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

摘要

The Middle to early Late Ordovician Hiswah and Dubaydib Formations in southern Jordan provide a well constrained 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 commonly 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 obliquity cycles predominant in icehouse conditions and the ~2.4 m.y. cycles with the long eccentricity cycle predominant 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.