Foreland basin systems

A foreland basin system is defined as: (a) an elongate region of potential sediment accommodation that forms on continental crust between a contractional orogenic belt and the adjacent craton, mainly in response to geodynamic processes related to subduction and the resulting peripheral or retroarc fold-thrust belt; (b) it consists of four discrete depozones, referred to as the wedge-top, foredeep, forebulge and back-bulge depozones – which of these depozones a sediment particle occupies depends on its location at the time of deposition, rather than its ultimate geometric relationship with the thrust belt; (c) the longitudinal dimension of the foreland basin system is roughly equal to the length of the fold-thrust belt, and does not include sediment that spills into remnant ocean basins or continental rifts (impactogens). The wedge-top depozone is the mass of sediment that accumulates on top of the frontal part of the orogenic wedge, including ‘piggyback’ and ‘thrust top’ basins. Wedge-top sediment tapers toward the hinterland and is characterized by extreme coarseness, numerous tectonic unconformities and progressive deformation. The foredeep depozone consists of the sediment deposited between the structural front of the thrust belt and the proximal flank of the forebulge. This sediment typically thickens rapidly toward the front of the thrust belt, where it joins the distal end of the wedge-top depozone. The forebulge depozone is the broad region of potential flexural uplift between the foredeep and the back-bulge depozones. The back-bulge depozone is the mass of sediment that accumulates in the shallow but broad zone of potential flexural subsidence cratonward of the forebulge. This more inclusive definition of a foreland basin system is more realistic than the popular conception of a foreland basin, which generally ignores large masses of sediment derived from the thrust belt that accumulate on top of the orogenic wedge and cratonward of the forebulge. The generally accepted definition of a foreland basin attributes sediment accommodation solely to flexural subsidence driven by the topographic load of the thrust belt and sediment loads in the foreland basin. Equally or more important in some foreland basin systems are the effects of subduction loads (in peripheral systems) and far-field subsidence in response to viscous coupling between subducted slabs and mantle–wedge material beneath the outboard part of the overlying continent (in retroarc systems). Wedge-top depozones accumulate under the competing influences of uplift due to forward propagation of the orogenic wedge and regional flexural subsidence under the load of the orogenic wedge and/or subsurface loads. Whereas most of the sediment accommodation in the foredeep depozone is a result of flexural subsidence due to topographic, sediment and subduction loads, many back-bulge depozones contain an order of magnitude thicker sediment fill than is predicted from flexure of reasonably rigid continental lithosphere. Sediment accommodation in back-bulge depozones may result mainly from aggradation up to an equilibrium drainage profile (in subaerial systems) or base level (in flooded systems). Forebulge depozones are commonly sites of unconformity development, condensation and stratal thinning, local fault-controlled depocentres, and, in marine systems, carbonate platform growth. Inclusion of the wedge-top depozone in the definition of a foreland basin system requires that stratigraphic models be geometrically parameterized as doubly tapered prisms in transverse cross-sections, rather than the typical ‘doorstop’ wedge shape that is used in most © 1996 Blackwell Science Ltd 105 P. G. DeCelles and K. A. Giles models. For the same reason, sequence stratigraphic models of foreland basin systems need to admit the possible development of type I unconformities on the proximal side of the system. The oft-ignored forebulge and back-bulge depozones contain abundant information about tectonic processes that occur on the scales of orogenic belt and subduction system. orogen (Fig. 1 A,B; e.g. Price, 1973; Dickinson, 1974; INTRODUCTION Beaumont, 1981; Jordan, 1981, 1995; Lyon-Caen & Molnar, 1985). The term ‘foredeep’ (Aubouin, 1965) is This paper addresses the existing concept of a foreland basin (Fig. 1A,B): its definition, areal extent, pattern of used interchangeably with foreland basin. Important ancillary concepts that are equally entrenched in the sedimentary filling, structure, mechanisms of subsidence and the tectonic implications of stratigraphic features in literature are: (i) foreland basin sediment fill is wedgeshaped in transverse cross-section, with the thickest part the basin fill. Our aim is to point out some inadequacies in the current conception of a foreland basin, propose a located directly adjacent to, or even partially beneath, the associated thrust belt (Fig. 1B; Jordan, 1995); (ii) more comprehensive definition and to elaborate upon some of the features of this expanded definition. foreland basin sediment is derived principally from the adjacent thrust belt, with minor contributions from the A foreland basin generally is defined as an elongate trough that forms between a linear contractional orogenic cratonward side of the basin (Dickinson & Suczek, 1979; Schwab, 1986; DeCelles & Hertel, 1989); and (iii) a belt and the stable craton, mainly in response to flexural subsidence that is driven by thrust-sheet loading in the flexural bulge, or forebulge, may separate the main part Fig. 1. (A) Schematic map view of a ‘typical’ foreland basin, bounded longitudinally by a pair of marginal ocean basins. The scale is not specified, but would be of the order of 102–103 km. Vertical line at right indicates the orientation of a cross-section that would resemble what is shown in part B. (B) The generally accepted notion of foreland-basin geometry in transverse crosssection. Note the unrealistic geometry of the boundary between the basin and the thrust belt. Vertical exaggeration is of the order of 10 times. (C) Schematic cross-section depicting a revised concept of a foreland basin system, with the wedge-top, foredeep, forebulge and back-bulge depozones shown at approximately true scale. Topographic front of the thrust belt is labelled TF. The foreland basin system is shown in coarse stipple; the diagonally ruled area indicates pre-existing miogeoclinal strata, which are incorporated into (but not shown within) the fold-thrust belt toward the left of diagram. A schematic duplex (D) is depicted in the hinterland part of the orogenic wedge, and a frontal triangle zone (TZ) and progressive deformation (short fanning lines associated with thrust tips) in the wedge-top depozone also are shown. Note the substantial overlap between the front of the orogenic wedge and the foreland basin system. © 1996 Blackwell Science Ltd, Basin Research, 8, 105–123 106 Foreland basin systems of the foreland basin from the craton (e.g. Jacobi, 1981; These accumulations generally are considered as ‘piggyback’ or ‘thrust top’ basins (Ori & Friend, 1984) that Karner & Watts, 1983; Quinlan & Beaumont, 1984; Crampton & Allen, 1995). Most workers in practice may or may not be isolated from the main part of the foreland basin fill. Although some piggyback basins are, consider the basin to be delimited by the thrust belt on one side and by the undeformed craton on the other indeed, geomorphically isolated for significant time periods (e.g. Beer et al., 1990; Talling et al., 1995), most side, although some well-known foreland basins ‘interfere’ with extensional basins orientated at a high angle active wedge-top accumulations are completely contiguous with the foreland basin. In the Upper Cretaceous to the trend of the orogenic belt (e.g. the Amazon and Alpine forelands; the ‘impactogens’ of Şengor, 1995). foreland basin fill of the western interior USA, isopach contours are continuous on both sides of the basin, Longitudinally, foreland basins commonly empty into marginal or remnant oceanic basins (Fig. 1A; Miall, 1981; demonstrating that the basin fill tapers toward the thrust belt and is not wedge-shaped in transverse cross-section Covey, 1986; Ingersoll et al., 1995) or zones of backarc spreading (Hamilton, 1979). Dickinson (1974) dis(Fig. 3). The type examples of piggyback basins cited by Ori & Friend (1984) in the Po–Adriatic foreland tinguished between ‘peripheral’ foreland basins, which form on subducting plates in front of thrust belts that completely bury the underlying thrust-related basement topography beneath >3 km of sediment (Fig. 4); in the are synthetic to the subduction direction, and ‘retroarc’ foreland basins, which develop on the overriding plates nonmarine part of the basin, tributaries from the northern Apennines thrust belt are graded across a smooth alluvial inboard of continental-margin magmatic arcs and associated thrust belts that are antithetic to the subduction plain to the modern Po River. The limit of topographic expression of the thrust belt is along the Apennine front, direction. Although this distinction has stood the test of two decades of research, only recently have the geobut seismicity associated with blind thrusting and related folding extends at least 50 km to the north and northdynamic differences between these two, fundamentally different, types of foreland basins been recognized (e.g. east (Ori et al., 1986). Thick and areally widespread synorogenic sediments on top of ancient thrust belts have Gurnis, 1992; Royden, 1993). The concept of a foreland basin as outlined above is been documented as well (Burbank et al., 1992; DeCelles, 1994; Pivnik & Johnson, 1995; among others). If these incomplete in two important respects. First, it is clear from many modern and ancient foreland settings that accumulations are included in the foreland basin fill, as we believe they should be, then the geometry of the fill sediment derived from the thrust belt, as well as sediment derived from the forebulge region and craton and intrabano longer has the wedge shape that is routinely used when modelling foreland basin subsidence and sedimensinal carbonate sediment, may be deposited over areas extending far beyond the zone of major flexural subsidtation patterns (e.g. Heller & Paola, 1992). Instead, in ence (i.e. cratonward from the forebulge). Examples transverse cross-section, the basin fill tapers toward both include the sediment accumulations associated with the craton and orogenic belt (Fig. 1C), and the asymmetric Cordilleran, Amazonian and Indonesian orogenic belts, wedge shape, where it exists, is a result of post-depositional which extend hundreds of kilometres beyond their structural processes (mainly truncation by thrust faults), respective limits of major flexural subsidence (Fig. 2; rather than a direct result of interacting depositional Ben Avraham & Emery, 1973; Jordan, 1981; Karner & processes and subsidence patterns. Watts, 1983; Quinlan & Beaumont, 1984). On the other Several problems in current foreland basin modelling hand, sediment accumulations in some foreland settings, and field studies, examples of which will be discussed in such as the Swiss molasse basin (Sinclair & Allen, 1992), this paper, can be traced to the inadequate concept of a the Taiwan foreland basin (Covey, 1986) and the foreland basin as outlined above. The remainder of this Po–Adriatic foreland basin (Royden & Karner, 1984; paper is devoted to proposing a more comprehensive Ricci Lucchi, 1986; Ori et al., 1986), are much narrower definition for foreland basins, and discussing some of the and confined to the zone of major flexural subsidence. implications of this definition for our understanding of This gives rise to the concept that foreland basins can foreland basin strata in terms of tectonic processes. exist in underfilled, filled or overfilled states (Covey, 1986; Flemings & Jordan, 1989). In practice, however, FORELAND BASIN SYSTEMS DEFINED the part of the basin fill that extends toward the craton beyond the flexural bulge is given only passing attention The discussion above highlights the fact that ‘foreland basins’ are geometrically complex entities, comprising in the literature (e.g. Quinlan & Beaumont, 1984; Flemings & Jordan, 1989; DeCelles & Burden, 1992). A discrete parts that are integrated to varying degrees. Thus, we introduce the concept of a foreland basin system. key question is what causes the widespread accommodation and sediment accumulation cratonward of the (i) Foreland basin systems are elongate regions of potential sediment accommodation that form on continental crust crest of the forebulge. Also ignored by the popular conception of foreland between contractional orogenic belts and cratons in response to geodynamic processes related to the orogenic basins is a substantial amount of sediment derived from the orogenic wedge that accumulates on top of the wedge. belt and its associated subduction system. (ii) Foreland © 1996 Blackwell Science Ltd, Basin Research, 8, 105–123 107 P. G. DeCelles and K. A. Giles Fig. 2. Generalized map of the Sunda shelf and Indonesian orogenic system, after Ben Avraham & Emery (1973) and Hamilton (1979). A complex, retroarc foreland basin system is present along the north-eastern and northern side of the Sumatra–Java magmatic arc and associated fold-thrust belts. Isopach contours indicate the thickness (in km) of Neogene sediment. In crosssection A–A∞ note the broad uplifted region of the Bawean and Karimunjava arches, which separates an obvious foredeep depozone from regions of lesser but broader scale subsidence. basin systems may be divided into four depozones, which subsidence mechanisms because sediment accommodation in each of the four depozones is controlled by a we refer to as wedge-top, foredeep, forebulge and backbulge depozones (Fig. 1C). Which of these depozones a different set of variables, which we will discuss below. The principal mechanisms of lithospheric perturbation sediment particle occupies depends on its location at the time of deposition (Fig. 5). Boundaries between depozones in foreland basin systems are flexure in response to orogenic loading and subsurface loads, but this flexure may shift laterally through time. In some foreland basin systems, the forebulge and back-bulge depozones may be may be manifested differently in each depozone. poorly developed or absent. (iii) The longitudinal dimension of the foreland basin system is roughly equal to the Wedge-top depozone length of the adjacent fold-thrust belt. We exclude masses of sediment that spill longitudinally into remnant oceanic In many continental thrust belts, the limit of significant topography is far to the rear of the frontal thrust, and basins (e.g. the Bengal and Indus submarine fans) or rifts, because they may not be controlled directly by large amounts of synorogenic sediment cover the frontal part of the fold-thrust belt (Fig. 4). This is because geodynamic processes related to the orogenic belt. Missing from this definition is any mention of specific frontal thrusts commonly are blind, tipping out in the © 1996 Blackwell Science Ltd, Basin Research, 8, 105–123 108 Foreland basin systems these deposits consist of the coarsest material in the basin fill, usually alluvial and fluvial sediments that accumulate proximal to high topographic relief; in subaqueous settings, wedge-top deposits typically consist of mass-flows and fine-grained shelf sediments (e.g. Ori et al., 1986; Baltzer & Purser, 1990). The wedge-top depozone tapers onto the orogenic wedge, and may be many tens of kilometres in length parallel to the regional tectonic transport direction. Examples abound: Upper Cretaceous to Palaeocene wedge-top sediments are widespread on top of the frontal 75 km of the Sevier thrust belt in Utah and Wyoming (Coogan, 1992; DeCelles, 1994); Eocene–Oligocene wedge-top sediments cover the frontal 30–40 km of the south Pyrenean thrust belt (Puigdefabregas et al., 1986); Pliocene–Quaternary wedge-top sediments bury the frontal 50 km of the active northern Apennines thrust belt (Ricci Lucchi, 1986); the frontal 50 km of the Zagros thrust belt are mantled by Pliocene–Quaternary sediments (British Petroleum, 1956); and #100–150 km of the active frontal thrust belt in northern Pakistan are covered by young syntectonic sediments (Burbank et al., 1986; Yeats & Lillie, 1991; Pivnik & Johnson, 1995). The main distinguishing characteristics of wedge-top deposits are the abundance of progressive unconformities Fig. 3. Isopach map of the upper Albian to Santonian fill of the (Riba, 1976) and various types of growth structures western interior USA foreland basin system (after Cross, 1986). (Fig. 4B), including folds, faults and progressively rotated Note that the basin fill tapers toward both the Sevier thrust belt cleavages (Anadon et al., 1986; Ori et al., 1986; DeCelles and the craton. et al., 1987, 1991; Lawton & Trexler, 1991; Suppe et al., 1992; Jordan et al., 1993; Lawton et al., 1993). These features indicate that wedge-top sediment accumulates cores of fault propagation anticlines (e.g. Vann et al., 1986; Mitra, 1990; Yeats & Lillie, 1991), triangle zones and is then deformed while at or very near the synorogenic erosional/depositional surface (as opposed to deeply (e.g. Jones, 1982; Lawton & Trexler, 1991; Sanderson & Spratt, 1992) or passive roof duplexes (Banks & buried and isolated from the surface). The wedge-top depozone actually is part of the orogenic wedge while it Warburton, 1986; Skuce et al., 1992) in the subsurface, whereas much larger, trailing fault-bend and faultis deforming, and hence it is useful for delimiting the kinematic history of the wedge. Aerially extensive aprons propagation folds develop above major structural ramps and duplexes further toward the hinterland (Fig. 1C;, of alluvial sediment or shallow shelf deposits commonly drape the upper surface of the orogenic wedge during e.g. Boyer & Elliott, 1982; Pfiffner, 1986; Rankin et al., 1991; Srivastava & Mitra, 1994). In addition, the rocks periods when the wedge is not deforming in its frontal part (Ori et al., 1986; DeCelles & Mitra, 1995), and that are involved in deformation along the fronts of thrust belts are usually relatively young, soft sediments, large, long-lived feeder canyons may develop and fill in the interior parts of orogenic wedges (Vincent & Elliott, whereas older, typically more durable rocks are exposed in the hinterland (DeCelles, 1994). The sediment that 1995; Coney et al., 1995). The frontal edge of a wedge-top depozone may shift accumulates on top of the frontal part of the orogenic wedge constitutes the wedge-top depozone (Fig. 1C). Its laterally in response to behaviour of the underlying orogenic wedge; thus it may be difficult to distinguish extent toward the foreland is defined as the limit of deformation associated with the frontal tip of the underfrom the proximal foredeep depozone in an ancient foreland basin system. Key distinguishing features of the lying orogenic wedge. This includes piggyback or thrustsheet-top (Ori & Friend, 1984) and ‘satellite’ (Ricci wedge-top depozone include progressive deformation, numerous local and regional unconformities, regional Lucchi, 1986) basins, large feeder canyon fills in the interiors of thrust belts (e.g. Vincent & Elliott, 1995; thinning toward the orogenic wedge and extreme textural and compositional immaturity of the sediment. Sediment Coney et al., 1995), deposits associated with local backthrusts and out-of-sequence or synchronous thrusts derived from the hinterland flanks of frontal anticlinal ridges may be shed back toward the hinterland (e.g. (Burbank et al., 1992; DeCelles, 1994), and deposits of regionally extensive drainage systems that are antecedent Schmitt & Steidtmann, 1990), and local lacustrine deposits may develop in geomorphically isolated piggyto younger structures and topography toward the foreland (Schmitt & Steidtmann, 1990). In subaerial settings, back basins (Lawton et al., 1993). Theoretically, the © 1996 Blackwell Science Ltd, Basin Research, 8, 105–123 109 P. G. DeCelles and K. A. Giles Fig. 4. (A) Isopach map of post-Messinian sediments and blind thrust faults beneath the Po alluvial plain in northern Italy, an active peripheral foreland basin system (after Pieri, in Bally, 1983). The topographic front of the northern Apennines trends WNW just south of Bologna. Note that the frontal 50+ km of the thrust belt are buried beneath as much as 8 km of wedge-top sediment. The Po delta is prograding eastward into the northern Adriatic Sea, which is the marine part of the system. (B) Interpreted seismic line across a part of the northern Adriatic Sea, off the coast of Conero, Italy, in the Po–Adriatic foreland basin system (after Ori et al., 1986). The topographic front of the Apennines thrust belt is to the left (south-west) of the section. Note the progressive deformation (growth fault-propagation folds) in Pliocene–Quaternary sediments on top of the frontal part of the orogenic wedge. Also note that this part of the basin is submarine, currently receiving shallow-marine