LITHOGRAPHIC MARLS FROM VALLECILLO, N.L. MEXICO: AGUA NUEVA FORMATION OR A NEW LITHOLOGIC UNIT?

In northeastern Mexico, in the vicinity of the Sabinas Hidalgo-Vallecillo area, state of Nuevo León, outcrops a sequence of dark gray limestones and carbonaceous laminated shales with individual beds of approximately 30 cm thick. This sequence has been reported as Agua Nueva Fm, although several workers have used the names Indidura Formation or even Eagle Ford Formation. Commonly, this unit is rich in planktic foraminifera and trace fossils, whereas macrofossils other than Inoceramus are rare.

Near to the village of Vallecillo, Nuevo León, outcrops a lithologic unit of at least 4 m thick of finely laminated pink marlstones with an average thickness of 6 cm, and 3 cm thick shale interlayers. This features contrast clearly with the beds mentioned above. The marlstones unit underlies concordantly with the limestone unit. The marlstones are widespread in the Vallecillo area and are characterized by the presence of well-preserved vertebrate fossils that includes new species and genera of fish as well as genera not previously reported for the Cretaceous of Mexico and even for the Cretaceous of North America, but present in other localities of the eastern Tethys.

The marlstone unit has not been previously recognized in the Agua Nueva, Eagle Ford or Indidura Formations, and is interpreted here to be a new lithologic unit within one of these Formations. According to lithological data and microfacies analysis, the fish bearing layers accumulated in a low energy condition on the outer part of a shelf under relatively low oxygen concentrations, and probably represent the maximum transgresive level during the Early Turonian. The presence of several fish genera such as Rhynchodercetis, Tselfatia, etc suggests that some marine faunas were probably able to exchange along the Western Interior Seaway during this transgresive event.

Finally, the different lithology and the extraordinary fish fauna makes necessary further stratigraphic analysis in this area.