Weathering and paleosol formation in the 1.1 Ga Keweenawan Rift

Precambrian Research 168 (2009) 271–283

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Weathering and paleosol formation in the 1.1 Ga Keweenawan Rift R.L. Mitchell a,∗ , N.D. Sheldon b a b

Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK Department of Geological Sciences, University of Michigan, 2534 CC Little Bldg, 1100 North University Ave, Ann Arbor, MI 48109-1005, USA

a r t i c l e

i n f o

Article history: Received 2 June 2008 Received in revised form 29 August 2008 Accepted 18 September 2008 Keywords: Paleosols Precambrian Keweenawan Weathering Paleoenvironments Midcontinental Rift

a b s t r a c t At ∼1.1 Ga, the North American Craton began to rift, resulting in flood-style basaltic volcanism. During the phase of active tectonism, some of that basalt was weathered and redistributed by fluvial processes. New observations near the northwestern shore of Lake Superior have also revealed the presence of seven paleosols formed on weathered, immature sediments derived from the basalt that are described here for the first time. Only three other paleosols have previously been described from this part of the geologic record, so these new paleosols represent a significant new discovery. The paleosols are weakly developed and exhibit physical and chemical characteristics similar to Phanerozoic Entisols. In contrast to many Precambrian paleosols, the Keweenawen paleosols have been subject to minimal postburial alteration. Among the paleosols, three distinct pedofacies are recognized: (1) waterlogged/gleyed fluvial-proximal paleosols, (2) dry fluvial-distal paleosols, and (3) cumulative fluvial-proximal paleosols. Despite these facies differences, using a combination of physical and chemical measures of weathering it is found that the paleosols are all derived from the same parental basalt, with one of the paleosols showing an additional extra-basinal rhyolitic component. By considering the mass-balance behaviour of a variety of alkali and alkaline earth elements, it is possible to rule out significant potassium metasomatism, suggesting that these paleosols may be particularly useful for paleoatmospheric and paleobiologic reconstructions, and that they form an important new source of data about Mesoproterozoic weathering. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Previous studies of weathering environments associated with Precambrian paleosols have mostly been aimed at reconstructing the atmospheric composition at the time of the paleosols’ formation, and in differentiating between metasomatic alteration of sedimentary rocks and pedogenesis (e.g., Retallack, 1986; Zbinden et al., 1988; Holland et al., 1989; Maynard, 1992; Retallack and Mindszenty, 1994; Sheldon, 2006b). In particular, paleosols have been used to understand the “Great Oxidation Event” (e.g., Rye and Holland, 2000a,b). Given that there is little dispute about the relative degree of oxygenation by the Mesoproterozoic, the geochemistry of Mesoproterozoic paleosols has been studied less thoroughly than their Archean and Paleoproterozoic counterparts. The north shore of Lake Superior, particularly in Minnesota, exhibits numerous outcrops associated with the failed Precambrian

∗ Corresponding author. Tel.: +44 1784 443581; fax: +44 1784 471780. E-mail address: [email protected] (R.L. Mitchell). 0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2008.09.013

Midcontinent Rift (MCR) which reached the height of its activity 1.1 Ga (Ojakangas et al., 2001; Green, 2005). Rocks associated with the rift on the north shore include volcanic and sedimentary deposits belonging to the North Shore Volcanic Group (NSVG), a series of flood basalt flows 6–9 km thick (Jirsa, 1984) accompanied by intermittent sedimentary deposition. The sedimentary lithologies are dominated by fluvial and alluvial sandstones, siltstones, mudstones, and some shales, deposited in a southeasterly direction towards present day Lake Superior (Jirsa, 1984; Green, 2005). In general, little attention has been paid to understanding the weathering environment, and most of the previously study has been of caliches in eastern Wisconsin and western Michigan (e.g., Kalliokoski and Welch, 1985; Kalliokoski, 1986). Only the “Sturgeon Falls” paleosol has been adequately documented (see Zbinden et al., 1988) and represents a single profile rather than a comprehensive study of the sedimentary environments at 1.1 Ga. No paleosol profiles have been fully documented and recognized within the NSVG and in total, only three paleosols of this age are known from the geologic record (Zbinden et al., 1988; Retallack and Mindszenty, 1994). This paper seeks to characterize both the weathering environments and paleosol formation within the NSVG, and to differentiate between pedogenesis and post-burial alteration.

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2. Geologic context 2.1. The MCR The 1.1 Ga Midcontinent Rift System (MCR) is a large failed continental rift thought to have originated from activity associated with a large mantle plume beneath the North American Craton (the supercontinent of Rodinia; Green, 1983; Cannon and Hinze, 1992; Davis and Green, 1997), at a time when the Keweenawan palaeolatitude was around 30◦ N (Kalliokoski and Welch, 1985; Kalliokoski, 1986). The rift subsequently failed to become an ocean basin due to compression associated with a continental collision to the east of the rifted area at the Grenville Front (Cannon and Hinze, 1992). Modern day Lake Superior follows a trend mimicking that of the failed rift, with outcrop on either shore exhibiting strikes that match the paleo-rift (Jirsa, 1984). From Bouger and magnetic anomalies, as well as seismic reflection data, the rift spans the north central part of the North American Craton, from Ontario, Canada, 1400 km south to Kansas (for example see Green, 1983; Van Schmus and Hinze, 1985; Anderson and McKay, 1989; Shay and Trehu, 1993). Volcanism took place in two pulses, between 1109 and 1105 Ma and between 1100 and 1094 Ma (Paces and Miller, 1993), with the climax of rifting being centred around 1100 Ma (Ojakangas et al., 2001). Sedimentation occurred in times of volcanic quiescence within these two pulses of volcanism, but not in the 5 million years gap between 1105 and 1100 Ma (Ojakangas et al., 2001).

resulted in long-lived outpourings of lava onto plains in the rifted area. 1 × 106 km3 of basalt is reported to have been erupted within the NSVG (Green, 1989). The NVSG (Fig. 1), the primary outcropping unit on the north shore of Lake Superior in Minnesota, is dominated by tholeiitic basalts (Green, 1982, 1989) emplaced in flood-style eruptions similar to Icelandic flood basalts erupted during the Tertiary (Green, 1989), with intermittent sedimentary rock emplacement during episodes of depositional quiescence. Limited dating has been undertaken on the rocks within the NSVG, however an age of 1097 ± 2 Ma was obtained from a rhyolite at Palisade Head (Green and Fitz, 1992; Vervoort, 1996; Fig. 1). Green (1982, 1989) document a complete change from mafic to felsic rocks spanning the range of tholeiitic basalt to rhyolite. In some areas up to 25% felsic volcanics are present (Jirsa, 1984; Green, 1989; Green and Fitz, 1993; Nicholson et al., 1997; Vervoort and Green, 1997; White, 1997; Vervoort et al., 2007), as well as interflow sedimentary rocks. Basalts are amygdaloidal and dominated by Ca-plagioclase phenocrysts, with minor amounts of olivine and clinopyroxene, and some iron oxide minerals. Amygdales are dominated by quartz, Caagates, and thomsonite (Jirsa, 1984; Green, 1989). Some large-scale intrusive episodes occurred simultaneously, notably the gabbroic Duluth Complex to the north of the NSVG, and the Beaver Bay Complex just north of Silver Bay, Minnesota, in addition to many other smaller intrusive events dominated by local diking. Associated sedimentary rocks are reportedly dominantly derived from the intrabasin mafic Keweenawan volcanics associated with the rift (Kalliokoski and Welch, 1985; Green, 1989; Ojakangas et al., 2001).

2.2. North Shore Volcanic Group (NSVG) 2.3. Good Harbor Bay sedimentary rocks Mesoproterozoic outcrop is found along the shores of present day Lake Superior, particularly in the Upper Peninsula of Michigan and the northeastern “Arrowhead” region of Minnesota (Fig. 1). The southern portion of the failed rift in southern Minnesota, Iowa and Kansas is covered by Phanerozoic strata (Green, 1983; Ojakangas and Dickas, 2002). Volcanic activity occurred from fissures within small subsided basins on low-angle slopes (Green, 1989), which

In the northeastern portion of Minnesota, a sequence of Keweenawan basalt-derived sedimentary rocks outcrop along a road cut at Good Harbor Bay (see Fig. 1). These were deposited in an elongate, east–northeast trending basin with sediment influx via fluvial action towards the shore of present day Lake Superior from inland source areas (Jirsa, 1984). Deposition occurred primarily in

Fig. 1. Geologic map of the Lake Superior region of the Midcontinent Rift. Location of the North Shore Volcanic Group (NSVG) and respective correlated formations on the south shore in Wisconsin and Michigan are shown (these include the Powder Mill Group ad the Portage Lake Volcanics). Localities visited during fieldwork are shown, as well as intrusive bodies to the northwest of the field areas, the Duluth and Beaver Bay Complex’s. Scale shown.

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Fig. 2. Photograph of the studied outcrop at Good Harbor Bay, Cook County, Minnesota, showing the contact between the basalt and underlying sediments. (A) Terrance Point Basalt is out of sight, above the below lava flow, (B) numerous smaller basalt flows, and (C) red sandstones, siltstones and mudstones. Cut Face Creek is seen in the far distance towards the end of the road. Scale shown.

a fluvial environment, with some accumulation in ponds and small lakes (Jirsa, 1984). As we will discuss in a subsequent section, during episodes of exposure, previously unrecorded soils also formed on the floodplains and margins of the lacustrine settings, as well as some paludal deposits, that are preserved throughout the 65 m thick section at Good Harbor Bay. 3. Methods The primary outcrop described in this paper is exposed along a road cut section at Good Harbor Bay, and along Cut Face Creek at the northeast end of the road section (see Fig. 2). Eighty-nine samples were collected from the 65 m thick road section comprising samples of seven paleosols (typically n = 5) as well as the associated sediments and basalt. Samples were collected by using a hammer and removing ∼10 cm of rock from the surface of the outcrop to allow “clean” samples to be collected. Sedimentary structures were recorded (including channel structures, cross beds, mudcracks, ripples, horizontal bedding and mottles), sketched, and photographed. Outcrop photos were made approximately every meter throughout the section. Paleosols were identified on the basis of grain size changes, clay content, and color changes. Evidence of depositional quiescence and exposure (e.g., desiccation cracks) were also used to locate paleosols. Geochemical analysis for major elements was undertaken on a PerkinElmer Optima 3300RL ICP-AES System, and a PerkinElmer Elan 5000 ICP-MS system for trace elements. Point count data from 29 thin sections (500 counts per section) was obtained from a James Smith Model F Point Count machine. SEM and EDX images were made with a Hitachi S3000 SEM, and collected in backscattered electron (BSE) mode. Munsell Soil colors were determined from dried samples once back in the laboratory. All of the above techniques were undertaken at Royal Holloway, University of London. 4. Results 4.1. Physical sedimentology 4.1.1. Sedimentary rock overview A variety of sedimentary rock lithologies including sandstone, siltstone, mudstone, and a small amount of shale, were deposited in the extensional basin at Good Harbor Bay, Minnesota (see Fig. 1 for localities) in a mixed fluvial and lacustrine setting with a gently

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dipping slope (less than 1◦ ) from the northwest to the southeast towards what is now present day Lake Superior (Fig. 3; Jirsa, 1984; Kalliokoski and Welch, 1985; Kalliokoski, 1986). Jirsa (1984) reported localized ponding which is supported by fine laminations and horizonation from this study (Fig. 3A and B). Paleosols (see below) were also identified in the Good Harbor Bay sequence. Desiccation cracks are frequently preserved on bedding surfaces (Fig. 3D), consistent with intermittent exposure of sediments during times when the floodplain dried out. Indicators of paleocurrent (e.g., Fig. 3E and F), though relatively rare, are present in the form of both sinuous and linguoid ripples and planar cross laminations (Table 2; Fig. 3; Jirsa, 1984). Fluvial processes are inferred on the basis of small, rare channels (typically <1 m width; Fig. 3C), scour surfaces, and overbank flood deposits. The overbank deposits occur frequently as laterally extensive, horizontally laminated sand and mudstone (Fig. 3A). A high-flood frequency may be inferred from the relative paucity of channel deposits and extensive overbank deposits and localized ponding. Scoured bases accompany channel structures with rip-up clasts derived from muddy/silty deposits below. Grain sizes range from silts, muds, and fine sands in the overbank deposits to medium sand in the channel deposits. Munsell colors of sedimentary rocks are consistently dark red in color (2.5YR to 5YR; e.g., Fig. 4). 4.1.2. Volcanogenic deposits Additional evidence for localized ponding is present at the south–south west end of the outcrop in the form of hyaloclastite deposition. Basalt (Fig. 2) has erupted from a single fissure beneath a mound of water-lain silts and sands as a mound of pillow basalt breccia and hydroclastite (Green, 1989), which indicates that the sediments must have been soft at the time of eruption, and precludes the possibility that the red color throughout the sedimentary sequence represents baking rather than oxidative weathering (Sheldon, 2003). 4.1.3. Paleosols in outcrop Seven paleosols are identified at Good Harbor Bay. Each of the seven paleosols exhibit grain size distributions (i.e., fining up), textures, and relative clast maturity similar to modern soils (Table 1). Though modern soil taxonomy is not necessarily appropriate (see Section 5.1 for further discussion), the paleosols are broadly similar in the field to Phanerozoic fluviatile Entisols (Soil Survey Staff, 2006). The paleosols developed in overbank and pond environments, and were distinguished from surrounding rocks on the basis of the destruction of sedimentary structures (i.e., little or no relict sedimentary structure), finer grain sizes relative to the surrounding deposits and the sediments from which they were derived, and color differences (typically between Munsell “Color” values but some times among “Hue” values). The identification of paleosols in the field was further confirmed by petrographic and geochemical observations. Field observations of the paleosol indicate the following fabric and textural differences among the paleosols (see Table 2): (1) three of the paleosols (4, 5, and 7) have granular textures and preserve some relict grains, consistent with weak development of soil structure; (2) two of the paleosols (1 and 3) have an intertextic fabric, consistent with weak to moderate development of soil structure; (3) one of the paleosols (2) has a pophyroskelic fabric (matrix > grains; confirmed petrographically), consistent with moderate development of soil structure. Three of the paleosols (1, 5, and 7) also have evidence of gleying in the form of blue and green mottles within the paleosol profile, consistent with waterlogged forming conditions (Retallack, 2001; Sheldon, 2005), though other features indicative of gleying and continual inundation such as pyrite were not present. The seven paleosols have been subdivided into three pedogenic facies, or pedofacies,

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Fig. 3. Photographs of some of the sedimentary features found in the Good Harbor Bay and Cut Face Creek sediments. (A) Horizontal laminations and reduction horizons in sedimentary near to the contact with the overlying basalt (Good Harbor Bay). (B) Fine-scale bedding and laminations, showing particularly black/dark horizons (Good Harbor Bay). (C) Thicker packages of sedimentary lithologies exist, some with fluvial channel features (Good Harbor Bay). (D) Fine-scale mudcracks on the tops and underside of beds are present, showing the periodic exposure and drying of the sediments (Good Harbor Bay). (E) Large scale and well-preserved ripples exist, supporting a south–south east paleocurrent direction (Cut Face Creek). (F) Fine-scale laminations and cross bedding are present, and also preserve paleocurrent directions (Good Harbor Bay). Scales shown.

that have accumulated in different parts of the fluvial/lacustrine system based on different characteristics of their formation (Fig. 4). Differences in the character of each of the paleosols are seen in Tables 1 and 2. 4.2. Petrography 4.2.1. Paleosols From point count data, the dominant grain/mineral composition of the paleosols includes feldspars (predominantly plagioclase, with minor amounts of microcline), iron/titanium oxides, lithic fragments (dominantly mafic over felsic igneous lithics, and some sedimentary lithics dominated by mudstones; Fig. 5B and G), calcite grains (Fig. 5G), and small amounts of olivine, clinopyroxene (Fig. 5C), quartz, and agate (Fig. 5). Matrix in the paleosol samples

is dominated by clay-sized matrix material. Samples are typically hematite-cemented, with some secondary cements (calcite) present also. Matrix:clast ratios vary significantly throughout the 65 m section, with some samples matrix-rich and classifiable as wackes under the scheme of Pettijohn (1975) (see Fig. 6). Clasts are for the most part composed of mineral grains derived from mafic igneous rocks, presumably from a local source, and are generally well rounded and moderately sorted (Fig. 5D), with the exception of paleosol profile 7, which is angular and poorly sorted (Fig. 5F and G). 4.2.2. Sedimentary lithologies Based on a QFL diagram (Pettijohn, 1975; Fig. 6), the primary clastic classification type is arkosic arenite (i.e., lithic arkose) as indicated by the dominance of feldspar in samples (sometimes up to 86%; e.g., Fig. 5B, C and E). Many of the feldspars show par-

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Fig. 4. The three different pedofacies found in the Good Harbor Bay section, shown by sedimentary logs, Munsell Soil Colours (Soil Survey Staff, 1975) sample numbers, petrology photos and various molecular weathering ratios. (A) Type A paleosols (profiles 1, 3 and 7; paleosol 1 shown here) are laterally extensive and fluvial proximal with thick A horizons and deep B horizons; (B) Type B paleosols (profiles 2 and 4; paleosol 4 shown here) are isolated profiles with more profound accumulation of material at the B horizon and towards the bottom of the profile; (C) Type C paleosols (profiles 5 and 6; paleosol 5 shown here) are thick paleosols developed on floodplain deposits that have shallow B horizons.

tial alteration to sericite (Fig. 5E) but maintain their relict crystal morphology. The lithics component is dominated by mafic and sedimentary clasts, where felsic lithics are rare, except for a lone sample (CFC61a) most probably derived from a different source

area. The preservation of large amounts of feldspar was previously used to suggest a semi-arid climate with limited water availability (Kalliokoski and Welch, 1985), which suggests that physical weathering was more important than chemical weathering.

Table 1 A comparison of some of the physical and chemical features of the seven paleosols compared with basalt. Paleosol 1

Paleosol 2

Paleosol 3

Paleosol 4

Paleosol 5

Paleosol 6

Paleosol 7

Overlying basalt

Thickness (cm) Sample numbers (CFC) Sedimentary features

214 9–14a Horiz. bed.

143 16–19 None

146 22–26 Cross beds, ripples

155 34–38 Mudcracks, horiz. bed.

SE/SSE (4) 50.0 1.48 0.79 0.26 0.001 0.18 0.07 0.008

None 44.6 1.24 1.19 1.77 0.55 0.16 0.11 0.009

None 41.9 1.03 0.95 2.29 0.56 0.16 0.14 0.012

None 44.3 1.12 0.83 4.61 0.52 0.17 0.14 0.014

134 53–56 Horiz. Bed, ripples, cross beds, mudcracks None 44.0 1.29 0.76 2.39 0.48 0.17 0.09 0.008

376 58–61a Horiz. bed. ripples, cross beds, mudcracks SSE (2) 43.4 1.11 1.92 1.80 0.54 0.16 0.15 0.01

N/A 1 N/A

Paleocurrent data (direction and no.) Maximum CIA Hydrolysis Leaching Salinization Calcification Clayeyness Ti/Al Ti/Zr

283 48–51 Horiz. Bed, Ripples, Rip-up clasts, cross beds SE (2) 48.1 1.49 0.48 1.12 0.42 0.18 0.06 0.004

N/A 43.0 1.16 0.35 0.20 0.21 0.07 0.014

Note: Molecular weathering ratio data is average of entire paleosol. Calcification mentioned but not important to study due to homogeneity. Comparison of each paleosol and its respective average molecular weathering ratio values (hydrolysis, drainage, salinization, calcification, clayeyness and provenance indicators Ti/Al and Ti/Zr) with those of the source basalt. Sedimentary features are also shown.

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Table 2 A comparison of the three pedofacies and the paleosols that comprise them. Depositional Environment Type A Paleosols Paleosol 1 Lacustrine

Dominant sediment type

Dominant texturea

Dominant fabricb

Features

Siltstone

Wacke

Intertextic

Color mottling, gleying, laterally extensive, slow deposition rate, rip-up clasts Relic sed. structures, poor drainage, scour marks Color mottling, relic sed. structures, laterally extensive

Paleosol 3

Fluvial proximal-floodplain

Siltstone

Wacke

Intertextic

Paleosol 7

Fluvial-floodplain

Siltstone

Wacke

Granular

Fine sandstone

Wacke

Porphyroskelic

Siltstone

Arenite

Granular

Siltstone

Wacke

Granular

Mudstone

Arenite

Argillasepic

Type B Paleosols Paleosol 2 Fluvial distal Paleosol 4

Fluvial distal

Type C Paleosols Paleosol 5 Lacustrine Paleosol 6

Fluvial

No relic sed. structures or mottling, isolated profile, poor drainage Coarse, isolated profile, halite structures, mudcracks, poor drainage Relic sed. structures, highly oxidized horizons, ripples, rip-up clasts Relic sed. structures, mudcracks, no mottling

Comparison of each paleosol, the assigned type of paleosol, and various sedimentary features. Environment of deposition has been inferred from various sedimentary and chemical features of each paleosol. a Denotes obtained from Pettijohn (1975). b Denotes the fabrics discussed in Retallack (2001).

The fluvial character of the sediments identified in the field (Fig. 3) is replicated in thin section. Grading is present in thin sections of overbank deposits at a mm-scale (seen in Fig. 3B and F, and also obvious enough to be seen in hand specimen). Well-rounded grains include iron/titanium oxides (Fig. 5C), which indicate significant transportation and reworking of sediments. These grains often form dark horizons, frequently visible to the naked eye, and of variable thickness. Quartz is also present as rounded detrital grains, with some well-preserved angular grains showing conchoidal fracture (Fig. 5G), as well as some well-preserved volcanic grains (Fig. 9A), likely from a local rhyolitic source such as that at Palisade Head (Fig. 1). 4.3. Scanning electron microscopy (SEM) Scanning electron microscope (SEM) images were collected from fourteen samples. A well-preserved volcanic quartz grain is found in sample CFC61a (Fig. 9A) consistent with a rhyolitic source, as well as typical weathering features such as etching and denticulated margins associated with silicate minerals (e.g., pyroxene; Fig. 9B). 4.4. Geochemistry 4.4.1. Loss on ignition In addition to the whole rock ICP analyses, loss on ignition (LoI) was measured. The mean LoI value for paleosol and surrounding sedimentary rock samples is 9.1 ± 1.1% (1). The mean loss is higher than that of the basalt sample (LoI of 4.7%). The higher LoI values for the paleosol and surrounding sediment samples are consistent with the presence of hydroxylated and hydrolized minerals in the weathered samples, and also with the presence of some secondary calcite (e.g., Section 4.2.1; Fig. 5G). 4.4.2. Molecular weathering ratios Five molecular weathering ratios (i.e., wt.% results are first converted to moles) were calculated to evaluate the degree of chemical weathering and to evaluate which pedogenic processes were most important (e.g., Sheldon and Tabor, in press; Retallack, 2001): (1) “hydrolysis”, (2) “clayeyness”, (3) “leaching”, (4) “salinization” and

(5) the chemical index of alteration (Nesbitt and Young, 1982). A comparison of the average values of these ratios for each paleosol is seen in Table 1. Hydrolysis (Retallack, 2001) is calculated with the following equation: Hydrolysis =

Al . Ca + Mg + K + Na

(1)

Application of the hydrolysis ratio is based on the idea that as minerals are weathered, base cations (denominators in Eq. (1)) are lost relative to Al during clay formation. Analyses of parental basalt values (n = 4) gives an initial hydrolysis value of 0.38 ± 0.04 (1). In contrast, paleosol and surrounding sediment analyses typically have values between 1 and 2, consistent with weak pedogenesis (Fig. 4; Sheldon et al., 2002; Hamer et al., 2007). This is supported by petrological evidence of the presence of easily weatherable minerals such as feldspars, clinopyroxene, and olivine (see Section 4.2; Figs. 5 and 9). The second molecular weather ratio that was applied was clayeyeness, which is given by the following equation: Clayeyness =

Al , Si

(2)

where the ratio should increase due to clay production during pedogenesis (e.g., Sheldon, 2006a; Sheldon and Tabor, in press). See clayeyness figure in data repository. The parental basalt value is 0.2 ± 0.02 (1), which is indistinguishable from most of the paleosol and surrounding sedimentary rock values, suggesting minimal clay formation. This is consistent with the lack of clay observed in thin section, point counts, and SEM (see Section 4.3). The degree of leaching may be assessed by considering the following equation (Retallack, 2001): Leaching =

Ba , Sr

(3)

where the solubility differences between the two elements (Ba < Sr) gives rise to higher values in more leached samples. The parental basalt leaching value is 0.33 ± 0.23 (1), whereas typical paleosol values are 0.5–2 (Fig. 4), indicating significant, though moderate, leaching during pedogenesis (Sheldon and Tabor, in press). Results calculated using Eq. (3) were also used to help define the different pedofacies.

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Fig. 5. Photomicrographs of various structures observed in the Good Harbor Bay samples. Lithologies include: (A) unaltered basalt (sample CFC1); (B) poorly sorted sandstone with lithic fragments (CFC24); (C) many relict volcanic grains including easily weatherable pyroxene crystals (plane-polarized light and cross-polarized light respectively, sample CFC14b; (D) well-sorted siltstones (sample CFC43); (E) represents CFC28, a highly altered feldspar crystal in ppl and xpl; (F) some large thomsonite crystals are present (e.g., sample CFC61a); (G) moderately sorted sandstone (CFC61a). Mineral labels are: pl: plagioclase; Io: iron oxide; Lf: lithic fragment; Q: quartz; Cx: clinopyroxene; Th: thomsonite; C: calcite. Scales shown.

Salinization (i.e., accumulation of soluble salts during weathering) may be assessed using the following equation (Retallack, 2001): Salinization =

Na + K . Al

(4)

Values in the paleosols are less than 0.2. This is well below the salinization threshold defined by Retallack (2001), where soils with values greater than 2 have been affected by salinization. There is also no increase in the ratio near the top of the paleosol profiles (Fig. 4), which is also consistent with minimal salinization.

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Dietrich (1987), is particularly useful for tracking elemental gains and loses relative to the paleosol’s parent material using an immobile index element. This method has been used previously in various ways, including with basalt-parented (Sheldon, 2003) and Precambrian (Driese, 2004) paleosols. Calculations of mass balance require the computation of two numbers, mass-transport and strain. The open system mass-transport function for an element of interest (j) in the weathered sample (w) is calculated as follows:



j,w =

w Cj,w



[εi,w + 1] − 1,

p Cj,p

(6)

where w is the density of the weathered material, Cj,w is the chemical concentration (weight percentage) of element j in the weathered material, p is the density of the parent material, and Cj,p is the chemical concentration (weight percentage) of element j in the parent material. If  j,w = 0 (i.e., element w was immobile), then εi,w can be solved for separately, thus bypassing volume (as in the classical definition of strain) as follows: Fig. 6. A quartz–feldspar–lithic (QFL) diagram classifying the Good Harbor Bay sandstones. The majority plot in the arkosic arenite sector (lithic arkose). Some outliers are more lithic-rich; none are quartz-rich. Circles represent sample points.

The final molecular weathering ratio applied is the chemical index of alteration (CIA herein) of Nesbitt and Young (1982), which is calculated as follows: CIA =

Al × 100. Al + Ca + Na + K

(5)

The CIA is a measure of the breakdown of feldspars to form clay minerals (Nesbitt and Young, 1982). Different parent materials have different initial CIA values. For example, basalts have a CIA of <45 (e.g., Sheldon, 2003), felsic rocks 55–60, and shales 70–75 (Maynard, 1992; Young and Nesbitt, 1998). Parental basalts (n = 4) have a mean CIA value of 41.3 ± 2.1 (1). In comparison to this, the paleosols in Good Harbor Bay exhibit a whole-profile average CIA of 41.6 ± 3.5 (1). The surrounding sediments and paleosols do not have a mean CIA dissimilar to the parent material, suggesting relatively little chemical weathering from parent basalt to daughter sediments, and that the majority of the material in the paleosols is derived from basaltic source rocks. However, depth profiles in paleosols do show significant variation (Fig. 4), and can help, along with physical sedimentology, to determine various soil horizons and weathering differences. As the horizonation is less distinct than that of modern soils and Phanerozoic paleosols, it is difficult to apply modern soil taxonomy. In this context we use the CIA values to name horizons as follows: the “A” horizon is the upper portion of a paleosol that is weakly leached and developed relative to the parent material (i.e., higher CIA), the “B” horizon is the most strongly leached and developed subsurface horizon and the zone of clay accumulation where relevant, and the “R” horizon is the extremely weakly to un-weathered, but eroded parent material (i.e., parental sandstones, siltstones, and mudstones derived from basalt). Because the paleosols are formed on sediments derived from earlier weathering of the parental basalt, grain size fines upprofile in the paleosols from the least weathered “R” horizon to the more weathered “B” and “A” horizons. An example of a typical source basalt in petrographic section is seen in Fig. 5A. 4.5. Mass-balance calculations for paleosols Mass-balance equations can be used to quantify the change in elements throughout a paleosol profile relative to their parent material. The approach used here, based on Brimhall and



εi,w =

p Cj,p w Cj,w



− 1,

(7)

where εi,w is the strain on immobile element i in the weathered sample. Density values for paleosol and parent material samples are based on Sheldon and Retallack (2001) and Sheldon (2003). Immobile elements were chosen based upon comparison of downprofile changes in the ratio of potentially immobile elements (Ti, Al, Zr), which indicated that Zr was the least mobile element (Al and Ti were somewhat mobile in comparison). Gains and loses of elements vary throughout each paleosol profile. Using basalt as the parent material, every paleosol has losses in major elements Al, Fe, Mg, Ca, and Na (Fig. 7), except paleosol 1, discussed in a later section. Using instead a parent material composition of sandstone derived from the basalt (but not weathered any further beyond its original derivation) results in the same weathering trends, but with smaller loses by around 10%. The relative sequence of how elements were lost from the paleosol profiles is: Mg ≈ Ca > Na > K. 5. Discussion 5.1. Pedofacies Three paleosol depositional types (pedofacies) are identified from the seven paleosols (see Table 2; Fig. 4) using the previously described physical sedimentology (see Section 4.2.1) and geochemistry (see Section 4.4.2) results. For simplicity’s sake, the pedofacies have been named Types A, B, and C, respectively. Type A paleosols (paleosol numbers 1, 3, and 7) are fluvialproximal and laterally extensive. The average grain composition indicates that they are wackes (Fig. 6; classification of Pettijohn, 1975) and include significant amounts of matrix. Down-profile, Type A paleosols have varying amounts of matrix (Fig. 4), with the highest proportion of matrix and finest mean grain sizes in point counts in the middle part (“B” horizon) of the paleosol profile. CIA results from that same horizon are the highest in the profile, and a significant amounts of leaching is also indicated (Fig. 4). Relative to the other paleosol types (Fig. 4), Type A paleosols show the moderately well-developed profiles and have their “B” horizons lowest in the profile, consistent with longer periods of pedogenic development than the other paleosols in the rest of this section. Moderate to poor drainage and a fluctuating water table is also indicated by color mottling (e.g., Kraus and Aslan, 1993) and by laminations of Munsell

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Fig. 7. Mass-balance graphs for all paleosols, with Zr immobile and basalt as the parent rock. (A)–(C) represent pedofacies type, respectively. Losses are represented by values less than 0. All show distinct losses, except FeO in paleosol 1, which is explained in the text. Error associated with analytical uncertainty is less than the width of the data points.

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colors consistent with variable oxidation (i.e., gley colors; cf. Sheldon, 2005). Salinization is non-existent and clayeyness is minimal. Type B paleosols (paleosol numbers 2 and 4) tend to form isolated profiles on local micro-paleotopography and have very few relict sedimentary structures. Paleotopography is indicated by dmto cm-scale changes in elevation. Grains in the profiles are well rounded and well sorted (Figs. 4 and 5). The weathering is less intense throughout the profile, but the CIA values at the “B” horizon are comparable to Type A paleosols, though the highest values occur nearer to the surface of the paleosol. Material has been leached and reprecipitated towards the bottom of these paleosols (see Section 5.2). Type B paleosols contain evidence of periodic exposure and drying based upon the presence of desiccation cracks (Figs. 3 and 4). The combination of all of these features and lack of evidence suggesting regular ponding or flooding, suggests that Type B paleosols were deposited relatively distal to the fluvial channel. Type C paleosols (paleosol numbers 5 and 6) formed on locally ponded parts of the floodplain, on finer-grained parent materials than the other two pedofacies (mudstones and siltstones; Table 1). Type C paleosols may have formed distal relative to the channel based on the lack of sand-sized grains in thin section, or may represent late-stage deposition of finer sediments during the periodic drying out of the floodplain. The CIA and leaching values of these paleosols exhibit very minor changes with depth in the profile and the profiles are generally much thicker than Types A and B paleosols, so it is possible that these paleosols developed in areas with a higher influx of sediment, or as cumulative profiles (Kraus, 1999). This is consistent with the lack of horizon development and large number of relict sedimentary structures present in the paleosols. The weak degree of development, both in terms of physical and chemical weathering, is most consistent with modern and

Phanerozoic Entisols (see Section 4.4). However, given the basic differences in the potential floral composition (i.e., vascular plants in the Phanerozoic versus microbial communities in the Proterozoic), it is not strictly possible to apply modern soil taxonomy to Precambrian paleosols. Nonetheless, one may speculate that the similarity between the Keweenawan and Phanerozoic paleosols may be due to a more extensive mid-late Precambrian biosphere than has been previously recognized. 5.2. Mass-balance and pedogenic processes Mass-balance calculations reveal important differences in how individual elements are gained, lost, or redistributed during pedogenesis. For example, throughout each paleosol profile, Mg is lost relative to the parent material to a constant degree and does not re-precipitate lower in the profiles, and was thus lost during pedogenesis (Fig. 7). Type A paleosols have significant losses of every major element except Fe throughout their profiles, losing up to 80% in some cases (Fig. 7). Fe is apparently gained in paleosol 1; this is simply due to the differential redox conditions within that paleosols, because this high value coincides with an apparently reduced horizon in this portion of the paleosol (Fig. 4A; based on redoximophic features; Kraus and Aslan, 1993). Waterlogged and gleyed paleosols, such as Type A paleosols, show a lesser loss of Fe compared with other paleosols due to the retention of material within the soil profile (see Figs. 4 and 7). Type B paleosols show a loss in every element, sometimes up to 80%. Fe behaves more consistently in these paleosol profiles in comparison to Type A paleosols, indicating a lack of strong redox gradients, more efficient drainage, and less fluctuation of the water table. Type C paleosols again show a loss of all the major elements, with the greatest degree of leaching in a thin leached horizon at the top of the profile (Fig. 4).

Fig. 8. Graph showing the Ti/Al ratios of all seven paleosols vs. profile depth. The data all indicate a local basaltic source rather than a rhyolitic one, with one exception as discussed in the text. Values for surrounding sediments are also consistent with the paleosol and basalt values. Four basalt and one rhyolitic sample also plotted for comparison. Error associated with analytical uncertainty is less than the width of the data points.

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5.3. Provenance of paleosols and surrounding sedimentary rocks

5.4. Metasomatic alteration?

Ti/Al ratios are useful for indicating whether the paleosols and surrounding sedimentary rocks are derived from uniform parent material (Sheldon and Tabor, in press; Sheldon, 2006a; Hamer et al., 2007). Jirsa (1984) suggests that the parental basalt that provided clasts to secondary sediments, as in the Good Harbor Bay sequence, may not have extended much further than the present outcrop location, which suggests that the sediments deposited in the Good Harbor Bay region should indicate a similar and consistent source. Fig. 8 shows the relationship between Ti/Al ratios and parental basalt samples, with a value of 0.11 ± 0.05 (1). The paleosols all have relatively constant values with depth. In addition, most of the Good Harbor Bay sedimentary rocks have Ti/Al values similar to the local basaltic value rather than with local rhyolitic sources. The slightly elevated Ti/Al values of the paleosols relative to the parental basalt is due to the balance between physical and chemical weathering. Ti is minimally weathered chemically (e.g., Sheldon, 2003) and is typically only removed by physical weathering, except under very acidic conditions. As a result, during pedogenesis where there is little physical weathering, Ti may accumulate (Sheldon and Tabor, in press). Separating out just the lithic portion of thin section samples into a mafic–quartz–felsic ternary diagram (Fig. 9) shows that there are a few samples with a significant proportion of felsic material. Those samples also are more alkali (Na and K)-rich than the parental basalt (see Fig. 9D), which suggests that either some of the source area contained felsic material as well, or that there was some wind-borne addition of volcanic ash. Paleosol 7 (sample CFC61a) has the highest felsic lithic grain component out of all the samples (near 25%) and an elevated Ti/Al ratio, again consistent with a slightly different parent composition and texture (i.e., lithic arenite; Fig. 6). This is supported by spikes in Ti and Zr in samples of Paleosol 7, different sediment character (i.e., poorly sorted and angular, with minerals include calcite, thomsonite, felsic and lithic sedimentary clasts, and large amounts of iron oxides (Figs. 5G and 8). However, the majority of the data centres around the dominantly mafic end of this ternary diagram, as one would expect from paleosols derived from mafic basaltic parent material (Fig. 9), with only one paleosol that appears to be clearly polygenetic.

The deposits in this region of Lake Superior have never been substantially buried over the past 1.1 Ga and so have not been subject to the same alteration as other parts of the rift. One common observation in Precambrian paleosols is an enhanced K2 O content due to post-burial metasomatism (e.g., Maynard, 1992). This is often supplemented by observations of K-rich minerals and growth fabrics, and by an increase in potassium up-profile in the sediments (i.e., potassium transported along bedding contacts; Sheldon, 2003). One way to address whether or not this process has been occurring is to compare the behaviour of K and Rb, another alkali element with similar chemical affinities, but which is less mobile in metasomatic settings (Sheldon and Tabor, in press; Sheldon, 2003). Mass-balance calculations (Eqs. (6) and (7); using paleosol 1) of changes in the alkali elements K and Rb indicate some addition relative to the parent material, but their distribution with depth actually shows the lowest enhancement near the surface of the paleosols (see data repository) and an accumulation of both elements low in the profile. Furthermore, distributions of both elements are parallel with depth and more Rb has typically been added than K, a pattern inconsistent with metasomatism (Sheldon, 2003). Paleosol 1 also has the largest gain in K in comparison to the other paleosols, and is the nearest paleosol to the top of the 65 m section. Consequently, it is most likely to have been affected by leaching of K from the overlying basalt flow during its emplacement. However, normalizing each element to Ti and plotting Rb/Ti versus K/Ti for each paleosol shows very consistent chemical behaviour (Fig. 10) for all of the paleosols and no significant difference between paleosol 1 and the other paleosols, which casts further doubt on a metasomatic explanation for the K enrichment. Plotting an A–CN–K ternary diagram (Fig. 11) shows that in comparison to basalt samples from the local vicinity, there has been a slight overall increase in K (<5% in ternary space) in the paleosols, but in combination with Fig. 10, suggests that the main change has been redistribution rather than significant addition. It also shows that Ca and Na have been removed relative to the parent material, which is consistent with mass-balance calculations (Fig. 7) and with typical pedogenesis of basalt and basalt-derived sediments (Sheldon, 2003, 2006c). The idea that “normal” pedogenic conditions are responsible for the K redistribution is further supported

Fig. 9. SEM Images. (A) SEM image of volcanic quartz grain from sample CFC68a. (B) SEM image of a denticulated margin is evidence of chemical weathering occurring on clinopyroxene grain. Scales shown. (C) QFL diagram showing the distribution of selected samples. Samples CFC49, CFC61a, CFC68a, and CFC81 show a break from the trend of other samples, due to a change in source. This concurs with Ti/Al ratios. White open symbols represent samples following trend, black closed symbol represent the four samples listed above. (D) Graph displays changes in abundances of different elements associated with felsic vs. mafic phases (or source areas).

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Fig. 10. Comparison of K/Ti with Rb/Ti for each paleosol. Most results are highly correlated; outliers are from paleosols 1 and 2 and have excess Rb relative to the general trend, which could also be interpreted as a depletion in K. Error associated with analytical uncertainty is less than the width of the data points.

Fig. 11. A–Cn–K ternary diagram. A: Al2 O3 ; CN: CaO + Na2 O; K: K2 O (Fedo et al., 1995). Slight addition of K to the system is shown, as well as a removal of Ca and Na. Circles represent the average of each paleosol, squares represent basalt samples. (Insert) CIA vs. CIA-K, showing strong correlation as indicated by the R2 value. Error associated with analytical uncertainty is less than the width of the data points.

by the strong correlation (R2 = 0.93) and slope of ∼1 on a plot that relates CIA and CIA–K values (Fig. 11, insert). Examination of thin sections and SEM images further supports the idea that there has been minimal post-burial alteration. The only mineral of any significance that has been identified is thomsonite (Fig. 5F), which is a tectosilicate zeolite group mineral that is present in some of the paleosol and surrounding sediment samples. The fibrous form is found dispersed in some of the sediments, and the banded form is present as clasts. Thomsonite typically forms in the amygdales of volcanic rocks and thus could represent some minor, low-temperature alteration, however, most of the clasts are moderately to well rounded (Fig. 5F) indicating that they formed elsewhere and were transported in. Thus, if metasomatic alteration is eliminated, then there are two other possible sources of the K distribution and redistribution within the paleosols: (1) incomplete weathering of alkali elements or (2) addition of alkali elements from a felsic source (e.g., Sheldon, 2003). For example, K could have been added from felsic volcanism that was also occurring in the local vicinity (for example, rhyo-

lite volcanism at Palisade Head). Though further work is needed to test this hypothesis, it is favored over incomplete alkali weathering because Na has been weathered “normally” in these paleosols (Fig. 7). 6. Conclusions Seven paleosols are identified in a ∼1.1 Ga fluvial sedimentary outcrop on the north shore of Lake Superior, and exhibit physical and chemical characteristics similar to Phanerozoic Entisols. Using a combination of the chemical index of alteration, mass-balance calculations, and molecular weathering ratios it is possible to estimate degrees of weathering on the paleosols and surrounding sediments. There were significant losses of most elements during pedogenesis, but relatively little clay formation and the overall degree of weathering was weak-moderate. Application of Ti/Al ratios also shows that the paleosols were derived from basaltic parent material, with a minor component of rhyolitic material in one of the paleosols. Three distinctive pedofacies are

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recognized, and the differing degree of waterlogging contributed to variable redox conditions within the paleosols. By considering mass-balance behaviour of a variety of alkali and alkaline earth elements, it is possible to rule out significant potassium metasomatism. Given that only three paleosols have previously been reported for this part of the geologic record and that there is a ∼700 Ma gap Mesoproterozoic gap between well-characterized paleosols (from the ∼1.8 Ga Flin Flon paleosol to these and the Sturgeon Fall paleosol), the pristine character of the Keweenawen paleosols suggests that they are an important source of evidence for understanding Mesoproterozoic weathering processes. Furthermore, because these paleosols have never been substantially buried or subjected to post-burial alteration, they may be particularly useful for paleoatmospheric reconstructions (e.g., Sheldon, 2006b) and are a good candidate to preserve evidence of a terrestrial biosphere if one was present. Acknowledgements NDS would like to acknowledge support from NERC and PSARC during a preliminary stage of this research. RLM would like to acknowledge financial support from the Kirsty Brown Fund at RHUL and technical support and training from Emma Tomlinson, Nick Walsh, Sue Hall and Neil Holloway. Both authors would also like to acknowledge helpful and thought provoking reviews of an earlier version of this manuscript from Wulf Mueller, Steven Driese, and an anonymous reviewer. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.precamres.2008.09.013. References Anderson, R.R., McKay, R.M., 1989. Clastic rocks associated with the Midcontinent Rift System in Iowa. USGS. Bull 1989-1. In: Day, W.C., Lane, D.E. (Eds.), Strategic and Critical Minerals in the Midcontinent Region. USGS Bull, United States, p. 1989. Brimhall, G.H., Dietrich, W.E., 1987. Constitutive mass balance relations between chemical composition, volume, density, porosity, and strain in metasomatic hydrochemical systems: results on weathering and pedogenesis. Geochim. Cosmochim. Acta 51 (3), 567–587. Cannon, W.F., Hinze, W.J., 1992. Speculations on the origin of the North American Midcontinent Rift. Tectonophysics 213, 49–55. Davis, D.W., Green, J.C., 1997. Geochronology of the North American Midcontinent rift in western Lake Superior and implications for its geodynamic evolution. Can. J. Earth Sci. 34, 476–488. Driese, S.J., 2004. Pedogenic translocation of Fe in modern and ancient Vertisols and implications for interpretations of the Hekpoort paleosol (2.25 Ga). J. Geol. 112, 543–560. Fedo, C.M., Nesbitt, H.W., Young, G.M., 1995. Unraveling the effects of potassium metasomatism in sedimentary rocks and paleosols, with implications for paleoweathering conditions and provenance. J. Geol. 23 (10), 921–924. Green, J.C., 1982. Geology of Keweenawan extrusive rocks. In: Wold, R.J., Hinze, W.J. (Eds.), Geology and Tectonics of the Lake Superior Basin. Geol. Soc. Am. Mem. 156, 47–55. Green, J.C., 1983. Geologic and geochemical evidence for the nature and development of the Middle Proterozoic (Keweenawan) Midcontinent Rift of North America. Tectonophysics 94, 413–437. Green, J.C., 1989. Physical volcanology of Mid Proterozoic plateau lavas: the Keweenawan North Shore Volcanic Group, Minnesota. Geol. Soc. Am. Bull. 101, 486–500. Green, J.C., Fitz III, T.J., 1993. Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition. J. Volc. Geol. Res. 54, 177–196. Green, J.C., 2005. Geology on Display—Geology and Scenery of Minnesota’s North Shore State Parks. Minnesota Department of Natural Resources. Hamer, J.M.M., Sheldon, N.D., Nichols, G.J., Collinson, M.E., 2007. Late Oligocene– Early Miocene paleosols of distal fluvial systems, Ebro Basin, Spain. Palaeogeogr. Palaeoclim. Palaeoecol. 247, 220–235.

283

Holland, H.D., Feakes, C.R., Zbinden, E.A., 1989. The Flin Flon paleosol and the composition of the atmosphere 1.8 Ga. Am. J. Sci. 289 (4), 362–389. Jirsa, M., 1984. Interflow sedimentary rocks in the Keweenawan North Shore Volcanic Group, Northeastern Minnesota. Minn. Geol. Surv. Rep. Inv., 30. Kalliokoski, J., Welch, E.J., 1985. Keweenawan-age Caliche Paleosol in the lower part of the Calumet and Hecla Conglomerate, Centennial Mine, Calumet, Michigan. Geol. Soc. Am. Bull. 96, 1188–1193. Kalliokoski, J., 1986. Calcium carbonate cement (caliche) in Keweenawan sedimentary rocks (∼1.1 Ga), Upper Peninsula of Michigan. Precam. Res. 32, 243–259. Kraus, M.J., 1999. Paleosols in clastic sedimentary rocks: their geologic applications. Earth Sci. Rev. 47, 41–70. Kraus, M.J., Aslan, A., 1993. Eocene hydromorphic paleosols: significance for interpreting ancient floodplain processes. J. Sed. Pet. 63, 453–463. Maynard, J.B., 1992. Chemistry of modern soils as a guide to interpreting Precambrian paleosols. J. Geol. 100, 279–289. Nesbitt, H.W., Young, G.M., 1982. Early Proterozoic climates and plate motions inferred from major element chemistry of lutites. Nature 299, 715–717. Nicholson, S.W., Shirey, S.B., Schulz, K.J., Green, J.C., 1997. Rift-wide correlation of 1.1 Ga Midcontinent Rift system basalts: implications for multiple mantle sources during rift development. Can. J. Earth Sci. 34, 504–520. Ojakangas, R.W., Morey, G.B., Green, J.C., 2001. The Mesoproterozoic Midcontinent Rift System, Lake Superior Region, USA. Sed. Geol. 141–142, 421–442. Ojakangas, R.W., Dickas, A.B., 2002. The 1.1 Ga Midcontinent Rift System, central North America: sedimentology of two deep boreholes, Lake Superior Region. Sed. Geol. 147, 13–36. Paces, J.B., Miller, J.D., 1993. Precise U–Pb ages of Duluth Complex and related mafic intrusions, Northeastern Minnesota—geochronological insights to physical, petrogenetic, palaeomagnetic, and tectonomagnetic processes associated with the 1.1 Ga Midcontinent rift system. J. Geophys. Res. Solid Earth 98 (B8), 13997–14013. Pettijohn, F.J., 1975. Sedimentary Rocks, 3rd ed. Harper and Row Publishers, New York. Retallack, G.J., 1986. Reappraisal of a 2200 Ma old paleosol near Waterval Onder, South Africa. Precam. Res. 32, 195–232. Retallack, G.J., Mindszenty, A., 1994. Well preserved Late Precambrian paleosols from Northwest Scotland. J. Sed. Res. A64 (2), 264–281. Retallack, G.J., 2001. Soils of the Past: An Introduction to Paleopedology, 2nd Rev. ed. Blackwell Science Ltd., April 2001. Rye, R., Holland, H.D., 2000a. Geology and geochemistry of paleosols developed on the Hekpoort Basalt, Pretoria Group, South Africa. Am. J. Earth Sci. 300, 85–141. Rye, R., Holland, H.D., 2000b. Life associated with a 2.76 Ga ephemeral pond? Evidence from Mount Roe #2 paleosol. J. Geol. 28, 483–486. Shay, J., Trehu, A., 1993. Crustal structure of the central graben of the Midcontinent Rift beneath Lake Superior. Tectonophysics 225, 301–335. Sheldon, N.D., 2006a. Abrupt chemical weathering increase across the Permian–Triassic boundary. Palaeogeogr. Palaeoclim. Palaeoecol. 231, 315–321. Sheldon, N.D., 2006b. Precambrian paleosols and atmospheric CO2 levels. Precam. Res. 147, 148–155. Sheldon, N.D., 2006c. Quaternary glacial–interglacial climatic cycles in Hawaii. J. Geol. 114, 367–376. Sheldon, N.D., 2005. Do red beds indicate paleoclimatic conditions? A Permian case study. Palaeogeogr. Palaeoclim. Palaeoecol. 228, 305–319. Sheldon, N.D., 2003. Pedogenesis and geochemical alteration of the Picture Gorge Subgroup, Columbia River Basalt, Oregon. Geol. Soc. Am. Bull. 115, 1377–1387. Sheldon, N.D., Retallack, G.J., 2001. Equation for compaction of paleosols due to burial. J. Geol. 29, 247–250. Sheldon, N.D., Retallack, G.J., Tanaka, S., 2002. Geochemical Climofunctions from North America soils and application to paleosols across the Eocene–Oligocene boundary in Oregon. J. Geol. 110 (6), 687–696. Sheldon, N.D., Tabor, N.J., in press. Quantitative paleoenvironmental and paleoclimatic reconstruction using paleosols. Earth Sci. Rev. Soil Survey Staff, 2006. Keys to Soil Taxonomy, 10th ed. USDA—Natural Resources Conservation Service, Washington, DC, 341 pp. Soil Survey Staff, 1975. Soil Taxonomy. United States Department of Agriculture Handbook, Washington, DC, 436 pp. Van Schmus, W.R., Hinze, W.J., 1985. The Midcontinent Rift System. An. Rev. Earth Planet. Sci. 13, 345–383. Vervoort, J.D., Green, J.C., 1997. Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd isotope evidence for melting of Archaean crust. Can. J. Earth Sci. 34, 521–535. Vervoort, J.D., Wirth, K., Kennedy, B., Sandland, T., Harpp, K.S., 2007. The magmatic evolution of the Midcontinent Rift: new geochronologic and geochemical evidence from felsic magmatism. Precam. Res. 157 (1–4), 235–268. White, R.S., 1997. Mantle temperature and lithospheric thinning beneath the Midcontinent rift system: evidence from magmatism and subsidence. Can. J. Earth Sci. 34, 464–475. Young, G.M., Nesbitt, H.W., 1998. Processes controlling the distribution of Ti and Al in weathering profiles, siliciclastic sediments and sedimentary rocks. J. Sed. Res. 68 (3), 448–455. Zbinden, E.A., Holland, H.D., Feakes, C.R., 1988. The Sturgeon Falls paleosol and the composition of the atmosphere 1.1 Ga BP. Precam. Res. 42, 141–163.