Three episodes of accumulation of organic-carbon-rich "black shales" occurred on the Iberia Margin of the North Atlantic during the Early-middle Cretaceous. The origins of the organic matter contents of these deposits have been investigated using elemental, isotopic, Rock-Eval, and biomarker analyses. The organic matter in Berriasian-Barremian turbiditic marlstones contains major proportions of continental plant material. Aptian-Albian turbiditic shale sequences include black layers similarly dominated by land-derived organic matter but also having important amounts of marine material. A thin layer of Cenomanian-Turonian black shale contains over 11% organic carbon, most of which is derived from marine sources. Downslope transport and rapid reburial of shelf-edge sediments within predominantly oxygenated deep-water settings contributed to deposition of the Early Cretaceous black shales. The Cenomanian-Turonian black shales evidently record a brief episode of intensified mid-water oxygen minimum zone and consequent improved preservation of marine organic matter during the middle Cretaceous.