Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Tectonics and sedimentation in foreland basins: results from the Integrated Basin Studies project ALAIN MASCLE 1 & CAI PUIGDEFABREGAS 2,3 IIFP School, 228-232 avenue Napoldon Bonaparte, 92852 Rueil-Malmaison Cedex, France (e-mail: [email protected]) 2Norsk Hydro Research Centre, Bergen, Norway. 3Institut de Ciences de la Terra, (?SIC, Barcelona, Spain. Why foreland basins? to a better understanding of some basic interact- ing tectonic, sedimentary and hydrologic pro- Over the last ten years or so, since the Fribourg cesses (More & Vrolijk 1992; Touret & van meeting in 1985 (Homewood et al. 1986), the Hinte 1992). Additional data have also been attention given by sedimentologists and struc- obtained through the development of analogue tural geologists to the geology of foreland basins and numerical models (Larroque et al. 1992; has been growing continuously, parallel to the Zoetemeijer 1993). The physical parameters increase of co-operative links between scientists controlling the forward propagation of d6colle- from the two disciplines. A number of reasons ments and thrusts (fluid pressure, roughness, lie behind this development. Attempting to sediment thickness, etc.) have been determined understand the growth of an orogen without and tested. The relationships between rapidly paying due attention to the stratigraphic record subsiding piggyback basins and growing ramp of the derived sediments would be unrealistic. It anticlines have also been imaged, although the would, moreover, be equally unrealistic to con- lack of deep-sea well control still prevents accu- struct restored sections across the chain without rate sedimentological studies. More significant considering the constraints imposed by the has been the progress in our understanding of basin-fill architecture, or to describe the basin- the role of fluids and pore pressure in the fill evolution disregarding the development of development of thrust belts. When the fluids the thrust sequence. As in other sedimentary escape from the mineral matrix of sediments basins, tectonics and sedimentation dynamically during sedimentary and tectonic burial, the interact in foreland basins. Additionally, as fore- resulting fluid flow can either be diffused land basins are incorporated in the growth through the sedimentary column if the average process of the orogen, they are more likely than permeability is high enough, or be channelled extensional basins to be subject to uplift and, along pathways such as d6collements and active therefore, more accessible to direct field obser- faults when the overall permeability is low. They vation. Foreland basins observed on the field may also remain trapped within the mineral may help in the understanding of non- matrix if such pathways are not available. With observable subsurface analogues. This is essen- increasing burial the pore pressure dramatically tially why foreland basins have always been increases, eventually approaching the vertical traditional field areas for sedimentological minimum stress, and catastrophic events such as research and training, and also why they have mud volcanoes may occur (e.g. Barbados Ridge, recently been considered as ideal field labora- Fig. 2). Migrating fluids will not only exert a tories, best suited to the study and understand- fundamental control on tectonic processes, but ing of the interplay between tectonics and will also contribute to the transfer of mineral sedimentation (Fig. 1). solutions, heat and hydrocarbons from the inner Further interest in foreland basins has also part of the thrust belt to the surface along verti- been triggered off by some spectacular results cal or more tortuous lateral pathways. from investigations of offshore accretionary Foreland basins are also important from the prisms. As a matter of fact, such prisms are con- point of view of hydrocarbon exploration. sidered to be very similar in many aspects to Source rocks are commonly provided by pre- onshore thrust belts. Extensive seismic and high- compressional rift basin sequences, whereas resolution bathymetric surveying of a few properly structured rock formations within the selected active margins (Barbados, Cascadian, foreland basin may eventually provide adequate Nanka'/, Middle America), calibrated with both reservoirs. Classical examples from the Urals, deep-sea drilling and deep-sea diving, have lead Caucasus and Carpathians together with those MASCLE, A. & PUIGDEFABREGAS,C. 1998. Tectonics and sedimentation in foreland basins: results from the Integrated Basin Studies project. In: MASCLE,A., PUIGDEF,M3REGAS,C., LUTERBACHER,H. P. & FERNANDEZ,M. (eds) Cenozoic Foreland Basins of Western Europe. Geological Society Special Publications, 134, 1-28. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 2 A. MASCLE & C. PUIGDEFA.BREGAS Fig. L Location map of the study areas. of the Canadian Rockies, have been recently Atlantic continental margins. Apart from the overtaken by the discovery of giant fields in still very speculative potential of non-con- Venezuela and Colombia, with estimated ventional gas production (coal-bed methane), reserves of well over the billion barrels of oil the next (and possibly the last) frontier will equivalent mark (Duval et al. 1995). It is obvious undoubtfully be the deep gas potential of Ceno- that in such a context, good knowledge of basin zoic thrust belts: there have already been some evolution and fluid flow in relation to thrust recent attempts to define prospective plays in sequence propagation will be required. Every the Pyrenees (Le Votet al. 1996) and Northern step in the progress of knowledge acquisition Alps (Greber et al. 1996). It is there where the might be, directly or indirectly, of prime rele- Foreland Basin Module of the IBS project hopes vance. Concerning the future of hydrocarbon to be of most use. exploration in Western Europe, while the classi- Additional interest in foreland basin geology cal plays are today at a stage of extensive explor- also stems from the fact that they are often ation, the present frontier areas are HP-HT associated with alluvial plains such as the Po prospects in deep stratigraphic intervals plain in Italy, the Parana river in Argentina and (5-8 km) of well-known basins such as the North the Ganges in India, to mention only three, Sea, and deep-water prospects in areas like the which support a high human population. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 INTRODUCTION 3 Alluvial plains in foreland basins are very sensi- It has long been recognized that in most cases, tive systems in relation to both predictable and orogens have adjacent elongated foreland basins less predictable changes in climate and land use. which are filled with the erosion products of the This is, perhaps, an issue of marginal interest to mountain chain. This observation defines the our project, but of much greater importance to crustal-scale approach. It is also widely accepted society. We believe that all the newly acquired (Riba 1964) that foreland basins are asymmetric knowledge, which is of relevance to hydro- in transverse section and that their depocentre carbon exploration, is sooner or later bound to axes migrate outwards through time as the have additional socio-economic applications. thrust sequence propagates - the earlier basin- fill sequences being thus incorporated into the Objectives and research premises thrust system (Puigdef~bregas et al. 1986). This constitutes the next scale of approach. Surpris- The Foreland Basin Module of the IBS project ingly enough, this evolution of the basin-fill has been planned to study the interplay, at geometry through time, which is the main dis- different scales, between tectonics and sedimen- tinctive feature, is not always taken into account tation during the construction of an orogen, and in the definition of the foreland basin (DeCelles also to study how control is exerted on the archi- & Giles 1996). It is also recognized that, on a tecture of the basin-fill down to the scale of the third and smaller scale, the propagation of a par- depositional sequence. ticular thrust interferes with sedimentation in Three main areas of work have been selected: the adjacent basin at sequence scale. In addition, the Guadalquivir basin related to the Betic as topography is created in the orogen, a orogen, the Ebro basin related to the Pyrenees, complex chain of geomorphic and climatic con- and the German Molasse related to the Alps. In trols is exerted through weathering, erosion and each of these areas, a number of research activi- sediment-transport processes, which are not ties have been planned to provide an insight to only relevant in predicting the nature of the sedi- some of the maj or problems such as the following. ments finally deposited in foreland basins, but ~ Is the tectonic load sufficient to account for also in the further propagation of the orogenic the observed lithospheric flexure? wedge itself. ~ Can we quantify amounts and rates of With these premises in mind, the objectives of erosion and topographic growth and incor- the Foreland Basin Module within the Inte- porate them in our time-step models? grated Basin Studies (IBS) project are to con- ~ Can we relate major sedimentary cycles and tribute to the understanding of how these thrust events? interactions work, which processes are involved, ~ Would modelling of growth structures help and at what rates they operate.