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Facies Architecture of a Bayhead Delta in the Cretaceous Ferron Notom Delta, Southern Utah


Plan view and cliff exposures of a river-dominated bayhead delta in the Turonian Ferron Notom Delta, Utah allowed a 3D reconstruction and analysis of mouth bar growth patterns and bar dimensions. Eighteen measured sections, 10 high-resolution photomosaics, 10 clinoform dip directions, and over 1000 paleocurrent directions allow for determination of mouth bar accretion directions. Lateral and downstream accretion account for more than 50% and 40% respectively, whereas less than 10% of mouth bars show upstream accretion. Facies analysis shows that the regional delta generally prograded toward the east while the bayhead delta prograded toward the west. The bayhead delta was protected from marine influence by a wave-dominated barrier system farther basinward. Four facies form an coarsening upward facies succession and include muddy prodelta, heterolithic distal delta front, sandy mouth bars, and terminal distributary channels. The mouth bar deposits are internally composed of planar beds, which pass upward into meter-thick low-angle cross beds, which gradually decrease in dimensions upward and finally change to dune-scale cross beds. The large cross beds form mega rib and furrow structures and are well exposed in plan view. Foreset ribs reach up to 30 m in width, and average about 5 m. This suggests that terminal distributary channels and associated mouth bars were up to 30 m wide. Planar-bedded and cross-bedded sandstones are interpreted respectively to be deposited in inertia-dominated and friction-dominated environments. The upwards decrease in dimensions of the cross bed sets above the planar beds is interpreted to represent a transition from an inertia-dominated to a friction-dominated environment, which is interpreted to be mainly due to the filling of accommodation and shallowing of the water as the delta progrades and builds. Friction-dominated bars are deposited at the river mouth in relatively shallow water. The inner side (upstream) of these friction-dominated mouth bars is characterized by upstream accretion, formed at bar diffluence zones, while the outer side (downstream) of the mouth bars is characterized by downstream and lateral accretion. Inertial bars are fed by hyperpycnal flows, which bypassed the river mouth and deposited detached bars at the foot of the delta-front slope.