L

Geo-Marine Letters (1989) 9:145-152 Geo-Marine Letters C 1989Spnnger-Vcrl

Evidence for Slope Instability and Current-Induced Sediment Transport, the RMS Titanic Wreck Search Area, Newfoundland Rise

P. Cochonat/ G . OBier/ and J . L. Mkhee

1IFREMER (Instirut Français de Recherche pour l'Exploitation de la Mer), Centre de Brest, BP 70, 29263 Plouzané, France, and ~IFREMER , Centre de Toulon, BP 330, 83507 La Seyne, France

Abstract touret and others 1974) and on the sou them t1an.k (Al­ lam a nd e thers 1981, 1987) of the soucheast The ftrSt map of the sea bed morphology and sedimentary fearures Newfoundland Ridge, but not within the srudied area wit.hin t.he RMS Titanic search area is proposed from the inter­ itself. The sediments recovered from the cores consist pretation of S AR side-scan sonar images. Downslope sedimencary of biogenic oozes overlying cenigenous sediments in­ fearures such as erosional furrows and crown scarps constituee a terbedded with turbidic sands and gravels, and ice rafted 7 km wide inscabilicy corridor. A large field (15 km') of asym­ debris. meoical sediment waves indicacing a downslope transport is iden­ tified. Currenc-induced fearures corresponding co associaced sand Three cruises were carried out in 1980, 198 !, and ribbons and barchan dunes resulting from t.he Western Boundary 1983 near and in the srudy area by Ryan (1982, 1983, Undercurrenc action are mapped. The morphology of the Titanic 1986). Bathymetrie and side-scan sonar surveys were Canyon is also precised from t.he SAR images. Finally, t.he origin compleced as weil during the three cruises, allowing of t.he sea bed fearures is discussed in an acrempt to link each bed the identification of a submarine canyon named Ti­ form co a sedimencary process. ranic Canyon, and unstable sedimentary bedforrns ap­ pearing as "crescent-shaped slump scars and debris Introduction flow aprons. " SeaMARC 1 sonar imagery also re­ vealed ~a lineaced bottom roughnwreck of the Titanic as part of a cooperative proj­ - (wavelength > 50 rn) and "migrating barchan dunes" ect berween. IFREMER (Institut Français_ de Re­ co the south of the Titanic Canyon. cherche pour l'Exploitacion de la Mer) and the Woods In 1985, the IFREMER SAR deviee (Systeme Hole Oceanographie Instirucion. Acoustique Remorqué, Deep Towed Acoustic Sys­ The surveyed area is locaced at the boctom of a sub­ tem) was used to investigate the present srudy area. marine valley near the intersection of two main bathy­ The SAR system pennitted the collection of a mosaic metrie features (Fig. 1): the southeasc Newfoundland of 1 km wide overlapping contiguous sonar images Ridge and the J anoma.ly ridge, about lOO km east of and 3.5 Khz subbocrom profiles. The SAR cruise pri­ the eastern edge of the Laurentian submarine fan. The marily resulted in focusing the Angus and Argo Ti­ orientation of the submarine valley is chought to result tanic wreck research (B allard 1986) oucside the SAR from a snuccural control by the J anoma.ly ridge. Sur­ survey area and contribuced significantly to ics sub­ face cores were collected on che northern flan.k (Pas- sequent discovery. Also, the 1985 SAR survey brought

,/ ..· 146 Geo-Marine Letters

board) cranducers and a 3.5 kHz subbottom profl.ler (Farcy and Voisset 1985). At the time of the croise, the system was able to produce one-kilometer wide swaths of the sea t1oor wüh a pixel size corresponding to a 30 cm field patch (today, the range bas been in­ creased up to 750 m). In soft sediments, subbottom profùes penetrate the sea t1oor to a depth of up co 80 m with a vertical resolution of about 0.75 m. The SAR is towed near che boctom at a heighc of about 70 me­ ters and a speed of 2 knocs . Ali the data are digitized and stored on magnetic tapes for reprocessing on­ shore. The navigation during the Titanic survey used Loran C for the ship, whereas acoustic transponders were used to navigate the SAR. Al! che data were pro­ cessed using IFREMER TRIAS software ("Traite­ ment des Images Acoustiques Sous-marines") which offers powerful capabilities previously described by Augustin ( 1986).

The Mosaic

The SAR side-scan sonar mosaic aids the identifica­ tion of chree physiographic regions (Fig. 2): l. The Ticanic canyon which traverses the encire mosaic from northeast co southwest (N220); Figure l. Bathymetrie map of the Grand Banks (after Uchupi and 2. A gentle slope east of the submarine canyon Hays. 1981) showing the srudy area. consisting of an eastern striped facies and sediment patches zone, and a western sediment wave field; 3. An area west of the canyon having an irregular data up to date, iroproving the quality of the sea t1oor topography where side-scan sonar images show scars mapping and adding to the geologica1 knowledge of and pronounced lineations orienced downs1ope. The a wide zone around the Titanic sire. The high reso­ western area is referred as the "instability corridor." lution SAR images has allowed us to resolve sea bed fearures of me aie scale range accurarely, to aid in the ir The Titanic Canyon identification as sand rib bons or sediment waves, to propose the frrst map of the sedimencary facies in che A 15 km long section of the Titanic Canyon crosses area, and suggest the primary sedimencary processes through the SAR mosaic with a slightly curved path: involved· on che Newfoundland rise. excepc at latitude 41 °42' where the strike of the can­ After the discovery of the Titanic wreck, Uchupi yo n changes abrupt! y from 230° to 195°. The upslope and others ( 1988) looked at the photographs and video continuation of the Titanic Canyon still remains un­ daca from a restricted area in the southern part of the certain. Uchupi and others ( 1988) think it is directly present study zone. They described severa! sea bed­ connected to the Cameron Canyon on the upper slope features like "sand ribbons, concinuous rippled sand of the Grands Banks. On the ocher band, recent bathy­ sheets, barchan dunes. mud waves, mud ripples, moats. metrie compilations (Canadian Hydrographie Service cornets and shadows." In the present report, these daca 1987) do not show clear evidence for the continuation were compared to the SAR images to incerpret the of the Titanic Canyon up to the Grands Banks upper sedimentary strucrures locaced ac the southeascern part slope. of the SAR survey area. The 3.5 kHz SAR sea floor cross-sections show thac the canyon bas a very distinct U shape. Going down­ Data Acquisition and Processing slope, the canyon tends to widen from 300 m in the northern section to about 1 ,000 m in the southem sec­ The SAR is a deep-wwed (up co 6,000 m) .'sidescan tion . The canyon depth remains constant (50 rn) in sonar equipped with 170 (port) and 190 kHz (star- the surveyed area. The mean slope value of the can- Vol. 9, No. 3, 1989 147

!FREMER

0 1

N 205 Current direc::tioo deduœd from sadimentaty bed1otms

Figur e 2. Map incerprecacion of rhe side-scan sonar mosaic showing rhe three different physiographic regions: area l-the canyon: area 2-a sourh east region wich gentle slope and specifie sedimencary fearures: area 3-a norrhward region having more irregular topography where side-scan sonar images show pronounced lineations oriented downslope.

yon floor is 0. 85 percent (0 .5°); in the northem sec­ posed of coarse material. This coarse material appears tion, the slope value is only 0.4 percent (0.2°) whereas as high reflectivity patches at the top of the sediment in the southem section the slope value is increasing waves. These waves are similar in scale to the grave! up co 1.55 percent (0.9°). Severa! hundred meter long waves previously described by Piper (1985) on the sections of the canyon banks are affected by slides Laurenrian fan and are thought to result from unusual which accumulate at the foot of the canyon walls (Fig. downslope flow conditions. 3). The wall slides appear co be overlaped with the The Striped Facies. To the east and south of the sediments filling the canyon giving a very flat topog­ sediment wave field, the sea floor is crossed by nu­ raphy at the canyon floor. merous streaks of low reflectivity, termed "the srriped facies . " These spectacular screaks are rypically 10 co East Section of the SAR Mosaic lOO meters wide and up to severa! kilomecers long (Fig. 5). They are sub-parallet to the isobaths which Sediment Waves. A field of sediment waves was run north-south on the east side of the valley. The mapped over an area of about 15 km2 east of the sub­ profiles do not exhibit any relief, chus, the striped fa­ marine canyon (Fig. 2) . The crescs of the bed forms cies are due to subtle surficial texrural variations. are oriented perpendicularly to the regional slope. The "The striped facies are thought to correspond to fine crests are about 400 rn to 500 rn long with anasw­ material resedimentation along current tines on a sub­ mosed undulating shapes. The wavelength of the bed­ srrarum consisring of coarse macerial of sandy to grave! forms (crest to crest) is about 60 m. The height of the type as described by Uchupi and others (1988). The waves calculated from the acoustic shadow length of description and the size of striped facies are in good the flanks and from the images of the sea t1oor pro­ agreement wirh the description of the sand ribbons by files (Fig. 4) is of the order of 3 m. Kenyon (1970). The accuracy of the SAR images al­ Differences in the sonar responses of the two flanks lows one to see chat the streaks are in sorne places of the waves indicate that they are strongly asym­ associated with 15 to 50 rn wide perpendicular ar­ metrical, with their sœep sides facing downslope and cuated bedforms (Fig. 5). their gently dipping sides facing upslope (Fig. 4). The The crescencic shape bedforms have the same size asymmetrical shape and size of the sediment waves and are deposited in the same place (Fig. 1) as the are confirmed by the sea floor profiles (Fig. 4). barchan dunes observed and described by Uchupi Poor penetration suggests chat the waves are corn- ( 1988) from the Angus photographs and video trac- 148 Geo-Marine Letters

1km

0

N-W s:.-E s!ide

0

100m Figure 3. Side-scan sonar sono­ graph and 3.5 kHz subbouom pro­ file of rhe box- shaped submarine canyon shown on Figure 2. Many closely-spaced slope îailures occur along the canyon walls.

ings. On the SAR images the barchan dunes are either The sediments in this area appear to have been re­ iso!ated or organized in group; they usually are iso­ rnoved along nurnerous parallel, rectilinear furrows lated when there is very little low reflecriviry rnaterial (Fig. 6). The furrows are 15 rneters wide and several on the sea floor and grouped when there is more low kilorneters long with a downslope orientation. The reflectivity material. According to the ir rnorpho!ogy, furrows show no appearance of significam sediment the associated sand ribbons and barchan dunes would filling (Fig. 6). According to Flood (1 983) such fur­ correspond to an intermediate type between b and c rows could be eroded by the coarse fraction of a sed­ of Ken yon 's sand rib bons classification (1970). Im~ irnentary flow. Several scarps facing downslope, in­ ages of a deep field of sand ribbons and associated terpreted to be rnass rnovernenc scars, occur in this barchan dunes (3,800 rn) have nor previous!y been area and rruncate straca crossed by the erosional fur­ pub!ished. rows (Fig. 6). At the bottorn of the corridor the topography is highly West Section of the SAR Mosaic irregular, showing chaotic facies on the sonar images (Fig. 2). The irregular topography is thought to result To the northwest of the srudy area, on the right bank fro m the accumulation of the sediments at the bottom of the canyon, a seven kilorneter wide corridor of of the valley after movernent along the instabiliry cor­ cornplex topography was studied. This may be the re­ ridor. The characteristics described above, and in par­ suit of large sedimentary instability events on the rise ticular the general direction oriented N200, suggest which leads to the canyon at the bottom of the re­ thar the instability corridor was initiated indepen­ gional valley. dently from canyon formation processes .

.... . · Vol. 9, No. 3, 1989 149

0 400m N- W S-E

0

Figure 4. Side-scan sonar sono­ graph and 3.5 kHz subboaom pro­ file showing a part of a field of asymmetricai sediment waves shown on Figure 2 wüh meir sreep 40m sides facing downslope towards base of the tïgure.

Discussion Directions of sedimentary transport can be inferred fr ;J"m the study of the sedimentary bedforms and the bottom topography in the area: Two lcinds of sedimentary transport are suggested on the basis of the SAR images descriptions. The tïrst • Mass transport processes: A N 195 direction given type is relaced to the " mass movemenc process" in­ by the lineacions of the instabilicy corridor indicating cluding the instability corridor of the west bank of the a downslope transport along the west flank of the sub­ canyon, the sediment waves of the east side, and the marine valley. The N205 direction of transport is per­ canyon icself. The second type of sedimentary trans­ pendicular to the sediments waves crests; A N220 di­ port is related co "currenc induced processes" on the rection given by the canyon indicacing a transport eastern section of the SAR mosaic wich the sand rib­ slightly oblique to the regional slope. bon area. • Borcom currenc induced fearures: The sand rib- !50 Geo-Marine Letters

Figure 5. Sonar image of a field of sand ribbons and associaœd barçhans dunes chac have been removed and deposiced by che Western Boundary Undercurrenc (WBU) whiçh is parallel co che isobachs.

bons and associared barchan dunes indicate a subbot­ ridor" result from one or severa! gravity events oc­ tom current from south to nonh. curing on the local slope. The erosional furrows ap­ pear to extend upslope beyond the boundaries of the Mass Transport Processes SAR mosaic suggesting gravity events with a crans­ port efficiency of the order of tens of kilometers. On The morphologie fearures such as mass movement scars the other hand the slides related to the scarps corre­ and erosional furrows constiruting the ~ instability cor- spond only to a maximum downslope transport of kil-

(.11 0 0 3

Figure 6. Sonograph of a part of the inscabiliry corridor: The scraight erosional furrows. orienced downslope. disseçc ançienc slides sçarps showing crunçaced scraca. ' VoL 9, No. 3, 1989 151

ometric scale range. Although no sampling was pos­ srudied area. After Schnitker (1979), Stow (1979), and sible within the surveyed area, che regional geological Fofonoff (1985), the WB U is foUowing the isobaths sening suggests thar coarse marerial such as pebbles of the lower slope of the north-west Atlantic. From and ice rafted debris can be present in che recent sed­ the bonom phorographs obtained during the 1984 iments. Gravity events with a transport efficiency up Woods Ho!e cruise, Uchupi (1988) indicates a north­ co cens of kilometers (lil

Figure 5. Sonar image of a field of sand ribbons and associated barchans dunes that have been removed and deposited by the Western Boundary Undercurrent (WBU) which is para!lel to the isobaths .

bons and associared barchan dunes indicace a subbot­ rider" result from one or several gravity events oc­ tom currenc from south co north. curing on the local slope . The erosi.onal furrows ap­ pear co extend upslope beyond the boundaries of the Mass Transport Processes SAR mosaic suggesting gravity events wi.th a crans­ port efficiency of the order of tens of kilometers. On The morphologie features such as mass movemenc scars the ocher hand the slides related to the scarps corre­ and erosion al furrows constituting the "instability cor- spond only to a maximum downslope transport of kil-

Figure 6. Sonograph of a part of the instabiliry corridor: The straight erosional furrows. oriented downslope. dissect ancient slides scarps showing truncated strata. .·