The Upper Cretaceous Point Lookout Sandstone separates the Cleary Coal Member of the Menefee Formation from the Gibson Coal Member of the Crevasse Canyon Formation in much of the Chaco slope and southwest San Juan basin. These coal-bearing members merge as the intervening Point Lookout Sandstone pinches out to the southwest in the direction of the Nutria monocline and Gallup coal field. The deposits of the merged coal interval supported widespread mining activity northwest and north of Gallup, New Mexico, near the turn of the century; however, renewed mining of these coals for the past few years has concentrated northwest of the town. Closely spaced outcrop sections provided data from which to infer the depositional settings of the merged coals.

Northwest of Gallup (Enterprise mine area), an alluvial facies of channel sandstones, deposited by northward-flowing streams is complexly interspersed with interfluvial siltstones and with laterally discontinuous coal deposits. North of Gallup (Gibson and Heaton mine areas), better integrated sandstones deposited in north-to-northeast flowing distributary channels, subordinate crevasse-splay sandstones, more widespread coal zones, and common bioturbations in laminated carbonaceous siltstones suggest coal accumulation in a deltaic to lagoonal transition environment. Further northeast of Gallup, where the Nutria monocline changes to an easterly strike toward the Chaco slope, well-developed coals remain associated with the deltaic and lagoonal environments. However, these coal-forming environments grade northeastward into the Point Lookout back-barrier to lagoonal transition environments, where coal deposits are poorly developed.

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