ABSTRACT

Approximately 3,500 feet of Cretaceous limestone crops out in the Sierra de Picachos. All of the limestone is lithified carbonate mud or micrite, which differs mainly in the types of contained allochems. The allochems consist principally of pelagic microfossils: calcispheres, foraminifers, tintinnids, and radiolarians. Intraclasts, pellets, and fragments of larger invertebrates occur in some of the limestone. Fossil allochems average fifteen percent of the micrite by volume and constitute more than fifty percent of a few limestones (packed biomicrite). Most of the limestone is quite pure (> 90% carbonate by weight) although that in the uppermost part of the section (San Felipe Formation) is very argillaceous. Some of the limestone (Cupido Formation) has been dolomitized, and some ( an Felipe Formation) has been extensively churned by burrowing organisms.

The carbonate mud was deposited in a basin in which the depositional interface was below wave base throughout deposition. The depth of water is unknown but was decidedly deeper than that to the west and northwest of the Picachos region in which contemporaneous shelf carbonates were deposited. Some of the carbonate mud may have been physicochemically precipitate, but much of it probably came from calcareous nannoplankton, principally nannoconids. Such nannofossils are now mostly unrecognizable due to diagenetic alteration and recrystallization.