A petrographic study of the Wildcat Group was undertaken to determine the sediment provenance of the Late Miocene to Pleistocene marine sediments within the Eel River basin. Previously, it was believed that the principle source terrane was the Franciscan and Yager complexes exposed to the south and to the southeast. The potassium feldspar content within the Wildcat Group appears high (range = 0.7% - 12.3%) for these sources. Compositional data plotted on ternary diagrams suggest sand derivation from transitional continental and mixed origin sources. ⁴⁰Ar/³⁹Ar laser probe analyses of Scotia Bluffs Sandstone micas yield ages of...