ABSTRACT: The Permo-Carboniferous Unayzah depositional system of Saudi Arabia

Heine, Christian J. , Saudi Aramco, Dhahran, Saudi Arabia

During the Permo-Carboniferous, Saudi Arabia, was part of the large Southern Hemisphere landmass Pangea. Two important events were taking place, the Arabian "Hercynian" tectonic event, coincident with the collision of Gondwana and Laurentia and continental glaciation. The Hercynian tectonics led to re-activation of pre-existing basement faults. Uplift and erosion associated with the Hercynian movement, removed most of the Paleozoic through Carboniferous section. Adding to the sculpting of the Pre-Unayzah paleotopography, was enhanced erosion associated with the drop in sea level related to the continental glaciation.

The Unayzah Formation of Saudi Arabia consists of three sandstone packages sandwiched between the Hercynian unconformity and the basal Khuff clastics. The lowermost unit, consists of stacked fluvial sandstones occupying the earliest and most deeply incised Carboniferous valleys. These sediments were deposited in a fluvial depositional environment and are most likely syn-orogenic, having undergone some deformation related to the Hercynian event. As Hercynian tectonics waned and sea level rose with the retreat of the glaciers, down cutting stopped and the backfilling began. This second unit is made up of "wadi" deposits, indicating an arid fluvial environment.

Erosion and deposition continued until peneplanation was achieved, after which the Unayzah changed from arid fluvial, to a distinctly eolian depositional environment. Image log data suggests a transverse to slightly barchanoid dune system. The planar tabular bed sets making up the dune slip-face deposits indicate a constant wind direction, west to east. Using present day global atmospheric circulation patterns as an analogue and Zeigler et al plate re-constructions (1997), the dune orientation recognized on logs could only have been deposited in Late Carboniferous to Early Permian time. Sparse but excellent palynology data, supports placing the fluvial-eolian transition very close to the Permian - Carboniferous boundary. An unconformity, at times visibly angular on seismic, punctuates the end of the Unayzah Formation.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia