The Khuff succession is a carbonate platform deposit with a well-developed shelf margin and slope break (Jallal, 1995). Furthermore, the regional distribution of stacked depositional sequences suggests the platform interior was hundreds of kilometers in width and had a cyclic facies development. Successive platform environments evolved in one location to include high-energy grainstone shoal, shallow restricted mudstone shelf, evaporitic tidal flat and subtidal evaporite salina facies accumulations. These stratal characteristics of the Khuff Formation across the Arabian Platform indicate there is an evolution of depositional systems involved in their cyclic formation. In particular it signals fundamental changes in water depth across the platform and in the position of shelf rim relative to sea level. The observed facies make-up of various Khuff depositional sequences reflects at least three models of deposition. Carbonate Research Consulting has developed three new depositional models to explain the observed facies make-up of various Khuff depositional sequences:

• Open Shelf Grainy Shoal Model
• Shallow Shelf Muddy Lagoon Model
• Evaporite Shelf Salina Model

The Open Shelf Shoal model, illustrated and described below, applies for the grain-dominated sequences that make up the Khuff B and C reservoirs on the platform interior.

OPEN SHELF SHOAL MODEL

The open shelf grain shoal model applies to the grain-dominated sequences that make up the Khuff-B and Khuff-C reservoirs on the platform interior. A requisite for the open shelf model is that the shelf lagoon setting be sufficiently deep for the propagation oceanic waves or local generation of vigorous wave action. It is difficult to conceive how open ocean wave energy can be transferred effectively to the shelf interior given that grainstone shoals and reefs rimmed the Khuff platform margin (Jallal, 1995). The implication of a high-energy rim is that ocean swells and storm-generated oceanic waves expend their energy at the platform margin. The loss of energy occurs there even if the margin is not a major barrier to wave energy. Hence, vigorous wave action on the interior of the Khuff platform necessitates a deep shelf with a significant wave-generating fetch area akin to modern carbonate platforms like the Belize Shelf or Australian Great Barrier Reef.

Water depth on platform interior sufficient to allow wind-generated waves.

The Belize carbonate platform features a shelf lagoon behind a nearly unbroken barrier reef rimmed margin. The reef rim reaches sea level and effectively absorbs the high wave energy of open ocean waves. Yet, high-energy conditions exist in the shelf-lagoon interior because of the presence of an extensive area of deep water (50-200 feet). The extensive fetch area in deep water allows for the development of significant waves whose energy is expended on platform interior shoals.

The depositional system shown on the left consists of four subenvironments (1) mobile sand shoal, (2) restricted lagoon, (3) tidal flat, and (4) open middle shelf. The mobile sand shoal complex features clean and variously cross-bedded grainstone accumulations that make up the shoal crest and burrowed muddy grainstone to packstone deposits form on the leeward side and along the base of the mobile sandbank. Tidal flats are attached to the leeward side of the mobile sand bank, specifically behind islands or portions of the sand bank that have become exposed. Restricted lagoon environments evolve locally between the attached tidal flats. The entire sand shoal complex develops on and progrades away from topographic highs extant on a muddy open shelf.

SHALLOW SHELF MUDDY LAGOON MODEL

The shallow shelf muddy lagoon model applies to the mud-dominated sequence below the Permo-Triassic boundary. Its definitive environmental requirement is a shallow shelf lagoon that inhibits the development of significant waves and hence the localized development of grainstone shoals. In general, the model proposes water depths across the platform were no more than 15 feet deep and that there was an effective barrier to open marine water circulation. While an effective barrier helps, the critical factor controlling wave energy on the platform interior is water depth itself. It is impossible to develop significant wave energy in shallow water regardless of the fetch area available for generating waves. This is true because waves quickly loose energy when they touch bottom. The available evidence suggests the platform margin rim was a fairly effective barrier to water circulation. Both the restricted faunal assemblage and presence of anhydrite pseudomorphs after hemipyrmidal gypsum in the strata are evidence for poor water circulation and hypersaline conditions. Much of the section is barren or has a sparse collection of ostracodes and rare brachiopods. Hemipyrmidal gypsum crystallization is an early diagenetic precipitate within the host sediment as exemplified in the Lake MacLeod evaporite basin of Western Australia (Logan, 1987). This form of gypsum crystallization is especially prominent along the landward margin of Lake MacLeod (Logan, 1987) and its distribution may have implications similar implications for the Khuff.

The aggraded nature together with the widespread distribution of mud-dominated and evaporitic strata across the Khuff platform documents the persistence of the shallow shelf muddy lagoon depositional environment through time. A persistent depositional setting of this nature implies that there must be a long-term balance between the rates of relative sea level rise and sediment accumulation. Otherwise, depositional facies evolution would take place if water depths changed significantly. Deepening of the shelf promotes the development of high-energy deposits on the platform interior, whereas increased restriction and reduced water depth would promote a switch to dominantly evaporite deposition. The most critical relationship is between the rates of relative sea level rise and the rate of mud accumulation on the shelf interior as the two must track one another.

If this relationship holds true, there are some significant implications for the geomorphology and sedimentation at the platform margin. Grainstone and reef depositional rates are far higher than the rate of mud accumulation on a platform interior. Hence, it seems reasonable to conclude that the shelf margin rim accumulations reached and were limited in upward growth by sea level. Similarly, it seems logical to think that the margin produced an excess of sediment that was transported down the platform slope. The overall stratal characteristics of the platform during this period of deposition would include an agradational platform sequence and a progradational shelf margin-slope succession.

Water depth on platform interior to shallow (>15 feet) to allow wind-generated waves.

Several horizons bear evidence of subaqueous evaporite deposition. In particular, the Khuff-D or Middle Anhydrite and the Khuff A member. Their model requirement is a salina evaporite basin, a closed, sub-sea level depression, isolated from the ocean by a barrier at the shelf margin. Seepage influx through the barrier from the ocean feeds the salina brine sheet, whereas evaporation concentration lowers the hydraulic head that in turn promotes the influx of sea water, maintains the elevated salinity and promotes evaporite deposition. Sea water is the primary source of ions.

Eustacy controls the development of the platform morphology. A rapid relative rise in sea level promotes "catch up" sedimentation and development of a raised rim at the shelf margin while sedimentation lags on the platform interior, to produce the "empty bucket" of Kendall and Shinn (1981). Isolation of the topographically low carbonate platform top takes place during a eustatic fall. At the same time, lithification occurs of the raised rim.

Relative highstands in sea level allowed sea water to inflow over the barrier and flood the salina. Such "freshenings" shut off evaporite deposition and promoted carbonate deposition.

Environmental factors and hydrologic states of the salina determine the various forms of subaqueous evaporite deposition featured in the Khuff. In general, a perennial salina pan is envisioned for the Khuff, but local topography may include ephemeral salina sites too. Optimal conditions for subaqueous gypsum precipitation developed when brines are unstratified. However, the presence of black carbonates and black evaporites indicate anoxia and brine stratification in a perennial salina. An association of fine-grained laminated gypsum (now anhydrite) could reflect spontaneous gypsum precipitation in a stratified brine sheet.

Interbedded carbonate deposits in the Khuff-A record major water-level changes in the salina basin. These reflect freshening of the salina waters during sea level highstands when open ocean water floods across the barrier and fills the salina. Waters in the shallow platform interior become well mixed, salinity decreases, and carbonate deposition prevails.