LA PENA FORMATION (APTIAN): A CONDENSED LIMESTONE‐SHALE SEQUENCE FROM THE SUBSURFACE OF NE MEXICO

An integrated stratigraphic study was made of the La Peña Formation, which is penetrated by a number of wells in NE Mexico. the study was conducted with the aid of ammonites (Cheloniceratidae, Deshayestidae, Acanthohoplitidae), microfossils (Colomiella, Nannoconus), and radioactivity logs to investigate the age of the unit ‐ late Aptian. A series of wells extending 400 Km NW ‐ SE were studied. The results show that the stratigraphic ranges of Colomiella recta, C. mexicana, and certain Nannoconus species are greater than currently believed. In contrast, the ammonite groups mentioned above are present only in the La Peña Formation. The formation is composed of a condensed section of limestone and shale in this area. The La peña and the units below it are cut by normal faults, in particular the Paras Fault System, which is described here for the first time.