rhodalgal carbonate sedimentary setting: a case history from the Miocene syn-rift Sardinia Basin, Italy

Sedimentary Geology 174 (2005) 1 – 30 www.elsevier.com/locate/sedgeo

Research paper

Anatomy of a submarine channel system and related fan in a foramol/rhodalgal carbonate sedimentary setting: a case history from the Miocene syn-rift Sardinia Basin, Italy Mario Vigoritoa,1, Marco Murrub,2, Lucia Simonea,* a

Dipartimento di Scienze della Terra, Universita` di Napoli bFederico II Q, Largo San Marcellino 10, 80138 Napoli, Italy b Dipartimento di Scienze della Terra, Universita` di Cagliari, via Trentino 51, 09126 Cagliari, Italy Received 11 August 2003; received in revised form 25 August 2004; accepted 22 October 2004

Abstract During Aquitanian–Burdigalian times, thick mixed carbonate–siliciclastic successions were deposited in basins located on the grabens and half-grabens along the Oligo-Miocene Sardinia Rift Basin. Locally active tectonics, sea level variations and ecological factors combined to control the development and distribution of foramol carbonate factories as well as the remobilisation and the redeposition of carbonate sediments into the adjacent deeper areas. In the Isili Basin, foramol/rhodalgal carbonate factories developed on submerged structural highs which resulted from preand syn-sedimentary tectonics. These carbonate factories were periodically shaved mainly during negative sea level oscillations and the sediments removed were funnelled towards the basin through a complex submarine channel network which included a tributary belt, one main channel (Isili Channel) and the related fan. The Isili Channel is up to 1 km wide, 60–100 m deep and includes two stacked channel complexes each built up by several minor-order channel units. Complex strata geometries characterise the Isili Channel and its related architectural elements (e.g., overbank, levee, margin and channel thalweg) which also include up to 15 m high bedforms. Individual channel complexes were temporally related to individual fan systems whose spatial distribution and internal geometry were strongly controlled by the type and rate of sediment accumulation and, in turn, by relative sea level oscillations. Facies associations include sandy to cobble-sized gravity flow and bottom current deposits as well as megabreccias characterised by impressive displaced and tilted blocks which resulted from major channel margin collapses. Detailed analysis has led to the reconstruction of the internal geometry and depositional architecture of these carbonate bodies and to the determination of the main controlling factors. The dimension and distribution of channel and channel-related

* Corresponding author. Tel.: +39 81 5473325; fax: +39 81 5525611. E-mail address: [email protected] (L. Simone). 1 Fax: +39 81 5525611. 2 Fax: +39 70 282236. 0037-0738/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2004.10.003

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depositional bodies have been accurately determined. This information provides a useful tool to analyse less extensively exposed analogues and to model foramol shelf to basin transitions and related channel and fan systems. D 2004 Elsevier B.V. All rights reserved. Keywords: Miocene; Sardinia; Syn-rift Basins; Foramol/rhodalgal facies; Submarine channels; Submarine fans

1. Introduction During the last 20 years, models for carbonate depositional systems have been proposed in which open shelf, skeletal debris covered sea bottoms, support biological assemblages of foramol/rhodalgal type (Carannante et al., 1981, 1988a, 1995, 1997; Nelson, 1978; see also Nelson, 1988; James and Clarke, 1997). Foramol/rhodalgal biological assemblages largely dominate carbonate shelves at high latitudes but they are able to colonise shelves at low latitudes (tropical/subtropical settings) when environmental factors favour their development in place of chlorozoan assemblages (Lees, 1975; Carannante et al., 1988a), thus resulting in reef-devoid shelves. Because of their calcite-dominated mineralogy, these skeletal carbonates are little affected by early diagenesis and this also contributes to the development of unrimmed shelves (Van de Poel and Schlager, 1994; Carannante et al., 1995, 1997, 1999). Fossil and recent examples have been described from these open shelves in which carbonate factories have characteristics which make them more prone to progradation than to aggradation with major resedimentation episodes occurring mainly during terminal high-stand, sea level falls and low-stand (Carannante and Simone, 1988; Carannante et al., 1988b, 1994, 1996, 1999; Nelson and Bornhold, 1983; Braga et al., 2001; Stossel and Bernoulli, 2000). Their response to sea level variation seems to be more akin to siliciclastic shelves than to the chlorozoan-dominated carbonate ones. In some cases, complex channel networks, which acted as sediment pathways, have been reported (see Passlow, 1997; Cherchi et al., 2000, Cathro et al., 2003) and illustrated (Carannante, 1982; Braga et al., 2001; Carannante and Vigorito, 2001). Carbonate submarine channels are poorly known compared with terrigenous analogues, and their architectural styles as well as facies associations and their 3D arrangement are far less understood. The

paucity of previous studies concerning this topic necessitates reference to studies on the depositional architectures of comparable submarine channels from siliciclastic sedimentary environments (see Cronin, 1995; Cronin et al., 1995, 2000; Clark and Pickering, 1996). Previous studies, from both siliciclastic and carbonate settings, led to simple and useful models, indicating that submarine channels may be erosional, aggradational or dmixedT (Normak, 1970; Clark and Pickering, 1996), sinuous, braided or straight, isolated or stacked, leveed or not, feeding lobes or not, and may in fact occur in any number of tectonic settings and basins with markedly different physiography and type and rate of sediment input (see Clark and Pickering, 1996, Cronin, 1995; Cronin et al., 1995, 2000a, Braga et al., 2001; Carannante and Vigorito, 2001; Vigorito, 2001). In some cases, syn-sedimentary tectonics strongly controlled longevity, trend and spatial distribution and the rate of sediment input to the channel systems both in carbonate (e.g., Braga et al., 2001; Carannante and Vigorito, 2001) and in siliciclastic (e.g., Kleverlaan, 1989; Cronin, 1995; Clark and Pickering, 1996) sedimentary environments. The aim of this paper is to define the spatial distribution and the 3D arrangement of the carbonate submarine channels occurring in the Isili area (southeastern Sardinia, Italy). In particular, by recognising all the architectural elements that make up the submarine channel system and by defining their shape, internal geometry, dimension, relationships and functions, it is possible to reconstruct the anatomy of this depositional system and to document its complexity. In order to achieve these objectives, channel and channel-related sub-environments and architectural elements (e.g., channel margins, channel levees, tributary and distributary channels, lobes, etc., see Clark and Pickering, 1996; Hurst et al., 1999, Johnson et al., 2001; Camacho et al., 2002) have been distinguished and described.

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2. Methods

3. Geological framework and tectonic constraints

Detailed geological mapping was carried out over an area which extends for 40 km2 in the vicinity of the village of Isili (southeastern Sardinia, Italy) and the adjacent Riu Corrigas canyon. In this area, spectacular canyon-walls with up to 50 m high, near to vertical cliffs offer a superb window of observation and facilitate the recognition of submarine channel bodies with reasonably good 3D control. Accurate bed to bed analysis, including sedimentological studies were performed on nine sections that were also logged. Thin sections were prepared from the collected samples and subsequently analysed with an optical microscope. Special attention was paid to indicative surfaces (e.g., hardgrounds, erosion, and drowning surfaces), sedimentary structures, sedimentary stacking patterns and vertical and lateral facies distribution patterns. Paleocurrent indicators (e.g., ripples, imbricated grains, scour marks) were recurrently measured throughout the investigated area.

During Oligocene–Miocene times, syn-rift sedimentary sub-basins formed on the top of fault-blocks along the margins of the main Sardinia Rift Basin (Fig. 1) following extensional phases that characterised much of the Mediterranean region (Doglioni et al., 1998, 1999; Se´ranne, 1999; Cherchi and Montadert, 1982, 1984; Casula et al., 2001). Active tectonics, relative changes in sea level and localised ecological factors combined to control the formation of complex depositional architectures in these subbasins (Cherchi et al., 2000). Carbonate factories developed locally on the topographically higher portions of certain blocks where environmental conditions were favourable. Tectonic instability and relative sea level variations promoted periodical sediment remobilisation through gravity flows and led to redeposition in deeper areas. According to Casula et al. (2001) the Isili area was located in the proximity of the btwisting zoneQ or btransfer zoneQ (Fig. 1) of the Oligo-Miocene Sardinia rift which

Fig. 1. Geological–structural sketch map of Sardinia. Location of the area investigated is shown.

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served to absorb differential motion between sets of faults thus dividing a graben system into different compartments of opposing-polarity half-grabens. One of the small sub-basins, formed during the rifting phases on grabens and half-grabens located on the margin of the main Sardinia graben system, was the Isili Basin (Figs. 1 and 2a, b) This latter extended approximately NNW–SSE along the Eastern side of the Oligo-Miocene Sardinia rift (Fig. 1) and was confined on its western side by a regional normal fault, the Isili Fault, and on the eastern side by the Sarcidano–Monte Rasu structural high (Figs. 1 and 2b). Syn-rift deposits laid down in the Isili Basin comprise up to a few hundred metres of deltaic to open marine deposits whose distribution and thickness were largely controlled by the palaeophysiography and, in turn, by tectonics. In the Isili Basin, in fact, NNW–SSE trending faults, parallel to the Isili Fault, concurred to form a complex palaeophysiography which favoured the development of a narrow NNW– SSE-trending, south-plunging trough (Isili Trough,

Fig. 2b) surrounded by structural highs. Since the Upper Oligocene–early Aquitanian times, these structural highs were partially and locally periodically exposed (e.g., the Monte Rasu–Sarcidano area, Fig. 2a, b) and underwent severe erosion with deposition of coarse syn-rift breccias and fan-delta conglomerates along the adjacent margins of the trough (Fig. 2b; see also Cherchi et al., 2000). These coarse deposits pass trough-ward and upward to marine tuffaceous sandstones (Fig. 2b). Since the late Aquitanian, foramol/rhodalgal carbonate sequences (Isili Limestones) were laid down in the Isili basin. On the northeastern margins of the Isili Trough, the carbonate deposits include large amount of skeletal debris as well as small patchy pioneer benthic communities gathered in primary biogenic concentration. These carbonate deposits rest in conformity over the tuffaceous sandstones, intercalated with terrigenous/carbonate sandy/pebbly deposits (Isili industrial area, Fig. 2a, b) and build up, together with the previous siliciclastic syn-rift deposits, large

Fig. 2. (a) Geological sketch map of the Isili area. Paleocurrent distribution patterns from the: (1) Tributary belt, Isili Village section; (2) Isili Channel: right margin and levee complexes West Is Cungiaduras); (3, 6) Isili Channel: right channel margin/mid-channel complexes (Pardu and Casa Cantoniera/Is Cungiaduras); (4) Eastern side of the Isili Trough (Isili Industrial Area); (5) Isili Channel: left channel margin/levee complexes (Casa Cantoniera section); (7) Isili Channel: left levee complexes (East Is Cungiaduras); and (8) Isili Channel: distributary zone (S’Acqua Salia–Nuraghe Maurus). (b) Schematic SW–NE oriented stratigraphic section for the Isili Basin (not in scale).

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trough-ward prograding sedimentary wedges (e.g., the Isili industrial area, Fig. 2a, b; see also Cherchi et al., 2000). On the northwestern and the western sectors of the Isili Trough (Fig. 2a, b), instead, large foramol/ rhodalgal carbonate factories developed on the palaezoic basement or on the previous syn-rift deposits (breccias and tuffaceous sandstones). These carbonate factories were located on the topographically highest portion of the structural highs of Punta Trempu and probably Nurallao (Fig. 2a), at water depth ranging between 30 and 80 m (Cherchi et al., 2000). At these water depths, which corresponded to the deep infralittoral/circalittoral sectors of the shelf, the rhodalgal benthic communities formed and lived in skeletal debris, produced mainly by bioerosion processes (Cherchi et al., 2000). The sediments derived were swept off by flushing currents or waves and/or periodically removed from the productive (source) areas probably in relation to negative relative sea level oscillations and/or tectonic events (Cherchi et al., 2000). Displaced sediments were redeposited as parallel to cross-stratified sedimentary bodies in marginal areas (tributary belts), corresponding to the outermost sectors of the shelf and to slope areas, or were transported into deeper areas through a complex network of channels (Isili Channel System, Murru et al., 2001). The distribution pattern and trend of these channels were also strongly controlled by palaeophysiography and, in turn, by rift-related synsedimentary tectonics (Murru et al., 2001). In the central and southern (deeper) sectors of the Isili Trough, the carbonate sequences are overlain by and pass eastwards to quartz-rich sandstones and siltstones locally associated with debris-flow deposits (Fig. 2a, b). Since the middle Burdigalian, rapid relative sea level rise had led to deposition of a few hundred metres of hemipelagic marls which filled the Isili Basin and are interpreted as post-rift deposits (Fig. 2b; Cherchi et al., 2000, Casula et al., 2001). This marly sequence commonly shows thin intercalations of clastic or mixed carbonate–clastic sandy tubidites in the lower and middle part. 3.1. The Isili Limestones During the late Aquitanian–early Burdigalian times, in the Isili Basin (Figs. 1 and 2a, b) foramol/

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rhodalgal (sensu Lees and Buller, 1972; Lees, 1975; Carannante et al., 1988a,b) sandy to pebbly carbonate deposits were laid down covering more than 30 km2 and ranging from 200 m in thickness in the productive areas to a few metres in basinal settings. According to Cherchi et al. (2000), carbonate production occurred mainly in the shelf areas located on the western and northern margins of the Isili Trough (Punta Trempu and Nurallao) and the bulk of the carbonate deposits laid down in the trough itself is proved to have been transported through gravity flows and/or bottom currents. Nevertheless, the same authors pointed out that limited amounts of carbonate sediments were locally and/or periodically produced within the Isili Trough in both marginal and channelised areas by limited extended bryozoan and/or bivalve dominated benthic communities. The Isili Limestones are commonly devoid of muddy fractions and comprise rudstones, floatstones, with grainstone to silty packstone matrix, grainstones and rarer packstones. Main skeletal components are red algae, bivalves, bryozoans, barnacles and benthonic foraminifers. Small coral colonies locally occur in the productive areas while planktonic foraminifers and glauconite grains occur occasionally in the marginal areas (e.g., Isili Village, Fig. 2a) and quite frequently in the distal basinal ones (e.g., Nuraghe Maurus, Fig. 2a). The Isili Limestones are commonly well-bedded with strata thickness ranging from a few centimetres to several decimetres. Parallel bedding is widespread in the productive areas while complex depositional architectures, which will be accurately described in this paper, characterise the adjacent outer shelf–slope sectors (tributary belts), the main channel and the basinal areas. Sharp erosive and ravinement surfaces are locally common throughout the sequences of the Isili Limestones (Cherchi et al., 2000). The erosive surfaces are commonly associated with reddened strata, locally bored and/or encrusted by oysters on top, which were interpreted as hardgrounds (Cherchi et al., 2000). These hardgrounds may occur as either individual beds or, more frequently, as up to several metres thick sets or packages of early hardened strata. The presence of hardgrounds in the study area appear to contrast with the behaviour of foramol carbonates, which, due to their overall calcitic

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mineralogy, are generally considered not prone to early cementation. Nevertheless, extensively earlycemented Foramol deposits have been recurrently reported from the Miocene of the central-southern Apennines, Italy (Barbera et al., 1980; Carannante and Simone, 1996) and of the Australian–New Zealand region (Nelson and James, 1995, 2000). In the Isili area, tectonic activity interacting with sea level changes controlled carbonate production and type and rate of sediment supply to marginal and distal areas and, in turn, strongly influenced the formation of hardgrounds (Cherchi et al., 2000). This conforms with the findings of Nelson and James (2000) who suggested that extensive sea-floor cementation may occur in foramol carbonate deposits during periods of starvation, by-pass and/or increased environmental energy which are related mainly, but not exclusively to sea level low-stands.

4. The Isili Submarine Channelised System Following the first reports of a complex network of channels in the Isili area (Cherchi et al., 2000), preliminary studies by Murru et al. (2001) and Simone et al. (2001) identified one main channel (the Isili Channel) and a conspicuous number of

tributary and distributary channels. The channels vary greatly in dimension and may be erosive, depositional or mixed, isolated or amalgamated (see Normak, 1970; Clark and Pickering, 1996). Channel and channel-related depositional elements (e.g., channel margins, lateral bars, levees, overbank, etc.), which are schematically represented in Fig. 3, were recurrently recognised at outcrop throughout the studied area. The channel complexes together with channel-related architecrtural elements (e.g., overbank, interchannel complexes, lobes, etc.) build up the Isili Submarine Channelised System which includes the tributary belts, the Isili Channel and its related fan. 4.1. Tributary belts Tributary channels were distributed along narrow belts (tributary belts), probably no wider than 1–2 km across, which extended through the outermost sectors of the shelf and the slope areas adjacent to the productive/source areas. These channels, which are markedly erosive, acted as sediment pathways and fed the Isili Channel. Tributary belt sequences are exceptionally exposed in the Isili Village area, where both N–S- and E–W-oriented fault scarps allow a reasonably good 3D control.

Fig. 3. Schematic block diagram showing main architectural elements for carbonate submarine channels.

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4.1.1. The Isili Village section The Isili Village area is located only 0.5–1 km east of the Punta Trempu carbonate production area (Fig. 2a). In this area, 60–80 m thick parallel-bedded rhodalgal rudstones and floatstones and rarer grainstones and packstones alternate with multiple channel and channel-related sedimentary bodies. Channels and scours, up to several tens of metres wide and 1–10 m deep (Fig. 4a–c), are alongside, overlap or intersect each other. These channels show margins sloping

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from 158 to 408 and are deeply cut into previous channel-fill and channel-related deposits or into the parallel-bedded units (Fig. 4a–c). The basal erosive surface is commonly reddened, early-hardened and/or is overlaid by early-hardened rudstone packages. The channels commonly show massive, trough-stratified or complex channel-fill architectures (see Vigorito, 2001). Clinostratified units were recognised locally and show up to 4 m high foresets. The channel-fill sequences commonly show a crude fining upward

Fig. 4. (a) Large-scale cross-stratified limestones which includes several tributary channels (Ch) and related interchannel ridges (IR), Isili Village section. (b) Detail of box (a), enlarged showing multiple stacked mixed erosional–depositional channel sequences. (c) Two stacked tributary channels (Ch) and related interchannel ridges (IR) overlying parallel-bedded rhodalgal limestones. Note the large-scale wedging and lensing geometry. Tributary belt, Isili Village section. (d) Clinoforms developed through lateral bar accretion downlapping on the ES surface (dashed line, see also Figs. 6 and 7). Isili Channel: Pardu–Casa Todde section. (e) Neptunian dyke (see arrows) developed within Isili Channel left margin sequences, Casa Todde section.

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trend and are mainly built up of coarse to fine rhodalgal rudstones and floatstones that exhibit chaotic or reverse graded texture and locally grain orientation and/or imbrication. Paleocurrent indications from scours and imbricated grains suggest flows mainly directed towards the eastern and northeastern sectors (Fig. 2a.1). Elongated ridges generally flank the channels and commonly separate adjacent channels (Fig. 4a–c). These ridges can be symmetric or asymmetric and exhibit a markedly convex transverse section with up to 408 steep flanks. The most prominent convex component of these ridges is usually an outer erosive surface which displays either a gently curved or a sharp angular apex (Fig. 4a–c) and commonly corresponds to the basal erosive surface of the adjacent channels. Multiple truncation and omission surfaces, often associated with sets of reddened and early-hardened strata, may be traced across the channels and/or the adjacent ridges forming complex internal geometries. Patchy and/or continuous hardgrounds are widespread throughout the area and occur mainly beneath or, less frequently, just above sharp erosive surfaces. Planktonic foraminifers and rarer glauconite grains are locally present. 4.1.1.1. Interpretation. The Isili Village area is interpreted as a relatively deep marginal area. Great quantities of sediments swept off from the inner shelf productive areas were deposited in this outer shelf and slope setting and arranged in east-dipping, parallelbedded, sheets of sediments to which large crossstratified bodies intercalated. The presence of numerous E–NE trending incisions (scours and channels) indicates a highly efficient, superimposed drainage system that allowed the removal and the re-deposition in deeper areas of large quantities of sediments. The ridges intercalated to the channels are interpreted as eroded interchannel ridges whose morphology was controlled by the shape, dimension, and relative position of the adjacent channels. Channel digression, avulsion and aggradation as well as repeated erosive-depositional events favoured the development of complex depositional architectures and concurred to create the wedging and lensing geometry (Fig. 4c) that characterises the carbonate deposits cropping out in the Isili Village

area. Similar geometries, reported by Quine and Bosence (1991), from the chalk sequences of Normandy (France) were interpreted as related to the formation of erosive structures, such as channels and interchannel ridges, in areas flushed by strong bottom currents. High energy environmental conditions are inferred to have also promoted early cementation episodes by pumping fluids into the pore-space. Sediment stabilization through sea-floor cementation also favoured the preservation of the high-angle cross-stratification which is widespread throughout the Isili Village area. 4.2. The Isili Channel The Isili Channel carbonate sequences crop out for about 3.5 km across the study area. These sequences were accurately mapped and logged allowing the reconstruction of the trend of the channel and its internal geometry. According to the field observations, the Isili Channel ran from the NW to the SE in its proximal reaches and sharply deflected to the SSE in its medial and distal reaches (Figs. 2a and 5). The Isili Channel fill sequence is on average 80 m thick and is divided vertically into two channel complexes (Channel Complexes A and B) which include several partly nested channel units which represent different fill stages. Up to nine channel units, each confined at the base by a sharp boundary surface, were identified. The boundary surfaces coincide with erosive surfaces and they separate the channel units, some which have markedly different lithology and/or internal geometry. Locally the boundary surfaces are less obvious, possibly due to sediment amalgamation. Up to 5 m thick packages of early-hardened strata occur locally beneath or pass laterally to the boundary surfaces. Channel Complex A is built up by the lower four channel units (A1–A4), which commonly exhibit trough-stratified to divergent fill architecture. Channel Complex A is truncated at the top by a main erosive surface (MES) which can be reliably traced throughout the modern Riu Corrigas Canyon allowing a fair correlation between the different sections studied of the Isili Channel (e.g., North Riu Corrigas, South Riu Corrigas, Casa Cantoniera and Is Cungiaduras sections). The MES surface separates Channel Complex A from Channel Complex B. This latter is formed by

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at least five channel units (from B1 to B5) and is characterised by complex internal geometries including widespread large-scale clinostratified units. Five sections, including exposures both parallel and transverse with respect to the inferred channel axis, are illustrated in the next paragraphs. 4.2.1. Pardu–Casa Todde section The Pardu–Casa Todde section is S–N oriented and extends for about 800 m across the most proximal outcropping portion of the Isili Channel. Mid-channel and the inner portions of both channel margin complexes are exposed in this section which is oblique at high angle (approximately transverse) with respect to the inferred channel axis (Figs. 2a and 5). Two detailed lithologic logs were measured in the Pardu and in the Casa Todde areas (Fig. 6). In the Pardu area (Fig. 5), the carbonate sequences overlay 2 m of tuffaceous sands with a sharp erosive contact (BES in Figs. 6 and 7). The first carbonate package is made up of a 1.5–2 m thick oyster bank with a mixed sandy siliciclastic–carbonate matrix (Fig. 6). This bank is overlain by 4 m thick tabular cross-bedded rhodolith-rich rudstones. These latter show 0.15–0.5 m thick strata and a crude fining and thinning upward

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trend. About 8 m of rhodalgal rudstones follow upwards. These are tabular cross-bedded (Fig. 6) and locally exhibit normal grading in the middle portion and reverse grading and/or grain oriented fabric in the upper part. The uppermost 1–2 m of this interval include several stacked reddish hardgrounds (Fig. 6). A sharp erosion surface (ES in Figs. 6 and 7) occurs at the top of this early-hardened package and is downlapped by clinoform foresets (Figs. 4d and 7) which dip NNE and are as steep as 20–258. These clinoforms, which extend from the channel margin towards the inferred channel axis (towards NE, Fig. 7) for about 250 m and down-channel for at least 300 m, are about 12 m thick and made up of fine floatstones/ rudstones and coarse grainstones, moderately to well sorted. Branching red algae fragments, small rhodoliths, serpulids, bryozoans and echinoids are among the main components. Rudstones rich in rhodolith, branching red algae fragments, and bivalves (Fig. 6) follow upwards. These deposits which onlap and cap the underlying clinostratified unit thicken towards the inferred channel axis and exhibit distinct concave-up to tabular bedding (Fig. 7).

Fig. 5. Isili Channel System sketch map. Modern-day physiography allows 3D geometrical analysis particularly along the steep walls of the Riu Corrigas Canyon, which represent an exceptional observation window for the Miocene Isili Channel. The location of the illustrated crosssections is also shown.

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Fig. 6. Lithological columns of the logged sections (cs: coarse sand; pb: pebble). The basal erosive surface (BES) and the main erosive surface (MES) correspond to prominent erosive surfaces that can be traced for long distances and allow correlation of different sections.

A sharp erosive surface which probably corresponds to the MES (Figs. 6 and 7) occurs at the top and is, in turn, overlain by 15–18 m of moderately sorted fine rudstones to grainstones. In the logged section, these deposits are commonly parallel-bedded but exhibit a sharp clinostratification only 150– 200 m eastwards. Up to 4 m thick bivalve rich

rudstones end the logged sequence. These deposits are commonly pebble to cobble-sized and show massive up to 1 m thick strata which dip 58 eastwards. Main transportation directions measured in the Pardu and adjacent areas are towards the E and SE (Fig. 2a.3).

Fig. 7. Schematic reconstruction of the stratigraphic and geometric relationships between Channel Complexes A and B (respectively CCA and CCB) and their relation to the basal tufaceous sandstones (gray unit). The illustrated section is oblique at high angle (approximately transverse) with respect to the inferred channel axis. Sketch of the Pardu–Casa Todde section.

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The carbonate sequence cropping out in the Casa Todde area shows lithostratigraphic patterns similar to those of Pardu area but is distinctly thinner (Fig. 6). The logged sequence also exhibits multiple erosional surfaces and locally 4–5 m thick clinoforms. These clinoforms are up to 20 m wide and 50–100 m apart and show southeastward dipping foresets. A sharp erosion surface (BES, Figs. 6 and 7) occurs at the base of the clinostratified unit whose top is truncated by another major erosion surface (MES, Figs. 6 and 7). Fractures, filled by rhodolith-rich rudstones occur locally within the carbonate sequence (Fig. 6e) and exhibit trends (NNW–SSE and E–W) largely consistent with the two major rift-related fault systems. Several metres thick sandy/silty turbidites followed by several tens of metres of hemipelagic marls cap the carbonate successions (Fig. 6). These siliciclastic to marly deposits are thin-bedded and are locally parallel- to ripple-scale cross-laminated. 4.2.1.1. Interpretation. According to strata geometries and sedimentological features, it appears that the carbonate successions cropping out in the Casa Todde and in the Pardu areas correspond, respectively, to the left and the right margin sequences of the Isili Channel (Figs. 5 and 7). The clinoforms recognised in both Pardu and Casa Todde areas are interpreted as lateral bars which prograded transversely toward the channel axis (Fig. 7). These clinoforms were onlapped and capped by concave up to tabular-bedded rhodalgal deposits which are interpreted as mid-channel sequences (Figs. 6 and 7). The logged carbonate sequence of the Casa Todde shows multiple erosional surfaces and a marked reduction in thickness (Figs. 6 and 7) when compared with the sequence cropping out in the Pardu area. This is reflected in a sharp asymmetry of channel morphology which is interpreted as relating to the presence of a major channel bend. In the Pardu area, which was located on the convex side (depositional) of the channel bend, thick and wide channel margin complexes were laid down through lateral bar accretion (Figs. 4d and 7). Conversely, on the concave (erosive) side of the channel bend (Casa Todde area), the prevalence of erosive regimes allowed only the deposition of thinner and poorly defined channel margin sequen-

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ces which were also locally and periodically truncated by erosive events and/or localised channel margin collapses. Lateral bars occasionally developed in the Casa Todde area, possibly in relation to channel thalweg digressions. Rhodolith-filled fractures (Fig. 4e) occurring in the Casa Todde area are interpreted as neptunian dykes produced by syn-sedimentary tectonics and localised channel margin collapses in early-cemented or at least partly consolidated sediments. 4.2.2. North Riu Corrigas section The North Riu Corrigas section is located about 700 m southeast of the Pardu area (Figs. 2a, 5 and 8). This section is 300 m wide and obliquely cut through the right channel margin and adjacent outer mid-channel deposits (Figs. 5 and 8). Two logs (Logs 1 and 2; Figs. 6 and 8) were measured from this section and include both Channel Complex A and B sequences which are separated by the MES surface. Channel Complex A sequences (CCA in Fig. 8) show at the base a few metres thick oyster bank which rests paraconformably on tuffaceous sandstones and rapidly pinch out southward (Figs. 6 and 9a). Parallel and tabular cross-bedded rudstones and floatstones, rich in red algae and bivalve fragments, follow upward. These are locally heavily bioturbated (Fig. 9b). In the lower part of this interval, the beds commonly show wavy basal contact and locally a crude Ta-c Bouma interval. The overlying deposits consist of fine floatstones and coarse grainstone/ packstones which are commonly parallel-bedded with rare small-scale scours. This interval includes several reddened hardgrounds and locally exhibits medium scale cross-stratification in the uppermost few metres. A sharp erosion surface (MES) truncates the top of Channel Complex A (Figs. 6 and 8) and, is in turn, overlaid by the Channel Complex B sequences (CCB in Figs. 8 and 9c). At the channel margin (log 1 in Figs. 6 and 8), the MES is downlapped by steeply inclined clinoforms which are made up of rhodolith-, serpulid- and branching red algae-rich floatstones/rudstones and grainstones. Multiple prominent erosion surfaces cut through the clinostratified unit and confine several vertically and laterally stacked minor channels which are filled by

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Fig. 8. Isili Channel right margin–outer mid-channel transition: Tabular to tabular cross-bedded Channel Complex A (CCA) sequences truncated at the top by the main erosive surface (MES). The MES is overlain by clinostratified lateral bar deposits of Channel Complex B (CCB) passing laterally to parallel-bedded outer mid-channel sequences. Note multiple incisions at the top of the carbonate sequence resulting from channel thalweg digression, channel margin collapses or tributary inputs. The section is oblique with respect to the channel axis, North Riu Corrigas section.

rudstones and subordinately floatstones rich in rhodoliths and/or bivalve fragments. Complex strata geometry and contorted beds were recognised beneath and between these minor channels (Fig. 8). Towards the channel axis (Fig. 8), the clinoforms pass laterally to or are onlapped by parallelbedded floatstones, rudstones and grainstones rich in rhodoliths, serpulids and branching red algae fragments which exhibit a crude fining upward trend (see log 2 in Figs. 6 and 8). These deposits are, in turn, overlaid by parallel stratified floatstone/rudstones and coarse grainstones rich in branching red algae and bivalve fragments which pass upward to coarse rhodolith-rich rudstones (Fig. 6 and 8). The carbonate succession is overlain by carbonate– siliciclastic pebbly breccias (Fig. 6) which exhibit clasts derived from both the Palaeozoic and Mesozoic basement locally associated with wood fragments. A few decimetres of fine silty-sandstones followed by

thinly bedded marls cap the succession (Fig. 6), marking the demise of carbonate sedimentation in the area. 4.2.2.1. Interpretation. The North Riu Corrigas section is located on the right margin of the Isili Channel. The exposed rock-wall allows 3D control on the transition from the right channel margin to outer mid-channel complexes (Fig. 8). The basal parallel-bedded sequences which build up the Channel Complex A deposits are here interpreted to correspond to channel-fill deposits. Following the main erosive event, which led to the formation of the MES surface, these deposits were overlaid by the fill sequences of the Channel Complex B (Figs. 6 and 8). These latter include well-defined clinostratified channel margin complexes which were deeply eroded into by minor-order stacked or intersecting channels. These latter are tributaries or minor channels developed following major channel

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Fig. 9. (a) A 2.5 m thick oyster bank (OB) resting on tuffaceous sandstones (TS) at the base of the North Riu Corrigas section carbonate sequence. (b) Small-scale cross-stratified, bioturbated bioclastic floatstones. Note the large burrow next to the pencil. Channel Complex A, North Riu Corrigas section. (c) Sharp erosive contact between the Channel Complexes A and B (see MES surface in Figs. 5 and 8), North Riu Corrigas section. (d) Sharp detach surface (see F1 in Fig. 10b) at the base of one of the tilted fault-blocks (hammer for scale); South Riu Corrigas section. (e) Detail of the Box in (d). (f) Bivalve-rich mid-channel deposits (MD) onlapping and capping lateral bar sequences (LB). Up-channel view of Channel Complex B, Casa Cantoniera section.

margin collapses and/or a relative sea level fall possibly boosted by tectonics. A crude inward (with respect to the channel axis) coarsening and thickening trend is recognised at the channel margin/outer mid-channel transition which is also characterised by the interfingering of steeply inclined channel margin elements and gently dipping mid-channel ones (Fig. 8). 4.2.3. South Riu Corrigas section The South Riu Corrigas section is located 200 m down-channel of the previously described North Riu

Corrigas section (Figs. 5 and 10a, b) and is orientated approximately NW–SE. The carbonate sequence includes 15 m of tabular, locally cross-stratified, red algae rich rudstones and floatstones locally intensively bioturbated (Fig. 6). This basal interval is overlain by about 5 m of coarse bioclastic limestones that show multiple reddened hardgrounds (Figs. 6 and 10b). The top of this earlyhardened package is truncated by the MES surface (Figs. 6 and 10a, b). This latter is downlapped by an approximately 12 m thick sequence of, moderately to well sorted, fine floatstones/rudstones and coarse

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grainstones (Figs. 6 and 10b). These deposits are made up mainly of branching red algae, rhodoliths and serpulids and arranged in large-scale clinoforms (up to 15 m high) dipping to E–SE (Figs. 6 and 10b). About 4 m of thick bivalve-rich rudstones onlap and partly cap the upper portion of the clinoforms and are, in turn, overlaid by a few metres of fine sandy siltstones followed by several tens of metres of hemipelagic marls (Fig. 6). Approximately 100 m south of the down-channel end of the clinostratified unit the carbonate succession is truncated by a sharp N20E-trending listric fault (F1 in Fig. 10b). The hanging-wall package appears to have been rotated and translated. Block disruption and fragmentation is documented by the presence of minor blocks with steeply inclined strata (Fig. 10a, b). Contorted bedding and possible injection structures occur locally beneath the fault, whose sole segment cuts through the base of the early-cemented package which occurs in the upper portion of Channel Complex A. Similar tilted blocks are also recurrently recognised down-channel for about 1.5 km along both sides of the modern Riu Corrigas Canyon. 4.2.3.1. Interpretation. The South Riu Corrigas section cuts through the right margin/levee and outer mid-channel complexes of the inferred Miocene Isili Channel (Fig. 10). The sedimentary succession, which comprises multiple hardgrounds, erosive as well as reactivation surfaces, indicates that multiple constructional phases occurred throughout channel life. The large-scale clinoforms which occur at the base of Channel Complex B are interpreted as resulting from the accretion of a lateral bar, which developed through progradation on channel margin/levee complexes deposited during the previous filling phases of Channel Complex A (Fig. 10a, b). The tilted blocks recognised along the walls of the modern Riu Corrigas Canyon are interpreted as megabreccias (Figs. 5, 10a, b, 11c and 12a–c). The troughs and pools created through block sliding and tilting are commonly draped and onlapped by sediments pertaining to the overlying channel units (Figs. 10a, b, 11b and 12a, b). These sediments were locally tilted through repeated fault reactivation events which are made obvious by the presence of multiple, tilted growth structures (Fig. 10b).

4.2.4. Casa Cantoniera section The Casa Cantoniera section (Figs. 5 and 10c) is located about 500 m down-channel of the South Riu Corrigas section. About 8 m of skeletal rudstone/floatstones and grainstones and are found at the base of the sequence (Fig. 6). These are rich in red algae and bryozoans with subordinate, though locally abundant, serpulid and bivalve remains. Lenses and levels of rhodolithrich rudstones are locally found at the base of this interval. Parallel and lenticular beds associated with minor-order wavy and cross-stratification prevail. Beds form sets which markedly thicken downchannel and towards the channel axis and exhibit sharp erosive bases (Fig. 10c). Multiple reddish hardgrounds occur in the last 2–3 m of this interval and underlie the MES surface (Fig. 6). This latter is downlapped by moderately to well sorted coarse grainstones and fine rudstones/floatstones, rich in branching red algae fragments, with minor amounts of serpulids, bryozoans and bivalves. These deposits, which correspond to the base of the Channel Complex B, are 15 m thick and exhibit sharp clinostratification (Fig. 6). Foresets become progressively gentler down-channel and towards the channel axis, showing a distinct concave-up profile (Fig. 10c). Towards the channel axis the clinoforms are truncated at the top and partially cut through by a sharp concave-up erosive surface which is, in turn, onlapped by parallel- to cross-bedded coarse rudstones rich in bivalves (Figs. 6 and 9f). The thickness of these rudstones passes rapidly from a few centimetres on the top of the clinostratified bodies to 5 m towards the channel axis over a distance of a few tens of metres. The carbonate sequence is capped by about 2 m of fine silty sandstones followed by 40–50 m of marls which exhibit a few sandy turbidite intercalations (Fig. 6). 4.2.4.1. Interpretation. The section described is approximately oblique at low angle with respect to the channel axis (Figs. 5 and 10c) and is considered to be cut into channel right margin/levee complexes and adjacent outer mid-channel complexes (Fig. 10c). Strata in Channel Complex A exhibit a distinct wedging geometry, with strata thickening toward the southeast. This strata geometry corresponds to the

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pp. 15–16

Fig. 10. (a) Isili Channel longitudinal section. Spectacular exposures of both Channel Complex A (CCA) and B (CCB) along the modern Riu Corrigas Canyon (window of observation) which intersect at different angles the Miocene Isili Channel allowing a reasonable 3D control. Trend of the exposures (dashed line) in relation to the Isili Channel trend is shown. Note the multiple tilted fault-blocks (Isili Megabreccias). (b) Lateral bar, up to 15 m high (CCB), prograding over previous channel margin/levee complexes (CCA). On the right side of the photo, large, tilted fault-block and related growth faults, appear (numbers indicate the chronology of block tilting events and related faulting). South Riu Corrigas section. (c) Lateral bar downlapping MES surface. Note the wedge-like geometry of the underlying Channel Complex A deposits which are interpreted to correspond to the transition from the channel margin to the channel axis. Location of the lithological log and of Fig. 9f is also shown. Casa Cantoniera section.

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pp. 17–18

Fig. 11. Up-channel view (view to north) of the transverse section of the Isili Channel: (a) Channel Complex A fill sequences truncated at the top by the MES surface. Note the levee complexes developed on previous channel margin complexes. Right margin of the Isili Channel. Western Is Cungiaduras section. (b) Transverse section through mid-channel and left margin/levee complexes of the Isili Channel. Note the multiple stacked, partly nested channel units. Eastern Is Cungiaduras section. (c) Detail of box in (a). Large tilted fault-block (TB-a, see also Fig. 12a) and related growth faults. See Fig. 10 for legend. (d) Detail of box in (b). Multiple partly nested channel units and related levee complexes (CCB) overlying the MES surface. Note the sharp asymmetric profile and the well-developed lateral accretion surfaces.

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transition from the channel margin to the channel axial portion (Fig. 10c). The logged clinoforms in Channel Complex B are interpreted as a lateral bar which resulted from progradation of channel margin complexes over previous channel margin and outer mid-channel complexes (CCA in Fig. 10c). Also in this case, a sharp erosive surface (MES in Fig. 10c) separates Channel Complexes A and B. A minor-order erosive event documented by a sharp erosive surface led to the partial erosion of previous clinostratified deposits and to deposition of bivalve-rich rudstones. These latter display a divergent channel-fill architecture and are interpreted as mid-channel deposits (Figs. 9f and 10c). 4.2.5. Is Cungiaduras section A transverse section of the Isili Channel is discontinuosly exposed in the Is Cungiaduras area along E–W-oriented cliffs located on both sides of the modern Riu Corrigas Canyon (Figs. 2a and 5). In this area, the Isili Channel shows a flat bottom and a sharp asymmetric profile with the right (western) side largely exceeding the left (eastern) one in width and height. Channel Complex A sequences are largely exposed in the western outcrops (Figs. 5 and 11a) and show a prominent erosion surface (up to 158 steep) at the base which cuts through tabular-bedded rhodalgal limestones. This basal erosive surface is followed upward by at least four channel units which build up Channel Complex A. Individual channel units are confined at the base by sharp erosive surfaces which are as steep as 208. These basal erosive surfaces are draped and then onlapped by rhodalgal rudstones, floatstones and grainstone which build up the bulk of each channel unit (Fig. 11a). Bedding is commonly sigmoidal to curved with strata dipping and crudely thickening towards the channel axis. Minor-order erosion and omission surfaces as well as sets of hardgrounds commonly occur within individual channel unit and locally are too numerous to be labelled with confidence. Mound-shaped units, up to 10 m high and 40–80 m wide, are commonly developed on the edge of individual channel units. These mound-shaped units are crudely convex and convex cross-stratified (Fig. 11a) and exhibit beds which extend laterally into the adjacent channels at least for several tens of metres.

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The upper portion of Channel Complex A shows a several metres thick package of multiple patchy to continuous reddish hardgrounds and is truncated by two, closely spaced, syn-sedimentary listric faults (Fig. 11c). These faults strike averagely N10E and their sole segments cut through the base of the hardground package. Bedding between the two faults is discontinuous, contorted, irregular or massive. A large tilted block (L=50 m, l=30 m, h=15 m; TB-a in Figs. 11c and 12a) crops out at the hanging wall of the easternmost fault. This latter is superbly exposed along the right-hand side of the modern Riu Corrigas Canyon (Figs. 5 and 12a) where the pool created through block-faulting is draped by rhogalgal limestones, possibly belonging to the lowermost portions (Fig. 12a, b) of Channel Complex B. Both faults are sealed by parallel-bedded coarse grainstones. These latter cap Channel complex A sequence and correlate to the lower portion of Channel Complex B sequence (Fig. 11a, c). The modern Riu Corrigas canyon truncates the exposures. An erosion gap, up to 150 m wide, lies between the western and eastern sections and thus a precise correlation between both sides proved difficult. Channel Complex B sequences are largely exposed along the eastern Is Cungiaduras section and show at least five partly nested channel units (Figs. 5 and 11b, d) which are up to 30 m thick as a whole. The lower units of Channel Complex B are made up of rhodolith- and serpulid-rich floatstones/ rudstones and exhibit, at the right (western) channel margin, sigmoidal to curved strata which slope 15– 208 eastward. Tabular to lenticular strata geometries are developed within the lower units fill sequences which are about 16 m thick in the logged section (Fig. 6). Three, closely spaced, locally fused, minororder erosion surfaces follow upward (Fig. 11d). The related fill sequences are partly nested and consist mainly of coarse rhodolith- and bivalve-rich rudstones and subordinate floatstones (Figs. 6 and 11d) which are altogether 10–15 m thick. Sandstones/siltstones, less than 1 m thick, appear on the top of the carbonate succession and pass rapidly into marls that locally intercalate sandy turbidite layers. On the easternmost portion of the section described, the basal erosion surfaces of the individual channel units become progressively steeper and start

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Fig. 12. (a) Tilted fault-blocks (TB-a and Tb-b) and related interblock pool. Note growth fault plane (dashed gray line). View to west, Is Cungiaduras. (b) Detail of box in (a). Rhodalgal limestones draping the growth fault plane (dashed line) and a tilted block (TB-b). (c) Distal deposits of the Isili Megabreccias. Note the complex architectural geometries created through the emplacement of several disrupted blocks, injection of underlying unlithified deposits and subsequent scouring. View to east, Is Cungiaduras. Gray box: Transverse section of the main channel (corresponding to the right side of Fig. 11c).

to dip averagely towards the west (Fig. 11b, d). Here the carbonate deposits as a whole form a flat-topped ridge which confine the Isili Channel to the east (Fig. 11b, d). This ridge is 20–25 m high and 250 m wide at outcrop and is, in turn, built up of several moundshaped, convex to cross-stratified units (Fig. 11d). These latter are stacked with minor lateral offsets testifying a progressive aggradation through time (Fig. 11d). The boundary surfaces that can be confidently traced through the eastern ridge appear to confine the mound-shaped units (Fig. 11d) and pass outward (with respect to the channel) into omission surfaces frequently reddened. The mound-shaped units cropping out on the eastern side (Fig. 11d) are analogous to those described in the western Is Cungiaduras

section (Fig. 11a) but smaller in size. Furthermore, the eastern units are mainly vertically stacked with minor lateral offsets (Fig. 11b, d) while the western ones show significant lateral offsets and minor vertical ones (Fig. 11a). An up to 20 m thick package characterised by deformed bedding, injection structures and small- to large-sized disrupted, displaced and/or floating blocks crops out for several tens of metres on both sides of the modern canyon, just down-channel of the Is Cungiaduras section (Fig. 12c). 4.2.5.1. Interpretation. The Is Cungiaduras section, which is cut transversely with respect to the channel axis, allows the identification of multiple, multistorey,

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nested, stacked channel units (Fig. 11a, b, d) which make up the Isili Channel succession. There are at least nine carbonate channel-fill sequences to which mound-shaped units locally correspond on one or both sides. These latter are interpreted as levee complexes which grew up and aggraded probably in relation to significant variations in the depositional rates which occurred during the deposition of each sequence (see Section 5.2.1 for further information). Some of the individual beds which build up the levees can be traced at least into the channel margins, displaying continuity for several hundreds of metres (Fig. 11a, d). These strata correlate with the upper portion of the fill sequence of the related channel unit and suggest that levees were developed only after the related channel unit was partially filled. Paleocurrents measured from levee sequences cropping out on both channel sides (Fig. 2a.2, 2a.7) indicate flows mainly directed outward (with respect to the channel) while inward and down-channel-trending flows were recurrently measured within the channel margin and mid-channel complexes (Fig. 2a.2, 2a.6). A crude coarsening trend from the channel margins towards the channel axis was recognised within each channel-fill sequence while no significant vertical trend was identified throughout the Isili Channel carbonate succession. This latter is draped by siltstones and marly deposits which are interpreted as abandon deposits which mark the progressive demise of the Isili Cannel as a sedimentary conduit. The Isili channel is sharply asymmetric with a higher and wider right side and a lower and narrower left one. The right channel margin was oversupplied with respect to the left one by multiple east-trending tributaries that connected the marginal areas (Isili Village area) with the Isili Trough. Preferential overspilling of gravity flows on the right side may have also occurred through flow deflection caused by the Coriolis force. The clinoforms developed at the base of Channel Complex B resulted from the progradation towards the channel axis of lateral bars following the formation of the MES (Fig. 11d). Transverse exposures (with respect to the channel axis) from cliffs located only 100 m down-channel of the Is Cungiaduras section show, in the medial and upper parts of the Isili Channel carbonate succession, a conspicuous number of minor-order channel bodies which intersect, flank and overlap each other. Bedding

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is here less obvious, poorly defined, lenticular to massive, suggesting extensive sediment re-working and amalgamation. These features are interpreted as resulting from the splitting of the main channel body into several minor-order incisions which show a scattered radial trend pattern directed towards the southern sectors. With regard to the tilted blocks, these have their detachment surfaces cut through the base of the earlycemented package which occur in the upper portion of the Channel Complex A and are interpreted as megabreccias (Isili Megabreccias, Fig. 5) resulting from a major channel margin/levee collapse. The floating blocks and boulders associated with contorted bed packages, as well as injection structures, cropping out south of the Is Cungiaduras section (Fig. 12c), are interpreted to correspond with the distal deposits of this gravitational event. It is worth noting that growth faults are parallel to the main fault system, which is NNW–SSE oriented and may document a WSW– ENE-distensive phase which, in turn, favoured the collapse of the channel margin/levee complexes and the emplacement of these megabreccias. Locally the same growth faults were repeatedly re-activated thereafter as testified by the presence of multiple growth structures (Fig. 11b). 4.3. Distributary zone—proximal fan South of the Is Cungiaduras area, multiple distributary channel complexes create a branching network fringed by overbank sheet-like deposits and planar-convex bodies, cropping out over 2 km downdepositional dip to the south. The carbonate sediments laid down in this area are gravelly to sandy and are divided by a sharp, probably erosive, surface (MES) in two vertically stacked complexes, here informally defined as Fan A and Fan B which relate, respectively, to Channel Complexes A and B. Both dip- and strikeoriented sections of Fan A and Fan B are extensively exposed along the wall of the Riu Corrigas Canyon throughout the S’Acqua Salia and Nuraghe Maurus area (Figs. 2a and 5). 4.3.1. S’Acqua Salia–Nuraghe Maurus Section The sedimentary succession cropping out in these distal areas shows, at the base, a few metres thick tufaceous sandstones overlain by thin-bedded skeletal

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grainstone/packstones to fine rudstones. These carbonate deposits build up the Fan A complex and exhibit a dominant sheet-like geometry and only occasionally minor, diverging, mixed erosional/depositional and depositional channels often associated with ripple- to dune-scale cross-stratified units (Fig. 13). The thickness of the Fan A complex sharply decreases down depositional dip (southward) from 10 to 15 m in the nearby of the Is Congiaduras area to less than 1 m and locally to 0 m (Nuraghe Maurus area) over a distance of about 1 km. A sharp probably erosive surface (labelled as MES in Figs. 13 and 14) separates the Fan A from the Fan B complex. This latter overlays the Fan A complex and extends far to the south of Fan A distal end. The Fan B complex is made up of coarse rhodalgal rudstones/floatstones and grainstone/packstones. Bedding is less obvious and averagely thicker with respect to the Fan A complex and massive to crossstratified units commonly occur (Fig. 13). The thickness of the Fan B complex ranges from 5 m in S’Acqua Salia area to about 1 m in the most distal exposed areas. The Fan B Complex includes several small asymmetric planar-convex bodies (Figs. 14 and 15a) which are separated and/or dissected by multiple channels, 10–50 m wide and up to 5 m deep (Fig. 15a–c). These channel bodies can be leveed or not and show, when mapped, a radial distribution with channels diverging and splitting into minor-order channels southward from the Is Cungiaduras area. The mixed erosional/depositional channels have Ushaped profiles (w/hb10:1) and have massive, trough-

stratified or transverse fill architectures (Fig. 15a–c). The depositional channels commonly show a flat lenticular morphology (w/hN10:1) and divergent to trough-stratified channel-fills. The filling sequences of the channels are made up of rudstones and subordinately floatstones that are commonly rich in rhodoliths and/or bivalve remains. Rudstones, made up mainly of bivalves, generally dominate at the top of the carbonate succession and fill the youngest channels. The planar-convex bodies are up to 300 m long, 200 m wide and 4 m high and are characterised by tabular to planar-convex stratification and rarely by crude cross-stratification in transverse section (Figs. 14 and 15a). Down-current dipping convex strata were locally recognised, in longitudinal section, on the down-current side of the planar-convex bodies. In the S’Acqua Salia–Nuraghe Maurus area, paleocurrent distribution shows a radial, significantly scattered, pattern with flows crudely directed towards the southern sectors (Fig. 2a.8). The logged section is located in the Nuraghe Maurus area, about 2 km southwest of the Is Cungiaduras section (Figs. 2a and 5). The logged sequence (Fig. 6) shows at its base 2 m of tuffaceous sandstones overlain by 0.9 m of bioclastic rudstones with a yellowish-greenish packstone–wackestone matrix. Rhodoliths and lithoclasts up to 0.04 m in diameter are common and average grain size range between 0.005 and 0.01 m. About 1.2 m thick rhodolith and pebble-rich (derived from both Palaeozoic sandstones and Mesozoic carbonates) rudstones/ floatstones follow upwards. The matrix is similar to

Fig. 13. Dip section of the Isili Fan System showing both Fan A (FA) and B (FB) complexes separated by the MES surface. Note the markedly different depositional architectures of Fan A and Fan B complexes.

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Fig. 14. Transverse section of a lobe within Fan B sequences, resting on tabular-bedded fine rudstone/floatstones and coarse grainstones (Fan A). S’Acqua Salia–Nuraghe Maurus section.

that of the underlying deposits and a sharp fining upwards trend is developed. The carbonate sequence ends with 1.6 m thick small-scale cross-stratified grainstones locally including small floatstone–rudstone lenses which pass upwards to marls. It is worth noting that planktonic foraminifers and glauconite grains are abundant throughout the logged succession. 4.3.1.1. Interpretation. The Nuraghe Maurus section represents the most distal outcrop of the Isili channel system. A well-developed distributary channel net-

work is recognizable at outcrop. This network consists of numerous hierarchically organised minor channels that overlap and lie alongside each other over an exposed area about 2 km long and 1.5 km wide. Two main sequences were recognised to correspond to the Fan A and Fan B complexes which relate to different phases of active fan-system growth separated by a main erosive event represented by the MES surface. The Fan A complex has a dominant sheet-like geometry while the Fan B is characterised by the coalescence of distributary channels, overbank ele-

Fig. 15. a) Interchannel-ridge (IR) confined between two distributary channels (Ch). Nuraghe Maurus area. (b) Detail of box in (c). Note the transverse channel-fill architecture of the distributary channel (Ch). (c) Transverse section of channel-complexes (Ch) and related levees (Le). Fan B complex, Nuraghe Maurus.

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ments and planar-convex bodies. These latter are interpreted as lobes and interchannel ridges (Figs. 14 and 15a) which locally exhibit poorly defined downcurrent prograding frontal accretion surfaces. The depositional architectures which characterise the S’Acqua Salia–Nuraghe Maurus section as well as the occurrence of a discrete number of sedimentary lobes, associated with numerous distributary channels diverging southward from the Isili Channel outlet, indicates a proximal fan environment.

5. Discussion From the analysis of the sedimentological and geometrical features of the limestones cropping out in the Isili area, all the architectural elements (levee, channel margin, channel-fill, tributaries, distributaries, etc.) that build the submarine channel system have been identified. The morphology and function of these elements, and their inter-relationships will be detailed discussed in order to reconstruct the anatomy of this channel system and its related fan. 5.1. Tributaries and other minor incisions The tributary channels are relatively short-lived incisions that heavily dissect the Isili fault-block, acting as transverse drains and feeding the main channel (Fig. 4a–c). The tributaries range from scours, a few metres wide and deep, to incisions up to 80 m wide and 15 m deep. Tributaries are, in turn, fed by minor-order incisions which show a rather scattered trend pattern. This results in a complex channel network characterised by a strict hierarchy. The smaller incisions commonly show a massive fill and are interpreted as cut and fill structures. The increase in channel dimension led to the development of more complex fill architectures. Concave-up, trough crossstratified, divergent-fill and rarely transverse channelfill, or a combination of these architectural styles (complex channel-fill, see Vigorito, 2001), characterise the tributaries and other major incisions in their proximal and medial reaches where they show a sharp V-shaped or flat-bottom transverse profile. In such proximal settings, channels are markedly erosive and locally steeply inclined (up to 308). Interchannel ridges commonly developed between adjacent inci-

sions from the erosion of previous channelised or nonchannelised deposits, giving rise to a complex medium- to large-scale cross-stratification (Fig. 4a– c). Filling sediments are, on average, coarser than those filling the main channel and locally reach cobble size. These sediments show a chaotic, or a normal or reverse graded texture. Sedimentological and geometric features suggest that gravity flows, such as debris flows, grain flows and high-density turbidites were the main transportation agents within the tributary channels. These gravity flows were largely favoured by the palaeophysiography and probably by the tectonic instability of the whole area. 5.2. The main channel body 5.2.1. Levees Levees are sediment embankments, developed on both sides of the Isili Channel, produced by repeated overspilling of sediments from the channel. The levees form convex depositional bodies which exhibit strata dipping towards the channel axis on their inner side and away from the channel axis on the outer side (back-levee). According to Kenyon et al. (1995), Clark and Pickering (1996), Hiscott et al. (1997) and Camacho et al. (2002), levees have a positive depositional relief above the sea-floor and related deposits show slump-folds and/or paleocurrent signatures directed away from the channel axis but with considerable local dispersion caused by topographic complexities and overspill from neighbouring channel bends. A similar current distribution pattern characterises the levee complexes recognised in the Isili area (Fig. 2a.2, 2a.6, 2a.7). In the Is Cungiaduras area (Figs. 2a and 5), where a complete transverse transect of the Isili Channel is exposed, each channel unit may show related levee sequences developed on one or both sides of the channel (Fig. 11a, b, d). Levees recognised within the channel units which build the Isili Channel succession are mound-shaped cross-stratified and may be symmetric to markedly asymmetric. On the right channel side, levees are 5–15 m high, up to 80 m wide and have a relatively low width/height ratio. The backlevee slopes are commonly 5–108 steep. The inner margin of the levee, on the other hand, is up to 258 steep but tends to become gentler up-sequence. On the left channel side, levees are averagely lower (max-

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imum 5 m high) and usually wider (up to 250 m) and flat-topped in the upper channel units (Fig. 11b, d). These levees show an inner slope no steeper than 208, while the back-levee may slope 5–128. Early-hardened surfaces are locally common within the backlevee sequences and are interpreted to correspond to periods of sediment starvation which allowed sediment stabilization. In siliciclastic sedimentary settings, the morphology of levees is commonly not preserved due to differential compaction that may lead to an inversion of the original depositional relief. In these cases, levee complexes are distinguished mainly on the basis of their higher shale/sand ratio with respect to the related sand-rich channel bodies. These architectural and sedimentological features were not recognised from the Isili Channel and other submarine channels elsewhere reported from foramol carbonate and mixed sedimentary environments (Braga et al., 2001; Carannante and Vigorito, 2001; Vigorito, 2001). These channel systems, in fact, are almost devoid of muddy fractions and levees, where documented, commonly present a positive relief and are made up of sand- to pebble-sized sediments, only on average finer than related channel-fill deposits. These levees did not suffer of significant differential compaction processes due to the paucity of muddy fractions and commonly show multiple early-cemented horizons which remarkably increase the preservation potential of their original morphology. The development of levee complexes in the Isili Channel is commonly related to the last filling phases in the related channel units (Fig. 11a, c). This implies that the formation of levees was temporally unrelated to the beginning of deposition in the adjacent channel unit. Lateral persistence of the strata of levees into the adjacent channels as well as the absence of any grain size break between axial and levee deposits suggest deposition from gravity flows which extended from the channel into the overbank areas. The thickness of these flows must have exceeded the channel depth allowing the spillage of sand- to pebble-sized sediments leading to the formation of coarse-grained levees. Laterally depleting flow conditions are inferred for pinchouttype strata terminations towards the outer fringes of levees. The spillage of coarse carbonate sediments is also favoured by their peculiar hydrodynamic behav-

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iour. These sediments, in fact, have characteristic (shape, internal structures, internal porosity, density) which strongly increase their buoyancy respect to siliciclastic sediments of analogue grain size. Hence, in carbonate sedimentary settings, gravity flows are able to transport sediments coarser and/or for longer distances respect to flows of similar magnitude in their siliciclastic counterparts. 5.2.2. Channel margin Levee complexes pass inward to channel margin deposits (Figs. 10a, b and 11a, b, d). These are generally coarser than related levee deposits, and are built up mostly of rudstones, floatstones and grainstones which crudely coarsen both inward and downward. The channel margin sequences drape and/or onlap the sloping margins of the sharp erosive surfaces (see Figs. 10a, b and 11a, b, d), which are deeply cut into previous levee/margin and/or channel-fill complexes. These erosive surfaces may show evidence for early sea-floor cementation or pass into early-hardened packages. Scouring is not largely developed and even the major erosive and/or nondepositional surfaces are more likely to be smooth. Lateral bars, up to 15 m high and up to 300 m long (Figs. 4d and 10a–c), were commonly developed mainly but not exclusively on the right channel margin following major erosive events. These bars are made up of moderately sorted, fine rudstone/ floatstones and coarse grainstones and may occasionally pass into down-channel prograding bedforms towards the channel axis. The bars, which are characterised by tangential to sigmoidal bedding with smooth strata surfaces, resulted from sediment deposition and reworking under sustained hydrodynamic regimes and relatively stationary flow conditions. The Channel Complex A sequences, made up mainly of rhodalgal coarse deposits, commonly exhibit a well-defined trough- to divergent-fill architecture. This rather simple geometry allows us to distinguish only minor geometrical features attributable to channel margin complexes. In this case, channel margin deposits initially drape and then eventually onlap the basal boundary surface without significant thickness variation or peculiar geometrical or sedimentological features. Extensive small-scale cross-bedding, as well as scouring and the occurrence

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of Ta-c Bouma’s intervals found in both channel margin and mid-channel sequences, suggest that turbidity currents were significant along-channel transportation agents at this early stage of channel life. Lateral bars were recognised at the channel margins of Channel Complex A, only in the most proximal section (Pardu–Casa Todde section) which was located within the major channel bend of the Isili Channel. Thus the development of the lateral bars was probably specifically promoted by this peculiar seafloor topography. Channel Complex B, in contrast, includes well-defined channel margin sequences, marked by significant variation in sediment thickness and by the presence of large-scale lateral bars (Figs. 8, 10a–c and 11b, d). These bars are widespread throughout the Isili channel and are inferred to reflect an increase of the sinuosity of the channel. We speculate that this increase of channel sinuosity was favoured by or resulted from the emplacement of megabreccias, which created an irregular sea-floor topography, and/or from successive tilting events of the megabreccia-related displaced blocks. Megabreccias, commonly associated with growth faults and sand injections, were recognised on the right (western) channel margin at the passage between Channel Complexes A and B. These megabreccias exhibit sharp detach surfaces which cut through the early-hardened bed package occurring at the top of Channel Complex A and resulted from a major channel margin collapse. According to Spence and Tucker (1997), tectonics and/or pore-water overpressures, generated in horizons hydrologically confined between early-cemented bed packages during relative sea level falls, have a prominent role in promoting the deposition of megabreccias in carbonate sedimentary environments. 5.2.3. Mid-channel At least nine channel units are easily recognizable at outcrop within the Isili Channel carbonate succession (Fig. 11a, d). The mid-channel sequences, mainly built up of coarse rhodalgal deposits, are commonly trough-stratified. A second-order cross-stratification developed locally within the channel axis deposits through multiple minor erosive/depositional events. Repeated major erosive events, probably related to relative sea level falls, are inferred for the erosion of previous channel-fill deposits and for the development

of multiple stacked and/or nested channel units which are recognised within the Isili Channel succession (Figs. 9f and 11a, b, d). Lenticular strata locally occur within the mid-channel sequences. These are interpreted as lag deposits produced through minor channel thalweg digression. Some of the individual channel units have mid-channel complexes which exhibit onlap-type strata terminations and subordinate interfingering at the transition with related channel margin complexes (Figs. 7, 8, 9f and 12a, b, d). This feature suggests that the bulk of mid-channel deposits postdate the formation of channel margin complexes. A two-stage model of channel filling is therefore invoked for those channel units showing evidences for lateral accretion at channel margins. The first phase involves the erosion of previous deposits and the development of a new channel conduit. At this stage, the depth of the channel exceeds the thickness of the sediment flows. These latter are hence confined into the channel resulting in bed erosion and high degrees of sediment by-pass in mid-channel areas. On the channel margins, instead, laterally depletive flow conditions may allow time equivalent deposition. The flows are thus able to deposit sediments preferentially into the marginal areas while eroding in the mid-channel ones favouring active lateral accretion of channel margin complexes through lateral bar progradation. During the second phase, the flows were more depositional overall and sedimentation rates were highest in the mid-channel areas. At this stage, channel-fills vertically aggraded leading to the onlap of mid-channel deposits on channel margin complexes. The first deposits were restricted to the channel floor but, as the channel filled, progressively increasing amounts of coarse sediments were spilled over the channel margins locally allowing the formation of levees. Planktonic foraminifer-rich marls cap the carbonate channel-fill succession and occur in the form of drapes that smooth the channel morphology. These marls are interpreted as corresponding to abandon deposits laid down after a major drowning event recorded throughout the Sardinia Rift Basin. Coarse sandy/pebbly turbidites as well as slumped intervals occur locally within the marly sequences, indicating that the channel was still active as a sedimentary conduit at least during the first phases of sandy/marly sedimentation.

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5.3. Distributary zone and fan The southernmost areas investigated (e.g., S’Acqua Salia–Nuraghe Maurus) of the Isili Channel system includes all the depositional elements indicative of a distributary zone/fan setting (Figs. 2a, 5, 14 and 15a– c); see Mutti, 1985, 1992; Mutti and Normak, 1987; Johnson et al., 2001). Two distinct fan complexes separated by a sharp probably erosive surface were recognised at outcrop. The lowermost complex, Fan A, is commonly thin-bedded and exhibits a dominant sheet-like internal geometry with subordinate distributary channels. The Fan B sequences, in contrast, are characterised by complex internal geometries resulting from the coalescence of multiple channel-levee complexes, interchannel ridges and lobes (Figs. 2a, 5 and 15a–c). Distributary channels in Fan B form a hierarchically organised branching network in which channel digression and avulsion must have been quite common as demonstrated by the occurrence of multiple stacked, intersecting or adjacent channel bodies (Fig. 15a, c). The markedly different internal geometries which characterise the Fan A and Fan B complexes and their relative positions suggest that the deposition of the Fan B sequences corresponded to a basin-ward shift of the fan system probably in relation to a major sea level fall.

6. Conclusions Following the extensional phases related to the formation of the Oligo-Miocene Sardinia Rift Basin, the Isili area was dissected into a series of small faulted blocks aligned approximately N–S on both sides of a relatively narrow trough (Isili Trough, Figs. 2b and 16) which sloped 10–208 southward. On the western side of the trough (e.g., Punta Trempu) and on its northward culmination (Nurallao area), foramol/rhodalgal carbonate factories set up on the topographic-highs. On the other hand, the northeastern side of the trough, the emersion of the Palaeozoic and Mesozoic basement (e.g., Monte Rasu and Sarcidano area) led to the deposition of thick fan-delta sequences (e.g., Isili Industrial area, Figs. 2a and 16; Cherchi et al., 2000; Murru et al.,

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2001; Simone et al., 2001). According to Cherchi et al. (2000) and by comparison with analogous foramol/rhodalgal carbonate factories documented elsewhere (see Carannante et al., 1981, 1996; Carannante and Simone, 1996), a water depth ranging from 30 up to 80 m is inferred for the productive shelf areas. Assuming that only minor significant differential vertical displacement between the outcropping areas occurred following the deposition of the Isili Limestones, a water depth of a few hundreds of metres is estimated for the more distal of the investigated areas (e.g., S’Acqua Salia– Nuraghe Maurus section) although shallower conditions may have periodically occurred during negative relative sea level oscillations. The abundance of planktonic foraminifers and reworked glauconite grains in the deeper areas (e.g., S’Acqua Salia–Nuraghe Maurus section) fits well with the hypothesized paleobathimetry. The depositional architecture of foramol carbonate channel-fill, as well as channel evolution, depends on sediment characteristics (sediment nature and grain size), transportation mechanisms (bottom currents and gravity flows), early diagenesis and topographic settings (see also Braga et al., 2001; Carannante and Vigorito, 2001; Vigorito, 2001). As far as the tributary channels are concerned, these ran across the tributary zone and were filled by coarsesized debris and/or grain flow deposits. High-energy environmental conditions, as well as multiple erosive/depositional events, boosted by relative sea level oscillations and/or tectonics, favoured the development of complex wedging and lensing depositional architectures formed through the overlapping and interfingering of multiple stacked and or nested channel bodies. Tributaries in such a proximal setting are markedly erosive and levee complexes are commonly absent. In more distal sectors (near the confluence with the Isili Channel), a sharp decrease in the slope dip angle is reflected in the tributaries by the development of a trough-stratified locally trough cross-stratified channel-fill. Channel aggradation, in addition to well-defined channel margin sequences, may be commonly recorded in this setting. The Isili Channel ran W–E draining both the Punta Trempu and Nurallao carbonate factories and deflected towards the S in its medial reaches (Fig. 16).

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Fig. 16. Block diagram of the Lower Miocene palaeophysiography of the Isili area. The Isili Channel and the related fan as well as main sediment transportation directions (arrows) are shown (modified after Casula et al., 2001).

The Isili Submarine Channel is a mixed, erosional/ depositional, channel which shows a multistorey, stacked, partly nested architecture produced by superimposition of multiple channel units, commonly confined by sharp erosive surfaces. The channel internal geometry documents a progressive shifting toward the east of its depositional axis (lateral migration of the channel thalweg) in relation to preferential accretion of its western (right) channel margin. Channel-fill architectures are locally complicated by the confluence of tributaries and by major levee/margin collapses. These latter resulted in the emplacement of megabreccias which created an irregular channel-floor topography and, in turn, favoured the development of the complex large-scale cross-stratified geometries which characterise the overlying deposits (Figs. 10a, b and 12a, c). The first fill sequences of the Isili Channel build up the Channel Complex A and have well defined channelscale trough-stratified to divergent-fill architecture, locally associated to clinostratified units or minororder cross-bedding, (Figs. 4d, 7, 9b and 11a), produced in turbidite-dominated depositional environments. The overlying Channel Complex B shows complex channel-fill architectures including widespread impressive lateral bar sequences (Figs. 10a–c

and 11a, d), which reflect an overall increase of channel sinuosity. Some of the individual channel units show two distinct filling stages. The first stage is characterised by lateral accretion of channel margin complexes which corresponded to a progressive increase of the sinuosity of the channel. The second stage involved mainly vertical aggradation of filling. These stages represent different flow conditions which were also function of channel topography. In its distal reaches, the Isili Channel split into a complex distributary network (Figs. 4 and 15a–c) fringed by pebbly/sandy sheet deposits and/or by lobes and interchannel ridges. A submarine fan system was built about 2 km south of this major channel bend (Figs. 5 and 16). Fan complexes were laid down as a consequence of slope gradient decrease and more probably by the expansion and collapse of down-channel directed sediment flows that were no longer confined by the Isili fault-block on the western side (Fig. 16). Two fan complexes were deposited during different stages of active fan-system growth. These latter are separated by a sharp erosive surface which probably relates to a major regressive event and corresponded to a basin-ward shift of the Isili Fan System.

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Acknowledgements We thank D. Bosence and G. Carannante who critically read the first draft of the manuscript and Mrs. M. Hullet who revised the English text. The authors are also indebted to the referees J.M. Martin and L. Pomar. M. Murru and M. Vigorito undertook the detailed geological mapping. M. Vigorito focused on the submarine channel bodies geometries as well as the sedimentary structures and paleocurrent indicator patterns. L. Simone undertook facies analysis with particular reference to sedimentological and taphonomic characteristics. Financial support for this research was provided by the MIUR-COFIN 2002 Grant to L. Simone.

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