Abstract

EGUILUZ Y DE ANTUNANO, Samuel; ARANDA-GOMEZ, José Jorge  and  JUAREZ-ARRIAGA, Edgar. Stratigraphy and depositional environments of the Ahuichila Formation in the Transversal Sector of Parras, Sierra Madre Oriental. Bol. Soc. Geol. Mex [online]. 2022, vol.74, n.1, e111121.  Epub May 02, 2023. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2022v74n1a111121.

Rocks in the Ahuichila Formation have been described in the literature for the last 80 years. In this time only generalized descriptions of the Ahuichila Formation have been published up to now. This paper includes the description of stratigraphic logs measured at the Bajío Ahuichila (type locality of this formation) and the Nazareno valley. The studied rocks are grouped into four lithofacies associations: I. Orthoconglomerate, which depending on the locality, occurs as either at the base or at the top of the formation, but never both in the same stratigraphic positions in the same section, II. Litharenite-limolite-shale, III. Gypsumdolomite-limolite-shale, and IV. Limestone with microbialites. The four lithofacies associations are interpreted as consistent with deposition in a closed continental basin, and correspond to fluvial, alluvial, and shallow lake sub-environments. The occurrence of evaporite and microbialite is consistent with an arid climate at the sedimentation time. The Planolites, Palaeophicus and Cochlichnus ichnogenera belong to the Mermia ichnofacies in a lacustrine environment. Intrabasinal volcanic lava flows are not founded in the Ahuichila Formation at the study area, this feature is a difference with similar lithologies in other continental red beds successions in central and northern Mexico, where intrabasinal volcanic rocks are common. Distal tuffs interlayered in the Ahuichila succession suggest extrabasinal volcanism contemporaneous, at least in part, with the sedimentation. The lower contact of the Ahuichila Formation, where is exposed out is always an erosional surface. Depending on the location, a disconformity with the beds in the underlying Mesozoic marine sedimentary rocks, or a clear angular unconformity can be observed. In those places where the contact is a disconformity, the structural attitude of the Mesozoic marine sediments and the continental Ahuichila rocks is the same, despite the fact that in some places, it might be sub-vertical; this is interpreted as clear evidence that the Ahuichila Formation was deformed together with the Mesozoic rocks. In those places where a clear angular unconformity exists it is obvious that at least one shortening pulse occurred prior to the Ahuichila deposit. The inferred shape of the Ahuichila paleobasin is elongated and parallel to the structures in the Parras Transversal Sector of the Sierra Madre Oriental, the paleobasin limits could be in the west at the Villa Juárez uplift and at the San Julián uplift in the east. The documented deformation in the Ahuichila Formation suggests the assumption of a syn-tectonic deposit, on a piggy-back type basin located atop the Parras Nappe, next to the Parras Tranversal Sector.

Keywords : piggy-back basin; alluvial-fluvio-lacustrine environments; microbialite; syn-tectonic depositional model; endorheic system.

        · abstract in Spanish     · text in Spanish     · Spanish ( pdf )