Major oil production in the San Juan basin is from isolated sandstone lenses in the Mancos shale of Upper Cretaceous age. These oil-productive sandstones have been termed Gallup, Tocito, or Horseshoe Gallup and have been described as offshore bars or strand-line features related to the regressive Gallup formation.

Detailed measured surface sections, electric log correlations, and faunal data indicate that the oil production is not from sandstones of the Gallup formation of

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Carlile age, but rather from basal sandstones of the Niobrara formation. The basal Niobrara sandstones are not the lithogenetic equivalent of the Gallup sandstone.

Evidence was obtained to establish the presence of an unconformity between the Carlile and Niobrara formations. In the northern part of the San Juan basin, in the vicinity of the Colorado-New Mexico state line, the pre-Niobrara unconformity has truncated approximately 400 feet of upper Carlile. This unconformity rises in the section, southward in the basin to a position approximately 100 feet above the type-Gallup formation.

Basal Niobrara sandstones were deposited on the pre-Niobrara unconformity as the Niobrara sea transgressed southwestward. During deposition of Niobrara sediments, structural uplift in the northern part of the present-day San Juan basin produced minor folding of the sea floor. As the Niobrara transgression progressed, the sea eroded, scoured, and deposited the Niobrara sandstones on the truncated, folded Carlile (pre-Niobrara) surface. Control for deposition of the basal Niobrara sandstones was the configuration of the erosional relief on the pre-Niobrara surface.

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