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The type Escondido Formation (Rio Grande section) has an aggregate outcrop thickness of about 900 ft and is sandwiched between the underlying coal-bearing Olmos Formation and the basal tabular limestone beds of the Midway Group (Tertiary). The lower three fourths of the outcrop Escondido is calcitic mudstone and shale separated by several fine-grained quartz arenite to lithic subarkose sandstone layers. Lithology, sedimentary structures, and macro-invertebrate faunal assemblages suggest a bay-lagoon-barrier bar depositional system. The upper part of the Escondido (Cuevas Creek Member) is a lithosome of calcitic mudstone, siltstone, very fine sandstone, and muddy to silty, commonly glauconitic limestone. Recurrent molluscan assemblages and abundant burrow structures sugges deposition in an inner-shelf setting of shallow neritic depths.
Sphenodiscid ammonite assemblages in the Escondido historically have been correlated with the lower part of the standard Maestrichtian stage of Europe. The uppermost sphenodiscid zone in the Escondido [zone of Sphenodiscus pleurisepta (Conrad)] has not been conclusively defined in other parts of Texas, the eastern Gulf Coast, or northeastern Mexico; it may represent higher Maestrichtian.
The Escondido-Midway contact is sharp, consisting of ledge-forming limestone above and mudstone below. However, the Cretaceous-Tertiary boundary does not coincide exactly with the contact. The boundary lies in a thin transitional interval, showing successions of glauconite concentrations and discontinuous levels of pholad borings.
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