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Isotope analysis of fluid inclusions de Graaf, S.
2018
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citation for published version (APA) de Graaf, S. (2018). Isotope analysis of fluid inclusions: Into subsurface fluid flow and coral calcification.
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Download date: 01. Oct. 2021 Chapter Fluid flow evolution in the Albanide fold- 3 and-thrust belt
ectonic forces generated during thrust emplacement along active margins may drive complex fluid flow patterns in fold-and-thrust belts and foreland basins. In the Albanide fold-and-thrust belt, fracture-controlled fluid flow Tled to the development of calcite vein networks in a sequence of naturally fractured Cretaceous to Eocene carbonate rocks. Fluid inclusion isotope data of these calcite veins demonstrate that fluids circulating in the carbonates were derived from an underlying reservoir that consisted of a mixture of meteoric water and evolved marine fluids, probably sourced from deep-seated evaporites. The meteoric fluids infiltrated in the hinterland before being driven outward into the foreland basin and ascended as soon as fracturing induced a sufficient increase in permeability. The contribution of fluids derived from evaporites increases towards the thrust front in association with elevated deformation, which is expected to be the main driver of expulsing connate waters from the evaporites. Structural and petrographic observations provide time constraints for the various phases of fracture infilling and reveal an increasing dominance of meteoric water in the system through time as migration pathways shortened and marine formation fluids were progressively flushed out. Similar fluid flow evolutions have previously been recorded in various fold-and-thrust belt settings elsewhere in the world.
Based on: De Graaf, S., Nooitgedacht, C. W., Le Goff, J., Van der Lubbe, H. J. L., Vonhof, H. B., and Reijmer, J. J. G. (accepted for publication in AAPG Bulletin) Fluid flow evolution in the Albanide fold-and-thrust belt: Insights from δ2H and δ18O isotope ratios of fluid inclusions Nooitgedacht, C. W., De Graaf, S., Van der Lubbe, H. J. L., Vonhof, H. B., Le Goff, J., and Reijmer, J. J. G. (in prep.) Tracking fluid migration in the external Albanides using hydrogen and oxygen isotopes of fluid inclusions in calcite veins 46 Chapter 3
3.1 Introduction Fold-and-thrust tectonics is commonplace along convergent plate margins and is associated with the development of intricate fracture and fault networks (Roure and Sassi, 1995). The structural complexity of fold-and-thrust belts opens up a wide array of conceivable fluid flow patterns (Travéet al., 2007; Morley et al., 2011; Lacroix et al., 2014). Tectonic loading due to thrust emplacement, for instance, may instigate basin-ward expulsion of fluids along large-scale migration pathways like faults or basement detachments (Oliver, 1986; Ge and Garven, 1992). The timing of fluid flow and circulating fluid types may differ considerably between foreland fold-and-thrust belts depending on their stratigraphic architecture and structural evolution (Ferket et al., 2003; Schneider et al., 2004; Barbier et al., 2012; Crognier et al., 2018). Predicting the evolution and interplay of deformation and fluid flow in foreland fold-and-thrust belts remains complicated, while it is of key importance when assessing the potential existence of aquifers or petroleum-bearing reservoirs (Agosta et al., 2010; Morley et al., 2011). Carbonate rock formations are of particular interest since the permeability of lithified limestone is generally governed by brittle deformation structures (i.e., fracture and faults) due to limited matrix porosities (Nelson, 2001). In this Chapter, fluid inclusion isotope data of calcite veins are used to delineate the fluid flow system of the Albanide foreland fold-and-thrust belt. Prominent calcite veining in the Albanide fold-and-thrust belt exists in fracture networks in Cretaceous to Eocene Ionian Basin carbonates, which are the main reservoir rocks in the area (Velaj et al., 1999) and an analogue to producing reservoirs in the Adriatic Sea and onshore Italy (Cazzola and Soudet, 1993; Zappaterra, 1994). The carbonate rocks are mainly composed of calciclastic gravity flows and mass transport deposits (Rubertet al., 2012; Le Goffet al., 2015). Events of fracturing provided discrete pathways facilitating the episodic migration of fluids and governing the emplacement of abundant hydrocarbon accumulations in the area (Graham Wall et al., 2006). Previous vein-based fluid flow reconstructions in the Ionian Zone focused on the areas of Kremenara (Van Geet et al., 2002; Swennen et al., 2003), Shpiragu (Graham Wall et al., 2006), Kelçyrë (Vilasi et al., 2009) and Saranda (Lacombe et al., 2009). These previous researches inferred multiple fluid flow episodes throughout the evolution of the foreland fold-and- thrust belt. Although the sequence of fluid flow and vein cementation events has quite elaborately been studied, the precise origin and migration pathways of fluids requires further constraints, especially for the early-stage fluid system. Excellent outcrops of Cretaceous to Eocene carbonate rocks are present in the Mali Gjere mountain ridge, which is a topographic expression of the Kurveleshi thrust sheet in the Ionian Zone of south Albania (Figure 3.1). In addition to studying veins from the Mali Gjere, calcite veins from a set of outcrops from the two other main thrust units in the Ionian Zone – the Çika and the Berati belt – were analyzed Fluid flow evolution in the Albanide fold-and-thrust belt 47
A B Berati BeltB Montenegro Albanian alps Kosovo
Mirdita Zone Mashkullorë Korabi 15 N Tirana Macedonia
Pre-Adriatic Depression Pre-Adriatic Gjirokastër
Mali Gjere
Korça Basin Kurveleshi Belt
Kruja Zone 20 Krasta Zone Vanister 40˚ Ionian 00’ Berati belt 18 Çika belt Sazani Zone Zone Terihat Kurveleshi belt 20 Frashtan Research area Greece Muzinë Çika Belt 22 Jorgucat Greece 41 Legend Quaternary Bodrishtë A Sarandë 14 Neogene
Paleocene Ionian Sea Cretaceous 39˚ 50’ Jurassic 10 km Triassic 20˚ 00’ 20˚ 15’
C SW Çika belt Kurveleshi belt Berati belt NE A Mali Gjere B
Jura Flysch ssic-Cr and m etace olass ous c e cove arbon r Delvina oil field ates an Triassic evaporites d shales 1 km 4 km
Figure 3.1 (A) Map showing the main structural units of Albania (Moisiu and Gurabardhi, 2004) with the research area indicated by the black square. (B) Geological map of southern Albania after Moisiu and Gurabardhi (2004). Seven outcrop locations (Mashkullorë, Vanister, Terihat, Frashtan, Muzinë, Jorgucat and Bodrishtë) were studied along the Mali Gjere mountain ridge, which exposes Cretaceous to Eocene carbonate rocks within the Kurveleshi thrust sheet. Red lines depict faults. (C) Simplified geological cross-section through southern Albania after Velaj (2015), indicated by the line A-B in the geological map.
to assess spatial variations in the fluid flow evolution throughout the entire fold-and- thrust belt. Besides fluid inclusion isotope analysis, complementary petrographic and δ18O and δ13C isotope analyses of the calcite vein cements were carried out. An analysis of the arrangement and cross-cutting relations of distinct structural sets was performed to acquire a chronological framework for the various generations of fracture-infilling calcite. 48 Chapter 3
3.2 Background 3.2.1 Geological evolution of the Ionian Zone The Ionian Zone is one of the structural domains that makes up the Al- banides (Figure 3.1) and refers to an assembly of folds and thrusts of Ionian Basin deposits situated in the southwest of Albania (Meço and Aliaj, 2000; Robertson and Shallo, 2000). Deposition in the Ionian Basin was closely linked to the structural evolution of the Adriatic plate (Channell et al., 1979). In Triassic times, at least 2000 m of evaporites and shallow-water carbonates were deposited in an epicontinental marine environment (Vlahović et al., 2005; Karakitsios, 2013). Subsequent exten- sional tectonics within the Adriatic plate from the Late Triassic onward led to the development of rift basins in a horst and graben system. The Ionian Basin was such an intracontinental rift basin and formed between the Apulian and Kruja carbonate platforms (Robertson and Shallo, 2000; Heba and Prichonnet, 2009). The syn-rift sequence in the Ionian Basin from the Jurassic until the Oligocene is characterized by sediment input from the bordering carbonate platforms (Hairabian et al., 2015; Le Goffet al., 2015). The sequence of Cretaceous to Eocene carbonate gravity flows that is presently exposed along the Mali Gjere mountain ridge exhibits a thickness of 900-1350 m (Velaj, 2015) and is mainly composed of decimeter to meter-scale strata of fine-grained mud- to wackestones with minor amounts (< 2%) of planktic foraminifera. Uplift of the hinterland provoked by the Alpine Orogeny led to an increasing input of terrigenous material into the basin and the deposition of up to 2000 m of Oligocene flysch, Neogene molasse, and Quaternary sand- and siltstones on top of the Ionian Basin carbonates (Meço and Aliaj, 2000). The position of the Ionian Zone in the southern extremity of the Alpine orogenic belt was crucial for its structural development from the Oligocene onward (Robertson and Shallo, 2000; Nieuwland et al., 2001). Southwest-verging fold-and- thrust tectonics involving an intricate network of structural elements (e.g., ramp anticlines, back-thrusts, strike-slip faults and salt diapirs) caused a regional uplift and the formation of three main NNW-striking anticlinal thrust sheets: the Çika, Kurveleshi and Berati belt. Triassic evaporites and to lesser extent Early Jurassic Posidonia shales act as decollement levels facilitating thin-skinned thrusting (Velaj, 2002; Swennen et al., 2003). The Ionian Zone is bordered by the Vlora-Elbasan lineament in the north and extends towards the south into the Hellenides (Neumann and Zacher, 2004; Karakitsios, 2013).
3.2.2 Fluid flow in the Cretaceous to Eocene carbonates Primary porosity of the Cretaceous to Eocene carbonate rocks is low (0-1%) due to compaction and calcite pore infill during early marine diagenesis (Van Geetet al., 2002; Vilasi et al., 2006; Dewever et al., 2007). Multiple events of brittle deforma- tion induced episodic increases in rock permeability and circulation of fluids in the Fluid flow evolution in the Albanide fold-and-thrust belt 49
Cretaceous to Eocene carbonates (Graham Wall et al., 2006). Episodes of fluid flow occurred before and during the main phase of folding and thrusting (Swennen et al., 2000). Fluids were overpressured and buffered by the host-rock during the early pre-folding fracture infilling events (Van Geet et al., 2002). Meteoric fluids played an important role in vein cementation events during the late stage, which is also associated with the development of karst networks (Vilasi et al., 2006). Evidence for leaching of evaporites indicates that fluids were supplied along deep-seated decolle- ment levels (Van Geet et al., 2002; Vilasi et al., 2009). Although burial histories may have varied throughout the entire Ionian Basin, peak burial of the carbonates is estimated to have been reached at depths of 2-4 km and temperatures of 80-100˚C in Early Miocene times (Rigakis and Karakitsios, 1998; Roure et al., 2004; Lacombe et al., 2009; Vilasi et al., 2009). Brittle deformation structures provided reservoir qualities to the Cretaceous to Eocene carbonates (Velaj et al., 1999). Hydrocarbon charge in the Late Miocene and Pliocene was derived from underlying Triassic to Jurassic shales and filled structural traps in strongly deformed parts in the north of the Ionian Zone (Zappaterra, 1994; Prifti and Muska, 2013). The Oligocene flysch deposits that overlie the carbonate reservoir rocks act as an effective seal (Velaj, 2015). Although the sealing cover has largely been removed in the central part of the Ionian Zone, hydrocarbon accumulations may still occur in structural traps associated with the impermeable Triassic evaporites (Vilasi et al., 2009). The Ionian Zone is renowned for hosting the most important petroleum accumulations in Albania (Curi, 1993; Velaj, 2015).
3.3 Methodology An evaluation of the structural development of the naturally fractured Cretaceous to Eocene carbonate rocks from the Mali Gjere may allow for establishing chronological relationships of distinct generations of calcite fracture infill and tracking the hydrogeological system through time. Orientations of fractures and stylolites were measured in seven distinct outcrops (Figure 3.1B; Table 3.1) using an iPad equipped with GeoID (Lee, 2014), a professional application for measuring geological structures. Fractures were subdivided in barren ones (joints) and infill-bearing ones (veins), with the extent of fracture infill being documented for the veins (i.e., infill width). The studied outcrops are all part of the upper part of the sequence of Cretaceous to Eocene calciturbidites (i.e., Upper Cretaceous to Eocene). An outcropping pavement or vertical section of typically 30-100 m2 (depending on structure density) was delimited and the structures within it were documented. Measurements were plotted in equal-area lower hemisphere projections using OSXStereonet (Cardozo and Allmendinger, 2013) to discern different structural sets. Abutment relationships and physical intersections between structural sets, referred to as cross-cutting relationships, were documented to determine relative time constraints for the structural sets. Thirty-four hand samples of veins were retrieved from the same outcrops studied in the structural analysis. Collected vein samples cover the various phases of 50 Chapter 3 fracture infilling recognized in the Mali Gjere. Isotope analyses of calcite vein cements and fluid inclusion water were performed following the analytical protocols as described in Chapter 1. Apart from the isotope analyses, thin sections were produced from 32 vein samples for petrographic analysis of vein textures, fluid inclusion assemblages and cross-cutting relationships. The petrographic study was performed using a polarizing microscope (Nikon Polarizing Microscope Eclipse 50i POL) and a confocal laser scanning microscope (Olympus Confocal Microscope - FV3000).
Table 3.1 Geographic coordinates and bedding orientations (dip direction/dip angle) per outcrop location (see Figure 3.1B)
Outcrop Coordinates Bedding Bodrishtë 39°53'57.93"N 20°18'24.56"E 013/14 Jorgucat 39°55'49.92"N 20°16'18.84"E 068/41 Muzinë 39°56'39.72"N 20°13'53.26"E 059/22 Frashtan 39°58'16.99"N 20°14'1.52"E 055/20 Terihat 39˚59'25.84"N 20˚12'58.35"E 064/18 Vanister 40°0'50.97"N 20°11'38.05"E 036/20 Mashkullorë 40°6'58.54"N 20°5'27.59"E 037/15
SSE NNW
SSE NNW
064/18
20 m
Figure 3.2 Interpretation of fracture patterns in a gently dipping pavement in the Mali Gjere (39˚59’19.76”N, 20˚13’4.84”E). Recent dissolution activity along fracture planes enhanced the visibility of their traces, which may be continuous over tens of meters. Two systematic sets predominate in this particular outcrop: a pervasive WNW-ESE pre-folding set (blue) and a set of NE-SW syn-folding fractures (black). Measured fracture orientations are presented in the lower hemisphere stereonet projection with thick lines representing averages for the pre-folding (blue) and syn-folding (black) set. Fluid flow evolution in the Albanide fold-and-thrust belt 51
3.4 Results 3.4.1 Structural elements Bedding in the Mali Gjere is consistently northeast dipping (Table 3.1) and devoid of major faulting and folding (Figure 3.1B). Across the seven studied outcrops, the orientations of in total 362 structural elements were measured. Orientations of fractures exhibit a high degree of systematics (Figure 3.2) and are organized in four recurring sets (Figure 3.3). An extensive network of WNW-ESE oriented opening- mode fractures is most prominent and recognized in every outcrop. Substantial calcite fracture infill of up to 2 cm thick occurs in 82% of the fractures that belong to this structural population. In the outcrops of Bodrishtë, Terihat and Vanister, another fracture set with similar infill characteristics is present at a slightly different angle (i.e., more E-W orientation). A third set of opening-mode fractures is oriented NNE-SSW, which is approximately perpendicular to the WNW-ESE fracture set. Although NNE-SSW fracturing is less pervasive, it is recognized in all outcrops except Terihat. Calcite infill occurs in 75% of the NNE-SSW fractures and is usually thinner (< 0.5 cm) compared to the WNW-ESE fractures. Fractures of the WNW-ESE, E-W and NNE- SSW sets display an orthogonal relationship to the bedding. A fourth deformational event recognized in the Mali Gjere is characterized by sub-vertical NE-SW oriented
Bodrishtë Jorgucat Muzinë Frashtan n = 54 n = 41 n = 10 n = 38
Terihat Vanister Mashkullorë Stylolite (plane) n = 46 n = 84 n = 75 Pole to fracture plane
Fracture sets
Syn-folding
Pre-folding 2
Pre-folding 1
Figure 3.3 Orientations of fracture planes (poles) and tectonic stylolitic surfaces (dashed great circles) presented in contoured lower hemisphere stereonet projections for the seven outcrops studied. Squares represent averages of opening-mode fracture sets; different colors correspond to the distinct structural sets. Interpretation of structural sets is based on structure orientations and cross-cutting relationships. Bedding-parallel burial stylolites are plentiful in all outcrops, but not included in the stereonet projections for clarity. Structures that are identified to be pre-folding were rotated back to their original position at the time of development (i.e., before inclination of bedding). 52 Chapter 3 fractures. Fractures related to this stress field were not identified in every outcrop and are predominantly barren; calcite infill occurs in 37% of the fractures and is typically thin (< 0.2 cm). Bedding-parallel stylolites are abundant in all outcrops. Besides, two sets of tectonic stylolites were recorded. One set strikes approximately NNW-SSE and displays a perpendicular relationship to the surface (Figure 3.4D). This set is well- defined and documented in all outcrops except Muzinë, being especially pervasive in the outcrops of Jorgucat and Mashkullorë. A second set of tectonic stylolites is oriented NE-SW and only observed in the outcrops of Mashkullorë and Bodrishtë. Across the outcrops, 33 cross-cutting relations between the various structural sets were recorded, in addition to countless relationships involving burial stylolites (Figure 3.4). WNW-ESE and E-W fractures are systematically cross-cut by the other two fracture sets. The major portion of these fractures (70%) post-date burial stylolites (Figure 3.4C), whereas fractures of the NNE-SSW set display a more ambiguous relationship with burial stylolites. NE-SW fractures systematically cut through the WNW-ESE, E-W and NNE-SSW fracture sets and post-date burial stylolites. Cross- cutting relations between fractures and tectonic stylolites are scarce. Only the outcrops of Mashkullorë and Terihat present clear evidence of veins of both the WNW-ESE and NNE-SSW set being dissolved by – and thus predate – NNW-SSE oriented tectonic stylolites.
A B WNW-ESE
228/67
273/69 NNE-SSW 147/86
C D
Figure 3.4 Compilation of small-scale structures observed in the field. (A) Cross-cutting relationships of three sets of pure-opening mode veins in Frashtan. (B) A WNW-ESE oriented vein that is cut by an NNE-SSW oriented vein in Bodrishtë. (C) A WNW-ESE oriented vein that is dissolved by a bedding- parallel burial stylolite in Vanister. (D) An NNW-SSE striking tectonic stylolite that is oriented vertically with respect to the present-day surface in Bodrishtë. Fluid flow evolution in the Albanide fold-and-thrust belt 53
A B C
1 mm 1 mm 1 mm
D E F
1 mm 0.5 mm 1 mm G H I
1 mm 1 mm 0.5 mm
Figure 3.5 Overview of photomicrographs. (A) Blocky calcite vein cement in a fracture from Muzinë (MU1). (B) Blocky calcite vein cement in a fracture from Muzinë (MU6). The vein is consumed on one side by a tectonic stylolite. (C) Disordered calcite cement in a fracture from Terihat (TE1), possibly due to recrystallization. (D) A rim of small crystals (<< 0.1 mm) at the walls of a vein from Vanister (VA2; XPL image). The vein displays opening along two distinct bands. (E) High-resolution banding in a vein from Vanister (VA3). (F) Elongated crystals growing inward perpendicular to the walls of a vein from Vanister (VA8). (G) Brecciated shards of host rock in a calcite vein from Muzinë (MU2). (H) Blackish aspect of a vein from Terihat (Te6 and Te7). This vein developed in a chert concretion. (I) Growth bands within a calcite crystal in a vein from Bodrishtë (BO6.3).
3.4.2 Vein petrography Mineral infill in fractures related to the three main deformational phases in the Mali Gjere is uniquely composed of calcite. Bitumen staining, although common in fractures in the north of the Kurveleshi belt (Graham Wall et al., 2006), is not observed in fractures from the Mali Gjere. Calcite fracture infill is typically blocky (Figure 3.5A-B) with well-defined growth zones (Figure 3.5I), while the infill of a minor number of samples exhibits a syntaxial texture (Figure 3.5F). A coarsening inward trend is common with rims of small crystals (<< 0.1 mm) at the vein walls (Figure 3.5D). More disorganized fine-grained infill characterized by poorly defined crystals makes up sample Te1 (Figure 3.5C) and is encountered in minor quantities 54 Chapter 3
(< 20%) in two other veins (Mu4 and Fr2). The vein from which Te6 and Te7 were sampled is hosted in a chert concretion and displays a blackish color with poorly defined crystals when studied under a polarizing microscope (Figure 3.5H). Deformational calcite twinning and banded textures are common in veins from all generations. Banding may be minor (Figure 3.5D) or highly frequent consisting of innumerable generations separated by thin bands of host rock (Figure 3.5E). Vein walls are in general matching (Figure 3.5A, 3.5C and 3.5D). Clear indications of shear were not observed in the infill of any of the fracture generations. Brecciated textures with shards of host rock dispersed in the calcite fracture infill occur in three WNW-ESE and one NNE-SSW vein (Figure 3.5G). Stylolitic surfaces developed along the walls of three E-W to WNW-ESE oriented veins (Figure 3.5B). Veins retrieved from the Mali Gjere are dominated by primary fluid inclusions. Primary fluid inclusions are typically small (< 2m m) and mostly occur as isolated inclusions in clusters within single crystals (Figure 3.6A-B) or organized along growth bands (Figure 3.5I). Although primary fluid inclusions are mostly monophase, larger inclusions may be two-phase with a low vapor-liquid ratio (< 0.1; Figure 3.6A). Primary fluid inclusions exhibit an equidimensional rounded or rectangular shape and do not display signs of post-trapping modifications (e.g., decrepitation or leakage). Secondary monophase fluid inclusions, which are arranged along trails that are unrelated to the crystal growth faces (Shelton and Orville, 1980; Smith and Evans, 1984; Bodnar, 2003), can reach a size of up to 10 mm and are less common than the primary ones.
A B
z = 5-6 μm
Gas phase
z = 6-11 μm 10 μm C
z = 10 μm
z = 7-9 μm z = 4-12 μm
50 μm 0.5 cm
Figure 3.6 Petrographic appearance of fluid inclusions hosted in calcite veins from the Mali Gjere. (A) Compiled image of a 3D confocal surface scanning analysis (up to 14 mm depth) of vein Va2 showing the distribution of primary fluid inclusions. Some of the larger inclusions display a minor gas phase (see inset). The scanning depth (z) is given for the different panels in the image. (B) Confocal image of all-liquid primary fluid inclusions in Bo6. (C) Petrographic view of secondary fluid inclusion trails (next to the red lines) in vein Te3. Fluid flow evolution in the Albanide fold-and-thrust belt 55
3.4.3 Isotope data Fluid inclusion isotope data were obtained for 26 calcite vein samples. The 18 2 δ Ow and δ Hw isotope signatures hold a significant positive correlation (Figure 3.7) 18 2 and range from -7.8 to 1.1‰ for δ Ow and -56.0 to -14.7‰ for δ Hw (Table 3.2). Isotope data plot either onto or to the right of the Global Meteoric Water Line (GMWL), which expresses the isotope fractionation of terrestrial meteoric waters as a global average (Craig, 1961). Veins belonging to the WNW-ESE, E-W and NNE-SSW fracture sets display higher isotope values than veins hosted by NE-SW fractures. Complementary carbon and oxygen isotope data were obtained for the calcite of 36 vein samples, as well as for five host rock samples. Average values of the measurements are presented in Table 3.2. Acquired isotope values of the veins 18 13 range from -6.5 to 0.9‰ for δ Oc and from -3.0 to 3.0‰ for δ Cc (Figure 3.8). Host rock samples display consistent isotope values throughout the various outcrops 18 13 (-2.3 to -1.5‰ for δ Oc and from 1.1 to 2.7‰ for δ Cc). Carbon isotope values of the veins closely resemble those of the host rock, apart from a number of samples from Frashtan, Bodrishtë and Mashkullorë, which display a depletion in 13C. Veins are mostly depleted in 18O with respect to the host rock (Figure 3.8). Especially veins 18 from the Frashtan outcrop display remarkably low δ Oc values.