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The AAPG/Datapages Combined Publications Database

Four Corners Geological Society


Cretaceous and Tertiary Rocks of the Southern Colorado Plateau, 1973
Pages 123-130

The Saga of the Ojo Alamo Sandstone; or The Rock-Stratigrapher and the Paleontologist Should be Friends

J. E. Fassett


The Ojo Alamo was named by Brown in 1910 for exposures south of the store at Ojo Alamo Arroyo. Brown said that the Ojo Alamo was a dinosaur-bearing shale unit overlain by a conglomerate bed. Brown put no lower boundary on the Ojo Alamo. In 1914 Sinclair and Granger raised the upper contact of the Ojo Alamo to include the overlying conglomerate because they found “the centrum of a dinosaurian caudal vertebra . . . loose on the surface ... ”of the “conglomeratic sandstone with much fossil wood.” They also described a lower conglomerate, 6 to 8 feet thick, 58 feet below the upper conglomerate. Sinclair and Granger, also, did not place a lower boundary on the Ojo Alamo.

In 1916 Bauer redefined the Ojo Alamo as “a sandstone including lenses of shale and conglomerate.” He agreed with Sinclair and Granger that the top of the Ojo Alamo was the top of the conglomeratic sandstone, but he placed the base of the Ojo Alamo at the base of the lower conglomerate of Sinclair and Granger. Reeside accepted Bauer’s redefinition in 1924 and mapped the Ojo Alamo as a conglomeratic sandstone throughout the western San Juan Basin. Reeside, however, designated the Ojo Alamo as Tertiary (?). Dane discussed the Ojo Alamo in 1936, and also agreed with Bauer’s definition of it, but suggested that it “should be classified as Cretaceous.” Dane mapped the Ojo Alamo throughout the southeastern San Juan Basin as a coarse-grained, in places conglomeratic, sandstone.

In 1966 Baltz, Ash, and Anderson suggested that “the name Ojo Alamo Sandstone be restricted to apply only to the upper conglomerate of Bauer’s type Ojo Alamo Sandstone on Ojo Alamo Arroyo.” This redefinition of the Ojo Alamo ironically eliminated the last remnant of Brown’s original Ojo Alamo on Ojo Alamo Arroyo from the restricted Ojo Alamo of Baltz, Ash, and Anderson.

In 1973 Powell recommended that the revision of the Ojo Alamo by Baltz, Ash, and Anderson be dropped and that the original rock-stratigraphic definition of the Ojo Alamo be revived. Powell further divided the Ojo Alamo into a lower member, the Naashoibito Member which was defined by Baltz, ash, and Anderson (1966) as a member of the Kirtland Shale and an upper member, the Kimbeto Member.

During the time that the definition of the Ojo Alamo was evolving, as briefly outlined above, the fossil vertebrate remains collected by Barnum Brown, C.W. Gilmore, and others in the Ojo Alamo Arroyo area were being referred to in paleontological papers in publications throughout the world. Nearly all these publications referred to the Ojo Alamo as a dinosaur-bearing shale unit in much the same sense as the original definition of Brown. Thus, today the name Ojo Alamo has two distinct meanings: to rock-stratigraphers, it is a conglomeratic sandstone and shale sequence; to biostratigraphers, it is a shale unit containing a distinctive vertebrate fauna.

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