High frequency eustatic sea-level changes during the Middle to early Late Ordovician of southern Jordan: Indirect evidence for a Darriwilian Ice Age in Gondwana Brian R. Turner a, ⁎, Howard A. Armstrong a , Charles R. Wilson a , Issa M. Makhlouf b a Department of Earth Sciences, Science Laboratories, Durham University, South Road, Durham, DH1 3LE, UK b Department of Earth and Environmental Sciences, Hashemite University, Zarqa, Jordan abstract article info Article history: Received 24 May 2011 Received in revised form 29 December 2011 Accepted 10 January 2012 Available online 18 January 2012 Editor: G.J. Weltje Keywords: Southern Jordan Ordovician Sequence stratigraphy Glacio-eustacy Ice sheets The Middle to early Late Ordovician Hiswah and Dubaydib Formations in southern Jordan provide a well con- strained sedimentary record of 4th and 3rd order relative sea level change along the northern, tectonically quiescent, subpolar Gondwana continental margin. Cyclic deposition occurred on a wave- and storm- dominated, microtidal shelf, dominated by shoreface and offshore transition zone facies. Cycles are common- ly bounded by “amalgamated” bounding surfaces (SB/ts or SB/mfs) and are characterised by stacked trans- gressive systems tracts (TSTs). The small facies dislocation across key sequence bounding surfaces, indicates a 15–50 m change in relative sea level. Fourth order sequences are hypothesized to be paced by the long eccentricity 405-kyr cycle and consequently 3rd order sequences are calculated to be ~ 1.2 m.y. and ~ 2.4 m.y. long. These correlate with the global eustatic sea level curve. In comparison with Mesozoic and Cenozoic sequence stratigraphical data we conclude that the ~ 1.2 m.y. cycles correspond with long obliq- uity cycles predominant in icehouse conditions and the ~ 2.4 m.y. cycles with the long eccentricity cycle pre- dominant during greenhouse conditions. We propose a Darriwilian Ice Age, during which, orbitally induced “cold snaps,” caused the expansion and amalgamation of small/medium-scale ice sheets. The reduced size of the correlative positive carbon isotope excursion, compared to that in the Hirnantian may indicate that the Darriwilian ice sheets only reached the sea in a few places. The absence of glacio-terrestrial deposits of Middle Ordovician age is most likely due to subsequent glacial erosion during repeated ice advances, up to and including those in the Hirnantian. The existence of Darriwilian ice sheets is consistent with zooplankton province distributions that indicate a Polar Front at ~ 40°S during the ice age. It is likely that polar ice existed throughout the Ordovician. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Palaeogeographical reconstructions show that during the Ordo- vician North Africa and Arabia formed part of the Gondwana Su- percontinent, largely located within subpolar–polar latitudes. Despite this high palaeolatitude, direct evidence of pre- Hirnantian Gondwana ice sheets is poorly constrained or absent (Armstrong, 2007); a fact traditionally explained by the high 8 to 22× PAL (present atmospheric levels) atmospheric CO 2 values, and an inferred intense greenhouse climate. Models now predict atmospheric CO 2 values of 5× PAL during the Middle Or- dovician; and a global mean air temperature of 15 °C (Godderis et al., 2001; Herrmann et al., 2004; Nardin et al., 2011) consistent with δ 18 O values (Shields et al., 2003; Trotter et al., 2008) and biogeographical studies (Vandenbroucke et al., 2009, 2010), that indicate a modern-style “cool” world climate for the late Middle and Upper Ordovician (Royer, 2006). If, by the Middle Ordovician, climate was essentially modern in character, polar ice would be expected and, if ice sheets were large enough, they would, exert a controlling influence on passive margin sedimentation. The origin of 3rd order eustatic cycles is critical to linking sed- imentary cyclicity in deep time records to glaciation. Glacio- eustacy provides the only mechanism that can account for changes in global sea level of > 10 m/m.y. (e.g. Pitman and Golovchenko, 1983). Boulila et al. (2011) recently proposed a glacio-eustatic origin for Mesozoic and Cenozoic 3rd order eustatic sequences, by comparing the sequence stratigraphical data to the best constrained astronomical model. From the Middle Eocene, icehouse sequences correspond to ~1.2 m.y. long obliquity cycles. Greenhouse sequences show a relationship to the ~2.4 m.y. eccen- tricity cycles, suggesting orbital forcing contributes to sea level change. During icehouse periods large ice sheets are associated with significant glacio-eustatic changes (> 25 m) and obliquity forcing was the strongest control on global sea level and deposi- tional sequences. Stratigraphically well-constrained 4th order se- quences may be linked to the astronomically stable (strongest Sedimentary Geology 251-252 (2012) 34–48 ⁎ Corresponding author. E-mail address: b.r.turner@durham.ac.uk (B.R. Turner). 0037-0738/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2012.01.002 Contents lists available at SciVerse ScienceDirect Sedimentary Geology journal homepage: www.elsevier.com/locate/sedgeo