Abstract

Analysis of sand-body distribution reveals that fluvial channel sands in the Upper Cretaceous lower Williams Fork Formation in the central Piceance Basin, Colorado, USA, are not randomly distributed but are predictable in their spatial and stratigraphic position. Sand bodies are organized in channel-belt clusters that are compensationally stacked on different temporal scales. While channel clustering occurred over a shorter period (channel and channel-belt scale 10s ky to 100s ky), compensational stacking occurred over a much longer time scale (channel cluster belts, ~ 400 ky).

The lower Williams Fork Formation consists of three distinct intervals of clustered channel belts, each ~ 400 ft (~ 122 m) thick, topped by a thin coal layer. The primary control on the cluster formation is autogenic channel avulsion during an overall aggradational phase. Short-lived changes in in basin accommodation caused by either changes in tectonic subsidence or high-frequency eustatic changes due to Milankovitch-band orbital forcing at the end of each cluster interval result in a distinct increase in channel thickness and coal formation towards the end of each cycle.

Coals at the end of each sub-cluster span the entire study area, but their overall thickness is greater in the inter-cluster, floodplain-prone areas. The resulting differential early coal compaction is greatest in the inter-cluster areas and has a significant control on the subsequent cluster-belt position overlying the coal. Channel sands overlying the coals are concentrated in the previously low net/gross floodplain-prone inter-cluster areas resulting in compensationally stacked cluster belts. On the longest time scale (~ 1 to 1.5 My), which encompasses the entire lower Williams Fork, changes in channel geometries and sand-body thickness seem to be controlled mainly by long-term changes in eustasy, and autogenic processes are of lesser importance.