At the Taitao Peninsula, in southern Chile (46.5°S), an active mid-ocean ridge is being subducted under the South American continent. Continental crust south of the current triple junction has experienced subduction of three ridge segments since the mid-Miocene. A series of Mid-Late Tertiary sedimentary basins lies inboard of the triple junction: the Cosmelli basin lies in the region which has experienced the earlier ridge subduction events. Within the Cosmelli basin fill, abrupt facies dislocations give clear evidence of marked base level changes. Four sequence boundaries are delineated, with an upward increase in degree of facies dislocation, which is believed to reflect increasing magnitudes of base level fall. The lower part of the basin fill is folded and then thrust eastward as a series of imbricate slices, while the overlying, greater thickness of fluvial sediments is only gently tilted westwards. This geometry indicates that the early basin fill was deforming due to contractional tectonics while the later basin fill was being deposited; these processes controlled accommodation space and thus stratigraphic architecture during much of the basin life. This complex basin history, plus the presence of basaltic sheet intrusions may reflect the underlying slab history as successive ridge segments were subducted.