Flood, R.D., Piper, D.J.W., Klaus, A., and Peterson, L.C. (Eds.), 1997 Proceedings of the Ocean Drilling Program, Scientific Results, Vol.

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1 Fld, R.D., Piper, D.J.W., Klaus, A., and Petersn, L.C. (Eds.), 1997 Prceedings f the Ocean Drilling Prgram, Scientific Results, Vl. 155 *6. MASS-TRANSPORT DEPOSITS OF THE AMAZON FAN 1 D.J.W. Piper, 2 C. Pirmez, 3 P. L. Manley, 4 D. Lng, 5 R.D. Fld, 6 W.R. Nrmark, 7 and W. Shwers 8 ABSTRACT Seismic reflectin prfiles shw at least fur majr mass-transprt depsits (MTDs) n the Amazn Fan that drilling has shwn date frm the late Pleistcene. Each depsit extends ver an area n the rder f 10 4 km 2 and is m thick. The entire thickness f individual MTDs was penetrated at Sites 931, 933, 935, 936, 941, and 944, and wireline lgs were cllected at mst f these sites. Mst depsits cnsist f large defrmed blcks (meters t decameters) f clayey sediment. A little matrix is recgnized between blcks, and sme weaker smaller blcks are highly defrmed. Thin matrix-rich depsits with small clasts near the tp f sme units are true debris flws. Prperties f clasts in the MTDs shw a bradly repetitive character vertically within the depsit, n a scale f meters t tens f meters. There is n evidence that a lng time span is represented by discntinuities in sediment prperties; rather, this repetitive pattern prbably represents retrgressive failure frm a headwall scarp. Majr units m thick within the MTDs can be crrelated between sites. Sediment prperties and micrfssils suggest that mst sediment was derived frm muddy channel-levee depsits n the cntinental slpe, but sme sediment (particularly near the base f flws) resembles lcal deep-water levee sediments. Mass-transprt events are inferred t have initiated in slpe and upper-fan levee sediments. This sediment was undercnslidated because f rapid prdeltaic depsitin during marine lwstands as well as a result f the presence f shallw gas and gas hydrates. Lcal steepening and weakening by diapiric intrusin may als have facilitated failure. The ages f the mass-transprt events may crrelate with times f falling sea level, when gas hydrate sublimatin culd destabilize sediments. MTDs were partly cnfined by pre-existing channel-levee tpgraphy n the fan. In places, high-relief levee depsits were erded by the mass-transprt flw and incrprated in the basal part f the depsit. INTRODUCTION Majr blcky mass-transprt depsits (MTDs) cvering areas f hundreds f square kilmeters, have been recgnized as majr cmpnents f cntinental margins frm the past tw decades f seaflr mapping with high-reslutin seismic-reflectin prfiling and sidescan snar (e.g., Walker and Massingill, 1970; Jacbi, 1976; Embley and Jacbi, 1986; Kenyn, 1987; Bugge et al., 1988; Lee, 1989). The surficial parts f nly a few such depsits have been investigated in detail by acustic imaging and cring (e.g., Nrmark and Gutmacher, 1988; Massn et al., 1993), but interpretatin f their dynamics remains speculative (Hamptn et al., 1996) in the absence f lithlgic and structural detail abut the deeper parts f these thick depsits. Althugh many f these MTDs have been lsely referred t as debris flws, ther prcesses have been imprtant in their frmatin and transprt in many cases, and many shw similarities t terrestrial landslides. The imprecise use f terminlgy results frm bth the cmmnly structureless character f the depsits n acustic-reflectin prfiles, the lack f samples, and because failures may incrprate elements f bth slides and debris flws. The entire thickness f majr MTDs was penetrated at six different sites n Leg 155, thus prviding imprtant new insights int the character, rigin, and transprt prcesses f such depsits. 1 Fld, R.D., Piper, D.J.W., Klaus, A., and Petersn, L.C. (Eds.), Prc. ODP, Sci. Results, 155: Cllege Statin, TX (Ocean Drilling Prgram). 2 Atlantic Gescience Centre, Gelgical Survey f Canada (Atlantic), Bedfrd Institute f Oceangraphy, P.O. Bx 1006, Dartmuth, N.S., B2Y 4A2, Canada. piper@agc.bi.ns.ca 3 Lamnt-Dherty Earth Observatry, Clumbia University, Palisades, NY 10963, U.S.A. 4 Gelgy Department, Middlebury Cllege, Middlebury, VT 05753, U.S.A. 5 British Gelgical Survey, West Mains Rad, Edinburgh EH9 3LA, United Kingdm. 6 Marine Sciences Research Center, State University f New Yrk, Stny Brk, NY , U.S.A. 7 U.S. Gelgical Survey, MS-919, 345 Middlefield Rad, Menl Park, CA 94025, U.S.A. 8 Department f Marine, Earth and Atmspheric Sciences, Nrth Carlina State University, 1125 Jrdan Hall, Bx 8208, Raleigh, NC 27695, U.S.A. On the Amazn Fan (Fig. 1), MTDs are intercalated with thick, predminantly muddy levee depsits (Fig. 2). Individual levee units, which rapidly prgrade dwnfan fllwing turbidity-current channel avulsin, are gruped int larger levee cmplexes, each f which appears t crrespnd t a majr lwstand in sea level (Fld, Piper, Klaus, et al., 1995, see Leg Synthesis ). Tw surficial MTDs are termed the Eastern and Western debris flws in recent publicatins (e.g., Manley and Fld, 1988). Each cvers an area extending ~200 km dwnslpe by 100 km in width, and each is ms thick. Nte that Damuth and Embley (1981) riginally referred t Western debris flw as the Central slump/debris flw cmplex and applied the term western t the cmplex n the rise at 50 W (which we refer t as the 50 W MTD). The Western Mass-transprt Depsit (WMTD) was cred at Site 941, whereas the Eastern Mass-transprt Depsit (EMTD) was nt sampled n Leg 155. A widespread buried MTD, the Unit R Debris Flw f Manley and Fld (1988) r Unit R Mass-transprt Depsit (URMTD) f this paper, underlies the Upper Levee Cmplex (istpic Stages 2 4) and was cred at Sites 935, 936, and 944. The crrelative interval was penetrated dwnfan at Site 946, where sediment cmprises thickly bedded sand with thin silt and mud beds. An MTD at the bttm f Hle 930C underlying the Orange Channel-levee Cmplex may be part f the URMTD, althugh this is nt demnstrated by seismic-reflectin crrelatin. On the eastern fan, an MTD verlying the crest f the Bttm Levee Cmplex (istpic Stage 8) was cred at Sites 931 and 933; it is referred t here as the Bttm Mass-transprt Depsit (BMTD). (We use this inelegant terminlgy because the MTD immediately verlies the scalled Bttm Levee Cmplex f Fld et al., 1991). The histrical nmenclature f marine MTDs is ptentially cnfusing. In this paper, we generally fllw the usage f Varnes (1978) and Nardin et al. (1979) in which the terms slide, slump, and debris flw each indicate a particular transprt prcess. We use the term mass-transprt depsit in a mre general sense r where mre than ne prcess is invlved. The characters f submarine slides and debris flws were riginally defined using 3.5-kHz high-reslutin reflectin prfiles (Jacbi, 1976) and have been refined as side-lking snar and cre data became available (Prir et al., 1984; Nrmark and Gutmacher, 1988; Nrmark, 1990). A slide invlves displacement Previus Chapter Table f Cntents Next Chapter 109

2 Channel D.J.W. PIPER ET AL. O 7N Surficial channels Diapir Surficial mass-transprt depsit Bttm-simulating reflectr Buried bathymetric high separating URMTD and BMTD Hummcky reflectins n BMTD 946 n A d e Guiana Shield Amazn Basin s Brazilian Amazn Fan Shield RC Fig m O 6N O 5N V W Mass-transprt Depsit V Western Mass-transprt Depsit V URMTD 930 Amazn BMTD RC Eastern Mass-transprt Depsit Ceara Rise O 4N shelf break 3000 m 49 O W Amazn Canyn 500 m 1000 m 2000 m 48 O W 47 O W 46 O W ODP site with masstransprt depsit Pistn cres in Fig. 29 Figure 1. Map f the Amazn Fan shwing lcatin and character f surficial MTDs and the tw majr buried MTDs (URMTD = Unit R Mass-transprt Depsit; BMTD = Bttm Mass-transprt Depsit). ODP sites penetrating MTDs and pistn cres illustrated in Figure 29 are als shwn. Map mdified frm Damuth et al. (1988) and Manley and Fld (1988). (shear) alng defined shear surfaces. A slump is a particular frm f slide in which there is rtatinal mvement n shear surfaces: many slumps n the cntinental slpe have mved nly shrt distances and preserve back-tilted benches. Debris flws may develp frm slides as a result f break up f blcks alng multiple shear surfaces, internal defrmatin f blcks, and inmixing f seawater. Acustic prfiles f slides may reveal the presence f cmpnent blcks (Prir et al., 1984; Nrmark, 1990). The size f the blcks cmmnly decreases dwnslpe, and slide depsits may merge dwnslpe with structureless, acustically transparent depsits with lw surface relief but steep margins that are interpreted as a debris flws n seismic-reflectin prfiles. This gradatin in acustic prperties frm slides t debris flw depsits has resulted in bth terms being used almst interchangeably in previus studies. In ancient depsits, lithlgic features can be used t infer transprt prcess. Debris flw depsits are mud-clast cnglmerates, cmmnly with scattered pebbles and rafted blcks (e.g., Piper et al., 1978), whereas slide depsits cmprise large blcks f sediment with defrmatin cncentrated at their leading edge (Cnigli, 1986). Depsits that include elements f bth slides and debris flws are als fund. Sme submarine slides and debris flws are thught t prduce turbidity currents (Middletn and Hamptn, 1976) as the mass-wasted sediment cntinues t defrm and lse chesin during transprt and water is entrained with the mving and defrming sediment. 110

3 MASS-TRANSPORT DEPOSITS OF AMAZON FAN Age (ka) Nann znes CN 15b Istpe stages 2-4 West WMTD 941 Pu Or 936 Am-Br Upper 935 Aq Levee Bl Cmplex Ch 6 Ye Ch 5 BMTD EMTD East 0 m 200 CN 15a 6 Middle Levee Cmplex URMTD Lwer Levee Cmplex Bttm Levee Cmplex CN 14b 8 sea level highstand pelagic sediment 0 km 50 mass-transprt depsit sandy unit Figure 2. Schematic crss sectin f the Amazn Fan thrugh selected ODP sites shwing stratigraphic setting f the MTDs and stratigraphic crrelatin with istpic stages and nannfssil stratigraphy. Names f turbidite systems (Ch 6, Ch 5, Or, Ye, Bl, Pu, Aq, Am-Br) are summarized in Table 1. Mdified frm Fld, Piper, Klaus, et al., 1995, Leg Synthesis, fig. 1). Purpse This paper addresses three questins, based n seismic-reflectin, cre, and dwnhle lgging data: (1) What is the age f the MTDs n the Amazn Fan? Des the age prvide any insight int what triggered the mass-transprt events, and was there any relatinship t sea-level change? (2) What is the surce area fr the sediment in the MTDs? (3) What are the transprt prcesses invlved in the masstransprt events? Is each depsit a single r a multiple event? The brader implicatins f each f these issues t the mre general questin f the rigin f MTDs n cntinental margins are als cnsidered. Stratigraphic Nmenclature n the Amazn Fan Table 1. Summary f stratigraphic nmenclature f the Amazn Fan. Upper Levee Cmplex Amazn (Am) Brwn (Br) Aqua (Aq) Purple (Pu) Blue (Bl) Yellw (Ye) Channel 5 (Ch 5) Orange (Or) Channel 6 (Ch 6) URMTD and BMTD Middle Levee Cmplex Red (Re) Lwer Levee Cmplex Gld (G) Green (Gr) Lime Gray Bttm Levee Cmplex Nte: Frm Damuth et al. (1983) and Manley and Fld (1988). The Amazn Fan has aggraded principally by the depsitin f thick levee depsits frm a turbidity-current channel. Peridic channel avulsin has resulted in the depsitin f a series f verlapping channel-levee depsits. Previus wrkers n the Amazn Fan have assigned clr names t all the recgnized channel-levee systems (Table 1; Fig. 2), with the exceptin f the yungest, which is termed the Amazn Channel-levee System, and tw systems n the eastern fan, termed Channels 5 and 6. On the upper fan, upstream frm any particular avulsin pint, tw successive channel-levee systems may be cntiguus, as is the case f the Amazn and preceding Brwn levee in Figure 2. Individual channel-levee units have been gruped int larger levee cmplexes: the Upper, Middle, Lwer, and Bttm levee cmplexes (Fig. 2). The prprtin f sheet sands increases dwnfan. Sandy units interbedded with channel-levee systems appear as high-amplitude reflectin packets (HARPs) n seismic reflectin prfiles. They were interpreted by Fld et al. (1991) as the first turbidite depsits fllwing an avulsin, acrss which the new channel-levee system prgraded. HARPs are thus assigned the same clr names as the verlying channel-levee depsits (Fig. 3). Chrnlgic cntrl fr this stratigraphic sequence is discussed elsewhere in this vlume. Briefly, framinifers, xygen istpes, and palemagnetic data prvide chrnlgy fr shallw cntinuusly sedimented sequences. Imprtant markers are the dwncre reappearance f the framinifer Pulleniatina bliquilculata at ~40 ka (Maslin et al., this vlume), the Lake Mung palemagnetic excursin at 32 ka (Ciswski and Hall, this vlume) and the heavy istpe peak f the last glacial maximum at 18 ka (Shwers et al., this vlume). Interglacial depsits are lithlgically distinct, cnsisting f hemipelagic calcareus clay, and cntain distinctive warm framinifer assemblages. These depsits can be assigned t nannfssil Znes CN15b (0 85 ka), CN15a ( ka) and CN14b ( ka), althugh the presence r absence f diagnstic species may be influenced by paleenvirnmental cnditins (Maslin and Mikkelsen, this vlume). The presence f istpe Stage 5 depsits at Sites 942 and 946 was cnfirmed by the presence f the Blake palemagnetic event. DISTRIBUTION Surficial Mass-Transprt Depsits The EMTDs and WMTDs have been mapped frm GLORIA lng-range sidescan snar, 3.5-kHz, and seismic-reflectin prfiles. Damuth and Embley (1981) described the 3.5-kHz acustic echcharacter f these surficial slump/debris flw cmplexes n the fan. The upslpe prtin f the WMTD (their central debris flw ) shws a hummcky acustic echcharacter (darker tne in Fig. 1), and they interpreted the regin bunded by scarps between 700 and 1500 meters belw sea level (mbsl) as a zne f slumping and sediment remval. The head f the WMTD is bunded by scarps up t 200 m high, frming a 15- t 25-km-wide valley subparallel t the 111

4 D.J.W. PIPER ET AL. Figure 3. Stratigraphic sectins f all sites penetrating buried MTDs shwing key stratigraphic markers. Amazn Canyn (Fig. 1). Lenses f acustically transparent material up t 75 m thick ccur within this valley (Damuth and Embley, 1981). Dwnslpe, the WMTD has a smther surface, althugh lcally with hummcky relief f 5 10 m. Within the WMTD, Damuth and Embley (1981) nted patches f acustically layered material, which they interpreted as turbidity current depsits generated at the same time r after the mass-flw event. In the vicinity f Site 941 (at 3400 mbsl), the depsit has a blcky surface with ~5 m f relief. Dwnslpe, blcks appear t be larger (10 15 m relief) befre an abrupt diminutin in thickness with a cncave up prfile and a transparent acustic character t a distinct snut at 3600 mbsl (Fld, Piper, Klaus, et al., 1995, Site 941 chapter, fig. 4). Seismic-reflectin data frm the WMTD indicate that scarps bunding the zne f sediment remval are assciated with defrmatin resulting frm rising diapirs r ther defrmatins caused by large-scale gravity tectnics (Figs. 4, 5). Immediately suth f the scarps, Manley and Fld (1988) identified a bttm-simulating reflectin (Fig. 1), suggesting the presence f gas hydrates. A wedgeshaped bdy f relatively lw acustic amplitude in the scar area is interpreted as an ld channel-levee system that was disrupted by the piercing diapirs. Seismic-reflectin data shw that the WMTD has a distinct eastern edge, abruptly terminating with a steep ramp against the lw slpe f Amazn Channel levee (Figs. 1, 6C, E). Seismicreflectin data shw that the upslpe part f the WMTD verlies the 112

5 MASS-TRANSPORT DEPOSITS OF AMAZON FAN SSW NNE 3 WMTD headscarp 0 10 km TWTT (s) buried diapir 4 buried diapir Figure 4. Lngitudinal seismic-reflectin prfile f the upper part f the WMTD, shwing the headscarp and relatinship t buried diapirs. (Farnella 81-5, 10 January 1982, ). Prfile lcated in Figure 5. 6N O Fig 12 URMTD Fig 11 Middle Fan BMTD 5N O Fig 10 Fig 9 Fig 13 WMTD Fig 6A Fig 6B Fig 6F Fig 6E Fig 6D Amazn Channel Fig 8 EMTD 4N O Fig 6C Fig 7B shelf break O 49 W Amazn Fig 4 Canyn 500 m 1000 m Fig 7A 2000 m Upper Fan O O 48 W 47 W 3000 m Figure 5. Map shwing lcatin f seismic prfiles illustrated in this paper. Dashed lines are thse prfiles used t identify reginal extent f the BMTD and URMTD. 113

6 D.J.W. PIPER ET AL. Figure 6. Series f strike (slpe-parallel) seismic-reflectin prfiles illustrating the eastern margin f the WMTD. Prfiles lcated in Figure 5. h = hummcky; s = smth; Am = Amazn Channel; cc = change curse. Hrizntal line indicates extent f the WMTD. 114

7 MASS-TRANSPORT DEPOSITS OF AMAZON FAN W Abandned Channel-Levee System E 3.5 EMTD TWTT (s) km W EMTD E 4.0 TWTT (s) Figure 7. Tw seismic-reflectin prfiles acrss the western margin f the EMTD, shwing the bunding ridge near the upper limit f the depsit (Farnella 81-5, and UCT, 11 January 1982). Prfiles lcated in Figure Amazn Channel-levee System (Pirmez, 1994, p. 352). On the dwnslpe part f the WMTD, high-frequency 3.5-kHz reflectin prfiles shw a thin veneer f sediment with parallel reflectrs that thins westward frm the distal end f Amazn Channel abve the WMTD (Pirmez, 1994, p ). This veneer indicates that the WMTD was emplaced during the late stages f activity within the Amazn Channel, and may pssibly represent several verbank depsitinal events. The WMTD has a maximum thickness f ~150 m, and the depsit is at least 100 m thick ver much f its extent. Ttal vlume is apprximately 2000 km 3. The vlume f missing sediment in the zne f sediment remval is perhaps km 3, suggesting that sediment has als been erded frm beneath the WMTD. The depsit tends t bury underlying tpgraphic irregularities, such as valleys between channel-levee systems (Fig. 4; see als fig. 19 f Damuth et al., 1988), and the surface f the WMTD rises nly slightly ver tpgraphic highs. Apparent verflw frm ne valley t the next is prbably a cnsequence f parallel flw dwn bth valleys and spillver at valley bends. The EMTD mapped n the fan surface by Damuth and Embley (1981) als has a zne f sediment remval assciated with an ancient channel-levee system (Fig. 7A). The upslpe part f the EMTD, shallwer than 3400 mbsl, is characterized by hummcky tpgraphy, bserved as verlapping hyperblae n 3.5-kHz reflectin prfiles. Unlike the WMTD, n clear ersinal headscarp is recgnized in acustic prfiles; rather, the upslpe limit f this zne is characterized by a sinuus, westward-facing steep slpe f stratified sediment that represents an ld levee crest (Fig. 7B). Seismic-reflectin data acrss this area shw a triangular-shaped wedge, which is acustically semi-transparent, suggesting that the material in the EMTD riginated frm failure f the eastern levee f this ld channel system. The depsits dwnslpe frm 3400 mbsl are acustically transparent and laterally thin gradually ver the underlying fan sediment. The blcky surface f the EMTD in 3.5-kHz prfiles can be recgnized at least t the 3700 m isbath. On GLORIA sngraphs, the depsit appears mre blcky than the WMTD. It is difficult t determine thickness frm available seismic data, s the ttal vlume can nly be estimated as ~1500 km 3. Buried Mass-Transprt Depsits Buried MTDs are recgnized reginally frm seismic-reflectin prfiles and include the URMTD and the BMTD. The URMTD ex- 115

8 D.J.W. PIPER ET AL. tends ver a large area f the present central Amazn Fan. The initial interpretatin f aerial distributin f the Unit R depsit and estimates f its vlume by Manley and Fld (1988) are inaccurate, because they included thse depsits that are nw identified as the BMTD (Manley and Fld, 1988). Recently btained seismic-reflectin prfiles (Figs. 8 11) allwed reevaluatin f the earlier data t delineate the bundaries f the URMTD and the BMTD (Fig. 1). The upfan limit f the URMTD is near the 2000 m isbath, which is 50 km farther than previusly published. There are uncertainties in the interpretatin f these buried MTDs in seismic-reflectin prfiles, because lcally the dipping levee sequences and MTDs have rather similar acustic character (Fig. 9). The interpretatins shwn in seismic prfiles and the ispach map (Fig. 12) represent the maximum extent f the URMTD. A mre cnservative interpretatin wuld identify sme f the thick depsits assigned t the eastern part f URMTD (e.g., the central part f Fig. 9) as levee depsits verlying HARPs that are interpreted as sand sheets by Fld et al. (1991). Given this interpretative uncertainty, sme care must be used in drawing far-reaching inferences frm seismic-reflectin data frm the buried MTDs. In general, the URMTD tends t have a nearly flat tp with a irregularly shaped base where it fills tpgraphic lws. It is characterized n acustic prfiles as a semi-transparent t chatic acustic facies with sme cherent reflectins within it (Manley and Fld, 1988; Pirmez, 1994). The URMTD is thinnest alng its western bundary, which can be apprximated by the eastern edge f the surficial WMTD. It wedges ut and verlies the Lwer Levee Cmplex r the Middle Levee Cmplex alng this western bundary. It thickens t the east (Fig. 9), and the eastern bundary is apprximated by the Blue Channel, althugh n the upper fan, the MTD is bserved lcally t extend eastward f the Blue Channel axis. The URMTD thins r is absent near the vicinity f the axis f the underlying Green levee system f the Lwer Levee Cmplex. On the midfan, where the URMTD thickens t the east, it abruptly changes acustic character frm semi-transparent t a cherent set f parallel t subparallel reflectins, and appears similar t a HARP (eastern part f seismic prfile in Fig. 9). This abrupt change has been interpreted t be an apparent fault (Pirmez, 1994), but it culd alternatively be the result f extensive ersin f Figure 8. Slpe-parallel single-channel reflectin prfile acrss the upper fan shwing the pssible cnnectin f the URMTD and BMTD. Prfile lcated in Figure 5. (Farnella UCT, 12 January). W Am 2 km URMTD?HARP E TWTT (s) Or URMTD Bl? base f Blue Figure 9. Seismic-reflectin prfile acrss upper fan (Line 1 f Pirmez, 1994), shwing distributin f the URMTD (duble arrws) and pssible interpretatins f the relatinship between the URMTD and the Blue Channel-levee System. Dashed line shws base f Blue Channel-levee System frm Pirmez (1994), but underlying HARP may be part f the same system. Am = Amazn, Bl = Blue, Or = Orange Channel-levee System (cf. Table 1). Prfile lcated in Figure

9 MASS-TRANSPORT DEPOSITS OF AMAZON FAN 4 W 2km E TWTT (s) Or Am URMTD dipping reflectins 5 Figure 10. Seismic-reflectin prfile shwing distributin f URMTD (duble arrws) n the middle fan (Line 3 f Pirmez, 1994). Dipping reflectins may be part f channel-levee system r may be allchthnus blcks. Am = Amazn, Or = Orange Channel-levee System. Prfile lcated in Figure 5. Figure 11. Seismic-reflectin prfile shwing distributin f the URMTD (duble arrws) acrss lwer middle fan (Line 7 f Pirmez, 1994). Am = Amazn, Pu = Purple, Gr = Green Channel-levee System. Prfile lcated in Figure 5. URMTD when the Blue Channel prgraded int this area (see belw). Prgressively farther dwnfan, URMTD still thickens t the east, but it appears t merge with levees f the Orange 2 (Figs. 10, 11) r in sme places, the Blue Channel system. Near the lwer part f the fan, URMTD thickens t the east and again merges with HARPlike depsits, which appear t be related t the Blue Channel-levee System (Fig. 13). The URMTD extends ver km 2 and has a vlume f ~610 km 3. The BMTD verlies the Bttm Levee Cmplex. Its aerial distributin is nt as well cnstrained as the URMTD because f sparse seismic-reflectin data. The acustic facies f the BMTD are similar t the URMTD, mstly semi-transparent t chatic in nature, flat tpped and filling valleys between channel-levee systems. The western extent f the BMTD appears cincident with the Yellw Channel and the Blue HARP unit farther dwnfan (Figs. 1, 13). As was bserved n the eastern edge f the URMTD, the BMTD n the middle fan is semi-transparent, thins, and merges abruptly int HARP-type reflectins n its western edge. Its upfan extensin appears t include a regin f hummcky reflectins near the 2000-m isbath (visible n Farnella 81-5, 4 January 1982, : nt illustrated here). These hummcky reflectins culd indicate the surce f failure fr this depsit r culd be an acustic characteristic f the Yellw Channel, as the seismic line is nearly directly ver the channel within this regin. The BMTD extends 200 km nrth frm its surce area n the cntinental slpe. Pckets f apparent MTDs are bserved farther dwnfan, but they cannt be directly crrelated with the BMTD. The eastern extensin f this unit is speculative. The main cnstraint is that it des exist 20 km east f Site 931 and is bserved n a nrthsuth seismic line just west f the surficial EMTD (Fig. 1). The BMTD becmes very thin near Site 933 (Fig. 13), which is lcated just west f a regin that is a highly reflective zne n GLORIA sidescan snar imagery. This reflective zne crrelates with a near-surface expressin f a buried channel-levee belnging t the Bttm Levee Cmplex ( 120 meters belw seaflr [mbsf]); see Fig. 13D), which prduces a m step in the bathymetry frm east t west ver a hrizntal distance f 3.5 km. Seismic-reflectin prfiles btained frm the JOIDES Reslutin (Fld, Piper, Klaus, et al., 1995, Site 931 chapter, fig. 3) shw a hummcky facies directly ver this buried channel-levee. T the east f this step, the BMTD has a transparent facies averaging 120 m thick (sampled at Site 931), that can be traced laterally and dwnfan. T the west f the step, the unit thins dramatically t less than 60 m at Site 933 and then wedges ut, being verlain by the Channel 6 levee f the Upper Levee Cmplex. The dwnfan limit f the BMTD is within the middle fan area and a first-rder apprximatin f its areal extent is km 2. The questin remains as t whether r nt the URMTD and the BMTD are the same unit. Only three seismic-reflectin lines running east-west are available t determine whether these tw units are equivalent (Fig. 5). The tw nrthern lines bth demnstrate the thickening f the URMTD t the east and the thinning f the BMTD t the west int the area f HARP-type reflectins. The suthernmst line (Fig. 8) is near the upper reaches f the Blue Channel where the HARP unit is nt well develped. This reflectin prfile suggests that these tw MTDs may be the same, and Figure 12 shws a pssible scenari relating depsitin f these tw MTDs. Prir t 40 ka (Piper et al., Chapter 39, this vlume), the surface tpgraphy f the fan had tw majr bathymetric highs: the Bttm Levee Cmplex in the east 117

10 D.J.W. PIPER ET AL. 600'N 540'N 520'N Crest f Green Levee and the Green levee f the Lwer Levee Cmplex t the west (Fig. 13A), representing the last active channel-levee system fr their respective levee cmplex. One r several mass-transprt events initiated n the upper slpe regin flwed dwn between these bathymetric highs. Near the bathymetric highs, the mass-transprt flw was either deflected away frm r lcally partially flwed ver these features (Fig. 13B). The lw f bth sides f the channel-levee system f the Bttm Levee Cmplex pnded n the flanks f this bathymetric high with a thicker sectin preserved n the steeper eastern side than the western. Near the Green Channel-levee System f the Lwer Levee Cmplex, the mass-transprt flw was able t verflw the levee crest, but the depsit rapidly thinned t the west. As the Upper Levee Cmplex began t frm, it develped n tp f the MTD. When Channel 6 and Yellw Channels prgraded acrss the lwer middle fan, they erded the MTD leaving nly small pckets n the western side f the Bttm Levee Cmplex. The develpment f the Blue Channel farther t the west, where the MTD was thicker, remved sme f the MTD by ersin (Fig. 13C). The HARP assciated with the develping Blue system wuld have been limited laterally by the limits f this ersin. The subsequent develpment f the Blue levee ver that HARP and merging with the URMTD wuld give the appearance f a thickening URMTD that abruptly changes int highamplitude parallel reflectins (Fig. 9). The lwer reslutin f sme f the seismic-reflectin prfiles, hwever, prevents definitive reslutin f the questin f whether the tw sampled MTDs (URMTD and BMTD) are the same. If these units are the same, then their emplacement was mdulated by bttm tpgraphy, and they were subjected t subsequent mdificatin by extensive ersin. Evidence fr Ersin Beneath Mass-Transprt Depsits 500'N 440'N 0.2 In several areas, seismic-reflectin prfiles shw lcal ersin beneath MTDs. At Site 935, the crest f the underlying levee appears t have been planed ff (Fld, Piper, Klaus, et al., 1995, Site 935 chapter, fig. 3). At Site 933, seismic-reflectin prfiles indicate that the surface f the Bttm Levee Cmplex is lcally quite irregular (Fld, Piper, Klaus, et al., 1995, Site 933 chapter, fig. 2), als suggesting pssible ersin. A seismic-reflectin prfile at Site 941 (Fld, Piper, Klaus, et al., 1995, Site 941 chapter, fig. 2) shws a very irregular base t the MTDs and the sediment f the underlying Purple Channel-levee System appears t be defrmed. Reginally, hwever, the base f the MTDs appears sharp in seismic-reflectin prfiles and shws little evidence fr ersinal truncatin f underlying reflectins, as shwn in Figures 9 11 fr the base f the URMTD. Lcally, hwever, the URMTD terminates abruptly against pre-existing channel-levee sediments, fr example in seismic Line 5 illustrated as fig. 16 f Pirmez and Fld (1995). We cannt agree n whether these abrupt cntacts reflect pnding against preexisting tpgraphy r whether they prvide evidence fr in situ failure cntributing t the MTDs; prbably bth situatins exist. Summary 420'N 47 40'W 47 20'W Figure 12. Ispach f the URMTD in tw-way traveltime (in secnds). Fr lcatin, see Figure 5. The URMTD is thickest t the east, reaching a maximum f 0.26 s. It is thinnest t the west and alng the crest f the Green levee system (cf. Table 1). The gridding, visualizatin, and vlumetric analysis f the URMTD data were accmplished thrugh the use f Earth Visins sftware created by Dynamic Graphics Inc., Alameda, Califrnia. Observatins summarized abve n bth surficial and buried MTDs depsits suggest the fllwing general characteristics inferred frm acustic mapping data: 1. MTDs are derived frm surce areas in water depths f mbsl where there are thick channel-levee depsits. The cntinental slpe t 1500 mbsl is cut by diapirs and in places shws bttm-simulating reflectins. 2. The upslpe parts f surficial MTDs appear very blcky n high-reslutin acustic prfiles. Blckiness decreases dwnslpe, but nly the extreme distal parts f the depsits appear smth. 118

11 MASS-TRANSPORT DEPOSITS OF AMAZON FAN W E D Amazn Site 933 Site 931 Middle Levee Cmplex Upper Levee Cmplex URMTD Lwer Levee Cmplex Bttm Levee Cmplex BMTD C Blue HARP Orange Blue Ch 6 Middle Levee Cmplex Lwer Levee Cmplex Bttm Yellw Levee Cmplex B Middle Levee Cmplex Lwer Levee Cmplex Bttm Levee Cmplex A Green Lwer Levee Cmplex Bttm Levee Cmplex Middle Levee Cmplex Mass-transprt depsit Figure 13. Schematic prfiles acrss the middle fan shwing the develpment f the URMTD and BMTD thrugh time. Prfile lcated in Figure 5. A. Fan mrphlgy befre the emplacement f the MTD. The Green Channel-levee f the Lwer Levee Cmplex and the unnamed levee f the Bttm Levee Cmplex were tw prminent bathymetric highs n the fan surface. B. Depsitin f the MTD, which pnded between the tw prminent bathymetric highs and thinned n the uter edges f these features. C. The emplacement f Channel 6 and Yellw Channel-levees erded mst f the MTD, leaving nly small pckets. The emplacement f Blue Channel erded part f the mass-transprt unit and subsequently filled it with a sandy HARP unit assciated with the Blue Channel. The Blue levee prgraded ver the HARP unit and verlies the mass transprt unit, apparently thickening the URMTD t the east. D. Present-day Amazn Fan mrphlgy based n seismic-reflectin prfiles and Leg 155 results. 3. As MTDs flwed dwnslpe, they appear t have lcally mved acrss the crests f channel-levee systems, but they generally fill valleys between channel-levee systems and have rather flat tps. The URMTD generally shws mre surface relief than the surficial MTDs. Ridges frmed by buried levee crests cmmnly mark the lateral bundaries f MTDs, but in places distinct lateral ramps are visible. Dwnfan, the margins f MTDs have been erded during the depsitin f HARPs assciated with prgrading channel-levee systems. 4. In places, seismic-reflectin prfiles indicate that the lwer cntact f MTDs is ersinal. In particular, levee crests lcally appear planed ff. Analgus bservatins have been reprted 119

12 D.J.W. PIPER ET AL. in slides elsewhere (Trincardi and Nrmark, 1989). Lateral cntacts f MTDs with lder sediments may als shw evidence fr sediment failure. LITHOLOGY Intrductin Our interpretatin f the character f the MTDs is based principally n descriptin f the split cre face and shipbard determinatins f physical prperties and micrfssils. Detailed descriptins are available in the individual site chapters f Fld, Piper, Klaus, et al. (1995) and are nt repeated here, but salient features are summarized in Figure 14 and Table 2. Dwncre variatins in these varius parameters are cmpared with wireline lgs, which are available fr parts f the buried MTDs. Lithlgic variability is determined frm cre descriptins f the split cre face (Fig. 15), dwncre variatin in physical prperties and micrfssils, and frm lgging data. Characteristics f sediment surce are prvided by labratry studies f sediment samples (e.g., grain size, gechemistry), micrfssil cntent, and sme physical prperties. The Frmatin MicrScanner (FMS), which measures micrresistivity f the brehle wall frm fur rthgnal pads with a reslutin f <1 cm, has been particularly useful fr analyzing the in situ character f the sediment and the defrmatin assciated with the emplacement f the MTDs. In the FMS images in Figure 16, mre resistive clasts and beds (typically f silt r sand) appear as a lighter tne. Crrelatin f a single bed between the fur pads allws the dip and azimuth f bedding t be calculated (tabulated n the right-hand side f the images in Fig. 16). Many FMS images btained during Leg 155 are degraded as a result f pr pad cntact against a rugse brehle wall (Fig. 16E). Frtunately, ver mst f the intervals lgged the images are f sufficient quality in at least ne pad s that ne can determine an electric facies that is clsely related t the nature f bedding and grain size in the sediment (see Pirmez et al., this vlume). Here, we present an initial analysis f the FMS images within the URMTD and BMTD intervals. In particular, we measured the rientatin f numerus beds and bed bundaries and determined the depths at which abrupt changes f dip/ azimuth ccurred. The main bjective is t make a preliminary assessment f defrmatin structures within the MTDs, which was nt feasible with the cre material alne n the ship. Lithlgic Variatin Thrugh Individual Mass-Transprt Depsits Mst f the MTDs are inferred t cnsist f mud blcks with a variety f physical scales. These scales can be inferred frm split cre surface, althugh biscuit defrmatin (Fld, Piper, Klaus, et al., 1995, see Explanatry Ntes ) makes this difficult in unifrm textured sediment (Fig. 15C). Analysis f abrupt changes in dip/azimuth f bedding frm FMS images allw blck bundaries t be identified (Fig. 16A) and smaller blcks r clasts can be recgnized in a single FMS image (Fig. 16B). In sme cases, blck size can be inferred frm the unifrmity f micrfssil assemblages and frm the cntinuity f index and strength prperties (e.g., Fig. 17). Resistivity, gamma and ther wireline lg data (Figs. 18, 19) can als be used t infer blck size and can be crrelated with shipbard measurement f physical prperties (Fig. 20). Such data shw that blck sizes lcally are n the scale f centimeters (Figs. 15D, 16B), whereas elsewhere the recvered sediment appears relatively unifrm ver tens f meters f thickness, such as the basal part f the MTDs in Sites 933 (Fig. 19) and 936 (Fig. 20). Althugh there is cnsiderable small-scale variability, it is pssible t classify the URMTD and BMTD int five main lithlgies (Fig. 14): 1. Intervals f unifrm mud typically m thick with rather unifrm characteristics (such as framinifer cntent, magnetic susceptibility, r uranium cntent). This unifrmity suggests that these intervals represent large blcks (Fig. 16A), f a scale similar t that inferred frm 3.5-kHz prfiles. In places, there are muddy r sandy intervals a few meters thick between these inferred blcks (Fig. 18). The large blcks lcally shw cmplete flds bth in cre sectin and FMS lgs (Fig. 16E). They cntain numerus small faults and evidence f shearing. In general, cres frm intervals with large blcks lack bvius matrix material and such blcks have the highest strength prperties and are fund at r near the base f MTDs. 2. Intervals with unifrm r gradatinal prperties in muddy sediment n a scale f a few meters, as fr example in unit B f the URMTD at Site 936 (Fig. 20) r the cyclical pattern nted at Site 941 (Fig. 17). Small flds are visible directly in FMS images (Fig. 16C) whereas larger flds can be detected frm dip reversals (203.5 mbsf in Fig. 16D). 3. Intervals where the variability in split cre surface, in lgs, and in physical prperties suggests blcks f sizes less than 1 m (Fig. 16B, F). This lithlgy is cmmner in the upper part f the MTDs, but is als fund at the base f the BMTD at Site 931 and at 253 mbsf at Site 935 (Fig. 14; Table 2). 4. Intervals with cmmn sand were generally prly recvered and brehle washut degraded lg data (e.g., Fig. 16B; 260 mbsf in Hle 936A in Fig. 18). 5. Intervals (e.g., at the tp f MTDs in Sites 931 and 935) where the sediment is distinctive because it appears highly flded and cnsists f mud with many silty laminae, in cntrast t the mre unifrm muds elsewhere in the MTDs. In places, the character f successive blcks appears repetitive. At Site 936, index prperty data and lg data shw five cycles frm 170 t 225 mbsf (Fig. 20). At Site 941, there is an alternatin f glacial and interglacial framiniferal assemblages in the blcks, with lwer water cntent in the interglacial sediments (Fig. 17). The buried URMTD and BMTD tend t have carser sediment near the base and finer sediment higher in the depsit, with several fining-up cycles visible at Sites 935 and 936 (Manley et al., this vlume). Matrix material is generally difficult t identify in the split cre face. Where small (<10 cm) clasts are present, a grey muddy matrix similar t the predminant blck lithlgy is visible (m in Fig. 15E) that cntains elngate clr patches suggesting shear. Individual less cmpacted blcks appear defrmed and penetrated by strnger blcks (e.g., at 99 cm in Fig. 15E, where blck c penetrates laminatin in the verlying defrmed blck). Matrix material may als be present between larger defrmed blcks (e.g., at 67 cm in Fig. 15F). Overall, hwever, it is nt pssible t systematically identify matrix. In many cases individual blcks appear t be in direct cntact with ne anther withut intervening matrix (e.g., at 88 cm in Fig. 15D). Defrmatin f Sediments Because f cring disturbance assciated with extended cre barrel (XCB) cring f the buried MTDs n the Amazn Fan, the FMS micrresistivity images btained at Sites 935, 936, and 944 (fr the URMTD) and at Site 933 (fr the BMTD) are f particular imprtance in understanding the characteristics f the defrmatin assciated with the emplacement f these depsits. During Leg 155, the XCB cres frm the MTDs were cmmnly characterized as having wdgrain facies (Fig. 15C), which were tentatively interpreted as resulting frm biscuiting f beds that dipped at varius angles (Fld, Piper, Klaus, et al., 1995, see Explanatry Ntes ). This defrmatin severely handicapped the shipbard party frm assessing the nature f the defrmatin within the MTDs. The nature f the defrmatin within the URMTD and BMTD can be assessed by examining the changes in dip and azimuth f the sediment layers measured by the FMS. In many cases, beds dipping in ne directin are superpsed by beds dipping at 180, suggesting the presence f flded structures (Fig. 16A). Multiple changes in azimuth 120

13 MASS-TRANSPORT DEPOSITS OF AMAZON FAN URMTD BMTD 200 Site 944 A 160 Site 936 A 200 Site 935 A 220 Site 931 A B 100 Site 933 A Depth (mbsf) B C D 16F 16E B C 16C 16D B C 16B C D B 16A 260 D E 340 E 1. Unifrm mud, large blcks 2. Variable mud, inferred m-size blcks 3. Abundant cm t dmsize blcks 4. Sand and silty mud 5. Cntrted mud with silty laminae Figure 14. Lithlgic interpretatin f the URMTD and BMTD. Lithlgies 1 5 are described in detail in text. A thrugh E are units defined in Table 2. Small bars indicate lcatin f images in Figure 16A thrugh 16F. Table 2. Subunits f mass-transprt depsits. Subunit Depth (mbsf) Distinctive prperties Hle 931A A Flded blcks f mud with silt laminae. Cmmn P. bliquilculata. B Sandier interval, cmmn mud clasts. C Large mud clasts. Plant fragments cmmn. Scatter in index prperties. P. bliquilculata absent in mst samples D Large mud clasts. At tp, change in magnetic susceptibility and water cntent prfile. Unifrm index prperties, break at 302 mbsf. P. bliquilculata cmmn in mst samples. E Mud blcks with decimeter- and centimeter-sized clasts including carbnate-rich muds. Hle 933A A Mud with decimeter-sized clasts, sme sand. N P. bliquilculata. FMS shws change in average dip at 108 mbsf (Fig. 21A). B Large mud blcks. Lwer magnetic susceptibility, higher U and higher N than in upper part f mass-transprt depsit. P. bliquilculata present. [Nte that index prperties (Fig. 27) and resistivity, velcity, prsity, and density lgs (Fig. 19) shw a change at mbsf. FMS images shw higher dips with higher scatter belw this level (Fig. 21A)]. Hle 935A A Flded blcks f mud with silt laminae. Cmmn P. bliquilculata. B Mud with decimeter- and centimeter-sized clasts, including carbnate-rich mud. C Large mud blcks, sme sandier intervals, rare smaller clasts. Few P. bliquilculata thrughut. Hle 936A A 154.5? 172 Abundant decimeter- and centimeter-sized mud clasts, wd, shell fragments, and fine pebbles. B? Large mud blcks. Five fining up cycles n gamma lg, sme crrespnding water cntent prfiles. P. bliquilculata cmmn in upper part, becming rarer lwer. C Large mud blcks. Abrupt increase in strength. Abundant bathyal framinifers. P. bliquilculata increases dwn-unit. D Sandier interval. Abundant P. bliquilculata. E Large mud blcks. Highest resistivity and gamma. Bathyal framinifers rare. Few P. bliquilculata. Hle 944A A Mud blcks, flded, prbably decimeter-sized. Rare smaller clasts. Rare bathyal framinifers. N P. bliquilculata. B Interval with cmmn carbnate-rich and ther decimeter-sized clasts. C Sandier interval. D Mud blcks. Different clr frm A. N bathyal framinifers, but cmmn P. bliquilculata. 121

14 D.J.W. PIPER ET AL. B A C Figure 15. Cre phtgraphs f selected intervals. Phtgraphs A D are als imaged by FMS lgs. A. Flded sediment, interval A-18X-1, cm, prbably crrespnding t the upper part f Figure 16A. The lack f flding in the FMS image suggests that this flding results frm drilling defrmatin, as interpreted by Fld, Piper, Klaus, et al. (1995, p. 211). B. Highly biturbated sandy mud interval (Sectin A-28X-4) crrespnding t a large blck in Figure 16B. C. Wdgrain texture in muds frm Sectin A-19X-6, resulting frm biscuit rtatin in flded blcks. This interval crrespnds apprximately t FMS image in Figure 16C. directin prbably indicate the presence f several blcks (Fig. 16E). Gradual changes in dip uphle are als bserved, suggesting perhaps gentler flded structures r larger scale flds. Small-scale flding that culd be bserved within a single pad is rare (Fig. 16C). The apparent scarcity f small-scale flding wuld indicate that the sediment within the MTDs may have been t stiff t generate such small-scale flds, and it must have been transprted as fairly large blcks, which were mre r less cherent. Within sme intervals, smaller blcks n a decimeter scale are visible (Figs. 16B, F) and crrespnd t rapid changes in lithlgy in recvered cre (Fig. 15D). In Hle 933A, a highly flded cre interval (Fig. 15A) was derived frm an interval that shws n flding n FMS, cnfirming the shipbard interpretatin that this particular flding resulted frm drilling disturbance. The MTDs differ frm autchthnus channel-levee depsits in shwing dips greater than 30 with azimuths that vary by up t 180 ver shrt intervals. Dip-azimuth ( tadple ) plts fr Hles 933A, 936A, and 944A are shwn in Figures 21A thrugh 21C. Fr Site 933, we shw in additin a plt f the azimuth f measured beds vs. depth (Fig. 21D). In each f thse plts, we marked apparent bundaries where either bed azimuth r dip changed significantly (greater than ~45 fr azimuth). These bundaries separate znes where bed 122

15 MASS-TRANSPORT DEPOSITS OF AMAZON FAN 29.1 kpa x? matrix cre disturbance 90 C C Fault 100 C M E Fld G Fault D Fault F x 26 kpa matrix Figure 15 (cntinued). D. Several small clasts in Sectin A-25X-5. This interval is just belw the image in Figure 16F. E. Pht and interpretatin shwing small clasts and prbable matrix (M) in Sectin A-4H-5. Clasts include relatively rigid light-clred clasts with 5% CaCO 3 (C) and mre defrmed clr-banded muds F. Pht f defrmed clr-banded mud blcks shwing verall fld defrmatin and small-scale fault ffset frm Sectin A-3H-7. Underlying lighter mud may represent matrix. G. Series f small faults in clr-banded mud clast frm Sectin A-6H

16 D.J.W. PIPER ET AL. Figure 16. Examples f particular features frm Frmatin MicrScanner lgs. In all plts, thin wavy lines indicate bedding surfaces that can be crrelated between pads. Numbers n right are dips and azimuths determined frm these bedding surfaces. A. Hle 933A, mbsf. Nte 180 change in azimuth acrss fault r blck bundary. 124

17 MASS-TRANSPORT DEPOSITS OF AMAZON FAN Figure 16 (cntinued). B. Hle 935A, mbsf, shwing abundant clasts and sandy interval. dip and azimuth tend t grup with a mre r less cnstant value. We interpret these znes t represent blcks that mved in a cherent fashin within the MTD. Further analysis is needed at this pint t determine the axis f flds and the exact nature f faults within the unit. The apparent blck bundaries ccur at intervals ranging frm a few meters up t 20 m. At Hle 933A, the blck dimensins appear t decrease uphle, but n such trend is visible at Sites 936 and 944. A summary plt f the azimuth f the measured features within the MTDs fr each hle is presented in map view in Figure 22. At Site 933, mst beds dip t the west-suthwest with secndary mdes t the sutheast and t the east. This crrespnds t the apparent dip f the Bttm Levee Cmplex levee at Site 933 as seen in a seismic-reflectin prfile crssing the site (Fld, Piper, Klaus, et al., 1995, Site 933 chapter, fig. 2). Sites 935, 936, and 944 display the majrity f beds dipping t the suth-suthwest, sutheast and east-sutheast, respectively. At Sites 935 and 936 the verall azimuth f the bed-dips is ppsite t the surface dwnslpe directin. At Sites 936 and 944, the URMTD verlies the western flank f the Red Channellevee System and the azimuth f dipping beds crrespnds t the apparent dip f the Middle Levee Cmplex at these lcatins. The dips determined frm seismic-reflectin prfiles are t the east at Site 936, cnsistent with the true dip directin bserved frm the FMS data at this site. Site 935 is lcated suth f the Gld-Green bifurcatin, a prminent tpgraphic high, and the azimuth f dip fr the MTD crrespnds t the bttm tpgraphy at the time it was emplaced. Sme bservatins f defrmatin can be made in split cre face. Matrix material (m in Fig. 15E) lacks internal stratificatin, but may cntain streaked blebs f different clr r grain size. Many individual blcks shw shearing (Fig. 18E) and flding (Fig. 18F), but are als ffset by brittle faults (Fig. 18F). There may be a cntinuum between defrmed less cnslidated blcks and true matrix in taking up shear within the MTD. Brittle faulting may in part develp during pst-depsitinal cnslidatin. Lithlgic Prperties f Clasts Grain-size data presented by Manley et al. (this vlume) shw that mst clasts are relatively clay-rich sediments, cmparable with upper fan levees. In general, samples are finer grained than the immediately 125

18 D.J.W. PIPER ET AL. Figure 16 (cntinued). C. Hle 936A, mbsf, shwing small-scale flding. underlying levees. Hwever, sme samples are indistinguishable in their grain size frm lcal levee sediments. The FMS images within the MTDs display a variety f electric facies (see Pirmez et al., this vlume), but are generally dminated by thinly laminated units (Fig. 16D). Thin resistive layers bserved n the images prbably crrespnd t silt and silty-sand intervals nted in many cres f URMTD (e.g., Fig. 15B). The resistive layers prvide gd markers that can be traced between pads fr measuring dips Lcally thicker resistive beds are bserved n the FMS images, ften cntained within the image f a single pad, and are interpreted as sandier intervals. The verall characteristics f the FMS images within the URMTD suggest that the electric facies f many f the 126

19 MASS-TRANSPORT DEPOSITS OF AMAZON FAN Figure 16 (cntinued). D. Hle 936A, mbsf shwing well-bedded silts and muds resembling thse fund in levee sequences. blcks is quite similar t channel-levee depsits examined n Amazn Fan. The images display typical characteristics f turbidite beds, such as grading (decreasing resistivity upward) and ersinal bases where thicker, resistive beds ccur. Sediments in the MTDs in general cntain a higher prprtin f benthic framinifers than d the autchthnus levee muds cred at the same site (see tables f framiniferal data fr individual sites in Fld, Piper, Klaus, et al., 1995). Many f the benthic framinifers appear t be mid- t upper bathyal, with rare uter shelf taxa (Vilela and Maslin, this vlume). Many MTDs cntain echinid spines. Biturbatin by echinids was recgnized nly at shallwer water sites such as 937, 938, and 942. Clasts f interglacial calcareus clay, 127

20 D.J.W. PIPER ET AL. Figure 16 (cntinued). E. Hle 944A, mbsf shwing abrupt cntacts and changes in azimuth between blcks. wherever recvered during Leg 155, are particularly distinctive because f their light clr. Many f these calcareus clays lack bathyal benthic framinifers and are therefre unlikely t have been transprted frm the cntinental slpe. At several sites, a few micrfssil samples are barren f bathyal benthic framinifers and cntain rare abyssal benthic framinifers (Samples A-25X-5, cm, and 27X-CC; A- 26X-CC and 28X-CC). These blcks presumably riginated in abyssal water depths. Clay minerals are nt generally diagnstic f surce, but the lwer part f the MTD at Site 931 has a distinctive chlrite that is nly fund in the levee sediment frm the Bttm Levee Cmplex during this leg. This supprts the lithlgic bservatin f the unusual ccurrence f small-scale blcks at the base f the depsit, suggesting that lcal ersin may have ccurred. Evidence fr Gas Hydrates Unequivcal evidence is lacking fr gas hydrates in sediment recvered during Leg 155. Extremely gassy sediment was recvered frm the WMTD at Site 941 that is assciated with lw-chlrinity pre water, interpreted as evidence f gas hydrates by Sh (this vlume). Shipbard chlrinity measurements were made at infrequent intervals: anmalusly lw values are fund in MTDs at Sites 941 (<500 mm), 936 (547 mm), and 933 (549 mm), but nt at ther sites. (Lw chlrinity ccurs in permeable HARPs at Sites 931 and 935.) 128

21 MASS-TRANSPORT DEPOSITS OF AMAZON FAN Figure 16 (cntinued). F. Hle 944A, mbsf, shwing abundant clasts. R = resistive clast (?hard clay). Crrelatin Between Hles In places, it seems pssible t crrelate specific features between nearby hles. At Site 941, crrelatin f cyclical variatins in framiniferal assemblages and water cntent appears t be pssible between the A and B hles, which are 500 m apart (Fig. 17). Magnetic susceptibility in the BMTD changes abruptly at 275 mbsf in Site 931 and 125 mbsf in Site 933 (Table 2; Fig. 14), but ther parameters (such as ttal nitrgen) d nt crrelate. Sites 935 and 936 bth have lng intervals f unifrm mud in the lwer part f the URMTD, suggesting that large blcks are present. In detail, hwever, the blcks are quite different at the tw sites. Unit C at Site 935 (Fig. 14) has abundant P. bliquilculata and Glbigerinides ruber and sparse bathyal benthic framinifers; in cntrast, the lwer part f URMTD at Site 936 has abundant bathyal benthic framinifers but P. bliquilculata and G. ruber are rare (Table 2). The basal part f URMTD at Site 944 is similar t that at Site 936 in lithlgy and many micrfssil prperties. Manley et al. (this vlume) suggest that there is a significant dwnfan increase in mdal size within the URMTD, frm ~7.2φ at Sites 935 and 936 t 5.8φ at Site 944. This change seems t represent higher percentages f sand (5% 20%; cf. 1% 5% upfan) at Site 944 and parallels a similar increase in autchthnus sediment. Similarities in the sequence f lithlgies between Sites 931, 933, and 935 supprt the suggestin frm seismic-reflectin data that the BMTD and URMTD may be crrelative. Unit A at Sites 931 and