JOURNAL OF SEDIMENTARY RESEARCH,VOL. 68, NO. 5, SEPTEMBER, 1998, P. 869–878 Copyright  1998, SEPM (Society for Sedimentary Geology) 1073-130X/98/068-0869/$03.00 OLIGO-MIOCENE ALLUVIAL-FAN EVOLUTION AT THE SOUTHERN PYRENEAN THRUST FRONT, SPAIN MATTHEW J. LLOYD 1 , GARY J. NICHOLS 2 , AND PETER F. FRIEND 1 1 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K. 2 Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 0EX, U.K. ABSTRACT: Oligocene to Miocene conglomerate bodies interpreted as alluvial-fan deposits are well exposed along the northern margin of the Ebro Basin in Spain. The source area was the southern Pyrenean thrust front, which was tectonically active at that time. Eleven separate conglomerate bodies have been identified and interpreted as the de- posits of individual alluvial fans. The smallest alluvial-fan deposit is less than 0.8 km radius, is about 460 m thick, and has a calculated volume of 0.1 km 3 ; the largest is up to 5.5 km radius, is at least 500 m thick, and has a calculated volume of over 3 km 3 . Fan evolution was strongly controlled by the lithologies exposed by erosion of the emer- gent thrust front: the size of the fan bodies was determined by the nature and extent of the bedrock lithologies, which were in turn struc- turally controlled. Growth structures in the strata are common, be- cause deformation continued along the thrust front during fan sedi- mentation. Stepwise tectonic reconstructions of one of the alluvial-fan conglomerate bodies and the adjacent thrust front suggest that the relief in the hinterland was around 500 m. INTRODUCTION The depositional models for sheetflood-dominated and debris-flow-dom- inated alluvial-fan sedimentation have largely been developed from exten- sional and transtensional settings. The processes and products of alluvial- fan sedimentation were summarized by Bull (1972) using examples from Recent alluvial fans in the Basin and Range province of the southwestern USA. The stratigraphy of fan deposits was addressed by Heward (1978) with the recognition of tectonically controlled sequences and megasequ- ences. However, all the examples cited and models developed by Heward (1978) were from extensional and transtensional settings. Alluvial-fan pro- cesses and their recognition in the stratigraphic record were further con- sidered by Blair and McPherson (1994), again largely using case studies from extensional basins. Many other studies on the stratigraphy of alluvial fans have been carried out (see Blair and McPherson 1994 and references therein), but it is striking that the tectonic setting is nearly always exten- sional. Exceptions come largely from the southern Pyrenees, with early work by Riba (1976) on syntectonic sedimentation in alluvial fans followed by similar studies by Anadon et al. (1986) and Nichols (1987a). THE SOUTHERN PYRENEES The Pyrenees are a part of the Alpine orogenic belt, extending across southern Europe (Fig. 1) to link with the Himalayan chain to the east. The Pyrenean mountain belt is the result of convergence between the Iberian sub-plate to the south and the main European plate (Boillot 1984) during latest Cretaceous and Tertiary time. It consists of a core of Hercynian basement rocks flanked to the north and south by thin-skinned tectonic belts, and a series of thrust sheets that have translated structural packages outwards to the north and south (Choukroune and Seguret 1973). Several distinct structural belts can be recognized in the central and western parts of the southern Pyrenees (Fig. 1; Teixell 1996). The Axial Zone comprises Paleozoic metamorphic rocks, Triassic limestone, shale, and evaporite deposits, and Cretaceous to Eocene clastic and carbonate strata. The Triassic evaporites are important de ´collement horizons for Al- pine thrust structures. To the south lies the Jaca Basin, a broad synform in Eocene to Oligocene turbidites and fluvio-lacustrine marls and clastic de- posits (Teixell 1996), which are part of the fill of the southern Pyrenean foreland basin. The External Sierras form the southern margin of the Jaca Basin and separate it from the Ebro Basin (Fig. 1). The general structural form of the External Sierras is a steep, narrow anticline in the west (Nichols 1987b; Teixell 1996), but farther to the east, the thrust-front structure is more complex (see below). The Oligocene to Miocene alluvial sediments in the Ebro Basin to the south of the External Sierras are largely undeformed, with the exception of units deposited immediately adjacent to the thrust front. These deformed beds are some of the younger southern Pyrenean foreland-basin deposits, and they include the interpreted alluvial-fan con- glomerates that are the subject of this paper. METHODS OF STUDY Exposures in the External Sierras and adjacent parts of the Ebro Basin north of Huesca (Fig. 1) are of excellent quality. Steep topography and deeply incised rivers provide extensive access to beds of Oligocene and Miocene conglomerate that lie adjacent to the folded and thrust-faulted older strata of the southern Pyrenean thrust front (Fig. 2). The extent of the conglomerate beds was determined by mapping the areas of outcrop. Detailed maps are presented in Hirst (1983), Nichols (1984, 1987a), and Lloyd (1994), and a summary of the distribution of these conglomerate units is shown in Figure 3. Eleven distinct conglomerate bodies were de- fined on the basis of this mapping and measured sedimentary sections drawn through the entire exposed thickness of conglomerate in five of these deposits at Agu ¨ero, Murillo, Riglos, Nueno, and Roldan (Hirst 1983; Nich- ols 1987a; Lloyd 1994). This stratigraphy formed the basis for the inter- pretation of these conglomerates in terms of processes and environments of deposition. The other six bodies were studied in less detail, and an assessment of the facies was made by comparison with the described strati- graphic sections. The structural evolution of the area presented in this paper is based on detailed work published elsewhere (Puigdefa ´bregas and Soler 1973; Nichols 1987b; Pocovı ´ et al. 1990; McElroy 1990; Teixell 1996). SEDIMENTOLOGY OF CONGLOMERATE UNITS ALONG THE NORTHERN MARGIN OF THE EBRO BASIN The conglomerate bodies at the northern edge of the Ebro Basin are striking geomorphological features, commonly consisting of vertical towers of reddish conglomerate rising several hundred meters above the subdued topography of the Ebro Basin (Fig. 2). Stratigraphically the conglomerates are part of the Uncastillo Formation in the west (Soler and Puigdefa ´bregas 1970) and the Sarin ˜ena Formation in the east (Quirantes 1969). These con- glomerate beds, containing associated and subordinate sandstone and mud- stone, form discontinuous outcrops along the northern margin of the basin (Fig. 3). The clast types present in the conglomerate beds consist of pre- dominantly carbonate lithologies that can be matched with pre-Oligocene strata exposed in the External Sierras (Hirst 1983; Hirst and Nichols 1986; Nichols 1984, 1987a; Lloyd 1994). Sandstone and mudstone beds associ- ated with the conglomerate are also calcareous and red in color (Hirst 1983; Nichols 1984). These rock types contrast with a second lithofacies asso- ciation in the Uncastillo and Sarin ˜ena Formations, which consists of pale brown to yellow sandstone and mudstone. These facies are interpreted as the deposits of two fluvial distributary systems, the Luna System in the west and the Huesca System in the east (Hirst and Nichols 1986; Nichols