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Closing the gap on Mesozoic extensional tectonics: the Torre

Poglina Lias carbonate system, western Sardinia

CHARLES R. SINGER1

1Department of Earth Science and Engineering, Imperial College London

ABSTRACT In western Sardinia, a Lower Jurassic carbonate tempestite section exhibits a series of NE dipping listric faults. Identification of lateral thickness variations within the rotated hanging wall blocks coupled with erosional intersections of marker horizons provides new evidence for syn-sedimentary faulting in the Lower Jurassic. A SE stereographic tensional regime supports the magnitude and direction of extensional tectonics in this chronostratigraphic interval following restoration attributed to the counter clockwise rotation of the Sardinian-Corsican block. A comparison with stratigraphic sections and structural orientations of eastern Sardinia and the French Maritime Alps is made, attempting to improve the continuity in reconstruction of the Western Tethyan margin with implications for an extensional regime on hydrocarbon bearing Lower Jurassic source rocks.

INTRODUCTION

In Western Sardinia, a sequence of Jurassic to Cretaceous carbonates overlies post-Hercynian Permian and Triassic siliciclastics and transgressive marine evaporites associated with localised basin subsidence. This cover represents the first (Early Jurassic) Mesozoic sedimentary cycle in western Sardinia.

Prior to Early Miocene anticlockwise rotation of the Corsican-Sardinian block, attachment of Sardinia to mainland Europe during the Mesozoic provides constraints with the geodynamic evolution of the French Maritime Alps (Stanley & Mutti, 1968). Palaeogeographic evidence of the Jurassic and Cretaceous periods highlights a large and spatially continuous carbonate megabank which occupied vast areas of the west-Mediterranean, as well as significant portions of Adria (Cherchi & Schroeder, 1985). However, closer reconstructions of restorations reveal the gradual westward propagation of the Neo-Tethys sea-floor spreading axis during the Late Triassic to early Jurassic. This is proposed to pre-date opening of the central Atlantic and the Piedmont segment of the oceanic Tethys (Ricou, 1994). Submergence produced isolated shallow water carbonate shelves sheltered from terrigenous clastics by deeper troughs and plateaus sharing affinities to the modern

Bahamas archipelago (Bernoulli & Jenkins, 1974; Ziegler, 1988).

The aim of this paper is to provide evidence for improved temporal and spatial resolution of Lower Jurassic extensional tectonics associated with the coeval opening with the Alpine Tethys. The results of direct field observations in western Sardinia attempt improve the accuracy of geographic restorations of past tectonic regimes across the French Maritime Alps and eastern Sardinia and provide further support against general consensus of tectonic stability in the Jurassic (Costamagna, 2015; Dardeau, 1988). The implications of sediment accumulation and extensional faulting on potential Liassic hydrocarbon deposits in the proximity of the central Mediterranean are discussed (Di Cuia & Riva, 2016). Focus is placed on a proximal marine carbonate succession composed of argillaceous limestones belonging to the Torre Poglina Lias located 7 km south of the town of Alghero (Fig. 1). This is where the formation is best developed both spatially and temporally. METHODS

Geological mapping in comparison to surrounding Jurassic and Cretaceous carbonate

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deposits is undertaken with close examination on sedimentological characterisation, facies interpretation, and structural analysis. Adjacent to the small town of Torre Poglina, an abandoned quarry provides a well-exposed succession of carbonate lithologies with evidence for alpine compressive tectonics and extensional faulting; the latter providing the basis for detailed assessment of syn-sedimentary structures. Interaction between sedimentary facies and normal faulting has allowed a localised palaeoenvironmental reconstruction to be developed.

Fault measurements are collected using a Suunto Compass Clinometer MC2 along a SW-NE transect along the quarry face with examination of slickenside orientations and slip directions. Strike errors are likely to have arisen when the compass datum line was not aligned horizontally coupled with misalignment due to their indistinction. Repeated observations and averaging aids to overcome this problem allowing stereographic projection. Woodcock (1976) concluded that high errors (>5ْ) could arise from trend measurements on steeply plunging lineation’s.

Scaled photographs of sedimentary structures and fault interaction are collated to assess their relationships within the depositional system. Images collected of sedimentary structures are on the centimetre scale and hence the scaling factor associated with parallax error is minimal between the top and bottom of investigated

sequences. Annotated digitised field sketches of the quarry face pose higher degrees of scaling errors of approximately 2 calculated between the top and bottom of the quarry face in accordance to the rules of triangulation. Sections of interest are predominantly towards the base of the outcrop minimising scaling errors associated.

RESULTS

The quarry exposure forms a 70 m by 13 m high outcrop of massive sub-horizontal carbonate beds with distinctly faulted marker horizons 0.4 to 1.4 m thick interbedded with a series of variably grey laminated micrites. The latter lithofacies is represented at its base by dark grey bioclastic grainstones up to 5 cm thick composed of laterally aligned fragmented bivalves 3 – 4 mm in length. These allochems typically concave downwards with occasional locking with adjacent non-inverted shells (60% vol.) set in a depleted mud matrix (Fig. 2a). In its lower part, a basal erosional surface marks a contact with beige micrites belonging to the underlying sequence. The basal part passes up sharply into gradationally fining carbonate silts with indistinct sedimentary structures. Laminations become increasingly defined and closely spaced with traces of hummocky cross stratification existing in the upper parts of each sequence up to 4 cm in amplitude (Fig. 2b).

Compilation of fault plane measurements and slickenside orientations are denoted in figure 3a.

Figure 1. Regional and local study map. General formation ages and spatial relationships mapped with investigated sections

highlighted.

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Fault planes are plotted and an average plane

calculated to correlate slickenside data. The directions of present fault planes are closely clustered towards the NE with an outlier orientated towards the ENE aligning with the slickenside orientations. Through accounting for the 95ْ counter clockwise rotation of the Sardinian block defined using palaeomagnetic evidence of Advokaat et al. (2014), a restored stereonet is constructed with principal vector directions displayed using data concentration on a restored rose diagram plot (Fig. 3b; Fig. 3c). A mean displacement towards the SE is shown.

A digitised field sketch is presented using computer overlay of scaled photographs (Fig. 4). Distinct marker horizons and fault intersections are highlighted to provide direct observation of potential sediment thickening within fault hanging walls. A series of NE verging synthetic listric faults dominate the quarry face varying significantly in length from sub-metre up to 10 m with conjugate antithetic faults steeply dipping towards the SW producing uplifted horst blocks. Maximum fault dips reach up to 70ْ. The majority of faults are concentrated within the bottom half of the quarry exposure with several protruding to intersect an upper brown bed used

Figure 2. Torre Poglina Lias deposits. (a) Succession composed of bioclastic grainstones in the lower part fining upwards into

laminated micrites. (b) Hummocky cross stratification.

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Figure 3. (a) Stereographic projections of present day fault and slickenside orientations and (b) restored projection corresponding to

~95ْ counter clockwise rotation (Avokaat, 2014). (c) Restored rose plot representing mean trends of fault planes and slickensides.

a

b

c

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Figure 4. Scaled photograph of quarry exposure and overlain digitised field sketch along studied transect. A prominent central grey

marker horizon has been used to assess syn-sedimentary thickening.

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as a further marker horizon beneath alluvium cover. The listric nature of faults is noted with increasing angle between the vertical with depth and appear to flatten to a lower detachment horizon comprised of a poorly sorted 20 cm thick fault breccia composed of angular micrite clasts up to 25 cm. An internal duplex structure detaching on the lower breccia sequence consists of NNW-NW verging fault striations, perpendicular to those associated with synthetic listric normal faults. The presence of bedding parallel styolites with amplitudes up to 2 mm support lithological discontinuity.

The magnitude of dip of the beds is strongly rotated within the hanging walls of successive normal faults in comparison to sub-horizontal laminae preserved towards the top of the exposure. Beds are recorded to dip as much as 25ْ towards the SW with the inclination of strata reducing towards the southern end of the outcrop associated with decreased fault concentration.

Associated displacements are also variable, but range only to several tens’ of centimetres. A positive correlation of the magnitude of displacement to fault distance is presented in figure 5. These offsets produce lateral thickness variations along a central grey marker bed. Over the region studied, the unit reaches a maximum thickness difference of approximately 0.93 m with evidence for 0.35 m of thickening across a single normal fault (Fig. 4). Thickness variations of marker horizons beyond the transect become

too indistinct to accurately measure. Underlying laminae exhibit evidence for oblique intersection with greater degrees of rotation than the truncating marker horizon (Fig. 4).

DISCUSSION

The data presented here provides improved constraint on the temporal extent of Jurassic rifting episodes, with evidence from direct observations to supplement compiled palaeogeographic reconstructions.

Lithofacies

The sedimentological characteristics of the Torre Poglina Lias reflect deposition within shallow, near-shore carbonate producing seas. The presence of an erosive basal bioclastic grainstone passing upwards to normally graded argillaceous silts and laminated micrites is analogous to turbidite deposits formed by density driven flows on shallow continental shelves. The repeated truncations of upper planar laminae with successive cycles represent episodic sedimentation phases triggered by pulses of sediment instability (Meiburg & Kneller, 2009). However, identification of hummocky cross stratification is believed to be diagnostic of storm-dominated shallow marine environments forming above the storm wave base, typically around 30 m in modern carbonate settings (Cheel & Leckie, 2009). Recent published data from Yang et al. (2006) has provided further evidence that HCS wavelength is controlled by the bottom orbital diameter (d0) of oscillatory wave motion, implying the maximum size of HCS sets should increase with decreasing water depth; according to the relationship:

λ ≈ 0.75 d0 The centimetre scale magnitude of hummocks observed within the Torre Poglina Lias suggests deposition offshore of the surf zone. The absence of overlying ripple cross laminations and underlying fragmented shell beds support an interpreted proximal to intermediate carbonate tempestite sequence (Fig. 6; Fig 7).

Figure 5. Graph constructed of the magnitude of

displacement in comparison to the perpendicular distance

between fault segments along the length of the studied

transect (Fig. 3).

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Figure 6. General sketch of interpreted stratigraphic sequence

of the Torre Poglina Lias, western Sardinia.

Extensional Structure

Collaboration of structural features and quantifiable bed thickening variations support the presence of syn-sedimentary deposition. Analysis of a grey marker horizon exhibits significant thickness variations along a lateral transect with nearly a metre scale variation in vertical accumulation. The rotation and thickening of beds into the hanging wall of listric faults coupled with discrete erosional truncations provides unequivocal evidence for syn-rift facies (Fig. 4). Calcite filled fractures up to 5 cm in length orientated parallel to macroscopic faults highlights the influence of a tensile regime even on small scale sedimentary deposition. Presence of a detachment horizon comprised of interpreted basal slump deposits also displays an internal horse structure. Slickenside striations suggest a WNW transport direction, approximately perpendicular to the vergence of the extensional regime. Hence, this could represent an inverted extensional duplex structure associated with Alpine orogenic events further supported by a gently folded antiformal structure to the north of the section. Interaction between these tectonic episodes lies beyond the scope of this paper.

Analyses of restored stereographic and rose diagram projections strongly support a NW-SE tensional regime producing a series of extensional listric faults presently dipping towards the NE. A component of oblique slip is noted through mean vector offsets between the average planes and slickenside orientations

approximately equal to 50ْ. The variability in fault displacement direction with a dominant oblique nature is likely related to the localised linkage of previously unconnected fault segments (Dawers & Anders, 1995). Cartwright et al. (1996) emphasised the displacement decrease on one fault segment is balanced by a sympathetic increase on another. Figure 5 denotes a correlation between lateral fault distance and offset magnitude. Within the studied section, closely spaced faults exhibit reduced displacements suggesting these may have experienced progressive linkage and strain uptake during formation. Fault concentrated regions further exhibit the thickening of sediments which could suggest implications of strain distribution maintaining accommodation space for sediment build-up prior to migrations in the loci of strain accumulation.

The interaction between structural development and sediment deposition is proposed through bed morphology and sedimentary structures. Erosional intersections may suggest the structural involvement in localised subsidence and uplift to subaerial exposure. Constraints on data collection means subsidence rates and magnitudes are difficult to ascertain solely from a 2D transect. Myrow & Southard (1996) discussed the catastrophic introduction of sediments as a result of earthquake activity. The non-uniformity in the thickness of tempestite deposits throughout geological history coupled with changes in the fluctuations of storm intensity corresponding to global environmental supercycles, means fault movement within the syn-sedimentary succession should not be ruled out as a triggering mechanism (Ito et al. 2001). However, the low subsidence suggested by the sedimentary record and small degrees of thickening, absence of large scale tectonic disturbances and presence of shallow marine HCS suggests influence of storm-induced waves as a more likely scenario (Durmas & Arnott, 2006). IMPLICATIONS

The prospect of active extensional tectonics in the Lower Jurassic period aids understanding of continental assemblage and restorations of palaeogeographies. In SE France and Western

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Figure 7. Palaeoenvironmental sketch of the western Sardinian carbonate ramp during the Liassic. Structural features and their

influence on sediment accumulation are annotated.

Sardinia, Cherchi & Montadert (1982) provided models to support the close proximity of these domains prior to Oligo-Miocene Sardinia-Corsica block drift. However, it has been proposed tectonic uplift was spatially and temporally limited resulting in the ephemeral replacement of Middle Jurassic carbonate shallow platform deposits (Cherchi & Schroeder, 1985). The results from this study argue for the role of active extensional tectonics within epeiric basins and interaction with carbonate platform sedimentation. Stereonet projections of faults and associated striation orientations strongly indicate a NW-SE extensional regime prior to combined Eocene and Miocene block rotation. According to Monleau (1986), the Jurassic Provençal facies of the Maritime Alps and those of eastern Sardinia can be correlated. Palaeogeographic reconstructions compiled by Dardeau (1988) indicate a NNW-SSE tectonic regime producing

SE verging extensional faults on the eastern Sardinian margin. The absence of significant rotation of the Argentiera massif means the comparable trends of half-graben structures are today preserved on the North-Provence edge (Fig. 8).

The Tacchi area, eastern Sardinia

Evidence for extensional tectonics in the early Middle Jurassic sedimentary cycle of eastern Sardinia is linked to the coeval opening of the Alpine Tethys (Bernoulli & Jenkins, 1974). The impact of active tensional tectonics on sedimentation is highlighted with the uplift of a temporary tectonic high with rapid collapse forming initial continental to transitional and finally shallow marine deposits comprising the Genna Selole Fm. Features including neptunian dykes and indications of sudden immersion provide further support of syn-sedimentary

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tectonics within the Middle Jurassic (Costamagna, 2015). Structural trends orientated towards the NE strongly correlate with those observed within the Torre Poglina Lias. Absence of Mesozoic sediments across present day central Sardinia makes basin analysis difficult but the comparison of structural trends from the province platform of the Maritime Alps to Eastern Sardinia supports an underlying extensional regime operating from the Liassic to Middle Jurassic periods across a subsiding NW Alpine Tethys passive margin (Dardeau, 1988). This suggests eastern Sardinian horst-and-graben structures developing during the Bajocian do not represent initiation of opening of the Alpine Tethys (Costamagna et al. 2007).

Triassic rifting Stratigraphic relationships of sediments

deposited during the Upper Triassic and Lower Jurassic on the western Tethys Iberian platform may suggest correlation of Jurassic extension with Late Triassic palaeogeographies. Seismic data has highlighted a series of depocentres controlled by syn-depositional faults (Gómez & Goy, 2005). The consenus of back-arc spreading associated with the Palaeo-Tethyan subduction zone operating until the Late Triassic may have provided pre-existing structures inherited by Jurassic extensional episodes (Stampfli et al. 2002). Further research of this chronostratigraphic relationship could

suggest a more continual extensional regime operating within the Mesozoic rather than individual tensional episodes.

Hydrocarbon exploration The direct evidence for rifting in Lower Jurassic sediments has further reaching implications beyond structural and palaeogeographic reconstructions, highlighting the geodynamic evolution of epicontinental basins. Jurassic carbonates and siliciclastic sediments are common source rocks for hydrocarbon exploration in areas including the Kimmeridgian-Brent system in the Northwest European shelf. Fourteen petroleum systems with Upper Jurassic source rocks contain one quarter of the world’s currently discovered oil and natural gas with eleven other smaller systems with Liassic carbonate source lithologies. It is estimated that one third of the Lower and Middle Jurassic source rocks lack an overlying Upper Jurassic source rock with two thirds of these hydrocarbon occurrences related to regional or local uplift (Klemme, 1993). Modelling of Triassic and Jurassic rifting in central north Bulgaria has emphasised the impact of significant subsidence during the Early-Middle Jurassic on the degree of source rock maturity (Botoucharov, 2014). Hence, understanding of syn-rift sediments deposited during the Liassic improves the application to proximal to distal basinal carbonate plays. This form of play is expected to consist of traps which could be pinch-outs of turbidite flows in distal marine environments or against structural highs that stop the flows associated with a fault-controlled platform margin; comparative to the previous Barbagia tectonic high in eastern Sardinia. Recent exploration studies have highlighted this play type could be present on the western Sardinian margin in addition to basinal deposits within the Gulf of Lion (Di Cuia & Riva, 2016). Improved constraints on rifting initiation aids the collaboration of continental reconstructions and hence the start of marine sedimentation during the Rhaetian time. The formation of widespread organic-rich (type II, marine) intervals could promote further interest in Liassic deposits with evidence for syntectonic deposition controlling the creation of accommodation space in rift basins

Figure 8. Schematic palaeogeographic reconstruction of the

Tethyan margin during Middle Callovian times (modified from

Costamagna et al. (2007), Middle Collovian).

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(Mascle & Vially, 1999; Gawthorpe et al. 1994). The potential of fault linkage within the Torre Poglina Lias may pose wider implications of fault interaction in controlling synrift stratigraphic sequences. CONCLUSIONS The lithofacies of the Torre Poglina Lias represents a shallow marine carbonate tempestite sequence deposited during an extensional tectonic regime associated with the development of the Alpine Tethys passive margin. The series of present day NE verging listric normal faults have controlled block rotation and wedge-shaped accumulation of sediments likely deposited from storm induced flows. Restoration of structural planes and lineation’s following the counter clock-wise rotation of the Sardinian-Corsican block during the Miocene to early Oligocene correlate strongly with palaegeographic reconstructions and the NW-SE trending extensional regime operating between the Provençal margin of the Maritime Alps and offshore eastern Sardinia. Hence, the evidence for syn-tectonic deformation of Liassic sediments bridges the gap between Upper Triassic and Middle Jurassic rifting episodes both spatially and temporally. This serves for future implications aiding to dismiss the stability of carbonate platforms during the Jurassic. Evidence for interaction between extensional tectonics and sedimentation within this chronostratigraphic interval provides further importance in understanding source rock maturity and the potential of hydrocarbon deposits within Jurassic source rocks within the central Mediterranean. ACKNOWLEDGEMENTS Many thanks to Dr. Matthew Genge and Dr. Mark Sutton for organisation of fieldwork in Western Sardinia. The assistance in data collection from Jennifer Reeves, and collaboration of structural measurements from Katherine Siuda, Qaitong Ren, Madeleine Hann, Sophie Munson and Bethany Mitchell-Bunce has greatly improved the quality of interpretations made.

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