Stratigraphic architecture of shallow-marine to coastal-plain parasequences: lower Williams Fork Formation, southeastern Piceance Basin, Colorado

The lower Williams Fork Formation in the southeastern Piceance Basin represents a transition from coastal-plain to shallow-marine environments of deposition. The stratigraphic architecture of the lower Williams Fork Formation was assessed by defining facies and facies associations in cores and outcrop exposures and then developing relationships between these facies and subsurface well-log signatures. The lower Williams Fork lies stratigraphically above the Rollins Sandstone Member of the Iles Formation and consists of several coal zones, two marine sandstone units informally named the middle and upper sandstones, and coastal-plain to alluvial-plain deposits. Parasequences were identified and correlated in the subsurface to characterize the geometries and depositional nature of marine sandstones in the middle and upper sandstones, and their relationship with associated coal zones. Parasequences of the lower Williams Fork Formation are composed of wave-dominated shoreface sandstones that transition landward into paludal (marsh) environments and farthest landward into environments of deposition that are characterized by low net-to-gross ratio coastal-plain deposits containing isolated channel sandstones and floodplain strata. This relationship appears to correspond to the paleogeographic movement of environments caused by movement of the position of the marine shoreline in association with changes in relative sea level. Tidally influenced deposits and brackish-water fauna exist landward of the marine shorelines within coal zones and indicate a fluctuating fresh water and marine-influence in thelower coastal plain and the presence of bays behind transgressive shoreline deposits.Two transgressive-regressive cycles are identified within the lower Williams ForkFormation and are composed of retrogradational and progradational parasequence sets of themiddle and upper sandstones. Differential compaction of underlying sediments generatesvariation in accommodation, which plays a major role in the deposition and preservation ofmarine sandstones within the middle sandstone 1 and middle sandstone 2 parasequences.Middle sandstone 3 and middle sandstone 4 are not influenced by differential compactionbecause these parasequences are deposited over relatively homogeneous shoreface deposits