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The AAPG/Datapages Combined Publications Database

Four Corners Geological Society

Abstract


Cretaceous and Tertiary Rocks of the Southern Colorado Plateau, 1973
Pages 131-143

The Gasbuggy Core — a palynological appraisal

Robert H. Tschudy

Abstract

Bore hole GB-1 (Gasbuggy-1), the first test hole for Project Gasbuggy in the San Juan Basin, was drilled to obtain core samples for laboratory examination of rocks that possibly might be affected by the proposed nuclear explosion. The hole was cored from a depth of 3,436 feet near the base of the Nacimiento Formation (Paleocene) to a depth of 4,316 feet, where it bottomed in the upper part of the Lewis Shale (Cretaceous). Chips from the cores were sent to the U.S. Geological Survey laboratory in Denver for palynological examination.

The pollen assemblages present in samples above 3,714.7 feet are indicative of an early Tertiary age. At and below this level all productive samples yielded many specimens of the genus Proteacidites which is considered a Late Cretaceous index fossil in the Rocky Mountain region. Isolated specimens of Proteacidites were found in samples from 3,638.1 and 3,655.8 feet. It is postulated that these are redeposited specimens in the base of the Ojo Alamo Sandstone near the unconformity separating the Ojo Alamo Sandstone from the Fruitland Formation. A second conspicuous palynological change was observed at about 4,060 feet, within the lower tongue of the Fruitland Formation which separates the Pictured Cliffs Sandstone in two parts.

The pollen assemblages obtained from the Cretaceous of the Gasbuggy core are distinctly different from those of the Judith River Formation of Montana and the Teapot Sandstone Member of the Mesaverde Formation of Wyoming, of late Campanian or early Maestrichtian age.

Comparison was made between the Gasbuggy section and a composite section from the Raton Basin to the northeast, which involved samples from the Raton and Vermejo Formations and the upper part of the Trinidad Sandstone.

A section of the Upper Cretaceous, present in the upper part of the Cretaceous in the Raton Basin, is absent from the Upper Cretaceous of the Gasbuggy core. This confirms the presence of a marked hiatus at the top of the Cretaceous, as previously postulated in the San Juan Basin.

Several palynomorph species, present in the Vermejo Formation and Trinidad Sandstone in the Raton Basin section, are absent from the lower part of the Pictured Cliffs Sandstone and the Lewis Shale of the Gasbuggy section — suggesting that the lower part of the Gasbuggy Cretaceous is older than any part of the Cretaceous examined from the Raton Basin.

A correlation chart is presented showing the postulated positions of the formations discussed in relation to the Western Interior invertebrate reference sequence of Gill and Cobban (1966).


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