Journal of Sedimentary Research, 2011, v. 81, 841 DOI: 10.2110/jsr.2011.67
DISCUSSION TRANSPORT MECHANISMS OF SAND IN DEEP-MARINE ENVIRONMENTS: INSIGHTS BASED ON LABORATORY EXPERIMENTS—DISCUSSION G. SHANMUGAM Department of Earth and Environmental Sciences, The University of Texas at Arlington, Arlington, Texas 76019, U.S.A. e-mail:
[email protected]
The experimental study by Breien et al. (2010) is important because it shows that massive sands can be deposited by laminar sediment flows. The massive sands in these experiments represent deposition from a ‘‘fluidized segment’’ of the flow. Breien et al. (2010, p. 977) consider fluidization as ‘‘…a mechanism where the mass moves like a fluid, and as the particles settle due to gravity, the pore fluid is displaced upwards, thus providing further grain support.’’ This fluidized-flow segment can also be termed a sandy debris flow if one defines debris flow as flow that is laminar. Illstad et al. (2004) do indeed refer to flows produced in similar experiments as debris flows. A problematic aspect of this study is comparison of these massive sands with the Ta division of the ‘‘Bouma Sequence,’’ which was designed for turbidites. These experimental massive sands are not turbidites, if one uses the definition of a turbidity current as a flow that is fully turbulent. Documentation of laminated Tb and Td divisions is not compelling in these experimental deposits. In Figures 4 (frame 1743) and 7D, for example, the Tb division corresponds to clay-rich divisions. In both cases, lamination (faint?) is not obvious. Importantly, there is no vertical scale for the photograph in Figure 7D. In the absence of a scale, it is difficult to discriminate a lamination (, 1 cm in thickness) from a bed (. 1 cm in thickness). The Figure 7 caption also fails to provide the necessary descriptive details of the Tb division. Therefore, the claim that this experimental study has developed Ta, Tb, Td, and Te divisions of the ‘‘Bouma Sequence’’ may not be correct. The lack of a ripple crosslaminated sandy deposit equivalent to Tc is striking. Further experiments would be needed to test the assertion of Breien et al. (2010) that this is simply due to a lack suitably sized silt particles in the experiments, and it could rather result from a lack of tractional reworking in such dense flows. Further experiments could also investigate whether sand with a wider range of sizes leads to a strongly graded deposit from the ‘‘fluidized’’ and laminar part of the flow. In order to provide further insight into their experiments it would be helpful if the authors could: 1. Provide new photographs (with scales) showing parallel lamination. 2. Label top and bottom contacts of laminated Tb and Td divisions, and consider whether the laminations really are equivalent to Tb.
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3. Provide data on thicknesses of massive Ta and laminated Tb and Td divisions. 4. Discuss in detail the depositional mechanics of laminated Tb and Td divisions, and clarify whether the observed planar laminations occur in sand or clay-size sediment. 5. Discuss the similarities and differences (e.g., grain size, division thickness, nature of contact, etc.) between experimental deposits and the original descriptions of each division by Bouma (1962). 6. State more precisely the grain-size range represented by their ‘‘medium sand,’’ inasmuch as grading of the sand fraction cannot occur in sand with a narrow range of sizes. This would mean that the lack of sand grading could potentially be an artifact of a narrow range of available sand sizes. 7. Discuss the deposition from the fluidized region that produced the massive clean sand occurred from predominantly laminar rather than turbulent flow. 8. Justify the need to apply the ‘‘Bouma’’ divisions to deposits of fluidized flows. ACKNOWLEDGMENTS
As an emeritus member of SEPM (member since 1970), I am grateful to Paul McCarthy (Editor), Peter Talling (Associate Editor), John Southard (Corresponding Editor), Melissa Lester (Managing Editor), and an anonymous journal reviewer for their dedicated editorial services. I thank Jean Shanmugam for her general comments. REFERENCES
BOUMA, A., 1962, Sedimentology of some Flysch Deposits: A Graphic Approach to Facies Interpretation: Amsterdam, Elsevier, 168 p. BREIEN, H., DE BLASIO, F.V., ELVERHØI, A., NYSTUEN, J.P., AND HARBITZ, C.B., 2010, Transport mechanisms of sand in deep-marine environments—insights based on laboratory experiments: Journal of Sedimentary Research, v. 80, p. 975–990. ILLSTAD, T., ELVERHØI, A., ISSLER, D., AND MARR, J.G., 2004, Subaqueous debris flow behaviour and its dependence on the sand/clay ratio: a laboratory study using particle tracking: Marine Geology, v. 213, p. 415–438. Received 9 November 2010; accepted 17 May 2011.
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