STRATIGRAPHY OF THE CUCHILLO FORMATION IN CHIHUAHUA, MÉXICO

A stratigraphic reconnaissance of the Cuchillo Formation in the type-section area, and detailed mapping on other outcrops from Chihuahua, have demonstrated that this formation comprises at least four mappable stratigraphic units which warrant designation as separate formations. They were already referred by Petroleos Mexicanos geologists from older to younger as La Virgen, Cupido, La Peña and Coyame formations.

La Virgen Fm. (350-616 m thick) is characterized by alternating thin-bedded carbonates and evaporites; the Cupido Fm. (19-250 m) consists of homogeneous medium- to thick-bedded light grey limestone; the La Peña Fm. (99-574 m) consists of laminar- thin- and medium-bedded, dark grey and black limestone, marlstone, and dominantly shale. It is considered as an index horizon for northern Mexico. This unit contains ammonites (Kazanskyella aff. arizonica) and bivalve’s fossil fauna (Exogyra quitmanensis, Aetostreon latissimum, Amphidonte obliquata, Gryphaea sp.) of late Aptian age; the Coyame Fm. (100-200 m) is composed predominantly of grey, thin- to medium-bedded limestone and black chert nodules, with alternating marlstone and shale intervals. The unit was deposited in a deep water environment. Planktonic foraminifera are abundant in the limestones of the Coyame as Hedbergella washitensis and Ticinella bejaouaensis, calcispherullids as Calcisphaerula innominata, Stomiosphaera sphaerica and Pithonella trejoi, colomiellids as Colomiella mexicana y Colomiella recta, and calcareous nannoplankton as Nannoconus minutus indicating an early Albian age.

About distribution, the Cuchillo Formation crops out north of Alamitos Lineament in the Cuchillo, Bronce, Aldama, Lagrima, Coyame, Navarrete, Alcaparra, Rancherias, Ojo Caliente, Aguilas, Presidio and San Ignacio ranges. South of Alamitos Lineament, in the Chorreras, Roque, Amargosa, Fernando, Benavides, Mojina and Magistral ranges.

R. H. Burrows (1910) defined the Plomosas, Las Vigas, Cuchillo, Aurora and Ojinaga formations from Eastern Chihuahua. All of them are made up by two or more mappable units, one of which Diego Cordoba (1969) raised to the group rank (Aurora Formation) as Chihuahua Group. Designation of formations within the Cuchillo Formation in Chihuahua will merit elevation to group rank as Cuchillo Group.