AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Facies Architecture of Sandstones and Shales in Mouth Bar Assemblages of the Late Cretaceous Ferron Notom Delta, Factory Butte, Utah
(1) Earth and Atmospheric Sciences, University of Houston, Houston, TX.
Facies architectural analysis of the Turonian Ferron Sandstone Member, near Factory Butte, UT, was conducted to determine the hierarchical spatial arrangement of mouth bars within a single deltaic parasequence. Five architectural elements were identified using measured sections, bedding diagrams, and paleocurrents: (1) prodelta fines, comprising interbedded mudstones, siltstones, and laminated to rippled sandstones, (2) frontal splay sandstones, comprising massive, deformed beds to parallel laminated beds with some hummocky cross stratified sandstones, (3) large single-foreset and compound cross-bedded unit mouth bars, (4) cross-bedded, erosionally-based terminal distributary channels, (5) hummocky cross-stratified storm sheet beds. Within the parasequence, lower facies record an inertia-dominated system, consisting of frontal splay sandstones and single-foreset-dominated unit mouth bars. The unit-mouth bars translate downstream and may be overlapped by adjacent bars, forming a bar assemblage. This bar assemblage is capped by an extensive shale drape, suggesting local abandonment. Upper facies show a distinct switch to a more friction-dominated system corresponding to a decrease in water depth. This is indicated by aggrading cross bedded mouth bars that overlie the inertial facies. Laterally, the sandstone bodies can be seen to transit from the proximal dominantly coarser cross-bedded sandstones into distal finer-grained planar bedded sandstones. Lateral overlap also suggests that the friction-dominated bars also group to form a bar assemblage, as separate distributary channels and their bars coalesce, ultimately to build the entire lobate deltaic parasequence. Mouth bars prograde locally southwest to west and lack fair-weather wave-formed sedimentary structures, suggesting a local embayment within the regionally northeast prograding shoreline. Low level of ichnofaunal diversity suggests a stressed, river-dominated setting. Initially, frontal splays and unit mouth bars dominated by storm-driven river-flood deposits are deposited over muddy prodelta facies. Inertial mouth bars are likely initiated several kilometers distal of the river mouth, in water depths of a few meters. Once this subaqueous platform is constructed by the inertial bar assemblage, the remaining shallow-water accommodation is filled by more proximal friction-dominated mouth bars. As the system continues to shallow, erosional distributary channels override the delta.