Volcanic ash contains 1%–10% FeO by weight and can be a significant contributor of Fe to the surface ocean. Fe fertilization by volcanic ash may have contributed to marine productivity in the past. A continuous core containing ash beds from the Eagle Ford Group (Texas, USA) was scanned by X-ray fluorescence spectroscopy to test for correlations between ash deposition and burial of Fe and other trace elements associated with biological productivity. Beds composed of mixed ash and shale contain much more Fe and Mo than interbedded black shales, implying that Fe fertilization promoted euxinia (elevated Mo and lower Cr). Moreover, foraminiferal assemblages were significantly depleted in deeper-water taxa following ash deposition. We conclude that Fe-bearing volcanic ash fertilized the southern Cenomanian Western Interior Seaway and promoted deposition of abundant black shales under anoxic conditions.