Environment of Deposition of Late Cretaceous Fruitland Formation Coal Deposits of San Juan Basin, New Mexico and Colorado

James E. Fassett

The Fruitland Formation in the San Juan basin contains large resources of subbituminous and bituminous coal. The thickest beds are in the lowermost part of the formation and in many places lie directly on the underlying Pictured Cliffs Sandstone. The coal beds were deposited in a coastal-swamp environment. This environment shifted northeast together with the Pictured Cliffs shoreline as the Late Cretaceous sea withdrew from the San Juan basin. The coal swamps trended northwest, parallel with the shoreline. The thickness of vegetal matter buildup in these coastal swamps is related closely to the manner in which the Pictured Cliffs shoreline retreated from the basin area. The retreat of the sea was primarily the result of sediment infilling along the edge of a subsiding dep sitional basin. When sediment influx was great, the shoreline regressed rapidly, swamps soon were filled and covered, and the resultant coal beds were thin. When sediment influx was moderate, the shoreline remained geographically stable, shoreline sandstone deposits built up vertically, swamp deposits of vegetal matter also built up vertically, and the resultant coal beds were thick. When sediment influx was small, the shoreline transgressed continental deposits, swamps were filled and covered by marine sandstone, and resultant coal beds were thin. Another paleoenvironmental factor affecting swamp configuration was streams passing through the coastal swamp area to the sea. This process tended to limit the length of swamps. In addition, variations in the paleotopography of the coastal swa p area due to the presence of bars and dunes limited the areas which the swamps occupied.

AAPG Search and Discovery Article #91051©2012 AAPG Rocky Mountain Section Meeting, Albuquerque, New Mexico, 23-26 February 1969