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Field Trip Guidebook




Day 6, Wednesday, July 5: Gallup to Grants, New Mexico (143 mi, 229 km)

A. C. Huffman Jr., S. M. Condon, and J. L. Ridgley

We begin by driving west on I-40 across the Gallup sag in continental Upper Cretaceous Crevasse Canyon Formation. The Gallup sag is a Laramide-age graben-like structure bounded on the east by the Nutria monocline and on the west by a monocline along the eastern edge of the Defiance uplift (pl. 3). It is not known if these monoclines are underlain by faults but that seems probable. As we proceed west along the Puerco River Valley, we pass Twin Buttes, an Oligocene-age minette intrusive being quarried for road metal. Just over a mile (2 km) west of town, the Gallup Sandstone is exposed on the east limb of the Torrivio anticline, the type section of the Torrivio Member of the Gallup Sandstone. We pass through the anticline; its axis is well exhibited on the north side of the highway. Beyond the anticline, coal and carbonaceous shale in the Dilco are exposed in roadcuts. The Dakota Sandstone and underlying Jurassic rocks are brought back up on the eastern limb of the Defiance uplift. The cliffs on either side of the highway are capped by Dakota Sandstone underlain by the eolian and fluvial sandstones of the Recapture, and eolian sandstone of the Cow Springs and Entrada at the base. We exit at Lupton, pass under the highway, and go east about 2 mi (3 km) on the frontage road.

Citation: Huffman, A. C., Jr., S. M. Condon, and J. L. Ridgley (1989), Day 6, Wednesday, July 5: Gallup to Grants, New Mexico (143 mi, 229 km), in Coal, Uranium, and Oil and Gas in Mesozoic Rocks of the San Juan Basin: Anatomy of a Giant Energy-Rich Basin: Sandia Mountains to Mesita, New Mexico June 30–July 7, 1989, Field Trip Guideb., vol. T120, edited by W. I. Finch et al., pp. 83–87, AGU, Washington, D. C., doi:10.1029/FT120p0083.

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