FAILURE MODES IN BASIN CARBONATES; MAJELLA MOUNTAIN, CENTRAL APENNINES, ITALY Marco Antonellini (1) *, Emanuele Tondi (2) , Fabrizio Agosta (1) , Atilla Aydin (1) , and Giuseppe Cello (2) (1) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA (2) Department of Earth Sciences, University of Camerino, 62032 Camerino MC, Italy *Corresponding author e-mail: [email protected]Abstract We document different modes of failure and the hierarchy of the fault development, in the slope-basinal sequence of marls and carbonate grainstones in the eastern part of the Majella Mountain, Central Apennines, Italy. In low-porosity breccia/grainstone units, the early formation of pressure solution seams (pss) and their subsequent shearing are the basic structures leading to fault zone development. In the micrite/marly units, sheared pss may form splays in closing mode (pss) or opening mode fractures (joints) and their subsequent higher order shearing results in faults development. In the high porosity grainstones, the earliest structures are deformation bands of both shearing and compaction modes. However, the later deformation involves pressure solution and the subsequent shearing of the pressure solution seams leading to cataclastic shear zones. We conclude, therefore, that fault zone architecture depends not only on the amount of slip, but also on lithology and rheology, which may have important implications for fluid flow properties of the faults in different carbonate reservoirs. Introduction Brittle deformation modes in carbonate rocks are quite variable. In some lithotypes, like dolomites, fault initiation and evolution are associated with opening mode fractures such as joints and veins (Mollema and Antonellini, 1999). However, because carbonate rocks are prone to dissolution under common geological loading conditions (Rawnsley et al. 1992), more often faulting initiates from and develops by pressure solution and the subsequent shearing of solution surfaces (referred to as pressure solution seams or in short pss in this paper) (Alvarez 1978, Rispoli 1981, Peacock and Sanderson 1995, Petit and Mattauer 1995, Willemse 1997, Graham et al. 2003). As a result, different generations of pss affect deformed carbonate rocks. Despite their origin as closing mode structures, pressure solution seams often represent conduits for water and oil (Graham et al. in press) and, therefore, are as important as open fracture networks for assessing the fluid flow properties of a reservoir. Geologic setting Majella Mountain is a thrust-related anticline composed of a carbonates sequence of Lower Cretaceous to Miocene age, and covered by silicoclastic sediments of Upper Miocene-Middle Pliocene age (Fig. 1). According to Ghisetti & Vezzani, (2002) and Scisciani et al., (2002), the development of the Majella thrust-related anticline occurred in the Middle-Upper Pliocene time. The study area is located near the village of Pennapiedimonte in the Tre Grotte Valley of the Avello Creek (Fig. 1). For a full acount of the regional and local geology refer to RFP volume 16b (2005). The Avello Creek gorge provides many vertical outcrops and a few pavement exposures in a sequence of Cretaceous and Tertiary slope-basinal carbonate rocks. Systematic structural analysis of these outcrops allowed us to reconstruct the 3D geometry of the exposed structures (Fig. 2). In the following section, we will briefly describe the lithologic characteristics of the rocks exposed in the Avello Creek, and their fracturing mechanisms leading to fault development. Fig. 1. Index map (a), stratigraphic column and cross-section of Majella Mt. Stanford Rock Fracture Project Vol. 17, 2006 PC-1
Transcript
FAILURE MODES IN BASIN CARBONATES; MAJELLA MOUNTAIN, CENTRAL APENNINES, ITALY
Marco Antonellini(1)*, Emanuele Tondi(2), Fabrizio Agosta(1), Atilla Aydin(1), and Giuseppe Cello(2)
(1) Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
(2) Department of Earth Sciences, University of Camerino, 62032 Camerino MC, Italy
Abstract We document different modes of failure and the
hierarchy of the fault development, in the slope-basinal sequence of marls and carbonate grainstones in the eastern part of the Majella Mountain, Central Apennines, Italy. In low-porosity breccia/grainstone units, the early formation of pressure solution seams (pss) and their subsequent shearing are the basic structures leading to fault zone development. In the micrite/marly units, sheared pss may form splays in closing mode (pss) or opening mode fractures (joints) and their subsequent higher order shearing results in faults development. In the high porosity grainstones, the earliest structures are deformation bands of both shearing and compaction modes. However, the later deformation involves pressure solution and the subsequent shearing of the pressure solution seams leading to cataclastic shear zones.
We conclude, therefore, that fault zone architecture depends not only on the amount of slip, but also on lithology and rheology, which may have important implications for fluid flow properties of the faults in different carbonate reservoirs.
Introduction Brittle deformation modes in carbonate rocks are
quite variable. In some lithotypes, like dolomites, fault initiation and evolution are associated with opening mode fractures such as joints and veins (Mollema and Antonellini, 1999). However, because carbonate rocks are prone to dissolution under common geological loading conditions (Rawnsley et al. 1992), more often faulting initiates from and develops by pressure solution and the subsequent shearing of solution surfaces (referred to as pressure solution seams or in short pss in this paper) (Alvarez 1978, Rispoli 1981, Peacock and Sanderson 1995, Petit and Mattauer 1995, Willemse 1997, Graham et al. 2003). As a result, different generations of pss affect deformed carbonate rocks. Despite their origin as closing mode structures, pressure solution seams often represent conduits for water and oil (Graham et al. in press) and, therefore, are
as important as open fracture networks for assessing the fluid flow properties of a reservoir.
Geologic setting Majella Mountain is a thrust-related anticline
composed of a carbonates sequence of Lower Cretaceous to Miocene age, and covered by silicoclastic sediments of Upper Miocene-Middle Pliocene age (Fig. 1). According to Ghisetti & Vezzani, (2002) and Scisciani et al., (2002), the development of the Majella thrust-related anticline occurred in the Middle-Upper Pliocene time. The study area is located near the village of Pennapiedimonte in the Tre Grotte Valley of the Avello Creek (Fig. 1). For a full acount of the regional and local geology refer to RFP volume 16b (2005).
The Avello Creek gorge provides many vertical outcrops and a few pavement exposures in a sequence of Cretaceous and Tertiary slope-basinal carbonate rocks. Systematic structural analysis of these outcrops allowed us to reconstruct the 3D geometry of the exposed structures (Fig. 2). In the following section, we will briefly describe the lithologic characteristics of the rocks exposed in the Avello Creek, and their fracturing mechanisms leading to fault development.
Fig. 1. Index map (a), stratigraphic column and cross-section of Majella Mt.
Fig. 2. WNW-ESE geologic cross section along the Tre Grotte valley that presents the formation exposed, the orientation of structures in the different lithologic units and their frequency. The average porosity of the different lithologic units is also reported.
Lithology and deformation The geological units exposed (westward down-
sequence) in the study area include: (i) the well-cemented bioclastic Miocene Bolognano Formation (about 200-300 m thick); (ii) the tertiary thinly bedded turbidites of the Upper Cretaceous Santo Spirito Formation (about 200-300 m thick), (iii) the poorly-cemented bioclastic Upper Cretaceous Orfento Formation (about 50-180 m thick); (iv) the well-bedded micritic-marly Lower Cretaceous Tre Grotte Formation (about 700 m thick) (Figs. 1 and 2). The complete stratigraphic sequence of the Majella Mountain exposed in the study area is shown in the cross section (Fig. 2) along the Tre Grotte Valley. This section defines the overall structure of the area, integrated with the distribution and orientation of lower-rank structural features. These include: (i) pressure solution seams, sometimes have columns and an orange or reddish insoluble residue on their surfaces; (ii) joints, which are characterized by the typical surface morphology such as hackle marks and hesitation lines (Pollard and Aydin 1988) and/or by the presence of dark manganese
dendrites on their surfaces; (iii) slip surfaces with striations and detectable offsets. A summary of the brittle structures observed in the Bolognano, Santo Spirito, Orfento, and Tre Grotte Formations is also reported in Figure 2.
We here report the main lithologic properties of the basinal carbonates, their modes of deformation, the exposed structure types, and the sequence of deformation events in the study area. We also present the spatial density of the structures, and the mean value of fracture spacing for bed thickness that are representative of each particular lithofacies.
3.1 Bolognano Formation 3.1.1Lithology and Petrophysics
The Miocene Bolognano Formation consists of shallow-water carbonates interfingering with marls and marly limestones (Crescenti, 1969; Mutti et al., 1997; Vecsei and Sanders, 1999, Pomar et al. 2004). A typical sequence in this formation is composed of a lower shallow-water limestone interval and a marly, deep-water, upper interval. The shallow-water intervals are composed of heterozoan-dominated bioclastic
Stanford Rock Fracture Project Vol. 17, 2006 PC-2
Bolognano quarry - PSS normal to bed and normal to bed strike-spacing frequency distribution
Fig. 3. Fractures in Bolognano formation. (a) Quarry outcrop where normal and parallel to bed strike pss are exposed. (b) Small thrust in cross section. (c) Mechanical layers thickness frequency distribution in quarry outcrop. (d) Pss normal to bed strike spacing frequency distribution in quarry outcrop. (e) Pss parallel to bed strike spacing frequency distribution in quarry outcrop. (f) Pss oblique to bed spacing frequency distribution in outcrop 100 m north of the quarry. limestones, rich in benthic foraminifera, bryozoans, echinoid and mollusc fragments (Pomar et al. 2004). Deep-water intervals consist of strongly bioturbated, planktonic foraminifera-rich marls with minor radiolarians and siliceous sponge spicules (Pomar et al. 2004).
The limestones in the lower part of the sequence are mostly composed by foraminifers (lepidocyclinids, small benthonic foraminifers and echinoids) and subordinately by a micritic marly matrix. The foraminifer’s fragments are cemented by early diagenetic calcite that has almost completely destroyed the rock’s porosity (porosity d 0.05) (Vecsei and Sanders 1999, Van Geet et al. 2002). Partly broken planktonic foraminifers embedded in a micritic marly matrix make up the limestones in the upper part of the sequence. The total porosity of this latter unit ranges between 0.05 and 0.10 (Vecsei and Sanders 1999, Van Geet et al. 2002); the effective porosity, however, is very low because pores are isolated as preserved cavities within foraminifers. The bioclastic limestones
are well cemented and have thicknesses up to a few meters (Fig. 3a, b, and c).
3.1.2. Failure modes and structures
The mesoscale structural elements which characterize the Bolognano Formation include bed-parallel pressure solution seams (ps) generally at bed boundaries, and three groups of early pss sets: two normal-to-bedding (ns, one bed strike parallel and one normal Figs. 3a, 3d and 3e) and the other at oblique-to-bedding (os) (Figs. 3a nad 3f). The os, which typically occur in clusters, have a prominent N-S orientation and westerly dip. In proximity to the lower boundary of the Formation, just above the top of the Santo Spirito, a NE oriented os with a SE dip can be observed. The ns have a prevalent NE orientation. A secondary set of ns in E-W orientation is also present (Fig. 2). There are sheared pss associated with os (Fig. 2); their primary orientations are similar to those of the os. The shearing is associated with a second deformation event and produced ‘splays’ formed in the compression quadrants at the end of the sheared pss patches.
1
0
0,2
0,4
0,6
0,8
frequency
3 0 1 2 4
c
classes (m) d
0
0,2
0,4
0,6
0,8
1
0 1 2 3 4
Bolognano quarry - PSS normal to bed and parallel to bed strike-spacing frequency distribution
e
frequency
classes (m)
Bolognano turnabout - PSS oblique to bedspacing frequency distribution
0
0,2
0,4
0,60,8
1
0 1 2 3 4
f
frequency
classes (m)
0 0,2 0,4 0,6 0,8 1
0 1 2 3 4
fre Thickness frequency
Bolognano quarry – mechanical layers quency distribution
classes (m)
Stanford Rock Fracture Project Vol. 17, 2006 PC-3
Fig. 4. Fractures in Orfento formation. (a) Mechanical layers thickness frequency distribution in marls within the Orfento formation. (b) Mechanical layers thickness frequency distribution in grainstones within the Orfento formation. (c) Different failure modes in the marls (above) and the high porosity grainstones (below) of the Orfento formation. Pressure solution seams and some joints are the only structures to develop in the marl. Compaction bands, shear bands and pressure solution seams develop in the grainstone.
The ns are usually bed confined and have a spacing ranging from 0.4 to 4 m (Fig. 3d, e). The os have a spacing varying from 0.3 to 2 m (Fig. 3f). Second order pss and sheared pss (2s) clustered at the termination of fault segments have a spacing ranging from 0.1 to 0.4 m. Third order pss (3s) have a spacing ranging from 0.5 to 0.15 m.
On the basis of the field relationships among structures in the Bolognano Formation, we propose the following sequence of events from old to young. Bed-parallel pss (ps) are the earliest structures and formed in response to overburden loading. The two sets of N50 and E-W oriented normal to bedding pss (ns) are pre-tilting structures similar to those described by Graham et al. (2003) in the nearby Fara S. Martino area. Typically, these structures are easy to observe in low strain areas (Fig. 2). As a result of folding, bed-parallel shear normal to the fold axis, with a prominent sense of shear top to the east (Fig. 2), is resolved in proximity to the thrust sheet front. Going down-section, the sense of shear changes to top-to-the-west/NW in proximity to the Santo Spirito contact (Fig. 2). Splay pss oriented oblique-to-bedding (os) and parallel-to-bed-strike are consistent with the direction of shear along the beds (Fig. 2). Shearing on os occur in a reverse dip-slip sense (Fig. 3a/b) and results in the second and third order splays of pss (2s and 3s) (Fig. 2). Shearing of the N50 oriented ns results in the formations of N60 to N70
striking left-lateral strike-slip faults and incremental slip on these faults form higher order pss in their compressional quadrants.
3.2 Orfento Formation 3.2.1. Lithology and petrophysics
The Orfento Formation was deposited during the Late Campanian and the Maastrichtian, in a thick wedge of bioclastic and lithoclastic carbonate sediments on a gently inclined (a few degrees) depositional slope that graded into a shelf. These deposits are today referred to as the Orfento Formation (Crescenti et al. 1969, Vecsei 1991, Mutti 1995, Vecsei and Sanders 1999).
The grains within the carbonate sand deposits of the upper part of the foresets, which are the most predominant in the Orfento Formation exposed in the Avello Creek, are well sorted and poorly cemented, so that the original depositional porosity has been preserved in large part reaching today about 0.15-0.2 (Tondi et al. 2006, Mutti 1995). The thickness of the mechanical layers within the grainstones ranges from 0.1 to 1 m (Fig. 4a). The micritic marly drapes, which are composed of mudstones and wackestones, had their original porosity destroyed during compaction, so that it is less than 0.05 at present time (Van Geet et al. 2002). The thickness of the drape marly deposits of the
Orfento MARLS - Mechanical layer Thickness Frequency Disteribution
.1 .1 .1 .1 .1m)
0
1
0-0 1.0-1 2.0-2 3.0-3 4.0-4classes (
frequency
Orfento GRAINSTON - Mechanical layer Thickness E Frequency Distribution
.1 .1 .1 .1 .1m)
0
1
0-0 1.0-1 2.0-2 3.0-3 4.0-4classes (
frequencyy
a
b
Shear bands
bedding
pssc
0.2 m
Stanford Rock Fracture Project Vol. 17, 2006 PC-4
Fig. 5. Fractures in marls of the Tre Grotte formation. (a) Mechanical layers thickness frequency distribution. (b) Pss normal to bed strike spacing frequency distribution in quarry outcrop. (c) Pavement map of a 0.23 m thick marl layer. The inset shows a detail of a small fault. The thin lines are bed strike parallel and normal pss, thicker lines are for the oblique to bed pss that are sheared forming short discontinuous joint segments (the thickest lines). (d) Pss parallel to bed strike spacing frequency distribution. (e) Pss oblique to bed spacing frequency distribution. (f) Road cut exposure of marls interlayered with chert. The chert layers form a boundary in which a fracture domain develops. Oblique to bed sheared pss start at chert pinch-outs and grow in the domain cutting across the ps but are confined within the chert layers. Joint clusters form at the termination of the slipping sheared oblique to bed pss. The layer thickness / joint spacing relationship is given in the inset. Note that joint saturation is reached around a bed thickness of 0.25 m.
c
Tre Grotte MARLS - Mechanical layer Thickness Frequency Distribution
.1 .1 .1 .1 .1m)
0
frequency
1
0-0 1.0-1 2.0-2 3.0-3 4.0-4classes (
a
Tre Grotte MARL pavement - PSS normal to bed and n to b
ormaled strike-spacing frequency distribution
m) 0 1 2 3 4
classes (
frequency
1
0
b
Tre Grotte MARL pavement - PSS normal to bed and n to b
Fig. 6. Fractures in grainstones of the Tre Grotte formation. (a) Pavement outcrop where normal to bed strike and parallel to bed strike pss are exposed. (b) Mechanical layers thickness frequency distribution in pavement outcrop. (c) Pss normal to bed strike spacing frequency distribution in pavement outcrop. (d) Pss parallel to bed strike spacing frequency distribution in pavement outcrop.
Orfento Formation in the Tre Grotte valley is rarely larger than 0.2 meter (Fig. 4b).
3.2.2. Failure modes and structures
The structures in the distal marly drapes are very similar to the structures observed in the Tre Grotte Formation and, therefore, we are not going to describe them here but refer the reader to the structures in the Tre Grotte Formation described later.
In the high porosity carbonate sands of the Orfento Formation, bed-parallel compaction bands (cb) are localized in high porosity grainstone layers (porosity > 0.15), which are similar to those described by Tondi et al. (2006). A bed-parallel pss (ps) is associated with each compaction band and, usually, is more developed on one side of the beds. Two sets of contemporaneous (on the basis of the mutually cross-cutting relationships) shear bands (sbss) are localized in the high porosity (> 10%) grainstone layers of the Orfento Formation (Fig. 4c): The first one trending N-S and dipping west, the second one striking WNW-ESE and 50-70° dip to the south (Fig. 2). A third set has also been observed in an E-NE direction and dipping about 70° to the south. In the high porosity layers, we also recognize bed-normal pss (ns) with a prominent E-W
orientation, which are different from those in other formations but similar to those described by Tondi et al. (2006) in the Orfento Formation cropping out in the Madonna della Mazza area to the north of the present study area. Normal faults with a N-S orientation and dipping west, NE-oriented and dipping SE, as well as NW-oriented and dipping SW, have developed from pre-existing sbss with similar orientations. E-W trending strike-slip left-lateral faults have developed from pre-existing E-W oriented ns.
Shear bands in the Orfento Formation have a narrow spacing that can be down to 3-5 cm. Compaction bands have a spacing ranging from 0.05 to 1.5 m.
On the basis of the field relationships among structures observed in the Orfento Formation, we propose the following sequence of events from old to young. Compaction due to overburden loading produce bed-parallel compaction bands (cb) in porous bioclastic carbonate sands. Shear bands (sbss), in the porous grainstones of the Orfento Formation, are the next in the formation sequence. They occur generally in two sets and accommodate non-systematic pre-tilting or sin-tilting deformation. These structures in the Orfento Formation may have formed contemporaneously. The
Tre Grotte GRAINSTONE gully pavement - PSS normalquencyto Bed and parallel to bed strike spacing fre
)
distribution
0 1 2 3 m
4classes (
frequency
d
Tre Grotte GRAINSTONE gully pavement - PSS normalto Bed and normal to bed strike spacing frequency
distribution
0 1 2 3 4classes (m)
frequency
c
ness Frequency Distribution
.1 .1 .1 .1 .1
1
0
frequency
0-0 1.0-1 2.0-2 3.0-3 4.0-4m)classes (b
1
0
1
0
Stanford Rock Fracture Project Vol. 17, 2006 PC-6
E-W oriented ns may have formed once the porosity has been reduced by compaction. Shearing of pss is the last episode of deformation mechanism.
3.3 Tre Grotte Formation 3.3.1. Lithology and petrophysics
Gravitational collapse of the platform margin during the Middle Upper Cretaceous (Turonian Santonian periods) caused the deposition of the Tre Grotte Formation (Accarie 1988). This Formation is composed of fine-grained micritic marls containing planctonic foraminifers. The micritic marls bed thickness is ususally less than 10 centimeters (Fig. 5a). Five well-cemented breccia and bioclastic arenite packages, up to a few tens meters in thickness, are interstratified with the marls. The breccias contain benthic and reef macro invertebrate’s fossils, such as rudists and echinoids articles. Biocalcarenites beds rich in rudists and Orbitolina foraminifers are also present; their thickness ranges between 0.1 and 0.5 meter (Fig. 6b).
The porosity of the micritic marls composed of wackestones and mudstones, is less than 0.05 (Van Geet et al. 2002). The breccia and bioclastic arenite layers composed by packstone and grainstone are well cemented by syntaxial rim calcite cements that have greatly reduced interparticle porosity (less than 0.05).
3.4.2. Failure modes and structures
3.4.2a. Structures in grainstones and breccias In the coarse-grained beds represented by breccia
and/or grainstones, bed-parallel pss (ps) are well developed at bed boundaries. In adition, there exist two groups of pss sets: one group normal-to-bedding (ns) and the other group at oblique-to-bedding (os). The os have a prominent N-S orientation and westerly dip (Fig. 6a). The ns have a prominent NE orientation (Figs. 6a).
The ns are usually bed confined, they have a spacing ranging from 0.1 to 1 m (Fig. 6c and 6d). The os, that are related to shearing across ps, are often linked across bed interfaces.
The os are usually sheared in a normal sense of motion and have a spacing ranging from 0.1 to 5 m. The typical fracture network in the coarse-grained beds is shown in Figure 6a. Here the relationship among os parallel to bed strike and ns normal to bed strike is clear. 3.4.2b. Structures in marls and micrites In the micritic/marl beds, bed-parallel pss (ps) are present at bed boundaries. Two groups of pss sets are also present: one group normal-to-bedding (ns) and the other group oblique-to-bedding (os): The os have a prominent N-S orientation with a westerly dip (Fig. 5c), and a second system of os with an E-W orientation and a south dip direction. The ns have a dominant
orientation of NE-SW and a subordinate NW-SE one. In the thinly bedded micrite layers of the Tre Grotte Formation, however, their orientation varies in a wide range and the NE-SW direction is often overprinted by a joint system (jo). A second order of pss (2s), mainly oriented NNE, and joints (jo), mainly oriented E-W, are associated to slip on NW and NE oriented ns respectively in left-lateral and right-lateral sense of motion. An additional set of joints, mainly oriented N-S, are associated to slip on os (Fig. 5f).
The ns are bed confined,and have a spacing ranging from 0.05 to 0.2 m (Fig. 5b). The os, that are related to slip on ps, are often linked across bed interfaces but they tend to be confined in domains bound at top and bottom by cheret layers (Fig. 5f). Their spacing varies from 0.2 to 0.8 m in the thinly bedded units. The bed-confined joints (jo1) have a spacing ranging from 2 to 4 cm; joint spacing decreases as bed thickness decrease, once the bed has reached a thickness of 0.25 m in the marls, joint density has reached saturation (at a value of one joint every 2 cm, see inset in Fig. 5e).
On the basis of the field relationships among structures in the Tre Grotte Formation, we propose the following sequence of events from old to young. Compaction due to overburden loading produced bed-parallel pressure solution seams (ps) in micrites/marls and less porous bioclastic carbonate sands. The N50 oriented pss normal to bedding (ns) are pre-tilting structures. Bed-parallel shear normal to the fold axis due to folding has a prevalent top-to-the-east sense of shear. Splay pss oriented oblique-to-bedding (os), and parallel-to-bed-strike, are consistent with the sense and direction of shear along the beds. Normal faults parallel to the strike of beds formed by linking pre-existing os and splays of os. N70 striking left-lateral strike-slip faults initiated by shearing of the N50 oriented ns.
Discussion 4.1. Hierarchy of structures and failure mechanisms
One of the characteristics of the deformation mechanism observed in the low porosity breccia/grainstones basinal carbonate rocks of the Tre Grotte Valley (Bolognano and Tre Grotte Formations) is the hierarchy that form down to top following this order: (1) pressure solution seams, (2) sheared pss, (3) small faults with associated pss, (4) intermediate faults and (5) fault zones. This hierarchy is very similar to that described in a thrust fold belt in Bolivia by Florez-NiĖo et al. (2005), with the difference that the fundamental deformation mechanism in the breccia/grainstones basinal deposits is the pressure solution seam and not the opening mode jointing. The
Stanford Rock Fracture Project Vol. 17, 2006 PC-7
modality is similar to what was described by Graham et al. (2003).
In the well-bedded micritic/marly units of the basinal carbonate rocks (Santo Spirito Spirito and Tre Grotte), the hierarchy that form down to top follows this order: (1) pressure solution seams, (2) sheared pss (3) small faults with associated pss and joints, (4) intermediate faults and (5) fault zones. In the high porosity Orfento Formation the hierarchy that form down to top follows this order: (1) shear bands, (2) zones of shear bands with associated psss, (3) faults with associated slip surfaces and pss and (4) fault zones (see also Tondi et al. 2006).
Given that fluid flow properties of faults are dependent on lithology characteristics and amount of slip, it is also important to define the hierarchical frequency of fault structures dependent on slip in the different lithologies. Figure 7 shows well the scaling between faults with different amount of separation and their spacing. Faults with small separations have a frequency higher than faults with large separations (Fig. 7); although the correlation is not linear, spacing increases roughly by an order magnitude as separation increases of an order of magnitude.
Fig. 7. Hierachy of faults in the Tre Grotte Valley
represented by the relationship between fault separation and average spacing of the faults in different separation classes. 4.2. Lithologic control on deformation mechanism
The description of different structures in the Bolognano, Orfento, and Tre Grotte formations in the slope-basinal deposits of the Majella thrust sheet to the west of Pennapiedimonte, offers an excellent opportunity to compare the failure modes and fault development processes within different carbonate lithologies.
Our new results show, that pss are more pervasive in the marls with respect to the breccia/grainstones deposits. Our statistical data show that there is a scaling between mechanical layer thickness and frequency of normal to bed pss (ns); the smaller the layer thickness, the narrower is the spacing among structures. The scaling between mechanical layer thickness and oblique
to bedding pss (os) is apparent in the marls but not in the low porosity grainstones. In the marls, there are many more sets of low-angle-to-bedding and normal-to-bedding pss than there are in the breccia/grainstones. In the marls, sheared pss may develop from both kinds of structures and not only from the low-angle one as it usually happens in the breccia/grainstones. Chert layers within the marls tend to form horizontal domain boundaries for the development of differently oriented structures (Fig. 5f).
Another characteristic for the deformation in the marls, is the presence of the opening mode fractures, such as joints and veins, associated with sheared pss and with more evolved large strike-slip faults. The opening mode structures are rarely observed in the breccia/grainstones layers. Joints at fault terminations are important structures in fault development within the micrite/marl layers of the Tre Grotte and S. Spirito Formations. Joints localize at fault segment’s terminations in the micrite even where the amount of shear is very small (Fig. 5f).
Lithology control on the deformation mechanism is also well demonstrated in the Orfento Formation. The grainstones layers of the Orfento Formation have the highest matrix porosity (0.1 to 0.2; Mutti 1995, Tondi et al. 2006) among the basinal deposits studied. Deformation banding is the preferred deformation mechanism in these high porosity layers (Fig. 4c), perhaps early in the deformation history. We have recognized both shear bands and compaction bands in the Orfento Formation. The deformation process change abruptly at a bed interface where there is a contrast between a high porosity grainstones (bottom of Fig. 4c) and a micritic layer (top of Fig. 4c). In the micrite, the deformation mechanisms are opening mode fractures and pressure solution, whereas in the high porosity grainstones they are deformation banding and pressure solution. It is also possible that there might be a change through time and space in the mode of deformation within the Orfento Formation. During compaction, in fact, the porosity of the rock and/or the stress state may change. Another important observation, that follows from the documentation of these different deformation mechanisms in different carbonate types, is that the orientation of structures may be different where the remote stress field or the local stress field are the same. The earlier structures provide an anisotropy for the structures formed later. Tondi et al. (2006) have shown how shear bands during localization and growth evolved into a stage in which pressure solution occurred along the shear bands. These pressure solution seams were subsequently sheared.
Lithology has also an important control on fault frequency. Figure 8 shows the frequency of faults with the same separation (0.5-5 m) for three different formations in a way to eliminate the dependence of
Fault Hierarchy in Tre Grotte Valley Average Spacing (m)
,05 ,5 0 0
110000
0,1
1 10
100
000
0-0 0,05-0 0,5-5 5-5 > 5vertical separation class (m)
average spacing (m)
Stanford Rock Fracture Project Vol. 17, 2006 PC-8
fault frequency on slip. If fault distribution was to be controlled only by structural position, we would expect a narrow spacing of the faults in this class next to the thrust front (in the Bolognano formation), because of the large strain that needs to be accomodfated in that area. This, however, is not the case. The narrower faults spacing is in the Orfento formation (Fig. 8) and not in the Bolognano as one would have expected. It appears, that faults localize more easily in the high porosity grainstone of the Orfento formation and the in marls of the Tre Grotte formation, rather than in the massive low porosity grainstones of the Bolognano formation.
Fig. 8. Average spacing for faults belonging to
the same separation class in different formations exposed along the Tre Grotte valley.
4.3. Types and patterns of the faults Well-developed bedding plane, reverse, normal and
strike-slip faults are present in the section that we studied. Bedding plane faults have been observed in all formations and we relate them to folding of the Majella thrust sheet, that induced shear stress resolved along bedding interfaces or bedding parallel discontinuities.
Reverse faults with a N-S orientation and a westerly dip are observed only in the steeply dipping (40-60°) Bolognano Formation close to the front of the thrust sheet. They could be also interpreted as normal faults rotated into reverse positions during the folding of the beds. Normal faults with a predominant N-S direction (Fig. 2) are present in all formations except in the Bolognano. There are also right-lateral strike-slip faults striking N40 to N50, and two sets of left-lateral strike-slip faults: one striking N70 and the other one (subordinate) striking N5 – N10.
Conclusions In the basinal carbonate sequence exposed in the
Tre Grotte Valley, we have observed a variety of structures and modes of failure that are to be related to different lithologic characteristics. A knowledge of the effects of lithology on structure types and failure modes is important to be able to use structural data for the
kinematic reconstructions and to determine the effects that different faults and structures have on fluid flow.
In our study, we show that in low matrix porosity carbonates of the Bolognano and Tre Grotte formations, a fracture hierarchy that starts from individual pss and their subsequent shearing initiates small faults. Eventually small faults grow into intermediate faults and then into large offset fault zones.
In the micrite/marl layers within the Tre Grotte and Santo Spirito Formations the mechanism is similar in that starts from the same high angle to bedding pss. Deformation, however, is also associated with opening mode fractures and pss that cluster in the extensional and compressional quadrants of slipping strike-slip fault segments. The variety of structures present in these carbonate lithologies leads to a large variation in structure orientation as more strain needs to be accomodated, because both pss and joints can be activated in shear and form other pss or opening mode fractures. This accounts for the large variability of orientation in structures within the micrites/marls.
Average Spacing for 0.5-5 m Vertical Separation Class F
In the high porosity grainstones of the Orfento Formation, fault evolution is controlled by the presence of initial or early deformation bands that provide for the anisotropy where pss eventually localize and then evolve into a fault.
Pressure solution seams normal to bedding have a spacing that scales with mechanical layer thickness. The thicker the layer, the larger the spacing.
Fault frequency is related both to lithological chracteristics and to amount of slip. Larger separation faults have a low frequency compared to smaller offset faults. Faults in the same separation class, however, have a spacing that is narrower in high porosity grainstones and marls than in low porosity massive grainstones.
Acknowledgments: This work has been supported
by the Rock Fracture Project at Stanford University (research funds for A. Aydin, M. Antonellini, and F. Agosta), by the University of Camerino (research funds to E. Tondi), by the MIUR, Cofin 2002 (research funds to G. Cello). We thank Pauline Mollema who revised an early version of this manuscript and Mauro Alessandroni for help during the fieldwork.
References Accarie, H., 1988. Dinamique sedimentaire et structurale au
passage plate-forme / bassin. Les facies cretaces et tertiaires: Massif de la Majella. Memoires des sciences de la terre, Ecole des mines de Paris, v. 5, 250 pp.
Alvarez, W., Engelder, T. & Geiser, P.A., 1978. Classification of solution cleavage in pelagic limestones. Geology, v. 6, p. 263-266.
Crescenti, U., Crostella, A., Donzelli, G., Raffi, G., 1969. Stratigrafia della serie calcarea dal Lias al Miocene nella
formation name
aults
rotte 0
40
80
average spacing (m) Tre G Orfento Bolognano
Stanford Rock Fracture Project Vol. 17, 2006 PC-9
regione marchigiano-abruzzese (Parte II, litostratigrafia, biostratigrafia, paleogeografia). Mem. Soc. Geol. Ital. 8, 343–420.
Florez-Nino, J. M., Aydin, A., Mavko, G., Antonellini, M. & Ayaviri, A., 2005. Fault and fracture systems in a fold and thrust belt: An example from Bolivia. AAPG Bulletin, v. 89, p. 471-493.
Ghisetti, F., Vezzani, L., 2002. Normal faulting, extension and uplift in the outer thrust belt of the central Apennines (Italy): role of the Caramanico fault. Basin Research, 14, 225-236.
Graham, B., Antonellini, M. & Aydin, A., 2003. Formation and growth of normal faults in carbonates within a compressive environment. Geology, v. 31, p. 11-14.
Graham, B. & Aydin, A., 2006 Lampert, S.A., Lowrie, W., Hirt, A.M., Bernoulli, D., Mutti
M., 1997. Magnetic and sequence stratigraphy of redeposited Upper Cretaceous limestones in the Montagna della Majella, Abruzzi, Italy. Earth and Planetary Science Letters, 150, 79-83.
Mollema, P. N. & Antonellini, M. A. 1999. Development of strike-slip faults in the Dolomites of the Sella Group, Northern Italy. Journal of Structural Geology, v. 21, p. 273-292.
Mutti, M., 1995. Porosity development and diagenesis in the Orfento supersequence and its bouding unconformities (Upper Creatceous, Montagna della Majella, Italy). AAPG Special Publications, p. 141-158.
Peacock, D.C.P., & Sanderson, D.J., 1995. Pull-aparts, shear fractures and pressure solution. Tectonophysics, v. 241, p. 1-13.
Petit, J. P., and Mattauer, M., 1995. Paleostress superimposition deduced from mesoscale structures in limestone: the Matelles exposure, Languedoc, France. Journal of Structural Geology, v. 17, p. 245-256.
Pollard, D. D., and Aydin, A., 1988. Progress over understanding joints over the past century. GSA Bulletin, v. 100, p.1181-1204.
Pomar, L., Brandano, M., and Westphal, H., 2004. Environmental factors influencing skeletal grain sediment associations: a critical review of Miocene examples from the western Mediterranean. Sedimentology, v. 51, p. 627-651.
Rawnsley, K.D., Rives, T., Petit, J.-P., Hencher, S.R., and Lumsden, A.C., 1992. Joint development in perturbed stress field near faults. Journal of Structural Geology, 14, 939-951.
Rispoli, R., 1978. Microtectoniques et champ de contraintes dans les calcaires fins du languedoc. These 3eme cycle, Montpelier.
Scisciani, V., Tavarnelli, E., Calamita, F., 2002. The interaction of extensional and contractional deformation in the outer zones of the Central Apennines, Italy. Journal of Structural Geology, 24, 1647-1658.
Tondi, E., Antonellini, M., Aydin, A., Marchegiani, L. and Cello, P., 2006. The roles of deformation bands and psss in fault development in carbonate grainstones of the Majella Mountain, Italy. Journal of Structural Geology, in press, 1-16.
Van Geet, M., Swennen, R., Durmish, C., Roure, F., and Muchez, P. H., 2002. Paragenesis of Cretaceous to Eocene carbonate reservoirs in the Ionian fold and thrust
belt (Albania): relation between tectonism and fluid flow. Sedimentology, v. 49, p. 697-718.
Vecsei, A., 1991. Aggradation und Progradation eines Karbonat plattform-Randes: Kreide bis mittleres Tertia¨r der Montagna della Maiella, Abruzzen. Mitt. Geol. Inst. ETH Univ. Zurich, N.F. 294, 169 pp. C appendices.
Vecsei, A., and Sanders, D. G. K., 1999. Facies analysis and sequence stratigraphy of a Miocene warm-temperate carbonate ramp, Montagna della Maiella, Italy. Sedimentary geology, v. 123, p. 103-127.
Willemse, E.J.M., Peacock, D.C.P. & Aydin, A., 1997. Nucleation and growth of strike-slip faults in limestones from Somerset, U.K. Journal of Structural Geology, v. 19, p. 1461-1477.
Stanford Rock Fracture Project Vol. 17, 2006 PC-10
