During the summers of 1948, 1949, and 1950 a study was made of the Supai formation which is a series of continental redbeds and marine limestones of Pennsylvanian (?)-Permian age. Detailed field work was limited to a small area east of Chino Valley in Yavapai County, Arizona.

Stratigraphic studies indicate that the Supai formation is an ancient delta deposit interbedded with marine limestones. Distribution of well sorted detrital sediments over wide areas, character of primary structures, and the interbedded relationship of redbeds to limestones are the basis for this conclusion.

The formation has been divided in the area studied into two members, easily recognized in the field. The lower member is 145–156 feet thick and consists of a slope-forming unit of red sandstones and siltstones, capped by a cliff-forming unit of cherty limestone. A basal conglomerate is locally present.

The upper member consists of 938+ feet of alternating red sandstones and siltstones interbedded with a few strata of structureless claystone and some aphanitic limestone. This member forms a cliff-and-slope type of topography.

Marine fossils collected from limestones of the lower member are of poor preservation but suggest a Lower Pennsylvanian age for the basal Supai. If further studies bear out such indications, it may be established that the basal Supai was deposited by a regressive Pennsylvanian sea.

The contact between the Supai formation and underlying Redwall limestone of Mississippian age is unconformable. A gradational contact is locally present between the Supai formation and the overlying Coconino sandstone of Permian age.

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