Finely textured, calcitic, terrigenous clastic rocks of the type Escondido Formation (Rio Grande section) afford perhaps the most complete sphenodiscid ammonite succession in North America. Documented superposition in the type section cannot be reconciled with the established Maestrichtian ammonite zonation for South Texas and northern Mexico. A new zonation is proposed that should provide a clearer basis for biostratigraphic correlation with sphenodiscid-bearing sections elsewhere.

Terrigenous clastic rocks previously referred to the Escondido Formation crop out in the eastern part of the Sabinas coal basin in northern Coahuila. In terms of facies and depositional style, these rocks bear more similarity to the Parras Shale-Difunta transition on the south than to the type Escondido. The lower part of the proposed zonal scheme can be recognized in the eastern Sabinas coal basin; the upper part is not expressed there because of facies change and missing section. If the ammonite zones have chronostratigraphic value then, during the Maestrichtian, the Sabinas coal basin was the site of far greater subsidence and sediment accumulation than the upper Rio Grande embayment.

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