Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
3-D Facies Architecture and Mouth Bar Development of a Flood-, Storm-Dominated Delta: Cretaceous Ferron Sandstone, Utah
(1) Earth and Atmospheric Sciences, University of Houston, Houston, TX.
Variably-oriented exposures of mouth bar deposits within the Cretaceous Ferron Notom Delta in southern Utah gives insight to sediment distribution and mouth bar formation and allows for a 3D facies architectural reconstruction. Photomosaic interpretations were integrated with 15 detailed sedimentological measured sections to assess the external bed geometry and internal facies variability. Each measured section shows a similar coarsening upward facies succession typical of deltaic deposits. The Notom Delta shows unit mouth bars that have coalesced to form compound bars (bar assemblages). The unit bars have been interpreted to be of two types, inertia-dominated and friction-dominated. A hierarchy of architectural elements includes 1. prodelta muds (i.e. suspension fallout beds, fluid mud beds, and hyperpycnites), 2. storm beds/sheets, frontal splays, and 3. unit mouth bars (i.e. inertia-, friction-, and/or buoyancy-dominated). These elements amalgamate and coalesce to form larger-scale bar assemblages and bar complexes, which then build the largest-scale delta lobe. The interpreted evolution of the mouth bar complexes begins with an inertia driven phase and ends with a frictional driven phase. Dimensions on average are ~420m long x 250m wide, suggesting elongate to lozenge-shaped bar deposits. Shoreline trajectory analysis shows portions of the Notom system to be strongly progradational and slightly aggradational. Cross-sectional mouth bar geometry consists of topsets, forests, and bottomsets. The term “unit bar” was first introduced within the fluvial realm to refer to bars that have a relatively unmodified history (Smith, 1974) and match the unit mouth bars described herein. Examination of braided bars in the South Saskatchewan River, Canada, showed that several unit bars were responsible for the creation of complex compound bars. These compound bars have a unit bar as a central core, and commonly have a distinct shape, often asymmetric, with elongated limbs or horns (Sambrook Smith et al., 2006) and are analogs to the bar assemblages in this study. Similar deltaic deposits have been previously-termed effluent-generated microdelta-type unit bars (Nemec, 1988). Deltaic modeling studies (Edmond and Slingerland, 2007) broadly match our outcrop data although it is unclear whether the models describes a unit bar that migrates through time or a larger-scale assemblage or bar complex.