Bajocian transgressive-regressive sequences of the tecocoyunca group, southern Mexico, with maximum flooding surfaces marked by thalassinoides

Abstract

Bajocian (Middle Jurassic) transgressive-regressive sequences (TRS), outcropping in the Rosario Nuevo Creek (Tezoatlán Basin, Tecocoyunca Group) in Oaxaca State, Mexico, represent one of the Jurassic phases of opening and widening of a trans-Pangaean marine corridor (called also the Hispanic Corridor) and show a retrogradational-progradational set of sedimentary successions with decipherable and diverse facies. Two TRSs have been distinguished. The lower one starts with fluvio-deltaic sandstones including pedogenic horizons. Drowning of the deltaic plain and gradual rising of the water table is marked by change in vegetation: from large trees to low-rise vegetation with characteristic clumps of dense roots cemented by siderite. The delta plain succession is topped by a thin coal seam, followed by a transgressive surface. Ensuing laminated mudstones of restricted marine origin pass into open marine deposits, represented by bioturbated heterolithic strata with ammonites followed by nearshore sandstones, deposited in a storm-dominated basin. A similar succession, although with-out the deltaic part, is repeated in the next TRS. Of note are two thin (15–20 cm) continuous beds with Thalassinoides isp. networks, present within open marine deposits. Although large Thalassinoides networks are mostly known from shallow-ma-rine and coastal environments, the case from Mexico represents less common occurrences from a deeper marine (offshore) setting, associated with maximum flooding surfaces, sediment starvation and firmgrounds (Glossifungites ichnofacies). Occurrences of Thalassinoides meshes, precisely marking maximum flooding surfaces, are helpful in defining the hierarchy of sequence stratigraphic cycles.