THE ROLE OF BUOYANCY REVERSAL IN TURBIDITE DEPOSITION AND SUBMARINE FAN GEOMETRY

Although recent work has shown that changing interstitial fluid density within turbidity currents is a frequently overlooked factor affecting the texture and internal architecture of turbidites, little is known about its influence on submarine fan morphology. Buoyancy reversal, or lofting, occurs when a turbulent underflow that is initially ground-hugging becomes less dense than the surrounding fluid and rises from the basin floor. Buoyancy reversal requires conditions in which the flow’s interstitial fluid is less dense than the ambient fluid, but bulk flow density is initially greater due to high concentrations of suspended sediment. These conditions occur in nature when fresh, sediment-laden rivers meet ocean basins or when turbidity currents initiated in warm, shallow environments travel into deeper and colder water. Despite an abundance of fluid dynamics studies describing lofting, few geologists have incorporated these findings into understanding turbidite systems. Lofting significantly alters the geometry of submarine flows and their deposits, and should no longer be overlooked in the context of submarine deposition.

Here, we present the results of three-dimensional flume experiments of turbidity currents that clearly demonstrate the role of low-density interstitial fluid, in combination with sediment concentration and basin gradient, on submarine fan geometry. The experiments show that flows with reversing buoyancy, and their resulting deposits, are narrower than those that remain ground-hugging. Furthermore, wider deposits result from increases in sediment concentration and/or basin floor gradient. We also show that Taylor-Görtler vortices associated with flows travelling over a break in slope lead to the deposition of wider lobes compared with those travelling over a constant gradient.