The Upper Cretaceous (Coniacian–lower Campanian) Chico Formation of the northeastern Sacramento Valley, California, includes three newly defined members at the type locality: (1) cobble conglomerate of the basal Ponderosa Way Member, (2) coarse-grained conglomeratic sandstone of the overlying Musty Buck Member, and (3) fine-grained silty sandstone of the uppermost Ten Mile Member. Other outcrops of the Chico Formation exhibit the same three members plus an additional unit, the Kingsley Cave Member, composed of mudstone. The Chico Formation resulted from a transgression onto the Sierra Nevada basement in the Late Cretaceous. The Kingsley Cave Member was deposited locally in quiet-water conditions below wave base in an intrashelf basin and may represent a marine connection with regions farther northeast.
Macrofossils allow correlation of the Chico Formation with marine deposits of the Great Valley sequence exposed along the western Sacramento Valley. Strata of the Guinda and Forbes Formations represent submarine fan-channel to outer-fan deposits of a shallowing Late Cretaceous fore-arc basin. The Santonian Guinda Formation, a massive sand unit deposited in fan-channel to distal-fan turbidite environments, is correlative with the Musty Buck and Kingsley Cave Members of the Chico Formation. The Dobbins Shale Member of the Forbes Formation, a widespread mudstone unit of hemipelagic outer-fan to basin-plain deposits, is correlative with the Kingsley Cave Member of the Chico Formation and reflects a Santonian transgressive event in the northern fore-arc basin. Mudstones and turbidites of the middle of the Forbes Formation are equivalent to the shallow-marine strata of the Ten Mile Member of the Chico Formation. Correlatives of younger Forbes Formation strata have not been positively identified from the Chico Creek region but may be represented by unfossiliferous, cross-bedded, coarse-grained sandstones found above sections of the Chico Formation.