A REGRESSIVE PERMIAN SEQUENCE IN THE SIERRA LAS LILAS, NORTHWEST CHIHUAHUA, MEXICO

In the northwest of the State of Chihuahua are exposed sedimentary rocks belonging to two different sequences of the Early Permian age. These rocks are well exposed all along the NE margin of the Sierra de Las Lilas, also called Sierra de Santa Rita.

This sedimentary sequence consists of two formations that are lithologically well distinguishable. The lower formation is a sequence of at least 200 m thick dolomites called formation La Concha and it is separated in three members. This formation shows a great diversity in fauna including the braquipods Wellerella lemasi (Cooper) and Dictyoclostus depressus (Cooper) with a Permian age and the in debris flows reworked echinoderms-crinoids Heterostelechus texanus (Miller), from lower Permian age, Lamprosterigma cf. L. erathense (Cooper), Cyclocrista martini (Miller) and Preptopremnum rugosum (Jeffords y Miller), which have an age of the Pensilvanian.

Overlying the La Concha Formation there are outcropped more than 100 m Quartz Sandstones from the Santa Rita Formation. The Santa Rita Formation consists of thin, well bedded Sandstones, which show abundant hematite nodules and locally at the upper part interbedded dolomites. Contrary to the La Concha Formation the Santa Rita Formation is poor in fauna, where are being identified only poorly preserved fusulinids.

The fauna and the lithology of both formations suggest a regressive Paleozoic se-quence with initial marine shallow environment in a carbonate ramp-type platform, which changed transitionally to an environment with clear continental siliclastic influ-ence. The Permian deposits represent the sedimentary coverage deposited on the North American craton of the Chihuahua terrane.