DEPOSITIONAL SETTING OF THE UPPER JURASSIC HITH ANHYDRITE OF THE ARABIAN GULF - AN ANALOG TO HOLOCENE EVAPORITES OF THE UNITED-ARAB-EMIRATES AND LAKE-MACLEOD OF WESTERN-AUSTRALIA

The Upper Jurassic Hith Anhydrite is a major hydrocarbon seal in the A rabian Gulf region. Outcrops, core samples from the subsurface, and th e literature indicate that the Hith Formation is composed mainly of an hydrite. In most locations where a section of the Hith Formation has b een measured, this unit contains less than 20% carbonate, much of whic h is in the form of thin laminations. This lack of carbonate, locally thick layers of salt, and the predominance of anhydrite favor a playa for the setting in which this sediment was accumulated. In fact, much of the Hith has the sedimentary characteristics of the Holocene Lake M acLeod playa of Western Australia, which is dominated by layers of gyp sum and halite (what little carbonate that occurs is found in layers a t the base of the section). Locally the Hith appears to have accumulat ed in a sabkha setting, particularly toward central Abu Dhabi where it pinches out into shallow-water, and peritidal carbonate. This sabkha setting is indicated by the interbedded relationship of the Hith anhyd rites with these carbonates and the local predominance of horizontally flattened nodules and enterolithic layers of anhydrite. These latter features match some of the characteristic fabrics found in the Holocen e coastal sabkhas of the United Arab Emirates. As with the local occur rences in the Hith, the Holocene sabkhas are dominated by carbonates a nd are divisible into a series of lateral facies belts. These are also expressed as equivalent vertical layers. Traced from seaward to landw ard, or from the base of the vertical sequence upward, these facies ar e characterized by (1) algal mat, (2) a layer of a gypsum crystal mush (3) active anhydrite replacement of gypsum (4) anhydrite with no gyps um mush, and (5) recycled eolianite and storm-washover sediments.