Turbidity Current Flow Out of Channels and Its Contribution to Constructing the Continental Slope

David Mohrig¹, Kyle M. Straub², and James Buttles^1
1Geological Sciences, University of Texas at Austin, Austin, TX
²St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN

We combine analysis of shallow seismic data from industry 3D volumes with results from physical models that resolve channel-to-overbank sedimentation by turbidity currents to explore the construction of regional surfaces by unconfined flows. Depositional patterns measured from seismic and laboratory data are used to define properties of proximal versus distal overbank sedimentation. Both data sets reveal a significant drop in variability of depositional thickness associated with the transition from levee to background overbank sedimentation. Laboratory data guide the development of relationships for levee-deposit thickness and lateral taper as functions of the absolute value for suspended sediment concentration and the gradient of the suspended sediment profile. These relationships are applied to seismically defined geometries to estimate properties of overbank-building currents.

Data from three laboratory channels with sinuosities of 1.00, 1.03 and 1.28, define how currents move out of channels. Overbank sedimentation is roughly axi-symmetric for the straight channel, but significantly asymmetric for the low channel sinuosity of 1.03. For this channel the levee deposits on the outer banks of bends are about 1.8 times thicker than inner bank deposits. For the channel with a sinuosity of 1.28 the outer bank deposits are about 3.4 times thicker than levee deposits on the inner banks of bends. These differences are related to bend-induced cross-channel flow. The style of cross-channel flow and the resulting distribution of particle sizes that move out of the channel will be discussed. Accurate modeling of currents moving out of channels at bends requires the use of realistic channel sidewall slopes and cross-sectional geometries. Laboratory-defined ties between overbank sedimentation and channel sinuosity will be compared to seismically resolved, natural systems.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas