Abstract
This paper presents a stratigraphic, sedimentological and palaeontological study of the Cambrian–Ordovician transition in the central Anti-Atlas in order to characterize the rift-drift transition of the Cambrian Atlas Rift. The event stratigraphy and sedimentology of the Cambrian–Ordovician transition in the central Anti-Atlas, and the endemic character of the latest shelly faunas from the Cambrian Atlas Rift have been reviewed, the latter by sampling the Bailiella Formation and the base of the Jbel Lmgaysmat Formation. In the vicinity of Foum Zguid, central Anti-Atlas, several unconformities (joined laterally into one single angular discordance or paraconformity) can be recognized across the Cambrian–Ordovician transition: (i) a ravinement surface capped by lenses of polymictic conglomerates marking the top of the Azlag Formation and reflecting reworking of the substrate under sharp transgressive conditions; (ii) a skeletal calcareous siltstone bed at the base of the Jbel Lmgaysmat Formation preserving surficial silicification (silcretization related to broad uplift, denudation and weathering of an exposed landscape dominated by silica-rich rocks), postdated by fissuring associated with hydrothermal mineralization of Fe (Ba-Mn-Pb) ore deposits, linked to the opening of the so-called El Graara graben; (iii) generalized uplift and denudation, followed by sedimentation of alluvial-to-fluvial and proximal coastal-plain deposits (lower member of the Lower Fezouata Formation); and (iv) a final drowning and shaly blanketing of the inherited palaeorelief by the upper member of the same formation, which marks the beginning of passive-margin conditions. The Bailiella Formation has yielded the trilobite species Planolimbus hessikouanus, Maladioidella? destombesi n.sp. and Bailiella sepulcra n.sp., and the Jbel Lmgaysmat Formation Bailiaspis? glabrata, which point to biogeographic connections with the neighbouring Avalonian and west-European margins of Northwest Gondwana and Baltica.
Resumen
Se presenta en este trabajo la estratigrafía, la sedimentología y el contenido paleontológico del tránsito Cámbrico–Ordovícico en el Anti-Atlas central, con el objetivo de caracterizar el paso a margen pasivo del Rift cámbrico del Atlas. Se realiza una revisión de la estratigrafía de eventos, la sedimentología y el contenido fósil del tránsito Cámbrico–Ordovícico en el Anti-Atlas central, así como el carácter endémico de las faunas esqueléticas más recientes registradas en el Rift cámbrico del Atlas a partir de un muestreo exhaustivo en la Formación Bailiella y la base de la Formación Jbel Lmgaysmat. En los alrededores de Foum Zguid se reconocen varias discontinuidades estratigráficas, que lateralmente se reúnen en una discordancia angular o paraconformidad única. Éstas son: (1) una superficie de abarrancamiento, recubierta por lentejones de conglomerados polimícticos, situados a techo de la Formación Azlag y que representan el retrabajamiento del sustrato bajo condiciones transgresivas súbitas; (2) una limolita calcárea con abundantes restos bioclásticos, situada a la base de la Formación Lmgaysmat y que ha registrado un proceso de silicificación subaérea (silcretización relacionada con un desarrollo de levantamiento cortical, denudación y meteorización de un sustrato dominado por rocas silíceas) seguido por otro de fisuración asociado al registro de mineralizaciones hidrotermales de Fe (Ba–Mn–Pb) relacionado con la apertura del denominado graben de El Graara; (3) un nuevo pulso de elevación cortical y denudación superficial relacionado con el registro de medios aluviales-fluviales a llanura costera proximal (miembro inferior de la Formación Fezouata Inferior); y (4) el colapso final de la cuenca, cuyo paleorelieve fue sellado por las lutitas del miembro superior de la Formación Fezouata, que marca el comienzo de condiciones de margen pasivo. Asimismo, la Formación Bailiella ha librado las especies de trilobites Planolimbus hessikouanus, Maladioidella? destombesi n. sp. y Bailiella sepulcra n. sp., y la Formación Jbel Lmgaysmat Bailiaspis? glabrata, lo que apunta a conexiones biogeográficas con los márgenes de Avalonia y Europa occidental de Gondwana occidental y Báltica.
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Acknowledgements
The authors thank Rodolfo Gozalo (Valencia University) and an anonymous reviewer for constructive criticism, Kai-Uwe Gräfe (Bremen University) for technical editing, Anna Żylińska (Warsaw) and Per Ahlberg (Lund) for information about disarticulated maladioidelloid pygidia, and Alberto Jorge (MNCN, Madrid) for SEM assistance. Agreement for using Google Earth images was yielded by Google Brand Permissions. Financial support for this work was provided by project CGL2017-87631-P from Spanish MINECO.
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Álvaro, J.J., Vizcaïno, D. The Furongian break-up (rift/drift) unconformity in the central Anti-Atlas, Morocco. J Iber Geol 44, 567–587 (2018). https://doi.org/10.1007/s41513-018-0066-2
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DOI: https://doi.org/10.1007/s41513-018-0066-2