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Clinoform Progradation by Turbidity Currents: Modeling and Experiments

¹ Division of EOS, Nicholas School of the Environment and Earth Sciences, Old Chemistry Box 90227, Duke University, Durham, North Carolina 27708, U.S.A.; tpg{at}duke.edu
² Division of EOS, Nicholas School of the Environment and Earth Sciences, Old Chemistry Box 90227, Duke University, Durham, North Carolina 27708, U.S.A.
³ St. Anthony Falls Laboratory, University of Minnesota Twin Cities, 2 Third Avenue SE, Minneapolis, Minnesota 55414, U.S.A.
⁴ Department of Geological Sciences, The University of Texas at Austin, 1 University Station, C-1110, Austin, Texas 78712, U.S.A.
⁵ Department of Geological Sciences and Large Lakes Observatory, University of Minnesota-Duluth, 229 Heller Hall, 1114 Kirby Dive, Duluth, Minnesota 55812, U.S.A.; present address: Barr Engineering Company, 332 W. Superior Street, Suite 600, Duluth, Minnesota 55802, U.S.A.
⁶ Department of Geological Sciences and Large Lakes Observatory, University of Minnesota-Duluth, 229 Heller Hall, 1114 Kirby Dive, Duluth, Minnesota 55812, U.S.A.
⁷ St. Anthony Falls Laboratory, University of Minnesota Twin Cities, 2 Third Avenue SE, Minneapolis, Minnesota 55414, U.S.A.

Clinoform morphologies and growth patterns are typically viewed as the product of a particular mode of sediment transport, but process-specific models for their generation from turbidity currents are few, despite observations of turbidity currents on modern clinoforms and turbidites in ancient clinoform deposits. We present a simple morphodynamic model, supported by laboratory experiments, which shows how net-depositional turbidity currents can build sedimentary clinoforms. Most conceptual models for clinoform evolution assume that under constant (sea level, sediment supply) forcing progradation occurs via continuous basinward migration of a depocenter localized near the clinoform rollover. Abrupt basinward shifts in depocenter location are therefore an indication of allogenic variability in forcing. In contrast, our results document a unique style of progradation driven by autogenic cycles of slope steepening, sediment bypass, and depositional backstepping on the foreset slope. In experiments designed to investigate this slope–flow feedback, deposition from a continuous turbidity current transporting fine sand and silt repeatedly steepened a clinoform foreset to a graded slope which bypassed sediment to the slope base, depositing a sediment lobe. Continued deposition then caused the lobe to backstep up the slope, building a lower-slope foreset and eventually reinitiating the cycle. Our model shows how this cyclic depositional pattern arises from a morphodynamic feedback between the foreset gradient and the rate of sediment resuspension by the overriding turbidity current. Using our model to scale experimental results to the field, we predict gradual foreset steepening in prograding turbidity-current- dominated clinoform strata, with slope grading and cyclic deposition favored where these clinoforms build over steep basin slopes under conditions of high sediment supply and/or lateral confinement. We demonstrate how our results can add new insight to process-based interpretation of clinoform strata by application to a modern and an ancient field example.