Black Mesa is an isolated remnant of Upper Cretaceous rocks lying in northeastern Arizona between more extensive areas of rocks of similar age in northwestern New Mexico and in southern Utah.

The Dakota sandstone is the basal Cretaceous formation of Black Mesa. It is overlain by the Mancos shale, which, in the Black Mesa area, represents only a small part of the type Mancos of southwestern Colorado. Overlying the Mancos in the Black Mesa area are three newly defined formations in the Mesaverde group. All these formations are older than any part of the Mesaverde in the type area in southwestern Colorado.

The basal formation herein defined as part of the Mesaverde group of Black Mesa is the Toreva formation. In the southern part of the area, the Toreva comprises a lower sandstone member, a middle carbonaceous member, and an upper sandstone member, and it is of late Carlile age.

In the northern part of Black Mesa the Toreva formation is somewhat younger than in the southern part; it is equal in age to the upper part of the Carlile shale and the basal part of the Niobrara formation of the Front Range. In that area also the formation is divided into three units, but these are not correlative throughout with those present on the south, and they are not herein recognized as map units or as members of the formation. It is realized however, that such recognition is possible and may be desirable at a later date.

Overlying the Toreva formation is the newly defined Wepo formation, of lower Niobrara age, a succession of continental shale, sandstone, and coal and some marine sandstone. The Wepo is overlain by the Yale Point sandstone of middle Niobrara age. The Yale Point sandstone is the youngest Cretaceous formation of the Black Mesa area. It is believed to be correlative with a similar unit in the middle of the Straight Cliffs sandstone of the Kaiparowits Plateau in southern Utah and with the Hosta tongue of the Point Lookout sandstone of the southern part of the San Juan Basin in northwestern New Mexico.

The Cretaceous rocks of Black Mesa are a result of neither the oldest nor the youngest deposition of Cretaceous time in the western interior of the United States, but represent deposition during the relatively short span of time involved in the greatest southwestward extension of Cretaceous seas in this part of the continent. They illustrate plainly that the initial inundation, although gradual, was persistent and was rather rapid in comparison to the long history of shoreline fluctuation that accompanied the slow and hesitant retreat of the sea from the interior of the United States.

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