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<p>The study of new collected, stratigraphically constrained ammonites from the uppermost Pliensbachian to Toarcian succession in three basinal sections of the Rich area (Central High Atlas, Morocco) allows refinement of the regional biochronostratigraphy for this interval and recognition of several unconformities with associated hiatuses of variable extent. Two of the sections (Azag and Ait el Abbas), located on the flanks of former diapiric anticlines, display relatively reduced thicknesses (110 and 91&#xa0;m, respectively), whereas the Agounou section, situated within a syndepositional, halokinetic-related syncline (minibasin), is nearly seven times thicker. Hiatuses are identified by the absence or condensation of stratigraphic intervals ranging from ammonite subzones to entire stages. Ammonites from the Azag section enabled characterization of the Polymorphum, Levisoni, Bifrons, Gradata, Bonarelli, and Speciosum zones beneath an erosional surface capped by a condensed, decimetric-thick calcarenitic-conglomeratic bed with a ferruginous hardground of early Bajocian (Discites–Laeviuscula zones) age. This indicates a hiatus spanning at least the upper part of the late Toarcian to the Aalenian. In Ait et Abbas, the hiatus related to erosional truncation is even larger, encompassing at least from the Gradata Zone (Middle Toarcian) to the Aalenian. Moreover, the Semicelatum Subzone appears to be absent or only partially developed in the studied sections, suggesting an additional hiatus affecting the upper part of the Polymorphum Zone, as observed in other Tethyan and European sections. These hiatuses are interpreted as resulting primarily from erosion and/or sedimentary thinning toward diapiric highs, as well as from gravity-driven sliding related to the synsedimentary mobility of Triassic evaporites. The interplay of these regional halokinetic processes with supraregional tectonic events, climatic perturbations, and sea-level fluctuations shaped the stratigraphic evolution of the area. The chronostratigraphic framework and interpreted Toarcian–Lower Bajocian tectonosedimentary evolution of the Rich area are likely applicable to the Central High Atlas basin as a whole.</p>

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New Toarcian ammonites from the Central High Atlas (Rich area, Morocco): implications for chronostratigraphy, stratigraphic architecture, and sedimentary gaps

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  • 发表日期：

    2025年12月29日

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  • Idoia Rosales,
  • María José Comas-Rengifo,
  • Antonio Barnolas,
  • Antonio Teixell,
  • María Luisa Arboleya,
  • Antonio Goy

摘要

The study of new collected, stratigraphically constrained ammonites from the uppermost Pliensbachian to Toarcian succession in three basinal sections of the Rich area (Central High Atlas, Morocco) allows refinement of the regional biochronostratigraphy for this interval and recognition of several unconformities with associated hiatuses of variable extent. Two of the sections (Azag and Ait el Abbas), located on the flanks of former diapiric anticlines, display relatively reduced thicknesses (110 and 91 m, respectively), whereas the Agounou section, situated within a syndepositional, halokinetic-related syncline (minibasin), is nearly seven times thicker. Hiatuses are identified by the absence or condensation of stratigraphic intervals ranging from ammonite subzones to entire stages. Ammonites from the Azag section enabled characterization of the Polymorphum, Levisoni, Bifrons, Gradata, Bonarelli, and Speciosum zones beneath an erosional surface capped by a condensed, decimetric-thick calcarenitic-conglomeratic bed with a ferruginous hardground of early Bajocian (Discites–Laeviuscula zones) age. This indicates a hiatus spanning at least the upper part of the late Toarcian to the Aalenian. In Ait et Abbas, the hiatus related to erosional truncation is even larger, encompassing at least from the Gradata Zone (Middle Toarcian) to the Aalenian. Moreover, the Semicelatum Subzone appears to be absent or only partially developed in the studied sections, suggesting an additional hiatus affecting the upper part of the Polymorphum Zone, as observed in other Tethyan and European sections. These hiatuses are interpreted as resulting primarily from erosion and/or sedimentary thinning toward diapiric highs, as well as from gravity-driven sliding related to the synsedimentary mobility of Triassic evaporites. The interplay of these regional halokinetic processes with supraregional tectonic events, climatic perturbations, and sea-level fluctuations shaped the stratigraphic evolution of the area. The chronostratigraphic framework and interpreted Toarcian–Lower Bajocian tectonosedimentary evolution of the Rich area are likely applicable to the Central High Atlas basin as a whole.