AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Architecture of Froude Critical-Supercritical Submarine Fans: Tank Experiments Versus Field Observations
(1) ExxonMobil Upstream Research, Houston, TX.
(2) School of Oceanography, University of Washington, Seattle, WA.
(3) ExxonMobil Exploration Company, Houston, TX.
(4) Imperial Oil Resources, Calgary, AB, Canada.
Experiments on submarine fans and comparison to the field (Tanqua Karoo, RSA) indicate that Froude supercritical to critical fans have a well constrained suite of hydrodynamic and morphodynamic processes and a characteristic architecture. The experiments using saline flow and low-density plastic sediment are bedload-dominated, but this may be reasonable in the channel mouth environment. For example, in typical deep-water fan outcrops (e.g., Karoo, Ross, Ainsa), a proliferation of climbing and aggrading bedforms indicate strong bedload transport, at least in the expanding flow - channel terminus region (Hoyal et al., this conference).
Outcrop architecture related to Froude supercritical flow includes:
1) Characteristic scour holes generated at the channel-lobe transition that rise and back step into a composite scour by vertical lobe accretion. This can lead to some very complex stratal geometry.
2) Backset bedding (e.g., Jopling, 1966) at multiple scales interpreted to be due to backing up of cyclic steps/antidunes, mouth bars and lobes due to upstream migrating hydraulic jumps
3) The combination of 1 & 2 lead to the well-known steers-head geometry described in other submarine fan outcrops like the Brushy Canyon (e.g., Gardner & Borer, 2000) and is characteristic of strongly inertial flows; hence the mounding.
4) Field observations suggest that the sandy lobe sequences are completely conformable with the underlying finer-grained strata indicating a facies change; any scour is typically associated with channel-mouth bar transitions stratigraphically higher in the sandy lobe.
5) The backstepping of the mouth bar/lobe is accompanied by the upstream migration of cyclic steps and antidunes that develop once bar mounding has been established. These are well preserved in the strongly accreting bar.
6) The proliferation of antidunes of different scales changing rapidly in wavelength with distance may be associated with shoaling cyclic step bars as the flow oscillates between supercritical to subcritical to near critical. These antidunes enable us to make new interpretations of paleoflow depth and the relation of the observed deposits (architecture and hierarchy) to the evolving flow.
These observations combined with experimental data support a cyclic process of fan formation typical of Froude supercritical fans including channel extension, mouth bar growth, back-stepping, and near-lobe avulsion/bifurcation (e.g., Gardner and Borer, 2000).