YARDANG, EOLIANITE, AND GYPSUM LUNETTE ALONG THE SOUTHERN ARABIAN GULF SHORELINES, NORTHEASTERN SAUDI ARABIA

The southern shorelines of the Arabian Gulf close to Dammam, the capital of Eastern Province of Saudi Arabia show a variety of eolian sedimentary structures and deposits including yardang, eolianite and gypsum lunette. Yardangs are common sedimentary structures observed on low-lying hills and ridges of the Upper Tertiary and Quaternary carbonate formations in the area including Rus, Dammam and Hofuf formations. The dominant orientation of the yardangs is to the SSE and roughly parallels to the present-day prevailing wind patterns along the Arabian Gulf coastlines. Eolianites, which often form low-lying ridges along the shorelines, are up to 10 m high, and made predominantly of well-sorted, poorly cemented ooids. Most section exposed in the ridge consists of SSW-trending, fine-grained cross-bed sets and often interbedded with medium to coarse-grained recessed laminae. Based largely on the coast-parallel orientation of the ridges, composition and paleocurrent data of the cross-bedded units, these ridges appear to represent the Quaternary eolianite deposits,which are common along the shorelines of the United Arab Emirates. Their origin also appears to be very similar to some of the well-documented eolianite ridges in the Bahamas and southern Australia. The noticeable difference is the widespread preservation of leached vadose textures, still in existence within the original depositional framework. This probably reflects their arid origin in contrast with the humid to semiarid coastal setting in the Bahamas and the semiarid environment in southern and Western Australia. The relatively low levels of intergranular cement, coupled with a high degree of sorting, broken cortices and leached nuclei usually result in producing very high effective porosity in the ridges (up to 60%). Relatively higher concentration of Sr++ (up to 5445 ppm) in the sands indicates that the final mineralogy was constrained by the meteoric leaching of the former aragonite nuclei in arid vadose setting. Gypsum lunettes are relatively less common eolian deposits along the Arabian Gulf shorelines. The only known lunette field in the area is located approximately 2 kilometers inland from the Qurraiyah beach next to a sabkha, typified by low-lying dunes of sandy gypsum with SSW-trending foresets (consistent with the orientation of the eolianites observed elsewhere along the shorelines).