Seismic stratigraphic analysis of a grid of 7860 km of shallow seismic Sleevegun data, plus lithological and biostratigraphic (foraminifera) data interpreted from core and sea-bed samples have been integrated into a detailed geological model for the Upper Cretaceous succession of the proximal Orange Basin. Emphasis has been placed on the palaeoenvironmental interpretation of incised valley fill sequences of this succession. Seismic reflection geometries for the Middle Albian to Lower Cenomanian interval show that the entire coast was dominated by aggradational fluvial braid-plain facies. The Lower Turonian to Upper Coniacian succession marks the appearance of repeated major incision into the exposed coastal plain by large fluvial drainage networks. This succession is characterized by fourth-order, fluvial-to-marine sequences, with valley incision up to 25 km wide and 70 m deep. Detailed palaeoenvironmental analysis indicates periodically waterlogged, highly reducing fluvial floodplain environments with valley systems that were flooded to create wave-dominated estuaries. Major fluvial activity during the Turonian was in an area that lies offshore of the present-day Swartlintjies to Groen River mouths in the southern part of the Orange Basin. During the Coniacian the site of fluvial activity changed to the central Orange Basin, offshore of the present-day Buffels River. These observations contradict previously published views that a dominant Orange River drainage system existed since the Albian, at a position coincident with the present-day mouth.