Lithologic Relationships of the Upper Cretaceous Gibson-Stratigraphic Interval: Gallup Coal Field, New Mexico, U.S.A.

Summary

In the Gallup Coal Field, the Upper Cretaceous Bartlett Member and Gibson Coal Member of the Crevasse Canyon Formation and the Cleary Coal Member of the Menefee Formation have been deposited in alluvial, deltaic and lagoon environments. The Bartlett Member is dominated by sandstones deposited from northward-flowing fluvial channels. These grade laterally into well-drained floodplain deposits consisting of overbank and crevasse splay channel sandstones, siltstones and shales. In this alluvial setting, thick, mineable coals and carbonaceous shales formed only in persistent, poorly-drained swamps that were localized in small abandoned areas of the alluvial plain.

Near Gallup, the Gibson and Cleary Coal Members exhibit better integrated, northerly flowing, distributary channels with subordinate crevasse splay deposits, coals, and carbonaceous shales intercalated with bioturbated, clean sandstones. These sediments represent deposits of a delta plain environment in which widespread coal beds accumulated in swamps flanking restricted interdistributary bays.

Further north-eastward, the Gibson and Cleary Coal Members become separated by the Hosta Tongue of the Point Lookout Sandstone. Lagoonal deposits contain burrowed sheet sandstones, small channel sandstones, thin coals, and carbonaceous shales. This lagoonal coal-forming environment was protected from marine processes by the back-barrier washover and flood-tidal delta deposits of the Hosta Tongue. Widespread thick coal beds are preferentially associated with transgression of the lagoonal facies across sites of waning fluvial activity.