The Upper Cretaceous (Turonian) Juana Lopez Beds of the D-Cross Tongue of the Mancos Shale in central New Mexico and their relationship to the Juana Lopez Member of the Mancos Shale in the San Juan Basin

Abstract

The distinctive rock types and faunas of the Juana Lopez Member of the Mancos Shale of the San Juan Basin can be recognized to the southeast in Socorro, Lincoln, and Otero Counties, New Mexico. Consequently, this lithostratigraphic name is extended formally into central New Mexico, where it is used as a bed-rank unit within the D-Cross Tongue of the Mancos Shale. The Juana Lopez Beds of the D-Cross Tongue of the Mancos Shale in central New Mexico consist of thin calcarenites and sandstones interbedded with thicker, noncalcareous shales as much as 21.4 ft (6.5 m) thick. Just as in the San Juan Basin, the base of the Juana Lopez Beds can lie in the middle Turonian Prionocyclus macombi Zone and its top, in the upper Turonian P. novimexicanus Zone. However, the unit is less than half as thick in central New Mexico as it is in the San Juan Basin, suggesting it is condensed to the southeast. Reduced thicknesses in units below and within the Juana Lopez Member and Beds suggest erosional unconformities throughout its geographic extent in New Mexico. The most distinctive fossil in the Juana Lopez is the small, ribbed oyster Cameleolopha lugubris (Conrad 1857), which is abundant in the basal part of the unit. The chronostratigraphy of the upper and lower contacts of the Juana Lopez in 11 outcrop sections extending from Mesa Verde National Park, Colorado, to northwest Otero County, New Mexico, indicate that neither contact represents a regional isochronous surface. An assemblage of three ammonite and two bivalve species from near the base of the Juana Lopez at four localities in New Mexico may provide the biostratigraphic resolution required for isochroneity. This assemblage includes the Boreal species Prionocyclus macombi, Coilopoceras inflatum, Inoceramus dimidius, and Cameleolopha lugubris along with the distinctive Tethyan ammonite Hourcquia mirabilis, described originally from Madagascar. However, this assemblage occurs in only three of the 11 profiled outcrop sections, although it is also known from the base of the Juana Lopez Member of the Carlile Shale in northeastern New Mexico.