Bol. Soc. Geol. Mexicana, Tomo XLVII. No. 1, 1986

Nuevo rudista (bivalvia-hippuritacea) del Cretácico Inferior de Pihuamo, Jalisco

http://dx.doi.org/10.18268/BSGM1986v47n1a4

Gloria Alencáster*

*Instituto de Geología, Universidad Nacional Autónoma de México.

Abstract

In Pihuamo, in the southern part of the State of Jalisco, near the El Encino iron mine, is a very thick Lower Cretaceous sequence of volcaniclastic rocks which is interbedded with lenses and banks of limestone, calcareous clay and submarine volcanic flows. These rocks comprise the Encino and Vallecitos formations. The caprinid rudist Coalcomana ramosa (Roehm), a guide species for the Early Albian, is verv abundant at different levels in both formations. A unit of green volcanic rock which is the lower member of the Vallecitos Formation, contains C. ramosa as well as species of Chondrodonta and Caprinuloidea. These occur in very extensive biostromes which are almost completely composed of rhe caprotinid rudist Jerjesia encina (new genus, new species). The individuals of this new taxon have a very unique hinge made of the curved distal end of the lower valve tooth which grows inside a small canal at the base of the upper valve's anterior rooth.

The top of the outcrop sequence contains species of Caprinuloidea of middle and late Albian age.