Depositional Environments of Fort Union Sediments (Tertiary, Northwest Colorado) and Their Relation to Coal

Abstract

Abstract The Fort Union Formation in the Sand Wash basin of northwestern Colorado is composed of subarkosic medium to fine-grained sandstone interbedded with siltstones, shales, and subbituminous coals. Surface and subsurface studies indicate that sandstones generally dominate along a north-trending central belt. On either side of this sandy belt, the upper two-thirds is shale, the lower part is sandy. Most sandstone beds appear to be lenticular, traceable over less than 0.25 mi (0.4 km); however, those in the lower part may be somewhat more continuous. Trend of the belt in which sandstone is dominant parallels a northward paleocurrent direction, as deduced from cross-bedding, and is perpendicular to an east-west basin margin, as indicated by lithofacies maps. Coal is present in all areas, but seems to be more abundant near the boundary between the sandy and shaly sections and near the sandy belt. To separate random depositional events from normal ordered events, a Markov analysis was performed using seven characteristics. Lithologies tend to succeed one another in a consistent order. From the base up, these are: irregular surface, sandstone, siltstone, shale, carbonaceous shale, coal. The typical fining-upward sequence—rootlet zones, carbonaceous debris, coal, lithologic contacts, sedimentary structures, and stratigraphic relations—is consistent with an origin within a fluvial system. The similarity in degree of sandstone lenticularity in the Fort Union Formation and recent deposits of the lower Mississippi River Valley indicates that the northward-flowing rivers that deposited the Fort Union probably changed from braided at the beginning of deposition to highly sinuous at the close. High sinuosity together with differential compaction caused meander belts to stabilize, leading to thick coal deposits in stabilized flood-plain areas. Differential compaction of flood-plain deposits formed undulations on the surface in which pools formed and, when conditions were proper limestone or siderite beds were precipitated. “Bumpy” surfaces on which vegetal debris accumulated are also believed to be responsible for variations in thickness of coal beds. Coarse alluvial clastic debris of the Wasatch Formation was shed into the basin as result of renewed tectonic activity in the area and signaled the close of Fort Union deposition.