Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: Paleoenvironments and sequence stratigraphy

    Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: paleoenvironments and sequence stratigraphy P.K. Pufahl, S.L. Anderson, E.E. Hiatt PII: DOI: Reference:

S0037-0738(14)00091-8 doi: 10.1016/j.sedgeo.2014.05.006 SEDGEO 4751

To appear in:

Sedimentary Geology

Received date: Revised date: Accepted date:

26 March 2014 17 May 2014 20 May 2014

Please cite this article as: Pufahl, P.K., Anderson, S.L., Hiatt, E.E., Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: paleoenvironments and sequence stratigraphy, Sedimentary Geology (2014), doi: 10.1016/j.sedgeo.2014.05.006

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Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: paleoenvironments and sequence stratigraphy

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P.K. Pufahla,*, S.L. Andersona,1 and E.E. Hiattb a

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Department of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia, Canada B4P2R6 b Department of Geology, University of Wisconsin Oshkosh, Wisconsin, 54901, USA

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_______________ * Corresponding author: telephone +1 902 585 1858; fax +1 902 585 1816; Email address [email protected] 1 Current address: Freeport-McMoRan Copper and Gold, 10861 N. Mavinee Drive, Suite A1, Oro Valley, Arizona, 85737

ACCEPTED MANUSCRIPT Abstract The Paleoproterozoic Sokoman Formation (ca. 1.88 Ga) of the Labrador Trough, eastern

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Canada, is a ca. 100-m-thick succession of interbedded iron formation and fine-grained,

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terrigenous clastic sedimentary rocks. Detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen-stratified water column, coastal upwelling of

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hydrothermally derived Fe and Si, as well as tide- and storm-generated currents controlled

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lithofacies character. Vertical and lateral facies stacking patterns record deposition through two relative sea-level cycles that produced seven distinct lithofacies comprising two unconformity-

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bounded sequences. Sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface. Sequence 2 is truncated by later erosion and encompasses the change to

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deeper-water accumulation of magnetite and Fe silicate-rich iron formation. The character and

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lateral distribution of redox-sensitive facies indicate that iron formation accumulation was

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controlled as much by shelf hydraulics as oxygen levels. The development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient-rich, upwelled waters offshore.

Deposition of other continental margin iron formations also occurred on Paleoproterozoic shelves that were favourably positioned for coastal upwelling. Variability between iron formations reflects intrinsic factors such as shelf profile, fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and nutrients. Aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline.

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ACCEPTED MANUSCRIPT As in the Phanerozoic, unconformities, and transgressive and maximum flooding surfaces frame iron formation sequences, but with important differences. The absence of trace and body

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fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult.

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Transgressive surfaces can also be easily mistaken for Phanerozoic-style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during

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ravinement. Outboard the accumulation of fresh precipitates increased sedimentation to produce

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a maximum flooding surface not usually marked by a prominent depositional hiatus. Understanding these differences is essential for establishing an accurate sequence stratigraphic

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framework. Such context is important because it is the backdrop for interpreting the sedimentology, oceanography, microbial ecology, and geochemistry of continental margin iron

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formations in proper paleoenvironmental, diagenetic, and metamorphic context.

Keywords: continental margin iron formation, Sokoman Formation, Labrador Trough, sedimentology, paleoceanography, sequence stratigraphy

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ACCEPTED MANUSCRIPT 1. Introduction Iron formation is a primarily Precambrian, Fe-rich, marine chemical sedimentary rock

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(Gross, 1980, 1983; Simonson, 2003; Clout and Simonson, 2005; Bekker et al., 2010; Pufahl,

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2010). It is generally subdivided into two broad categories based on tectonic environment: exhalative and continental margin types (Pufahl, 2010). Exhalative (Algoma type; Gross, 1980,

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1983) iron formation is mostly Archean in age and occurs in deformed and metamorphosed

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greenstone belts. Continental margin (Superior type; Gross, 1980, 1983) iron formation is generally Paleoproterozoic in age. The use of non-geographic terms such as “exhalative” and

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“continental margin” is preferred because emphasizing depositional context is more useful than simply comparing various iron formations to their type localities (Pufahl, 2010).

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Continental margin iron formation records the accumulation of iron formation and

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associated sediments on tectonically stable, unrimmed shelves that developed at the end of the

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Archean (Trendall, 2002; Simonson, 2003; Trendall and Blockley, 2004; Klein, 2005; Bekker et al., 2010; Pufahl, 2010). These giant deposits are of economic importance since they contain most of the world’s Fe for steel manufacture (Clout and Simonson, 2005). Scientifically, continental margin iron formation is significant because its appearance approximately coincides with increasing oxygen during the Great Oxidation Event (Bekker et al., 2004; Canfield et al., 2005; Holland, 2006). These events, together with the upwelling of anoxic, deep-ocean water rich in hydrothermally derived Fe and Si, are interpreted to have led to the widespread precipitation of iron formation on continental margins (Cloud, 1973). Because some iron facies are direct precipitates from seawater, iron formation chemistry can yield important insight into the changing composition of the early oceans and atmosphere. Although not normally applied to iron formations, a prerequisite for understanding their chemical composition is the interpretation of geochemical data in proper depositional context 4

ACCEPTED MANUSCRIPT using sequence stratigraphy (Pufahl and Hiatt, 2012; Eriksson et al., 2013). Sequence stratigraphy emphasizes lithofacies relationships and stratal architecture within a chronological

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framework (e.g. Catuneanu et al., 2009, 2011; Eriksson et al., 2013). Such an approach is

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especially timely given the widespread interest in continental margin iron formation for information about ocean-atmosphere evolution (Bekker et al., 2010; Pufahl and Hiatt, 2012). This

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permits assessment of whether geochemical anomalies represent conditions at the time of

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deposition, are truly global in character, result of local environmental factors, or are the consequence of alteration (Pufahl and Hiatt, 2012).

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We present and interpret the sedimentology, oceanography, and sequence architecture of continental margin iron formation from the Paleoproterozoic Sokoman Formation. The ca. 1.88

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Ga Sokoman Formation accumulated in the Labrador Trough in eastern Canada (Fig. 1) and is

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considered one of the best examples of continental margin iron formation. Lithofacies stacking

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patterns and depositional textures are exceptionally well preserved in parautochthonous regions of the Labrador Trough where low-grade metamorphism of horizontal strata is limited to prehnite-pumpellyite facies (Klein and Fink, 1976; Klein, 1978). A primary aim is to highlight differences in the character of key stratigraphic surfaces used to frame Phanerozoic and continental margin iron formation sequences.

2. General Geology The Sokoman Formation accumulated in the Labrador Trough, which is a belt of sedimentary and volcanic rocks along the margin of the Paleoproterozoic supercontinent Columbia (Fig. 2; Gross, 1968; Wardle and Bailey, 1981; Gross and Zajac, 1983; Clark and Wares, 2004; Evans and Mitchell, 2011; Edwards et al., 2012; Meert, 2012). This succession of iron formation belongs to the Ferriman Group of the Kaniapiskau Supergroup (Fig. 3). The 5

ACCEPTED MANUSCRIPT Kaniapiskau Supergroup is divided into three “cycles” of sedimentation interpreted to record rifting, drifting, and ultimate collision between the Superior Province, an Archean microcontinent

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known as the Core Zone, and the Nain Province in the east (Fig. 1; Hoffman, 1988; St-Onge et al.,

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2000; Wardle et al., 2002).

Terrigenous clastic sediments of the first cycle accumulated during rifting between the

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Superior and Nain Provinces at ca. 2.2 Ga (Wardle et al., 2002). Second-cycle sedimentation

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began by ca. 1.88 Ga and is characterized by clastics and iron formation of the Ferriman Group (Fig. 3). The Ferriman Group reflects passive margin development, upwelling of bottom water

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enriched in hydrothermally derived Fe (Clark and Wares, 2004), and the eventual telescoping of this margin during the onset of the Torngat Orogen. The Torngat Orogen occurred between ca.

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1.87 and 1.81 Ga as the Nain Province and Core Zone collided and produced the terrigenous

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clastic sedimentary rocks of the third and final cycle (Hoffman, 1988; Clark and Wares, 2004).

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The entire Kaniapiskau Supergroup was deformed again between ca. 1.82 and 1.77 Ga during the New Québec Orogen resulting in tectonic shortening through folding and faulting (Fig. 4; Wardle et al., 2002). In the vicinity of Schefferville, Québec, where this study is located (Fig. 1), metamorphism in northwest-southeast trending imbricate thrusts (Fig. 4) is limited to prehnitepumpellyite facies (Zajac, 1974; Klein and Fink, 1976; Dimroth and Dressler, 1978; Machado et al., 1997). High-grade alteration is restricted to the southern Labrador Trough near Labrador City (Fig. 1) where amphibolite grade metamorphism and refolding of these previously deformed rocks occurred during the Grenville Orogeny at ca. 1.0 Ga (Wardle et al., 2002; Clark and Wares, 2004).

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ACCEPTED MANUSCRIPT 3. Ferriman Group The Ferriman Group can be subdivided from base to top into the Wishart, Nimish, Ruth,

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Sokoman, and Menihek formations (Fig. 3). The Wishart Formation consists of medium- to

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coarse-grained quartz sandstone to arkose and interbedded siltstone and rests unconformably on first-cycle rocks or Archean basement of the Superior Province (Dimroth and Dressler, 1978),

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and is interpreted as a high-energy shelf deposit (Chauvel and Dimroth, 1974; Simonson, 1984,

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1985a; Clark and Wares, 2004). In the vicinity of Schefferville pyritic shale and chert of the Ruth Formation (Zajak, 1974; Klein and Fink, 1976; Edwards et al., 2012) unconformably

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overlie the Wishart Formation (Fig. 5A; Edwards et al., 2012; this study). This depositional hiatus implies the Ruth Formation is a transgressive shallow, muddy lagoon deposit that

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accumulated in calm embayments along the paleocoast, and is not a deep-water equivalent to the

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shelf sandstones of the Wishart Formation (e.g. Simonson, 1984, 1985a). The Ruth Formation is

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absent in the southern Labrador Trough where alkali volcaniclastics of the Nimish Formation overlie the Wishart Formation (Clark and Wares, 2004). The Nimish Formation is thought to record episodes of transtensional volcanism on this otherwise passive margin (Skulski et al., 1993; Findlay et al., 1995; Watanabe, 1996). Iron formation of the Sokoman Formation, the focus of this research, is in conformable contact with the Ruth Formation in the vicinity of Schefferville and is interbedded with the Nimish Formation to the south (Simonson, 1985a; Findlay, 1995; Clark and Wares, 2004). In the study area the Sokoman Formation is ca. 100-m-thick and grades from chert-rich mudstone at its base to granular and parallel-laminated iron formation at the top (Dimroth and Chauvel, 1973; Zajac, 1974; Klein and Fink, 1976; Klein, 2005; Edwards et al., 2012). It represents a range of shallow shelf environments from supratidal and lagoonal to middle shelf settings.

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ACCEPTED MANUSCRIPT A U-Pb age of 1877.8 ± 1.3 Ma for the Sokoman Formation is derived from zircons in syenitic cobbles forming an interbedded polymict conglomerate (Findlay et al., 1995). These

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cobbles are petrographically and geochemically identical to intercalated alkalic volcanics of the

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Nimish Formation.

Shale and turbidites of the overlying Menihek Formation are ca. 1000-m-thick and rest

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unconformably on the Sokoman Formation (Fig. 5B; Clark and Wares, 2004; this study). In

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many places this is now a tectonic contact between imbricate thrust sheets. The deep-water deposits of the Menihek Formation are interpreted as foreland fill shed from rising mountains

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during the onset of the Torngat Orogen (Hoffman, 1988).

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4. Methods

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Eleven drill cores through the Sokoman Formation near Schefferville, Québec, were

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described in detail at the cm-scale and sampled for sedimentologic, stratigraphic, and petrographic analysis (Fig. 4; drill cores HL1016D, HL1022D, HR1008D, HL1009D, HR1175D, HR1195D, HR1197D, HR1233D, HR1236D, HR1266D, HR1279D). Drill cores were selected to provide the most information about paleoenvironments of deposition, regional stratigraphic trends, and sequence architecture. Their distribution forms a transect through the Sokoman Formation that is sub-parallel to the inferred paleoshoreline (Fig. 4). Because iron formation lithofacies are texturally similar to many limestones, a modified version of Dunham’s classification (Dunham, 1962) was used to describe chemical sedimentary facies (Pufahl, 2010). Outcrop descriptions augmented drill core data to better understand facies changes and assist in distinguishing sequence stratigraphic surfaces necessary for regional correlation. Exposures of the Sokoman Formation commonly occur in imbricate thrust complexes of the New Québec Orogen (Fig. 4). The rocks have weathered into a series of north-south trending ridges 8

ACCEPTED MANUSCRIPT with bedding dipping between 10 and 80º to the east. Consequently, outcrops are best preserved on the steep western sides of ridges.

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Percentages of chemical sedimentary and terrigenous grains were estimated from 85 thin

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sections and were given an abundance index of rare (<10% particles), uncommon (11-40% particles), common (41-70% particles), and abundant (>70% particles). Grain mineralogies and

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paragenetic relationships were investigated using a Nikon Type 104 transmitted and reflected

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light microscope and a Nikon Eclipse E400 POL petrographic microscope equipped with a Reliotron III cold-cathode cathodoluminescence (CL) system. CL textures were observed using a

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beam current of 0.2-1.2 mA and an accelerating voltage of 8-10 kV under vacuum pressure of 60 mTorr. A JEOL JSM-5900LV scanning electron microscope (SEM) with a Princeton Gamma-

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Tech IMIX-PC EDS system confirmed mineralogies and paragenesis. Backscattered electron

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images showing compositional and textural associations were taken under a high vacuum of 10-7

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Torr with a working distance of 16-18 mm and accelerating voltages of 15-28 kV.

5. Sedimentology and Paleoenvironments The succession studied is composed of seven lithofacies (Table 1). All are primarily chemical in nature except for black shale of Facies F1. Facies F1 represents the Ruth Formation, and all others comprise the Sokoman Formation. This array of lithofacies reflects the wide range of hydrodynamic and oceanographic conditions across the paleoshelf.

5.1 Facies F1 – Pyritic black shale Interlaminated pyritic, black shale and microbially layered chert of the Ruth Formation unconformably overlies nearshore coarse-grained clastic rocks of the Wishart Formation (Table 1; Figs. 5A & C). Shale laminae contain sedimentary organic matter, fine sand-sized Fe9

ACCEPTED MANUSCRIPT carbonate and angular quartz grains, as well as framboidal pyrite. Rare, interbedded, waverippled grainstone beds are 2 to 4-cm-thick and formed of microcrystalline chert and Fe

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carbonate grains. Greenalite and coarsely crystalline anhedral to subhedral Fe-rich dolomite are

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common matrix constituents (Fig. 5D). Sedimentary apatite or francolite occur as phosphatic crusts in microbially layered chert, as in situ peloids in shale laminae, and as discrete, fine sand-

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sized grains within grainstone beds (Fig. 5D).

Interpretation. Facies F1 is interpreted to record suspension rain deposition of

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phytoplankton debris and terrigenous clastic silt within shallow, protected lagoons. The presence of wave-rippled grainstone suggests episodic storm transport of grainy sediment from more

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energetic coastal environments. The occurrence of framboidal pyrite indicates that pore water

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sulfate concentrations were sufficient to degrade sedimentary organic matter via bacterial sulfate

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reduction (Schieber, 2002). Such degradation liberated organically bound phosphate to pore water promoting the precipitation of phosphatic crusts and in situ peloids (Froelich et al., 1988; Arning et al., 2009) that were subsequently reworked and incorporated into grainstone beds (Föllmi et al., 1991; Pufahl, 2010). The microbially laminated chert likely reflects bacterial colonization of freshly precipitated opal-A, a hydrous chert precursor, formed during evaporitic concentration of silica in lagoonal waters (Pufahl, 2010).

5.2 Facies F2 – Chert mudstone This facies is formed of black or green microcrystalline chert that is commonly microbially laminated with rare subaerial exposure surfaces. Microbial laminae are generally composed of silicified, filamentous bacterial communities (Knoll and Simonson, 1981; Edwards et al., 2012) that form crenulated layers resembling modern pustular mats (Allen et al., 2009). 10

ACCEPTED MANUSCRIPT Desiccation cracks, phosphatized microbial laminae, and phosphatic crusts mark exposure surfaces (Fig. 5E). Some chert laminae contain fabric destructive dolomite, sedimentary organic

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matter, and framboidal pyrite (Fig. 5F).

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Thinly to thickly bedded, silicified, cross-stratified grainstone and feldspathic arenite beds are common. Grainstone beds are formed of sand-sized chert, quartz, Fe- carbonate, microcline,

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and glauconite grains that are cemented by microcrystalline chert. The thickest and coarsest

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grainstone beds also contain rounded francolite grains and pebble-sized, microcrystalline chert rip-up clasts. Cross-stratified sandstones are composed of subrounded detrital quartz grains and

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microcrystalline chert cement with dolomite rhombs. Rare stilpnomelane, pyrite, acicular greenalite, subrounded rutile grains and blocky calcite cement also occur in some beds. Stylolites

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are common where fabric destructive dolomite rhombs overprint primary sedimentary textures.

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Interpretation. This cherty facies is also interpreted to represent the suspension rain of phytoplankton debris and freshly precipitated opal-A in an evaporitic setting (Maliva et al., 2005; Pufahl, 2010). The filamentous morphology of fossilized bacterial communities suggests colonization of the seafloor by cyanobacteria, similar to Siphonophycus septatum, and/or Feoxidizing bacteria such Gallionella, Leptothrix, and Mariprofundus (Edwards et al., 2012). The presence of desiccation cracks, however, constrains deposition to periodically emergent inter- and supratidal environments (Pratt, 2010). As in Facies F1, the francolite crusts and framboidal pyrite are also interpreted to be the products of bacterial sulfate reduction. Reworking of siliceous intertidal flats by tides and storms is interpreted to have produced siliceous intraclasts that were subsequently reworked by traction currents into cross-stratified grainstones. Where silicified sandstones are common, terrigenous clastic sedimentation rates were higher along these portions of the paleoshoreline. 11

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5.3 Facies F3 – Stromatolite-bearing flaser-bedded chert grainstone

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Flaser- and lenticular-bedded, fine- to medium-grained chert grainstone and sandstone

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(Fig. 6A) with common domal stromatolites (Fig. 6B) comprise this facies. Stromatolites are generally jasperlitic (Edwards et al., 2012) and, depending on organic matter content, contain

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variable amounts of fabric destructive magnetite. Ripple cross-laminated grainstones are

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composed of subrounded, microcrystalline chert and hematite grains when associated with stromatolites (Fig. 6C), and chert and magnetite-replaced ankerite grains in areas away from

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these microbial structures. Thicker, slightly coarser beds also contain rare hematite and francolite intraclasts.

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Wavy laminated chemical mudstones between grainy beds are organic-rich and composed

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of either jasper or varying proportions of subhedral fabric retentive magnetite, euhedral fabric

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destructive magnetite, greenalite, stilpnomelane, microcrystalline chert, and silt-sized ankerite rhombs. Microbial textures and stylolites are common in more carbonate-rich laminae. These mudrocks are also rich in sedimentary organic matter, but composed of detrital quartz grains and muscovite cemented with ankerite.

Interpretation. Facies F3 reflects tidal reworking of seawater and authigenic precipitates, as well as terrigenous clastic silt and sand, in intertidal and shallow subtidal environments. In modern environments variable hydraulic conditions and bi-directional flow in peritidal environments produces flaser- and lenticular-bedding (Reineck and Singh, 1980). Differences in grainstone and chemical mudstone mineralogy are interpreted to record the paleo-redox conditions of ocean bottom and pore water. Grainstones and mudstones containing hematite are closely associated with stromatolites, likely reflecting photosynthetic oxygen 12

ACCEPTED MANUSCRIPT production and deposition under suboxic conditions (Klein 2005; Pufahl, 2010; Akin et al., 2013). The presence of reduced mineral phases such as magnetite, Fe-silicates, and ankerite in sediments

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represent accumulation in anoxic settings away from oxygen oases (Klein 2005; Pufahl, 2010;

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Akin et al., 2013). The interbedding of hematitic grainstone with magnetite-rich mudstone suggests that tides and storms transported intraclasts from suboxic, intertidal environments into

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anoxic, shallow subtidal settings where magnetite was being produced.

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The subhedral, fabric retentive nature of hematite and magnetite crystals suggests that these phases formed diagenetically from a Fe-(oxyhydr)oxide precursor (Beukes and Klein 1990;

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Klein, 2005; Akin et al., 2013). Hematite is interpreted to have formed during burial beneath the seafloor via the transformation of Fe-(oxyhydr)oxide that precipitated in the water column or at

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the seafloor (Pufahl, 2010). Magnetite probably formed authigenically in anoxic pore water

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through the addition of ferrous Fe to Fe-(oxyhydr)oxide by diffusion from overlying seawater

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(Laberge, 1964) or via microbial processes such as dissimilatory Fe reduction in the sediment (Walker, 1984; Raiswell, 2006; Johnson et al., 2008). Dissimilatory Fe reduction involves the oxidation of sedimentary organic matter by heterotrophic bacteria that use Fe-(oxyhydr)oxide as an electron acceptor (Johnson et al., 2008). Authigenic hematite and magnetite were later recrystallized to fabric destructive magnetite. Fabric destructive magnetite overprints all pre-existing textures, indicating it was the last phase formed during burial diagenesis and/or prehnite-pumpellyite facies metamorphism.

5.4 Facies F4 – Cross-stratified hematitic chert grainstone Composed of cross-stratified, reddish, hematitic chert grainstone (Fig. 6D) that is sometimes interbedded with metallic black, wavy and microbially laminated, magnetite-rich chemical mudstone. Grainstones are thinly to thickly bedded and composed of medium sand13

ACCEPTED MANUSCRIPT sized, subrounded microcrystalline chert and hematite intraclasts (Fig. 6E). Some intraclasts are concentrically coated with cortical layers composed of chert and hematite (Fig. 6F). Grainstones

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are cemented with subhedral magnetite crystals and microcrystalline chert with disseminated,

in some beds has produced a distinctive mottled texture.

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silt-sized dolomite rhombs. The intergrowth of diagenetic magnetite and microcrystalline chert

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Wavy and microbially laminated magnetite separating grainstone beds occurs in 0.5 to

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2.0-cm-thick packages. Individual laminae are formed of silt-sized, subhedral magnetite crystals

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in a matrix of sedimentary organic matter, microcrystalline chert and ankerite.

Interpretation. Facies F4 is a subtidal deposit that is transitional from the shallower,

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flaser- and lenticular-bedded grainstones and sandstones of Facies F3 to the deeper, magnetite-

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rich packstones of Facies F6. Cross-stratified grainstones of this facies are interpreted to record

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the migration of subaqueous dunes by tides and storms (Simonson, 1985b; Pufahl, 2010; Akin et al., 2013). Traction currents reworked chemical muds of freshly precipitated opal-A and Fe(oxyhydr)oxide to form rounded, sand-sized intraclasts (Simonson, 1985b, 1987; Pufahl, 2010) that were later converted to microcrystalline chert and hematite during diagenesis (Klein, 2005). In grainstones not associated with chemical mudstone, these grains were transported to produce stacked, trough cross-stratified beds that record dune migration across a grain-covered seafloor (Simonson, 1985; Akin et al. 2013). Coated hematite and chert in cross-stratified beds are interpreted to have formed by rolling intraclasts through accumulating Fe-(oxyhydr)oxide and opal-A muds (Pufahl, 2010), authigenic precipitation of cortical layers just beneath the seafloor (Akin et al., 2013), or a combination of both. The mineralogy and style of cortical layers forming many types of coated grains reflects variations in saturation state, Eh, and degree of sediment reworking (Pufahl and 14

ACCEPTED MANUSCRIPT Grimm, 2003). Thus, the formation of coated hematite and chert grains was likely the result of differences in the concentrations of Fe and silica, the redox potential of bottom- and pore-water,

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and tide and storm currents that reworked intraclasts across the seafloor.

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The interbedded, wavy and microbially laminated magnetite that occurs between some grainstone beds is interpreted to reflect background sedimentation, microbial colonization of the

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seafloor, and subsequent diagenetic conversion of Fe-(oxyhydr)oxide to magnetite in more

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muddy, anoxic areas. The extremely low oxygen concentrations required for the creation of magnetite (ca. PO2-water = 10-20; Klein, 2005) were likely fueled in part by the bacterial degradation

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of accumulating sedimentary organic matter (Froelich et al., 1988).

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5.5 Facies F5 – Concretionary, hematitic chert grainstone

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In situ septarian chert concretions (Figs. 7A) are interbedded with medium- to coarse-

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grained, hematitic grainstone (Fig. 7B) in this facies. Concretions are flattened in the plane of bedding and internally are concentrically coated, changing from a thick, chert centre through hematite to a magnetite-rich outer rim (Fig. 7C). Enveloping grainstone is composed of intraclastic chert and hematite grains and rare acicular stilpnomelane crystals that are cemented with microcrystalline chert. Well-indurated, pebble- and cobble-sized grainstone rip-ups are abundant (Fig. 7D). Grainstones in this facies are rarely organized into beds, but when present are similar to the cross-stratified beds in Facies F4. All grainstones are cemented with microcrystalline chert with subhedral ferroan dolomite and ankerite rhombs.

Interpretation. The presence of chert concretions, abundance of large, grainy intraclasts, and lack of well-organized beds suggest this is a condensed facies (Brett and Baird, 2002). The abundance of hematitc grains and similarity of bedded grainstones to those in Facies F4 point to 15

ACCEPTED MANUSCRIPT reworking and winnowing in a shallow, suboxic environment (Klein, 2005; Akin et al., 2013). As in limestones, such arrested sedimentation allowed hardgrounds to develop (Simonson, 1987)

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that were later ripped-up by tides and storms to form grainy intraclasts (Fig. 7D). Hardground

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cement is interpreted to have formed in a silica-saturated zone just beneath the seafloor. Slow sedimentation resulted in extended periods of shallow burial that allowed opal-A to precipitate to

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produce well-indurated grainstones.

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Such conditions are interpreted to have also produced concretions. The progressive change in mineralogy from chert through hematite to magnetite suggests concretion precipitation

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began in the silica-saturated zone just below the sediment-water interface. The lack of large intraclasts at their centres indicates initial nucleation around sand-sized chert and hematite grains.

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During burial concretions were removed from this zone to precipitate hematite and later

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magnetite in increasingly reducing pore waters. The septarian nature of concretions and absence

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of evidence for subaerial exposure in this facies suggests shrinkage cracks developed subaqueously through diagenetic volume loss when opal-A dehydrated and recrystallized to quartz (Pufahl, 2010).

5.6 Facies F6 – Wavy- and cross-bedded magnetite-rich packstone In this facies wavy- and cross-bedded, pink and dark grey chert packstone is interbedded with thin packages (2 to 4-cm-thick) of magnetite chemical mudstone (Figs. 8A & B). Packstones are thinly bedded and composed of rounded, medium sand-sized, microcrystalline chert and hematite intraclasts (Fig. 8C). Rare francolite and glauconite grains also occur in some beds. Unlike grainstones in Facies F5, intraclasts formed packstones that are generally matrixsupported. The thickest and coarsest beds also have erosive bases, abundant pebble-sized chert and hematitic mudstone rip-ups, and a pinkish mottling produced by pervasive carbonate 16

ACCEPTED MANUSCRIPT alteration. Common cements include microcrystalline chert and ankerite with disseminated dolomite rhombs. In chemical mudstones between packstone beds magnetite laminae are formed

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of abundant silt-sized, subhedral magnetite crystals that are also cemented by microcrystalline

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chert and ankerite (Fig. 8D).

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Interpretation. The wavy and cross-bedded nature of packstone beds suggests agitation,

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reworking, and dune migration by fair weather waves in deep subtidal environments (Dott and Bourgeois, 1982). Beds containing abundant rip-ups and hematitic intraclasts are interpreted as

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tempestites derived from shallow suboxic peritidal environments that were transported into deeper, anoxic settings. The abundance of magnetite and lack of hematite in interbedded

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prominent oxygen chemocline.

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chemical mudstones suggests this facies accumulated in a stratified water column below a

5.7 Facies F7 – Parallel-bedded, magnetite-rich chert grainstone Parallel-bedded, dark greyish green, fine- to medium-grained grainstone separated by wavy laminated magnetite (Fig. 8E) characterize this facies. Grainstones are thinly bedded and composed of abundant chert and Fe-silicate grains cemented with microcrystalline chert and magnetite. Rare francolite grains, stilpnomelane, Fe chlorite, and euhedral dolomite rhombs also occur in some grainstones (Fig. 8F). The thickest and coarsest beds have angular, pebble-sized, chert intraclasts along erosive bases. Magnetite laminae separating grainstones are composed of interpenetrating magnetite crystals with microcrystalline chert, blocky calcite, and rare euhedral pyrite.

17

ACCEPTED MANUSCRIPT Interpretation. Lithofacies F7 is interpreted to record erosive storm deposition on the anoxic middle shelf. The accumulation of granular tempestites composed of redeposited peritidal

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sediments punctuated the background suspension rain of Fe-(oxyhydr)oxide, producing

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interbedded grainy and muddy deposits (Nelson et al., 2010). As in Facies F6, the ubiquity of magnetite suggests that Fe-(oxyhydr)oxide on the seafloor was diagenetically converted to

SC

magnetite below a prominent oxygen chemocline. The rare occurrence of pyrite suggests that

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there was enough sulfate in pore water for bacterial sulfate reduction to accompany the reduction

MA

of Fe (Nelson et al., 2010; Edwards et al., 2012).

6. Sequence Stratigraphy

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Vertical and lateral lithofacies stacking patterns indicate that the Sokoman Formation

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records deposition through two relative sea-level cycles (Fig. 9). Systems tracts were correlated

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using a distinctive bed of evaporitic green chert (Facies F2) as a datum (Fig. 10). This horizon occurs at the base of Sequence 2 and is correlative throughout the study area.

6.1 Sequence 1

Sequence 1 is 50 to 70-m-thick and consists of terrigenous clastic sedimentary rocks and iron formation that rest either conformably or disconformably over quartz arenite and minor arkose of the Wishart Formation (Figs. 9 & 10). This contact is interpreted as a forced regression surface that is conformable in inferred deeper-water environments (Edwards et al., 2012). Lowstand deposits are composed of lagoonal black shale (Facies F1; Ruth Formation) and supratidal chert (Facies F2; Sokoman Formation) that are overlain by tidally deposited iron formation (Facies F3, F4; Sokoman Formation) of the transgressive systems tract. Highstand

18

ACCEPTED MANUSCRIPT sedimentary rocks are characterized by subtidal granular iron formation (Facies F4, F6; Sokoman

PT

Formation).

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6.1.1 Systems tracts

Lagoonal and supratidal sedimentary rocks (Facies F1, F2) of the lowstand systems tract

SC

(LST) are 5 to 20-m-thick and occur in all drill cores (Figs. 9 & 10). These deposits consist of

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variably phosphatic (francolite) siltstone and shale interlaminated with desiccation-cracked chert and microbialite (Edwards et al., 2012). A transgressive lag along the base of the LST is absent,

MA

which likely reflects either erosion along this low-energy energy segment of arid paleocoastline or lack of long-term exposure.

D

The transgressive systems tract (TST) is 5 to 30-m-thick, depending on antecedent

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topography (Fig. 10). It is thickest in drill cores HL1008D, HR1266D, and HR1236D where its

AC CE P

base is defined by a well-developed transgressive surface. This surface is a sharp, but conformable contact between tidally deposited, flaser- and lenticular-bedded chert grainstone (Facies F3) and overlying cross-stratified hematitic grainstone (Facies F4) of the TST. The maximum flooding surface is interpreted to be the contact between wavy bedded, magnetite-rich packstone (Facies F6) and basal HST deposits composed of hematitic cross-stratified grainstone (Facies F4). This stacking relationship likely reflects deposition in an oxygen-stratified water column where hematitic facies accumulated under suboxic conditions, and magnetite- and Fesilicate-rich sediments were deposited beneath anoxic waters (Klein, 2005; Pufahl, 2010). The highstand systems tract (HST) is 5 to 30-m-thick (Fig. 10) and records eventual progradation of cross-stratified grainstone (Facies F4) over deeper subtidal, magnetite-rich packstone (Facies F6). These grainy deposits are interpreted to reflect current- and wavereworking of ripples and dunes above fair-weather wave base. 19

ACCEPTED MANUSCRIPT

6.2 Sequence 2

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Sequence 2 is 20 to 60-m-thick and rests conformably on the HST of Sequence 1 (Figs. 9

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& 10). It is incomplete because most of the TST and all of the HST have been removed by subsequent erosion. As in Sequence 1, the LST contains supratidal chert (Facies F2), but instead

SC

of shale (Facies F1) is interbedded with peritidal grainstone (Facies F3). The base of the TST is

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characterized by a condensed facies of concretionary grainstone (Facies F5) interpreted to record hardground development on a wave- and current-swept transgressive surface. Where the

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remainder of the TST is preserved, it is composed almost entirely of magnetite-rich, chert grainstone (Facies F7). This parallel-bedded chemical sediment is finer-grained and interpreted

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6.2.1 Systems tracts

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D

to record storm deposition on the middle shelf.

The LST is generally 20 to 30-m-thick, but is as thin as 5-m in HL1022D (Fig. 10). Supratidal chert (Facies F2) with rare phosphatic crusts becomes more granular and coarsens upward over 30 to 40 cm into flaser- and lenticular-bedded chert grainstone (Facies F3). These ripple cross-stratified deposits are interpreted to have accumulated on evaporitic tidal flats and in shallow subtidal environments. The TST is absent in drill cores HL1009D, HL1008D, and HR1179D, with its greatest thickness of 55 m preserved in HL1022D (Fig. 10). The transgressive surface defining its base records diminished or net negative sedimentation in the zone of wave abrasion. Such conditions allowed silica and Fe to concentrate in pore water resulting in the authigenic precipitation of concretions (Facies F5) in the lower shoreface. With continued deepening the zone of wave abrasion migrated landward allowing tempestites to accumulate over this condensed surface. 20

ACCEPTED MANUSCRIPT These graded storm beds consist of grains derived locally and transported from peritidal environments. The conspicuous absence of traction deposits and abundance of authigenic

PT

magnetite and Fe-silicates suggests deposition under anoxic conditions below fair weather wave

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base (Klein, 2005; Pufahl, 2010).

The top of the TST is expressed as either the present-day erosion surface or a prominent

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disconformity between iron formation of the Sokoman Formation and shales of the overlying

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Menihek Formation. The Sokoman-Menihek contact is preserved in drill cores HR1266D,

MA

HR1236D, and HR1143D.

7. Depositional Model

D

Facies associations and lithofacies stacking patterns in the Sokoman Formation show that

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the LST and lower TST of the two sequences are characterized by stromatolites, evaporitic chert,

AC CE P

and hematite-rich, granular peritidal facies (Facies 3, 4, & 5). The upper portion of each TST is dominated by laminated facies containing chert, magnetite and Fe-silicates (Facies 6 & 7). Because the Sokoman Formation experienced only very low metamorphic grade (prehnitepumpellyite facies; Klein and Fink, 1976), this change from a chert-hematite-dominated nearshore to a chert-magnetite-Fe-silicate offshore likely reflects the differing redox conditions of bottom and pore water at the time of deposition (Klein, 2005; Pufahl and Hiatt, 2012). Such a preserved relationship is rare in iron formations that are hydrothermally altered, of higher metamorphic grade, and/or complexly weathered because paragenetic associations are often ambiguous (Pufahl and Hiatt, 2012; Rasmussen et al., 2014). Hematite is interpreted to record deposition in shallow, suboxic paleoenvironments where stromatolites produced photosynthetic oxygen in seawater that was otherwise anoxic (Fig. 11). Freshly precipitated Fe-(oxyhydr)oxide formed authigenically beneath oxygen oases and later 21

ACCEPTED MANUSCRIPT was converted to hematite in suboxic pore water during burial (Klein, 2005; Pufahl, 2010). “Suboxic” in this context is a relative measure of oxygen level in the water column and sediment

PT

and does not refer to specific authigenic reactions or oxygen concentrations (cf. Canfield and

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Thamdrop, 2009). In addition to the physiochemical precipitation of Fe-(oxyhydr)oxide through the abiotic oxidation of ferrous Fe by photosynthetic oxygen, preserved fossil microbial

SC

communities in suboxic, hematitic lithofacies also imply that precipitation by Fe-oxidizing

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bacteria such as Gallionella, Leptothrix, or Mariprofundas may have occurred (Edwards et al., 2012). Today, these Fe-oxidizing bacteria exploit the suboxic-anoxic redox interface, which in

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Paleoproterozoic oxygen oases could have occurred just beneath the seafloor, to oxidize ferrous to ferric Fe and passively precipitated Fe-(oxyhydr)oxide on the exterior of their cell walls

D

(Emerson and Moyer, 1997; Konhauser et al., 2002; Emerson et al., 2007).

TE

The shallow, wave-swept seafloor in oxygen oases was the grainstone factory (Pufahl,

AC CE P

2010). Sand- and granule-sized intraclasts were formed here by reworking freshly precipitated Fe-(oxyhydr)oxide and opal-A by waves and currents. Fine-grained sediment was winnowed and coarser grains were concentrated into subaqueous dunes to produce the widespread hematitic and cross-stratified grainstone that characterizes much of the Sokoman Formation. The thinly bedded to laminated magnetite and Fe-silicates that typify calmer, deeperwater environments are interpreted to reflect precipitation under anoxic conditions. The stability of these minerals during mudstone formation suggests that bottom and pore waters had extremely low dissolved oxygen levels that varied between ca. 10-70 and 10-20 PO2-water (Mel’nik, 1982; Klein, 2005). Authigenic magnetite is interpreted to have formed when Fe-(oxyhydr)oxide precipitated in suboxic surface waters incorporated ferrous Fe either from diffusion of overlying seawater (Pufahl, 2010) or through bacterial processes in the sediment (Walker, 1984; Raiswell, 2006; Johnson et al., 2008). The latter involves the respiration of sedimentary organic matter by 22

ACCEPTED MANUSCRIPT heterotrophic bacteria that use Fe-(oxyhyrdr)oxide as an electron acceptor to produce magnetite. Fe-silicates such as greenalite and stilpnomelane are also interpreted as synsedimentary

PT

precipitates, but formed from primary Fe-rich silica gels that formed and accumulated with Fe-

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(oxyhydr)oxides. The lateral distribution of redox-sensitive, authigenic minerals implies that facies character was controlled as much by shelf hydrodynamics as precipitation in an oxygen-

SC

stratified water column.

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The development of a suboxic surface ocean probably reflects photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that

MA

thrived in offshore environments (Pufahl and Fralick, 2004). The sharp transition from hematitic, peritidal grainstones to laminated magnetite- and Fe-silicate-rich chemical mudstones from more

D

distal settings further suggests that a prominent oxygen chemocline extended no deeper than fair-

TE

weather wave base. This relationship indicates that wave mixing of photosynthetic oxygen was

AC CE P

effective to a depth of ca. 20 m (Fig. 11). Such mixing during storms probably promoted the abiogenic precipitation of Fe-(oxyhydro)oxide by temporarily destroying the oxygen chemocline, permitting photosynthetic oxygen and ferrous Fe to react (Pufahl and Fralick, 2004; Edwards et al., 2012).

Minor negative Ce anomalies in rare earth element (REE) data from hematitic facies not present in magnetite- and Fe-silicate-rich deposits (Fryer, 1977) support this interpretation. Because the concentration of Ce in seawater is low, oxidative scavenging of Ce on the surface of freshly precipitated Fe-(oxyhyr)oxide removed Ce from seawater to produce the observed negative anomalies (Elderfield and Greaves, 1982; Ohta and Kawabe, 2001; Planavsky et al., 2009). Anoxic bottom waters like those of deeper-subtidal environments are interpreted to have precluded these anomalies from forming.

23

ACCEPTED MANUSCRIPT Upwelling of the bio-essential element P probably stimulated the high surface ocean productivities necessary for widespread oxygen stratification. Some paleogeographic

PT

reconstructions place the Labrador Trough on the west coast of Columbia in a low to temperate

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latitude south of the equator (Evans and Mitchell, 2011; Meert, 2012), which would be ideal for prolonged and sustained coastal upwelling. Such a position is analogous to the present-day west

SC

coasts of South America and Africa where two of the great upwelling regions of the world are

NU

centered. Here, prevailing shore-parallel winds and Ekman transport drive coastal upwelling. Ekman transport maintains high primary productivities by replacing surface waters pushed

MA

offshore with P-rich intermediate and bottom waters (Sverdrup, 1938; Burnett et al., 1983). In the Paleoproterozoic, coastal upwelling is interpreted to have also delivered a constant

D

supply of ferrous Fe and Si required for the precipitation of continental margin iron formation

TE

(Klein, 2005; Pufahl, 2010). In addition to seawater redox state, REE data from the Sokoman

AC CE P

Formation indicate that this upwelled Fe and Si was of hydrothermal origin in the Labrador Trough (Klein, 2005). A positive Eu anomaly (Fryer, 1977), similar to those in other continental margin iron formations (Klein, 2005), is interpreted to reflect the hydrothermal alteration of spreading centers and input of Fe and Si (Danielson et al., 1992; Isley and Abbott, 1999). More recently, Ge and Si ratios have been used to show that at least some of the Si in continental margin iron formation was also derived from continental weathering (Hamade et al., 2003). Although alkalic volcanism in the Labrador Trough was penecontemporaneous with iron formation deposition (Findlay et al., 1995), localized volcanic activity is not a prerequisite for iron formation accumulation. A generally anoxic Paleoproterozoic ocean with a lack of silica secreting organisms would have allowed hydrothermal Fe and Si to concentrate over millions of years, producing an oceanic reservoir that was periodically tapped by upwelling (Pufahl and Hiatt, 2012; Akin et al., 2013). 24

ACCEPTED MANUSCRIPT

8. Sequence stratigraphy and bounding surfaces in continental margin iron formation

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Sequence stratigraphy emphasizes lithofacies relationships and stratal architecture within

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a chronological framework (Catuneanu et al., 2009, 2011, and references therein). This framework links changes in the vertical and lateral stacking of facies to variations in

SC

accommodation and sediment supply through time. Thus, depositional sequences are defined

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based on stratal stacking patterns and recognition of key bounding surfaces formed during prominent inflections during the rise and fall of sea level. The recurrence of similar types of

MA

bounding surfaces through time indicates that the mechanisms forming cycles of accommodation change or sediment supply have largely remained the same (Catuneanu et al., 2011). The rates

D

and intensities of these processes have differed, however, resulting in dissimilarities in the

TE

amount of time represented by Precambrian and Phanerozoic successions (Eriksson et al., 2005,

AC CE P

2013; Miall, 2005; Catuneanu and Eriksson, 2007; Catuneanu et al., 2012). These contrasts are amplified in Precambrian depositional systems, such as continental margin iron formation that have no Phanerozoic analogs.

In spite of these differences, and lack of chronostratigraphic control in the Precambrian, the development of a robust sequence architecture can be based on a good understanding of facies relationships (Eriksson et al., 2013). Although not usually applied to Precambrian biochemical sediments, such an approach is timely given the widespread interest in iron formation chemistry and the potential for information about Earth’s early ocean-atmosphere (e.g. Pufahl and Hiatt, 2012). A sequence stratigraphic framework comprises the necessary foundation for interpreting sedimentology, oceanography, microbial ecology, and geochemistry in paleoenvironmental, diagenetic, and metamorphic context (Pufahl and Hiatt, 2012).

25

ACCEPTED MANUSCRIPT In Phanerozoic successions key surfaces used to frame sequences include the unconformity, transgressive surface, and maximum flooding surface (Catuneanu et al., 2009;

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Catuneanu et al., 2011). An unconformity reflects subaerial exposure and erosion during relative

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sea level fall, separating HST deposits or, when present, the falling stage systems tract (FFST) from the overlying LST. A transgressive surface marks the inflection between the LST and TST.

SC

It represents ravinement and shoreward migration of facies belts related to rapidly rising sea level

NU

during the onset of transgression. The maximum flooding surface also develops at an inflection point, but between the TST and HST. This surface forms when marine sediments reach their

MA

most landward position (e.g. Catuneanu et al., 2011).

Although these same bounding surfaces are present in continental margin iron formations,

D

there are three important differences (Pufahl, 2010): 1) the lack of trace and body fossils, as well

TE

as terrestrial vegetation, can make recognition of key bounding discontinuities difficult; 2)

AC CE P

transgressive surfaces can be easily mistaken for Phanerozoic-style maximum flooding surfaces; and 3) the maximum flooding surface is not usually marked by a prominent depositional hiatus. In the absence of body fossils, bioturbation, and terrestrial vegetation, key bounding surfaces in continental margin iron formations such as the Sokoman Formation are recognized by sharp facies transitions recording rapid changes in accommodation that mark inflection points between systems tracts. Rapid rise or fall of relative sea level can produce easily discernable surfaces because lithofacies deposited in very different paleoenvironments are often juxtaposed. One of the most obvious differences in key bounding discontinuities is the nature of the transgressive surface. Because this surface is especially well developed in the Sokoman Formation, there is excellent insight into its attributes. The transgressive surface is preserved as an easily recognizable indurated, concretionary horizon (Facies F5) in the heart of the grainstone factory of Sequence 2. While the processes of formation are different, this horizon has many of 26

ACCEPTED MANUSCRIPT the same physical attributes as condensed intervals associated with maximum flooding in Phanerozoic sequences. Rather than sediment starvation during maximum flooding, however,

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ravinement during transgression is interpreted to have resulted in stratigraphic condensation.

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Constant wave-sweeping of the seafloor diminished sedimentation enough for precipitation and reworking of authigenic, chert-hematite-magnetite concretions and associated siliceous

SC

hardgrounds. Similar to the development of carbonate hardgrounds, the maintenance of the zone

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of saturation just beneath the sediment-water interface, and the wave-pumping of saturated seawater through the sediment were probably prerequisites for producing a lithified seafloor

MA

(Sami and James, 1996).

Fe-redox pumping likely enhanced the precipitation of ubiquitous chert cement that

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further indurated the seafloor in this shallow, suboxic environment (Pufahl, 2010). Fe-redox

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pumping is a cyclic mechanism that concentrates dissolved silica in pore water by releasing silica

AC CE P

adsorbed onto freshly precipitated Fe-(oxyhdr)oxides (Fischer and Knoll, 2009; Pufahl, 2010). During burial beneath the Fe-redox boundary the dissolution of Fe-(oxyhydr)oxides liberated silica to saturate pore waters. The escape of silica out of the sediment was prevented by readsorption onto Fe-(oxyhydr)oxides just above this redox interface. The final major difference of key bounding surfaces in continental iron formation sequences relative to Phanerozoic carbonate and clastic systems is the character of the maximum flooding surface. In these Proterozoic systems the maximum flooding surface is rarely discernable by a depositional hiatus or typical condensed facies that may include phosphatic lags or hardgrounds. This is because of the generally high sedimentation rates of chemical precipitates across the shelf (Pufahl, 2010). Thus, the maximum flooding surface is identifiable only as the sharp change from deeper to shallower-water lithofacies. In Sequence 1 of the Sokoman Formation the maximum flooding surface is interpreted as the contact between wavy 27

ACCEPTED MANUSCRIPT bedded, magnetite-rich packstone (Facies F6) and overlying cross-stratified, hematitic grainstone (Facies 4). Such juxtaposition implies that hematitic grainstones reflect the onset of progradation

PT

as accommodation began to diminish at the start of the HST.

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The lack of phosphatic material along the maximum flooding surface probably reflects a combination of two processes. Sedimentation rates were likely too high for phosphate to saturate

SC

pore water, and more importantly, critical phosphogenic redox boundaries were suspended in the

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anoxic water column rather than in the sediment (Nelson et al., 2010; Pufahl, 2010; Pufahl and Hiatt, 2012). Phosphatic deposits did, however, accumulate in shallow environments beneath

MA

photosynthetic oxygen oases because important redox interfaces were depressed into the suboxic sediment (Nelson et al., 2010, Pufahl and Hiatt, 2012). In the Sokoman Formation, peritidal

D

phosphatic mudstones, cherts, and grainstones (Facies F1, F2, F3, F6) are similar to other

TE

phosphorus enriched, coastal iron formation facies (Nelson et al., 2010). The development of

AC CE P

phosphate-associated maximum flooding surfaces likely occurred at ca. 580 Ma when oxygenation of the deep ocean (Canfield et al., 2007; Narbonne, 2010) finally forced phosphogenic redox processes beneath the seafloor across the entire shelf (Nelson et al., 2010; Pufahl and Hiatt, 2012).

Appreciation of these differences will assist in establishing accurate facies architectures. Such a depositional framework is essential to making meaningful interpretations about the sedimentology, oceanography, microbial ecology, geochemistry, and alteration of continental margin iron formation (Pufahl and Hiatt, 2012).

10. Continental margin iron formation Lithofacies associations and stratal architecture of other continental margin iron formations support the notion that deposition occurred on Paleoproterozoic shelves that were 28

ACCEPTED MANUSCRIPT favourably positioned for coastal upwelling. In addition to the Labrador Trough, excellent examples are preserved in the Animikie Basin, Lake Superior region of North America and the

PT

Earaheedy Basin of Western Australia (Gross, 1980, 1983; Trendall and Blockley, 2004; Fralick

RI

and Barrett, 1995; Simonson, 2003; Bekker et al., 2010; Pufahl, 2010; Rasmussen et al., 2012; Akin et al., 2013). Older Paleoproterozoic examples of continental margin iron formation (ca.

SC

2.5 Ga) occur in the Transvaal, Hamersley, Krivoy Rog, Kursk, and the Quadrilátero Ferrífero

NU

regions of South Africa, western Australia, Ukraine, Russia, and Brazil, respectively (Gross,

2008; Bekker et al., 2010; Pufahl, 2010).

MA

1980, 1983; Simonson and Goode, 1989; Trendall and Blockley, 2004; Beukes and Gutzmer,

The abundance of laminated, magnetite-rich facies and rarity of granular iron formation in

D

these older contemporaneous iron formations suggest deposition on the distal shelf or slope. The

TE

uncommon occurrence of hematitic, granular event beds derived from shallower, suboxic

AC CE P

environments (Pufahl and Hiatt, 2012) imply deposition below a well-developed oxygen chemocline (Klein and Beukes, 1989; Simonson and Hassler, 1996; Pufahl and Hiatt, 2012). Although the nature and complexity of such stratification can be debated (Beukes and Gutzmer, 2008), the interbedding of jasperlitic grainstones with laminated, deep-water magnetite (Pufahl and Hiatt, 2012) suggests that oxygen concentration and water depth were primary controls on lithofacies character. Younger iron formations from the Labrador Trough, Animikie Basin, and Earaheedy Basin (ca. 1.9 Ga) contain the full range of granular and laminated, redox-sensitive facies because they accumulated in the complete spectrum of shelf environments. Nevertheless, each is a distinctive depositional system with unique sedimentologic attributes. Not surprisingly, the ca. 1.88 Ga iron formations from the Animikie Basin (Simonson, 1985b, 2003; Morey and Southwick, 1995; Pufahl, 1996; Ojakangas, 2001; Fralick et al., 2002; Pufahl and Fralick, 2004) are most similar to the Sokoman Formation since they accumulated 29

ACCEPTED MANUSCRIPT coevally along the margin of Columbia (Fig. 2). Like the Sokoman Formation, these iron formations contain a variety of granular, hematitic, shallow-water lithofacies and laminated,

PT

magnetite and Fe-silicate-rich, deeper deposits that define a sedimentary wedge that fines and

RI

thickens away from the paleocoastline (Pufahl, 1996). Differences in the Animikie Basin include the presence of interbedded limestone and organic-rich shale.

SC

The ubiquity of peritidal, Fe-carbonate grainstone in the Gunflint Formation,

NU

northwestern Ontario, is interpreted to record the saturation and precipitation of calcium carbonate from Fe-rich coastal waters. The absence of such carbonate in the Sokoman Formation

MA

suggests that shallow paleoenvironments in the Labrador Trough were seldom saturated with respect to calcium carbonate. The abundance of chert grainstone supports this interpretation

D

since the low pH conditions that promote silica precipitation also cause carbonate undersaturation.

TE

In the Trommald Formation of southern Minnesota, the occurrence of interbedded

AC CE P

organic-rich shale and laminated, magnetite and chert-rich iron formation suggests distal deposition near the upwelling front (Pufahl, 1996). Even during maximum flooding, these facies did not accumulate in the more proximal deposits preserved in the Sokoman Formation. As in modern coastal upwelling environments, the inferred high primary productivities are interpreted to have resulted in an elevated flux of organic C to the seafloor and deposition of organic-rich sediment (Raymont, 1980; Barber and Smith, 1981). Phanerozoic black shales often host phosphorite and are important hydrocarbon source rocks (Glenn et al., 1994; Pufahl, 2010). The association of contemporaneous organic-rich shale and iron formation in the Animikie Basin also highlights that, as in the Phanerozoic, the preservation and accumulation of sedimentary organic matter is environment specific. Thus, the commonly held view that continental margin iron formations are universally barren of organic C is not entirely true (Klein, 2005). The apparent dearth of non-upwelling-related, organic-rich facies probably reflects the 30

ACCEPTED MANUSCRIPT slow settling rates of planktic bacteria (Edwards et al., 2012) and its degradation in the water column before reaching the seafloor (Porter and Robbins, 1981). Ballasting of organic matter,

PT

which results in higher settling velocities, was limited through much of the Precambrian by the

RI

absence of zooplankton and their ability to concentrate C in fecal pellets (Porter and Robbins, 1981; Logan, 1995; Butterfield, 1997). Some organic matter may have also been destroyed

SC

during the diagenetic conversion of hematite to magnetite (Perry et al., 1973). However, this loss

NU

was likely too small to account for the ubiquitous presence of magnetite in some facies (Beukes et al., 1990), further suggesting that organic carbon was remineralized in the water column before

MA

it could be deposited.

In addition to upwelling, the lack of diluting terrigenous clastics was also a prerequisite

D

for iron formation deposition. As in carbonate depositional systems, accumulation occurred

TE

because the rate of precipitation was much higher than siliciclastic input. In the younger, ca. 1.9

AC CE P

Ga, continental margin iron formations sedimentologic data indicate this was the consequence of aridity. An arid climate promotes higher chemical sediment precipitation rates via evaporative concentration and decreases clastic input by reducing fluvial activity. Arid intertidal facies are ubiquitous in the Labrador Trough, Animikie, and Earaheedy basins where desiccation cracked chemical mudflat facies and evaporitic chert characterize coastal deposits (Pufahl, 1996; Pufahl, 2010; Edwards et al., 2012; Akin et al., 2013). In the Labrador Trough, the lack of chemical weathering of feldspar in arkosic sandstone of the Wishart and Sokoman formations, and the virtual absence of riverine and deltaic sediments (Simonson, 1985a; Edwards et al., 2012) also indicate aridity. The conspicuous absence of sulfate evaporites typical in arid, Phanerozoic peritidal environments likely reflects the very low sulfate concentrations of Precambrian seawater (Pope and Grotzinger, 2003; Zentmyer et al., 2011).

31

ACCEPTED MANUSCRIPT When an arid climate was coupled with significant eolian input, as in the Frere Formation of the Earaheedy Basin, iron formation deposition was restricted to shallow suboxic

PT

environments (Akin et al., 2013). The high flux of wind-blown sediment to middle and distal

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shelf environments precluded the accumulation of deeper, laminated Fe-rich facies (Akin et al., 2013). The lack of terrigenous clastic sediment in hematitic, peritidal grainstones is interpreted

SC

to reflect the ease with which coastal currents transported fine-grained, eolian sands offshore.

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This process, in conjunction with the suspension settling of wind-blown sediment below the oxygen chemocline, diluted anoxic precipitates to form a sandy distal shelf (Akin et al., 2013).

MA

It is difficult to ascertain if aridity played a significant role in the deposition of the older, ca. 2.5 Ga continental margin iron formations. Nevertheless, the presence of coeval platform

D

carbonates in the Transvaal and Hamersley basins (Krapez et al., 2003; Rasmusssen et al., 2005;

TE

Beukes and Gutzmer, 2008) suggests either an arid climate and minimal input of terrigenous

AC CE P

clastics or trapping of siliciclastics in nearshore settings to prevent the dilution of accumulating limestone and iron formation.

9. Conclusions

1) The Paleoproterozoic Sokoman Formation consists of seven lithofacies that define two depositional sequences through two sea-level cycles. Sequence 1 is 50 to 70-m-thick and consists of peritidal terrigenous clastic sedimentary rocks and chert- and hematite-rich, granular iron formation that rest conformably to disconformably over tidally deposited sandstone of the underlying Wishart Formation. Sequence 2 is 20 to 60-m-thick, but is incomplete due to postdepositional erosion. It rests conformably on Sequence 1 and is also formed of hematitic, peritidal grainstone. Unlike Sequence 1, however, these shallow deposits are overlain by deeper water, chert-magnetite-Fe-silicate-rich, laminated iron formation. 32

ACCEPTED MANUSCRIPT 2) The stacking pattern and mineralogy of redox sensitive lithologies implies that facies character was controlled first and foremost by shelf hydrodynamics and secondly by the

PT

overlying oxygen-stratified water column. The mineralogy and close association of hematitic

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granular facies with stromatolites suggests “suboxic” deposition in photosynthetic oxygen oases. The shallow, wave-swept seafloor in oxygen oases served as the grainstone factory where

SC

granular iron formation was born by reworking authigenic precipitates. In addition to the abiotic

NU

oxidation of ferrous Fe by photosynthetic oxygen, preserved fossil microbial communities in shallow lithofacies also imply precipitation by Fe-oxidizing bacteria. Laminated magnetite and

MA

Fe-silicates accumulated beneath the oxygen chemocline in “anoxic” middle shelf environments. Interbedded granular tempestites indicate storm redeposited peritidal sediments punctuated the

D

background suspension rain of chemical precipitates formed in the water column. Upwelling is

TE

interpreted to have provided the constant supply of hydrothermal Fe and Si for iron formation

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precipitation. The high surface productivities necessary for widespread oxygen stratification were also probably related to upwelling through the delivery of P, a biolimiting nutrient element. 3) Sequence stratigraphy provides the necessary framework for interpreting sedimentology, petrography, and geochemistry in paleoenvironmental, diagenetic, and metamorphic context of continental margin iron formations. As in the Phanerozoic, key bounding discontinuities used to define sequences include unconformities, transgressive surfaces, and maximum flooding surfaces. However, the lack of trace and body fossils as well as terrestrial vegetation make the recognition of these key surfaces difficult. Instead, surfaces are recognized by sharp facies transitions that record rapid changes in accommodation marking inflection points between systems tracts. The identification of the transgressive surface is further confounded because extensive synsedimenatry cementation creates a discontinuity that has many of the same attributes as a Phanerozoic-style maximum flooding surface. In continental margin 33

ACCEPTED MANUSCRIPT iron formation the maximum flooding surface also differs since it is not marked by a prominent depositional hiatus. A condensed interval could not develop because of copious precipitation in

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the distal water column. The maximum flooding surface is identifiable only as a sharp change

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from deeper to shallower-water lithofacies reflecting diminished accommodation and onset of progradation.

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4) The sequence architecture of other continental margin iron formations also suggests

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accumulation occurred on Paleoproterozoic shelves that were favorably positioned for coastal upwelling. Dissimilarities between iron formations reflect intrinsic factors such as shelf profile,

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fluvial contribution, eolian input, evaporation rates, and coastal current systems, which influenced upwelling dynamics and the delivery of Fe, Si, and P. In addition to upwelling, aridity

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was also apparently a prerequisite for deposition because it controlled the type and abundance of

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diluting terrigenous clastic sediment as well as the evaporative precipitation of minerals along the

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shore.

Acknowledgements

This research was funded by a Natural Sciences and Engineering Research Council of Canada Discovery Grant to PKP, an Acadia Graduate Award to SLA, and a University of WisconsinOshkosh Faculty Development Research Grant (FDR375) to EEH. LabMag-New Millennium Corporation provided field support and access to drill core. Discussions with Cole Edwards, Tom Clark, Sara Akin, Gabe Nelson, Dean Rossell, and Phil Fralick assisted in refining our interpretations. Thoughtful reviews by Bruce Simonson and Pat Eriksson also improved the paper.

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ACCEPTED MANUSCRIPT References Akin, S.J., Pufahl, P.K., Hiatt, E.E., Pirajno, F., 2013. Oxygenation of shallow marine

RI

Formation, Western Australia. Sedimentology 60, 1559-1582.

PT

environments and chemical sedimentation in Palaeoproterozoic peritidal settings: Frere

SC

Allen, M.A., Goh, F., Burns, B.P., Neilan, B.A., 2009. Bacterial, archeal and eukaryotic diversity

NU

of smooth and pustular microbial mat communities in the hypersaline lagoon of Shark Bay.

MA

Geobiology 7, 82-96.

Arning, E.T., Luckge, A., Breuer, C., Gussone, N., Birgel, D., Peckmann, J., 2009. Genesis of

TE

D

phosphorite crusts off Peru. Mar. Geol. 262, 68-81.

AC CE P

Barber, R.T., Smith, R.L., 1981. Coastal upwelling ecosystems. In: Longhurst, A.R. (Ed.), Analysis of Marine Ecosystems. Academic Press, New York, pp. 31-68.

Bekker, A., Slack, J.F., Planavsky, N., Krapez, B., Hofmann, A., Konhauser, K.O., Rouxel, O.J., 2010. Iron formation; the sedimentary product of a complex interplay among mantle, tectonic, oceanic, and biospheric processes. Econ. Geol. 105, 467-508.

Beukes, N.J., Klein, C., 1990. Geochemistry and sedimentology of a facies transition, from microbanded to granular iron-formation, in the early Proterozoic Transvaal Supergroup, South Africa. Precambrian Res. 47, 99-139.

35

ACCEPTED MANUSCRIPT Beukes, N.J., Klein, C., Kaufman, A.J., Hayes, J.M., 1990. Carbonate petrography, kerogen distribution, and carbon and oxygen isotope variations in an early Proterozoic transition from

PT

limestone to iron-formation deposition, Transvaal Supergroup, South Africa. Econ. Geol. 85,

RI

663-690.

SC

Beukes, N.J., Gutzmer, J., 2008. Origin and paleoenvironmental significance of major iron

NU

formations at the Archean-Paleoproterozoic boundary. In: Hagemann, S., Rosière, C., Gutzmer, J., Beukes, N.J. (Eds.), Banded iron formation-related high-grade iron ore. SEG Reviews in

MA

Economic Geology 15, pp. 5-47.

D

Brett, C.E. and Barid, G.C., 2002. Revised stratigraphy of the Trenton Group in its type area,

TE

central New York state: sedimentology and tectonics of a Middle Ordovician shelf-to-basin

AC CE P

succession. Physics and Chemistry of the Earth 27, 231-263.

Burnett, W.C., Roe, K.K., Piper, D.Z., 1983. Upwelling and Phosphorite Formation in the Ocean. Plenum Press, New York.

Butterfield, N.J., 1997. Plankton ecology and the Proterozoic-Phanerozoic transition. Paleobiology 23, 247-262.

Canfield, D.E., Thamdrup, B., 2009. Towards a consistent classification scheme for geochemical environments, or, why we wish the term "suboxic" would go away. Geobiology 7, 385-392.

36

ACCEPTED MANUSCRIPT Canfield, D.E., 2005. The early history of atmospheric oxygen; homage to Robert M. Garrels.

PT

Annu. Rev. Earth Planet. Sci. 33, 1-36.

SC

oxygenation and the rise of animal life. Science 315, 92-95.

RI

Canfield, D.E., Poulton, S.W., Narbonne, G.M., 2007. Late-Neoproterozoic deep-ocean

NU

Catuneanu, O., Abreu, V., Bhattacharya, J.P., Blum, M.D., Dalrymple, R.W., Eriksson, P.G., Fielding, C.R., Fisher, W.L., Galloway, W.E., Gibling, M.R., Giles, K.A., Holbrook, J.M., Jordan,

MA

R., Kendall, C.G.S.C., Macurda, B., Martinsen, O.J., Miall, A.D., Neal, J.E., Nummedal, D., Pomar, L., Posamentier, H.W., Pratt, B.R., Sarg, J.F., Shanley, K.W., Steel, R.J., Strasser, A.,

D

Tucker, M.E., Winker, C., 2009. Towards the standardization of sequence stratigraphy. Earth-Sci.

AC CE P

TE

Rev. 92, 1-33.

Catuneanu, O., Eriksson, P.G., 2007. Sequence stratigraphy of the Precambrian. Gondwana Research 12, 560-565.

Catuneanu, O., Martins-Neto, M.A., Eriksson, P.G., 2012. Sequence stratigraphic framework and application to the Precambrian. Mar. Pet. Geol. 33, 26-33.

Chauvel, J., Dimroth, E., 1974. Facies types and depositional environment of the Sokoman Iron Formation, central Labrador Trough, Quebec, Canada. J. Sed. Pet. 44, 299-327.

Clark, T., Wares, R., 2004. Synthèse lithotectonique et métallogénique de l'Orogène du NouveauQuébec (Fosse du Labrador). Ministère des Ressources naturelles, Québec, MM 2004-01.

37

ACCEPTED MANUSCRIPT Cloud, P., 1973. Paleoecological Significance of the Banded Iron-Formation. Econ. Geol. 68,

PT

1135-1143.

Clout, J.M.F., Simonson, B.M., 2005. Precambrian Iron Formations and Iron Formation-Hosted

SC

RI

Iron Ore Deposits. Society of Economic Geologists, 100th Anniversary Volume, 643-679.

NU

Danielson, A., Moeller, P., Dulski, P., 1992. The europium anomalies in banded iron formations

MA

and the thermal history of the oceanic crust. Chem. Geol. 97, 89-100.

Dimroth, E., Dressler, B., 1978. Metamorphism of the Labrador Trough. Paper - Geological

TE

D

Survey of Canada 78-10, 215-236.

AC CE P

Dimroth, E., Chauvel, J., 1973. Petrography of the Sokoman Iron Formation in Part of the Central Labrador Trough, Quebec, Canada. Bull. Geol. Soc. Am. 84, 111-134.

Dott, R.H.,Jr, Bourgeois, J., 1982. Hummocky stratification; significance of its variable bedding sequences. Bull. Geol. Soc. Am. 93, 663-680.

Edwards, C.T., Pufahl, P.K., Hiatt, E.E., Kyser, T.K., 2012. Paleoenvironmental and taphonomic controls on the occurrence of Paleoproterozoic microbial communities in the 1.88 Ga Ferriman Group, Labrador Trough, Canada. Precambrian Res. 212-213, 91-106.

Elderfield, H., Greaves, M.J., 1982. The rare earth elements in seawater. Nature 296, 214-219.

38

ACCEPTED MANUSCRIPT Emerson D, Moyer CL, 2002. Neutrophilic Fe-oxidizing bacteria are abundant at the Loihi Seamount hydrothermal vents and play a major role in Fe oxide deposition. Appl. Environ.

PT

Microbiol. 68, 3085-3093.

RI

Emerson, D., Rentz, J.A., Lilburn, T.G., Davis, R.E., Aldrich, H., Chan, C., Moyer, C.L., 2007. A

SC

novel lineage of proteobacteria involved in formation of marine Fe-oxidizing microbial mat

NU

communities. PloS One 2007, 1-18.

MA

Eriksson, P.G., Banerjee, S., Catuneanu, O., Corcoran, P.L., Eriksson, K.A., Hiatt, E.E., Laflamme, M., Lenhardt, N., Long, D.G.F., Miall, A.D., Mints, M.V., Pufahl, P.K., Sarkar, S.,

D

Simpson, E.L., Williams, G.E., 2013. Secular changes in sedimentation systems and sequence

AC CE P

TE

stratigraphy. Gondwana Research 24, 468-489.

Eriksson, P.G., Catuneanu, O., Nelson, D.R., Rigby, M.J., Bandopadhyay, P.C., Altermann, W., 2012. Events in the Precambrian history of the Earth; challenges in discriminating their global significance. Mar. Pet. Geol. 33, 8-25.

Eriksson, P.G., Catuneanu, O., Sarkar, S., Tirsgaard, H., 2005. Patterns of sedimentation in the Precambrian. Sediment. Geol. 176, 17-42.

Evans, D.A.D., Mitchell, R.N., 2011. Assembly and breakup of the core of PaleoproterozoicMesoproterozoic supercontinent Nuna. Geology 39, 443-446.

39

ACCEPTED MANUSCRIPT Findlay, J.M., Parrish, R.R., Birkett, T.C., Watanabe, D.H., 1995. U-Pb ages from the Nimish Formation and Montagnais glomeroporphyritic gabbro of the central New Quebec Orogen,

PT

Canada. Canadian Journal of Earth Sciences 32, 1208-1220.

RI

Fischer, W.W., Knoll, A.H., 2009. An iron shuttle for deepwater silica in Late Archean and early

NU

SC

Paleoproterozoic iron formation. Bull. Geol. Soc. Am. 121, 222-235.

Föllmi, K.B., Garrison, R.E., Grimm, K.A., 1991. Stratification in Phosphatic Sediments;

MA

Illustrations from the Neogene of California. Springer Verlag, Berlin.

D

Fralick, P., Davis, D.W., Kissin, S.A., 2002. The age of the Gunflint Formation, Ontario, Canada;

AC CE P

Sciences 39, 1085-1091.

TE

single zircon U-Pb age determinations from reworked volcanic ash. Canadian Journal of Earth

Froelich, P.N., Arthur, M.A., Burnett, W.C., Deakin, M., Hensley, V., Jahnke, R., Kaul, L., Kim, K.H., Roe, K., Soutar, A., Vathakanon, C., 1988. Early diagenesis of organic matter in Peru continental margin sediments; phosphorite precipitation. Mar. Geol. 80, 309-343.

Fryer, B.J., 1977. Trace element geochemistry of the Sokoman Iron Formation. Canadian Journal of Earth Sciences 14, 1598-1610.

Glenn, C., Follmi, K., Riggs, S., Baturin, G., Grimm, K., Trappe, J., Abed, A., Galli-Olivier, C., Garrison, R., Ilyin, A., 1994. Phosphorus and phosphorites: sedimentology and environments of formation. Eclogae Geologicae Helvetiae 87, 747-788.

40

ACCEPTED MANUSCRIPT Gross, G.A., 1968. Geology of iron deposits in Canada; Volume 3, Iron ranges of the Labrador

PT

geosyncline. Economic Geology Report, Geological Survey of Canada.

Gross, G.A., 1980. A classification of iron formations based on depositional environment. Can.

SC

RI

Min. 18, 215-222.

NU

Gross, G.A., 1983. Tectonic systems and the deposition of iron-formation. Precambrian Res. 20,

MA

171-187.

Gross, G.A., Zajac, I.S., 1983. Iron-Formation in Fold Belts Marginal to the Ungava Craton. In:

D

Trendall, A.F., Morris, R.C., Iron formation: facts and problems. Elsevier, Amsterdam, pp. 253-

AC CE P

TE

294.

Hamade, T., Konhauser, K., Raiswell, R., Goldsmith, S., Morris, R.C., 2003. Using Ge/Si ratios to decouple iron and silica fluxes in Precambrian banded iron formations. Geology 31, 35-38.

Hoffman, P.F., 1988. United plates of America, the birth of a craton; early Proterozoic assembly and growth of Laurentia. Annu. Rev. Earth Planet. Sci. 16, 543-603.

Holland, H.D., 2006. The oxygenation of the atmosphere and oceans. Philosophical Transactions - Royal Society of London 361, 903-915.

Isley, A.E., Abbott, D.H., 1999. Plume-related mafic volcanism and the deposition of banded iron formation. Journal of Geophysical Research 104, 15-15,477.

41

ACCEPTED MANUSCRIPT Johnson, C.M., Beard, B.L., Klein, C., Beukes, N.J., Roden, E.E., 2008. Iron isotopes constrain biologic and abiologic processes in banded iron formation genesis. Geochim. Cosmochim. Acta

PT

72, 151-169.

RI

Klein, C.,Jr, Fink, R.P., 1976. Petrology of the Sokoman Iron Formation in the Howells River

NU

SC

area, at the western edge of the Labrador Trough. Econ. Geol. 71, 453-487.

Klein, C., Beukes, N.J., 1989. Geochemistry and sedimentology of a facies transition from

MA

limestone to iron-formation in the early Proterozoic Transvaal Supergroup, South Africa. Econ.

D

Geol. 84, 1733-1774.

TE

Klein, C., 2005. Some Precambrian banded iron-formations (BIFs) from around the world; their

1473-1499.

AC CE P

age, geologic setting, mineralogy, metamorphism, geochemistry, and origins. Am. Mineral. 90,

Knoll, A.H., Simonson, B., 1981. Early Proterozoic microfossils and penecontemporaneous quartz cementation in the Sokoman Iron Formation, Canada. Science 211, 478-480.

Konhauser, K.O., Hamade, T., Raiswell, R., Morris, R.C., Ferris, F.G., Southam, G., Canfield, D.E., 2002. Could bacteria have formed the Precambrian banded iron formations? Geology 30, 1079-1082.

42

ACCEPTED MANUSCRIPT Krapez, B., Barley, M.E., Pickard, A.L., 2003. Hydrothermal and resedimented origins of the precursor sediments to banded iron formation; sedimentological evidence from the early

PT

Palaeoproterozoic Brockman Supersequence of Western Australia. Sedimentology 50, 979-1011.

RI

LaBerge, G.L., 1964. Development of magnetite in iron formations of the Lake Superior region.

NU

SC

Econ. Geol. 59, 1313-1342.

Logan, G.A., Hayes, J.M., Hieshima, G.B., Summons, R.E., 1995. Terminal Proterozoic

MA

reorganization of biogeochemical cycles. Nature 376, 53-56.

D

Machado, N., Clark, T., David, J., Goulet, N., 1997. U-Pb ages for magmatism and deformation

AC CE P

TE

in the New Quebec Orogen. Canadian Journal of Earth Sciences 34, 716-723.

Maliva, R.G., Knoll, A.H., Simonson, B.M., 2005. Secular change in the Precambrian silica cycle: Insights from chert petrology. Bull. Geol. Soc. Am. 117, 835-845.

Meert, J.G., 2012. What's in a name? The Columbia (Paleopangaea/Nuna) Supercontinent. Gondwana Research 21, 987-993.

Mel'nik, Y.P., 1982. Precambrian banded iron formations, physiochemical conditions of formation. Elsevier, New York.

Miall, A.D., 2005. Testing for eustatic sea-level control in the Precambrian sedimentary record. Sediment. Geol. 176, 9-16.

43

ACCEPTED MANUSCRIPT Morey, G.B., Southwick, D.L., 1995. Allostratigraphic relationships of early Proterozoic iron-

PT

formations in the Lake Superior region. Econ. Geol. 90, 1983-1993.

RI

Narbonne, G.M., 2010. Ocean chemistry and early animals. Science 328, 53-54.

SC

Nelson, G.J., Pufahl, P.K., Hiatt, E.E., 2010. Paleoceanographic constraints on Precambrian

NU

phosphorite accumulation, Baraga Group, Michigan, USA. Sediment. Geol. 226, 9-21.

MA

Ohta, A., Kawabe, I., 2001. REE(III) adsorption onto Mn dioxide (delta -MnO2) and Fe

D

oxyhydroxide; Ce(III) oxidation by delta -MnO (sub 2). Geochim. Cosmochim. Acta 65, 695-703.

TE

Ojakangas, R.W., Morey, G.B., Southwick, D.L., Eriksson, P.G., Catuneanu, O., Aspler, L.B.,

AC CE P

Chiarenzelli, J.R., Martins-Neto, M.A., 2001. Paleoproterozoic basin development and sedimentation in the Lake Superior region, North America. Sediment. Geol. 141-142, 319-341.

Perry, E.C.,Jr, Tan, F.C., Morey, G.B., 1973. Geology and Stable Isotope Geochemistry of the Biwabik Iron Formation, Northern Minnesota. Econ. Geol. 68, 1110-1125.

Planavsky, N., Rouxel, O., Bekker, A., Shapiro, R., Fralick, P., Knudsen, A., 2009. Ironoxidizing microbial ecosystems thrived in late Paleoproterozoic redox-stratified oceans. Earth and Planetary Science Letters 286, 230-242.

44

ACCEPTED MANUSCRIPT Pope, M.C., Grotzinger, J.P., 2003. Paleoproterozoic Stark Formation, Athapuscow Basin, northwest Canada: record of cratonic-scale salinity crisis. Journal of Sedimentary Research 73,

PT

280-295.

RI

Porter, K.G., Robbins, E.I., 1981. Zooplankton fecal pellets link fossil fuel and phosphate

NU

SC

deposits. Science 212, 931-933.

Pratt, B.R., 2010. Peritidal Carbonates. In: James, N.P., Dalrymple, R.W. (Eds.), Facies Models 4.

MA

Geological Association of Canada, St. John's, pp. 401-420.

D

Pufahl, P.K., 1996. Stratigraphic architecture of a Paleoproterozoic iron formation depositional

AC CE P

Canada.

TE

system: the Gunflint, Mesabi, and Cuyuna iron ranges. M.Sc. Thesis, Lakehead University,

Pufahl, P.K., Fralick, P.W., 2004. Depositional controls on Palaeoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA. Sedimentology 51, 791-808.

Pufahl, P.K., Grimm, K., 2003. Coated phosphate grains: Proxy for physical, chemical, and ecological changes in seawater. Geology 31, 801-804.

Pufahl, P.K., 2010. Bioelemental Sediments, in James, N.P., Dalrymple, R.W. (Eds.), Facies Models 4. Geological Association of Canada, St. John's, pp. 477-503.

45

ACCEPTED MANUSCRIPT Pufahl, P.K., Hiatt, E.E., 2012. Oxygenation of the Earth's atmosphere-ocean system; a review of

PT

physical and chemical sedimentologic responses. Mar. Pet. Geol. 32, 1-20.

Raiswell, R., 2006. An evaluation of diagenetic recycling as a source of iron for banded iron

RI

formations. In: Kesler, S.E., Ohmoto, H. (Eds.), Evolution of the early Earth’s atmosphere,

SC

hydrosphere, and biosphere – constraints from ore deposits. Geological Society of America,

NU

Boulder, Colorado, pp. 223-238.

MA

Rasmussen, B., Blake, T.S., Fletcher, I.R., 2005. U-Pb zircon age constraints on the Hamersley spherule beds: evidence for a single 2.63 Ga Jeerinah-Carawine impact ejecta layer. Geology 33,

TE

D

725-728.

AC CE P

Rasmussen, B., Fletcher, I.R., Bekker, A., Muhling, J.R., Gregory, C.J., Thorne, A.M., 2012. Deposition of 1.88-billion-year-old iron formations as a consequence of rapid crustal growth. Nature 484, 498-501.

Rasmussen, B., Krapez, B., Meier, D.B., 2014. Replacement origin for hematite in 2.5 Ga banded iron formation: evidence for postdepositional oxidation of iron-bearing minerals. GSA Bulletin 126, 438-446.

Raymont, J.E.G., 1980. Planton and Productivity in the Ocean, Phytoplankton, v. 1. Pergamon, New York.

46

ACCEPTED MANUSCRIPT Reineck, H., Singh, I.B., 1980. Depositional Sedimentary Environments with Reference to

PT

Terrigenous Clastics. Springer-Verlag, Berlin.

Sami, T.T., James, N.P., 1996. Synsedimentary cements as Paleoproterozoic platform building

SC

RI

blocks, Pethei Group, northwestern Canada. Journal of Sedimentary Research 66, 209-222.

NU

Schieber, J., 2002. Sedimentary pyrite: A window into the microbial past. Geology 30, 531-534.

MA

Scott, C., Lyons, T.W., Bekker, A., Shen, Y., Poulton, S.W., Chu, X., Anbar, A., 2008. Tracing

D

the stepwise oxygenation of the Proterozoic ocean. Nature 452, 456-459.

TE

Simonson, B.M., 2003. Origin and evolution of large Precambrian iron formations. Special Paper

AC CE P

- Geological Society of America 370, 231-244.

Simonson, B.M., 1987. Early silica cementation and subsequent diagenesis in arenites from four early Proterozoic iron formations of North America. J. Sediment. Petrol. 57, 494-511.

Simonson, B.M., 1985a. Sedimentology of cherts in the early Proterozoic Wishart Formation, Quebec-Newfoundland, Canada. Sedimentology 32, 23-40.

Simonson, B.M., 1985b. Sedimentological constraints on the origins of Precambrian ironformations. Bull. Geol. Soc. Am. 96, 244-252.

47

ACCEPTED MANUSCRIPT Simonson, B.M., 1984. High energy shelf-deposit; early Proterozoic Wishart Formation, northeastern Canada. Special Publication - Society of Economic Paleontologists and

PT

Mineralogists 34, 251-268.

RI

Simonson, B.M., Goode, A.D.T., 1989. First discovery of ferruginous chert arenites in the early

NU

SC

Precambrian Hamersley Group of Western Australia. Geology 17, 269-272.

Simonson, B.M., Hassler, S.W., 1996. Was the deposition of large Precambrian iron formations

MA

linked to major marine transgressions? J. Geol. 104, 665-676.

D

Skulski, T., Wares, R.P., Smith, A.D., 1993. Early Proterozoic (1.88-1.87 Ga) tholeiitic

AC CE P

TE

magmatism in the New Quebec Orogen. Canadian Journal of Earth Sciences 30, 1505-1520.

St-Onge, M.R., Scott, D.J., Lucas, S.B., 2000. Early partitioning of Quebec: Microcontinent formation in the Paleoproterozoic. Geology 28, 323-326.

Sverdrup, H.U., 1938. On the process of upwelling. Journal of Marine Research 1, 155-164.

Trendall, A.F., 2002. The significance of iron-formation in the Precambrian stratigraphic record. Special Publication of the International Association of Sedimentologists 33, 33-66.

Trendall, A.F., Blockley, J.G., 2004. Precambrian iron-formation. Developments in Precambrian Geology 12, 403-421.

48

ACCEPTED MANUSCRIPT Wardle, R.J., Bailey, D.G., 1981. Early Proterozoic sequences in Labrador. Paper - Geological

PT

Survey of Canada 81-10, 331-358.

Wardle, R.J., James, D.T., Scott, D.J., Hall, J., 2002. The southeastern Churchill Province;

RI

synthesis of a Paleoproterozoic transpressional orogen. Canadian Journal of Earth Sciences 39,

NU

SC

639-663.

Watanabe, D.H., 1996. Petrology and Geochemistry of the Nimish Formation, western Labrador,

MA

Newfoundland. M.Sc. Thesis, University of Ottawa, Canada.

D

Zajac, I.S., 1974. The stratigraphy and mineralogy of the Sokoman Formation in the Knob Lake

AC CE P

TE

area, Quebec and Newfoundland. Bulletin - Geological Survey of Canada 220.

Zentmyer, R.A., Pufahl, P.K., James, N.P., Hiatt, E.E., 2011. Dolomitization on an evaporitic Paleoproterozoic ramp: widespread synsedimentary dolomite in the Denault Formation, Labrador Trough, Canada. Sedimentary Geology 238, 116-131.

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ACCEPTED MANUSCRIPT Figure Captions Figure 1. General geology of the Labrador Trough with location of study area near Schefferville,

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Québec.

Figure 2. The occurrence of continental margin iron formation around the Paleoproterozoic

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supercontinent Columbia (modified from Gross and Zajac, 1983).

(modified from Clark and Wares, 2006).

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Figure 3. Tectono-stratigraphic cycles of the Labrador Trough and Ferriman Group nomenclature

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Figure 4. Geologic map of the Schefferville area with drill core locations and orientation of

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stratigraphic cross-section in Figure 10 (A-A’).

Figure 5. A) Unconformable contact between the Wishart (W) and Ruth (R) formations. This boundary is accentuated by the contrast between silicified sandstone of the Wishart Formation and black shale of the Ruth Formation. Arrow indicates younging. Modified from Edwards et al., (2012) B) Unconformable contact between the Menihek (M) and Sokoman (S) formations. The top of the Sokoman Formation is a weathered surface composed cobble-sized clasts supported in siltstone of the basal Menihek. Arrow indicates younging. C) Pyritic black shale of the Ruth Formation (Facies F1). Arrow indicates younging. D) Acicular greenalite nucleated on reworked francolite peloids in the Ruth Formation (Facies F1) PPL. E) Sediment-filled desiccation cracks in chert mudstone (Facies F2). Modified from Edwards et al. (2012). F) Interlayered microcrystalline chert and organic-rich laminae (dark layer at bottom). Fabric destructive dolomite rhombs and framboidal pyrite are common (D). XN. 61

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Figure 6. A) Stromatolite-bearing flaser bedded chert grainstone (Facies F3). Cross-bedded and

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ripple laminated grainstones are draped with magnetite mudstone (dark laminae). Arrow indicates

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younging. B) Stromatolites associated with tidally deposited grainstone (Facies F3). C) Hematite intraclasts composing Facies F3. Grains are cemented with microcrystalline chert (white) and

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ankerite (brown). PPL. D) Cross-stratified hematitic grainstone (Facies F4). Dashed lines

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highlight foresets in trough cross-stratified bed. E) Jasperlitic rip-ups (reddish fragments) in hematitic grainstone (Facies F4). F) Coated grains composed of alternating cortical layers of

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chert and hematite in cross-stratified grainstone (Facies F4). Some grain nulcei have been replaced with secondary mosaic hematite. Composite grains also occur (left). Interparticle pores

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are filled with microcrystalline chert cement. PPL.

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Figure 7. A) Septarian, chert-hematite-magnetite concretion in hematic grainstone (Facies F5). B) Concretionary hematitic grainstone (Facies F5). C) Cross-section through concretion showing transition from concentrically coated chert core (C) through hematite (H) to a magnetite-rich outer rim (M). This progressive change in mineralogy likely records precipitation in the zone of silica saturation just beneath the seafloor and subsequent growth during burial into progressively more reducing pore-water. D) Pebble-sized grainstone rip-ups produced from reworking hardgrounds (Facies F5).

Figure 8. A) Exposure of interbedded wavy- and cross-bedded magnetite-rich packstone (Facies F6) and parallel bedded magnetite-rich chert grainstone (Facies F7). Wavy- and cross-bedded magnetite-rich grainstones contain some hematite and are therefore reddish in outcrop. Parallelbedded magnetite-rich chert grainstone contain abundant Fe-silicates and thus have a greenish 62

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mudstone. Arrow indicates younging. Hammer is 30-cm-long. C) Wavy- and cross-bedded

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magnetite-rich packstone (Facies F6) showing variation in thickness of magnetite mudstone (dark grey) and granular chert layers (white). D) Carbonate replaced packstones (Facies F6). Grain

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structure is almost completely obliterated by replacement with Fe carbonate. Opaque euhederal

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crystals (dark) are secondary fabric destructive magnetite. PPL. D) Parallel bedded magnetiterich grainstone (Facies F6). Lighter, grainstone layers are composed of chert and Fe silicate

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intraclasts. Darker laminae between these beds are wavy laminated magnetite. E) Magnetite-rich laminae in parallel bedded magnetite-rich grainstone (Facies F7) with fabric destructive

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magnetite (M), Fe chlorite (I), and stilpnomelane (S).

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Figure 9. Composite stratigraphic section showing stacking of lithofacies in the Ruth and Sokoman formations. Sea-level curve in lower right defines sequence stratigraphic surfaces and systems tracts highlighted in the composite stratigraphic section. mdst = mudstone; wkst = wackestone; pkst = packstone; grst = grainstone; rdst = rudstone; SB = sequence boundary; TS = transgressive surface; MFS = maximum flooding surface; LST = lowstand systems tract; TST = transgressive systems tract; HST = highstand systems tract; FSST = falling stage systems tract.

Figure 10. Stratigraphic correlations and sequence stratigraphy of the Ruth and Sokoman formations through the Schefferville area of the Labrador Trough. Locations of stratigraphic sections are shown in Figure 4. The chert horizon at the base of Sequence 2 is the stratigraphic datum. Lithofacies, increments of rock type, and sequence stratigraphic surfaces are the same as Figure 9. 1 = Sequence 1; 2 = Sequence 2; both are discussed in the text. 63

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Figure 11. Depositional and paleoceanographic model for deposition of the Ruth and Sokoman

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formations. A combination of upwelling ferrous Fe and Si and cyanobacterial oxygen production in the nearshore produced a prominent oxygen chemocline interpreted to have controlled

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lithofacies mineralogy. The term “suboxic” and “anoxic” are used as a relative measure of

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oxygen levels in the water column and sediment and does not refer to specific authigenic

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reactions or oxygen concentrations (cf. Canfield and Thamdrup, 2009). Hematite and chert-rich facies dominate suboxic peritidal environments whereas chert, magnetite, and Fe-silicates

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characterize anoxic offshore settings. Lithofacies are the same as in Figure 9. Sea-level cycle in

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lower right highlights point on curve that the model represents. FWWB = fair weather wave base.

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Table 1. Facies description and interpretation.

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ACCEPTED MANUSCRIPT Table 1 Facies description and interpretation.

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Chert mudstone

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Stromatolite-bearing flaser-bedded chert grainstone

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Cross-stratified hematitic chert grainstone

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Concretionary hematitic chert grainstone

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Wavy- and cross-bedded magnetite-rich packstone

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Parallel bedded magnetite-rich chert grainstone

Description Interlaminated pyritiferous black shale and microbially laminated chert. Rare, thinly bedded, wave-rippled grainstones. Chert laminae and intraclastic grainstone are composed of microcrystalline chert. Sedimentary apatite (francolite) occurs as crusts or reworked grains. Greenalite common in matrix and nucleated on francolite grains. Laminated black or green chert with subaerial exposure surfaces marked by desiccation cracks and phosphatized microbial laminae. Common cross-stratified chert grainstone and feldspathic arenite. Rare pyrite, greenalite, stilpnomelane in matrix. Fabric destructive dolomite overprints primary sedimentary textures. Chemical mud-draped, ripple cross-laminated, hematitic grainstone. Common stromatolites. Mudstones near stromatolites are hematitic whereas those away contain magnetite, greenalite, and stilpnomelane. Some grainstone beds also contain rare francolite intraclasts. Medium scale cross-bedded, chert grainstone composed of medium sand-sized microcrystalline chert and hematite intraclasts, coated chert-hematite grains, and rare oncolites. Beds are cemented with microcrystalline chert and overprinted with fabric destructive magnetite. Laminae draping some beds are organic rich with magnetite. Decimeter-scale, concentrically zoned concretions in hematite-rich chert grainstone. Concretions change from a chert-rich centre through hematite to a magnetite-rich outer rim. Associated grainstones are composed of intraclastic chert and hematite and well indurated to form reworked hardgrounds. Thinly bedded chert packstone and wavy laminated magnetite mudstone. Packstones are formed of microcrystalline chert and hematite intraclasts. Characteristic pinkish mottling from pervasive carbonate alteration. Thinly bedded, Fe silicate-rich grainstone and wavy laminated magnetite. Chert and Fe silicate intraclasts forming beds are cemented with microscrytalline chert and magnetite. Fe chlorite and stilpnomelane also replaces and cements grains. Fabric destructive dolomite in some beds.

Interpretation Suspension rain of phytoplankton and silt and precipitation of evaporitic opal-A in coastal lagoons. Episodic storms transported grainstones from more energetic settings. Bacterial sulfate reduction facilitated pyrite and francolite precipitation.

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Facies Pyritic black shale

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Precipitation of evaporitic opal-A in inter- and supratidal environments to produce laminated chert. Tidal and storm reworking of siliceous tidal flats produced grainstones. Sandstone deposition where terrigenous clastic sedimentation was higher. Tidal reworking of chemical sediment in inter- and subtidal settings. Hematite occurs where stromatolites enriched seawater with photosynthetic oxygen. Magnetite and iron silicates record anoxic conditions. Migration of subaqueous dunes in suboxic, subtidal environment. Traction currents reworked freshly precipitated opal-A and Fe-(oxyhydr)oxide to form grainstone. Coated grains likely reflect precipitation in pore-water with fluctuating redox and saturation. Condensed facies formed in shallow, suboxic environment. Reworked hardgrounds and concretions suggest deposition in the zone of wave abrasion. Concretion mineralogy records progressively more reducing conditions with burial. Dune migration by fairweather currents in deep subtidal environment. Preponderance of magnetite mudstone suggests accumulation under anoxic conditions. Storm transport and mixing of coastal sediments with intraclasts produced on anoxic middle shelf. The ubiquity of magnetite suggests the accumulation of chemical mud below a prominent oxygen chemocline.

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ACCEPTED MANUSCRIPT Dynamic sedimentation of Paleoproterozoic continental margin iron formation, Labrador Trough, Canada: paleoenvironments and sequence stratigraphy One of a few studies that uses sequence stratigraphy to understand iron formation

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Sequence architecture suggests oxygen-stratification controlled facies character

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Differences in surfaces framing sequences reflect unique conditions of deposition

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Sequence stratigraphic context is critical to properly interpreting iron formations

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