Abstract

The Gallup Sandstone of Upper Cretaceous age is a regressive clastic wedge that prograded about halfway across the San Juan Basin from a probable source in southwestern Arizona. Numerous closely spaced detailed surface sections measured along the outcrop belt on the west and south sides of the San Juan Basin and extending about 70 miles south of Gallup, New Mexico, into the Zuni basin have facilitated detailed correlations of the complex of marine coastal barrier sandstones and associated nonmarine paludal mudstones, coal beds and fluvial channel sandstones that comprise the formation. South of Gallup the Pescado Tongue of the Mancos Shale separates the Gallup into two parts; a lower part called the Atarque Member of early Carlile to early-late Carlile age, and an upper part of latest Carlile to possibly earliest Niobrara (?) age. Only the upper part is present in the San Juan Basin.

Physical tracing of some of the units indicates discrepancies of former correlations. The Horsehead Tongue of the Mancos is nonexistent or is the same unit as the Pescado Tongue and hence the term should be abandoned. The widely distributed Juana Lopez (Sanostee) Member of the Mancos Shale was traced southeast of Gallup where it grades into a nonmarine sequence below the Pescado Tongue. Hence, the Pescado Tongue is younger than formerly considered, and undoubtedly is equivalent to the lower half of the D-Cross Tongue to the southeast. The upper pink fluvial sandstone of the type area near Gallup has been traced to the north where it merges with the coastal barrier sequence north of Sanostee. The name Torrivio Sandstone Member of the Gallup is proposed for this unit. To the east along with the outcrop belt, it pinches out into nonmarine paludal mudstones of the Dilco Coal Member of the Crevasse Canyon Formation.

The basal transgressive upper Mancos Shale of early Niobrara age, which extends to a point a few miles northeast of Gallup, overlies the Gallup and associated nonmarine deposits of the Dilco. Up to 300 feet of strata have been truncated in the northern San Juan Basin as a result of an unconformity at the base of this transgression. A belt 15 to 20 miles wide in which the rate of truncation is greatest coincides with the location of the so-called Gallup oil fields or pools which produce from lenticular marine sandstone bodies associated with the Niobrara transgression. On the south side of the San Juan Basin, another type of transgressive sandstone, which has been informally referred to as the “Stray” sandstone and has recently been designated as the Borrego Pass Lentil, is an offlap sequence associated with the Niobrara transgression. All these sandstones are genetically unrelated to the type Gallup Sandstone.

A northwesterly trending shoreline prevails for most of the units of the Gallup and associated formations.