Host rock diagenetic framework

Chapter 5 124

¹³C values are comprised between -2.88 ‰ and -0.40 ‰. Fault core rocks are shear bands (CFDolCalSB) composed of a mixture of calcite and dolomite (their isotopic values have to be considered with caution). ¹⁸O values range between -2.71 ‰ and -0.38 ‰ and ¹³C values are comprised between -0.25 ‰ and 1.45 ‰. Calcite cement (CFCalV) filling veins and vugs in the fault zone shows both¹⁸O and ¹³C depleted compared with the matrixes, with values ranging from -8.30 ‰ to -6.06 ‰ for oxygen and from -3.86 ‰ to 0.54 ‰ for carbon.

Bituminous Dolostones host rocks (BDHR)¹⁸O values range from -0.09 ‰ to 2.20

‰ and ¹³C shows values in the range 1.40 ‰ to 2.31 ‰. Only four samples collected at site CF3, have ¹⁸O values comprised between -2.22 ‰ and -1.05 ‰. Dolomite fault-veins in bituminous dolostones (BDDolFV) in the haging wall of the Vado di Ferruccio Thrust have ¹⁸O and ¹³C values comprised between -0.48 ‰ and 0.83 ‰, and between 1.69 ‰ and 2.22 ‰, respectively. A fault-vein included in bitumen layers shows depleted values with a¹⁸O value of -2.65 ‰ and a ¹³C value of -1.60 ‰. Calcite micro-veins in bituminous dolostones (BDCalV) show depleted values of ¹⁸O and ¹³C compared to the host and have values ranging from -6.86 ‰ to -2.93 ‰ and from -4.68 ‰ to 2.02 ‰, respectively. Dolostones host rocks sampled at site PF1 display heavier¹⁸O values, comprised between 3.26 ‰ and 3.79 ‰, and the associated fault-veins showheavier ¹³C values, from 2.24 ‰ to 2.58 ‰, compared to the bulk of the samples.

In the Vado di Ferruccio fault core, like in the Monte Camicia Thrust, data were collected from a limestone-dolostone mixed material (VFCore). Isotopic values are comprised between -0.48 ‰ and 0.39 ‰ for¹⁸O, and from 1.55 ‰ to 2.13 ‰ for ¹³C, respectively. Vein sets VFCal1 and VFCal2, in the footwall damage zone, have depleted

¹⁸O values, ranging from -5.82 ‰ to 0.13 ‰, respectively. Their ¹³C values range from -1.01 ‰ to 1.61 ‰. The marly limestones of the Corniola Fm. in the footwall of the thrust, show ¹⁸O values comprised between -1.15 ‰ and 0.00 ‰, and ¹³C values ranging between 1.62 ‰ and 2.65 ‰, respectively. Only at site VF1, two ¹³C values of 0.52 ‰ and 0.56 ‰ were obtained. Veins CoCalV typically have ¹³C values ranging from 1.98 ‰ to 2.55 ‰ and only at site VF1 they decrease to a range varying between 0.25 ‰ and 0.36

‰. CoCalV show ¹⁸O values comprised between -1.43 ‰ and 0.22 ‰.

8.2. Strontium isotopes data

Eight samples were selected for strontium isotopes analysis, including two dolostone host rocks (BDHR), two dolomite fault-veins (BDDolFV), two limestone host rocks (RCHR and CoHR), a calcite vein (CoCalV), and a dolomitic-calcareous micritic matrix in the Monte Camicia Thrust fault core (CFDolCalSB). Results are illustrated in Fig.

17A, where host rock strontium isotope ratios are plotted vs. their depositional age (McArthur et al., 2012). Bituminous dolostones display ^87/86Sr values of 0.707968 and 0.707843, and show a good correlation with the seawater strontium ratio of their depositional ages. A similar feature occurs also for the Corniola sample, with a ^87/86Sr value of 0.707157, whereas Rudist Calcarenites show a ^87/86Sr value of 0.707035, which is depleted compared to the age of this formation. Plotting ^87/86Sr values versus the corresponding ¹⁸O values indicates that thrusting-related dolomite fault-veins have ^87/86Sr values of 0.707707 and 0.707500, which are depleted compared to their host rocks (Fig.

17B). The ^87/86Sr value of the calcite vein CoCalV from Corniola is 0.707397, i.e. enriched compared to the host rock. The ^87/86Sr value from the Monte Camicia fault core is 0.707786.

8.3. Noble gases isotopes data

Results of noble gas analyses are reported as ratios in Table 1. ³He and ⁴He were below instrumental detection limits in all samples. ⁴⁰Ar/³⁶Ar ratios range between 280.2 and 309.8, i.e. typical values of the atmosphere. Fig. 18 shows the measured heavy noble gas ⁸⁴Kr and ¹³²Xe data plotted as a function of ³⁶Ar values. ⁸⁴Kr/³⁶Ar are comprised between 0.0228 and 0.0347 and ¹³²Xe/³⁶Ar show values ranging from 0.00126 to 0.00378.

Almost all samples lie on an ideal mixing trend whose end-members are represented by atmosphere (Air) and air-saturated water (ASW). The bituminous dolostone host rock sample deviates from this trend and shows a higher ¹³²Xe concentration, reaching a

132Xe/³⁶Ar value of 0.01126. Massive dolostone samples provided ⁸⁴Kr/³⁶Ar values always lower than 0.0282, which are closer to the Air end-member. On the other hand, limestone and calcite cement samples have ⁸⁴Kr/³⁶Ar values always higher than 0.0279, closer to the ASW end-member.

Chapter 5 125

-5,00 -4,00 -3,00 -2,00 -1,00 0,00 1,00 2,00 3,00

-9,00 -8,00 -7,00 -6,00 -5,00 -4,00 -3,00 -2,00 -1,00 0,00 1,00 2,00 3,00 4,00

Late calcite in Dol Late Calcite in T1 Zone Calcite Veins T7 T7 Fault Zone Corniola &AmmVer Veins Corniola & AmmVer HR Bit. Dol. HR Bit. Dol. Veins T1 Fault Zone Meteoric Calcarenites HR Prena_Dol_HR Prena_Dol_Breccia-Veins Mixed_T1_material

0 1 2 3 4 5 6

-9,00 -8,00 -7,00 -6,00 -5,00 -4,00 -3,00 -2,00 -1,00 0,00 1,00 2,00 3,00 4,00

0 1 2 3 4 5 6

-9,00 -8,00 -7,00 -6,00 -5,00 -4,00 -3,00 -2,00 -1,00 0,00 1,00 2,00 3,00 4,00

0 1 2 3

-4 -3 -2 -1

-5-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4

-5 -4 -3 -2 -1 0 1 2 3 4

-9 -8 -7 -6

-5 -4 -3 -2 -1 0 1 2 3 4

-9 -8 -7 -6

C.F.

E.C.

V.F.F Cor B.Dol.

C.F.

E.C.

V.F.F Cor B.Dol.

δ13C ‰

δ¹³C ‰ δ¹⁸O ‰

δ¹⁸O ‰

A

B

C

CoHR CoCalV CFCalSB

CFDolCalSB

VFCore RCHR

CFCalV

VFCal1 VFCal2 BDHR

BDCalV BDDolFV

Eight samples were selected for strontium isotopes analysis, including two dolostone host rocks (BDHR), two dolomite fault-veins (BDDolFV), two limestone host rocks (RCHR and CoHR), a calcite vein (CoCalV), and a dolomitic-calcareous micritic matrix in the Monte Camicia Thrust fault core (CFDolCalSB). Results are illustrated in Fig.

17A, where host rock strontium isotope ratios are plotted vs. their depositional age (McArthur et al., 2012). Bituminous dolostones display ^87/86Sr values of 0.707968 and 0.707843, and show a good correlation with the seawater strontium ratio of their depositional ages. A similar feature occurs also for the Corniola sample, with a ^87/86Sr value of 0.707157, whereas Rudist Calcarenites show a ^87/86Sr value of 0.707035, which is depleted compared to the age of this formation. Plotting ^87/86Sr values versus the corresponding ¹⁸O values indicates that thrusting-related dolomite fault-veins have ^87/86Sr values of 0.707707 and 0.707500, which are depleted compared to their host rocks (Fig.

17B). The ^87/86Sr value of the calcite vein CoCalV from Corniola is 0.707397, i.e. enriched compared to the host rock. The ^87/86Sr value from the Monte Camicia fault core is 0.707786.

8.3. Noble gases isotopes data

Results of noble gas analyses are reported as ratios in Table 1. ³He and ⁴He were below instrumental detection limits in all samples. ⁴⁰Ar/³⁶Ar ratios range between 280.2 and 309.8, i.e. typical values of the atmosphere. Fig. 18 shows the measured heavy noble gas ⁸⁴Kr and ¹³²Xe data plotted as a function of ³⁶Ar values. ⁸⁴Kr/³⁶Ar are comprised between 0.0228 and 0.0347 and ¹³²Xe/³⁶Ar show values ranging from 0.00126 to 0.00378.

Almost all samples lie on an ideal mixing trend whose end-members are represented by atmosphere (Air) and air-saturated water (ASW). The bituminous dolostone host rock sample deviates from this trend and shows a higher ¹³²Xe concentration, reaching a

132Xe/³⁶Ar value of 0.01126. Massive dolostone samples provided ⁸⁴Kr/³⁶Ar values always lower than 0.0282, which are closer to the Air end-member. On the other hand, limestone and calcite cement samples have ⁸⁴Kr/³⁶Ar values always higher than 0.0279, closer to the ASW end-member.

9. Discussion

9.1 Host rock diagenetic framework

Figure 16. Oxygen and car-bon stable isotopes data.

(A) d¹⁸O vs. d¹³C plot; axes are in ‰ V-PDB. (B) d¹⁸O vs. structural unit plot; black ticks and lines are average d¹⁸O ‰ and ± 1s values of the seawater for the structu-ral unit host rock age. (C) d¹³C ‰ vs. structural unit plot; black ticks and lines are average d¹³C ‰ and ± 1s values of the seawater for the structural unit host rock age; seawater referen-ce values from Veizer et al., 1999.

Eight samples were selected for strontium isotopes analysis, including two dolostone host rocks (BDHR), two dolomite fault-veins (BDDolFV), two limestone host rocks (RCHR and CoHR), a calcite vein (CoCalV), and a dolomitic-calcareous micritic matrix in the Monte Camicia Thrust fault core (CFDolCalSB). Results are illustrated in Fig.

17A, where host rock strontium isotope ratios are plotted vs. their depositional age (McArthur et al., 2012). Bituminous dolostones display ^87/86Sr values of 0.707968 and 0.707843, and show a good correlation with the seawater strontium ratio of their depositional ages. A similar feature occurs also for the Corniola sample, with a ^87/86Sr value of 0.707157, whereas Rudist Calcarenites show a ^87/86Sr value of 0.707035, which is depleted compared to the age of this formation. Plotting ^87/86Sr values versus the corresponding ¹⁸O values indicates that thrusting-related dolomite fault-veins have ^87/86Sr values of 0.707707 and 0.707500, which are depleted compared to their host rocks (Fig.

17B). The ^87/86Sr value of the calcite vein CoCalV from Corniola is 0.707397, i.e. enriched compared to the host rock. The ^87/86Sr value from the Monte Camicia fault core is 0.707786.

8.3. Noble gases isotopes data

Results of noble gas analyses are reported as ratios in Table 1. ³He and ⁴He were below instrumental detection limits in all samples. ⁴⁰Ar/³⁶Ar ratios range between 280.2 and 309.8, i.e. typical values of the atmosphere. Fig. 18 shows the measured heavy noble gas ⁸⁴Kr and ¹³²Xe data plotted as a function of ³⁶Ar values. ⁸⁴Kr/³⁶Ar are comprised between 0.0228 and 0.0347 and ¹³²Xe/³⁶Ar show values ranging from 0.00126 to 0.00378.

Almost all samples lie on an ideal mixing trend whose end-members are represented by atmosphere (Air) and air-saturated water (ASW). The bituminous dolostone host rock sample deviates from this trend and shows a higher ¹³²Xe concentration, reaching a

132Xe/³⁶Ar value of 0.01126. Massive dolostone samples provided ⁸⁴Kr/³⁶Ar values always lower than 0.0282, which are closer to the Air end-member. On the other hand, limestone and calcite cement samples have ⁸⁴Kr/³⁶Ar values always higher than 0.0279, closer to the ASW end-member.

9. Discussion

9.1 Host rock diagenetic framework

Corniola limestones show ¹⁸O and strontium isotope ratios values in the range of Lower Jurassic seawater and ¹³C values distributed in two clusters, slightly higher and

0,707 0,7072 0,7074 0,7076 0,7078 0,708

-2 -1 0 1 2 3 4

0.7072 0.7074 0.7076 0.7078

0 1

-1

-2 2 3 4

δ¹⁸O ‰

87/86Sr

0.707 0.708 0.7075 0.7085

0.7065

A B

87/86Sr

Triassic Jurassic Cretaceous

Figure 17. (A) Age vs.

87/86Sr plot of host rock sam-ples; black line represents strontium isotopes ratio of seawater. (B) d¹⁸O vs. ^87/86Sr plot; d¹⁸O in ‰ V-PDB. See legend in Fig. 16; seawa-ter reference values from McArthur et al., 2012.

0 0.002 0.004 0.006 0.008 0.01 0.012

0.02 0.025 0.03 0.035 0.04

10

0 8 6 4 2 12

Air

ASW

4

2 2.5 3 3.5

132Xe/36Ar * 10-3

84Kr/³⁶Ar * 10^-2

Bituminous Dol. fault-veins Bituminous Dol. HR CFDolCalSB (shear bands)

VFcore VFCal1 vein Corniola HR CoCal vein

Figure 18. ⁸⁴Kr/³⁶Ar vs.

132Xe/³⁶Ar plot of all analyzed samples; all samples exclu-ding Bituminous Dolostones lie on a mixing trend between Air and ASW (Air Saturated Water).

Eight samples were selected for strontium isotopes analysis, including two dolostone host rocks (BDHR), two dolomite fault-veins (BDDolFV), two limestone host rocks (RCHR and CoHR), a calcite vein (CoCalV), and a dolomitic-calcareous micritic matrix in the Monte Camicia Thrust fault core (CFDolCalSB). Results are illustrated in Fig.

17A, where host rock strontium isotope ratios are plotted vs. their depositional age (McArthur et al., 2012). Bituminous dolostones display ^87/86Sr values of 0.707968 and 0.707843, and show a good correlation with the seawater strontium ratio of their depositional ages. A similar feature occurs also for the Corniola sample, with a ^87/86Sr value of 0.707157, whereas Rudist Calcarenites show a ^87/86Sr value of 0.707035, which is depleted compared to the age of this formation. Plotting ^87/86Sr values versus the corresponding ¹⁸O values indicates that thrusting-related dolomite fault-veins have ^87/86Sr values of 0.707707 and 0.707500, which are depleted compared to their host rocks (Fig.

17B). The ^87/86Sr value of the calcite vein CoCalV from Corniola is 0.707397, i.e. enriched compared to the host rock. The ^87/86Sr value from the Monte Camicia fault core is 0.707786.

8.3. Noble gases isotopes data

Results of noble gas analyses are reported as ratios in Table 1. ³He and ⁴He were below instrumental detection limits in all samples. ⁴⁰Ar/³⁶Ar ratios range between 280.2 and 309.8, i.e. typical values of the atmosphere. Fig. 18 shows the measured heavy noble gas ⁸⁴Kr and ¹³²Xe data plotted as a function of ³⁶Ar values. ⁸⁴Kr/³⁶Ar are comprised between 0.0228 and 0.0347 and ¹³²Xe/³⁶Ar show values ranging from 0.00126 to 0.00378.

Almost all samples lie on an ideal mixing trend whose end-members are represented by atmosphere (Air) and air-saturated water (ASW). The bituminous dolostone host rock sample deviates from this trend and shows a higher ¹³²Xe concentration, reaching a

132Xe/³⁶Ar value of 0.01126. Massive dolostone samples provided ⁸⁴Kr/³⁶Ar values always lower than 0.0282, which are closer to the Air end-member. On the other hand, limestone and calcite cement samples have ⁸⁴Kr/³⁶Ar values always higher than 0.0279, closer to the ASW end-member.

9. Discussion

9.1 Host rock diagenetic framework

Corniola limestones show ¹⁸O and strontium isotope ratios values in the range of Lower Jurassic seawater and ¹³C values distributed in two clusters, slightly higher and

Table 1: Noble gases isotopes ratios of analyzed samples and reference ratios of natural reservoirs; Air-atmosphere; ASW-air saturated water Sample 3He/4He ± R/Ra± *21Ne/22Ne ± 40Ar/36Ar ± 84Kr/36Ar ± 132Xe/36Ar ± CFCalDolSB1,13E-061,43E-066,58E-018,33E-014,50E-024,36E-033,00E+02 1,45E+00 2,79E-021,51E-043,78E-033,28E-05 CFCalDolSB5,46E-071,55E-063,18E-019,03E-012,82E-024,89E-033,00E+02 2,13E+00 2,29E-021,89E-041,40E-032,44E-05 CFCalDolSB1,47E-071,22E-068,53E-027,08E-013,44E-026,00E-032,98E+02 6,52E-012,60E-027,95E-051,34E-031,24E-05 CFCalDolSB-5,29E-07-1,24E-06-3,07E-01-7,19E-012,31E-022,63E-023,09E+02 5,44E+00 3,06E-025,72E-042,15E-035,86E-05 Bit.Dol.HR8,50E-071,11E-054,94E-016,45E+00 4,48E-023,66E-023,10E+02 3,17E+00 2,82E-023,15E-041,13E-021,41E-04 Bit.Dol.V-9,43E-06-4,77E-05-5,48E+00 -2,77E+01 3,50E-026,43E-033,02E+02 1,41E+00 2,36E-021,26E-041,23E-031,78E-05 Bit.Dol.V5,14E-064,36E-062,99E+00 2,53E+00 3,66E-023,90E-033,00E+02 1,13E+00 2,53E-021,11E-041,98E-031,99E-05 VFCore7,54E-055,99E-054,38E+01 3,48E+01 3,00E-023,45E-032,87E-039,45E-022,58E-026,03E-042,56E-037,54E-05 VFCal1-5,88E-06-2,62E-05-3,42E+00 -1,53E+01 2,14E-022,59E-032,98E+02 3,42E+00 3,21E-023,88E-042,07E-033,58E-05 VFCal23,06E-061,08E-051,78E+00 6,27E+00 2,91E-023,72E-03- - - - - - CoHR4,38E-063,98E-062,55E+00 2,31E+00 3,69E-025,71E-032,96E+02 1,15E+01 2,98E-021,18E-033,08E-031,39E-04 CoCalV-1,09E-06-1,28E-02-6,36E-01-7,45E+03 3,19E-023,15E-033,06E+02 3,09E-013,47E-021,49E-042,52E-032,61E-05 Reference Ratios Air 1 0,0291298,5 0,0210,000784 ASW (25°C)0,02910,0370,002340 Mantle8 < 0.0291 Crust 0.01-0.05> 0.0291>298.5

lower than seawater values, respectively (Figs. 16A-B-C, 17A). This indicates that bacterially-mediated processes occurred during early-diagenesis in the bioclastic-rich Corniola Fm (Hoefs, 1997).

Bituminous Dolostones in the thrust sheet bounded by the Vado di Ferruccio and Monte Camicia thrust faults, respectively, have ¹⁸O values enriched by +2 to +3.5 ‰ compared to the Upper Triassic seawater ratio (Fig. 16B). ¹⁸O fractionation of +3 to +4 ‰ in dolomite compared to calcite at low temperature (Land, 1980), supports the hypothesis that seawater was likely the main fluid involved in early-diagenetic dolomite crystallization, despite recrystallization of the dolomite host during burial can’t be excluded based on our stable isotopes data (Machel, 2004). Samples from site PF1, characterized by heavier

¹⁸O values, reasonably interacted with slightly evaporated seawater during early-diagenesis. This inference is supported by ¹³C values and also by the strontium isotope ratios (Figs. 16C-D, 17A). The high ¹³²Xe/³⁶Ar ratio is likely due to preferential adsorption of ¹³²Xe by organic matter (Fig. 18, Hohenberg et al., 2002).

Rudist Calcarenites in the hanging wall of the Monte Camicia thrust show ¹⁸O and

¹³C values that correspond to the mean ranges of the Lower Cretaceous seawater stable isotopic composition (Fig. 16B-C). However, their strontium isotopes ratio does not fit with this age (Fig. 17A). This feature could be due to the detrital origin of the Rudist Calcarenites.