The Benue—Abakaliki trough and the Anambra basin of southern Nigeria were two major depocenters of sediments during the Cretaceous Period. The first depositional cycle of Albian to Santonian time was confined mainly to the Benue—Abakaliki trough and the lithic fill is characterised by feldspathic sandstone. The second sedimentary cycle, commencing after the Santonian tectonic episode and lasting till the Early Eocene, filled the Anambra basin and the small Afikpo syncline, and the lithic fill is distinguished by quartz arenite. These petrographic characteristics of sandstone units can be used as evidence of a major change in the sedimentary tectonism in this area, in recognising the stratigraphic boundary between the two sedimentary cycles, establishing correlation among subsurface well sections, and delineating margins of pre-Santonian and post-Santonian basins where other criteria are not reliable or readily available.The feldspathic sandstone of the first cycle is thought to be the product of high relief and rapid erosion of the Cameroun basement complex. The quartz arenites of the second cycle, largely generated in the same provenance, were the product of an intense chemical weathering of granitic basement rocks, both at the source and during a longer period of transportation to the Anambra basin, in a humid climate. These observations may have other palaeoclimatic significance.