LOWER TURONIAN FOSSIL LAGERSTÄTTEN IN MEXICO: THEIR RELATIONSHIP TO OAE-2

Sediments that crop out at several localities in the vicinity of Vallecillo, Nuevo Leon, Múzquiz, Coahuila (northeastern Mexico), and Xilitla, San Luis de Potosi (central Mexico) consist of interbedded marlstones and biocalcilutites varying between 10cm and 20cm in thickness. They show primary lamination at the millimeter scale, with disseminated pyrite, and the absence of bioturbation. Benthic organisms other than inoceramids are not reported at these three localities. However, the presence of the ammonite Watinoceras coloradoense, indicates an early Turonian age for the deposits at Vallecillo. The most important characteristic common to these deposits is the presence of exceptionally well preserved vertebrate assemblages or LAGERSTÄTTEN that consist mainly of well-preserved fishes in both two and three dimensions, depending on the sediment texture. These LAGERSTÄTTEN include complete and well-articulated skeletons, as well as details of soft parts such as muscles and gills. The high degree of preservation of the fossils implies anoxic conditions and rapid burial.

Lithological, paleontological and microfacies data indicate that the sediments were deposited in open-marine platform environment under low-energy conditions. The presence of primary lamination, pyrite and excellent degree of preservation of fishes, suggests that low oxygen concentrations lead to the formation of anoxic/dysoxic conditions during the accumulation of these exceptional deposits, which are apparently coeval with OAE-2.