Abstract

Alluvial plain environments landward of Late Cretaceous maximum transgressive and regressive coastlines represent two contrasting coal-forming settings in the Gallup sag, New Mexico, and in the Raton basin, Colorado and New Mexico. In the Gallup sag, the alluvial coal-bearing deposits are in the Crevasse Canyon and Menefee Formations. In the Raton basin, corresponding deposits are in the lower part of the Raton Formation. Lithofacies analysis of these alluvial coal-bearing deposits suggests similar meanderbelt, floodplain, and backswamp facies in both basins. Meanderbelt facies consist of channel sandstone, abandoned channel shale and siltstone, cutbank slumped deposits, and levee deposits. These facies grade laterally into floodplain facies of crevasse splay-channel sandstone, siltstone, and shale. Floodplain facies are interbedded with backswamp facies of coal and carbonaceous shale.

Economic coals deposited in the alluvial plain landward of deltaic and barrier complexes accumulated in poorly drained backswamps of slightly elevated platforms where peat was neither drowned nor oxidized. These swamp platforms were best developed in interfluvial floodbasins underlain by abandoned meanderbelt facies that consisted of relatively incompactible channel sandstones. In the Gallup sag, the poorly drained backswamps in the lower alluvial plain were sustained by a regional rise of ground-water table related to coastal lagoonal transgression accompanied by maximum landward advance of coastal barriers. In the Raton basin, the poorly drained backswamps in the upper alluvial plain were sustained by a regional rise of ground-water table due to basin subsidence accompanied by uplift of source area during the time of Laramide orogeny.