Experimental Studies of Channel Mouth Bar Formation and Evolution in Deep and Shallow-Water Clastic Sedimentary Systems

David C.J.D. Hoyal, Ben A. Sheets, and Neal L. Adair
ExxonMobil Upstream Research Company, Houston, TX

Mouth bars are an important component of terminal distributive complexes in clastic sedimentary systems. The processes associated with mouth bar initiation and evolution in both shallow and deep-water settings are the subject of ongoing physical experimental research at ExxonMobil Upstream Research Company.

Experiments show that there are many similarities in the evolution of shallow-water and deep-water mouth bars due to the universal morphodynamic process of a flow interacting with topography. Mouth bars are initiated by deposition from a turbulent jet but rapidly aggrade vertically to the point where the interaction between topography and flow dominates the later stages of bar growth. This evolution from jet (inertial) to boundary layer (gravitationally and frictionally dominated) produces a universal, identifiable and repeatable stratal pattern characteristic of mouth bars. There are, however, important variations on this pattern.

One variation is associated with channel-bank strength, which controls the mobility of the channel mouth (jet orifice). Weak banks promote laterally mobile orifices, which rapidly deflect from the incipient mouth bar, creating a complex of laterally accreting jet-derived deposits. If the orifice is strongly fixed, however, the mouth bar evolves beyond simple jet deposition, through backstepping and lateral growth due to the interaction of topography with flow. These contrasting cases produce characteristic stacking patterns. Another geometric variation is associated with a wake effect produced by the bar, which may be enhanced during low-flow, as is typical during observations of modern systems.