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Stud. Geophys. Geod., 56 (2012), 677−704, DOI: 10.1007/s11200-011-9005-9 677 © 2012 Inst. Geophys. AS CR, Prague

Magnetic susceptibility and its relationship with paleoenvironments, diagenesis and remagnetization: examples from the Devonian carbonates of Belgium

ANNE-CHRISTINE DA SILVA1, MARK J. DEKKERS2, CÉDRIC MABILLE3 AND FRÉDÉRIC BOULVAIN1

1 Pétrologie sédimentaire, B20, Boulevard du Rectorat, 15, University of Liège, 4000 Liège,

Belgium ([email protected], [email protected]) 2 Faculty of Earth Sciences, Geodynamic Research Institute, Paleomagnetic Laboratory Fort

Hoofddijk, Utrecht University, Utrecht, The Netherlands ([email protected]) 3 TOTAL E&P, CSTJF, 18 avenue Larribau, 64018 Pau, France

Received: January 27, 2011; Revised: June 20, 2011; Accepted: September 13, 2011

ABSTRACT

To better understand the origin of the initial magnetic susceptibility (χin) signal in carbonate sequences, a rock magnetic investigation that includes analysis of acquisition curves of the isothermal remanent magnetization (IRM) and hysteresis parameters, was undertaken on Devonian carbonates from the Villers and Tailfer sections, Belgium. Both sections are divided into a lower unit, dominated by biostromal and external ramp facies (biostromal unit) and an upper unit, only consisting of lagoonal facies (lagoonal unit). The variations in χin signal are mainly driven by magnetite variation, mostly pseudo-single-domain (PSD) magnetite. Clay minerals, pyrite, hematite and obviously calcite and dolomite are also present but their contribution to the χin pattern is not significant. There is a correlation between detrital proxies (Zr, Rb, Al2O3, TiO2) and χin for the Tailfer biostromal unit and the entire Villers section. The pervasive presence of fine-grained magnetite is interpreted as related to remagnetization. In absence of external fluids, the iron released during the smectite to illite transition remains in situ. In those situations χin may reflect an inherited primary synsedimentary signal. In the lagoonal unit of the Tailfer section, remagnetization appears to have obscured the original detrital information prompting the need for an evaluation of the composition of the susceptibility signal for individual case studies.

Ke y wo rd s : magnetic susceptibility, Devonian, carbonate, hysteresis loop,

isothermal remanent magnetization (IRM) acquisition curves, detrital, diagenesis

1. INTRODUCTION

The use of initial magnetic susceptibility (χin) records for high-resolution stratigraphic correlation across Palaeozoic sedimentary successions seems attractive because data acquisition is straightforward and rapid (e.g. Ellwood et al., 1999). The application to

A.-C. Da Silva et al.

678 Stud. Geophys. Geod., 56 (2012)

well-bedded Palaeozoic carbonate sediments has shown its potential for correlating continuous sedimentary sequences (Hladil, 2002; Da Silva and Boulvain, 2006; Whalen and Day, 2008; Boulvain et al., 2010; Da Silva et al., 2009, 2010; Hladil et al., 2010). Furthermore, sea-level variations could be reconstructed from χin records (Devleeschouwer, 1999; Zhang et al., 2000; Racki et al., 2002; Da Silva and Boulvain, 2006) and climatic variations were inferred (Curry et al., 1995; Arai et al., 1997). Importantly, for the interpretation of χin records the evolution of sedimentary facies should be taken into account; several studies highlight a close link between facies and χin trends (Ellwood et al., 1999; Da Silva and Boulvain, 2006; Babek et al., 2010).

However, before χin records can be interpreted faithfully in terms of sea-level changes or paleoclimate it must be verified whether they still contain the original palaeoenvironmental information. This is not trivial as it is well known that the sedimentary paleomagnetic signal is potentially affected by (early) diagenetic processes (e.g. Karlin and Levi, 1985; Canfield and Berner, 1987; Machel and Burton, 1991; Roberts et al., 1999; Passier et al., 2001; Rowan and Roberts, 2006; Rowan et al., 2009) and later pervasive remagnetization (e.g. McCabe and Elmore, 1989; Katz et al., 1998; Zegers et al., 2003; Elliot et al., 2006). During these processes, the ferrimagnetic minerals magnetite, greigite and pyrrhotite, all with a very high specific χin, are formed and/or removed. One of the more common processes to form magnetite associated with remagnetization is linked to the transformation of smectite into illite between 2 and 4 km burial depth (Jackson et al., 1988; Katz et al., 1998). Illite contains less iron than smectite and the liberated iron is the source of the magnetite in which the remagnetization resides. Zegers et al. (2003) identified such processes in the Devonian from Belgium. So, the magnetic susceptibility signal may reveal a convolved expression of detrital, diagenetic and later remagnetization processes. Therefore, they potentially bias the interpretation of the χin records and their contribution to χin must be established on a case-by-case basis. Papers dealing with the nature and origin of the different components that contribute to χin, along with potential variations through time are, however, surprisingly rare (Devleeschouwer et al., 2010; Riquier et al., 2010).

A full understanding of what is driving the magnetic susceptibility signal is critical to a better understanding of its origin. This allows an assessment of the influence of primary sedimentary processes and secondary processes, i.e. diagenesis and potential remagnetization. To further investigate the link between magnetic parameters, facies and diagenesis an extended rock-magnetic characterization was performed on a selection of samples from the Frasnian Tailfer and Villers sections. Hysteresis loops and acquisition curves of the isothermal remanent magnetization (IRM) were determined at room temperature in order to explain the origin of the magnetic susceptibility signal in these sediments. To further assess the potential detrital origin of the initial susceptibility signal, χin and the high-field susceptibility χHF will be compared with the trends of some elements like Ti, Al, Rb and Zr, which are acknowledged proxy parameters for detrital input variations (e.g. Tribovillard et al., 2006; Riquier et al., 2010).

Magnetic susceptibility and remagnetization in Devonian carbonates

Stud. Geophys. Geod., 56 (2012) 679

2. GEOLOGICAL SETTING

The two studied outcrops are part of the western zone of the Rhenohercynian fold-and-thrust belt (Tailfer at the northern border of the Dinant Synclinorium and Villers in the Philippeville Anticlinorium; Fig. 1). Both sections (105 m thick each) are Frasnian of age, the Tailfer section is part of the Lustin Formation and the Villers section part of the Philippeville Formation.

The Rhenohercynian fold-and-thrust belt was formed during the Carboniferous to Permian, as a consequence of the collision between Laurentia and Gondwana. The main metamorphic event reached a highest temperature of 450°C in the south, in the early Palaeozoic massifs. However, the area around the Tailfer section hardly reached the

Fig. 1. Geological setting of the Frasnian of Belgium. a) Geological map with studied outcrop locations (stars). b) South-North section of the Frasnian basin before Variscan deformation, with the different Formations and Members (referred to as section X-Y in a).



anchizone (temperatures around 200−220°C). The Villers section, further to the southwest, possibly experienced slightly deeper burial conditions; the whole Philippeville Anticlinorium reached the anchizone (200−250°C) (Fielitz and Mansy, 1999). Furthermore, a main remagnetization event was recognized throughout the entire Belgian Ardennes, carried by superparamagnetic (SP) to small single domain (SD) magnetite (10−30 nm grain size) (e.g. Zegers et al., 2003). The remagnetization mechanism is interpreted to be tied to burial whereby the smectite to illite conversion has played an important role in providing the source of the iron for the remagnetized magnetic minerals. This transition goes via a series of discrete mixed layer smectite-illite compositions (e.g. Lanson et al., 2009) whereby the availability of K plays an important role. Fully ordered illite represents high diagenetic temperatures with its crystallinity improving with metamorphic temperature (synthesis in Kübler and Jaboyedoff, 2000).

Detailed magnetic susceptibility results for the Tailfer and Villers sections and their relation with the sedimentological evolution were published elsewhere (Da Silva and Boulvain, 2006), with data from 510 samples (maximum sampling interval (unoriented samples) is fifty centimetres). As often recorded in carbonate sediments (e.g. Borradaile and Lagroix, 2000; Riquier et al., 2010; Devleeschouwer et al., 2010), magnetic susceptibility data are low, between −0.2 × 10−8 and 30 × 10−8 m3/kg. Observation of mineralogy in thin section shows of course calcium carbonate occurring in large majority. The occurrence of dolomite is relatively low (a few scattered small dolomitic rhombohedric crystals), except at the basis of the Tailfer section, where a few beds of dolomite are observed. A few argillaceous joints are noted in both sections but the carbonates seem to be generally relatively pure. Pyrite and hematite are sometimes observed (reflected light microscope observations) and they are present in a higher amount in some paleosol levels.

Tailfer represents shallow paleoenvironmental conditions; it is composed of biostromes and lagoonal deposits with occasional paleosols. The Villers section presents deeper paleoenvironmental conditions, with outer ramp deposits, crinoidal marls capped by biostromes and lagoonal deposits. The sedimentological succession in both sections (complete facies analyses in Da Silva and Boulvain, 2004) is characterized by small (dm to m-thick) and medium-scale (m-thick) shallowing-upward sequences, as commonly observed in other Devonian successions (e.g. Elrick, 1995; Whalen et al., 2000). The Tailfer and Villers sections could both be divided in two parts, a first part dominated by

Fig. 2. (Facing page) Stratigraphical evolution of various magnetic parameters, compared to facies and TiO2 content for the Tailfer section. The lithological column and complete magnetic susceptibility and facies results are from Da Silva and Boulvain (2006) and are presented here to illustrate complete variations of initial magnetic susceptibility χ in with facies and sequences (shallowing trend leads to increasing susceptibility). χFerro - ferromagnetic susceptibility, χHF - high field susceptibility, Bc - coercive force, Ms - magnetization at saturation, Mrs - saturation remanent magnetization, SIRM-1 - saturation isothermal remanent magnetization for Component 1, S-ratio - the forward S-ratio at 300 mT according to the Bloemendal et al (1992) definition, TiO2 - titanium oxide content in % (XRF results). In the S-ratio column, sample with a dotted arrow framed with a rectangle is considered as outlier (Sample L23c, S-ratio = 0.52).



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external and biostromal deposits (biostromal unit) and a second part dominated by lagoonal deposits (lagoonal unit). The link between χin and sedimentological trends is as follows (cf. Fig. 2 for Tailfer and Fig. 3 for Villers and detailed results in Da Silva and Boulvain, 2006): 1) The χin curve is clearly divided in two distinct parts, with low χin in the biostromal unit and high values in the lagoonal unit; 2) there is a correlation between the χin variation and fourth-order sequences (a regressive trend corresponds to a peak in the χin curve). However, this trend is the most obvious in the biostromal unit of Tailfer, and is still present but more convolved in the Villers section and in the lagoonal unit of Tailfer; 3) there is a strong relationship between χin and microfacies, with increasing mean χin related to proximality.

Table 1. Results of hysteresis measurements: initial susceptibility (χin), saturation magnetization (Ms), coercive force Bc, high-field susceptibility (paramagnetic + diamagnetic contributions, χHF), ferromagnetic susceptibility (χFerro) and magnetization at saturation to remanent magnetization ratio (Mrs/Ms). * considering this sample, χHF is higher than χin (which lead to a negative χFerro) presumably because of different samples used for measurement χin (KLY-3S) and the small sample used for hysteresis and χHF measurements (MicroMag).

Sample χin [m3/kg] Ms [Am2/kg] Bc [mT] χHF [m3/kg] χFerro [m3/kg] Mrs/Ms

Tailfer 20c −1.901E−09 2.00E−04 11.946 −4.025E−09 2.124E−09 0.105 23b 1.138E−09 1.05E−04 11.383 −4.125E−09 5.263E−09 0.153 25b 1.473E−08 2.84E−04 16.859 2.213E−08 * 0.231 28d 1.923E−08 5.99E−04 11.058 −2.619E−09 2.185E−08 0.233 29e 1.746E−08 4.65E−04 4.946 −3.730E−10 1.784E−08 0.089 41c 3.607E−08 9.10E−04 8.982 −2.789E−09 3.886E−08 0.176 49 2.337E−08 6.87E−04 5.579 −2.731E−09 2.610E−08 0.104 50b 5.167E−08 2.00E−03 5.365 −1.115E−09 5.278E−08 0.101 54c 1.187E−07 2.36E−03 3.250 −2.250E−09 1.210E−07 0.069 60 8.304E−08 1.31E−03 4.148 −2.133E−09 8.518E−08 0.103 66 7.973E−08 1.93E−03 7.312 −1.119E−09 8.085E−08 0.147 70d 5.189E−08 1.12E−03 11.892 2.373E−09 4.952E−08 0.216 73d 6.126E−08 7.01E−04 6.921 2.153E−08 3.973E−08 0.154 87 1.469E−07 8.50E−05 13.708 2.121E−09 1.448E−07 0.132 94 9.156E−08 1.80E−03 4.823 2.121E−08 7.034E−08 0.095 105 1.546E−07 3.16E−03 8.301 1.057E−08 1.440E−07 0.155 110 1.084E−07 2.78E−03 6.360 2.678E−09 1.057E−07 0.116 114d 8.971E−08 6.08E−04 7.240 2.403E−09 8.730E−08 0.141 118c 6.861E−08 1.24E−03 4.884 −7.116E−10 6.932E−08 0.111 119c 1.209E−07 1.72E−03 4.984 3.957E−09 1.170E−07 0.118 123 1.898E−07 5.65E−03 9.984 2.396E−09 1.874E−07 0.187 133 1.291E−07 2.02E−03 3.531 1.702E−09 1.274E−07 0.089 140 1.090E−07 2.16E−03 11.145 −1.338E−09 1.104E−07 0.218



3. ANALYTICAL METHODS AND INSTRUMENTATION

As indicated earlier, a detailed χin data base was available on the two sections, with 510 samples (Da Silva and Boulvain, 2006). Out of these 510 samples, 47 (23 from Tailfer and 24 from Villers) samples were selected for extended magnetic and geochemistry measurements. Extra samples were collected when the available material was insufficient in amount and a regularly spaced data base with representative facies was formed. On each sample, two cores were drilled and the remaining sample was crushed for geochemistry analyses. Standard 2.5 cm diameter and 2.2 cm long samples were prepared for the analysis of the alternating field (AF) demagnetization spectrum of the natural remanent magnetization (NRM). Furthermore, a small chip of the bigger cores was used for hysteresis measurements. Samples of 1 cm diameter and 2.2 cm long were prepared for the IRM acquisition data (to remain within the dynamic range of the DC-SQUID magnetometer). In order to compare χin with the other data on exactly the same sample, χin was measured three times. One set of χin measurements on the bigger

Table 1. Continuation.

Sample χin [m3/kg] Ms [Am2/kg] Bc [mT] χHF [m3/kg] χFerro [m3/kg] Mrs/Ms

Villers 8c 3.079E−08 5.48E−04 13.405 2.599E−09 2.820E−08 0.223 9c 9.563E−09 5.47E−04 8.202 −1.946E−09 1.151E−08 0.155 15 4.397E−08 9.38E−04 13.804 −1.749E−09 4.572E−08 0.225 23 8.818E−09 2.54E−04 13.578 −2.262E−09 1.108E−08 0.271 31 1.881E−08 4.86E−04 13.986 −1.978E−09 2.079E−08 0.163 34 2.893E−08 5.61E−04 12.314 −2.122E−09 3.105E−08 0.214 37 9.471E−09 5.90E−04 11.051 −2.375E−09 1.185E−08 0.110 47d 4.027E−10 3.58E−04 16.697 −2.965E−09 3.368E−09 0.216 50b 5.239E−09 3.32E−04 7.900 −4.837E−09 1.008E−08 0.171 V59 −3.172E−09 9.94E−05 12.368 −5.050E−09 1.878E−09 0.207 64 −2.084E−10 1.04E−03 5.516 −2.229E−09 2.020E−09 0.169 66c 2.554E−09 1.70E−04 14.272 −2.500E−09 5.053E−09 0.148 68b 4.576E−10 2.21E−04 17.671 −2.886E−09 3.344E−09 0.244 73 7.616E−09 2.16E−04 2.688 −7.518E−10 8.368E−09 0.098 76 2.093E−08 4.60E−04 9.564 −1.032E−09 2.196E−08 0.145 V76c 4.870E−08 7.97E−04 6.500 −2.521E−09 5.122E−08 0.130 79 1.787E−08 2.69E−04 5.081 −2.423E−09 2.030E−08 0.144 86 5.111E−08 8.15E−04 7.343 −7.502E−10 5.186E−08 0.147 89 −1.351E−09 9.82E−04 8.086 −3.159E−09 1.809E−09 0.168 89b 8.011E−09 6.50E−04 7.597 −4.850E−09 1.286E−08 0.186 98b 1.610E−08 6.80E−04 9.556 −2.752E−09 1.885E−08 0.191 114 8.024E−08 2.69E−03 4.987 −1.774E−09 8.201E−08 0.131 121b 5.716E−08 8.51E−04 5.418 −4.767E−09 6.193E−08 0.125 124 7.446E−08 1.67E−03 4.176 −8.581E−10 7.532E−08 0.104



cores was compared with NRM and hysteresis measurements (Table 1) (we realize that hysteresis data are necessarily acquired on very small samples). That acquired on the smaller cores will be compared with the saturation IRM (SIRM) results (Table 2), and the final set on the crushed samples was compared with geochemical analyses (Table 3).

It is important to note that the magnetic parameters (hysteresis and SIRM) will help to get a better idea of the proportion of minerals in terms of their magnetic amounts. For conversion to molar proportions conversion factors are required that depend on grain size which is beyond the scope of this contribution.

Table 2. Results of IRM measurements after fitting with cumulative log-Gaussian (CLG) functions, using the excel workbook developed by Kruiver et al. (2001): Comp-1 (and Comp-2): proportion of Component 1, interpreted as magnetite (and proportion of Component 2 interpreted as hematite); SIRM-1 (SIRM-2): SIRM of Component 1 (or 2); B1/2-1 (B1/2-2): the field at which half of the SIRM is reached for Component 1 (or 2); DP-1 (DP-2): dispersion parameters of Component 1 (or 2); S-ratio: expected forward S-ratio for the modelled components, calculated following the definition provided by Bloemendal et al. (1992).

Sample χin

[m3/kg] Comp-1

[%] SIRM-1

[Am2/kg]B1/2-1[mT]

DP-1[mT]

Comp-2[%]

SIRM-2[Am2/kg]

B1/2-2 [mT]

DP-2 [mT] S-ratio

Tailfer L20c 1.6E−08 82.50 5.61E−04 53.70 0.35 17.50 1.19E−04 707.95 0.20 0.87 L23c 5.7E−10 40.76 8.21E−05 58.88 0.31 59.24 1.19E−04 776.25 0.18 0.52 L25b 1.7E−09 75.11 1.36E−04 53.70 0.29 24.89 4.50E−05 1000.00 0.35 0.87 L28d 3.0E−08 89.39 9.97E−04 53.70 0.34 10.61 1.18E−04 794.33 0.28 0.93 L29e 2.1E−08 87.92 3.61E−04 52.48 0.32 12.08 4.95E−05 562.34 0.30 0.92 L41c 3.2E−08 94.13 9.57E−04 51.88 0.34 5.87 5.97E−05 288.40 0.20 0.96 L49 1.5E−08 91.00 5.61E−04 66.83 0.36 9.00 5.55E−05 562.34 0.24 0.90 L50b 5.1E−08 81.67 7.61E−04 60.26 0.32 18.33 1.71E−04 562.34 0.40 0.89 L54c 1.0E−07 86.96 1.35E−03 53.70 0.31 13.04 2.02E−04 239.88 0.22 0.95 L60 7.7E−08 91.33 1.47E−03 53.70 0.33 8.67 1.39E−04 199.53 0.28 0.97 L66 7.7E−08 89.01 2.56E−03 72.44 0.30 10.99 3.15E−04 331.13 0.33 0.91 L70d 6.0E−08 79.79 2.34E−03 67.61 0.35 20.21 5.93E−04 234.42 0.30 0.91 L73d 5.9E−08 84.85 8.89E−04 54.95 0.28 15.15 1.59E−04 158.49 0.35 0.97 L87 1.6E−07 84.53 3.55E−03 66.07 0.34 15.47 6.49E−04 199.53 0.30 0.94 L94 9.4E−08 89.43 1.28E−03 66.07 0.35 10.57 1.52E−04 199.53 0.35 0.94 L105 1.6E−07 83.33 5.07E−03 75.86 0.27 16.67 1.01E−03 251.19 0.28 0.93 L110 1.2E−07 93.52 2.88E−03 79.43 0.37 6.48 2.00E−04 316.23 0.28 0.92 L114d 8.2E−08 94.98 3.23E−03 74.13 0.31 5.02 1.71E−04 316.23 0.30 0.95 L118c 7.4E−08 90.03 1.38E−03 83.18 0.25 9.97 1.52E−04 208.93 0.25 0.96 L119c 1.2E−07 91.94 3.00E−03 74.13 0.32 8.06 2.63E−04 354.81 0.24 0.93 L123 2.1E−07 81.03 7.95E−03 89.13 0.31 18.97 1.86E−03 316.23 0.27 0.86 L133 1.3E−07 78.11 2.02E−03 60.26 0.31 21.89 5.67E−04 407.38 0.38 0.88 L140 1.3E−07 77.97 3.58E−03 63.10 0.31 22.03 1.01E−03 251.19 0.35 0.91



3 . 1 . M a g n e t i c S u s c e p t i b i l i t y a n d G e o c h e m i s t r y

Initial magnetic susceptibility measurements were performed on a KLY-3S instrument (AGICO, noise level 2 × 10−8 SI) at the University of Liège (Belgium). Magnetic susceptibility is expressed in m³/kg; each data point is the average of three measurements. Sample mass is at least 10 g (weighed with a precision of 0.01 g).

Major and trace element geochemistry were done by X-ray Fluorescence (ARL 9400 XP XRF instrument, University of Liège) on the same 47 selected samples (complete analytical process in Duchesne and Bologne, 2009). All the rock samples were carefully cleaned prior to all treatment: weathered surfaces were removed. They were then crushed with a Bauknecht crusher and milled in agate mortars. The major elements (in this paper TiO2 and Al2O3 will be used) were measured on lithium tetra- and meta-borate fused glass discs, with matrix corrections following the Traill-Lachance algorithm and are expressed in elemental oxide concentration. Trace elements (in this paper Zr and Rb) were measured on pressed powder pellets and data were corrected for matrix effects by

Table 2. Continuation.

Sample χin

[m3/kg] Comp-1

[%] SIRM-1

[Am2/kg] B1/2-1[mT]

DP-1[mT]

Comp-2[%]

SIRM-2[Am2/kg]

B1/2-2[mT]

DP-2 [mT] S-ratio

Villers V8c 3.2E−08 82.40 8.99E−04 61.66 0.39 17.60 1.92E−04 707.95 0.18 0.85 V9c 1.1E−08 84.59 4.13E−04 56.89 0.32 15.41 7.53E−05 707.95 0.18 0.88 V15 4.0E−08 64.71 1.39E−03 60.26 0.36 35.29 7.59E−04 724.44 0.19 0.71 V23 1.3E−08 90.09 6.14E−04 56.23 0.31 9.91 6.75E−05 501.19 0.32 0.93 V31 1.8E−08 70.67 7.69E−04 50.12 0.34 29.33 3.19E−04 758.58 0.20 0.78 V34 2.3E−08 96.50 1.08E−03 52.48 0.34 3.50 3.90E−05 537.03 0.20 0.97 V37 1.3E−08 90.82 6.89E−04 48.98 0.31 9.18 6.97E−05 630.96 0.35 0.94 V47c 3.2E−10 96.19 3.64E−04 57.54 0.29 3.81 1.44E−05 398.11 0.35 0.97 V50b 4.1E−09 94.10 4.36E−04 56.23 0.32 5.90 2.73E−05 630.96 0.30 0.95 V59 −3.4E−09 84.47 5.02E−05 60.26 0.29 15.53 9.23E−06 794.33 0.35 0.91 V64 4.2E−10 97.12 1.53E−04 57.54 0.31 2.88 4.55E−06 562.34 0.30 0.97 V66 2.9E−09 92.99 2.10E−04 54.95 0.32 7.01 1.59E−05 575.44 0.13 0.94 V68b 3.4E−09 90.82 2.89E−04 50.12 0.34 9.18 2.92E−05 316.23 0.40 0.95 V73 −8.2E−10 97.40 1.05E−04 63.10 0.31 2.60 2.80E−06 630.96 0.30 0.97 V76 1.5E−08 91.44 3.68E−04 60.26 0.31 8.56 3.44E−05 630.96 0.40 0.95 V76c 6.3E−08 89.50 1.15E−03 64.57 0.35 10.49 1.35E−04 630.96 0.40 0.92 V79 9.7E−09 94.69 3.11E−04 54.95 0.34 5.31 1.75E−05 630.96 0.30 0.95 V86 5.4E−08 65.24 9.16E−04 66.07 0.34 34.76 4.88E−04 691.83 0.20 0.72 V89 9.0E−10 59.17 1.49E−04 55.59 0.36 40.83 1.03E−04 707.95 0.17 0.68 V89b 5.0E−09 68.91 2.41E−04 57.54 0.31 31.09 1.09E−04 794.33 0.20 0.78 V98b 8.2E−09 81.78 2.90E−04 53.70 0.35 18.22 6.46E−05 691.83 0.13 0.86 V114 7.6E−08 84.68 1.48E−03 61.66 0.34 15.32 2.67E−04 676.08 0.15 0.87 V121b 4.3E−08 94.55 8.28E−04 64.57 0.37 5.45 4.78E−05 467.74 0.30 0.93 V124 7.0E−08 94.13 1.15E−03 64.57 0.37 5.87 7.17E−05 602.56 0.22 0.93



Compton peak monitoring and are expressed in elemental concentration. Accuracy is estimated as better than 1% for major elements and 5% for trace elements as checked with 40 international and in-house standards.

3 . 2 . H y s t e r e s i s

Hysteresis loop parameters were obtained from the samples of the Tailfer section (23 samples), and the Villers section (24 samples). A Princeton Measurements Corporation (Princeton, USA) alternating gradient force magnetometer MicroMag Model2900 was utilized (Paleomagnetic Laboratory ‘Fort Hoofddijk’, Utrecht University, The Netherlands; instrumental noise level 2 × 10−9 Am2, typical signals were at least two orders of magnitude higher). A P1 phenolic or P1 silica probe (for magnetically very weak samples) were used to mount the small sample chips (5−40 mg that were weighed before with a semi-micro balance). The maximum field was 1.5 T, field increment 15 mT and the averaging time 50 ms. The following parameters were extracted from the hysteresis loops: saturation magnetization Ms (Am2/kg); remanent saturation magnetization Mrs (Am2/kg); high-field magnetic susceptibility (χHF, m3/kg) and coercive force Bc (mT). χHF corresponds to the high-field magnetic susceptibility values and represents the paramagnetic plus diamagnetic contributions. Ms and Bc were determined after slope correction based on data points at field values > 1.0 T. The ferromagnetic susceptibility χFerro (m3/kg) which corresponds to the ferromagnetic s.l. contribution, was calculated by subtracting χHF from χin (e.g. Walden et al., 1999). The parameters extracted from the

Table 3. Correlation coefficient between main geochemical elements linked to detrital input and χHF in the upper part of the table and χin in the lower part of the table.

TiO2 [%] Al2O3 [%] Rb [ppm] Zr [ppm]

Correlation Coefficients between χHF and Detrital Proxies

Tailfer whole section 0.63 0.68 0.67 0.62 Tailfer Biostromal unit 0.13 0.11 0.07 0.03 Tailfer Lagoonal unit 0.84 0.87 0.87 0.78 Villers whole section 0.45 0.43 0.35 0.40 Villers biostromal unit 0.84 0.85 0.87 0.73 Villers lagoonal unit 0.06 0.06 0.30 0.01

Correlation Coefficients between χin and Detrital Proxies

Tailfer whole section 0.25 0.19 0.13 0.30 Tailfer Biostromal unit 0.92 0.68 0.92 0.86 Tailfer Lagoonal unit 0.31 0.27 0.36 0.19 Villers whole section 0.68 0.64 0.62 0.62 Villers biostromal unit 0.81 0.75 0.77 0.51 Villers lagoonal unit 0.59 0.57 0.54 0.57



hysteresis curves are often interpreted through the classic “Day-plot” of Mrs/Ms vs, Bcr/Bc (Day et al., 1977). However, the Day-plot is equivocal concerning the grain size in case of mixed magnetic mineralogy or bimodal grain-size distributions. The exact value of the remanent coercive force (Bcr) is subject to experimental conditions due to time-dependent effects caused by small magnetic grains (Fabian and von Dobeneck, 1997). To circumvent this possible bias Tauxe et al. (2002) proposed to use instead the “squareness versus coercive field plot” (SQC), Mrs/Ms vs. Bc, which delineates a number of magnetic domain state regions with clear analytical explanations (see Section 4 for detailed results).

3 . 3 . A c q u i s i t i o n o f I s o t h e r m a l R e m a n e n t M a g n e t i z a t i o n

IRM acqusition curves were determined on 24 samples for each section. They were generated with the robotized magnetometer set-up at the ‘Fort Hoofddijk’ paleomagnetic laboratory; each curve consists of 55 field steps up to a maximum field of 700 mT, more or less logarithmically spaced over the field range. A ‘2G’ DC-SQUID magnetometer (instrumental noise level 3 × 10−12 Am2) with in-line alternating field (AF) and IRM acquisition facilities is interfaced with an automatic sample handler that allows the processing of batches up to 96 samples without operator interference. The magnetic starting state is the static three-axial-AF-demagnetized state with the last demagnetization direction parallel to the direction in which the subsequent IRM is acquired. This ensures minimal deviation from lognormality for the low levels of magnetic interactions anticipated (Heslop et al., 2004). They were fitted with cumulative log-Gaussian (CLG) functions according to Kruiver et al. (2001). Each coercivity distribution is characterized by B1/2 (the field at which half of the SIRM is reached, indicative of the magnetic mineralogy and grain size), SIRM (saturation IRM; the magnetic concentration of the respective phase) and DP (dispersion parameter, provides information on the distribution of grain sizes and/or crystal defects). From the IRM acquisition curve the expected forward S-ratio for the modelled components can be calculated, following the definition

provided by Bloemendal et al. (1992): ( )0.3T 1T1 2IRM IRM⎡ ⎤−⎣ ⎦ . Because the Kruiver

et al. (2001) package allows fitting of symmetric components in the log-field space, distributions that appear to be skewed at very low applied fields need to be fitted with an extra component that is not assigned physical meaning. It is a consequence of the magnetic behaviour of fine ‘semi-SP’ particles (magnetic interaction is irrelevant for the low concentrations here) (cf. Heslop et al., 2004). Its SIRM is added to that of the dominant low-coercivity component.

Before IRM acquisition the natural remanent magnetization (NRM) of the samples was AF demagnetized in 14 steps up to 100 mT with the same robotized magnetometer. Since samples were unoriented an analysis of the paleomagnetic directions is not possible but we can compare the NRM coercivity distribution with that of the IRM for the very same sample. To create directionally unbiased decay curves the vector differences between subsequent AF steps were added algebraically and subsequently normalized to the total sum. In this manner the AF coercivity spectrum of the NRM can be compared amongst the samples.



4. MAGNETIC RESULTS

4 . 1 . M a g n e t i c H y s t e r e s i s

In this section, we will first consider some magnetic parameters individually; involving χFerro, Ms, Bc, Mrs/Ms and χHF (cf. hysteresis data in Table 1). We will compare them with χin, to identify the main magnetic parameters controlling the initial susceptibility.

Hysteresis loops from Tailfer appear to be distinctly variable, whereas those from Villers are all relatively similar. They are dominated by diamagnetic (Fig. 4a,c) or paramagnetic (Fig. 4b) contributions. Hysteresis loop shapes are: either (a) sigmoid-shapes slightly open, to straight lines (not open) at low fields (7 samples in Tailfer and 13 in Villers; Fig. 4b,c), with a positive or negative high-field magnetic susceptibility; or (b) wasp-waisted shapes (constricted at the middle of the loop; 17 samples in Tailfer and 10 in Villers, Fig. 4a) with a positive or negative high-field slope. After slope correction, the ferromagnetic contribution is visible and all loops appear to be wasp-waisted (Fig. 4, right column). The sigmoid shape and the fact that the curve is hardly open (hysteresis curve type a) imply that the ferromagnetic behaviour is masked by the para- or diamagnetic contribution. The wasp-waistedness can either result from two coercivity populations due to different magnetic mineralogy or due to grain size (Wasilewski, 1973; Jackson, 1990; Jackson et al., 1993; Roberts et al., 1995; Tauxe et al., 1996). In Tailfer, hysteresis loops before slope correction have positive slopes (paramagnetic contribution) in 11 samples out of 23 and 10 of them are from the lagoonal unit. In Villers, there is only one paramagnetic sample (out of 24) and this sample belongs to the biostromal unit. In the total 12 paramagnetic samples, for 5 of them, the positive slope is very small, suggesting that the amount of paramagnetic minerals is probably reduced.

The contribution of the ferromagnetic susceptibility (χFerro) is low (between 0.18 × 10−8 and 18.74 × 10−8 m3/kg). Also Ms is low (between 0.85 × 10−4 and 56.54 × 10−4 Am2/kg; equivalent to ~1−100 ppm magnetite based on Ms values taken from Dunlop and Özdemir, 1997), as often occurs in limestones. The highest Ms values are observed in the lagoonal unit of Tailfer. Ms data are relatively well correlated with χin for both sections (correlation coefficient r = 0.75 for Tailfer and for Villers; Fig. 5a); in Tailfer, the correlation coefficient is higher for the biostromal unit (r = 0.94) than for the lagoonal unit (r = 0.67) but this is the opposite in Villers (r = 0.51 for the biostromal unit and 0.75 for the lagoonal unit). This correlation of χin versus Ms (a hysteresis parameter only controlled by the ferromagnetic fraction) suggests that the variations in the magnetic signal are mainly driven by the ferromagnetic minerals. A very high linear correlation between χFerro and χin is observed (r = 0.99 for both sections, without differences between the units; Fig. 5b), also a strong argument in favour of a major influence of ferromagnetic minerals on the χin signal despite their low amount. Three samples deviate from the χin vs. χFerro linear trend (L25b, L73d, L94; Fig. 5b).

χHF provides a quantification of the contribution of the paramagnetic and diamagnetic minerals to the bulk magnetic susceptibility. Most of the samples have a low χHF



(Table 1: χHF between −0.51 × 10−8 and 2.21 × 10−8 m3/kg). In a plot of χHF vs. χin for the Tailfer samples (Fig. 5c), four samples with very high χHF (L25b, L73d, L94, L105: χHF > 1.00 × 10−8 m3/kg), appear to be out of the main trend obscuring correlation inferences. With the four samples included there is a subtle positive correlation (r = 0.12) that increases to 0.69 without these samples, indicating that the initial magnetic signal is also partly controlled by para and/or diamagnetic fraction. This correlation occurs in the biostromal unit (r = 0.60) but not in the lagoonal unit (r = 0.31). There is no significant correlation for the whole Villers section (r = 0.34), but as there is almost no paramagnetic signal in that section (signal always dominated by diamagnetic minerals, except for sample V8c), this is not surprising. There is a correlation for the biostromal unit (r = 0.60) but this is related to the only sample with paramagnetic signal which pull the trend. This absence of correlation between χHF and χin means that the diamagnetic fraction does not control the χin.

Fig. 4. Examples of hysteresis curves: a) and b) Tailfer section, c) Villers section. Left column: plots before slope correction; right column: plots after slope correction.

c) V23

b) L114d

a) L54c

B [ T ] B [ T ]

M [A

m/k

g]2

M [A

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B [ T ] B [ T ]

B [ T ] B [ T ]

-1.5

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Coercivity Bc is in the range of 2.69 to 17.67 mT (Fig. 5d) which is low but common for carbonate platform sediments (Borradaile et al., 1993; Riquier et al., 2010). The occurrence of two grain-size populations could explain the relatively large coercivity distribution (e.g. Borradaile et al., 1993).

4 . 2 . I R M A c q u i s i t i o n C u r v e s

Most IRM acquisition curves have a relatively similar shape. However, they could be divided into two groups based on the behaviour in the LAP (linear acquisition plot): whether or not there is an appreciable increase in IRM beyond 300 mT. In Group I, the IRM acquisition curve still markedly increases with applied field (Fig. 6a), corresponding to the influence of a high-coercive mineral phase (4 samples out of 23 in Tailfer and 10 out of 24 in Villers). In Group II, the IRM acquisition curves are nearly horizontal at the high-field end, indicating that the samples almost reach saturation (Fig. 6b), and that the high coercive mineral is present in small amount (20 samples out of 23 in Tailfer and 14

Fig. 5. Relationship between χin and different magnetic parameters. a) Saturation magnetization (Ms) versus χin, correlation coefficient r for Tailfer and for Villers is 0.75; b) ferromagnetic susceptibility (χFerro) versus χin, for Tailfer and for Villers r = 0.99; c) high-field magnetic susceptibility (χHF) versus χin, for Tailfer r = 0.12 (r = 0.7 without the 4 samples with χHF > 1 × 10−8 m3/kg) and for Villers r = 0.34; d) coercivity (Bc) versus χin, for Tailfer r = 0.23 and for Villers r = 0.41.



Fig. 6. Examples of isothermal remanent magnetization (IRM) acquisition curves from Tailfer. Induced field (in mT) is represented in logarithmic scale. LAP corresponds to the linear acquisition plot; GAP to the gradient of acquisition plot and SAP to the standardised acquisition plot (terminology of Kruiver et al., 2001). SIRM (saturation IRM; the magnetic concentration of the respective phase) expressed as a percentage and in Am2/kg, B1/2 (the field at which half of the SIRM is reached, indicative of the magnetic mineralogy and grain size), and DP (dispersion parameter, provides information on the distribution of grain sizes and/or crystal defects) are provided for each component (Comp). a) The panels correspond to the type of IRM acquisition curve which does not reach saturation after 700 mT acquisition, corresponding to an appreciable contribution of a high-coercive mineral phase (Group I in the text). b) The panels correspond to the IRM acquisition curves that are nearly saturated after application of 700 mT, dominated by low coercivity mineral (Group II in the text).

0

4

8

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16

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Log Field [mT] Log Field [mT]

GAP

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Log Field [mT] Log Field [mT]

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Sum of individual componentsData points

a) Sample Tailfer L23c

b) Sample Tailfer L54c

IRM

[10

Am

/kg]

-52

IRM

[10

Am

/kg]

-42

Comp % SIRM [Am /kg]2

SIRM [Am /kg]2

B1/2 [mT]

B1/2 [mT]

DP mT[ ]1 37.0 7.46 10× -5 58.9 0.332 59.2 1.19×10-4 776.2 0.233 3.7 7.54×10-6 8.9 0.30

S-ratio 0.521

S-ratio 0.947

Comp % DP T[m ]1 81.0 1.26×10-3 53.7 0.312 13.0 2.02×10-3 251.2 0.223 5.9 9.19×10-5 10.0 0.30

Gra

dien

t [10

]8G

radi

ent [

10]

10



out of 24 in Villers). With the Kruiver et al. (2001) software three ferromagnetic components could be distinguished. Component 1 represents the main part of the IRM acquisition curve (commonly >80%); B1/2 values (B1/2 ranges between 48.98 and 89.13 mT, Fig. 7d) are compatible with magnetite. Component 2 represents commonly between 5 and 20% of the IRM signal and is responsible for the non-saturating IRM curve type; it is interpreted as a high coercivity mineral (B1/2 between ~160 and ~1000 mT), likely hematite. As indicated in Section 3, there is a third component which has no physical meaning and its amount has been added to Component 1 when comparing amounts of Component 1 to those of Component 2.

SIRM of Component 1 ranges between 0.82 × 10−4 and 79.50 × 10−4 Am2/kg for the Tailfer section and between 0.50 × 10−4 and 14.76 × 10−4 Am2/kg for Villers. SIRM of Component 2 is between 0.45 × 10−4 and 18.61 × 10−4 Am2/kg for Tailfer and between 0.03 × 10−4 and 7.60 × 10−4 Am2/kg for Villers. DP of Component 1 is between 0.25 and 0.37 log mT in Tailfer and 0.29 and 0.39 log mT in Villers and DP of Component 2 is between 0.18 and 0.40 log mT in Tailfer and 0.13 and 0.40 in Villers. Component 3 has very low B1/2, commonly around 10.00 mT and it represents generally about 5% of the total IRM.

Fig. 7. a) SIRM of Component 1 compared with χin (r = 0.89 for Tailfer and Villers); b) SIRM of Component 2 compared with χin (for Tailfer r = 0.79 and for Villers r = 0.49); c) relationship between S-ratio and proportion of Component 1 (for Tailfer r = 0.92 and for Villers r = 0.99), sample with a dotted arrow framed with a rectangle is considered as an outlier (Sample L23c, S-ratio = 0.52); d) B1/2 of Component 1 compared with χin (for Tailfer r = 0.64 and for Villers r = 0.59).



The SIRM of Component 1 is correlated with χin (Fig. 7a; r = 0.89 for Tailfer and Villers sections, similar in both units from both sections), showing again that the variation in χin is driven by the ferromagnetic minerals and mostly by magnetite. There is also a correlation between SIRM of Component 2 and χin for Tailfer (Fig. 7b, r = 0.79), but not for Villers (r = 0.49). In the Tailfer and Villers sections, most samples have a proportion of high coercivity mineral (hematite) lower than 20%, only 5 samples have a proportion higher than 20% (in Tailfer sample L23c reaching 58%).

The distribution and the amount of the high-coercivity minerals (Component 2) in relation with stratigraphic levels can be deduced from the observation of the shape of the acquisition curves, from the parameters deduced from this shape (DP, B1/2, SIRM), or from the S-ratio. However, the S-ratio may depend on the coercivity and DP values of the component or mixture of components rather than on the relative contribution of magnetite and hematite (Kruiver and Passier, 2001). In our case, there is a good correlation between the S-ratio and the proportion of the first component (low-coercivity component) (Fig. 7c; r = 0.92 for Tailfer and 0.99 for Villers). Most of the samples with a S-ratio < 0.9 have a proportion of high coercivity mineral > 20% (Table 2). So, in this case, the S-ratio can be considered as a good indicator of the proportion the minerals with different coercivities. For both sections, samples with a higher content of high-coercivity minerals are equally distributed over the lagoonal and biostromal units, as well as over the various facies (cf. S-ratio variations shown in Figs. 2−3).

4 . 3 . N R M A l t e r n a t i n g F i e l d C o e r c i v i t y S p e c t r a

NRM across the Ardennes is remagnetized and resides in magnetite (e.g. Zwing et al., 2002; Zegers et al., 2003). The decay curves of the Tailfer section are rather variable; those of the Villers section are more similar to each other (Fig. 8). In the Villers section, the demagnetization starts at low fields and around 50 mT demagnetization between 20% and 40% of the original NRM is left (slightly higher for the lagoonal unit). In Tailfer there is a clear difference between the samples from the biostromal unit and those from the lagoonal unit. At 50 mT, for the biostromal unit the NRM remaining is between 20% and

Fig. 8. Decay curves of alternating field (AF) demagnetization of the normalized NRM for: a) Tailfer and b) Villers.

0

0,2

0,4

0,6

0,8

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mal

ized

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M

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Peak lternating ield mTA F [ ]Peak lternating ield mTA F [ ]

Biostromal unitLagoonal unit

a) b)



40% (as for Villers section). For the lagoonal unit in Tailfer, more of the NRM is remaining after demagnetization at 50 mT, between 35% and 50%. These results point to a broad coercivity distribution of the magnetite, with a broader distribution for the lagoonal unit of Tailfer. There is no relationship between the NRM decay curve and the amount of hematite provided by IRM component analysis (Table 2). There is a good link between SIRM-Component 1 and normalized NRM (r = 0.96 for Tailfer section and 0.91 for Villers section). Furthermore, as observed before, the data from the lagoonal unit of Tailfer are showing a different behaviour than the biostromal unit of Tailfer and the whole Villers section, with significantly higher SIRM and NRM for the lagoonal unit.

4 . 4 . S y n t h e s i s : N a t u r e o f t h e I n i t i a l M a g n e t i c S u s c e p t i b i l i t y

No strong differences for the χin values were observed between the two sections, except that the lagoonal unit of Tailfer has the highest χin values (and so also the highest Ms, χFerro, χHF and SIRM, Figs, 2, 5a−c and 7a). For the two sections, the hysteresis and IRM parameters point to:

1. a minor influence of paramagnetic minerals except in the lagoonal unit of Tailfer section (paramagnetic signal stronger than the diamagnetic signal in only 12 out of 40 samples with 10 of them in the lagoonal unit of Tailfer);

2. a major influence of ferromagnetic minerals, despite their low amount. The main ferromagnetic mineral is magnetite and in most of the samples hematite was also recognized (less than 20% of the ferromagnetic amount, except in a few cases). The occurrence of hematite appears without evident relationship with facies or stratigraphic level.

The shape of the hysteresis loops, Bc and Mrs/Ms provide information concerning the magnetic grain size. As already indicated before, the wasp-waisted shapes could be related to a bimodal magnetic mineral population or to a mixture of different grain sizes. However, the wasp-waistedness of hysteresis loops is not systematically associated with samples that have a higher ratio of low coercivity minerals (data from the IRM acquisition curves). Samples showing more than 50% of hematite have wasp-waisted shapes whereas those are also observed for samples with less than 10% of hematite. The influence of magnetic mineralogy on the shape of the hysteresis loops is therefore interpreted to be insignificant in comparison to the influence of grain size. The occurrence of two distinct grain-size populations could also explain the relatively large coercivity distribution and the comparatively high DP. We use a squareness vs coercivity plot (Fig. 9) to interpret the grain size and shape (Tauxe et al., 2002). As with the more classical ‘Day’ plot representation, also in the SQC plot it is difficult to ascertain the difference between large MD particles on the one hand and mixtures of SD-PSD and SP particles on the other. The lagoonal samples plot on the ‘vortex’ trendline (Tauxe et al., 2002) that is equivalent to PSD particles. Since their extrapolated behaviour trends toward the origin, there could be influence of SP particles which concurs with the wasp-waisted shape of the hysteresis loops. A vortex structure is a non-collinear magnetic structure that is calculated to be transitional between SD and particles that contain a few domains separated by Bloch domain walls (Schabes and Bertram, 1988). Williams and Dunlop (1995) suggested that grains whose remanent state is a vortex are responsible for PSD behaviour. The



biostromal samples of both sections seem to plot slightly to the right of the lagoonal samples, i.e. they have a higher Bc for a given Mrs/Ms ratio. The influence of SP particles could be less.

5. ORIGIN AND NATURE OF MAGNETIC MINERALS AND INITIAL SUSCEPTIBILITY

As indicated earlier, the main magnetic carrier is magnetite, mostly between SP and PSD size. Paramagnetic minerals have a minor influence on the in-field magnetic properties, except for the lagoonal unit in Tailfer section. We examine various hypotheses to explain the origin of these categories. To assess the influence of detrital input on χin and on the paramagnetic fraction (χHF), we compare the magnetic trends with some chemical proxies for detrital input and with data from other studies (Devleeschouwer et al., 2010; Riquier et al., 2010). Elements like Ti, Al, Rb and Zr are classically interpreted

Fig. 9. Squareness Mrs/Ms vs. coercivity Bc (SQC) plots. a) Overall plot with domains according to Tauxe et al. (2002). b) Enlargement of the boxed section in a). c) Comparison of the data from this paper with data from Riquier et al. (2010) and Devleeschouwer et al. (2010).



as proxies for detrital input (e.g. Calvert and Pedersen, 1993; Tribovillard et al., 2006; Riquier et al., 2010). It is important to note that results of TiO2, Al2O3 and Zr are all showing globally the same trends (Table 3).

Bacterial magnetite is also known to result in small magnetic grains, preferentially SD (Kopp and Kirschvink, 2008). The possible influence of bacterial magnetite is also to be considered here. A mixture of PSD and SP grains could be interpreted as being related to remagnetization processes (the remagnetization in the Ardennes was considered as carried by SD and SP magnetite, but the precise grain-size distribution was not clear; Zegers et al., 2003).

5 . 1 . P r i m a r y S i g n a l : D e t r i t a l I n f l u e n c e s

The paramagnetic minerals are mostly composed of pyrite and clay minerals. The paramagnetic contribution is relatively high for the lagoonal unit of Tailfer, but weak in the biostromal unit of Tailfer and for the entire Villers section. A comparison of the χHF with detrital proxies (Fig. 10b) allows to determine whether the main paramagnetic minerals are clay minerals of detritic origin (as opposed to pyrite of diagenetic origin). For the Tailfer section, a plot of χHF vs. Zr shows a very good correlation if one excludes one sample out of the main trend (L25b; with that sample included the correlation coefficient r = 0.62; without that sample it becomes 0.82). The correlation is slightly higher for the lagoonal unit which is probably related to the higher proportion of paramagnetic minerals in this unit. For the Villers section, the correlation between χHF and Zr is low (r = 0.40) but this is related to the fact that the paramagnetic signal is not detectable in this section (Fig. 10 and Table 3). The correlation is seemingly higher for the biostromal unit but this is interpreted as a data artifact: just one single sample has a positive high-field susceptibility (this sample is so deviating that creates its own ‘trend’). So, this means that the main paramagnetic minerals are presumably dominated by clays and are of detrital origin for the Tailfer section but as the paramagnetic signal is so low this link cannot be observed in Villers.

To constrain how we are allowed to interpret χin in terms of detrital input variation, we plot χin against some generally recognized detrital proxy elements (Fig. 10). The plot of TiO2, Al2O3, Rb and Zr are showing the same trends (the Zr plot was selected for Fig. 10) and indicate moderate correlation for Villers (r around 0.65; Table 3); and no correlation between magnetic susceptibility and detrital proxies for the whole Tailfer section (r between 0.13 and 0.32). Considering the Tailfer section, there is a strong difference between the behaviour of χin versus detrital proxies in the biostromal and lagoonal units. The correlation between χin versus TiO2, Al2O3, Rb and Zr is high for the samples from the biostromal unit (r around 0.85) but very low for the samples from the lagoonal unit (r around 0.25; complete trends in Table 3). In the Villers section, the correlation is lower for the lagoonal unit than for the biostromal unit. To summarize, considering the relationship between initial magnetic susceptibility, three groups could be defined: a) biostromal unit of the Tailfer section strongly linked with detrital proxies; b) the entire Villers section moderately linked with detrital proxies and c) the lagoonal unit of Tailfer section, not apparently linked with detrital proxies. A stronger influence of detrital material on the biostromal unit of Tailfer and for the Villers section could also be deduced



from the SQC plot (Fig. 9), as this unit plots slightly off-trend which might be interpreted as an indication for a detrital origin.

Fig. 9c provides a comparison of the data from the present research with those published in Devleeschouwer et al. (2010) and in Riquier et al. (2010). Data presented in Devleeschouwer et al. (2010) are from the Givetian to Frasnian Nismes section (southern Belgium). The hysteresis data were interpreted as corresponding to an evolution from a mixture of fine grained authigenic SP and coarser detrital grains associated to hematite in the Givetian to an enrichment of hematite and a coarser grained magnetite contribution, and a small contribution of smaller particles in the Frasnian. The coarser magnetite is interpreted as primary detrital magnetite. The data from Devleeschouwer et al. (2010), mostly those from the Frasnian having Mrs/Ms < 0.05 are clearly different from our data and are falling in the MD zone. Data from Riquier et al. (2010) are all from the Frasnian/Famennian boundary (Coumiac in France, Anajdam in Morocco, Beringhauser Tunnel and Steinbruch Schmidt in Germany) and these sections were in most cases affected by significant burial diagenesis and noticeable deformation. Riquier et al. (2010) show that their hysteresis data are not showing classical detrital non-remagnetized signature but are similar to what was observed by McCabe and Channel (1994) and Zwing et al. (2005) in some remagnetized limestones from the UK and Germany. It corresponds to a mixture of (1) a primary detrital fraction of mainly fairly coarse-grained magnetite and (2) a secondary diagenetic fraction including authigenic fined-grained magnetite (SD + SP). Riquier et al. (2010) argue that the post-depositional authigenic magnetite fraction does not significantly distort the primary depositional χin signal based on the absence of correlation between χin variations and illite concentration and crystallinity, in conjunction with a robust correlation of χin with Zr and Th. Except for the lagoonal unit of Tailfer, data from Riquier et al. (2010) are very close to our data and a similar interpretation is proposed here (see Section 5.3).

Fig. 10. Plots of selected geochemical proxy data (XRF) with selected magnetic properties. a) Zirconium concentration (Zr in ppm) versus χin (correlation coefficient r for Tailfer is 0.30 and for Villers 0.62); b) Zr (in ppm) concentration versus χHF (for Tailfer r = 0.62 and for Villers r = 0.4).



5 . 2 . P r i m a r y S i g n a l : I n f l u e n c e o f B a c t e r i a l M a g n e t i t e

As indicated before, magnetotactic bacteria produce small SD magnetite (or greigite) particles. We should therefore consider their influence on the initial susceptibility as a possibility. Bacterial magnetite is common in Quaternary environments (Kirschvink and Chang, 1984; Stolz et al., 1986; McNeill et al., 1988), preferentially under specific redox conditions such as suboxic or the conditions at the oxic-anoxic boundary (Kopp and Kirshvink, 2008). Bacteria produce SD magnetite or greigite with a very specific narrow distribution of size (they have small DP < 0.18, e.g. Kruiver and Passier, 2001; Egli, 2004). Depending on the geochemical regime, this fine-grained magnetite can occur near the surface of modern sediments decreasing strongly with depth because of reductive dissolution (e.g. Stolz et al., 1986; McNeill et al., 1988). In the Devonian sediments of this investigation, magnetotactic magnetite has probably been dissolved during (early) diagenetic conditions but its former presence is very difficult to infer. DP values are fairly large not hinting at a persisting magnetotactic magnetite contribution.

5 . 3 . S e c o n d a r y S i g n a l : I m p a c t o f R e m a g n e t i z a t i o n

Remagnetization events were recognized in the Paleozoic rocks of the Ardennes (e.g. Molina-Garza and Zijderveld, 1996; Zwing et al., 2002; Zegers et al., 2003); and were described in some details for Givetian rocks by Zegers et al. (2003): two remagnetized NRM components are identified: an early Permian P-component and a Carboniferous C-component. The P-component likely resides in pyrrhotite and is spatially correlated with Mississippi Valley Type (MVT) ore deposits. Two main MVT districts have been recognized in Belgium, the northern Namur-Verviers district and the more southern Dinant district. The Tailfer section is outside of an MVT zone, and is located about 25 km to the south of the Namur-Verviers district. The Villers section is located outside of the Dinant MVT district, at its northern boundary. For both of these sites, pyrrhotite was not identified in line with their location and therefore the P-component is not important as is its influence on the χin. The C-component is carried by SP-SD magnetite and is interpreted as formed during the smectite to illite transition. This process does not need the presence of an external fluid and could have been enhanced by pressure solution (Zegers et al., 2003). The iron produced during this conversion would partly remain in the clay aggregates, where it would crystallize as fine-grained magnetite. The iron could also have been transported over short distances by fluid migration and would precipitate along fluid pathways such as cracks, grain boundaries and interconnected voids (e.g. Weil and Van Der Voo, 2002 in the Devonian of the Catabrian mountains). The entire Villers section and the biostromal unit of the Tailfer section are showing a strong to moderate link between χin and detrital proxies (Fig. 10). This indicates that even under the effects of the C-component, with the formation of SP-SD magnetite, the system remained probably in situ or isochemical, without strongly affecting the primary trends (leading possibly even to enhance trends). Concerning the lagoonal unit in Tailfer, the link between χin and detrital input is not observed (Fig. 10 and Table 3) and the signal is dominated by SP-PSD magnetite (Fig. 9). The original amount of clay minerals available for the smectite-to-illite conversion was higher in this unit as shown by the higher χHF (Fig. 5c) and Zr contents (Fig. 10b). Two aspects may have obscured potentially primary paleoenvironmental



trends: 1) the originally present clay mineral suite contained an appreciably varying amount of smectite (unfortunately this remains hard to test) and 2) prolonged reaction has blurred the original lithological expression. So, in this scenario, a portion of the released iron has migrated over a larger distance, say up to several decimeters (under the action of fluid that is considered to be buffered by the nearby lithology), leading to a partially hidden primary signal. If by chance the amount of available smectite was lower in the Villers section and in the biostromal unit of Tailfer, such processes occurred in that setting to a lower extent, leading toward a still recognizable detrital, albeit overprinted to some degree.

6. CONCLUSIONS

The Frasnian Belgian carbonate sediments from the Tailfer and Villers sections are showing external, biostromal and lagoonal sediments organized in fourth order shallowing upward sequences. Sequences are piled up to form a lower part, dominated by deep external and biostromal facies (biostromal unit) and an upper part dominated by shallow lagoonal facies (lagoonal unit). In addition to magnetic susceptibility measurements published elsewhere (Da Silva and Boulvain, 2006) here hysteresis loops and IRM acquisition curves were determined on these carbonates to get a better understanding on the nature and the origin of the initial magnetic susceptibility signal. It appears that its variations are mainly driven by magnetite variations, with minor influence of clay minerals (except in the Lagoonal unit of Tailfer). This magnetite occurs as a mixture of two grain-size ranges, PSD and SP range interpreted as related to remagnetization. The influence of detritism is shown through the strong link between facies and χin. This influence is confirmed in the entire Villers section and the biostromal unit of the Tailfer section by the link between χin and detrital geochemical proxies (Zr, Rb, Al2O3 and TiO2). The influence of the Carboniferous remagnetization in the Ardennes (described by Zegers et al., 2003) is evident from the pervasive occurrence of PSD to SP magnetite. As long as the iron-releasing minerals are producing this magnetite in situ, without diffusion over appreciable distance, it does not strongly disturb the primary signal as observed for the Villers section and for the biostromal unit of the Tailfer section. It seems, however, that when such iron is produced in higher amount as in the lagoonal unit of Taifler (higher χHF and Zr content), it could lead to a stronger mobility of the iron and to a blurred primary signal.

In conclusion, the initial magnetic susceptibility signal shows up as a mixture of primary and secondary influences and is more complicated than expected. It implies that caution is recommended when remagnetized limestones are to be correlated by the initial susceptibility approach. The interpretation of initial susceptibility records must be done in conjunction with the outcome of other techniques like magnetic property analysis and geochemical detrital proxy records. Only then it can be assessed whether the initial susceptibility has not been affected by remagnetization effects.

Acknowledgements: A.C. Da Silva acknowledges the Belgian National Research Foundation

(FRS-FRNS) for her postdoctoral research position. This paper is part of the UNESCO International Geoscience Program (IGCP) number 580; entitled “Application of magnetic susceptibility as



a paleoclimatic proxy on Paleozoic sedimentary rocks and characterization of the magnetic signal”. Special thanks to the Paleomagnetic laboratory of Fort Hoofddijk, Utrecht University for the access and help on the hysteresis, IRM and NRM measurements and to the laboratory of magmatic petrology of Liège University (N. Delmelle, O. Namur and J. Vander Auwera) for access and help on the geochemical measurements (X-ray fluoresence).

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