In the southwestern Pyrenees, distinctive alluvial mega-sequences correlate with the development of a shallow, mountain front thrust structure, the Peña flexure. The flexure evolved during the Oligocene to early Miocene and is interpreted as a duplex marking the limit of thrust front advance. Three main depositional facies, preserved completely across the Peña flexure, are described: a pre-thrust emergence, high-sinuosity fluvial sequence (the c. 2300 m thick Sheet Sandstone Facies of the Campodarbe Formation); a syn-thrust emergence, conglomeratic alluvial fan sequence (the c. 1100 m thick Conglomeratic Facies of the Bernués Formation); and a post-thrust emergence, low sinuosity fluvial sequence (the > 700 m thick Ribbon Sandstone Facies of the Uncastillo Formation). Collectively, their continuous exposure across the Peña flexure provides a more or less complete record of the sedimentary evolution of a mountain front. Syn-deformational features in the alluvial fan sequence of the Bernués Formation and marked contrasts in palaeochannel morphology, sandstone body geometry and palaeocurrent patterns between the two fluvial sequences are best understood by invoking a strong tectonic control over their deposition. An increased rate of vertical sediment accretion was a key process in determining fluvial style. The onset of major thrusting led to more rapid basin floor subsidence, thereby promoting a change from lateral to vertical accretion as channels responded to the rising base level.