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Bajocian transgressive-regressive sequences of the Tecocoyunca Group, southern Mexico, with maximum flooding surfaces marked by Thalassinoides

Pieńkowski Grzegorz, Martini Michelangelo, Zepeda-Martínez Mildred

Geological Quarterly

2019

Vol. 63, No. 3

449--459

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 without 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-marine 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.

Jurassic stage definitions: the value of global sequence stratigraphy

Simmons D., Sharland P., Casey D.

Volumina Jurassica

2006

Vol. 4, no. 1

215-215

Five years ago we demonstrated the occurrence across the Arabian Plate of 11 synchronous Maximum Flooding Surfaces (MFS) during Jurassic time (Sharland et al. 2001, 2004). Ongoing work, incorporating all the stratigraphy of North Africa and examination of other regions, can now demonstrate the occurrence of 15 Jurassic 3rd order surfaces, and intervening sequence boundaries. Each of our MFS is defined in a Reference Section. This is a location with good sedimentological and/or wireline log evidence for a MFS, supported by biostratigraphy. The biostratigraphy also provides constraint on the correlation of the MFS to its occurrence in other locations. Each of our MFS can be shown to be occurring within the same biozone, or correlative, in different basins across the Middle East and North Africa region. Data from differing fossil groups have been calibrated to the standard zonation for each period. Sequence boundaries are age-calibrated and correlated in a similar manner via their correlative conformities. The ability to recognize and correlate depositional sequences across Arabia and beyond, across basins with differing subsidence and sedimentation rates, indicates that these sequences are driven by synchronous eustatic sea-level change. This observation is of value in providing guidance for the definitions of the fundamental units of chronostratigraphy - stages. Each Phanerozoic stage requires a Global Stratotype Section and Point (GSSP), that is to say, a location and bedding plane where the base of each stage is defined. This definition is tied to an event in the rock record useful for correlation. Progress in defining Jurassic GSSPs has been delayed because of difficulties in choosing the most appropriate (bio)event to relate to definition. It is recommended that stage boundaries be related to bioevents associated with correlative conformities of sequence boundaries. This links chronostratigraphy and sequence stratigraphy and honours the original concepts upon which many stages were first described in the 19th Century.