An integrated structural, stratigraphic, and sedimentological analysis of the West Crocker formation in northwest Borneo suggests that it is best interpreted as an accretionary prism. The structural geology provides clear evidence of at least two episodes of syndepositional folding and thrust faulting. A probable Eocene age, indicated by foraminiferal and palynological assemblages, differs from the generally accepted Oligocene to early Miocene age and is consistent with deposition of the West Crocker formation during a phase of tectonism at the northwest Borneo margin. Sandstones within the West Crocker formation were deposited by high-density turbidity currents that constructed relatively small, progradational lobes in a slope apron environment, and trace fossil assemblages confirm bathyal water depths of ∼1000 m or more. The composition of the sandstones, which contain abundant feldspars and lithic fragments, suggests that their provenance was the first-cycle product of an eroded orogenic belt, whereas immature textures indicate a short distance of transport.