Allocyclicity of Sediment Volume and Composition Provide the Basis for a Predictive Model of Turbidite Channel Architectures
T. McHargue1, J. Clark1, M. Sullivan2, M. Pyrcz2, A. Fildani1, M. Levy1, H. Posamentier2, B. W. Romans1, and J. A. Covault1
1Chevron ETC, San Ramon, CA, [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]
2Chevron ETC, Houston, TX, [email protected], [email protected], [email protected]
Cyclicity in sediment supply volume and composition can strongly influence channel hierarchy, avulsion, stacking pattern, fill architecture, and net/gross. It is rarely possible to confirm that allocyclicity is the dominant control on these variables, but a reasonable and predictive model results from the assumption. Cyclicity is expressed as a phase of waxing energy followed by a phase of waning energy that appear to be present on at least three scales in most turbidite settings: 1) a channel element cut-and-fill cycle, 2) a system (an overbank aggradation cycle of multiple elements), and 3) a sequence (multiple systems). At each scale, a cycle is separated from both the preceding and following cycle by avulsion. A cycle of channel element cut-and-fill is expressed by erosion of the channel element along with sediment bypass during the waxing phase, followed by sediment deposition and back-stepping during the waning phase. A system-scale cycle may be initiated by erosion of a slope valley (negative aggradation) followed by a low rate of overbank aggradation during which time multiple channel element cycles contribute to the development of an amalgamated channel complex. Waning energy and increasing clay composition of turbidite flows commonly result in a gradual elevation of the equilibrium profile and an increase in the rate of overbank aggradation.
The expression of a sequence is highly variable depending on gradient, but typically consists of a relatively sand-rich waxing phase with elevated potential for erosion followed by an increasingly mud-rich waning phase with diminishing erosion potential.
The convolution of these multiple cycles strongly constrains the potential depositional architectures that are likely to develop on a given topography. These allocycles can be expressed as predictive rules that define a framework for the construction of forward models of turbidite architecture within which the contributions of other variables can be incorporated.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009