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The Influence of Flow Rheology from the Architecture of Deep-Water Depositional Systems*
Donald R. Lowe1
Search and Discovery Article #50097 (2008)
Posted October 24, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008.
1 Geological & Environmental Sciences, Stanford University, Stanford, CA ([email protected])
The rheology of submarine mass flows and mass movements plays a key role in controlling the large- and small-scale surface morphology and internal architecture of the resulting deep-water depositional systems. Turbulent flows, including turbidity currents and some watery slurry flows, have the ability to erode, transport, and deposit individual particles based on their hydraulic properties. As a result, deep-water systems dominated by sedimentation from such flows show the development of complex morphological architectural elements including channels, bars, levees, crevasses, splays, lobes or distal splays, and broad out-of-channel areas. In contrast, more viscous and cohesion-dominated slurry flows and debris flows are generally laminar during their depositional stages or dominated by thick laminar near-bed layers and generally show minimal ability to scour and fractionate transported particles. Depositional systems dominated by these flow types tend to be lobe-like accumulations with flat or irregular surfaces that show poor development of channels, levees, and other organized features of turbulence-dominated systems. Where periods of deposition from turbidity currents 
alternate
with intervals of debris-flow activity, the debris flows often bury and obliterate the complex surface architecture of the preceding turbidity-current phase
of sedimentation, effectively resetting the surface system. Initial flows of the ensuing turbidity-current stage are forced to follow courses determined by debris-flow deposit surface morphology. The resulting turbidity-current deposits may show architectural features and sediment distributions that differ from those of more mature constructional turbidite systems, making predictive modeling of the deposits and reservoir sands difficult.
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The contrasting rheologies of turbidity currents (turbulent) and debris flows (generally laminar):
The basic geometry and heterogeneity of deepwater systems is controlled by the rheology of the flows that deliver and deposit sediment. End members include the complex depositional systems that we associate with turbulent flows (turbidity currents) and the simple, lobe-like debris-flow dominated systems. They can also bury pre-existing topography and reset the depositional surface. Complex channel-levee systems produced by turbidity currents may be buried. The result may include low-relief surfaces such that overlying sands may have a tabular HARP-like distribution or hummocky surfaces that may provide local, irregularly distributed sites for sediment accumulation during passage of subsequent turbidity currents. Reservoir modeling and development in complex mixed systems depends on an evaluation of the nature of the depositional processes and the ways that turbidity currents, slurry flows, and debris flows have interacted to control the sites of sand deposition and the geometry of the resulting deposits. Moore, J.G., and W.W. Chadwick, Jr., 1995. Offshore geology of Mauna Loa and adjacent areas, Hawaii, in J.M. Rhodes and J.P. Lockwood, eds. Mauna Loa revealed: Structure, composition, history, and hazards. Geophys. Monograph 92, p. 21-44. Posamentier H.W., and V. Kolla, 2003, Seismic geomorphology and stratigraphy of depositional elements in deep-water settings: Journal of Sedimentary Research, v. 73, p. 367 388. Colleagues and Students in Deep-Water Studies at Stanford Stanford Project on Deep-water Depositional Systems (SPODDS)
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