The Cambro-Ordovician Sedimentary Sequence around Petra (Jordan): Coastal Sedimentary Development at the Edge of Gondwana

Luthi, Stefan M.¹; Hagadorn, Whitey ²; Donselaar, Marinus E.¹
(1)Geotechnology, Delft University of Technology, Delft, Netherlands. (2) Department of Earth Sciences, Denver Museum of Nature & Science, Denver, CO.

The northeastern edge of the Gondwana superplate underwent steady subsidence starting in the Ediacaran and continuing through the Cambro-Ordovician. The Arabian-Nubian Shield (ANS) hinterland was peneplained by erosion starting at about 750 my, but neotectonism in Southern Jordan created a significant topography. The Cambro-Ordovician sedimentary sequence in the well-exposed in the Petra area illustrates the transition from this late Ediacaran topography with islands of igneous rocks in a shallow sea to the vast fluvial peneplains in the late Cambrian to Ordovician that are characteristic for this part of the Arabian peninsula. Detailed field surveys demonstrate the gradual infill of local basin lows in a shallow-marine rocky coast with two sediment sources, one of mature sediment from the ANS hinterland to the South, and a second one with immature sediment from the local topography. The first one was brought in by rivers and was reworked by marine currents, while the latter was flushed in through episodic flash floods. Initially subsidence outpaced sedimentation, leading to a large-scale fining-upward cycle with fine-grained marine deposits at its culmination. Subsequent slowing of subsidence led to deltaic progradation, resulting in an almost complete blanketing of the topography. Eventually a vast fluvial system prograded onto the deltaic deposits and a widespread low-gradient braidplain developed with massive, progressively maturing fluvial sandstones that lasted into the Ordovician and that outcrop prominently in the area of the Nabataean city of Petra. Slight sea level rises led to occasional marine incursions into embayments, producing fine-grained deposits with rich ichnofossil assemblages. This second-order sequence is a good example of pre-Silurian coastal sedimentation at the edge of Gondwana, with the lack of vegetation resulting in great sediment mobility and early arthropods exploring the shallow seas and the intertidal areas. It shows an astonishing balance of sedimentation and subsidence, resulting in close to 1000 m of sediment accumulation that can be widely traced across the Arabian peninsula and the North African region.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.