Boletín de la Sociedad Geológica Mexicana

Volumen 66, núm. 2, 2014, p. 397-412

Evidencias petrográficas de estructuras de origen algal/bacteriano en carbonatos de la Formación Agua Nueva (Cenomaniano/Turoniano: Cretácico Superior) en Xilitla, S.L.P. México central

Alberto Blanco-Piñón1, Florentin Jean-Marie Robert Maurrasse2, Francisco Javier Zavala Díaz-de la Serna3, Rubén Alfonso López-Doncel4, Susana Abigail Ángeles-Trigueros4, Juan Hernández-Ávila1, Edgar Juárez Arriaga1,5

1 Área Académica de Ciencias de la Tierra y Materiales, Universidad Autónoma del Estado de Hidalgo, Apdo. Postal 1-288, Admón. 1, Pachuca, Hidalgo, C.P. 42001, México.
2 Department of Earth and Environment, Florida International University, 11200 SW 8th Street, PC-344, Miami, Florida, Estados Unidos de América.
3 Facultad de Ciencias Químicas, Universidad Autónoma de Chihuahua, Nuevo Campus Universitario, Circuito Universitario, C.P. 31125, Chihuahua, Chihuahua, México.
4 Instituto de Geología, Universidad Autónoma de San Luis Potosí, Av. Dr. Manuel Nava 5, Zona Universitaria, 78240, San Luis Potosí, San Luis Potosí, México.
5 Gerencia de Estudios Especiales e Investigación, Servicio Geológico Mexicano, Blvd. Felipe Ángeles, km 93.50-4, Col. Venta Prieta, C.P. 42080, Pachuca, Hidalgo, México.

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This paper documents and describes fossil microbial structures inferred to be of algal/bacterial origin in the Upper Cretaceous Agua Nueva Formation in the Xilitla area (San Luis Potosi, Mexico), located in the southern part of the Tampico-Misantla basin. The sequence consists of alternating decimeter-thick beds of limestone with occasional brown shale and green bentonite layers. The limeston also include intermittent beds of black chert. The sedimentary succession shows two calcareous facies: 1) dark laminated limestones containing fossil remains of holosteans, teleosteans and shark teeth, as well as lenticular layers of sedimentary pyrite, and high content of organic matter (Corg1 ‒ 8 wt%); and 2) gray, non-laminated, bioturbated limestones with low content of organic matter (Corg < 1.0 wt%). Petrographic and scanning electron microscope analyses of the laminated limestone reveal a micritic matrix with sub-parallel wavy lamination, continuous and discontinuous folded laminae with shreds of organic matter, as well as different kinds of filaments, coated grains and planktonic foraminifera. On the other hand, the non-laminated, bioturbated limestone comprises infrequent filaments and pyrite. Both textures and biomorphic structures are very similar to those previously described for ancient and recent sediments representative of microbial origin. Furthermore, the occurrence of filaments embedded in the organic-matter-rich laminae suggests in situ production related to benthic microbial activity.

Concurrence of lamination due to the absence of bioturbation, pervasive framboidal pyrite, and the high concentration of organic matter in the laminated limestones are consistent with persistent dysoxic/anoxic conditions, whereas the bioturbated limestones might represent recurrent relatively well-oxygenated episodes. The presence of the planktonic foraminifera Rotalipora cushmani concomitant with Inoceramus labiatus indicates a time interval from the Late Cenomanian to the Earliest Turonian for the Agua Nueva Formation at Xilitla, thus indicating that this interval of intermittent severe oxygen deficient conditions is coeval with the global Oceanic Anoxic Event 2 (OAE-2).

Keywords: filaments, microbial activity, OAE2, Turonian, Agua Nueva Formation, central Mexico.