Process Control on Grain Size Trends in Turbidite Levee Sequences

Abstract

Submarine channel-levee systems allow turbidity currents to transport sediment for large distances into oceanic basins because the self-formed channel-levee shape provides confinement that sets up a pathway for the bypass of sediment. The levees of channels do not only facilitate sediment transfer but also form significant sediment bodies in their own right. Upward bed thinning and fining have been observed widely in levee sequences and are commonly attributed to progressive increase of channel depth combined with vertical grain size sorting in turbidity currents. Alternatively, changes in flow size or composition due to upstream forcings may have the same effect on deposit sequences. However, these conceptual models cannot unequivocally be tested in outcrop, subsurface, or seafloor datasets because flow conditions and morphological evolution are commonly not synchronously constrained. Outcrop and subsurface datasets provide no direct information on flow processes, and monitoring of presently active systems rarely covers sufficient time to monitor the co-evolution of turbidity currents and channel morphology. Here we use Shields scaled experimental turbidity currents to study the development of a self-formed leveed channel that becomes progressively deeper. Combined measurements of flow processes and depositional products allow for a process interpretation of the levee grain size and deposit geometry. Sediment samples show that the levees formed by the turbidity current are fining upward and measurements of the turbidity current show an upward decrease in sediment concentration and grain size. The vertical grain size gradient in the flow and the decrease in levee grain size with channel depth are highly similar. The channel depth evolves due to a combination levee growth and channel floor aggradation and degradation. The movement of the channel floor appears to be a significant factor that cannot be ignored when linking flow stratification to levee grain size trends. The vertical stratification structure of the turbidity currents is shown to be the fundamental control on the lateral transition from a bypassing channel into an aggrading levee. The transition from bypassing flow to capacity-driven deposition occurs on the channel wall at the point where concentration exceeds near-bed capacity. Thus, we have a viable method to predict both the morphological evolution of a channel and the composition of the levees from the turbidity current flow structure.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017