An incomplete, 570-m-thick volcano-sedimentary section measured in the locality of Rancho La Colgada in north-central Sonora corresponds to the upper part of the Cucurpe Formation. This section, composed of strongly deformed, interbedded thinly laminated to thick bedded shale, calcareous shale, siltstone, tuff beds, and basaltic flows, is unconformably (?) overlain by the shallow-marine La Colgada Formation of probable Early Cretaceous age. The La Colgada Formation is gradationally overlain by the Morita Formation of the Bisbee Group which is also represented in the area by the Mural and Cintura Formations. The Cucurpe Formation has several rich fossiliferous intervals with ammonites and/or bivalves. The ammonites are representatives of the middle Oxfordian and the early Tithonian ages. Oxfordian genera include Perisphinctes (Dichotomosphinctes) and cf. "Discosphinctes" Tithonian genera include Mazapilites, Schaereria, Subplanitoides, Glochiceras (Lingulaticeras), Sublithacoceras, Franconites, Aulacosphinctoides, and Discosphinctoides (Pseudodiscosphinctes). The Tithonian fauna has a Tethysian, Mediterranean and Submediterranean affinity while Dichotomosphinctes has been previously recognized in central Mexico in Durango and San Luis Potosi, "Discosphinctes" was reported from San Luis Potosi and Cuba, and Mazapilites and Schaereria are also present in San Luis Potosi, Zacatecas and Durango. The new ammonite data from Sonora extend the age of the Cucurpe Formation, formerly considered as exclusively Oxfordian, into the lower Tithonian.