Available online at www.sciencedirect.com Sedimentary Geology 202 (2007) 796–820 www.elsevier.com/locate/sedgeo Impact of synsedimentary metre-scale normal fault scarps on sediment gravity flow dynamics: An example from the Grès d’Annot Formation, SE France ⁎ Stéphane Pochat a, , Jean Van Den Driessche b,1 a LPG Nantes, UMR-CNRS 6112, Université de Nantes, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France b Géosciences - Rennes, CNRS - UMR 6118, Université Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France Received 20 February 2007; received in revised form 18 September 2007; accepted 28 September 2007 Abstract Synsedimentary faults result in the direct interaction between tectonic and sedimentary processes at similar spatio-temporal scales. Sedimentological analysis of sediment-laden gravity flows in the northern part of the Grès d'Annot Formation (Sanguinière sub-basin, Col de la Moutière/Tête Ronde) has revealed the presence of fault scarps of metre-scale height. These synsedimentary fault scarps were sufficient to disturb the sediment gravity flow dynamics resulting in (i) a strong variation of the erosive behaviour of a concentrated flow and (ii) the transformation of a strongly stratified, laminar hyperconcentrated flow into a turbulent flow, in short distance (less than 500 m). These disturbances develop without the flows being deviated by the fault scarps but produce great facies heterogeneity, the least homogeneous facies (mixing sand and clay) being localized on the upstream obstacle side, the most homogeneous (massive sand) downstream. © 2007 Published by Elsevier B.V. Keywords: Synsedimentary fault; Fault scarp; Sediment gravity flow; Flow transformation; Facies variation; Grès d’Annot Formation 1. Introduction Typically, synsedimentary faults are analysed in terms of finite strain on time scale exceeding one million years so Considerable work has investigated normal fault that displacement along faults is considered to be kinematics using the geometry of syntectonic deposits. continuous on shorter time scales (Nicol et al., 1997; Fault finite throw may result from both instantaneous and Van der Woerd et al., 2000; Tapponnier et al., 2001). continuous displacements (co-seismic displacement, and Vertical instantaneous displacement ranges from milli- fault creeping or aseismic displacement, respectively). metres to a few metres maximum (e.g. Bonilla, 1988; Leeder and Jackson, 1993; Koukouvelas et al., 2001). Similarly, instantaneous deposition in a deep-water environment ranges from micrometre (clay suspension) to decimetres (turbiditic flow), and up to ten’sofmetres ⁎ Corresponding author. Tel.: +33 2 51 12 54 77; fax: +33 2 51 12 52 68. E-mail addresses: [email protected] (S. Pochat), for a single debrite event (e.g. Piper and Normark, 1983; [email protected] (J. Van Den Driessche). Middleton and Neal, 1989; Allen, 1991; Leigh and 1 Tel./fax: +33 2 23 23 56 86. Hartley, 1992; Piper and Savoye, 1993; Savoye et al., 0037-0738/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.sedgeo.2007.09.005 S. Pochat, J. Van Den Driessche / Sedimentary Geology 202 (2007) 796–820 797 1993; Praston et al., 2000). Consequently, tectonics and mud-rich deposits, with a maximum thickness of 1000– sedimentation interact in the same scale of space and 1200 m (Fig. 1). The Grès d'Annot Formation is the top time. of the Nummulitic trilogy (Lutetian–Rupelian) which Synsedimentary fault scarps are widely documented begins by the transgressive shallow marine limestones on the present-day sea-floor and in ancient deposits Formation (the Calcaire Nummulithique) followed by through their influence on sedimentary processes in the transitional to deeper marine marlstones Formation various depositional settings (e.g. Piper and Normark, (the Marne Bleues) (e.g. Elliot et al., 1985; Ravenne 1983; Petit and Beauchamp, 1986; Thornburg et al., et al., 1987; Sinclair, 1994; Hilton, 1995; Sinclair, 1997; 1990; Leeder and Jackson, 1993; Edwards, 1995; Pickering and Hilton, 1998; Sinclair, 2000; Sztrakos and Morris et al., 1998; Anderson et al., 2000; Armentrout Du Fornel, 2003; Du Fornel et al., 2004)(Fig. 2). et al., 2000; Hodgetts et al., 2001; Radies et al., 2005). The development of the foreland basin is related to In deep-water environments, most of studies have Alpine tectonics that induced the uplift of the Corsica– focused on large-sized tectonic obstacles, of metres to Sardinia and the Maures–Esterel areas (Fig. 1)(Elliot hundred metres in height and kilometres to 10 km in et al., 1985; Sinclair, 1997; Ford et al., 1999; Apps et al., length (e.g. Cope, 1959; Craig and Walton, 1962; 2004; Ford and Lickorish, 2004). The later westward Hersey, 1965; Ryan et al., 1965; Piper and Normark, propagation of the Alpine thrust system (Embrun– 1983; Siegenthaler et al., 1984; Aarseth et al., 1989; Ubaye nappes), throughout the Grès d'Annot basin, has Thornburg et al., 1990; Kneller et al., 1991; Ricci resulted in the sedimentation of the deep-water Schistes- Lucchi and Carmenlenghi, 1993; Alexander and Morris, à-Blocs Formation (Kerchkove, 1969)(Fig. 2). During 1994; Haughton, 1994; Cronin, 1995; Kneller and deposition of the Grès d'Annot Formation, the initial E–W McCaffrey, 1999; Haughton, 2000a; Haughton, 2000b; direction of shortening turned towards the SE–NW and the Kneller and Buckee, 2000; Gee et al., 2001; Haughton, maximum subsidence area migrated to the west–northwest 2001; Felletti, 2002; Hooper et al., 2002; Booth et al., part of the basin (Elliot et al., 1985; Ford et al., 1999). Both 2003; Hodgson and Haughton, 2004; Kubo et al., 2004; phenomena induced the creation of numerous confined Smith, 2004; Edwards et al., 2005; TuZino and Noda, sub-basins of several tens of square kilometres (Eastern 2007). The influence of a large-sized obstacle is much Italian, Conte–Peïra Cava, Mont Tournairet, Quatre more pronounced than that of a small-sized one, insofar Cantons–Sanguinière, Annot and Barrême basins, from as the relationship between the flow thickness and the the east to the west respectively) (Fig. 1). These basins were obstacle height is a fundamental control parameter in the partially and successively filled from the east to the west disturbance intensity (e.g. Pantin and Leeder, 1987; during middle Eocene to early Oligocene times. (Elliot Alexander and Morris, 1994; Edwards et al., 1994; et al., 1985; Ravenne et al., 1987; Pickering and Hilton, Kneller and Buckee, 2000). This is probably why the 1998;DuForneletal.,2004). Palaeocurrent analysis shows studies that concern the influence of small obstacles are aSE–NW direction of transport, implying a dominant infrequent. Nevertheless small obstacles show undeni- source of the sediment flux from Corsica–Sardinia and able effects such as (1) localized or enhanced erosion, Maures–Esterel massif (Fig. 1)(Kuenen et al., 1957; (2) granulometric sorting between upstream and down- Gubler, 1958; Stanley, 1965; Ivaldi, 1974; Bouma and stream side of the obstacle and (3) morphological Coleman, 1985; Elliot et al., 1985; Ravenne et al., 1987; changes in sedimentary features (Mutti, 1992; Alexan- Hilton, 1995; Ford et al., 1999), with very little sediment der and Morris, 1994; Morris et al., 1998; Armentrout supply from the Alpine thrust sheets to the east et al., 2000; Kneller and Buckee, 2000). Here, the (Campredon, 1972; Ivaldi, 1974). influence of metre-scale synsedimentary normal fault Alpine deformation before the Grès d’Annot depo- scarps on sediment gravity flow dynamics in the Grès sition, i.e. during the Marne Bleue and the Calcaire d'Annot Formation (French Alps) is investigated. Nummulithique deposition, resulted in a complex sea- floor topography (Pairis, 1987; Ravenne et al., 1987) 2. Regional geological and palaeogeographical settings and the compartmentalization of each sub-basin into regional topographic highs (areas of sediment by- The Grès d'Annot Formation (French Alps) was passing and erosion) and topographic lows (areas of deposited in the South East Alpine foreland from the sediment accumulation) (Sinclair and Tomasso, 2002). middle Eocene (early Bartonian) to the early Oligocene These movements also resulted in the development of (middle Rupelian) and is a sand-rich deep-water system, local fault-controlled topography of ten's of metres mainly comprising sediment gravity flow deposits (i.e. height, which have resulted in strong flow disturbances. turbidites, debrites and mass transport deposits) and This influence can be seen in the deposits with evidence 798 S. Pochat, J. Van Den Driessche / Sedimentary Geology 202 (2007) 796–820 Fig. 1. Geological map of the Tertiary Grés d'Annot and palaeogeography of the corresponding foreland basin (modified from Apps (1987)). Palaeocurrent directions are derived from Bouma and Coleman (1985), Ravenne et al. (1987), and Ford et al. (1999). They show that the principal source of sediment was Corsica–Sardinia and Maures–Esterel massifs. Several sub-basins (Eastern Italian (1), Conte–Peïra Cava (2), Mont Tournairet (3), Quatre Cantons–Sanguinière (4), Annot (5) and Barrème (6)) have been developed successively from the east to the west, from the middle Bartonian to the middle Rupelian. The studied area is situated in the northern part of the Grés d'Annot basin, in the Quatre Cantons– Sanguinière sub-basin (noted number 4). such as (i) flow deflection and flow reflection, with formities, local sand body pinch-out, and limited variable orientation between erosive structures