Ordovician and Devonian microfacies of Sierra Agua Verde, central Sonora, Mexico: A new locality of the Nuia Province from western Laurentia

Abstract

The Lower Ordovician carbonate rocks exposed in the Tuntunudé hill north of the Sierra Agua Verde, northeast of the town of Mátape, in central Sonora, consists of intraformational breccia, conglomeratic limestone, sandy limestone and limestone, which are overlain by a mixed succession of quartz sandstone and dolomitic sandstone. This succession is in turn unconformably overlain by an Upper Devonian sequence of dolomitic limestone, marl, conglomeratic limestone and sandy dolomite. The Lower Ordovician rocks contain abundant bioclasts of gastropods, brachiopods, trilobites, echinoderms, and sponges. Their microfacies show a marine medium to low energy environment, with sedimentation dominated by distal tempestites in laminar stages and intraclast debris formation during deposition. The depositional paleoenvironment corresponds to a marine middle-to-outer ramp. These strata are characterized by conodonts belonging to the Oepikodus communis and Reutterodus andinus biozones, as well as cyanobacteria incertae sedis Nuia sibirica. Devonian rocks overlying the Ordovician strata contain bioturbation and abundant bioclasts of echinoderms, brachiopods, bryozoans, gastropods, chaetetiform sponges, stromatoporoids; and also, colonial corals like Hexagonaria attenuata, Phillipsastrea hennahii and Phillipsastrea jachowiczi, monothalamous foraminifera like Vicinosphaera sp., monoserials such as Tikhinella measpis and planispiral such as Nanicella gallowayi. The microfacies show a high-energy environment in marine tropical shallow open-sea lagoonal waters, with ooids and reef building organisms.