Boletín de la Sociedad Geológica Mexicana Volumen 69, núm. 3, 2017, p. 739 ‒ 770

Paleoenvironment and biostratigraphy of the Upper Sinemurian (Lower Jurassic) of the Huayacocotla Formation in East-Central Mexico

Carlos Esquivel-Macías^1,*, Rita Gabriela León-Olvera, Kinardo Flores-Castro¹

¹ Cuerpo Académico de Ciencias de la Tierra, UAEH Pachuca, Hidalgo. C.P. 42184.
² Independent researcher.

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Abstract

The Lower Jurassic Huayacocotla Fm. (Upper Sinemurian), was first studied by several American and European geologist during the twentieth century (1948), whom eventually concluded that these rocks were not commercially productive. Consequently, the interest in this unit faded a long time ago. However, studies resumed in the 2000’s by Mexican researchers, this time including a wide range of paleontological, paleoenvironmental, paleoecological, geologic, and structural aspects, incorporating taphonomic, petrologic and geochemical techniques. They concluded that the Huayacocotla Fm. is not a condensed sequence, but can be better regarded as a single unit, including several facies rather than a complex unit of several formations. Today, it is well known that the Huayacocotla Fm. was deposited in a semi-restricted back arc paleobasin, with suboxic to euxinic conditions. Its protolith of igneous intermediate composition reveals tectonic features related to the Pangea collision phase and the first stages of the Gulf of Mexico opening. The paleobasin with high rate of sedimentation, generated this sequence siliciclastic, simultaneously with the process of expansion of the Gulf of Mexico. The modern biostratigraphical analyses developed in the 2000’s, accurate the biostratigraphic zonation proposed by Burckhardt and Erben. These new studies revealed that the deposition of the Huayacocotla Fm. spanned several Late Sinemurian diverse chronozones, especially Oxynotum , Densinodulum , and Raricostatum subzones. Nevertheless, some local divergences with respect to the standard biozonation were noticed, and these anomalies are tentatively explained to be the result of Early Sinemurian homotaxial events. There is a gap in the Lower Obtusum faunas, but the presence of Lower Sinemurian deposits was reported from at least one outcrop. This Jurassic field trip guide was written to lead and encourage interested researchers in this field to contribute to this subject, and maybe other related topics, as well as to encourage academic discussions about the knowledge of Early Jurassic marine paleobasins, in the context of the 10th International Jurassic Congress.

Keywords: Jurassic, Sinemurian, Huayacocotla, Mexico, Biostratigraphy.