Fluvial-shallow marine-glacio¯uvial depositional environments of the Ordovician System in Jordan B.S. Amireh a, * , W. Schneider b , A.M. Abed a a Department of Geology, University of Jordan, Amman, Jordan b Institut fu Èr Geowissenscaften, TU Braunschweig, Postfach 3329, Braunschweig, Germany Abstract The Ordovician System, cropping out in southern and west-central Jordan, consists entirely of a 750 m thick clastic sequence that can be subdivided into six formations. The lower Disi Formation starts conformably above the Late Cambrian Umm Ishrin Formation. According to Cruziana furcifera occurring in the upper third of the Disi Formation, an Early Ordovician age is con®rmed. The Disi Formation, consisting mainly of downstream accretion (DA) ¯uvial architectural element, was deposited in a proximal braidplain ¯owing N±NE from the southerly-located Arabian±Nubian Shield towards the Tethys Seaway. The braidplain depositional environment evolved into a braid- plain-dominated delta through the middle and upper parts of the Disi Formation and the lower part of the overlying Um Saham Formation. The delta was replaced by siliciclastic tidal ¯ats, that in turn evolved into an upper to lower shoreface environment through the upper part of the Um Saham Formation. The depositional environment attained the maximum bathymetric depth during the deposition of the lower and central parts of the third unit, the Hiswa Formation, where offshore graptolite-rich mudstone with intercalated hummocky cross-strati®ed tempestites were deposited. The Tethys Seaway regressed back through the upper part of the Hiswa Formation promoting a resumption of the lower±upper shoreface sedimentation. Oscillation between the lower to upper shoreface depositional environment characterized the entire fourth unit, the Dubaydib Formation, as well as the Tubeiylliat Sandstone Member of the ®fth unit, the Mudawwara Formation. The depositional history of the Ordovician sequence was terminated by a glacio¯uvial regime that ®nally was gradually replaced by a shoreface depositional environment throughout the last unit, the Ammar Formation. q 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction An Ordovician sequence consisting invariably of sand- stone and silt±mudstone sediments crops out in southern Jordan and the area along the eastern margin of Wadi Araba (Fig. 1). The Ordovician System in Jordan has been studied by many authors, including Qunnel (1951), Bender (1968), Lloyd (1968), Selley (1970), Masri (1988), Powell (1989) and Khalil (1994). Some of these studies has been summarized and correlated with Ordovician outcrops throughout the Middle East countries by Alsharhan and Nairn (1997). Most of the above studies involved lithostrati- graphic subdivision of the Ordovician System into various units, but without giving details of their depositional envir- onments. Amireh (1993) conducted a sedimentological investigation aimed at distinguishing between the Ordovi- cian Disi Sandstone and the similar overlying Early Cretac- eous Kurnub Sandstone, and consequently extended the known occurrence of the Ordovician System 60 km north- ward of the limit recorded in the previous geologic maps. Makhlouf (1992, 1998) determined the depositional envir- onment of random parts of the Ordovician System. Therefore, it appears that a comprehensive sedimentolo- gical study of the Ordovician System has not been under- taken up until now. Thus, the present work documents the detailed sedimentology of the Ordovician System based on a systematic facies analysis and aims to determine the devel- opment of depositional environments during the Ordovician Period. 2. Geologic setting In southern Jordan, the clastic Ordovician System starts conformably above the Cambrian Umm Ishrin Formation, and is overlain, also conformably, by the Silurian Batra Mudstone Member of the Ordovician±Silurian Mudawwara Formation. The contact with the underlying Cambrian sedi- ments is problematic. It is based mainly on the color change from brown, characteristic of the Cambrian Umm Ishrin Sandstone, to white which is diagnostic of the Disi Sand- stone (Bender, 1968). Very likely, this lithologic contact does not represent the Journal of Asian Earth Sciences 19 (2001) 45±60 1367-9120/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(00)00010-9 www.elsevier.nl/locate/jseaes * Corresponding author. Fax: 1962-6-5348932. E-mail address: bamireh@sci.ju.edu.jo (B.S. Amireh).