Lower Aeronian (Llandovery, Silurian) graptolitic carbonate concretions from the Qusaiba Shale Formation, Tabuk Basin, Saudi Arabia, and their significance

David Loydell, Mohamed Abouelresh

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    Abstract

    A unique layer of carbonate concretions within the Qusaiba Shale Formation, north-west of Tayma in north-west Saudi Arabia, is dated to the early Aeronian (within the lowermost magnus Biozone, close to the triangulatus/magnus Biozone boundary) based upon its diverse (18 species) assemblage of graptolites. This assemblage is dominated by biserial taxa, many showing current alignment. The studied concretion included a 6 mm thick layer composed entirely of graptolites, potentially significant, if laterally extensive, for the burial of organic carbon. Carbonates of a similar age occur in northernmost Saudi Arabia, suggesting that the concretions may be a useful stratigraphical marker and are likely to result from similar environmental conditions. A few localities on Avalonia and Baltica, in sections otherwise dominated by outer shelf or basinal clastics, also have limestone layers/concretions and/or unusually high incidences of preservation of shells (mostly orthocones) within the lower Aeronian. It is considered likely that the limestone layers and concretions resulted from dissolution then reprecipitation of carbonate from some of the shelly fauna. This may be a stratigraphically very early and poorly developed expression of the cephalopod limestone facies so characteristic of higher levels in the Silurian of peri-Gondwanan Europe and Gondwana. The deposition/preservation of the limestones occurred within the positive Early Aeronian Carbon Isotope Excursion (EACIE) which itself may reflect an interval of slightly lower eustatic sea-level. One new graptolite species, Paraclimacograptus crameri, is described, distinguished from Pa. libycus by its narrower rhabdosome.
    Original languageEnglish
    Pages (from-to)251-263
    Number of pages13
    JournalBulletin of Geosciences
    Volume96
    Issue number3
    Early online date30 May 2021
    DOIs
    Publication statusPublished - 1 Jul 2021

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