The Darriwilian Hiswah fauna of western Gondwana (Jordan): Biostratigraphy, palaeogeography and palaeoecology

Abstract

The fauna and facies of the Hiswah Formation in southern Jordan were investigated. The formation indicates deposition under low energy, open-marine, siliciclastic shelf conditions and reflects the maximum of the first regional post-Cambrian marine transgression on the northern edge of the Arabian-Nubian Shield. This eustatic signal accompanied by a maximum flooding surface can be traced over the entire Arabian Plate and Turkey. The lower part of the Hiswah Fm. consists of pelagic mudstones with only occasional thin beds of rippled, hummocky cross-bedded siltstone and mudstone concretions, and contains a distinct fauna of low diversity. The upper part of the formation is characterized by sandstone-siltstone alternations with some hummocky cross-stratification in its lower portion and trough cross-bedding and ripple marks above indicating somewhat shallower conditions. A very detailed re-investigation of the fauna from the lower part of the Hiswah Fm. and extensive new fossil material indicates that the most remarkable faunal element is the planktonic graptolite Didymograptus murchisoni (Beck in Murchison, 1839in) which, in the past, has been determined erroneously as D. bifidus (Hall in Berry, 1962). Numerous specimens of small obolid brachiopod Palaeoglossa sp. cf. P. attenuata (Sowerby, 1839) represent the most common benthic element within the Hiswah fauna. The exceptional arthropod Hanadirella cf. bramkampi El-Khayal, 1985, is reported for the first time from Jordan, representing its fifth known region of occurrence in addition to Saudi Arabia, Algeria, Spain, and France. For the first time, cephalopod remains are described from the Ordovician of Jordan by the occurrence of ?Dideroceras sp. and a second, indeterminate species. The biostratigraphic age of the Hiswah fauna is revised to late Darriwilian (Dw 3, upper Middle Ordovician) according to our re-investigation of the graptolites. The Hiswah fauna represents the oldest body fossils of the Ordovician marine sedimentation in southern Jordan and is palaeogeographically strongly related to northern and northeastern Saudi-Arabia and to high/mid-latitude Gondwana margins in general.