--> Development of Banding in the Deposits of Experimental Transitional Flows

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Development of Banding in the Deposits of Experimental Transitional Flows

Abstract

Transitional flows deposits have been described in a wide range of deepwater basins such as the Agadir Basin (offshore Morocco), Britannia Formation in the North Sea and the ultra-deepwater sub-salt Wilcox Group (USA). These types of deposits do not follow the ‘classic’ deepwater facies models (i.e., Bouma Sequence). One of the most enigmatic aspects of transitional flow deposits lies in an alternation between clay-rich and clean sand bands. Distinguishing between laminae and true banding in argillaceous deepwater deposits is often unclear due to uncertainty in the processes controlling the development of banding. Surging within depositing sediment gravity flows may provide a process-based explanation for why banded intervals are common in facies descriptions of transitional flow deposits. To test this hypothesis, we induced intermittent surges on clay-rich, sand-laden transitional flows in a two-dimensional experimental flume. Surging produces unsteady deposition of alternating clay-rich and clean sand bands in an overall fining upward event bed. Millimeter-scale bands are connected to fluctuating bed aggradation rates which wax and wane in response to surging. High-resolution videos taken during the experiments show that rapid bed aggradation results in clay-rich bands while lower aggradation rates deposit relatively clean sands. The character of banding differs across the tested flow compositions, from high-concentration turbidity currents to strong transitional flows. Variability in the spatial distribution of clay within banded deepwater deposits has implications for predicting both permeability structure and overall reservoir quality.