Sedimentary environments in the prelude to the lagerstätten conditions of the Tlayúa Formation (Albian) in central Mexico: A microfacies approach

Abstract

This contribution presents the first detailed microfacies analysis of the Lower Member of the Albian Tlayúa Formation outcropping in southern Mexico. The basis for this study consists of two measured stratigraphic sections named Tlayúa and Barranca Abuelo. The Tlayúa section is a composite section ∼48 m thick that includes the sedimentary record of the Lower Member and the first 12 m of the Middle Member. This carbonate succession directly precedes the fossil Lagerstätten deposits of the Middle Member exposed at Tlayúa Quarry. A second stratigraphic section of ∼23 m outside of Tlayúa Quarry includes the transitional contact between the Lower and Middle members and is documented herein for the first time. A detailed microfacies analysis combined with stratigraphic and sedimentologic data suggests that the Tlayúa deposits are part of a tropical shallow-water carbonate platform similar to the modern Great Bahama Banks.Microfacies analysis of the studied sections reveals the evolution of a protected carbonate setting affected by recurring storm events of different intensities. The succession records shelf lagoon sedimentation for most of the Lower Member and its gradual change to tidal flat deposition in its upper part. Shelf lagoon deposits consist of a diverse set of subtidal microfacies characterized by a carbonate fabric with low taxonomic group diversity that indicates stressed environmental conditions. Tidal flat deposits consist of shallowing upward cycles, each less than 1 m thick. Commonly, the peritidal cycles are incomplete and consist of mud-rich shallow-subtidal intertidal microfacies, which are clearly observed at the top of the Lower Member. Both microfacies are similar to those observed in the complete cycles described below. The complete cycles consist of mud-rich shallow-subtidal microfacies with low diversity of bioclasts that gradually change to intertidal microfacies characterized by the association of microbialites with desiccation features that suggest intermittent subaerial exposure. Intertidal microfacies record an important change in the composition of the carbonate factory because microbial processes generated most of the sediments in these microfacies. Supratidal deposits characterized by the presence of subaerial exposure structures and calcified pedogenic products, complete the peritidal succession and were identified at the base of the Middle Member. Thus, the presence of soil pisoid-like structures, clotted micrite, possible Microcodium structures, and alveolar textures indicate a sufficiently long subaerial exposure for the formation of pedogenic products, and therefore, a break in the marine sedimentation of the Tlayúa Formation.