Quaternary Eolianite Sequence                                              


A Quaternary eolianite sequence in the Arabian Gulf coastal region, northeastern Saudi Arabia:

a modern analogue for oomoldic porosity development in an arid setting    

Close to the City of Dammam, the Gulf Coastal Province of the Eastern Saudi Arabia are several isolated outcrops of linear ridges that have been previously identified as the part of the Eocene-age Dammam Formation.  The outcrops lie 3-4 km inland form the present shoreline. They are up to 12 m high and made up of cross-bedded oolitic grainstones and calcareous quartzose arenites with foreset packages up to 5 m thick and dip consistently (up to 30) to the SSE. Most of the well-defined bedsets in these ridges are interbedded combinations of eolian sandflow and grainfall units. The ridges are interpreted as Quaternary coastal eolianites that define one or more former sealevel highstands; their origin is similar to documented eolianite ridges in the Bahamas and southern Australia. The noticeable difference is the widespread preservation of leached vadose textures, still in the original depositional framework. This probably reflects their arid hydrogeochemical genesis compared to the humid to semiarid coastal setting in the Bahamas and the semiarid setting in southern and western Australia.

 

        

The ridge sands are made predominantly of well-sorted, poorly cemented ooid or skeletal grains. Some of the former ooids are characterized by the presence of over-sized quartz nuclei. Dissolution and breakage, however, damage the great majority of the grains. Many grains now lack a nucleus, probably reflecting the leaching of the aragonitic precursor (oomouldic and skelmoldic porosity). The remaining carbonate in the rinds that outline the former ooid/skeletal cores is now low-Mg calcite.  The relatively low levels of intergranular cement, coupled with a high degree of sorting, broken cortices and leached nuclei results in very high effective porosity in the ridges (up to 60%). The bulk and trace element chemistry, especially the distribution of Sr++  (up to 5445 ppm), in the sands indicates that the diagenetic waters were derived by the meteoric leaching of the former aragonite nuclei in this arid vadose setting. The high aridity of the setting means the same aragonite probably also acted as the local source for the calcite cement rinds. This is reflected in the very high Sr ++ content in the current ridge sands; unlike their more humid counterparts, the Sr++ is simply not leached from this arid-zone coastal eolianite system.

The sands are a modern analog for early diagenetic alteration processes that may have created early moldic porosity in many ancient oolitic reservoirs such as; the Smackover Formation of the Gulf of Mexico, the Arab C of Saudi Arabia, and the Saint Genevieve Limestone of Indiana.