About 2,000 m of Upper Jurassic (Tithonian) to Upper Cretaceous (Maestrichtian) rocks are exposed in Santa Rosa Canyon. The Tamaulipas is composed of resistant, light-gray to black, thin to thick-bedded, well-indurated lime mudstones about 800 m thick.

A medial unit (64 m) of black, laminated, thin-bedded lime wackestones allows division of the succession into three parts which appear to be homotaxial with the lower Tamaulipas, the La Pena, and the upper Tamaulipas.

Microfossils are rare to sparse in the lowest unit but are abundant in the medial unit and common in the upper unit. Extreme induration has prevented disaggregation and recovery of individual specimens; consequently this study is limited to thin sections of the rock. The general aspect of the fauna is pelagic and is dominated by radiolarians and foraminifers, although colomiellids, nannoconids, calcispheres, and pelagic pelecypods(?) are abundant at some levels in the upper unit.

Identifiable and chronostratigraphically useful taxa include Colomiella mexicana Bonet, C. recta Bonet, Favusella washitensis (Carsey), Globigerinelloides algerianus Cushman and Ten Dam, G. barri (Bolli, Loeblich, and Tappan), G. ferreolensis (Moullade), Planomalina cheniouriensis (Sigal), Microcalamoides diversus (form B) Bonet, M. diversus (form C) Bonet, Nannoconus steinmanni Kamptner, and N. wassalli Bronnimann.

The distribution of these taxa indicates that virtually all of the lower unit of the Tamaulipas is Hauterivian and Barremian. The middle unit (= La Pena?) is Aptian, and the upper unit is lower Albian. Some of the earliest Albian may be missing.

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