Eel River Basin, a westerly trending Cenozoic basin in Humboldt County, was the site of deposition of 13,000 feet of dominantly marine upper Miocene to Recent clastic sediments.

Pre-Tertiary formations in the region include the Franciscan and Yager formation (Cretaceous). Deposition of the Wildcat group on an eroded surface cut on Franciscan and Yager rocks began in Mohnian time with the basal beds of the Pullen formation (Mohnian-lower Pliocene). The Pullen, 600-1,000 feet thick, locally has a 200-foot thick basal sand, but is dominantly made up of massive diatomaceous mudstone. Overlying the Pullen disconformably is the Eel River formation (600-2,000 feet) which is characterized by glauconitic sandstones and dark gray mudstones containing a Repettian fauna. From 3,000 to 6,000 feet of middle to upper Pliocene mudstones, siltstones, and sandstones of the Rio Dell formation overlie the Eel River formation and overlap onto pre-Tertiary rocks in the northeaster part of the basin. Numerous thin, friable, permeable sands are present in the middle part of this unit; gas is produced from them in the Tompkins Hill gas field. Gradationally above the Rio Dell is the Scotia Bluffs sandstone, an upper Pliocene shallow marine to non-marine massive sandstone unit, 1,000-2,000 feet thick, noted for forming spectacular cliffs. Carlotta formation's non-marine massive conglomerates, sandstones, and claystones grade up from Scotia Bluffs sandstone. This 500-3,000-foot series of beds ranges in age from upper Pliocene to Pleistocene.

Rapid changes take place in thickness, character, and distribution of these units. Sedimentary and faunal evidence indicate deep-water deposition in the early history of the basin with gradual shallowing and eventual regression of the sea by late Pliocene.

Faulting probably aided in the early development of the depositional trough. The principal structural features of the basin are the major Eel River syncline and smaller anticlines on the north and south, and the northwest-trending Little Salmon fault.

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