Grainsize Control on Depositional Style in Deepwater Depositional Systems

Eggenhuisen, Joris T.; Hofstra, Menno; Cartigny, Matthieu

We aim to link turbidity current flow processes to turbidite sandstone style, architecture, and variability through quantitative study of experimental sandy turbidity currents and their deposits. In this contribution we focus on grainsize control on depositional style of deepwater systems.

Systematic compilation of types of deepwater depositional systems in the 1990's led to the recognition of large scale architectural trends that depend on the predominant sedimentary composition of the flows within the systems: With decreasing grain sizes the fan systems become larger and have characteristically lower slopes.

These trends are commonly attributed to "increased transport efficiency" due to the fine grainsizes within the turbidity currents, without explaining how this transport efficiency is governed by grainsize, or by which process fine grains may increase it.

Previous experiments have confirmed that adding small quantities of fine grains to a mono-disperse coarser grained turbidity current increases the flow's capacity to transport the coarser material over larger distances, but the processes that trigger the enhancement of the efficiency remain unclear, mainly due to lack of control on flow dynamics and limitations to the used experimental approach.

Here we present data from bypassing, sustained, sandy turbidity currents that clarify the increased flow efficiency due to the addition of fine grains. The equilibrium slope of the currents has been mapped as a function of grainsize distribution and sediment concentration. The results show a strong decrease of the equilibrium slope with the addition of up to 25% fines. Adding more fines triggers some further lowering of the equilibrium slope, but the effect is less obvious than the addition of the first 25%. This is consistent with a ratio of 30% fine and 70% coarse sediment that has been suggested as the boundary between sand-rich and mixed mud-sand systems. The explanation of this effect in terms of the flow dynamics can be found in increased stability of the upper flow interface, an effect that partitions friction to the base of the flow.

Deposit analyses add a new twist to the capacity vs. competence debate: the fine grained component of the sediment can be bypassed effectively, while the coarse grains are partitioned into the deposit. This indicates that close to bypass conditions, flow competence governs the constitution of the bed and thereby reservoir quality of turbidite sandstones.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013