Laramide basement blocks of the U.S. Rocky Mountain Province have long been considered the sediment source of the Paleocene–Eocene Wilcox Group, implying that large-scale progradation driven by Laramide uplift induced a transition from carbonate to clastic deposition in the Gulf of Mexico. Detrital zircon U-Pb age spectra from 10 samples collected from the Wilcox Group in south Texas reveal a complex grain assemblage with major age populations at ca. 1800–1300 Ma and 350–50 Ma and minor populations at 1300–950 Ma, 3250–1850 Ma, and 900–400 Ma. Modal data for Wilcox Group sand grain populations from the same region suggest a diverse provenance, including sources from the Cordilleran magmatic arc and subordinate reworked sedimentary rocks. Sediment was primarily derived from a mix of crystalline basement blocks in the U.S. southern Rocky Mountains and northern Mexico, the Cordilleran arc of western Mexico and the southwestern United States, and Mesozoic–Paleogene magmatic rocks of northern Mexico. Secondary sources include recycled strata from Sevier-Laramide basins in the southwestern United States and northern Mexico. Comparisons of detrital zircon U-Pb age spectra show that the Lower Wilcox subunit contains significant magmatic arc–derived sediment, whereas the Upper Wilcox subunit is more enriched in basement and recycled sedimentary detritus. Therefore, the relatively quartzose composition of the bulk detrital assemblages throughout the Wilcox Group of south Texas is not an accurate reflection of the range of volcanic and basement sources indicated by the associated U-Pb ages, suggesting significant attrition of unstable grains during long-distance (>500–1000 km) transport.