AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Downstream Controlled Avulsion and Local Base Level in Submarine Fans: The Role of Hydraulic Choke and Backwater — A Scaling Analysis
(1) ExxonMobil Upstream Research, Houston, TX.
(2) Department of Earth and Atmospheric Science, St Cloud State University, St Cloud, MN.
(3) School of Oceanography, University of Washington, Seattle, WA.
(4) Department of Civil Engineering, University of Houston, Houston, TX.
Outcrop observations from Karoo (RSA), Ross (Ireland), and Ainsa (Spain), (Hoyal et al., this conference), suggest that terminal submarine fans are likely to develop Froude supercritical to critical conditions; however, we know that further upstream channel depth increases rapidly and that Froude numbers in the mid-fan region are typically subcritical (e.g., Fr’ ~ 0.2-0.5, Pirmez and Imran, 2003). These two observations potentially create ideal conditions for a well-known phenomenon in open channel hydraulics, i.e. vertically choked flow. The condition may originate with the growth of a mouth bar at the fan terminus that contracts the flow area. With enough contraction, the flow can become choked and lead to the upstream propagation of a backwater wave and flow deepening along the subcritical channel. Theoretically the condition of choke is imposed once the mouth bar reaches a condition of critical flow (Fr’ ~ 1). We propose that this choke condition, and the associated backwater, may be important controls on the organization of deep-water channel fills and a driver on avulsion, avulsion cycles and associated stratigraphy. To test this idea we perform a scaling analysis for density stratified flows and turbidity currents to derive the basic length and thickness scales. These scales are compared with sea floor data of avulsion lengths and thicknesses from high-resolution bathymetric and seismic studies of deep-water fans published over the last decade.
This analysis indicates that backwater in the submarine environment scales closely with measured avulsion lengths and therefore is a plausible mechanism to set up avulsion. Potential flow depth changes are significant and may lead to intermittent bursts of bed aggradation and levee growth (e.g., Hoyal & Sheets, 2009). Therefore, while low order (feeder) deep-water channels will eventually avulse due to over-extension and slope decrease below grade, backwater may be a contributing process that structures the avulsion deposits, channel fills, and intra-lobe avulsions. The onset of choke will likely depend on the availability of bedload at the channel mouth. The new model suggests that local baselevel (e.g., Pirmez et al., 2000) may be cyclic process associated with onset by choke and backwater (mouth bar growth) and release by supercritical flow (avulsion), leading to some very interesting dynamics of avulsion cycles.