DENSITY FLOW AS THE PRIMARY PROCESS OF SEDIMENT EMPLACEMENT IN THE DELTA FRONT SUBENVIRONMENT

Re-sedimentation via density flow was the primary process of sediment emplacement in the delta front subenvironment of the Cretaceous Panther Tongue, Star Point Formation, central Utah. Prior to re-sedimentation, sediment was temporarily stored in the stream mouth bar subenvironment, immediately seaward of the distributary channel. Sand was transported to the stream mouth bar during periods of low discharge in the distributary channel, and redistributed to the delta front during major river-flooding events. These major river-flooding events destabilized the stream mouth bar and triggered density flows which spread out in a seaward direction, depositing upward-fining fan-shaped sheets of sand and silt in the delta front. Stream mouth bars are characterized by large scale soft sediment deformation, including meter-scale flame structures. Failure is attributed to excess pore pressure resulting in liquefaction. Beds within the delta front are sharp-based and show waning flow attributes. In the most proximal setting of the delta front, the basal few centimeters of each bed is structureless, and overlain by planar bedding. Tools marks occur commonly on the base of beds. When traced basinward, beds are planar bedded, and are often amalgamated. In the more distal areas of the delta front, planar bedding is overlain by either wave-ripple laminae, wave modified current ripple laminae, or current ripples, which are then overlain by burrowed to bioturbated siltstones. Siltstones are interpreted as deposits of hypopycnal processes, which occurred during fair weather processes as sand was being stored in the steam mouth bar. We propose that river dominated deltas are dominated by two temporarily distinct stages of deposition. The first stage is when sediment is stored in the stream mouth bar, and the delta front is dominated by hypopycnal processes and corresponds to fair-weather conditions within the drainage basin. The second stage corresponds to major river-flooding events, when sediment is remobilized from the stream mouth bar to the delta front and deposited via density flows. A few beds record waxing then waning flow conditions, implying isolated deposition from hyperpycnal flows.