THE RIOJA TROUGH (N SPAIN) - TECTOSEDIMENTARY EVOLUTION OF A SYMMETRICAL FORELAND BASIN

The Rioja Trough is the foreland basin of the western Pyrenees (to the north) and the Cameros-Demanda Massif (to the south). This E-W elonga ted trough is about 120 x 35 km. It was filled with Tertiary continent al deposits (upper Eocene to upper Miocene), reaching thicknesses betw een 2500 and 5000 m. Both margins of the Rioja Trough are large thrust s with horizontal displacements of more than 20 km basinward. Rocks th at fill the basin originated in alluvial fan and playa-lake environmen ts, with conglomerates in the proximal sectors grading into sandstones , mudstones, lacustrine limestones and evaporites in distal sectors. T he Tertiary series are horizontal in the central parts of the basin, w ith several E-W monoclines caused by north-verging thrusts in the base ment of the basin. Near the basin margins, the Tertiary units are fold ed and thrusted, with several syntectonic unconformities. Calculated v elocities for the Cameros-Demanda thrust range from 0.02 tol.1 mm yr(- 1) (average 0.7 mm yr(-1)). The sedimentation rate near the southern b asin margin varies between 2 and 20 cm 1000 yr(-1) (average 10 cm 1000 yr(-1)). Deposition in the Rioja Trough was strongly controlled by te ctonic activity throughout the Tertiary. Eight tectosedimentary units (R1 to R8) have been characterized. These are bounded by angular uncon formities at the margins and breaks in the vertical trend of the sedim entary record toward the basin centre. Every tectosedimentary unit (ex cept R6 and R8) shows a fining-upward/coarsening-upward trend, corresp onding to tectonic retrogradations and progradations, respectively. Th e main source area during the Palaeogene was the Cameros-Demanda Massi f, whose unroofing sequence was strongly dependent on tectonic activit y. During the Neogene a longitudinal WNW-ESE drainage system, with sho rt alluvial fans in the northern and southern margins, developed. The final shape and the evolution of the Rioja Trough are the result of cr ustal flexure in the northern border of the Iberian plate, linked to t he emplacement of the southern Pyrenean thrust system, and intraplate thrusting with basement uplift at its southern margin.