subordinate volcanic rocks. The volcanic and sedimentary lithofacies indicate a subaqueous setting, with the unconformity possibly proximal to the subaerial–subaqueous interface, because the exposed gneisso-plutonic basement would be the locus of the eroded subaerial deposits. The geometry of the Slave basement (Fig. 9B) suggests a large-scale division into a western epeiric sea, analogous to Hudson Bay of Canada, accessing the eastern open ocean system formed on continental crust, as for the modern shelf segment of the Atlantic Ocean. These larger sub-basin structures are bounded by normal to listric faults, represented in the Slave craton by the ancestral Jackson Lake and Beniah Lake faults (Fig. 1). The Kaapvaal craton, South Africa, at 2.67 to 2.1 Ga, had a well-defined epeiric sea, with several cycles of rifting and a major flood basalt event, which drowned the Transvaal basin three times over a protracted period of 600 My (Eriksson et al., 2001). In order to produce such large-scale basins, crustal attenuation must occur steadily, and as individual rift basins span up to 280 My (Ziegler and Cloetingh, 2004), this could allow for sufficient time to form the Slave craton sub-basins. A western epeiric sea, herein referred to as the Anton Sea, and an eastern open ocean system, herein referred to as the Augustus Ocean, represent these craton-cover subbasins. The configuration of Slave sub-basins, the unconformable but autochthonous basement–cover relationships and an absence of thick conglomerate sequences (e.g., as in the Moodies Group) favour a rifted continental margin with subsequent drowning of the platform via basalt volcanism, rather than a thrustcontrolled foreland basin setting. The Slave craton sub-basins exhibit a conspicuous rift–drift transition. 5.2. Beniah Lake depositional setting The Beniah Lake succession, based on the lithofacies and geometry of Slave sub-basins (Fig. 9), represents the coastline of the Augustus Ocean. The envisaged setting is an irregular coastline with tidecontrolled estuaries (e.g., Reinson, 1992) and/or fandeltas (e.g., Davies et al., 2003) adjacent to topographic highs that delivered coarse clastic detritus to the coast (Fig. 10A–F). The Beniah Lake quartz arenites, a product of intensive weathering of the

uplifted gneisso-plutonic basement, were deposited along a stable marginal platform, and are comparable to quartz arenite successions of the Dharwar (Srinivasan and Ojakangas, 1986) and Zimbabwe cratons (Eriksson et al., 1994; Fedo and Eriksson, 1996). Few coastal settings accommodate the sedimentological complexity of the three studied siliciclastic lithofacies and their repetition in 7–120 m thick CUsequences. Differentiating between estuaries and fandeltas is difficult and a function of the sedimentology and lithofacies stacking during transgression and regression (Catuneau, 2002), but as suggested by Mueller et al. (2005), both geomorphological settings were present along the Augustus Ocean coast. Of major significance is the constant argillite, or mudstone–siltstone component in the medium- to very coarse-grained sandstone beds. Argillite, deposited on foresets of cross-beds, bounded composite tabular cross-strata, and where it formed millimetric siltstone– mudstone couplets, is a suspension deposit, in contrast to current-deposited sandstone, so that tidal currents best explain these highly variable energy conditions (e.g., Nio and Yang, 1991; Allen, 1991; Richards, 1994; Davies et al., 2003). The prominent very coarse-grained sandstones in the Beniah Lake siliciclastic lithofacies, with ubiquitous argillite, may be a trait of Archean macrotidal shorelines because the constant proximity of topographic highs, a weathering-aggressive Archean atmosphere and an absence of vegetation inhibiting runoff would promote rapid detrital input. Also, high-energy currents are required to transport the coarse clastic debris. The tripartite division of estuaries into fluvial, mixed fluvial-marine and marine influenced portions in a macrotidal system (Reinson, 1992; Fig. 10 inset) may have a modified Archean analogue in the Beniah Lake succession. The macrotidal Gironde estuary with its 2.5–6 m tide amplitudes (Allen, 1991) is strikingly comparable. The argillite–sandstone lithofacies with abundant mudstone signals low-energy conditions, therefore favouring an estuarine or embayment setting (Fig. 10A). This lithofacies is removed from the major tidal channels, possibly corresponding to the calmer part of the mixed fluvial-marine zone. The sandstone– argillite lithofacies combines principal sandbar and channel migration in a tide-controlled estuary with recurrent suspension deposition (Fig. 10B). A more evolved estuarine setting with tidal channels and local

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Fig. 10. Paleogeographic reconstruction of the Beniah Lake area with the up-section change from an estuarine to fan-delta complex, as defined by the decametric CU-sequence (A–D). Terminal mafic volcanism (E–F) is associated with the estuarine stage where maximum crustal thinning occurred.

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subtidal sandbars is envisaged. In contrast, the quartz arenite lithofacies is consistent with tide-influenced channels and sandbars debouching from the river system directly into the ocean (Fig. 10C). The argillite component, although present, is generally subordinate fuchsitic mudstone and siltstone. Furthermore, the conglomerate lithofacies with the quartz-pebble conglomerate and sedimentary breccia are explained by flash floods discharging gravels onto the shelf (Fig. 10D). The latter two lithofacies suggest a transition from an estuary into a fan-delta type setting. 5.3. Slave rift volcanism Volcanism requires either regional or local extension for magma to propagate through the crust. The mafic volcanic units located at the top of Bell Lake group stratigraphy (Figs. 3, 7A) indicate sufficient attenuation to allow for magma ascent via dykes, and suggest the change from passive rifting, indicated by cyclic sedimentation (Fig. 4), to active rifting, indicated by volcanism (Miall, 2002; Ziegler and Cloetingh, 2004). The mafic pillowed units and volcaniclastic deposits capping the stratigraphy are consistent with drowning of the Slave shelf, thereby terminating siliciclastic sedimentation. Mafic volcanism during the end of Bell Lake group development attests to extensive crustal attenuation, with a change from rift to drift tectonics (Fig. 10E, F). The precursor CU- and FU-sequences support crustal thinning. Felsic volcanism, identified by Isachsen and Bowring (1997) and Pickett (2002) at Dwyer and Bell Lakes, respectively, which precedes mafic volcanic activity, is part of the FU-sequences at the basal (?) unconformity (Mueller et al., 2005). The felsic volcaniclastic units near the base of the stratigraphy are local, but significant in that they attest to an early phase of crustal extension or major fracture zones facilitating magma ascent. Silicic volcanism may initiate fault propagation in intracontinental rifts (e.g., East African rift; Lahitte et al., 2003), but the dearth of felsic volcanism (b1%), as documented on a pan-Slave scale (Mueller et al., 2005) supports the notion of a thick continental crust. Slave felsic volcanism, restricted to point sources, straddled along zones of weakness in the crust, which are represented by the N-trending ancestral Jackson and Beniah Lakes faults.

The subordinate felsic and dominant mafic volcanism and associated dykes (Fig. 8) in the Slave craton are consistent with intracontinental rift (Lahitte et al., 2003) or continental break-up sequences (Bellieni et al., 1984; Fahrig, 1987; Hald and Tegner, 2000; Ojakangas et al., 2001), with the inference being that mantle upwelling caused crustal attenuation. In the Archean, continental break-up is suggested indirectly by the quartz arenite–komatiite assemblage, initially identified by Thurston and Chivers (1990). Komatiites, a product of plume activity (Campbell et al., 1989), although presently contested (Grove and Parman, 2004), support the rifting concept if associated with quartz arenite (Mueller et al., 2005). A comparative study of quartz arenite–basaltic–komatiite assemblages on known Archean cratons by Mueller et al. (2005), noted that break-up, just as during the Phanerozoic, is governed in large by mantle plumes. The subaqueous basalts of the Bell Lake group are associated with the argillite–sandstone lithofacies and therefore extruded during maximum basin subsidence. They may represent remnant subaqueous flood volcanism extruded during transgression and sea level highstands, as advocated by Arndt (1999) to explain the abundance of subaqueous flood basalts rather than continental flood basalts in early Precambrian greenstone belts. 5.4. Lithofacies stacking: Beniah Lake CU-sequences The CU-sequences (7–120 m thick) are an integral component of a coastal embayment–estuary–fan-delta sequence with water depths of b30 m (and probably b20 m), as inferred by the Beniah Lake tidal deposits and modern macrotidal estuary counterparts (e.g., Gironde estuary; Allen, 1991). Such depositional sites are prime loci where a change in accommodation can be monitored, because even subtle sea level changes have major implications for grain size and lithofacies distribution. The well-defined Beniah Lake CUsequences indicate transgression (Haq, 1991; Reinson, 1992; Reading and Collinson, 1996), either due to rapid tectonic uplift/basin subsidence or glacioeustasy (Plint et al., 1992). Transgression in tectonically active areas is characterized by marine flooding and fine-grained clastic sediments, and may be cyclical in rift, pull-apart and foreland basins (e.g., Blair and Bilodeau, 1988). The

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base of the Beniah Lake CU-sequences, composed of up to 3 m thick massive to laminated argillite, overlying directly conglomerate or quartz arenite, argue for sudden marine incursion and flooding, forming an estuarine complex, which is by definition transgressive (Reinson, 1992). The up-section change from coarse-grained sandstone, into quartz arenite and conglomerate signal a sequential infilling and a change in accommodation, supporting progradation with fan-delta formation (Fig. 10A–D). The local felsic volcanic input may have accelerated the progradational sequence, but did not significantly affect a change in accommodation at Beniah Lake, nor in the FU-sequence at the unconformities (e.g., Bell Lake; Mueller et al., 2005). The consistent lithofacies stacking at the decametric scale, important lithofacies contact relationships, and CU-sequences, indicating a repetition from estuarine to fan-delta complexes, are collectively strong arguments for event stratigraphy. A sequence stratigraphic approach thus seems viable. 5.5. Precambrian sequence stratigraphy A sequence stratigraphic approach for Precambrian rocks seems a viable concept as more studies are being conducted (e.g., Krapez, 1993; Catuneau, 2004; Catuneanu et al., this issue). Generally, five orders of cycles of sea level change have been defined (Vail et al., 1977; Plint, 1991; Plint et al., 1992; Catuneau, 2004), and are briefly addressed to put the Beniah Lake CUsequences and basal unconformity into context. Firstorder cycles, spanning 200–400 My, are related to supercontinent accretion and break-up; second-order cycles, ranging between 10 and 100 My group thirdorder cycles and are associated with the rate of midocean ridge spreading; 1–10 My third-order cycles are attributed to waxing and waning of continental ice masses, as well as smaller scale oceanic ridge changes. Fourth-order cycles, between 0.2 and 0.5 My in duration, are considered the response to growth and decay of ice sheets on the continents that control delta progradation and transgression, and fifth-order cycles, occurring at 10,000–2000, 000 ka intervals, are commonly considered to be orbitally forced glacioeustatic cycles. Low-order, high frequency 4th and 5th order periodicities are related to Earth’s orbital rhythms, referred to as Milankovitch cycles (see Plint, 1991; Reading and Levell, 1996). However, as

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discussed by Catuneanu et al. (this issue), the duration of cycles (orders) during the Archean may have been significantly different, as shifts in basin accommodation and sedimentation are time independent. The strongest evidence for Precambrian sequence stratigraphic events is associated with passive margin, foreland and intracratonic basins. Sound documentation stems from recent studies that show high-order sequence (low frequency) stratigraphic boundaries in the 2.7 to 2.1 Ga Transvaal Supergroup, South Africa (Catuneanu and Eriksson, 1999), and transgressive to regressive systems in the 2.8–3.0 Ga Witwatersrand basin, consistent with highstand and lowstand systems tracts (Catuneau and Biddulph, 2001). Establishing whether tectonic uplift/basin subsidence, or glacioeustatic sea level rise (or a combination thereof) affected sedimentation is problematic, as shown by Fulford and Busby (1993), because fluvial progradation and subsequent marine transgression was primarily a function of tectonism (rate 5 larger; subsidence 86–161 m/Ma) rather than sea level rise (17 m/Ma). Only abundant 40Ar/39Ar single crystal analyses of sanidine in tuff horizons, which spanned b2 My, permitted such an interpretation for these Late Cretaceous deposits. This level of resolution is not possible in the Archean. The 7–120 m thick Archean CU-sequences that form estuary–fan-delta complexes, compare favourably in scale with: (i) 50–150 m thick wave- and riverdominated progradational deltaic successions (Reading and Collinson, 1996), (ii) 20–100 m thick progradational tide-dominated foreland delta sequences (Davies et al., 2003), and (iii) transgressive or regressive 10–100 m thick estuarine complexes (Richards, 1994; Reading and Collinson, 1996). Additional comparisons include 10–80 m thick, Silurian proximal offshore to tidal flat CU-sequences with notable volcanic input in a successor basin of the Iapetus suture zone (Sloan and Williams, 1991), and 30–80 m thick CU-sequences in a non-barred shoreline of a thrust-controlled foreland basin (Duke and Prave, 1991). Similar thicknesses and setting argue for similar processes, as well as the same hierarchy of sequence stratigraphic cycles. Unconformities are a principal criterion for identifying various orders of sequence boundaries, as they can have local, regional or global extent. The panSlave unconformity between the gneisso-plutonic

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basement and Bell Lake group (Fig. 1) is interpreted as a first-order boundary consistent with N2.8 Ga continental break-up. The timeframe coincides with the Witwatersrand basin evolution, Kaapvaal craton (Eriksson et al., this issue) and is within 100 My of the quartz arenite successions on the Karelian, Dharwar, Zimbabwe, and Superior cratons. How these successions correlate remains elusive, but a global supercontinent break-up straddling 100 My is a plausible timeframe (Ziegler and Cloetingh, 2004). The secondorder cycles are best explained by combining stacked sets of Beniah CU-sequences (and FU-sequences at the unconformity) with early felsic and terminal mafic volcanism. The change from rift to drift represents a large-scale geodynamic tectonic process, coincident with ridge spreading. Individual CU-sequences, representing relative sea level change, are interpreted as third- to fourth-order cycles associated with continental rifting, and the sedimentary response to crustal attenuation. The smaller 7–20 m thick remnant CUsequences are probably a subset of the 20–120 m thick sequences (fourth-order?). The Beniah Lake sequence (Fig. 3B) is controlled by tectonism and documents break-up of the Slave continent. Although high frequency glacio-eustatic events probably played a role during the Archean, and especially with decametric third-order CU-sequences along a passive margin setting, it is not possible to gauge their importance and duration without volcanic markers permitting radiometric age determinations.

6. Conclusions The Bell Lake group at Beniah Lake, thus far the best studied Archean quartz arenite-dominated succession worldwide because of excellent exposure, displays salient sedimentary and volcanic responses to continental rifting. A sequence stratigraphic record could be established based on careful mapping of volcano-sedimentary lithofacies even though the stratigraphic record is fragmental, and the rocks have been submitted to upper greenschist to amphibolite metamorphism, and deformation with vertical dipping strata is prominent. The Beniah Lake CU-sequences, representing estuaries and fan-deltas, are considered essentially a product of tectono-eustasy with rapid basin subsidence due to crustal attenuation caused by

a mantle plume or convective mantle upwelling. Glacio-eustasy cannot be discounted and was probably operative, if compared to similar modern coastal passive margin settings, yet this scale of observation cannot be quantified to date. Using a sequence stratigraphic approach that considers the importance of bounding surfaces, unconformities, lithofacies architecture and repetition of lithofacies architecture, the following inferences are tenable. The pan-Slave basement–cover contact, a major unconformity, is the expression of first-order sea level change and coincident with (super) continent break-up. The up to 1 km thick Bell Lake group stratigraphy, composed of both FU-sequences at the basal unconformity (Mueller et al., 2005) and CUsequences up-section (this study), represents a secondorder sea level change, related to oceanic ridges and was a function of the spreading rate. Subaqueous mafic flooding of the siliciclastic sedimentary rocks at the top of the succession supports this notion. The subaqueous mafic volcanism signals the change from rift to drift tectonics. The individual CU-sequences are considered as representing third-order sea level changes, which reflect the immediate response to crustal thinning. The three levels of sequence stratigraphic cycles (orders) are representative of Slave craton evolution, but it must be kept in mind that, as mentioned by Catuneanu et al. (this issue), Precambrian time-scales of events may be significantly different. The Slave basement–cover unconformity may be traceable on a global scale with the quartz arenite as a marker horizon. Similarly on a global scale of dispersed Archean cratons, this siliciclastic continental break-up succession generally features an upper basalt–komatiite succession attesting to rifting. The clastic 2.7–3.0 Ga Witwatersrand basin on the Kaapvaal craton, and quartz arenite–basalt–komatiite assemblages on the Slave, Superior, Zimbabwe, Dharwar cratons, which developed between 2.7 and 2.9 Ga, may all be part of a supercontinent that rifted diachronously.

Acknowledgements The authors thank Bill Padgham and Carolyn Relf of the Department of Indian and Northern Development (DIAND), Northwest Territories for generous

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