GASTROPODS, BIOSTRATIGRAPHY, AND PALEOGEOGRAPHY, LOWER CRETACEOUS, IN A RUDIST-BEARING FORMATION, JALISCO, MEXICO

In the southeastern part of the Jalisco State, Southwestern Mexico, a great abundance of diverse reefal fauna is found in the Tamazula area. It is composed of numerous species of Mollusk bivalves (rudist, trigonids, veneroids) and gastropods biocenosis. The species are typical of the Cretaceous shallow tropical seas of the Tethys Realm, and show a strong affinity with species of the Mediterranean Realm.

The fossils are contained in the Encino Formation, which in this area consists of an approximately 1,200 m thick sequence of marine volcaniclastic rocks interbedded with lenses of limestone. These deposits indicate a history of periods of volcanic activity and tectonism alternating with intervals of stability on a shelf covered by shallow water with a low rate of sedimentation that allowed the formation of rudist biostromes and the development of diverse benthic fauna. The gastropods are found in the upper half part of the column, in horizons of calcareous rocks slightly argillaceous. The overlying beds are massive limestones, rudists occur in great abundance and hundreds of specimens of caprinids are found.

The gastropod fauna, of Aptian-Albian age, includes Microschiza (Cloughtonia) scalaris (Conrad), Mesoglauconia (Mesoglauconia) burnsi (Stanton), Gymnentome (Gymnentome) helvetica (Pictet & Campiche), G. (G.) paluxiensis Stanton, Nerinella boehmi Blanckenhorn, Aptyxiella supracostata (Stanton), Cossmannea (Eunerinea) hicoriensis (Cragin), C. (E.) pauli (Conrad), Lunatia pedernalis Roemer, Natica conradi (Hill) and Peruviella dolium (Roemer). Interlayered with the early Albian carbonated reefal horizon contain the caprinid Coalcomana ramosa (Boehm) and several species of Caprinulidea, there are mudstones with Nerinella dayi Blanckenhorn and Plesioptyxis fleuriaui (d’ Orbigny).

This study contributes to the confirmation of the Aptian-Albian age for the sedimentary rock of the Tamazula region and, therefore, to the discarding of the Cenomanian age assumed by Palmer in 1928.