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6. MASS-TRANSPORT DEPOSITS OF THE AMAZON FAND.J.W. Piper, C. Pirmez, P. L. Manley, D. Long, ...ABSTRACTINTRODUCTIONDISTRIBUTIONLITHOLOGYPHYSICAL PROPERTIESAGEDISCUSSIONCONCLUSIONSACKNOWLEDGMENTSREFERENCESFIGURESFigure 1. Map of the Amazon Fan showing location ...Figure 2. Schematic cross section of the Amazon Fan ...Figure 3. Stratigraphic sections of all sites ...Figure 4. Longitudinal seismic-reflection profile ...Figure 5. Map showing location of seismic profiles ...Figure 6. Series of strike (slope-parallel) seismic ...Figure 7. Two seismic-reflection profiles across ...Figure 8. Slope-parallel single-channel reflection ...Figure 9. Seismic-reflection profile across upper ...Figure 10. Seismic-reflection profile showing ...Figure 11. Seismic-reflection profile showing ...Figure 12. Isopach of the URMTD in two-way traveltime ...Figure 13. Schematic profiles across the middle fan ...Figure 14. Lithologic interpretation of the URMTD ...Figure 15. Core photographs of selected intervals....Figure 15 (continued). D. Several small clasts in ...Figure 16. Examples of particular features from ...Figure 16 (continued). B. Hole 935A, 251–254 mbsf, ...Figure 16 (continued). C. Hole 936A, 179.5–182.5 ...Figure 16 (continued). D. Hole 936A, 200–204 mbsf ...Figure 16 (continued). E. Hole 944A, 263–266 mbsf ...Figure 16 (continued). F. Hole 944A, 225.5–227.5 ...Figure 17. Downcore variation in water content and ...Figure 18. Wireline logs of the URMTD at Sites 935 ...Figure 19. Wireline logs of the BMTD at Site 933.Figure 20. Selected index and wireline log properties ...Figure 21. Dip-azimuth (“tadpole”) plots for MTDs ...Figure 21 (continued).Figure 21 (continued).Figure 21 (continued).Figure 22. Map of Amazon Fan showing regional ...Figure 23. Oedometer consolidation tests from Site ...Figure 24. Plot of ratio of undrained shear strength ...Figure 25. Plot of ratio of undrained shear strength ...Figure 26. Void ratio plot for Site 931. The BMTD ...Figure 27. Water content plot for Site 933. The BMTD ...Figure 28. Water content for Site 933 with the MTD ...Figure 29. Oxygen isotope determinations and numbers ...Figure 30. Inferred age of source sediment and ...Figure 31. Drawing illustrating source and flow of ...Figure 32. Age of MTDs plotted against sea-level c ...TABLESTable 1. Summary of stratigraphic nomenclature of ...Table 2. Subunits of mass-transport deposits.Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 155109*6. MASS-TRANSPORT DEPOSITS OF THE AMAZON FAN1D.J.W. Piper,2 C. Pirmez,3 P. L. Manley,4 D. Long,5 R.D. Flood,6 W.R. Normark,7 and W. Showers8ABSTRACTSeismic reflection profiles show at least four major mass-transport deposits (MTDs) on the Amazon Fan that drilling hasshown date from the late Pleistocene. Each deposit extends over an area on the order of 104 km2 and is 50–100 m thick. Theentire thickness of individual MTDs was penetrated at Sites 931, 933, 935, 936, 941, and 944, and wireline logs were collectedat most of these sites. Most deposits consist of large deformed blocks (meters to decameters) of clayey sediment. A little matrixis recognized between blocks, and some weaker smaller blocks are highly deformed. Thin matrix-rich deposits with small clastsnear the top of some units are true debris flows. Properties of clasts in the MTDs show a broadly repetitive character verticallywithin the deposit, on a scale of meters to tens of meters. There is no evidence that a long time span is represented by disconti-nuities in sediment properties; rather, this repetitive pattern probably represents retrogressive failure from a headwall scarp.Major units 20–50 m thick within the MTDs can be correlated between sites. Sediment properties and microfossils suggest thatmost sediment was derived from muddy channel-levee deposits on the continental slope, but some sediment (particularly nearthe base of flows) resembles local deep-water levee sediments. Mass-transport events are inferred to have initiated in slope andupper-fan levee sediments. This sediment was underconsolidated because of rapid prodeltaic deposition during marine low-stands as well as a result of the presence of shallow gas and gas hydrates. Local steepening and weakening by diapiric intrusionmay also have facilitated failure. The ages of the mass-transport events may correlate with times of falling sea level, when gashydrate sublimation could destabilize sediments. MTDs were partly confined by pre-existing channel-levee topography on thefan. In places, high-relief levee deposits were eroded by the mass-transport flow and incorporated in the basal part of thedeposit.INTRODUCTIONMajor blocky mass-transport deposits (MTDs) covering areas ofhundreds of square kilometers, have been recognized as major com-ponents of continental margins from the past two decades of seafloormapping with high-resolution seismic-reflection profiling and side-scan sonar (e.g., Walker and Massingill, 1970; Jacobi, 1976; Embleyand Jacobi, 1986; Kenyon, 1987; Bugge et al., 1988; Lee, 1989). Thesurficial parts of only a few such deposits have been investigated indetail by acoustic imaging and coring (e.g., Normark and Gutmacher,1988; Masson et al., 1993), but interpretation of their dynamics re-mains speculative (Hampton et al., 1996) in the absence of lithologicand structural detail about the deeper parts of these thick deposits. Al-though many of these MTDs have been loosely referred to as debrisflows, other processes have been important in their formation andtransport in many cases, and many show similarities to terrestriallandslides. The imprecise use of terminology results from both thecommonly structureless character of the deposits on acoustic-reflec-tion profiles, the lack of samples, and because failures may incorpo-rate elements of both slides and debris flows. The entire thickness ofmajor MTDs was penetrated at six different sites on Leg 155, thusproviding important new insights into the character, origin, and trans-port processes of such deposits.On the Amazon Fan (Fig. 1), MTDs are intercalated with thick,predominantly muddy levee deposits (Fig. 2). Individual levee units,which rapidly prograde downfan following turbidity-current channelavulsion, are grouped into larger levee complexes, each of which ap-pears to correspond to a major lowstand in sea level (Flood, Piper,Klaus, et al., 1995, see “Leg Synthesis”). Two


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