Stratigraphy, Coal Occurrence, and Depositional History of the Paleocene Fort Union Formation, Sand Wash Basin, Northwestern Colorado

Abstract

The Fort Union Formation in the Sand Wash basin is divided into the massive Cretaceous and Tertiary (K/T) sandstone unit, lower coal-bearing unit, gray-green mudstone unit, basin sandy unit, and upper shaly unit. Lithofacies and coal-occurrence maps of the stratigraphic units indicate that sandstone bodies and coal beds occur along south-north oriented, intermontane fluvial systems. Net-sandstone-thickness trends of the massive K/T sandstone unit reveal laterally extensive channel-fill sandstones formed in north-flowing fluvial systems. The massive K/T sandstone unit's dominant source was in the Sawatch Range. Sandstones within the lower coal-bearing unit consist of similar north-flowing fluvial systems, but they are laterally discontinuous and have several tectonically active source areas, including the Uinta and Sierra Madre-Park uplifts, and Sawatch Range. Coal-occurrence maps of the lower coal-bearing unit indicate that maximum coal-bed thicknesses are greatest along the south-north-oriented fluvial axes. Coal beds thin and split to the east and west, confirming a direct relation between the position of thick, fluvial-sandstone bodies, which form a stable platform for peat accumulation, and the location of the thick coal beds. Above the lower coal-bearing unit, the gray-green mudstone unit forms north-trending belts centered R91W and R100W. The gray-green mudstone thins to the north and intomore » the basin center and probably is lacustrine in origin, reflecting tectonic quiescence and cessation of coarse clastic sedimentation. The basin sandy unit is best developed in the central parts of the basin, where its fluvial depositional axis is oriented south-north. The upper shaly unit directly overlies the basin sandy unit and includes a thin Cherokee coal zone. The upper shaly unit has variable thicknesses due to erosion at the base of the Wasatch Formation and lateral facies changes.« less