Sedimentology and Authigenesis of the Lower Devonian Torbrook Formation Ironstone, Torbrook, Nova Scotia, Canada

Home , Ironstone, Southern Nova Scotia

Luke A. Marshall

Thesis submitted in partial fulfillment of the requirements for the degree of Bachelor of

Science with Honours in Geology

Department of Earth and Environmental Science

Acadia University

Wolfville, Nova Scotia

This thesis by Luke A. Marshall

is accepted in its present form by the

Department of Earth and Environmental Science as satisfying the thesis requirements for the degree of

Bachelor of Science with Honours

Approved by the Thesis Supervisor

______(Dr. Peir K. Pufahl) Date

Approved by the Head of the Department

______(Dr. Robert Raeside) Date

Approved by the Honours Committee

______(Dr. Sonia Hewitt) Date

I, Luke A. Marshall, grant permission to the University Librarian at Acadia University to reproduce, loan, or distribute copies of my thesis in microform, paper, or electronic formats on a non-profit basis. I however retain the copyright in my thesis.

______

Luke A. Marshall

______

Date

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Acknowledgements

I would like to thank my supervisor, Dr. Peir Pufahl, for his guidance and assistance. Don Osburn is gratefully acknowledged for preparing thin and polished sections. I would also like to thank Drs. Chris White and Sandra Barr for providing information related to the Torbrook Formation (Rockville Notch Group). Haixin Xu assisted with the scanning electron microscopy and Sara Akin helped while in the field.

Ivan Trimper is acknowledged for the historical background on mining in Torbrook. I would also like to thank Acadia University and the Department of Earth and

Environmental Science for use of their laboratory equipment. Funding was provided by a

NSERC Discovery Grant to Dr. Peir Pufahl.

Table of Contents Approval Page…………………………………………………………………………….ii Permission for Duplication page…………………………………………………………iii Acknowledgements………………………………………………………………………iv Table of Contents…………………………………………………………………………v List of Figures……………………………………………………………………………vii Abstract……………………………………………………………………………...…..viii 1. Introduction……………………………………………………………………………1 2. Background…………………...……………………………………………………….2 2.1 Regional Geology and Stratigraphy…...……………………………………………...2 2.2 Paleoenvironments of the Torbrook Formation ..……………………………...... 5 2.3 Mining……………………………………………………………………….………...5 2.4 Phanerozoic Ironstones ………………………………………...………………….....6 3. Methods………………………………………………………………………………...7 4. Sedimentologic attributes and Paleoevnironments…….…………...……………….8 4.1.1 Lithofacies 1 – Interbedded laminated mudstone and ripple cross laminated sandstone .………………………………………...……………………………………...10 4.1.2 Lithofacies 2 – Dark-grey to black, laminated, pyritic, mudstone…………..……..11 4.1.3 Lithofacies 3 – Brachiopod-rich, medium-grained sandstone…………….……….12 4.1.4 Lithofacies 4 – Nodular ironstone hummocky cross-stratified, fine-grained sandstone……………...………………………………………………………………….13 4.1.5 Lithofacies 5 – Iron-rich, hummocky cross-stratified, fine grained sandstone…....15 4.1.6 Lithofacies 6 – Bioturbated mudstone……………………………………..………18 4.2 Paraseqence descriptions ……………………………………....……………...... 19

5. Discussion………………………….………………………………………...……….20 5.1 Authigenesis in the Torbrook Formation…………………………...………………..20 5.2 Coated iron grains and seafloor processes………………………………….……....24

6. Conclusions…………..…………………………………………………...………28 7. References……………………….…………………………………………...... 30 Appendix I (Sample Descriptions)..……………………………...…………………..33 Appendix II (Thin Section Microscopy Descriptions) ……………………………....35 Appendix III (Stratigraphic Field Logs)……………………………...…….………..37

List of Figures

1. Stratigraphic map of Spinney Brook study area……………………………..…………4 2. Torbrook Formation stratigraphic column showing parasequences, lithofacies and stratigraphic sample locations.………………………………...……..…………….……...9 3. Transmitted light photomicrograph, crossed polars (XP) of stylotitic mudstone and burrow filled with lithified coarser sediments…………………………………………...10 4. Transmitted light photomicrograph (XP), pyrite crystal with crinoid grain…………..11 5. Transmitted light photomicrograph (XP), brachiopod shell fragment………………...12 6. Transmitted light photomicrograph (XP) of an iron nodule…………………..………13 7. SEM backscatter X-ray image of an iron nodule……………………………...………14 8. An Iron-rich, hummocky cross-stratified, fine-grained sandstone with scour surfaces, and corresponding transmitted light photomicrograph (XP) with quartz grains cemented with hematite and francolite…………………………………..………………………….16 9. Transmitted light photomicrographs of coated grains………………………..……….17

10. Hematite cementing coarser silt-sized grains of in-filled burrow….…………..….....18 11. Formation of iron oxide, iron silicate and francolite………………..…………….....21 12. Redox aggraded and Uncnformity bounded grain formation through redox pumping……………………………………………………………………………….…25

13. SEM backscatter X-ray image of an unconformity-bounded grain.………..………..27 14. SEM backscatter X-ray images of several unconformity-bounded grains……..……28

vii

Abstract

Ironstone in the lower Devonian Torbrook Formation of Nova Scotia accumulated in an array of storm-dominated, middle and distal shelf environments. Lithofacies stacking patterns indicate deposition occurred through the development of seven parasequences. Paraseqeunces range in thickness from 40 to 250 m thick and become progressively thicker through the Torbrook Formation. Individual parasequences coarsen from parallel laminated mudstones and ripple cross-laminated sandstones to iron cemented hummocky cross-stratified sandstones that are capped by marine flooding surfaces. Ironstone firmgrounds that mark flooding surfaces contain in situ and transported brachiopods and crinoids, suggesting mesotrophic nutrient levels predominated across the shelf.

Ironstone development is interpreted to have been restricted to flooding surfaces because low or net negative rates of sedimentation stabilized the iron redox interface within the sediment, which allowed pore waters to become saturated with iron.

Saturation and precipitation of iron minerals across this redox interface is interpreted to have occurred through iron redox pumping. Iron redox pumping is a cyclic mechanism that concentrates iron in pore water through the dissolution of iron (oxyhydr)oxide below the iron redox interface. Precipitation occurs when ferrous iron diffuses upward through the sediment to combine with oxygen in suboxic pore water. This process links the cycling of iron to phosphorus and is likely responsible for the high levels of bioavailable phosphorus necessary to sustain firmground populations of brachiopods and crinoids.

Important authigenic phases in these ironstone firmgrounds include hematite, chamosite, brethierine, and francolite.

viii

1. Introduction

Ironstone is a marine chemical sedimentary rock composed of more than 15 weight % Fe (Young and Taylor 1989). Ironstones are commonly associated with black shale that accumulated during the Devonian-Ordovician and Jurassic-Paleogene (Young and Taylor 1989). Rising sea level increased accommodation space and starved submerging shelves of diluting terrigenous clastics (Young 1989), preconditioning shelves for ironstone deposition (Taylor et al. 2002). Ironstone precipitates as peloids and cements within condensed sediments when iron is concentrated in pore water (Pufahl

2010). An oxygenated seafloor is a prerequisite for concentration because such conditions permit pumping of iron in pore water. Iron redox pumping is a cyclic mechanism that concentrates iron in pore water through the dissolution of iron

(oxyhydr)oxide below the iron redox interface. In general, ironstones are Phanerozoic in age and contain more silica than Precambrian iron formations (Pufahl 2010). Iron formations also precipitated from an anoxic water column or at the seafloor from hydrothermally derived Fe and Si (Simonson and Hassler 2003).

The purpose of this thesis is to investigate the sedimentology and stratigraphy of ironstone in the lower Devonian Torbrook Formation, Torbrook Mines Nova Scotia (Fig.

1). This 950 m thick succession provides an excellent opportunity to examine the depositional and authigenic processes of ironstone formation. An important aspect of this research is to understand the relationship between the benthic phosphorus cycle and iron precipitation, which is a common association in many Phanerozoic ironstones. Such information promises to yield new insights into the importance of firmground

development, nutrient cycling, and benthic productivity in middle and distal shelf environments.

2. Background

2.1 Regional Geology and Stratigraphy

Sedimentary rocks in Nova Scotia form two major Gondwana Terranes with different Early Paleozoic depositional histories. The Avalon Terrane extends from northeastern mainland Nova Scotia to Cape Breton. The Meguma Terrane encompasses nearly all of southwestern mainland Nova Scotia. They were structurally juxtaposed during the Devonian Appalachian orogeny along the Cobequid–Chedabucto fault (Force and Barr 2005).

The original relationship between the Avalon and Meguma terranes in the

Paleozoic Appalachian orogen is controversial. Two major hypotheses exist: (1) these terranes developed along different parts of the Gondwanan margin in the late

Neoproterozoic and were accreted to Laurentia as separate terranes. The accretion of

Avalonia and/or the Meguma terrane was related to the Devonian Acadian orogeny (e.g.,

Schenk 1997; Robinson et al. 1998); (2) Meguma terrane rocks were deposited on

Avalonian basement along the same part of the Gondwanan margin, traveled as a single tectonic unit, and together were accreted to Laurentia-Baltica by the Early Silurian (e.g.,

Keppie et al. 1997).

The Meguma terrane has recently been subdivided into three groups and nine formations (White 2010). The Early Cambrian to Early Devonian Goldenville Group

(Church Point and Tupper Lake Brook Formations) conformably underlies the Halifax

Group (Canard, Lumsden Dam, Elderkin Brook, and Hellgate Falls Formations), which in turn unconformably underlies the Rockville Notch Group (White Rock, Kentville, and

New Canaan/Torbrook formations), (Fig. 1). Approximately 30 Ma is interpreted to be missing between the Halifax and Rockville Notch groups (White 2010).

The Early Devonian Torbrook Formation, the focus of this thesis, and the correlative New Canaan Formation are the youngest units in the Meguma Group. Once thought to be an Appalachian sedimentary deposit, re-examination of brachiopods by

Boucot and Stevens (1960) indicated a Lower Devonian age North Atlantic Rhenish provenance, referring to the paleo-equatorial Rhenish Massif of western Germany, eastern Belgium, Luxembourg and northeastern France. The Torbrook Formation consists of interbedded medium and fine-grained, hematite [Fe2O3] cemented sandstone that is interbedded with siltstone and minor thin tuffs. It is interpreted to have accumulated in a shallow marine environment (Hickox 1958; Smitheringale, 1960;

Taylor 1965, 1969). The most hematitic sandstone beds are true ironstones that contain authigenic phosphate nodules, hematite concretions, uncommon invertebrate fossils, and rare coated ironstone grains (senso Pufahl and Grimm 2002; White 2010).

The Torbrook Formation is metamorphosed to greenschist facies and lies within a tightly folded syncline produced during the Acadian Orogeny. The southern limb of the fold is conveniently incised by Spinney Brook approximately normal to the fold axis

(Fig. 1). The Spinney Brook section is 1434 m thick (Jensen 1976) with a sedimentologically interpretable thickness of approximately 950 m. Sedimentologic interpretations in this thesis are based on the Spinney Brook section.

Nova Scotia

Spinney Brook

Figure 1. Stratigraphic map of Spinney Brook study area, adapted from (White 2010).

2.2 Paleoenvironments of the Torbrook Formation

The Torbrook Formation conformably overlies the graptolitic, dark grey silty shale of the Upper Silurian Kentville Formation. The contact between the two is defined as the first appearance of sandstone in the Torbrook Formation (Jensen 1976). The stratigraphic section logged by Jensen (1976) describes the Torbrook Formation as starting with a basal silty mudstone, coarsening upwards to massive sandstone that is marked at its top by a sharp return to finer grained sedimentary rocks. Above this is a coarsening upwards succession that is interpreted as either a storm influenced shoreface or inner shelf environment. A rapid deepening is recorded by a return to fine-grained sedimentation that gradually coarsens and shallows to interpreted deltaic sandstones with plant fragments and ostracods. Another rapid deepening event produced fine-grained siltstones and pyritic mudstones that are iron and phosphate-rich. Limey sandstones within this interval also contain minor hydrocarbon. The remainder of the section consists of cross-bedded sandstones interlayered with bioturbated mudstones, with fossil assemblages and bidirectional paleocurrents indicating deposition on intertidal-flats and within tidal channels (Jensen 1976). A central focus of this thesis is to use modern stratigraphic principles to further constrain depositional environments and their relationship to ironstone accumulation.

2.3 Mining

The Torbrook Formation was first mined in the early 1800s and processed into pig iron, with limestone flux shipped from Saint John, New Brunswick (Goodrich 1900).

It was also mined extensively in the early 1900s, at a time when Phanerozoic ironstones

were still an important economic source of iron ore, a source that has since been replaced by Precambrian iron formations. Torbrook ore contained 30-55% Fe and were of four types: (1) shelly (mainly brachiopods) hematite; (2) massive hematite; (3) shelly magnetite [Fe3O4], and (4) massive magnetite (Goodrich 1900). The magnetite is interpreted to have formed by hydrothermal alteration of hematite during the intrusion of the South Mountain Batholith (Goodrich 1900; Woodman 1909).

2.4 Phanerozoic Ironstones

The formation of ironstone requires a source of Fe and specific depositional and post-depositional environments. Both volcanic (Sturesson et al. 2000) and terrigeneous sources (Taylor and Curtis 1995) have been proposed. Depositional environments with low sedimentation rates are essential because iron must be concentrated to saturation within pore water to cause precipitation (Van Houten & Arthur 1989). Stratigraphic condensation allows the establishment and stabilization of the iron redox interface within sediment. If the sedimentation rate is too high pore waters become diluted with siliciclastics, which moves the iron redox boundary too swiftly through the sediment to promote precipitation. Precipitation occurs across this redox interface as iron diffuses above to combine with oxygen in suboxic pore waters (Pufahl 2010). Iron precipitates as either iron (oxyhydrox)oxide or iron silicate cement, discrete laminae, nodules, or peloids

(Taylor et al. 2002). Some peloids may be coated, indicating multiple episodes of reworking, transport, and redeposition of grains back into the zone of iron precipitation beneath the seafloor (Pufahl and Grimm 2002; Pufahl 2010).

Ironstones from the Cretaceous Western Interior Seaway, Alberta and Utah, show that ironstone formation not only involves dissolution and reprecipitation of iron oxides above and below the iron redox interface, but also requires low sulfide levels (Taylor et al, 2002). When sulfide levels are high pyrite forms instead of iron (oxyhydr)oxide. This occurs most commonly in organic-rich sedimentary rocks with high rates of bacterial sulfate reduction. The sulfide that is produced readily combines with reduced iron to precipitate framboidal pyrite (Berner 1981). Thus, ironstones precipitate in sediments with low organic matter content, that are generally well oxygenated (Taylor et al. 2002).

3. Methods

Interpretations are based on detailed measurement and sampling of the Spinney

Brook section (Fig. 1). Strata are generally well preserved and close to their original thickness, but have been deformed by the Acadian Orogeny. Where possible strata were traced by walking out units along the banks of the brook. Emphasis was placed on field relations, vertical stratigraphic trends, and collection of 60 hand samples for fossil identiﬁcation and petrographic analysis of thin sections. Sample locations were recorded using a handheld GPS. Although not entirely accurate, the stratigraphy of Jensen (1976) was used as a rough guide to delineate stratigraphic units.

Macrofossil characteristics were determined qualitatively in the ﬁeld by assessing type, abundance and diversity over the expanse of individual outcrops. Percentages of bioclastic and terrigeneous silt- and sand-sized grains were estimated from 33 thin sections and were given a ranking of rare (1–5% of particles), uncommon (6–30% of

particles), common (31-60% of particles) or abundant (> 60% particles). Thin sections intended for scanning electron microscopy were carbon coated and imaged using back- scattered electron imaging on a JEOL JSM-5900LV. Qualitative analyses of iron and phosphatic phases were performed with a dedicated Princeton Gamma-Tech IMIX-PC

EDS system.

4. Sedimentologic attributes and paleoenvironments

The Torbrook Formation can be separated into six distinct lithofacies that accumulated in an array of middle and distal shelf environments. Lithofacies are stacked in a series of seven parasequences that coarsen from parallel laminated muds to massive and hummocky cross-stratified sandstones. When ironstone is present it occurs within condensed horizons along marine flooding surfaces at parasequence tops (Fig. 2).

Lithofacies associations suggest that deposition occurred near storm wave base in deeper environments than originally interpreted by Jensen (1976).

Figure 2. Torbrook Formation stratigraphic column showing parasequences, lithofacies and stratigraphic sample thin section locations.

4.1.1 Lithofacies 1 – Interbedded laminated mudstone and ripple cross laminated sandstone

This facies consists of interbedded, parallel laminated mudstone and fine-grained, quartz sandstone. Sandstones are bioturbated and current ripple cross-laminated with common scour surfaces. Anastomosing stylolites are common (Fig. 3). Bioturbated sandstones contain abundant rhizocorallid burrows. The interbedding of mudstone and fine sandstone suggests that deposition occurred between fair-weather and storm wave base in a middle shelf environment. The presence of bioturbated sandstones indicates that the seafloor was generally well oxygenated.

1.5 mm

Figure 3. Lithofacies 1 - transmitted light photomicrograph, crossed polars (XP) of stylotitic mudstone and burrow filled with lithified coarser sediments, thin section (SB-7)

4.1.2 Lithofacies 2 – Dark-grey to black, laminated, pyritic, mudstone

Thinly laminated, dark-grey to black mudstones of facies 2 contain abundant pyrite (Fig. 4). Fish scales and abraded crinoid columnals are common in some laminae suggesting transport from shallower water environments by storms. The laminated character of this organic-rich lithofacies and the presence of pyrite indicate accumulation on an anoxic seafloor (Wilkin and Barnes 1997). The occurrence of transported crinoid fragments and fish scales suggests mesotrophic nutrient levels prevailed across much of the shelf.

0.3 mm

Figure 4. Lithofacies 2 - transmitted light photomicrograph (XP), pyrite crystal with crinoid columnal, thin section SB-8.

4.1.3 Lithofacies 3 – Brachiopod-rich, medium-grained sandstone

This brachiopod-rich sandstone is a minor facies that is interpreted to record stratigraphic condensation at the top of some parasequences. It is weakly indurated with minor authigenic iron (oxyhydr)oxide cement (Fig. 5). Brachiopods generally occur as shell lags that mark winnowing and reworking along iron-cemented firmgrounds. Some brachiopods are articulated indicating minimal transport on a well-oxygenated seafloor.

The presence of rare, abraded crinoid fragments, however, suggests some storm transport and redeposition into deeper water environments.

0.75 mm

Figure 5. Lithofacies 3 - Transmitted light photomicrograph (XP), brachiopod shell fragment, thin section SB2-10A.

4.1.4 Lithofacies 4 – Nodular ironstone

Reddish, parallel laminated to ripple cross-laminated fine-grained sandstone with abundant iron nodules characterize this facies. Like facies 3, this lithofacies also marks the top of some parasequences and is interpreted to be the product of stratigraphic condensation associated with marine flooding. Transmitted light microscopy reveals that nodules are formed of authigenic, microcrystalline iron silicate cement and sedimentary apatite (francolite) [(Ca, Mg, Sr, Na)10(PO4, SO4, CO3)6F2−3] (Fig. 6).

These phases did not grow displacively as nodules formed, but instead encapsulated surrounding sediment grains. SEM-EDS analysis of nodules confirms the presence of francolite and indicates the iron silicate is likely chamosite

2+ 3+ [(Fe ,Mg,Fe )5Al(Si3Al)O10(OH,O)8 ] (Fig. 7). Chamosite and francolite commonly co-

0.75 mm

Figure 6. Lithofacies 4 - transmitted light photomicrograph (XP) of an iron nodule, thin section SB-33.

occur in many Phanerozoic phosphatic successions (Glenn and Arthur 1988). Because chamosite contains both reduced and oxidized iron it indicates that the precipitation of iron and phosphorus occurred at the iron redox interface within suboxic pore waters

(Pufahl and Grimm 2002; Pufahl 2010).

B C D

Figure 7. (A) Back-scattered electron image of an iron nodule and EDS spectra. (B) Silicate (clay mineral). (C) Chamosite/berthierine with calcium phosphate (francolite). (D) Hematite with chamosite/berthierine. Thin section SB-33.

4.1.5 Lithofacies 5 – Iron-rich, hummocky cross-stratified, fine-grained sandstone

This hummocky cross-stratified, fine-grained sandstone is in places well indurated with hematite and francolite cement (Fig. 8). Reworked, authigenic peloids include

2+ 3+ chamosite, berthierine [(Fe ,Fe ,Al,Mg)2-3(Si,Al)2O5(OH)4], and francolite (Fig. 9).

Some berthierine peloids are coated, indicating multiple episodes of reworking, exhumation and reburial into the zone of iron precipitation during episodes of stratigraphic condensation (Pufahl and Grimm 2002). Because such grains require long residence times near the iron redox interface, low or net-negative sedimentation rates are a prerequisite for their formation (Pufahl and Grimm 2002). The presence of redeposited plant fragments and rounded quartz grains suggests transport from shallower paleoenvironments during storm deposition of hummocky cross-stratified sandstone (Dott and Bourgeois 1979).

1.5 mm

Figure 8. Lithofacies 5 - (A) Iron-rich, hummocky cross-stratified, fine-grained sandstone with scour surface. (B) Corresponding transmitted light photomicrograph (XP) with quartz grains cemented with hematite and francolite, thin section SB-28.

A B

0.5 mm 0.5 mm C D

0.25 mm 0.25 mm E F

0.5 mm 0.5 mm

Figure 9. Lithofacies 5 - transmitted light photomicrographs of coated grains. (A) PPL. (B) XP. (C) PPL. (D) XP. (E) PPL. (F) XP. Thin section PM-1.

4.1.6 Lithofacies 6 – Bioturbated mudstone

This facies consists of interbedded bioturbated mudstone and siltstone. Although not directly observed, Jensen (1976) also notes abundant plant fragments in some laminae. This facies is best indurated in coarser layers where hematite cement binds bioturbated silt-sized quartz grains (Fig. 10). The fine-grained nature and presence of plant fragments suggests that this is a prodelta facies (Jensen 1976).

1.5 mm

Figure 10. Lithofacies 6 – hematite cementing coarser silt-sized grains within filled burrow. Thin section SB-28.

4.2 Parasequence descriptions

Seven parasequences are identified based on lithofacies stacking patterns.

Parasequences range in thickness from 40 to 250 m and become progressively thicker stratigraphically upward through the Torbrook Formation (Fig. 2). They coarsen from parallel laminated mudstones and ripple cross-laminated sandstones of lithofacies 1 and 2 to iron cemented hummocky cross-stratified sandstones of lithofacies 5. At the top of some parasequences where stratigraphic condensation is most pronounced brachiopod shell lags of lithofacies 3 and nodular ironstone of lithofacies 4 indicate widespread firmground development. These horizons are interpreted as marine flooding surfaces that formed through punctuated relative sea level rise during the deposition of the Torbrook

Formation. Parasequences near the top of the Torbrook Formation are capped by prodelta mudstones and siltstones of lithofacies 6, indicating an overall shoaling and reduction in accommodation space with time. Lithofacies associations and the stacking of parasequences suggests that the maximum flooding surface occurs between parasequence 4 and 5 (Fig. 2). Abundant scours, firmgrounds and iron cemented brachiopod lags suggest an intense episode of stratigraphic condensation associated with trapping of terrigenous clastics in nearshore environments (McLaughlin et al. 2008). The coarser nature of parasequences above this surface further suggests that this is indeed the inflection point between the transgressive and highstand systems tracts, signaling the shift from aggradational to progradational deposition (Plint and Nummedal 2000).

5. Discussion

5.1 Authigenesis in the Torbrook Formation

Ironstone is interpreted to form during periods of pronounced low or net negative rates of sedimentation (Young and Taylor 1989). Such conditions characterize flooding surfaces because the iron redox boundary is stabilized beneath the seafloor, allowing the ubiquitous precipitation of iron (oxyhydr)oxide and iron silicate cements (Taylor and

Macquaker 2000; Pufahl 2010). Precipitation is interpreted to have occurred via iron- redox pumping (Pufahl and Grimm, 2003; Pufahl 2010), which is a cyclic mechanism that concentrates iron in pore water by dissolving iron (oxyhydr)oxide below the iron redox interface (Fig. 11). Authigenic iron cements precipitate when dissolved ferric iron diffuses up across the iron redox interface to combine with oxygen in suboxic pore water.

The resultant cement is either a finely disseminated iron (oxyhydr)oxide or iron silicate, which in the Torbrook Formation binds silt and fine sand-sized quartz grains in lithofacies 3, 4, and 5. During burial and subsequent diagenesis iron (oxyhydr)oxide converts to subhedral hematite (Klein 2005; Pufahl 2010). Euhedral magnetite forms in regions where the Torbrook Formation was contact metamorphosed by the South

Mountain Batholith.

Figure 11. Within precipitated iron (oxyhydr)oxide, Fe2+ and Fe3+ are concentrated by repeated dissolution and precipitation at the redox boundary. They then combine with fine quartz to form 2- iron silicate at the redox boundary. HPO4 sorbs to iron (oxyhydr)oxide above the redox boundary and is concentrated and liberated again by the same redox cycling, eventually combining with F- to precipitate francolite below the redox boundary. Remaining iron (oxyhydr)oxide is altered to hematite through subsequent diagenesis.

Iron redox pumping also links the cycling of iron to the benthic phosphorus cycle.

Phosphorites that are not associated with prominent upwelling are often interpreted to accumulate as a consequence of this cyclic mechanism (Heggie et al. 1990). In addition to maintaining high iron concentrations, iron redox pumping also saturates pore water with phosphate. During burial dissolution of iron (oxyhydr)oxide below the iron redox boundary liberates sorbed phosphate to porewater. The escape of phosphate out of the sediment is prevented by re-adsorption of phosphate onto iron (oxyhydr)oxides just above this redox interface. Iron redox pumping is very efficient at transferring phosphate from the water column to the sediment because of the high affinity phosphate has for iron

(oxyhydr)oxide (Heggie et al. 1990; Pufahl et al. 2010). This process was likely important in producing the francolite cements in lithofacies 4 and 5. It is also interpreted to have maintained the high levels of bioavailable phosphorus necessary to sustain firmground populations of brachiopods and crinoids.

For fe-redox pumping to operate efficiently as a P pump requires either repeated mixing of iron (oxyhydr)oxide below the iron redox boundary through bioturbation or else an iron redox boundary that oscillates vertically with time (Glenn and Arthur 1988).

Such oscillations in pore water Eh are attributed to seasonal fluctuations in the deposition of sedimentary organic matter (Pufahl and Grimm 2003). Increased delivery of organic matter produces a rise in the biological oxygen demand at the seafloor, which causes the

Fe-redox boundary to move upward through the sediment dissolving iron

(oxyhydr)oxide. Once the bulk of the organic carbon is microbially respired, oxygen diffuses deeper into the sediment causing this redox boundary to move downward. This

allows iron (oxyhydr)oxides to re-precipitate and re-adsorb phosphate before it diffuses out of the sediment (Fig. 11).

Iron redox pumping can only occur in suboxic pore water because the iron redox interface must reside within the sediment to promote precipitation (Nelson et al. 2010).

Under anoxic conditions this suboxic redox boundary is suspended in the water column, precluding the precipitation of authigenic iron (oxyhydr)oxide and iron silicate beneath the seafloor (Nelson et al. 2010; Pufahl 2010). Under anoxic conditions pyrite precipitates in organic-rich sediments. The organic-rich and pyrite-rich character of lithofacies 2 indicates that primary productivities and the export of organic carbon to the seafloor were too high to sustain the suboxic conditions necessary for ironstone formation.

The presence of interbedded tuffaceous horizons observed by Jensen (1976) suggest the source of iron for ironstone in the Torbrook Formation may have been from volcanic sources. Alternatively, chemical weathering of laterites during the warm and equitable climate of the early Devonian may have delivered the necessary iron. The latter seems more plausible since the early Devonian is a major episode of ironstone accumulation (Taylor and Young 1989). This peak in ironstone accumulation has been attributed to transgressive events that produced widespread stratigraphic condensation in a variety of basin types (Young and Taylor 1989). These occurrences also correspond to major phosphogenic episodes (Pufahl 2010).

5.2 Coated iron grains and seafloor processes

Coated iron grains are ubiquitous on the most condensed flooding surfaces

(lithofacies 5). Debate about their origins has, at times, led to misinterpretations about how coated grains formed (Pufahl and Grimm 2003). Unlike ooids, which precipitate while in turbulent suspension, coated iron grains are authigenic, precipitating just beneath the seafloor near the iron redox interface (Pufahl and Grimm 2003). They are considered the granular equivalent of condensed beds, and, as such, preserve a record of physical and chemical change in bottom water and pore water (Pufahl 2010).

Figure 12. A redox aggraded grain forming through redox pumping just above the redox interface, and an unconformity-bounded grain exhumed, and eroded (often adsorbing fine quartz grains) reburied to the redox interface.

There are two types of coated grains: redox aggraded and unconformity-bounded

(Fig. 12). Redox-aggraded grains consist of concordant, concentric laminae interlayered with redox sensitive minerals such as chamosite and berthierine (Fig. 6). Both contain

ferric and ferrous iron indicating formation close to the Fe-redox boundary during suboxic authigenesis. In some ironstones siderite, pyrite, and francolite are important cements and replacement products. Differences in cortical mineralogy record changes in porewater Eh during authigenesis that result from variations in the sedimentation rate of organic matter. Redox-aggraded grains are thus sensitive indicators of surface ocean productivity and biological oxygen demand at the seafloor.

Unconformity-bounded grains (lithofacies 5) exhibit internal discordances and erosional surfaces, attributable to multiple episodes of iron precipitation, exhumation, and erosion, followed by reburial to the iron redox interface (Figs. 13, 14). Therefore, they contain a record of substrate reworking/winnowing and indicate breaks in calm-water deposition caused by storms and episodic undercurrents, conditions that are common during marine flooding.

In the Torbrook Formation hybrid grains also exist along some flooding surfaces

(lithofacies 4). Hybrid grains contain attributes of both redox-aggraded and unconformity bounded coated grains, suggesting that changes in pore water redox conditions accompanied repeated episodes of exhumation. Such grains are the granular equivalent to condensed beds and provide independent information on the nature and magnitude of hiatuses marking marine flooding surfaces. The most condensed section occurs between parasequences 4 and 5 (thin section SB33). It is interpreted as the maximum flooding surface because it is characterized by thin lags of coated grains, and is the most indurated and scoured in comparison to the other parasequences. It is thought to mark the transition from purely aggradational to progradational depositional process.

B C D

E A F

G H I

Figure 13. (A) Back-scattered electron image of unconformity-bounded grain and EDS spectra: (B) Chamosite/berthierine. (C) Chamosite/Berthierine. (D) Ilmenite?. (E) Francolite. (F) Quartz. (G) Apatite (francolite). (H) Hornblende? (I) Magnetite. Thin section PM-1.

A B

C D

Figure 14. Back-scattered electron image of unconformity-bounded grains, thin section PM-1.

6. Conclusions

1. The lower Devonian Torbrook Formation accumulated in storm-dominated, middle and distal shelf environments between fair-weather and storm wave base. Such an interpretation differs with previous interpretations that place Torbrook lithofacies in shallower environments (Jensen 1976).

2. Lithofacies stacking patterns indicate deposition occurred through the development of seven paraseqeunces. Parasequence tops are marked by marine flooding surfaces that are well indurated with authigenic iron and phosphate minerals. Parasequences thicken and

coarsen upwards through the Torbrook Formation and reflect a shift from aggradational to progradational deposition. This transition is interpreted to record the change in accommodation space associated with the shift from the transgressive to highstand systems tract.

3. Stratigraphic condensation was a prerequisite for ironstone formation on flooding surfaces because low or net negative rates of sedimentation stabilized the iron redox interface within the sediment, allowing the concentration of iron in pore water. Iron redox pumping across this redox boundary redox further facilitated the precipitation of authigenic iron and phosphate minerals. This cyclic process linked the cycling of iron to phosphorus and is likely responsible for the high levels of bioavailable phosphorus necessary to have sustained firmground populations of brachiopods and crinoids.

4. Based on the presence of interbedded tuffaceous beds in the Torbrook Formation the source of iron for ironstone in the Torbrook Formation may have been from contemporaneous volcanic activity. A more probable source, however, was the intense chemical weathering that typified the warm and equitable climate of the early Devonian.

The early Devonian was a major episode of ironstone accumulation that has been attributed to transgressive events that produced widespread stratigraphic condensation in a variety of basin types. These occurrences also correspond to major phosphogenic episodes.

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Appendix I: Torbrook Formation Samples

Sample H Type Notes (m) SB2-1 0 silty shale Phyllitic, ripple laminated SB2-2 8 silty Centimeter sand beds with smaller silty/mud interbeds, sandstone laminated/ripple cross-laminated SB1 29 silty Ripple cross-laminated shales, in situ stromatoporoid sandstone SB32 41 silty Centimeter-scale, massive, interbedded with smaller sandstone ripple cross-laminated shale beds. Adjacent to thin tuffaceous layer. SB2 52 siltstone Parallel/ripple cross laminated SB2-3 101 f.g. Meter-scale sand, mostly massive with some cross sandstone stratification, HCS? Scour surface SB3 109 f.g Meter-scale sand, mostly massive with some cross sandstone stratification, HCS? Scour surface SB2-4 113 f.g. Parallel/ripple cross laminated, bioturbated, coquina? sandstone SB2-5 146 siltstone Cross stratification,whispy, and cm-scale lenticular A beds, numerous brachiopods, dark grey SB2-5 147 siltstone Coquina/shell bed, brown B SB4 175 siltstone Parallel laminated, brachiopod fossil, pyrite? phosphatic pebbles? SB2- 180 silty Ripple cross laminated, bioturbated, phosphatic 6A sandstone pebbles? SB2-6B 180 silty Ripple cross laminated, bioturbated, phosphatic sandstone pebbles? SB2-7 181 f.g. Bioturbated, phosphatic pebbles? sandstone SB5 186 igneous SB2-8 203 igneous SB6 219 f.g. Ripple cross laminated sandstone SB7 223 shale Laminated, phyllitic SB2-9 251 f.g. HCS? Fe ooids? “interference ripples”, bioturbation sandstone SB8 345 shale Dark grey, laminated/ripple cross laminated, pyrite, sulfur smell, micacious, fish scales, highly limonitic SB9 463 f.g. Very hematitic, cross stratified, 20 cm beds, intercalated sandstone with cm phyllitic shale, hematite nodules SB2- 466 silty Dark grey, limonitic, phosphatic pebbles? Grades into 10A sandstone phyllitic/hematitic shales, was drilled for dating in past SB2- 467 siltstone Phyllitic/ hematitic ripple cross-laminated 10B

SB10 477 shale Hematitic, thinly laminated/ripple cross-laminated SB11 549 shale Hematitic, phosphatic? SB12 553 shale Hematitic, phosphatic? SB13 565 shale Phyllitic, 1mm laminated/whispy SB13B 587 shale Hematitic, ripple cross laminated SB14 609 Sandy Hematitic, bioturbated siltstone SB15 662 v.f.g Hematite concretions, phosphatic? sandstone SB16 693 mudstone Hematitic, ripple cross-laminated SB17 694 f.g. sand Parallel laminated, cm bedding, hematite concretions and 1-2 cm discontinuous hematite beds. SB18 695 shale Heavy bioturbation, dark grey to green, parallel laminated, cm bedding, hematite concretions and 1-2 cm discontinuous hematite beds. SB33 699 siltstone Hematite concretion SB19 715 f.g. Highly hematitic, parallel lamination, sandstone SB20 724 shale Ripple cross-stratified, grey/green, SB21 726 f.g. Hematitic, ripple cross-laminated,bioturbated, HCS? sandstone SB22 731 mudstone Hematite nodule SB23 735 mudstone Hematitic/phyllitic, bioturbated, plant fossils SB24 753 f.g. Highly hematitic sandstone SB25 774 siltstone Highly bioturbated, light red/orange, SB2- 794 siltstone Highly bioturbated, grey, whispy hematite laminations, 11A plant fossils SB2- 795 siltstone Highly hematitic/bioturbated, plant fossils 11B SB26 818 mudstone Highly bioturbated, grey, whispy hematite laminations, plant fossils SB27 841 f.g. Highly bioturbated, hematitic, ripple cross-laminated, sandstone HCS?, SB28 842 silty Highly bioturbated, hematitic, ripple cross-laminated, mudstone HCS?, SB29 871 f.g. Alternating +/- 60 cm beds intercalated with 2-8 cm sandstone beds, hematitic, bioturbated, “wavy contacts) scour?, plant fossils SB30 927 siltstone Hematitic, highly bioturbated, with whispy sands, Plant fossils. SB31 973 f.g. Ripple cross-stratified, whispy cm-scale hematitic silts sandstone

Appendix II – Optical Microscopy Descriptions

SB2-1 Not thin section

SB2-2 FOV-3mm, Image1637-on, Image1638-off. FOV-0.5mm, Image1640-on muscovite grain – vfL, undulatory extinction visible in some qtz grains, very well sorted qtz grains. Muscovite visible and conforms to bedding planes. Very small opaques present. SB-2 FOV-1.2mm, Image1642-on, Image1643-off, same features as SB2-2, vfL SB2-3 FOV-3mm, Image1644-on, Image1645-off, Image1651-reflected light on opaque FOV-1.2mm. vfU, both quartz grains and recrystallized quartz veins show undulatory extinction, minor muscovite, minor opaques, Minor hematite in “blotches” throughout, and along the sides of microfractures. SB2-4 FOV-3mm Image1647-on, Image1648-off. Image1650-reflected light on opaque FOV-1.2mm. Intergranular chlorite present, otherwise same features as SB2-3, vfU. SB2- FOV-3mm Image1652-on, Image1653-off, same as SB2-4 5A SB2- FOV-3mm Image1655-on, Image1656-off. FOV~0.75mm Image1654 reflected 6A opaque (hematite rhomb?) Same as SB2-5A SB2-7 FOV-3mm Image1657-on, Image1658-off. Calcite veins and patches of chlorite, small opaques are more common than in the older strata. SB-5 FOV-3mm Image1659-on, Image1660-off. FOV-1.5mm opaque Image1666-on, Image1667-off, Image1665-reflected. Intrusive, 60 % sericitized plagioclase, 25% altered clinopyroxene, 10% chlorite, 5% opaques SB-6 FOV-3mm Image1668-on, 1669-off. Quartz arenite, well sorted, undulatory extinction, fine intergranular chlorite and oxide staining. SB-7 FOV-3mm Inage1670-on, Image1671-off. Similar to SB-6, yet with many cross- cutting with what appear to be boreholes with oxide stained sidewalls. Interestingly, opaque rhombs (pyrite) seem to be in close proximity to the ends of these holes Images1673-1678. Image1679 FOV-1.5mm reflected pyrite grain. SB2-9 No thin section SB-8 Echinoderm FOV 1.5mm Image1680-1681-on. Brachiopod shell impunctate- pseudopunctate showing both thick fibrous layer, and outer primary layer with fibers oriented perpendicular to the shell surface FOV~1.5mm Image1694-on 1695-0ff. Stylolites Image1696-0n, 1697-off FOV 3mm. Crinoid Columnal Image1698-on, 1699-off FOV 3mm. Pyrite grain, cubic, Image1700-on 1701- reflected FOV 0.75mm. Carbonate cemented. SB-9 No thin section SB2- Carbonate of some sort, minor “clumpy” stylolites. Opaque, Image1702-off 10A Image1703-reflected FOV~0.75mm. Horizontal burrows? Image1704-on 1705-off FOV3mm. minor stylolites? Image1706-on, 1707-off FOV-3mm. brachiopods and or crinoid fragments. Interesting grain with opaque inclusions Image1709-on 1710-off 1711 reflected, FOV 0.75mm. SB2- No thin section 10B SB-10 vfL unknown matrix with <5% mud sized quartz grains, extinction of matrix from dark to bright yellow,orange,red. Image1715-on 1716-on (extinct), 1717-off FOV 3mm with crinoid fragment. Brachiopod spine? Image1718-on 1719-on (rotated) Image1720-off, FOV 1.5mm

SB-11 Several calcite filled micro fissures cut and offset by abundant stylolites, francolite/chlorite matrix?. Image 1723-on 1724-off, FOV 3mm. Opaque grain FOV~0.75mm Image1725-off, Image 1726 –reflected SB-13 No thin section SB-14 Chlorite/hematite filled stylolites Image1728-on Image1729-on (rotated) FOV 3mm. SB-17 Sporadic hematite staining. Sharp contact between vfL and vfU Image1730-on 1731-on (rotated), Image1732-off, vfL is chlorite/phosphorite? FOV3mm SB-33 Hematite nodule, sporadic qtz grains Image1734-on, 1735-off, 1736-reflected FOV1.5mm SB- Same as SB-17 with rip up or filled burrow. Image1737-on. 1738-off FOV 3mm 33B SB-19 Quartz grains (sutured contacts), evidence of dissolution and recrystallization within veins, minor opaques, minor muscovite. Image1739-on, 1740-off FOV3mm SB-20 Filled burrows, one hematite filled. Image1741-on 1742-off FOV 3mm, 1745- reflected FOV1.5mm. Minor hematite in burrow Image1743-on, 1744-off SB-21 Same as SB-19 Image1746-on, 1747-off FOV 1.5mm SB-23 vfL, minor stylolites w/hematite/chlorite? Image 1748-on 1749-off SB-24 Similar to SB-21 but coarser and more hematite, and quartz grains not in contact (sutured), but rather matrix supported. Image1750-reflected, 1751-on, 1752-off FOV 1.5mm SB-25 vfL, reddish in hand sample, hematite staining. Image1753-on, 1754-off FOV 1.5mm. SB2- Similar to SB17 with coarser patches of quartz, hematite in stylolites, and chlorite 11A in matrix. Image1755-on, 1756-off FOV 3mm SB-26 vfL to vfU patches of quarts grains, minor hematite. Image1757-on. 1758-off, FOV 3mm SB-28 vfL-FL quartz silt grain hematite patches in burrows, minor hematite and minor stylolites. Image1759-on, 1760-off FOV 3mm SB-29 vfL, minor hematite, even distribution of hematite, same as SB-25. Image1761-on, 1762-off, FOV1.5mm SB-30 fL to fU, minor hematite, suturing of quartz grains Image1765-on 1766-off TB-7 No thin section PM-1 Hematite bearing sample from Potter Mine, dark/olive green matrix (chamosite?). Ooids! Concentrically layered, radial extinction. Image1767-on 1768-off FOV 1.5mm. Lone ooid close to a small calcite anomaly Image1769-on, 1770-off FOV1.5mm, close-up 1771-on 1772-off, quartz tiny grains appear within the ooid wall* FOV 0.75mm

Appendix III –Stratigraphic Field Logs

Stratigraphic log symbols

Stratigraphic log 0-200m

Stratigraphic log 200-400m

Stratigraphic log 400-600m

Stratigraphic log 600-800m

Stratigraphic log 800-1000m