Hydrodynamics of Fluvial Strata in the Cretaceous Williams Fork Formation, Colorado

Abstract

Stratal patterns in foreland basins are interpreted to form in response to changes in allogenic controls (climate, sea-level change, subsidence), however deciphering which controls were active from the preserved stratigraphic record is a continuing challenge. This study evaluates depositional controls on stratal patterns within fluvial strata of the US Western Interior foreland basin. Hydrodynamics (i.e. paleoflow depths, shear stress) and morphological styles (i.e. braided vs. meandering, amalgamated vs. distributary) of fluvial sandstones in the Cretaceous Williams Fork Formation were quantified. The role of sediment supply and relative distance from the shoreline position on alluvial architecture was also investigated. Emphasis was placed on laterally extensive (up to 10km), amalgamated sandstones at the base of stratal packages identified in this formation. Sections were measured in detail with the aid of a laser range finder to collect quantitative data for hydrodynamic analysis. Bar form deposits within the amalgamated sandstones contain cross-bed sets averaging 40 cm in height. Using an empirical approach, cross-bed set thicknesses were used to calculate dune height and flow depth. Flow velocities are calculated from grain size, flow depth and interpretations of sedimentary structures. Amalgamated fluvial sandstones have paleoflow depths estimated at 5 meters. Flow velocities of 87-130 cm/s are estimated to have formed cross-bed set thicknesses of 40 cm during bankfull flow conditions. Using grain size and paleo-flow depth, a mean slope of 0.0004 is calculated for these fluvial systems. These hydrodynamic calculations and others suggest a suspended-load fluvial system for multi-storey channel-belt sand bodies. The age of the Williams Fork Formation is contested, and samples were collected for U-Pb dating of detrital zircons to test existing geochronological framework of these strata. Hydrodynamics will be used to guide subsurface prediction within the Williams Fork Formation with known channel scaling relationships. This study will serve as a predictive tool to assist in exploration and development of amalgamated sandstones in foreland basins.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016