‘Nature’s Juggling Act’: Glaciers, Sand Dunes and Limestone: A Post Glacial Sea-level Rise Captures in Rock a Record of the Early Permian Stratigraphy of Saudi Arabia

Christian J. Heine
Saudi Aramco, Dhahran, Saudi Arabia

‘Natures Juggling Act’ the deposition and preservation of the climate-controlled facies is in response to three continually changing conditions, at variable rates of speed:
       Northward movement of the Arabian-plate in the southern hemisphere through the Carboniferous into the Permian. (Relatively fast as plate movements go and continuous)
       Post-glacial transgression (Fast rise in base level but cyclical or punctuated)
       Post-glacial polar expansion of the atmospheric circulation patterns. (Probably in phase with post glacial transgression cyclicity)

Together these three variables capture and preserve the movement of the Arabian plate northward through a variety of climate-controlled depositional environments, each with a distinct lithological signature reflecting the passage from one climatic region to the next.

Three distinct and radically different climate zones are preserved along this journey. Evidence for glacial ice is seen in the striated glacial pavements in the outcrops of Oman. On the Saudi side, glacial evidence is limited to the Unayzah ‘C&B’ reservoir units. The Unayzah ‘C’ is characterized by stacked fluvial channels with evidence for multiple glacial re-advances. With the continued retreat of the glaciers, the waning stages of the cold-wet glacial phase are represented by lacustrine deposits of the Unayzah ‘B’ characterized by shale, varved sediments and drop-stones. An unconformity separates the wet-cold Unayzah ‘C&B’ from the overlying cool-dry Unayzah ‘A’. The Unayzah ‘A’ reservoir comprises arid mid-latitude eolian and associated ephemeral fluvial and playa deposits. This is the main hydrocarbon bearing unit which includes several fields containing over 1-billion barrels of oil and several multi-TCF gas fields. A major unconformity separates the arid Unayzah ‘A’ deposits from the overlying humid, tropical environment of the basal Khuff clastics and transgressive Khuff carbonates.

The most impressive reservoir is the eolian and associated desert deposits of the Unayzah ‘A' package. Five distinct facies can be recognized in core and on image log: dune, sand-sheet, paleosol, playa and ephemeral-fluvial deposits. Although the eolian facies is not present in outcrop, the Permian Cedar Mesa Formation of Southern Utah serves as an excellent outcrop analog for the Permian eolian Unayzah ‘A’ reservoir and will be used extensively to illustrate the nature of the wet eolian cyclicity. Barren of consistent datable organic material, the Unayzah ‘A' reservoir is sandwiched between the Lower Permian glacial deposits of the Unayzah ‘C&B’, and the Middle Permian tropical deposits of the Khuff carbonates, both of which have very good palynological control for the age of the units. A dominant west-to-east wind direction is identified on image logs for the Unayzah ‘A’ transverse dune system which would place the eolian reservoir in a mid-latitude desert setting at the time of deposition around 45 degrees south latitude in the location of the prevailing westerlies (~285Ma).

AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010