The Fort Apache Member of the Supai Formation is a marine unit of variable lithologic composition, and is limited areally to east-central Arizona. The member was measured, described, and sampled at 17 surface exposures along the Mogollon Rim from the Fort Apache--White River area northwest to Oak Creek Canyon. Examination of samples and well logs from 22 oil tests indicates that the member is extensive in the subsurface of the southern Mogollon Plateau. The Fort Apache Member is thickest in the extreme east-central part of Arizona and thins northwestward.

The type section at Kelly Butte is incomplete and relatively inaccessible; therefore the Corduroy Creek and Middle Cedar Creek sections were established as reference localities.

Distinct limestone, dolomite, and evaporite-dolomite lithofacies are present within the Fort Apache Member. The limestone facies is confined mainly to the Fort Apache Indian Reservation and adjacent areas, and consists of dark, dense, very finely crystalline, fossiliferous limestone with a biomicrosparite-intramicrosparrudite texture. The facies contains a mollusk-echinoderm-foraminifer fauna. Terrigenous admixtures generally are less than 5 per cent; clay minerals are illite and minor montmorillonite.

The dolomite facies is present only in the central and western Mogollon Rim region and areas adjacent on the north. The strata consist of light-colored, dense, silty, and very finely crystalline dolomite bearing a fauna composed predominantly of mollusks, foraminifers, and ostracods. The abrupt change from the limestone to the dolomite facies is accompanied by a marked thinning of the Fort Apache Member and an increase in the amount of quartz silt. Illite and montmorillonite are the principal clay minerals, and kaolinite is abundant locally.

The evaporite-dolomite facies is present only in the Holbrook basin and consists mainly of dolomite with interstratified and intergrown anhydrite and halite. Foraminifers and ostracods are the principal biogenic constituents. Clay minerals consist of an illite-montmorillonite-chlorite assemblage.

Because the rocks of the Fort Apache Member are predominantly dolomite, it is recommended that use of the lithologic qualifying term "limestone" be discontinued and that the unit be referred to as the Fort Apache Member of the Supai Formation.

Petrographic studies indicate that the Fort Apache sedimentary rocks were deposited originally as an aragonitic or calcitic, allochem-bearing or microcrystalline, allochemical mud. Most of the textures are characteristic of carbonate accumulation in a shallow-marine environment free of persistent currents or wave motion. Intraclastic textures are indicative of intermittent, vigorous agitation of the sea bottom. An early diagenetic (syngenetic) origin is suggested for the dolomite.