The fill of an incised valley in the subsurface beneath Beaver County, Oklahoma, preserves the sedimentary response to a major cycle of change in relative sea level that took place in the Anadarko basin at the end of the Mississippian and the beginning of the Pennsylvanian (Morrowan). The valley-fill sequence includes heterolithic clastic sediments of the lowstand systems tract (LST) and estuarine and marine sediments of the transgressive system tract (TST). Deltaic sediments within the TST record a minor cycle of base-level change and a small drop in relative sea level within the major cycle of sea-level change that marks the Mississippian-Pennsylvanian boundary in the Anadarko basin. Conglomerates above the heterolithic facies of the LST formed as a transgressive lag on the ravinement surface at the base of the TST. Siliciclastic tidal sediments overlie the conglomerates and in turn are overlain by crinoidal limestones that record relative deepening of the water and establishment of open marine conditions within the estuary. Above the limestones are deltaic deposits indicating a minor drop in relative sea level during which fluvial conditions were established within the estuary. The sediments of the LST and the initial stages of the TST are confined to the axis of the paleovalley and do not extend high onto its walls. The LST does not extend upstream into the higher reaches of the paleovalley. In most places, shales deposited as shelf muds overlie the unconformity between the Mississippian and Pennsylvanian. The shales represent the later stages of the TST when sea-level rise resumed following the episode of deltaic sedimentation, and marine waters flooded and drowned the pre-existing topography.