Magnetic susceptibility variations in carbonates of the La Vid Group (Cantabrian Zone, NW-Spain) related to burial diagenesis

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Abstract

The carbonates of the Lower Devonian La Vid Group in the Cantabrian Zone (NW-Spain) reveal distinct variations in low-field magnetic susceptibility (MS) from base to top. There is good correlation between MS-variations and bulk Fe-content. A predominance of paramagnetic minerals (Fe-carbonate cements, pyrite, Fe-chlorite), mainly responsible for these MS-variations, is evidenced by optical methods, temperature-dependent MS-measurements and high-field magnetisation behaviour. These minerals are members of the diagenetic mineral assemblage formed during migration of a reducing Fe-bearing fluid. We interpret the variation in MS to reflect two stages of Fe-bearing mineral precipitation with an earlier Fe-carbonate and a later Fe-chlorite crystallisation; the latter restricted to interbedded carbonates and shales. Furthermore, porosity has an additional influence on the MS-signature, with high values in coarse-grained sandstones and carbonates, and lower values in fine-grained dolostones and limestones. This study highlights the influence of diagenetic mineral formation on MS-variations in carbonate-bearing successions.

Introduction

During the last two decades, low-field magnetic susceptibility (MS) has become one of the most commonly measured petrophysical properties in geosciences. Fields of application range from environmental and palaeoclimate studies of soils and lake deposits to provenance and climate reconstructions of sedimentary rocks in terrestrial (e.g., Bloemendal et al., 1992, Bloemendal and de Menocal, 1989, Kukla et al., 1988) as well as in marine geological settings (e.g., Schmieder et al., 2000, Stage, 2001).

During burial and subsequent diagenetic processes, sedimentary rocks are exposed to large-scale fluid circulation. This may induce the formation of mineral phases that can significantly influence the primary magnetic susceptibility record. High porosities in carbonate rock successions facilitate the percolation of fluid phases documented in multiple cement successions in these type of rocks (cf. Meyers, 1974, Zeeh et al., 1997). Despite the widespread application of magnetic susceptibility measurements in sedimentary geology, the effects of secondary processes on the MS of carbonate rocks are not well known. Bityukova et al. (1998) and Shogenova (1999) documented increases in MS-values due to dolomitisation of carbonate rocks in Estonia, the consequence of diagenetic processes. Hydrocarbon migration may lead to changes in the chemical properties of rocks and cause reducing conditions, changing Fe-minerals from ferric to ferrous. Consequently, a shift of the magnetic properties can be observed (Machel, 1996). Most work concerning the influence of diagenesis on magnetic properties is concentrated on the effects of remagnetisation of the natural remanent magnetisation of rocks (e.g., Ellwood et al., 1989, Elmore et al., 1993, Haubold, 1999).

This study demonstrates that the MS-signature in carbonates, monitored along a stratigraphic profile at the type section of the Lower Devonian La Vid Group (Cantabrian Zone, NW-Spain), is significantly influenced by diagenetic processes and cementation.

The Cantabrian Zone (Fig. 1) in NW-Spain is composed of several nappes containing Palaeozoic sediments from a shelf to basin transition. This succession was thrusted and folded during the Variscan and Alpine orogenies Lotze, 1945, Julivert, 1971, Pérez-Estaún et al., 1988.

The stratigraphic succession studied here at the La Vid type section (Fig. 2) comprises sandstones and carbonates from Upper Ordovician to Lower Devonian age. Silurian Formigoso Shales were deposited on a discontinuity atop Ordovician Barrios Sandstones (Comte, 1959). These shales are built up by sandstones and mudstones from a shallow marine environment (Evers, 1967) grading upwards into black shales with graptolites. The transition to the overlying ferrugineous sandstones of the San Pedro Formation is gradual and is marked by an increase in coarse-grained particles. The lower part and the top of the San Pedro Formation are composed of red oolithic sandstones. In the middle part, shales intercalate with green and red sandstones, and towards the top, shales intercalate with white quartz sandstones (van den Bosch, 1969). These sediments were deposited in a shallow marine environment.

Transitional sandstones cemented by carbonates at the base of the Lower Devonian La Vid Group mark the return to carbonate-dominated successions of the Devonian and lower Carboniferous since the Cambrian. The basal member consists of sandstone which passes with a decrease in detritus into the dolostones of the La Vid Group. At the type section, the La Vid Group displays cycles of intercalating carbonates and shales from a carbonate ramp (Keller, 1997). It is subdivided into three units from bottom to top (Fig. 3): the transitional sandstone unit (TS), the dolostone unit (DST) and the limestone unit (LST). Shales overlie the succession.

The TS, consisting of sandstones alternating with fossil-rich carbonates, was deposited in an open-marine environment. Late diagenetic dolomites and calcites are common cements in this part of the succession. Regionally, the first layer of the TS is composed of siderite (Keller, 1988). The overlying DST represents a succession of peritidal lagoonal to sabkha sediments, associated with detrital quartz and showing nodular moldic porosity. The latter probably represents dissolved former evaporites (Keller, 1988). The DST reveals complex fabrics due to dolomitisation, recrystallisation and cementation. Upwards the DST passes gradually into the LST. The upper part of the DST characterised by detrital quartz and nodular molds, and the lower LST unit are repeated due to reverse faulting.

The limestones display a broad variation in their sedimentary composition and are subdivided into three units (LST 1, LST 2, LST 3; see Fig. 4). LST 1 is composed predominantly of mudstones at the base, followed by packstones and grainstones; in contrast, LST 2 consists of fine-grained wackestones. The LST 3 unit, consisting of coarse-grained crinoidal grainstones intercalating with shales, indicates the change from restricted to open marine settings. A gradual transition to overlying shales occurs. In association with a transgression, these shales were partly deposited under euxinic conditions (Keller, 1988).

During subsequent burial of the basin, the La Vid Group was overlain by about 2500 to 3000 m of sediments. According to Brime et al. (2001), maximum temperatures in the Cantabrian Zone did not exceed the field of diagenesis and were reached during the late basin stage prior to orogenesis. Although major fluid flow events were active in the Cantabrian Zone Gasparrini, 2003, Grimmer, 2001, Schneider, 2002, the La Vid Group was only affected by a reducing Fe-bearing fluid during the basin stage and an oxidising low-temperature post-Variscan fluid (Schneider, 2002).

Low-field magnetic susceptibility measurements were undertaken using the KLF-3 Minikappa (Geofyzika Brno) in an ac-field of 300 A/m and a constant operating frequency of 2 kHz. This equipment provides a sensitivity of 1×10−6 SI for the magnetic susceptibility record. As rock fragments different in size and porosity were measured, we related the sample susceptibility to a given mass. In total, 78 samples were measured (Barrios: 2; Formigoso/San Pedro: 6/6; La Vid: 64). The sampling resolution was increased for the upper part of the La Vid Group (LST). This method allows a rapid monitoring of magnetic susceptibility variations along sedimentary profiles.

For evaluation of the magnetic behaviour and discriminating ferrimagnetic mineralogy, the temperature dependency of magnetic susceptibility [MS(T)] was measured in the temperature range of −192 to 700 °C using the KLY-2 Kappabridge (sensitivity for specimen with nominal volume of 10 cm3: 4×10−8 SI), combined with the CS-2/CS-L furnace apparatus of AGICO (Hrouda, 1994). During the cooling run, temperatures and susceptibilities were recorded as the sample warmed up from −192 to 0 °C. Heating/cooling cycles from room temperature to 700 °C (heating rate: 10 °C/min) were performed in an argon atmosphere flow (100 ml/min) in order to minimise mineral reactions with atmospheric oxygen that may occur during the heating process. The raw data were corrected for the empty furnace and normalised to the susceptibility magnitude at room temperature.

Petrographic studies (reflected and transmitted light) were conducted on polished thin sections. Backscatter electron images were taken with a Leo 440 scanning electron microscope (SEM) at the Institute for Environmental Geochemistry of the University of Heidelberg. The samples were coated with carbon in an argon atmosphere to generate a conductive layer. Estimations on the major cation composition of carbonates were carried out using an EDX-apparatus (Oxford, Link Isis 300) with an acceleration voltage of 20 kV.

The bulk Fe-content of the carbonates was determined by an Inductive Coupled Plasma Emission Spectrometer (ICP-ES). Drilled sample powder was analysed by ACME (Analytical Laboratories) in Vancouver, Canada. Samples were digested for 1 h in concentrated agua regia (HCl–HNO3–H2O in a ratio of 3:1:2) at a temperature of 95 °C.

The MS-logs of the type section and profiles to the east and west (Fig. 2) exhibit characteristic variations within the different formations, showing mass susceptibilities (Table 1) ranging from <1 to 44 (10−6 SI/g). The quartz sandstones of the Barrios Formation at the base of the measured profile show negative (diamagnetic) values below 1 (10−6 SI/g), whereas the overlying shales of the Formigoso Formation display variations between 1 and 10 (10−6 SI/g). The San Pedro Formation is characterised by strong MS-variations from negative diamagnetic to positive values in the range of 10−5 SI/g. Low values are related to white sandstones while higher values are found in greenish to reddish sandstones. The transition to the La Vid Group is marked by the highest value in the whole MS-profile with a subsequent drop to very low paramagnetic values. Proceeding stratigraphically upwards, the MS generally increases towards the top of the profile. The transition to the overlying shales is characterised by a distinctly higher value, followed again by a lower MS-value. In order to understand these magnetic susceptibility variations in the context of fluid migration and diagenesis of carbonates, a detailed magneto–mineralogical study was performed.

A characteristic feature of the La Vid section measured in this study is the general increase of MS from the upper part of the dolostones to the top of the measured succession Fig. 2, Fig. 3 with some distinct anomalies. We observed a correlation between MS and bulk Fe-contents in the investigated samples (Fig. 3) and related the MS-variation to observed lithological and compositional variations. Table 1 compiles the MS-data of the Lower Devonian La Vid Group together with the maximum and minimum Fe-contents for bulk rock samples. Two samples with high susceptibilities (P1 and P3) and one typical carbonate sample (P2) are investigated in detail in order to evidence the hypothesis that diagenetic fluid-controlled processes can have a significant influence on MS (see Fig. 3).

The prevailing lithological and fabric characteristics and their typical magneto-mineralogy are discussed as deduced from optical and thermomagnetic analysis along with hysteresis loop measurements. At the base of the La Vid Group, a sandstone layer occurs, cemented by carbonates showing MS-values of +5 to +44 (10−6 SI/g) and a varying Fe-content ranging from 6 to 12 wt.%. Diamagnetic quartz is the main detrital mineral phase in the diagenetic Mg-rich siderite- and chlorite-cemented transitional sandstones (Fig. 4a). Accessory idiomorphic pyrite occurs as secondary phase in the matrix revealing ghost structures of the former (possibly detrital) Fe-bearing minerals (Fig. 4b). Two types of Mg-rich siderite were distinguished based on their Mg/Fe ratio of 1:3 and 1:1, respectively (Fig. 4a). EDX spectra exhibit a low Ca composition of (Mg0.25–0.5Fe0.75–0.5)CO3 (Fig. 5).

The MS(T)-curve (Fig. 4c) indicates dominantly paramagnetic behaviour between −192 and 450 °C. Above that temperature, the susceptibility increases and drop down at a Curie temperature (TC) of about 560 °C (Fig. 4c). This susceptibility increase documents the formation of a ferrimagnetic phase from a previous mineral during heating. The cooling curve shows a strong increase of MS at a Curie temperature (TC) of 530 °C followed by a pronounced Hopkinson peak. This feature points to the formation of a magnetite-near phase (possibly maghemite) with predominantly single-domain grains.

The MS of the overlying sandstone beds are significantly lower than in P1, with values of 2.5 (10−6 SI/g). These values remain relatively constant from the TS through the DST, the lower carbonate member of the La Vid Group. The bulk Fe-content of these successions is around 1 wt.%.

A significant drop in MS to values of 1 (10−6 SI/g) is observed in the middle part of the DST. A characteristic feature of this part of the DST is the abundance of syn-depositional sulfate nodules (now filled by later diagenetic cements) crosscut by veins of Fe-rich saddle dolomite (Fe-Dol). The Fe-content of this layer is also lower, similar to the underlying carbonates with values around 0.6 wt.%. Above this distinct low, MS-values increase (see Fig. 3). However, a small drop in MS occurs at the transition from the DST into the LST.

P2 (Fig. 6a), composed of partly fragmented fossils up to 3 mm in diameter, is a representative sample from the packstone unit of LST 1. The matrix surrounding these fossil fragments is predominantly composed of calcite and quartz. Fe-rich saddle dolomite (Fe-Dol) occurs as cement in a fossil cavity. Accessory framboidal pyrite appears in the matrix and is enclosed in Fe-Dol. The bulk rock Fe-content of sample P2 is about 2 wt.%. EDX investigation of the Fe-rich saddle dolomite evidences a (Ca0.64Mg0.33Fe0.03)CO3 chemical composition (Fig. 5).

The thermomagnetic curve of the limestones in the La Vid Group is characterised by constant MS-values up to 400 °C. Above 420 °C, a transformation into a ferrimagnetic phase occurs (Fig. 6b). The heating and cooling curves exhibit a TC at 560 °C, and, in the cooling branch, a subsequent decrease in MS below 300 °C was observed. Compared to Fig. 4d, this suggests only a small formation of a ferrimagnetic phase, apparently unstable during cooling to room temperature. This behaviour has been observed in several limestone samples from this study and is typical for the host rocks from the La Vid Group. Possible minerals serving as precursor for this transition are Fe-rich saddle dolomite or pyrite as they are the Fe-bearing minerals evaluated by SEM and EDX.

A reverse fault zone (see Fig. 3) causes a repetition of the transition from the DST to the LST. The MS-values in the hanging wall of the fault correlate well with the MS-pattern in the equivalent part of the footwall. The MS-pattern for the first limestone succession (LST 1), composed of mud-, pack- and grainstones, shows a continuous rise in MS-values up to 8.6 (10−6 SI/g) which is interrupted by two drops.

The following LST 2 succession is characterised by a significant decrease in MS to values ranging from 3.1 to 5.2 (10−6 SI/g). The contact between the coarser grainstones and packstones of the LST 1 to the fine-grained wackestones and grainstones of LST 2 is marked by a sharp discontinuity. The bulk Fe-content of these carbonates is about 0.3 wt.%. In LST 2, Fe-rich saddle dolomite occurs locally in veins and, as cement, replacing the rarely occurring fossils. The transition to the overlying coarse-grained crinoidal grainstones of LST 3 distinct and is marked by a jump from MS-values of 6.6 to 37 (10−6 SI/g). MS-data for this succession also displays a trend of increasing MS with respect to that of the underlying units. The top of the grainstone layer is coincident with the highest MS-values of the limestone interval followed by a decrease in values of 10.6 (10−6 SI/g) at the base of the overlying shales. Similar to the trend for MS, bulk Fe-content increases in LST 3 evidenced by higher values than in the units below. The highest Fe-value of 10 wt.% is measured in the uppermost layer (P3). Sample P3 is a coarse-grained crinoidal grainstone composed of calcitic fossil components up to 3 mm in diameter. Open cavities and pore spaces are cemented by idiomorphic Mg-enriched ankerite, pyrite and chlorite. Pyrite and chlorite occur around the ankerite rhombs and in partly dissolved calcitic fossil fragments (Fig. 6c). EDX-analysis of the “ankerite“ reveals a composition of (Ca0.58Mg0.27Fe0.15)CO3 (Fig. 5). Detrital Fe-bearing minerals were not observed by SEM.

The contribution of paramagnetic minerals to the MS is also reflected in the temperature-dependent MS-curve of P3 (Fig. 6d). Between 0 and ∼350 °C, a parabolic decrease of MS with increasing temperature occurs which is typical for paramagnetic behaviour. Above 420 °C, a transition into a higher susceptible phase with TC at 580 °C is observed. The strong increase in MS during cooling (inlay in Fig. 6d) points to the formation of a magnetite-near phase comparable to that observed in sample P1.

According to MS(T)-measurements, no magnetite was identified in the original samples; hence, the MS-variation in the La Vid Group is related to paramagnetic minerals. This is confirmed by the analyses of high-field magnetisation behaviour (Fig. 7). All three samples (P1, P2, P3) show a positive linear relationship between applied field and magnetisation typical for paramagnetic behaviour, and no indication for the presence of ferrimagnetic minerals, especially magnetite, was found.

Pure paramagnetic rocks should show a positive linear correlation between paramagnetic ion content and MS. This has been confirmed by numerous studies on paramagnetic rocks (e.g., Rochette, 1987, Gleizes et al., 1993). In order to verify this relationship for the carbonates of the La Vid, the measured MS was plotted versus the Fe+Mn content (Fig. 8). The samples with low Fe+Mn content (TS and DST, LST 1, LST 2) show distinctly lower MS than samples from the LST 3 unit. Highest Fe+Mn content and MS, respectively, are found in samples P1 and P3. Overall, there is a good linear correlation between the MS and the (Fe+Mn) content of the different lithologies. An occurrence of ferrimagnetic minerals would have caused a significant deviation from this linear correlation trend.

In summary, the mineralogical and magnetic analyses indicate paramagnetic behaviour of the carbonate rocks. For a better understanding of the temperature-dependent MS-behaviour of these carbonates Fig. 4, Fig. 6, the temperature-behaviour of relevant paramagnetic minerals that might contribute to the curves are investigated. From petrographic analyses, a contribution to MS is inferred by Fe-carbonates (siderite, Fe-Dol, ankerite), chlorite and pyrite.

The course of the MS(T)-curves for all carbonate samples are mainly determined by two features, namely, a constant or parabolic course followed by a strongly increasing MS above about 400 °C. In order to strengthen the interpretation that the MS(T)-curves are related to Fe-bearing paramagnetic minerals, we isolated and measured individual minerals of Fe-Dol, Fe-Mg-chlorite, ankerite and pyrite. We interpret the transformation temperature of the investigated samples above 400 °C characteristic for one of these paramagnetic phases.

Fe-rich saddle dolomite has been separated from the host limestone in open cavities of brachiopods (Fig. 9a). Nonluminescent behaviour here indicates an enriched Fe-content, confirmed by the ICP-values ranging from 2 to 2.7 wt.% (Fig. 3). Investigations by EDX exhibits a similar composition as P2 with (Ca0.66Mg0.3Fe0.04)CO3 (Fig. 5).

Above room temperature, the susceptibility is nearly constant during heating (Fig. 9c). However, the cooling branch shows a small increase in MS below 520 °C that we interpret as the formation of very small amounts of ferrimagnetic minerals during the heating procedure.

Fe-Mg-chlorite (CCa-2) from the Flagstaff Hill Area, California (cf. Post and Plummer, 1972) was used for the temperature-dependent MS-measurement. Based on XRD analyses, Warr and Peacor (2002) estimated a Fe-content of 1.7 (1.2 Fe in the silicate layer and 0.5 Fe in the hydroxide sheet) distributed among the six available octrahedral sites of CCa-2. This chlorite shows a typical paramagnetic curve (Fig. 10a) with an exponential decrease of the MS at low temperatures and a reversible heating/cooling curve with no transformation into a ferrimagnetic phase during heating of the sample. Néel temperatures for thuringite and chamosite are reported to be −256 and −261 °C, respectively (Ballet et al., 1985).

For a pure ankerite sample from the Erzgebirge near Freiberg (Germany), the MS(T)-curve was measured in the temperature interval from −192 to 700 °C. Fig. 10b shows a clear paramagnetic course between −180 and 350 °C and an increase of MS beginning above 350 °C. The Curie temperature of nearly 580 °C points to the existence of magnetite. Ellwood et al. (1989) also described this breakdown of Fe-carbonates and postulated a formation of maghemite. The strong resemblance of the curve of Fig. 10b to our sample P3 (Fig. 6d) confirms the influence of Fe-carbonates on the magnetic behaviour.

Pyrite shows a weakly positive MS independent of temperature (Serres, 1953). This behaviour is also documented from the MS(T)-curve of pyrite pieces isolated from borehole cutting samples of the KTB drilling, Germany. The heating and cooling cycles done in an argon atmosphere (Fig. 10c) and ambient atmosphere (Fig. 10d) are each distinct. Heating in an argon atmosphere revealed constant values for MS, but during cooling, ferrimagnetic pyrrhotite formed with a Curie temperature of ∼310 °C. The MS(T)-curve measured in an ambient atmosphere shows an increase above 420 °C and a drop in MS at 580 °C, similar to what we have observed in samples P2 and P3. Although the limestone samples were measured in an argon atmosphere, no pyrrhotite formation was observed but the transformation into a ferrrimagnetic oxide phase, which we interpret to be magnetite. This indicates that additional to the ambient conditions, under which the measurements were performed, the sample material itself may influence the reactions during heating because adhered water is released, and internal oxidation occurs. Although pyrite is only a minor component it contributes to the behaviour of P1, P2 and P3 during heating, as observed in pure pyrite above 420 °C in an ambient atmosphere.

Comparing the MS(T)-curves for Fe-rich saddle dolomite, Fe-Mg-chlorite, ankerite and pyrite with the complex curves of the carbonates from the La Vid section Fig. 4, Fig. 6 we interpret the latter to reflect a combination of the different Fe-bearing minerals. A ferrimagnetic contribution of minerals similar to magnetite can be excluded. The dominantly paramagnetic behaviour, especially for that of sample P3, is related to chlorite and ankerite, while in samples P1 and P2, this paramagnetic influence is diminished in the thermomagnetic curves. According to Rochette (1987), chlorite and siderite show higher susceptibilities than do antiferromagnetic minerals like hematite, due to the paramagnetic behaviour of the ferric Fe. We assume that the increase of MS at TC > 420 °C observed in the samples is related to a transformation of Fe-carbonates and pyrite into magnetite (or maghemite) during heating.

Carbonates of the Lower Devonian La Vid Group at the type section in the Cantabrian Zone (NW-Spain) show paramagnetic magnetic susceptibility (MS) values with characteristic patterns from bottom to top Fig. 2, Fig. 3. These MS-variations correlate with the variations in the bulk Fe-content. From the petrographic observations and geochemical data presented, we imply that the carbonate minerals were precipitated from a Fe-bearing fluid active during the basin stage (Schneider, 2002) and driven stratigraphically upward by compaction. Two phases of Fe-bearing mineral formation are recognised. In the first phase, Fe-bearing carbonates and pyrite formed. Later on, they were overgrown by Fe-bearing chlorite and a second pyrite generation (Fig. 6c). Considering the observed variations in MS and in the Fe-content, we propose the following interpretation (Fig. 11). During burial diagenesis (phase I), ferric Fe from the hematite-bearing San Pedro sandstones was reduced to ferrous Fe and transported by a fluid into the overlying La Vid Group. The predominant precipitation of siderite in the TS in contrast to the subordinate occurrence of pyrite implies a low content of sulfide in the original fluid (cf. Burton et al., 1993). A similar reduction process in hematite-bearing sandstones in relation with subsequent precipitation of Fe-minerals is also reported in studies of Chan et al. (2000) and Garden et al. (2001) where they describe bleached zones in reddish sandstones along the Moab Fault in southeastern Utah.

The reduction of the ferric Fe can be attributed to hydrocarbons associated with this Fe-bearing fluid and having originated from the underlying shales of the Formigoso Fm. (Schneider, 2002). Primary petroleum inclusions in Fe-rich saddle dolomite (Schneider, 2002) support this interpretation. Hydrocarbons have a low redox potential. This is related to the absence of diagenetic magnetite in this layer of the transitional sandstones. Based on stability diagrams for temperatures of 100 °C and a pressure of 30 MPa (Burton et al., 1993), magnetite is only stable in a small field with an intermediate redox potential, in contrast to pyrite and siderite occurring in the stability field with a lower redox potential. These conditions correspond to fluid inclusion data of Fe-rich saddle dolomite with homogenisation temperatures at of about 114 °C (Schneider, 2002). At the base of the succession, Mg-rich siderite, poor in Ca, was formed. Subsequently the fluid inherited Ca during its ascent from the surrounding carbonates; thereafter, Fe-rich saddle dolomite and, finally, dolomitic ankerite was precipitated (Fig. 11).

The upward movement of Fe-bearing fluids cannot be taken as the only explanation for the distinct increase in MS within the LST units of the La Vid Group. Normally, it is expected that a fluid would become depleted in Fe during its migration and hence displays a decreasing trend with respect to MS. But the opposite trend is observed in the limestones of the La Vid Group. We interpret the increase in the LST 1 and abrupt decrease in the LST 2 units to reflect variations in the rock porosity due to fabric changes from coarser-grained grainstones/packstones to finer-grained wackestones. Ferroan saddle dolomite crystallised in interparticle pore spaces, e.g., fossil molds, which remained open after compaction and replaced aragonitic and high Mg-shells. Thus, grain-supported rocks exhibit significant concentrations of Fe-carbonate cements responsible for the high Fe-content of the bulk rock samples and, as a consequence, high MS-values for the succession. In contrast, matrix-supported mudstones, wackestones and dense DST occurring at the base and above the fault zone in the succession are characterised by low MS-values. Open pore spaces are rare. Thus, Fe-Dol precipitated and crystallised only in veins during the ascent of the fluid and did not replace the matrix.

The distinct and continuous increase of MS in the LST 3 unit to values significantly higher than those in LST 1 and 2 is here assigned to various processes of Fe-enrichment in the La Vid carbonates. The LST 3 unit is characterised by carbonate layers intercalating with shales which become more abundant to the top. We propose that in a second phase (phase II), water-rock interaction within the shales and adjacent carbonates result in the formation of Fe-bearing chlorite and a second pyrite generation. During this increased rate of rock burial (phase II), Fe-, SiO2- and sulfur-bearing fluids, which originated from prograde mineral transformations in the shales (e.g., smectite to illite), migrated into the carbonate layers. This additional influx of Fe caused the distinct increase in MS towards the top of the La Vid Group. Such an interpretation of a small-scale water rock interaction is also evidenced by the observation that chlorite disappears in the carbonates stratigraphically below LST 3.

Magnetic susceptibility variations in marine sedimentary rocks were also described by Ellwood et al., 2000, Ellwood et al., 2001, and related to climate-driven erosion effects. According to their interpretation, the influx of detrital grains into the marine environment is the most important factor that controls the variation in MS. In our carbonate samples, we did not observe any detrital Fe-bearing minerals. We propose here that the detrital minerals were destabilised during burial diagenesis and thus released Fe which was thereafter incorporated into the carbonates. However, such a process is considered to be of minor importance as a source of Fe in the investigated carbonates of this study.

Section snippets

Conclusions

Two stages of Fe-bearing mineral precipitation that caused paramagnetic behaviour are recognised in the La Vid carbonates. The MS-pattern can be explained in terms of the following:

  • (1)

    Decreasing Fe-content in a Fe-bearing fluid moving stratigraphically upward caused the formation of siderite at the base and ankerite at the top of the La Vid Group carbonates.

  • (2)

    Variation in rock porosity due to facies changes from grainstones to wackestones are responsible for different amounts of Fe-bearing carbonate

Acknowledgements

Financial support for this study was provided by the DFG-Graduiertenkolleg 273. We acknowledge Z. Veselovsky (Heidelberg) for providing the stratigraphic profiles of the La Vid type section, J. Just, and C. Vahle (Heidelberg) for helping with the magnetic measurements and their interesting discussions and K.L. Carrière (Heidelberg) for comments and corrections. We are grateful to T. Frederichs (Bremen) for high-field measurements, to L.N. Warr and R. Altherr (Heidelberg) for providing the

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