Cretaceous coals of the western Gallup field, New Mexico, occur with detrital wedges that interfinger to the northwest with brackish-marine sediments of the San Juan basin. This study documents detailed stratigraphic relations and the relation of coal occurrence to depositional environments of the Gallup, Crevasse Canyon, and Menefee formations. One hundred and fifteen sections form the basis for three-dimensional reconstructions of a 30-sq km area northwest of Gallup.

The regressive Gallup Sandstone represents reworking of river-mouth sands into coastal barriers where coals accumulated in back-barrier subfacies. This formation grades upward into the Dilco Coal Member of the Crevasse Canyon Formation characterized by varve-like fine-grained sediments, thin sandstones, and coals, which pass upward into thick, northwesterly transported fluvial sandstones. These merge laterally and upward into northerly oriented, thick paleochannel sandstones of the Bartlett Barren Member of the Crevasse Canyon Formation. The coal-bearing Gibson Coal Member of the Crevasse Canyon Formation, which cannot be differentiated from the Cleary Coal Member of the overlying Menefee Formation, contains finer grained sediments and coals deposited in a broad interfluvial depression bounded on the west by Bartlett alluvial channel facies. Coal accumulation in this depression was terminated by southeasterly oriented, crevasse-like deposits associated with thick fluvial sandstones (Menefee Formation).

Stratigraphic variations of coal beds are directly related to their proximity to contemporaneous channel facies. Uniformly thick coals trend subparallel to channel facies; near the channel facies, coals become erratic and pass into rooted, carbonaceous overbank detritus. Coals are offset locally by faults caused by differential compaction beneath overlying channel sandstones.

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