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An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits

An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits
An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits
The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass-ballotini sand of particle size 35·5 ?m < d < 250 ?m and volume fractions, ?s, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 ?m and volume fractions, ?m, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at ?s ? 0·2 and entirely inhibited at ?s ? 0·6. In noncohesive and cohesive mixtures with low sand concentrations (?s < 0·2), particle segregation was initially suppressed at ?m ? 0·07 and entirely suppressed at ?m ? 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear-flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The 'abrupt' change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud-poor, graded sandy deposits or mud-rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean 'turbidite' or muddy 'debrite' sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal 'linked' debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ? ?s ? 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near-bed sediment concentrations.

0037-0746
1411-1434
Amy, Lawrence A.
857f581c-1529-424c-a948-91ab2fd476eb
Talling, Peter J.
1cbac5ec-a9f8-4868-94fe-6203f30b47cf
Edmonds, Victoria O.
93de75dd-2ae0-4d73-ae5e-cf8f051b00f3
Sumner, Esther J.
dbba4b92-89cc-45d9-888e-d0e87e5c10ac
Lesueur, Anne
77fc4d7d-a9f2-4f4c-abe7-5f89765d9836
Amy, Lawrence A.
857f581c-1529-424c-a948-91ab2fd476eb
Talling, Peter J.
1cbac5ec-a9f8-4868-94fe-6203f30b47cf
Edmonds, Victoria O.
93de75dd-2ae0-4d73-ae5e-cf8f051b00f3
Sumner, Esther J.
dbba4b92-89cc-45d9-888e-d0e87e5c10ac
Lesueur, Anne
77fc4d7d-a9f2-4f4c-abe7-5f89765d9836

Amy, Lawrence A., Talling, Peter J., Edmonds, Victoria O., Sumner, Esther J. and Lesueur, Anne (2006) An experimental investigation of sand–mud suspension settling behaviour: implications for bimodal mud contents of submarine flow deposits. Sedimentology, 53 (6), 1411-1434. (doi:10.1111/j.1365-3091.2006.00815.x).

Record type: Article

Abstract

The settling behaviour of particulate suspensions and their deposits has been documented using a series of settling tube experiments. Suspensions comprised saline solution and noncohesive glass-ballotini sand of particle size 35·5 ?m < d < 250 ?m and volume fractions, ?s, up to 0·6 and cohesive kaolinite clay of particle size d < 35·5 ?m and volume fractions, ?m, up to 0·15. Five texturally distinct deposits were found, associated with different settling regimes: (I) clean, graded sand beds produced by incremental deposition under unhindered or hindered settling conditions; (II) partially graded, clean sand beds with an ungraded base and a graded top, produced by incremental deposition under hindered settling conditions; (III) graded muddy sands produced by compaction with significant particle sorting by elutriation; (IV) ungraded clean sand produced by compaction and (V) ungraded muddy sand produced by compaction. A transition from particle size segregation (regime I) to suppressed size segregation (regime II or III) to virtually no size segregation (IV or V) occurred as sediment concentration was increased. In noncohesive particulate suspensions, segregation was initially suppressed at ?s ? 0·2 and entirely inhibited at ?s ? 0·6. In noncohesive and cohesive mixtures with low sand concentrations (?s < 0·2), particle segregation was initially suppressed at ?m ? 0·07 and entirely suppressed at ?m ? 0·13. The experimental results have a number of implications for the depositional dynamics of submarine sediment gravity flows and other particulate flows that carry sand and mud; because the influence of moving flow is ignored in these experiments, the results will only be applicable to flows in which settling processes, in the depositional boundary, dominate over shear-flow processes, as might be the case for rapidly decelerating currents with high suspended load fallout rates. The 'abrupt' change in settling regimes between regime I and V, over a relatively small change in mud concentration (<5% by volume), favours the development of either mud-poor, graded sandy deposits or mud-rich, ungraded sandy deposits. This may explain the bimodality in sediment texture (clean 'turbidite' or muddy 'debrite' sand or sandstone) found in some turbidite systems. Furthermore, it supports the notion that distal 'linked' debrites could form because of a relatively small increase in the mud concentration of turbidity currents, perhaps associated with erosion of a muddy sea floor. Ungraded, clean sand deposits were formed by noncohesive suspensions with concentrations 0·2 ? ?s ? 0·4. Hydrodynamic sorting is interpreted as being suppressed in this case by relatively high bed aggradation rates which could also occur in association with sustained, stratified turbidity currents or noncohesive debris flows with relatively high near-bed sediment concentrations.

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Published date: 2006
Organisations: Geology & Geophysics, Marine Geoscience

Identifiers

Local EPrints ID: 54757
URI: https://eprints.soton.ac.uk/id/eprint/54757
ISSN: 0037-0746
PURE UUID: 4fbf9167-c00f-4873-ad64-ee02ed208dcb

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Date deposited: 18 Jul 2008
Last modified: 13 Mar 2019 20:38

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