Article publication date: 1983-03-01
Vol. 1 No. 1 (yearly), pp. 143-156.
Saudi Arabia. sedimentary, environments
The depositional environments of the Haddat Ash Sham and Usfan
Fonnations, exposed at Haddat Ash Sham area NE of Jeddah have been identified
on the basis of the vertical variation in primary sedimentary structures.
The base of the Haddat Ash Sham Fonnation (Kheslef Member) is characterized by bioturbation, Cross-bedding of Planar, herringbone and longitudinal types, ripple marks, cross-lamination and rare mud-cracks, which might reflect deposition on shallow intertidal-subtidal subenvironment with numerous channels and sand bars. On the other hand, the top of the same fonnation (Bonna Member) shows planar and' channel cross-bedding, graded bedding, scour and fill structures, Also within this member few horizons with poor bioturbation and cross-lamination are also encountered. These features might indicate alluvial fan (water laid and debris flow sediments) depositional environment under arid and semi-arid climates.
The Usfan Fonnation exhibits moderate to poor bioturbation, ripple marks, cross-lamination, lenticular and flaser-bedding, planar cross-bedding, and rare trough-cross-bedding, mud-cracks and rain drop prints. Consequently, the lower and upper members of the fonnation were probably deposited on shallow intertidal-subtidal zones. The middle member, however, was probably accumulated as a pelecypod bank. in shallow water, under moderate to high agitation, with good current circulation.