Stratigraphic architecture and preservation of shallow-marine parasequences: lower Williams Fork Formation, southeastern Piceance Basin, Colorado
Rachel Shaak, Matthew Pranter, and Edmund (Gus) Gustason
The lower Williams Fork Formation in the southeastern Piceance Basin represents a transition from coastal-plain to shallow-marine environments of deposition. The sequence stratigraphy and 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 farther landward into low net-to-gross ratio coastal-plain deposits containing isolated channel sandstones and floodplain strata. 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 the lower coastal plain and the presence of bays behind transgressive shoreline deposits. Two transgressive-regressive sequences are identified within the lower Williams Fork Formation and are composed of retrogradational, aggradational, and progradational parasequence sets of the middle and upper sandstones. Anomalous isopach patterns of several parasequences reflect differential compaction of underlying mud and peat generated by variation in accommodation, which plays a major role in the deposition and preservation of marine sandstones within the middle sandstone parasequences. Other parasequences are not influenced by differential compaction because they were deposited over relatively homogeneous shoreface deposits.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012