The Mesaverde and upper Hilliard Formations form a regressive sequence, from northwest to southeast, of lagoonal coal-bearing siltstone and thick sandstone beds, a littoral sandstone complex, and a sequence of marine siltstone and shale. Truncated, upturned littoral sandstone beds at the Late Cretaceous-Paleocene unconformity, lateral permeability changes, and the gentle structure of the La Barge arch, together form the traps within the Mesaverde Formation. The major part (Fort Union Formation) of the basal Paleocene rocks forms a regressive lacustrine sequence of lacustrine shale; marginal-lacustrine sandstone; paludal shale, siltstone, and thin sandstone; and variegated mudstone and thick sandstone probably deposited in a fluvial environment. Traps are formed as a result of updip and lateral pinchouts of individual sandstone bodies into lacustrine shale along the east flank and crest of La Barge arch.

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