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Neogene sedimentation history of the Congo Fan
Marine and Petroleum Geology 04 "0887# 524Ð549
Neogene sedimentation history of the Congo Fan G[ Uenzelmann!Neben Alfred!Wegener!Institut fur Polar! und Meeresforschung\ Postfach 019050\ 16461 Bremerhaven\ Germany Received 01 December 0886^ revised 11 May 0887^ accepted 18 May 0887
Abstract High!resolution seismic re~ection data were interpreted to develop a model for the Neogene sedimentation in the Congo Fan area[ Identi_cation of two re~ectors representing marine isotope stages 5 and 01 allows a distinction between the Early and the Late Quaternary[ A single sediment source system during the Late Paleogene:Early Neogene gradually changes into a two sediment source system in the Late Neogene[ Since the Late Quaternary\ the Congo River has been the prevailing sediment source[ The evolving Benguela Current further modi_ed the system and led to a high biogenic productivity[ Evidence for gas in the sediments can be found in the whole area of investigation[ Additionally\ indications for a tectonic control of the Congo Canyon have been found in the distribution of sedimentary units and unit thicknesses[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords] Seismostratigraphy^ Sedimentation history^ Congo Fan evolution
0[ Introduction The Congo Fan is one of the world|s largest fan systems[ It is the largest depocentre in the Angola Basin and dominates this continental margin "Uchupi\ 0878^ 0881#[ The fan is now being fed mainly by material from the Congo river that is transported through the Congo Canyon to be deposited on the lower fan[ Little has been known hitherto about the development of the Congo Fan and the Congo Canyon[ Seismic sur! veys carried out in the early |69s resulted in a general view of the fan and revealed the presence of numerous salt diapirs "Leyden\ Bryan + Ewing\ 0861^ Emery\ Uchupi\ Phillips\ Bowin + Mascle\ 0864#[ Although the oil indus! try has worked o}!shore Gabon\ Cabinda and Angola only few seismic data have been published "Brice\ Coch! ran\ Pardo + Edwards\ 0871^ Edwards + Bignell\ 0877^ Teisserenc + Villemin\ 0878^ Liro + Coen\ 0884#[ Recent investigations have dealt with the Quaternary morphology and evolution of the depositional systems and environments "Droz\ Rigaut\ Cochonat + Tofani\ 0885^ Uenzelmann!Neben\ Spiess + Bleil\ 0886#[ A stratigraphy for the Congo Fan was developed by Uenzelmann!Neben et al[ "0886# which is followed here in slightly modi_ed form[ Indications for a more complex evolution of the depositional system in the Congo Fan area have been given by Droz et al[ "0885# and Corresponding author[ Tel[] 9938 360 3720 197^ fax] 9938 360 3720 038^ e!mail] uenzelÝawi!bremerhaven[de S9153Ð7061:87 ,08[99 Þ 0887 Elsevier Science Ltd[ All rights reserved PII] S 9 1 5 3 Ð 7 0 6 1 " 8 7 # 9 9 9 2 3 Ð 7
Uenzelmann!Neben et al[ "0886#[ In this paper\ this aspect is investigated more closely and a model is presented for Neogene:Quaternary sedimentation[ To give a short summary on the general geologic evol! ution] Mezosoic rifting in the South Atlantic produced a succession of _ve distinct tectonic regimes] Prerift\ Synrift "I and II#\ Postrift and Regional Subsidence "Brice et al[\ 0871#[ Up to 0999 m of evaporites were deposited during the Aptian[ This was followed by initial movement of the salt in Albian time "Leyden et al[\ 0861^ Emery et al[\ 0864^ Emery + Uchupi\ 0873^ Emery\ 0881#[ The seaward edge of the salt diapirs is formed by the prominent Angola Escarpment "Emery + Uchupi\ 0873^ Emery\ 0881#[ Crossing the escarpment\ the sea~oor drops sharply by up to 0999 m "Fig[ 0#[ During the middle Cretaceous "AptianÐTuronian#\ black shales and bituminous sand! stones were deposited on top of the evaporites "Bolli et al[\ 0867^ Brice et al[\ 0871^ Emery + Uchupi\ 0873^ Uchupi\ 0881#[ Deposition of turbidites started in the Oligocene "Brice et al[\ 0871^ Uchupi\ 0881#[ The Benguela Current\ the major oceanic current in the eastern South Atlantic\ commenced in the Late Miocene "Dean\ Hay + Sibuet\ 0873#[ A large amount of organic material was produced by associated upwelling and led to mass movement in the southern Angola Basin "Emery + Uchupi\ 0873^ Uchupi\ 0881#[ 1[ Data acquisition and processing About 0199 km of high!resolution re~ection seismic data were gathered on the Congo Fan during a joint
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Fig[ 0[ Location of seismic pro_les shot on the Northern Congo Fan[ Bathymetry according to ETOPO!4 "National Geophysical Data Center\ 0877#[ The italicized numbers refer to the parts of the lines displayed in Figs 1\ 2\ 4\ 6 and 8[ The dots show the locations of the recently drilled Sites 0964\ 0965 and 0966 of ODP leg 064[ The dot!dashed line marks the position of the Congo Canyon according to Shepard + Emery "0862#[ The dotted line shows the location of the Angola Escarpment[ A fault zone beneath the Congo Canyon as proposed by Reyre "0873# is marked by the hatched area[ The parts of the fan referred to in the text as middle and upper fan are separated by the dashed line[ Asterisks in the insert map show the locations of DSDP Sites 253 and 429[
study made by the Alfred!Wegener!Institute and the Uni! versity of Bremen "Fig[ 0#[ A single GI!gunTM in the true GI mode "an airgun with two chambers] one to generate the seismic pulse and a second one triggered delayed to destroy the bubble pulse# was _red every 14 m as seismic source generating frequencies up to 299 Hz[ This pro! vided a vertical resolution of ½1[4 m[ The data were received employing an 799 m streamer with 13 channels in a 599 m active section and recorded on a EG+G ES 1319 multichannel digital recording system[ Positioning was by GPS "Global Positioning System#[ In parallel with the seismic measurements\ continuous records were acquired with the ParasoundTM system\ a narrow beam echosounder of only 3> beam width with high vertical and lateral resolution[ This system was run with a signal of 3 kHz and 149 ms pulse length[ The shot interval was discontinuous and depended on the water depth\ but was generally every 0Ð1 s[ This system has a vertical resolution of 09 cm\ while penetrating the upper 49Ð049 m of the sedimentary column[ Thus\ high quality images of recent sedimentation and the surface structures were also collected "Spiess + Uenzelmann!Neben\ in prep[#[ The processing of the multichannel seismic data com! prised demultiplexing\ de_nition of the geometry invok!
ing the navigation data "CDP interval 14 m# and CDP sorting[ Due to the very good seismic signal "short double pulse\ bubble signal suppressed# and a negligible ocean bottom multiple\ no deconvolution was applied[ Despite the fact that the small streamer length gave only a rela! tively small move out\ a detailed velocity analysis "every 49 CDP# was carried out[ This was required because of the complex subbottom morphology[ After correcting both spherical divergence and normal move out and stacking\ a _nite di}erence omega!x migration was applied to the data[ This algorithm was chosen because it is accurate for dips up to 54>\ while simultaneously being able to handle lateral velocity variations "Stolt\ 0867^ Kjarstansson\ 0868^ Yilmaz\ 0876#[ This method was especially useful in regions with steeply dipping re~ectors due to salt diapirism[ For display\ no gain "i[e[\ AGC# or _ltering except for an anti!aliasing!_lter "259 Hz during aquisition# were applied to the data[ The amplitudes shown on the pro_les therefore represent values relative to the maximum amplitude of the whole section[ 2[ Stratigraphy of re~ector CF!A!int In general\ the seismic stratigraphy as de_ned by Uenzelmann!Neben et al[ "0886# is followed in this paper[
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The recent ODP leg 064 has supplied new information about the nature and age of the sediments drilled at three Sites 0964Ð0966 in the Congo Fan area "Berger\ Wefer + Richter\ 0887# resulting in modi_cation of the strati! graphy presented by Uenzelmann!Neben et al[ "0886#[ Uenzelmann!Neben et al[ "0886# de_ned four seismo! stratigraphic units according to their re~ection charac! teristics ranging in age from Eocene to Quaternary "Fig[ 1#[ The lowermost unit CF3 "Eocene\ 269Ð429 mbsf\ 029Ð 104 m thick# is characterized by weak internal re~ections and is interpreted to document an increasing terrigenous sediment input[ Unit CF2 "Middle Eocene:Lower Oli! goceneÐMiocene\ 109Ð209 mbsf\ 059Ð119 m thick# again shows weak internal re~ections[ It is separated from unit CF3 by a prominent re~ector CF!B "Eocene:Oligocene\ Figs 1\ 2 and 8# and is inferred to be related to the development of the Congo River drainage system[ The build!up of the Benguela Current "Late MioceneÐ Pliocene\ Dean et al[\ 0873# is represented by unit CF1 "79Ð039 mbsf\ 029Ð069 m thick#[ Unit CF0 "Late Pliocene\ 79Ð039 m thick# is interpreted to consist of deep marine hemipelagic clays and oozes[ The sites of ODP Leg 064 located within our area of investigation "see Fig[ 0# have all penetrated the seabed only to a subbottom depth of 191Ð196 m "Table 0^ Fig[ 1^ Berger et al[\ 0887#[ This lies still within unit CF1 and thus\ nothing could be learned about the nature of units CF2 and CF3 and re~ector CF!B[ The sites show rela! tively homogeneous material of Pleistocene!Pliocene age
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"Table 0^ Berger et al[\ 0887#[ Sedimentation rates were derived for all sites and a sonic log was obtained at Site 0966 "Berger et al[\ 0887#[ Using this information the age of re~ector CF!A was re!dated from Pliocene to marine isotope stage 01 "see lower part of this section# and unit CF0 from Late Pliocene:Quaternary to Quaternary[ Within unit CF0 a strong continuous re~ection CF!A! int can be observed "Figs 2 and 3#[ This re~ector is easily mapped and lies at a mean depth of 29 ms below sea~oor "¼12 m using a 0499 m:s conversion velocity as deduced from the analysis of the seismic data and ODP well log! ging "Berger et al[\ 0887##[ The re~ector shows slightly larger amplitudes than the base re~ector CF!A of unit CF0 but no change in re~ection characteristics can be observed "Figs 2] CDPs 0499Ð0599 and 0699Ð1999\ and 3] CDPs 0099Ð0299 and 0599Ð0799#[ Re~ector CF!A!int is very smooth and shows no indications of erosion\ or of trans! or regression or non!depositional "toplap# character[ This implies that the re~ector is primarily related to a change in lithology and hence in the com! position of the sediments[ The re~ector shows only minor deformation due to salt movement[ Anomalies in re~ec! tion amplitude observed south of the Congo Canyon "Figs 3b] CDPs 0699Ð0799\ 6b] CDPs 3349Ð3499\ and 4^ note the yellow!red colours indicating a change in re~ection polarity# are probably related to the rise of thermogenic gas and:or the formation of gas hydrates "Uenzelmann!Neben et al[\ 0886#[ According to Berger et al[ "0887#\ sedimentation rates
Fig[ 1[ Correlation of information from ODP Leg 064 Sites 0964Ð0966 "Berger et al[\ 0887# with the seismic data[ For location see Fig[ 0[ The hatched blocks show the location and penetration of the drill sites within the seismic lines[ The stratigraphy according to Uenzelmann!Neben et al[ "0886# has been included with CF0!CF3 referring to the seismic units and CF!A!int!CF!Ca to the seismic re~ectors[
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Fig[ 2[ Line GeoB 82!992:AWI!82912 showing the stratigraphy adapted from Uenzelmann!Neben et al[ "0886# and\ additionally\ re~ector CF!A!int within unit CF0[ Note the bottom simulating re~ector "BSR# and the loss in re~ection amplitude over the salt diapir indicating the presence of gas hydrates and ~uids[ Further note the debris ~ow in unit CF2 "chaotic internal structure with continuous re~ections only on the western side#[ Modi_ed from Uenzelmann!Neben et al[ "0886#[
are 099 m:My on average "Site 0964# and 199Ð069 m:My "Sites 0965 and 0966\ respectively# for the depth range of unit CF0[ Sonic log velocities range from 0369Ð0409 m:s "Berger et al[\ 0887#\ and velocities deduced from seismic re~ection data lie in the same range "0449 m:s#[ Thus\ an age of about 064 ky "Sites 0965 and 0966# is estimated for re~ector CF!A!int with a strong deviation at Site 0964 "119 ky#[ This deviation is probably due to the fact that only average sedimentation rates have yet been published for Site 0964 and average values for Sites 0965 and 0966 would lead to more similar estimated ages of 084 ky and 194 ky\ respectively[ The detected age of re~ector CF!A! int corresponds to marine isotope stage 5[5 "Sites 0965 and 0966#[ Changes in sediment composition have been reported for stage 5[5 "Schneider\ 0880^ Jansen\ Ufkes + Schneider\ 0885^ Schneider et al[\ 0885#[ The accumulation rates of marine organic carbon were much higher during stage 5[5 than stage 6[ Higher accumulation rates of opal and an increase in CaCO2 production were observed[ Fur! thermore\ the paleoproductivity increased in stage 5[5 "Schneider\ 0880#[
Since these changes in composition result in a sig! ni_cant shift in density "lower density of opal and higher porosity due to higher opal content\ Gerland\ Kuhn + Bohrmann\ 0886# they can cause a conformable re~ector[ According to Schneider "0880#\ stage 5[5 represents a glacial as well as a period of minimum precession[ This led to a maximum in the monsoon which in turn moved the Angola!Benguela!Front northwards "Schneider\ 0880#[ As a result\ the Benguela Current probably reached farther north\ leading to increased upwelling and thus production of organic material deposited in this area[ All this could initiate modi_cations in sediment composition[ Jansen et al[ "0885# present a modi_cation of Schneider|s "0880# model with the Angola!Benguela! Front shifting southwards from its northernmost pos! ition "north of 8> N# during stage 5[5 in contrast to the northward shift assumed by Schneider "0880#[ Non! etheless the shift of the Angola!Benguela!Front as a result of a glacial and minimum precession led to changes in the composition of the sediments[ Those changes are documented as a re~ector which is assumed to be re~ector CF!A!int\ i[e[\ a climatically!enhanced condensed zone\
074 096 69 32 069 in 9Ð019 m 033 in 019Ð029 m 039 in 029Ð199 m Computed from sonic log information for Site 0966[ For Site 0966 computed from magnetostratigraphy[
0369Ð0409 Diatom!rich\ diatom!bearing\ nannofossil!rich\ nannofossil! bearing clay 0 191 0966
169
0 193 0965
169
0 169 196 0964
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and the event can thus be mapped regionally[ So\ the upper unit CF0 is supposed to represent the Late Quat! ernary[ However\ other glacial stages are less easily docu! mented as seismic re~ectors[ Marine isotope stage 1 can! not be resolved with the available seismic re~ection data[ From the sedimentation rates deduced by ODP leg 064 "Berger et al[\ 0887# a corresponding re~ector would be expected at 4 ms to 09 ms below sea~oor and lies within our seismic wavelet "Figs 2 and 3#[ In contrast\ the Par! asound data show weak indications of a re~ector "Fig[ 5#[ Stage 7 represents a glacial which was less severe than stage 5 "Imbrie et al[\ 0873#[ Thus\ the resulting changes in sediment composition and sedimentation rate were probably not su.cient to generate a seismic re~ector[ Nevertheless\ converting the age into traveltime via sedi! mentation rates and sonic log velocities "Berger et al[\ 0887# any re~ector would lie at a depth of 49Ð69 ms depth below sea~ow[ This is not always distinguishable from re~ector CF!A!int in our seismic re~ection data due to the length of the seismic wavelet[ However\ the Par! asound data do show a thin re~ector at an appropriate depth "Fig[ 5#[ Marine isotope stage 01 covers the period 339Ð369 ky "Imbrie et al[\ 0873#[ From sedimentation rates from ODP leg 064 "Berger et al[\ 0887#\ a corresponding re~ector would be expected at traveltimes around 099Ð009 ms depth below sea~oor[ Re~ector CF!A lies at this depth and marks signi_cant changes in re~ection characteristics "Figs 2] CDPs 0499Ð0599 and 0699Ð1999\ 3] CDPs 0099Ð 0499 and 0699Ð0799\ and 5\ Uenzelmann!Neben et al[\ 0886#[ Thus\ the age inferred for this re~ector by Uen! zelmann!Neben et al[ "0886# is corrected "Pliocene to marine isotope stage 01#[ Furthermore\ not only move! ments of the CCD and changes from high variability to longer lasting conditions "Uenzelmann!Neben et al[\ 0886# seem to be responsible for re~ector CF!A but in addition similar changes in composition as observed for stage 5[
Pliocene
067 000 69 34 199 in 9Ð79 m 49 in 79Ð019 m 109 in 019Ð199 m Carbon!rich olive gray clay
Pliocene
119 099 49 29 Pleistocene! Pliocene 099 diatomaceous clay\ high gas concentrations "CO1\ Methane# mainly below 099 m "¼019 ms#
Computed age for CF!A!int "ky# CF!A Stage 7 Traveltime "ms# to CF!A!int Age Sedimentation rate "m:m[y[# Velocity "m:s# Lithology Lithologic units Traveltime to base TWT "ms# Subbottom depth "m# ODP leg 064 site
Table 0 Drill information from ODP Leg 064 Sites 0964Ð0966 "Berger et al[\ 0887#[ The traveltimes to re~ectors CF!A!int\ CF!A and marine isotope stage 7 have been added[ Furthermore\ the age for re~ector CF!A!int using sedimentation rates and velocity information from the sites "Berger et al[\ 0877# have been computed
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3[ Distribution of Neogene:Quaternary sediments To better de_ne and describe the sedimentary environ! ments\ the seabed re~ection and re~ectors CF!A!int\ CF! A\ CF1!base and CF!B were mapped\ thicknesses of the di}erent units computed and maps of re~ector depth\ re~ection amplitude and unit thickness compiled[ Since no reliable seismic velocities could be deduced from our data due to the recording con_guration "short streamer length vs great target depth# both maps of re~ector depth and unit thickness are still in ms two waytime and strictly represent neither depth nor thickness[ Both terms are used here for better clarity[ To carry our a rough depth:thickness conversion velocity values at 0499 m:s\ 0699 m:s and 0649 m:s can be used for sedimentary units upper CF0\ lower CF0\ CF1 and CF2\ respectively[
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Fig[ 3[ Line GeoB 82!909:AWI!82929 showing the Angola Escarpment "a#[ Movement of the salt led to faulting and deformation of the sediments "b#[
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Fig[ 4[ Map of the re~ection amplitude of re~ector CF!A!int[ Note the anomalies in the area south of the Congo Canyon[ There\ even positive re~ection amplitudes were observed "note the yellow to red colours#[ This is interpreted as an indication for the presence of gas in the sediments[
steps "Fig[ 3] CDP 0199Ð1199#[ This might be a result of downfaulting along the escarpment "Fig[ 3] CDPs 0199Ð 1999#[ One can clearly see that the escarpment itself is formed by salt diapirs which lead to deformation and inclination "{block faulting|# of the sedimentary layers[ The sedimentary units on top of the salt diapirs wedge out towards the escarpment "Figs 3b] around CDP 0899\ and 7b] around CDP 3599#[ Indications for this were given by\ e[g[\ Emery + Uchupi "0873# and Uchupi "0881#[ The morphology of the di}erent re~ectors varies more to the south of the Congo Canyon than to the north of it[ This is true even for the sea~oor where small elevations and depressions can be seen close to the Angola Escarp! ment while the re~ector appears very smooth in the north[ The only re~ector to display small scale morphological variations in the north is re~ector CF!B "Fig[ 6b#[ This re~ector shows a few elevations and depressions in the north[ South of the canyon the re~ector distribution is heavily a}ected by this high!frequency sinuosity[ This observation implies that the salt diapirs lie shallower south of the Congo Canyon than north of it[ Thus\ the
The mapped re~ectors in general show an even seaward increase in depth "Fig[ 6#[ They do not reveal a paleoshelf\ i[e[\ no strong progradation of the shelf took place during the late Neogene:Quaternary[ Moreover\ the slope!rise transition remained fairly stationary] it only moved from 00> E "re~ectors CF!B and CF1!base# to 09>49? E "re~ec! tor CF!A# and back via 09>44? E "re~ector CF!A!int# to 00> E "re~ector Sea~oor#[ This indicates sediment trans! port and deposition controlled by channel and turbidite systems for the late Neogene:Quaternary[ The Angola Escarpment can be easily recognized in all re~ectors mapped south of the Congo Canyon "Figs 3] CDPs 0199Ð1199\ 6 and 7] CDPs 3599Ð3799#[ In contrast\ no indications for the escarpment can be found on the northern lines[ The question of whether the escarpment terminates at the canyon or whether its course is modi_ed by the canyon "Emery + Uchupi\ 0873^ Emery\ 0881# remains open since our lines do not cover the area across the canyon[ The escarpment appears to mark the seaward edge of salt diapirism "Figs 3] around CDP 0899\ and 7] around CDP 3599^ also see Emery et al[\ 0861^ Leyden et al[\ 0864#[ In some places\ the escarpment occurs in two
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Fig[ 5[ Parasound record of line GeoB 82!990:AWI!82910[ Re~ectors representing di}erent marine isotope stages "stage 1\ CF!A!int ¼ stage 5\ stage 7\ CF!A ¼ stage 01# can be identi_ed[
ascent of the salt was either stronger or longer lasting or both in the south[ Above and in the close vicinity of salt diapirs\ the re~ectors show variations in re~ection amplitude[ Strong amplitudes and phase conversions are found on the sou! thern lines for re~ectors CF!A!int and CF!A "Figs 4 and 7c] CDPs 2149\ 2[4 s TWT\ and 2349Ð2699\ 2[64 s TWT#[ They can be linked with elevations of the re~ectors[ This is an indication for either condensed sections or small amounts of gas which has risen at the ~anks of the highs "Uenzelmann!Neben et al[\ 0886#[ The interpretation of gas is favoured since BSRs "bottom simulating re~ectors# have additionally been observed "Fig[ 2] CDPs 0599Ð 0549\ 3[1Ð3[04 s TWT^ also see Uenzelmann!Neben et al[\ 0886#[ Thermogenic gas generated from middle Cre! taceous "AlbianÐTuronian# black shales "Foresman\ 0867^ Hunt\ 0867^ Kendrick\ Hood + Castan½o\ 0867^ Stow\ 0873# is interpreted to have risen along faults and salt diapirs[ The oil and gas potential of this region has been known for a number of years with established o}! shore and onshore oil and gas production "Brice et al[\ 0871^ Teisserenc + Villemin\ 0878^ Liro + Coen\ 0884#[ Furthermore\ the high productivity introduced by the evolving Benguela Current "Bolli et al[\ 0867^ Schneider\
0880^ Schneider et al[\ 0885# and the high sedimentation rates observed for the Pliocene:Quaternary "Berger et al[\ 0886# may also have favoured the generation of biogenic gases which assembled in the local highs[ Although no traces of methane hydrates were found at either of the recently drilled ODP leg 064 Sites 0964Ð0966 "Berger et al[\ 0887# modest amounts of methane and carbon diox! ide detected at all three sites are the result of microbial degradation in the sediment[ Apart from these local variations associated with salt diapirs\ in general re~ection amplitudes are slightly larger in the north "e[g[\ Fig 4\ note the regional trend#[ This is best observed for the sea~oor and re~ectors CF!A!int and CF!B[ There\ the strongest amplitudes can be found in a water depth of 499Ð1699 m on the inner fan where the isobaths broaden from the north into the sediment cone "Fig[ 0#[ The increased re~ection amplitude is inter! preted as the result of winnowing by bottom currents which removed all _nes from coarse!grained sediments "Reading\ 0885#\ probably turbidites\ leaving material with a higher re~ectivity[ Droz et al[ "0885# found coarse sediment associated with high re~ectivity in the active channel[ Condensed sections due to very slow sedi! mentation rates show high re~ectivity as well[ Miall
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than 699 ms TWT#[ To better describe the isopachs a depocentre was de_ned[ Depocentre in this context depicts the area where the thickness of the unit reaches values signi_cantly higher than the root mean square "rms# thickness of the unit[ Similar observations can be made for units CF1\ lower and upper unit CF0 "Figs 2\ 3\ 7\ 09 and 02#[ Moreover\ the size of the depocentre is reduced "½069 km1 for unit CF2\ ½89 km1 for upper unit CF0#\ and it was shifted into the northeastern corner of our area of investigation "Figs 8 and 01#[ The change in size and location of the depocentre indicates alterations in the sediment source and supply[ Debris ~ows identi_ed on the seismic data "Figs 2\ 09 and 00# give further signs for a re!location of the sediment source[ Although debris ~ows are not directly related to ~uvial input and are often generated from the slope\ a change in their general distribution gives indications for a re!location of the sedi! ment source[ These observations are interpreted as evi! dence for a prevailing sediment supply via the Kouilou:Niari turbidite system "KNTS# during the Late Paleogene as indicated by Droz et al[ "0885# and Uen! zelmann!Neben et al[ "0886#[ The fact that nearly all debris ~ows encountered in unit CF3 "e[g[\ Fig[ 09] CDPs 1499Ð1799\ 2[2 s TWT^ Fig[ 00# lie north of 4> N and east of 8>49? E\ i[e[\ the main Paleogene sediment body is found near the Kouilou:Niari rivermouth\ supports this suggestion[ Furthermore\ the depocentre in unit CF2 "Early Neogene# lies directly in front of the Kouilou:Niari estuary[ The sediment was deposited near the source and later re!deposited in the middle fan area[ Debris ~ows found in units CF2 and CF1 are located in greater water depths "Figs 2] CDPs 0699Ð1999\ 3[0Ð3[1 s TWT\ and 00# farther south and west[ This indicates that the sediment source was no longer solely from the Kouilou:Niari River "KNR# system[ More sediments were derived from the south and deposited in the middle fan[ This southern component is interpreted as contributions via the Congo River and the evolving Benguela Current system "Fig[ 01#[ Since this modi_cation in sediment source can be observed from unit CF2 to unit CF1\ it is placed around Middle:Late Miocene[ As the build!up of the fan pro! gressed the sediment was transported via the Congo Can! yon farther to the west "Figs 01 and 02#[ This westward transport _nally led to deposition in an area not covered by our lines[ The question remains why so much less of the sediment was deposited south of the Congo Canyon[ One reason might be the fact that in the Late Paleogene the dominant sediment source was located in the northeast thereby encouraging deposition in the north and re!deposition in the north and west[ With the onset of the Benguela Coastal Current and its migration to the north\ the material distributed by the Congo and not captured by the canyon\ i[e[\ _ne material\ was de~ected northwards[ The general transport of the sediments has been towards the west for unit CF0 which prevents the deposition of a
Fig[ 6[ Maps of depth to re~ector CF!A!int "a# and CF!B "b#[ Note the smooth surface of re~ector CF!A!int whereas re~ector CF!B is strongly a}ected by the salt topography[
"0886# pointed out that a number of high re~ectivity horizons appear to owe their origin to current erosion associated with the growth of the Antarctic ice cap[ Re~ectors CF!B and CF!A!int are correlated with the onset of the Antarctic Bottomwater in Eocene:Oligocene times and marine isotope stage 5[5\ respectively\ and thus their origin due to condensed sections is not favoured[ A thick pile of sediment that gradually thins seaward can be observed in the north for unit CF2 "Figs 2\ 8 and 09# To the south of the Congo Canyon\ the unit is more evenly distributed with only small highs and lows linked to salt diapirs and generally thinner than to the north "highest values of 139 ms TWT contrasting with more
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Fig[ 7[ Line GeoB 82!902:AWI!82922 showing the Angola Escarpment "a# and the salt diapirs landward of the escarpment in enlarged sections "b and c#[ Note the faulting and deformation of the sediments\ the ~at spot and the strong re~ection amplitudes hinting on gas!bearing sediments[
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Fig[ 7[ Continued[
thick sequence in the south[ Still\ the discrepancy between unit thicknesses in the north and south is too large to be due solely to these mechanisms[ Two further explanations could be di}erential sub! sidence within the Congo Basin resulting in either stron! gly northwards tilted sequences and:or displacement of the layers along a fault[ A similar setting has been observed in the Gabon Basin "Teisserenc + Villemin\ 0878#[ There\ the basin is divided into the Northern and Southern subbasins by the N|Komi Fracture Zone which show di}erent depths to continental basement "01 km and 8 km\ respectively#[ The postsalt deposits rapidly plunge to the north from the platform south of the frac! ture zone into the Northern subbasin|s o}shore portion "Teisserenc + Villemin\ 0878#[ The N|Komi Fracture Zone was created in the early rifting of the Atlantic and remained active throughout the whole rift phase "Teiss! erenc + Villemin\ 0878#[ The coastal basins of west Africa have been found to be broadly similar in tectonic style "Ala + Selley\ 0886#[ Such fracture and fault zones can be found all along the West African coast "Emery + Uchupi\ 0873^ Teisserenc + Villemin\ 0878^ Liro + Coen\ 0884#\ and especially Reyre "0873# suggests one at the location of the Congo Canyon "Fig[ 0#[ It appears plaus! ible for a river to take up its path along a fault since the
penetration into the rock in order to create a riverbed is easier along a zone of weakness[ This tectonic control has been observed for other submarine canyons\ e[g[\ the Cascadia Canyon "Clark\ Kenyon + Pickering\ 0881# or a number of canyons in the Bay of Biscay "Kenyon\ Belderson + Stride\ 0867#[ Furthermore\ a structural con! trol on distribution of the Eocene and Miocene sedi! mentary facies has been reported for the Cuanza Basin in the south as well "Ala and Selley\ 0886#[ The existence of a fault beneath the canyon would enable di}erential subsidence within the Congo Basin[ This was probably assisted by the location of the sedimentary source[ With the main amount of material being contributed initially by the Kouilou:Niari Turbidite System the northern sub! basin acquired thicker sedimentary sequences "Figs 8 and 01a# leading to stronger subsidence[ When the main sedi! ment source changed from the KNTS to the Congo River and contributions from the Benguela Current "Figs 01bÐ d and 02# and the material was distributed more equally to the north and south subbasins and the di}erential subsidence ceased[ The frequent reactivation of transverse zones often results in the gathering of terrigenous clastics in deltaic systems or submarine canyons "Reyre\ 0873#[ This way the build!up of the Congo Fan might have been encour!
) 535
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Fig[ 8[ Isopach map of unit CF2[ Note the strong di}erence in thickness north and south of the Congo Canyon and the big depocentre near the estuary of the Kouilou:Niari River[
Kouilou Niari River and the Congo River[ The open arrow marks material transported by the Benguela Coas! tal Current[ In the Late Paleogene:Early Neogene the Kouilou Niari River appeared to be the main sediment source[ Indications for this were already given by Droz et al[ "0885# and Uenzelmann!Neben et al[ "0886#\ and the iso! pachs of unit CF2 verify this "Fig[ 8# showing the main pile of sediments in the northeastern part of our area of investigation[ Huge amounts of sediment were deposited on the shelf and slope in front of the KNR estuary and re!deposited via the KNTS[ This resulted in a large thick depocentre on the slope and rise on the northern rim of the Congo Fan "Figs 8 and 01a#[ The Late Neogene "Late Miocene<# saw a modi_cation in prevailing sediment source[ Less material was con! tributed by the KNR[ Moreover\ the evolving Benguela Coastal Current led to a northward de~ection of the sediment and the build!up of a smaller thinner depocentre near the estuary[ This is documented in the isopachs of unit CF1[ Another developing sediment source was fed by the Congo River[ Doming associated with rifting in the East African Rift in Miocene time led to a strong
aged[ Very few lines crossing the Congo Canyon have been published "e[g[\ Leyden et al[\ 0861^ Emery et al[\ 0864^ Droz et al[\ 0885#[ The seismic refraction data of Leyden et al[ "0861# are unfortunately not detailed enough and located obliquely to resolve the proposed fault zone whereas the high!resolution seismic re~ection pro_les of Droz et al[ "0885# do not penetrate deep enough to con_rm our theory[ Additional data are needed to con_rm our hypothesis[
4[ Neogene:Quaternary deposition The interpretation of sedimentary structures and maps of re~ector depth\ re~ection amplitude and unit thickness led to the construction of a model for the Neogene and Quaternary deposition in the Congo Fan area which re_nes the ideas of Droz et al[ "0885# and Uenzelmann! Neben et al[ "0886#[ Figure 01 shows the di}erent stages of the scenario from unit CF2 "Late Paleogene:Early Neogene\ Fig 01a# to upper unit CF0 "Late Quaternary\ Fig 01d#[ The shaded areas show the depocentres while the black arrows represent the contributions from the
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Fig[ 09[ Line GeoB 82!996:AWI!82916[ Note the debris ~ow in unit CF3 and the strong faulting in unit CF1 due to di}erential compaction[
Fig[ 00[ Map shows the distribution of mass transport features "slumps and debris ~ows# as a function of sedimentary unit[
536
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Fig[ 01[ Model for the evolution of the deposition in the Congo Fan since the Late Paleogene[ The shaded areas show the depocentres for "a# unit CF2 "Late Paleogene:Early Neogene#\ "b# unit CF1 "Late Neogene#\ "c# lower unit CF0 "Early Quaternary#\ and "d# upper unit CF0 "Late Quaternary#[ The black arrows represent the distributions by the Kouilou:Niari River and the Congo River\ and the open arrow marks the material contributed by the Benguela Current "BC#[
pick!up of the Congo River drainage away from the East African Rift "Frostick + Reid\ 0878^ Frostick\ 0886#[ As a result\ more sediment was carried into the Atlantic[ The main material was transported via the Congo Canyon and also deviated to the northwest where a second depo! centre was created "Fig[ 01b#[ Little material found its way to the area south of the canyon "Fig[ 01b#[ The depocentre of lower unit CF0 again shrank as can be seen in Figs 01c and 02a[ Obviously\ less and less material was distributed by the KNR during Early Quat! ernary while the amount of sediment carried by the Congo River increased[ The Congo Fan\ and thus the canyon as well\ was built out farther to the west\ and a large amount of the contributed sediments was trans! ported into the middle fan where our lines do not cover it "Fig[ 02a#[ A signi_cant component of the material was de~ected to the north by the Benguela Coastal Current "Fig[ 02a#[ The onset of the Benguela Current in Late Miocene "Dean et al[\ 0873# led to a further modi_cation of the sedimentary environment[ The establishment of estuarine!in~uenced upwelling "Berger et al[\ 0887# led to an increased productivity and\ moreover\ reworking of shelf sediments as observed in ODP Leg 064 Site 0965 "Berger et al[\ 0887#[ The depocentre observable in our
data thus moved westward with insigni_cant quantities being laid down south of the canyon "Fig[ 01c#[ During the Late Quaternary\ the size of the depocentre at the northern rim of the Congo Fan shrank further "Fig[ 01d#[ The KNR has contributed even less material while the main sediments contributed by the Congo River have been guided to the middle and lower fan by the Congo Canyon[ A small component has again been de~ected to the north by the Benguela Coastal Current and deposited on the northern rim of the fan "Figs 01d and 02b#[ This could explain the observed distribution of upper unit CF0[ Since the amount of sediment seized by the Benguela Coastal Current is less than during the Early Quaternary "lower unit CF0# the size of the depocentre is further reduced while its location has moved upslope[
5[ Conclusions Neogene sedimentation on the Congo Fan has been interpreted using a regional set of high resolution seismic lines[ The stratigraphy developed by Uenzelmann!Neben et al[ "0886# is extended to four seismic lines located south of the Congo Canyon[ Information supplied by Berger et
) G[ Uenzelmann!Neben:Marine and Petroleum Geolo`y 04 "0887# 524Ð549
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enable a depositional pattern as diverse as that observed[ Fault zones initiated during the rifting of the Atlantic have been observed all along the West African coast\ and it appears plausible for a river to take up its path along a fault[ Re~ection amplitude anomalies have been observed in the area south of the Congo Canyon[ Those anomalies are connected to the presence of both thermogenic and biogenic gas in the sediments[ Thermogenic gas originates from middle Cretaceous black shales\ the biogenic gas has been created in younger sediments deposited by the Benguela Current[ A model for the evolution of Neogene and Quaternary sedimentation proposes that the pattern changed from a single sediment source\ the KNR\ in Late Pale! ogene:Early Neogene time to two sediment sources rep! resented by the KNR and the Congo River\ in Middle:Late Miocene[ Since the Early Quaternary\ the Congo River has been the prevailing sediment source with the Congo Canyon guiding the material farther to the west onto the lower fan[ Acknowledgements I am grateful for the support and assistance of the o.cers and crew of RV Sonne during the expedition[ Furthermore\ the manuscript has greatly bene_ted from the comments of three unknown reviewers[ This is Alfred! Wegener!Institut contribution No 0330 and contribution No of the Joint Research Project 150 The Late Quat! ernary of the South Atlantic[ The work was funded by the German Federal Ministry of Research and Technology "BMBF# under contract No 92R316[ The author is responsible for the contents of this paper[
References Fig[ 02[ Isopach maps of lower unit CF0 "a# and upper unit CF0 "b#[ Note the di}erence in isopach interval[
Ala\ M[ A[\ + Selley\ R[ C[ "0886#[ The West African Coastal Basins[ In Selley "Ed[#\ African Basins[ Sedimentary Basins of the World\ "pp[ 062Ð075#[ Berger\ W[\ Wefer\ G[\ Richter\ C[ et al[ "0887#[ Le` 064 Preliminary Report[ Ocean Drilling Program\ Preliminary Report\ 064\ College Station\ TX "Ocean Drilling Program#[ Bolli\ H[ M[\ + Ryan\ W[ B[ F[ et al[ "0867#[ Initial Reptors of the Deep Sea Drillin` Project\ 39\ Washington] U[S[ Govt Printing O.ce[ Brice\ S[ E[\ Cochran\ M[ D[\ Pardo\ G[\ + Edwards\ A[ D[ "0871#[ Tectonics and Sedimentation of the South Atlantic Rift Sequence] Cabinda\ Angola[ In Watkins and Drake "Eds[#\ Studies in Con! tinental Mar`in Geolo`y\ "pp[ 4Ð07#[ AAPG Mem[\ 23[ Clark\ J[ D[\ Kenyon\ N[ H[\ + Pickering\ K[ T[ "0881#[ Quantitative analysis of the Geometry of Submarine Channels] Implications for the Classi_cation of Submarine fans[ Geolo`y\ 19\ 522Ð525[ Dean\ W[ E[\ Hay\ W[ W[\ + Sibuet\ J[!C[ "0873#[ Geologic Evolution\ Sedimentation\ and Paleoenvironment of the Angola Basin and Adjacent Walvis Ridge] Synthesis of Results of Deep Sea Drilling Project Leg 64[ In W[ W[ Hay\ J[!C[ Sibuet et al[ Init[ Rept[ DSDP\ 64\ "pp[ 498Ð433#[ Washington] U[S[ Govt Printing O.ce[ Droz\ L[\ Rigaut\ F[\ Cochonat\ P[\ + Tofani\ R[ "0885#[ Morphology
al[ "0887# has allowed a modi_cation of the previous de_nition of unit CF0 and re~ector CF!A[ Furthermore\ the identi_cation of an internal re~ector CF!A!int within unit CF0 as representing marine isotope stage 5[5 is now possible[ Additionally\ marine isotope stages 7 and 01 "re~ector CF!A# are identi_ed[ All units are much thinner south of the Congo Canyon than in the north[ This is probably due to both a longer lasting salt diapirism in the south and a preferred depo! sition north of the canyon resulting in stronger subsid! ence[ A hypothesis regarding the tectonic control of the Congo Canyon has been presented[ A tectonic control of the canyon\ i[e[\ a fault zone beneath the canyon would
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and Recent Evolution of the Zaire Turbidite System "Gulf of Guinea#[ Geol[ Soc[ Am[ Bull[\ 097\ 142Ð158[ Edwards\ A[\ + Bignell\ R[ "0877#[ Hydrocarbon Potential of West African Salt Basin[ Oil and Gas Journ[\ 75\ 60Ð63[ Emery\ 0881[ Emery\ K[ O[\ + Uchupi\ E[ "0873#[ The Geolo`y of the Atlantic Ocean[\ Springer\ New York[ Emery\ K[ O[\ Uchupi\ E[\ Phillips\ J[\ Bowin\ C[\ + Mascle\ J[ "0864#[ Continental Margin o} Western Africa] Angola to Sierra Leone[ AAPG Bull[\ 48\ 1198Ð1154[ Foresman\ J[ B[ "0867#[ Organic Geochemistry DSDP Leg 39\ Con! tinental Rise of Southwest Africa[ In Bolli\ Ryan et al[ "Eds[#\ Initial Reports of the Deep Sea Drillin` Project\ "pp[ 446Ð456#[ Washington] U[S[ Govt Printing O.ce[ Frostick\ L[ E[ "0886#[ The East African Rift Basins[ In Selley "Ed[# African Basins[ Sedimentary Basins of the World\ "pp[ 076Ð198#[ Frostick\ L[ E[\ + Reid\ I[ "0878#[ Is Structure the Main Control of River Drainage and Sedimentation in Rifts< J[ Afr[ E[ Scien[\ 7\ 054Ð071[ Gerland\ S[\ Kuhn\ G[\ + Bohrmann\ G[ "0886#[ Physical Properties of a Porcellanite Layer "Southwest Indian Ridge# constrained by Geophysical Logging[ Mar[ Geol[\ 039\ 304Ð315[ Hay\ W[ W[\ + Sibuet\ J[!C[ et al[ "0873#[ Initial Reports of the Deep Sea Drillin` Project\ 64\ Washington] U[S[ Govt Printing O.ce[ Hunt\ J[ M[ "0867#[ Light Hydrocarbons in Holes 250 and 253\ Leg 39[ In Bolli\ Ryan et al[ "Eds[#\ Initial Reports of the Deep Sea Drillin` Project\ 39\ "pp[ 538Ð549#[ Washington] U[S[ Govt Printing O.ce[ Imbrie\ J[\ Hays\ J[ D[\ Martinson\ D[ G[\ McIntyre\ A[\ Mix\ A[ C[\ Morley\ J[ J[\ Pisias\ N[ G[\ Prell\ W[ L[\ + Shackleton\ N[ J[ "0873#[ The Orbital Theory of Pleistocene Climate] Support from a revised Chronology of the Marine d07 O Record[ In Berger et al[ "Eds[#\ Milankovitch and Climate\ "pp[ 158Ð294#\ Part I\ Reidel[ Jansen\ J[ H[ F[\ Ufkes\ E[\ + Schneider\ R[ R[ "0885#[ Late Quaternary Movements of the Angola!Benguela Front\ SE Atlantic\ and Impli! cations for Advection in the Equatorial Ocean[ In Wefer et al[ "Eds[#\ The South Atlantic*Present and Past Circulation\ "pp[ 442Ð 464#[ Springer[ Kendrick\ J[ W[\ Hood\ A[\ + Castan½o\ J[ R[ "0867#[ Petroleum!Gen! erating Potential of Sediments from Leg 39\ Deep Sea Drilling Project[ In Bolli\ Ryan et al[ Initial Reports of the Deep Sea Drillin` Project\ 39\ "pp[ 560Ð565#[ Washington] U[S[ Govt Printing O.ce[ Kenyon\ N[ H[\ Belderson\ R[ H[\ + Stride\ A[ H[ "0867#[ Channels\ Canyons and Slump Folds on the Continental Slope between South! West Ireland and Spain[ Oceanol[ Acta\ 0\ 258Ð279[ Kjarstansson\ E[ "0868#[ Modeling and Migration by the Mono! chromatic 34!Degree Equation[ Stanford Exploration Project Report No 04\ Stanford University[ Leyden\ R[\ Bryan\ G[\ + Ewing\ M[ "0861#[ Geophysical Reconnais! sance on African Shelf] 1[ Margin Sediments from Gulf of Guinea to Walvis Ridge[ AAPG Bull[\ 45\ 571Ð582[
Liro\ L[ M[\ + Coen\ R[ "0884#[ Salt Deformation History and Postsalt Structural Trends\ O}shore Southern Gabon\ West Africa[ In Jack! son et al[ "Eds[#\ Salt Tectonics] a Global Perspective\ AAPG Memoir 54\ "pp[ 212Ð220#[ Miall\ A[ D[ "0886#[ The Geolo`y of Strati`raphic Sequences[ Springer\ 322 pp[ National Geophysical Data Center "0877#[ ETOPO!4 Bathy! metry:Topography Data\ Data Announce[ 77!MGG!91\ Natl Oce! anic and Atmos[ Admin[\ U[S[ Dep[ Commer[\ Boulder\ Colorado[ Reading\ H[ G[ "0885#[ Sedimentary Environments] Processes\ Facies and Strati`raphy[ Blackwell\ p[ 577[ Reyre\ D[ "0873#[ Petroleum Characteristics and Geological Evolution of a Passive Margin[ Example of the Lower Congo!Gabon Basin[ Bull[ Res[ centre Exploration Production Elf Aquitaine\ 7\ 292Ð221[ Schneider\ R[ "0880#[ Spatquartare Produktivitatsveranderungen im ostlichen Angola Becken] Reaktion auf Variationen im Passat!Mon! sun!Windsystem und in der Advektion des Benguela!Kustenstroms[ Berichte aus dem Fachbereich Geowissenschaften\ 10\ 079[ Schneider\ R[ R[\ Muller\ P[ J[\ Ruhland\ G[\ Meinecke\ G[\ Schmidt\ H[\ + Wefer\ G[ "0885#[ Late Quaternary Surface Temperatures and Productivity in the East!Equatorial South Atlantic] Response to Changes in Trade:Monsoon Wind Forcing and Surface water Advection[ In Wefer et al[ "Eds[#\ The South Atlantic*Present and Past Circulation\ "pp[ 416Ð440#[ Springer[ Shepard\ F[ P[\ + Emery\ K[ O[ "0862#[ Congo Submarine Canyon and Fan Valley[ AAPG Bull[\ 46\ 0568Ð0580[ Spiess\ V[\ + Uenzelmann!Neben\ G[ "in prep[# Fluid and gas migration in hemipelagic sediment o} the Congo River*Evidence from a high!resolution seismoacoustic survey[ Stolt\ R[ H[ "0867#[ Migration by Fourier Transform[ Geophysics\ 32\ 12Ð37[ Stow\ D[ A[ V[ "0873#[ Cretaceous to Recent Submarine Fans in the Southeast Angola Basin[ In Hay\ Sibuet et al[ Initial Reports of the Deep Sea Drillin` Project\ 64\ Washington] U[S[ Govt Printing O.ce[ pp[ 660Ð673[ Teisserenc\ P[\ + Villemin\ J[ "0878#[ Sedimentry Basin of Gabon* Geology and Oil System[ In Edwards and Santogrossi "Eds[# Diver! `ent:Passive Mar`in Basins\ AAPG Memoire 37\ 006Ð088[ Uchupi\ E[ "0878#[ the Tectonic Style of the Atlantic Mesozoic Rift System[ J[ Afr[ E[ Scien[\ 7\ 032Ð053[ Uchupi\ E[ "0881#[ Angola Basin] Geohistory and Construction of the Continental Rise[ In Poag + de Graciansky "Eds[# Geolo`ic Evol! ution of Atlantic Continental Rifts\ "pp[ 66Ð88#[ New York] Nostrand Reinhold[ Uenzelmann!Neben\ G[\ Spiess\ V[\ + Bleil\ U[ "0886#[ A Seismic Reconnaissance Survey of the Northern Congo Fan[ Mar[ Geol[\ 039\ 172Ð295[ Ý [ "0876#[ Seismic Data Processing[ Soc[ Explor[ Geophys[\ Yilmaz\ O Investi`ations in Geophysics\ 1\ Tulsa[
Neogene sedimentation history of the Congo Fan G[ Uenzelmann!Neben Alfred!Wegener!Institut fur Polar! und Meeresforschung\ Postfach 019050\ 16461 Bremerhaven\ Germany Received 01 December 0886^ revised 11 May 0887^ accepted 18 May 0887
Abstract High!resolution seismic re~ection data were interpreted to develop a model for the Neogene sedimentation in the Congo Fan area[ Identi_cation of two re~ectors representing marine isotope stages 5 and 01 allows a distinction between the Early and the Late Quaternary[ A single sediment source system during the Late Paleogene:Early Neogene gradually changes into a two sediment source system in the Late Neogene[ Since the Late Quaternary\ the Congo River has been the prevailing sediment source[ The evolving Benguela Current further modi_ed the system and led to a high biogenic productivity[ Evidence for gas in the sediments can be found in the whole area of investigation[ Additionally\ indications for a tectonic control of the Congo Canyon have been found in the distribution of sedimentary units and unit thicknesses[ Þ 0887 Elsevier Science Ltd[ All rights reserved[ Keywords] Seismostratigraphy^ Sedimentation history^ Congo Fan evolution
0[ Introduction The Congo Fan is one of the world|s largest fan systems[ It is the largest depocentre in the Angola Basin and dominates this continental margin "Uchupi\ 0878^ 0881#[ The fan is now being fed mainly by material from the Congo river that is transported through the Congo Canyon to be deposited on the lower fan[ Little has been known hitherto about the development of the Congo Fan and the Congo Canyon[ Seismic sur! veys carried out in the early |69s resulted in a general view of the fan and revealed the presence of numerous salt diapirs "Leyden\ Bryan + Ewing\ 0861^ Emery\ Uchupi\ Phillips\ Bowin + Mascle\ 0864#[ Although the oil indus! try has worked o}!shore Gabon\ Cabinda and Angola only few seismic data have been published "Brice\ Coch! ran\ Pardo + Edwards\ 0871^ Edwards + Bignell\ 0877^ Teisserenc + Villemin\ 0878^ Liro + Coen\ 0884#[ Recent investigations have dealt with the Quaternary morphology and evolution of the depositional systems and environments "Droz\ Rigaut\ Cochonat + Tofani\ 0885^ Uenzelmann!Neben\ Spiess + Bleil\ 0886#[ A stratigraphy for the Congo Fan was developed by Uenzelmann!Neben et al[ "0886# which is followed here in slightly modi_ed form[ Indications for a more complex evolution of the depositional system in the Congo Fan area have been given by Droz et al[ "0885# and Corresponding author[ Tel[] 9938 360 3720 197^ fax] 9938 360 3720 038^ e!mail] uenzelÝawi!bremerhaven[de S9153Ð7061:87 ,08[99 Þ 0887 Elsevier Science Ltd[ All rights reserved PII] S 9 1 5 3 Ð 7 0 6 1 " 8 7 # 9 9 9 2 3 Ð 7
Uenzelmann!Neben et al[ "0886#[ In this paper\ this aspect is investigated more closely and a model is presented for Neogene:Quaternary sedimentation[ To give a short summary on the general geologic evol! ution] Mezosoic rifting in the South Atlantic produced a succession of _ve distinct tectonic regimes] Prerift\ Synrift "I and II#\ Postrift and Regional Subsidence "Brice et al[\ 0871#[ Up to 0999 m of evaporites were deposited during the Aptian[ This was followed by initial movement of the salt in Albian time "Leyden et al[\ 0861^ Emery et al[\ 0864^ Emery + Uchupi\ 0873^ Emery\ 0881#[ The seaward edge of the salt diapirs is formed by the prominent Angola Escarpment "Emery + Uchupi\ 0873^ Emery\ 0881#[ Crossing the escarpment\ the sea~oor drops sharply by up to 0999 m "Fig[ 0#[ During the middle Cretaceous "AptianÐTuronian#\ black shales and bituminous sand! stones were deposited on top of the evaporites "Bolli et al[\ 0867^ Brice et al[\ 0871^ Emery + Uchupi\ 0873^ Uchupi\ 0881#[ Deposition of turbidites started in the Oligocene "Brice et al[\ 0871^ Uchupi\ 0881#[ The Benguela Current\ the major oceanic current in the eastern South Atlantic\ commenced in the Late Miocene "Dean\ Hay + Sibuet\ 0873#[ A large amount of organic material was produced by associated upwelling and led to mass movement in the southern Angola Basin "Emery + Uchupi\ 0873^ Uchupi\ 0881#[ 1[ Data acquisition and processing About 0199 km of high!resolution re~ection seismic data were gathered on the Congo Fan during a joint
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Fig[ 0[ Location of seismic pro_les shot on the Northern Congo Fan[ Bathymetry according to ETOPO!4 "National Geophysical Data Center\ 0877#[ The italicized numbers refer to the parts of the lines displayed in Figs 1\ 2\ 4\ 6 and 8[ The dots show the locations of the recently drilled Sites 0964\ 0965 and 0966 of ODP leg 064[ The dot!dashed line marks the position of the Congo Canyon according to Shepard + Emery "0862#[ The dotted line shows the location of the Angola Escarpment[ A fault zone beneath the Congo Canyon as proposed by Reyre "0873# is marked by the hatched area[ The parts of the fan referred to in the text as middle and upper fan are separated by the dashed line[ Asterisks in the insert map show the locations of DSDP Sites 253 and 429[
study made by the Alfred!Wegener!Institute and the Uni! versity of Bremen "Fig[ 0#[ A single GI!gunTM in the true GI mode "an airgun with two chambers] one to generate the seismic pulse and a second one triggered delayed to destroy the bubble pulse# was _red every 14 m as seismic source generating frequencies up to 299 Hz[ This pro! vided a vertical resolution of ½1[4 m[ The data were received employing an 799 m streamer with 13 channels in a 599 m active section and recorded on a EG+G ES 1319 multichannel digital recording system[ Positioning was by GPS "Global Positioning System#[ In parallel with the seismic measurements\ continuous records were acquired with the ParasoundTM system\ a narrow beam echosounder of only 3> beam width with high vertical and lateral resolution[ This system was run with a signal of 3 kHz and 149 ms pulse length[ The shot interval was discontinuous and depended on the water depth\ but was generally every 0Ð1 s[ This system has a vertical resolution of 09 cm\ while penetrating the upper 49Ð049 m of the sedimentary column[ Thus\ high quality images of recent sedimentation and the surface structures were also collected "Spiess + Uenzelmann!Neben\ in prep[#[ The processing of the multichannel seismic data com! prised demultiplexing\ de_nition of the geometry invok!
ing the navigation data "CDP interval 14 m# and CDP sorting[ Due to the very good seismic signal "short double pulse\ bubble signal suppressed# and a negligible ocean bottom multiple\ no deconvolution was applied[ Despite the fact that the small streamer length gave only a rela! tively small move out\ a detailed velocity analysis "every 49 CDP# was carried out[ This was required because of the complex subbottom morphology[ After correcting both spherical divergence and normal move out and stacking\ a _nite di}erence omega!x migration was applied to the data[ This algorithm was chosen because it is accurate for dips up to 54>\ while simultaneously being able to handle lateral velocity variations "Stolt\ 0867^ Kjarstansson\ 0868^ Yilmaz\ 0876#[ This method was especially useful in regions with steeply dipping re~ectors due to salt diapirism[ For display\ no gain "i[e[\ AGC# or _ltering except for an anti!aliasing!_lter "259 Hz during aquisition# were applied to the data[ The amplitudes shown on the pro_les therefore represent values relative to the maximum amplitude of the whole section[ 2[ Stratigraphy of re~ector CF!A!int In general\ the seismic stratigraphy as de_ned by Uenzelmann!Neben et al[ "0886# is followed in this paper[
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The recent ODP leg 064 has supplied new information about the nature and age of the sediments drilled at three Sites 0964Ð0966 in the Congo Fan area "Berger\ Wefer + Richter\ 0887# resulting in modi_cation of the strati! graphy presented by Uenzelmann!Neben et al[ "0886#[ Uenzelmann!Neben et al[ "0886# de_ned four seismo! stratigraphic units according to their re~ection charac! teristics ranging in age from Eocene to Quaternary "Fig[ 1#[ The lowermost unit CF3 "Eocene\ 269Ð429 mbsf\ 029Ð 104 m thick# is characterized by weak internal re~ections and is interpreted to document an increasing terrigenous sediment input[ Unit CF2 "Middle Eocene:Lower Oli! goceneÐMiocene\ 109Ð209 mbsf\ 059Ð119 m thick# again shows weak internal re~ections[ It is separated from unit CF3 by a prominent re~ector CF!B "Eocene:Oligocene\ Figs 1\ 2 and 8# and is inferred to be related to the development of the Congo River drainage system[ The build!up of the Benguela Current "Late MioceneÐ Pliocene\ Dean et al[\ 0873# is represented by unit CF1 "79Ð039 mbsf\ 029Ð069 m thick#[ Unit CF0 "Late Pliocene\ 79Ð039 m thick# is interpreted to consist of deep marine hemipelagic clays and oozes[ The sites of ODP Leg 064 located within our area of investigation "see Fig[ 0# have all penetrated the seabed only to a subbottom depth of 191Ð196 m "Table 0^ Fig[ 1^ Berger et al[\ 0887#[ This lies still within unit CF1 and thus\ nothing could be learned about the nature of units CF2 and CF3 and re~ector CF!B[ The sites show rela! tively homogeneous material of Pleistocene!Pliocene age
526
"Table 0^ Berger et al[\ 0887#[ Sedimentation rates were derived for all sites and a sonic log was obtained at Site 0966 "Berger et al[\ 0887#[ Using this information the age of re~ector CF!A was re!dated from Pliocene to marine isotope stage 01 "see lower part of this section# and unit CF0 from Late Pliocene:Quaternary to Quaternary[ Within unit CF0 a strong continuous re~ection CF!A! int can be observed "Figs 2 and 3#[ This re~ector is easily mapped and lies at a mean depth of 29 ms below sea~oor "¼12 m using a 0499 m:s conversion velocity as deduced from the analysis of the seismic data and ODP well log! ging "Berger et al[\ 0887##[ The re~ector shows slightly larger amplitudes than the base re~ector CF!A of unit CF0 but no change in re~ection characteristics can be observed "Figs 2] CDPs 0499Ð0599 and 0699Ð1999\ and 3] CDPs 0099Ð0299 and 0599Ð0799#[ Re~ector CF!A!int is very smooth and shows no indications of erosion\ or of trans! or regression or non!depositional "toplap# character[ This implies that the re~ector is primarily related to a change in lithology and hence in the com! position of the sediments[ The re~ector shows only minor deformation due to salt movement[ Anomalies in re~ec! tion amplitude observed south of the Congo Canyon "Figs 3b] CDPs 0699Ð0799\ 6b] CDPs 3349Ð3499\ and 4^ note the yellow!red colours indicating a change in re~ection polarity# are probably related to the rise of thermogenic gas and:or the formation of gas hydrates "Uenzelmann!Neben et al[\ 0886#[ According to Berger et al[ "0887#\ sedimentation rates
Fig[ 1[ Correlation of information from ODP Leg 064 Sites 0964Ð0966 "Berger et al[\ 0887# with the seismic data[ For location see Fig[ 0[ The hatched blocks show the location and penetration of the drill sites within the seismic lines[ The stratigraphy according to Uenzelmann!Neben et al[ "0886# has been included with CF0!CF3 referring to the seismic units and CF!A!int!CF!Ca to the seismic re~ectors[
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Fig[ 2[ Line GeoB 82!992:AWI!82912 showing the stratigraphy adapted from Uenzelmann!Neben et al[ "0886# and\ additionally\ re~ector CF!A!int within unit CF0[ Note the bottom simulating re~ector "BSR# and the loss in re~ection amplitude over the salt diapir indicating the presence of gas hydrates and ~uids[ Further note the debris ~ow in unit CF2 "chaotic internal structure with continuous re~ections only on the western side#[ Modi_ed from Uenzelmann!Neben et al[ "0886#[
are 099 m:My on average "Site 0964# and 199Ð069 m:My "Sites 0965 and 0966\ respectively# for the depth range of unit CF0[ Sonic log velocities range from 0369Ð0409 m:s "Berger et al[\ 0887#\ and velocities deduced from seismic re~ection data lie in the same range "0449 m:s#[ Thus\ an age of about 064 ky "Sites 0965 and 0966# is estimated for re~ector CF!A!int with a strong deviation at Site 0964 "119 ky#[ This deviation is probably due to the fact that only average sedimentation rates have yet been published for Site 0964 and average values for Sites 0965 and 0966 would lead to more similar estimated ages of 084 ky and 194 ky\ respectively[ The detected age of re~ector CF!A! int corresponds to marine isotope stage 5[5 "Sites 0965 and 0966#[ Changes in sediment composition have been reported for stage 5[5 "Schneider\ 0880^ Jansen\ Ufkes + Schneider\ 0885^ Schneider et al[\ 0885#[ The accumulation rates of marine organic carbon were much higher during stage 5[5 than stage 6[ Higher accumulation rates of opal and an increase in CaCO2 production were observed[ Fur! thermore\ the paleoproductivity increased in stage 5[5 "Schneider\ 0880#[
Since these changes in composition result in a sig! ni_cant shift in density "lower density of opal and higher porosity due to higher opal content\ Gerland\ Kuhn + Bohrmann\ 0886# they can cause a conformable re~ector[ According to Schneider "0880#\ stage 5[5 represents a glacial as well as a period of minimum precession[ This led to a maximum in the monsoon which in turn moved the Angola!Benguela!Front northwards "Schneider\ 0880#[ As a result\ the Benguela Current probably reached farther north\ leading to increased upwelling and thus production of organic material deposited in this area[ All this could initiate modi_cations in sediment composition[ Jansen et al[ "0885# present a modi_cation of Schneider|s "0880# model with the Angola!Benguela! Front shifting southwards from its northernmost pos! ition "north of 8> N# during stage 5[5 in contrast to the northward shift assumed by Schneider "0880#[ Non! etheless the shift of the Angola!Benguela!Front as a result of a glacial and minimum precession led to changes in the composition of the sediments[ Those changes are documented as a re~ector which is assumed to be re~ector CF!A!int\ i[e[\ a climatically!enhanced condensed zone\
074 096 69 32 069 in 9Ð019 m 033 in 019Ð029 m 039 in 029Ð199 m Computed from sonic log information for Site 0966[ For Site 0966 computed from magnetostratigraphy[
0369Ð0409 Diatom!rich\ diatom!bearing\ nannofossil!rich\ nannofossil! bearing clay 0 191 0966
169
0 193 0965
169
0 169 196 0964
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and the event can thus be mapped regionally[ So\ the upper unit CF0 is supposed to represent the Late Quat! ernary[ However\ other glacial stages are less easily docu! mented as seismic re~ectors[ Marine isotope stage 1 can! not be resolved with the available seismic re~ection data[ From the sedimentation rates deduced by ODP leg 064 "Berger et al[\ 0887# a corresponding re~ector would be expected at 4 ms to 09 ms below sea~oor and lies within our seismic wavelet "Figs 2 and 3#[ In contrast\ the Par! asound data show weak indications of a re~ector "Fig[ 5#[ Stage 7 represents a glacial which was less severe than stage 5 "Imbrie et al[\ 0873#[ Thus\ the resulting changes in sediment composition and sedimentation rate were probably not su.cient to generate a seismic re~ector[ Nevertheless\ converting the age into traveltime via sedi! mentation rates and sonic log velocities "Berger et al[\ 0887# any re~ector would lie at a depth of 49Ð69 ms depth below sea~ow[ This is not always distinguishable from re~ector CF!A!int in our seismic re~ection data due to the length of the seismic wavelet[ However\ the Par! asound data do show a thin re~ector at an appropriate depth "Fig[ 5#[ Marine isotope stage 01 covers the period 339Ð369 ky "Imbrie et al[\ 0873#[ From sedimentation rates from ODP leg 064 "Berger et al[\ 0887#\ a corresponding re~ector would be expected at traveltimes around 099Ð009 ms depth below sea~oor[ Re~ector CF!A lies at this depth and marks signi_cant changes in re~ection characteristics "Figs 2] CDPs 0499Ð0599 and 0699Ð1999\ 3] CDPs 0099Ð 0499 and 0699Ð0799\ and 5\ Uenzelmann!Neben et al[\ 0886#[ Thus\ the age inferred for this re~ector by Uen! zelmann!Neben et al[ "0886# is corrected "Pliocene to marine isotope stage 01#[ Furthermore\ not only move! ments of the CCD and changes from high variability to longer lasting conditions "Uenzelmann!Neben et al[\ 0886# seem to be responsible for re~ector CF!A but in addition similar changes in composition as observed for stage 5[
Pliocene
067 000 69 34 199 in 9Ð79 m 49 in 79Ð019 m 109 in 019Ð199 m Carbon!rich olive gray clay
Pliocene
119 099 49 29 Pleistocene! Pliocene 099 diatomaceous clay\ high gas concentrations "CO1\ Methane# mainly below 099 m "¼019 ms#
Computed age for CF!A!int "ky# CF!A Stage 7 Traveltime "ms# to CF!A!int Age Sedimentation rate "m:m[y[# Velocity "m:s# Lithology Lithologic units Traveltime to base TWT "ms# Subbottom depth "m# ODP leg 064 site
Table 0 Drill information from ODP Leg 064 Sites 0964Ð0966 "Berger et al[\ 0887#[ The traveltimes to re~ectors CF!A!int\ CF!A and marine isotope stage 7 have been added[ Furthermore\ the age for re~ector CF!A!int using sedimentation rates and velocity information from the sites "Berger et al[\ 0877# have been computed
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3[ Distribution of Neogene:Quaternary sediments To better de_ne and describe the sedimentary environ! ments\ the seabed re~ection and re~ectors CF!A!int\ CF! A\ CF1!base and CF!B were mapped\ thicknesses of the di}erent units computed and maps of re~ector depth\ re~ection amplitude and unit thickness compiled[ Since no reliable seismic velocities could be deduced from our data due to the recording con_guration "short streamer length vs great target depth# both maps of re~ector depth and unit thickness are still in ms two waytime and strictly represent neither depth nor thickness[ Both terms are used here for better clarity[ To carry our a rough depth:thickness conversion velocity values at 0499 m:s\ 0699 m:s and 0649 m:s can be used for sedimentary units upper CF0\ lower CF0\ CF1 and CF2\ respectively[
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Fig[ 3[ Line GeoB 82!909:AWI!82929 showing the Angola Escarpment "a#[ Movement of the salt led to faulting and deformation of the sediments "b#[
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Fig[ 4[ Map of the re~ection amplitude of re~ector CF!A!int[ Note the anomalies in the area south of the Congo Canyon[ There\ even positive re~ection amplitudes were observed "note the yellow to red colours#[ This is interpreted as an indication for the presence of gas in the sediments[
steps "Fig[ 3] CDP 0199Ð1199#[ This might be a result of downfaulting along the escarpment "Fig[ 3] CDPs 0199Ð 1999#[ One can clearly see that the escarpment itself is formed by salt diapirs which lead to deformation and inclination "{block faulting|# of the sedimentary layers[ The sedimentary units on top of the salt diapirs wedge out towards the escarpment "Figs 3b] around CDP 0899\ and 7b] around CDP 3599#[ Indications for this were given by\ e[g[\ Emery + Uchupi "0873# and Uchupi "0881#[ The morphology of the di}erent re~ectors varies more to the south of the Congo Canyon than to the north of it[ This is true even for the sea~oor where small elevations and depressions can be seen close to the Angola Escarp! ment while the re~ector appears very smooth in the north[ The only re~ector to display small scale morphological variations in the north is re~ector CF!B "Fig[ 6b#[ This re~ector shows a few elevations and depressions in the north[ South of the canyon the re~ector distribution is heavily a}ected by this high!frequency sinuosity[ This observation implies that the salt diapirs lie shallower south of the Congo Canyon than north of it[ Thus\ the
The mapped re~ectors in general show an even seaward increase in depth "Fig[ 6#[ They do not reveal a paleoshelf\ i[e[\ no strong progradation of the shelf took place during the late Neogene:Quaternary[ Moreover\ the slope!rise transition remained fairly stationary] it only moved from 00> E "re~ectors CF!B and CF1!base# to 09>49? E "re~ec! tor CF!A# and back via 09>44? E "re~ector CF!A!int# to 00> E "re~ector Sea~oor#[ This indicates sediment trans! port and deposition controlled by channel and turbidite systems for the late Neogene:Quaternary[ The Angola Escarpment can be easily recognized in all re~ectors mapped south of the Congo Canyon "Figs 3] CDPs 0199Ð1199\ 6 and 7] CDPs 3599Ð3799#[ In contrast\ no indications for the escarpment can be found on the northern lines[ The question of whether the escarpment terminates at the canyon or whether its course is modi_ed by the canyon "Emery + Uchupi\ 0873^ Emery\ 0881# remains open since our lines do not cover the area across the canyon[ The escarpment appears to mark the seaward edge of salt diapirism "Figs 3] around CDP 0899\ and 7] around CDP 3599^ also see Emery et al[\ 0861^ Leyden et al[\ 0864#[ In some places\ the escarpment occurs in two
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Fig[ 5[ Parasound record of line GeoB 82!990:AWI!82910[ Re~ectors representing di}erent marine isotope stages "stage 1\ CF!A!int ¼ stage 5\ stage 7\ CF!A ¼ stage 01# can be identi_ed[
ascent of the salt was either stronger or longer lasting or both in the south[ Above and in the close vicinity of salt diapirs\ the re~ectors show variations in re~ection amplitude[ Strong amplitudes and phase conversions are found on the sou! thern lines for re~ectors CF!A!int and CF!A "Figs 4 and 7c] CDPs 2149\ 2[4 s TWT\ and 2349Ð2699\ 2[64 s TWT#[ They can be linked with elevations of the re~ectors[ This is an indication for either condensed sections or small amounts of gas which has risen at the ~anks of the highs "Uenzelmann!Neben et al[\ 0886#[ The interpretation of gas is favoured since BSRs "bottom simulating re~ectors# have additionally been observed "Fig[ 2] CDPs 0599Ð 0549\ 3[1Ð3[04 s TWT^ also see Uenzelmann!Neben et al[\ 0886#[ Thermogenic gas generated from middle Cre! taceous "AlbianÐTuronian# black shales "Foresman\ 0867^ Hunt\ 0867^ Kendrick\ Hood + Castan½o\ 0867^ Stow\ 0873# is interpreted to have risen along faults and salt diapirs[ The oil and gas potential of this region has been known for a number of years with established o}! shore and onshore oil and gas production "Brice et al[\ 0871^ Teisserenc + Villemin\ 0878^ Liro + Coen\ 0884#[ Furthermore\ the high productivity introduced by the evolving Benguela Current "Bolli et al[\ 0867^ Schneider\
0880^ Schneider et al[\ 0885# and the high sedimentation rates observed for the Pliocene:Quaternary "Berger et al[\ 0886# may also have favoured the generation of biogenic gases which assembled in the local highs[ Although no traces of methane hydrates were found at either of the recently drilled ODP leg 064 Sites 0964Ð0966 "Berger et al[\ 0887# modest amounts of methane and carbon diox! ide detected at all three sites are the result of microbial degradation in the sediment[ Apart from these local variations associated with salt diapirs\ in general re~ection amplitudes are slightly larger in the north "e[g[\ Fig 4\ note the regional trend#[ This is best observed for the sea~oor and re~ectors CF!A!int and CF!B[ There\ the strongest amplitudes can be found in a water depth of 499Ð1699 m on the inner fan where the isobaths broaden from the north into the sediment cone "Fig[ 0#[ The increased re~ection amplitude is inter! preted as the result of winnowing by bottom currents which removed all _nes from coarse!grained sediments "Reading\ 0885#\ probably turbidites\ leaving material with a higher re~ectivity[ Droz et al[ "0885# found coarse sediment associated with high re~ectivity in the active channel[ Condensed sections due to very slow sedi! mentation rates show high re~ectivity as well[ Miall
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than 699 ms TWT#[ To better describe the isopachs a depocentre was de_ned[ Depocentre in this context depicts the area where the thickness of the unit reaches values signi_cantly higher than the root mean square "rms# thickness of the unit[ Similar observations can be made for units CF1\ lower and upper unit CF0 "Figs 2\ 3\ 7\ 09 and 02#[ Moreover\ the size of the depocentre is reduced "½069 km1 for unit CF2\ ½89 km1 for upper unit CF0#\ and it was shifted into the northeastern corner of our area of investigation "Figs 8 and 01#[ The change in size and location of the depocentre indicates alterations in the sediment source and supply[ Debris ~ows identi_ed on the seismic data "Figs 2\ 09 and 00# give further signs for a re!location of the sediment source[ Although debris ~ows are not directly related to ~uvial input and are often generated from the slope\ a change in their general distribution gives indications for a re!location of the sedi! ment source[ These observations are interpreted as evi! dence for a prevailing sediment supply via the Kouilou:Niari turbidite system "KNTS# during the Late Paleogene as indicated by Droz et al[ "0885# and Uen! zelmann!Neben et al[ "0886#[ The fact that nearly all debris ~ows encountered in unit CF3 "e[g[\ Fig[ 09] CDPs 1499Ð1799\ 2[2 s TWT^ Fig[ 00# lie north of 4> N and east of 8>49? E\ i[e[\ the main Paleogene sediment body is found near the Kouilou:Niari rivermouth\ supports this suggestion[ Furthermore\ the depocentre in unit CF2 "Early Neogene# lies directly in front of the Kouilou:Niari estuary[ The sediment was deposited near the source and later re!deposited in the middle fan area[ Debris ~ows found in units CF2 and CF1 are located in greater water depths "Figs 2] CDPs 0699Ð1999\ 3[0Ð3[1 s TWT\ and 00# farther south and west[ This indicates that the sediment source was no longer solely from the Kouilou:Niari River "KNR# system[ More sediments were derived from the south and deposited in the middle fan[ This southern component is interpreted as contributions via the Congo River and the evolving Benguela Current system "Fig[ 01#[ Since this modi_cation in sediment source can be observed from unit CF2 to unit CF1\ it is placed around Middle:Late Miocene[ As the build!up of the fan pro! gressed the sediment was transported via the Congo Can! yon farther to the west "Figs 01 and 02#[ This westward transport _nally led to deposition in an area not covered by our lines[ The question remains why so much less of the sediment was deposited south of the Congo Canyon[ One reason might be the fact that in the Late Paleogene the dominant sediment source was located in the northeast thereby encouraging deposition in the north and re!deposition in the north and west[ With the onset of the Benguela Coastal Current and its migration to the north\ the material distributed by the Congo and not captured by the canyon\ i[e[\ _ne material\ was de~ected northwards[ The general transport of the sediments has been towards the west for unit CF0 which prevents the deposition of a
Fig[ 6[ Maps of depth to re~ector CF!A!int "a# and CF!B "b#[ Note the smooth surface of re~ector CF!A!int whereas re~ector CF!B is strongly a}ected by the salt topography[
"0886# pointed out that a number of high re~ectivity horizons appear to owe their origin to current erosion associated with the growth of the Antarctic ice cap[ Re~ectors CF!B and CF!A!int are correlated with the onset of the Antarctic Bottomwater in Eocene:Oligocene times and marine isotope stage 5[5\ respectively\ and thus their origin due to condensed sections is not favoured[ A thick pile of sediment that gradually thins seaward can be observed in the north for unit CF2 "Figs 2\ 8 and 09# To the south of the Congo Canyon\ the unit is more evenly distributed with only small highs and lows linked to salt diapirs and generally thinner than to the north "highest values of 139 ms TWT contrasting with more
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Fig[ 7[ Line GeoB 82!902:AWI!82922 showing the Angola Escarpment "a# and the salt diapirs landward of the escarpment in enlarged sections "b and c#[ Note the faulting and deformation of the sediments\ the ~at spot and the strong re~ection amplitudes hinting on gas!bearing sediments[
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Fig[ 7[ Continued[
thick sequence in the south[ Still\ the discrepancy between unit thicknesses in the north and south is too large to be due solely to these mechanisms[ Two further explanations could be di}erential sub! sidence within the Congo Basin resulting in either stron! gly northwards tilted sequences and:or displacement of the layers along a fault[ A similar setting has been observed in the Gabon Basin "Teisserenc + Villemin\ 0878#[ There\ the basin is divided into the Northern and Southern subbasins by the N|Komi Fracture Zone which show di}erent depths to continental basement "01 km and 8 km\ respectively#[ The postsalt deposits rapidly plunge to the north from the platform south of the frac! ture zone into the Northern subbasin|s o}shore portion "Teisserenc + Villemin\ 0878#[ The N|Komi Fracture Zone was created in the early rifting of the Atlantic and remained active throughout the whole rift phase "Teiss! erenc + Villemin\ 0878#[ The coastal basins of west Africa have been found to be broadly similar in tectonic style "Ala + Selley\ 0886#[ Such fracture and fault zones can be found all along the West African coast "Emery + Uchupi\ 0873^ Teisserenc + Villemin\ 0878^ Liro + Coen\ 0884#\ and especially Reyre "0873# suggests one at the location of the Congo Canyon "Fig[ 0#[ It appears plaus! ible for a river to take up its path along a fault since the
penetration into the rock in order to create a riverbed is easier along a zone of weakness[ This tectonic control has been observed for other submarine canyons\ e[g[\ the Cascadia Canyon "Clark\ Kenyon + Pickering\ 0881# or a number of canyons in the Bay of Biscay "Kenyon\ Belderson + Stride\ 0867#[ Furthermore\ a structural con! trol on distribution of the Eocene and Miocene sedi! mentary facies has been reported for the Cuanza Basin in the south as well "Ala and Selley\ 0886#[ The existence of a fault beneath the canyon would enable di}erential subsidence within the Congo Basin[ This was probably assisted by the location of the sedimentary source[ With the main amount of material being contributed initially by the Kouilou:Niari Turbidite System the northern sub! basin acquired thicker sedimentary sequences "Figs 8 and 01a# leading to stronger subsidence[ When the main sedi! ment source changed from the KNTS to the Congo River and contributions from the Benguela Current "Figs 01bÐ d and 02# and the material was distributed more equally to the north and south subbasins and the di}erential subsidence ceased[ The frequent reactivation of transverse zones often results in the gathering of terrigenous clastics in deltaic systems or submarine canyons "Reyre\ 0873#[ This way the build!up of the Congo Fan might have been encour!
) 535
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Fig[ 8[ Isopach map of unit CF2[ Note the strong di}erence in thickness north and south of the Congo Canyon and the big depocentre near the estuary of the Kouilou:Niari River[
Kouilou Niari River and the Congo River[ The open arrow marks material transported by the Benguela Coas! tal Current[ In the Late Paleogene:Early Neogene the Kouilou Niari River appeared to be the main sediment source[ Indications for this were already given by Droz et al[ "0885# and Uenzelmann!Neben et al[ "0886#\ and the iso! pachs of unit CF2 verify this "Fig[ 8# showing the main pile of sediments in the northeastern part of our area of investigation[ Huge amounts of sediment were deposited on the shelf and slope in front of the KNR estuary and re!deposited via the KNTS[ This resulted in a large thick depocentre on the slope and rise on the northern rim of the Congo Fan "Figs 8 and 01a#[ The Late Neogene "Late Miocene<# saw a modi_cation in prevailing sediment source[ Less material was con! tributed by the KNR[ Moreover\ the evolving Benguela Coastal Current led to a northward de~ection of the sediment and the build!up of a smaller thinner depocentre near the estuary[ This is documented in the isopachs of unit CF1[ Another developing sediment source was fed by the Congo River[ Doming associated with rifting in the East African Rift in Miocene time led to a strong
aged[ Very few lines crossing the Congo Canyon have been published "e[g[\ Leyden et al[\ 0861^ Emery et al[\ 0864^ Droz et al[\ 0885#[ The seismic refraction data of Leyden et al[ "0861# are unfortunately not detailed enough and located obliquely to resolve the proposed fault zone whereas the high!resolution seismic re~ection pro_les of Droz et al[ "0885# do not penetrate deep enough to con_rm our theory[ Additional data are needed to con_rm our hypothesis[
4[ Neogene:Quaternary deposition The interpretation of sedimentary structures and maps of re~ector depth\ re~ection amplitude and unit thickness led to the construction of a model for the Neogene and Quaternary deposition in the Congo Fan area which re_nes the ideas of Droz et al[ "0885# and Uenzelmann! Neben et al[ "0886#[ Figure 01 shows the di}erent stages of the scenario from unit CF2 "Late Paleogene:Early Neogene\ Fig 01a# to upper unit CF0 "Late Quaternary\ Fig 01d#[ The shaded areas show the depocentres while the black arrows represent the contributions from the
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Fig[ 09[ Line GeoB 82!996:AWI!82916[ Note the debris ~ow in unit CF3 and the strong faulting in unit CF1 due to di}erential compaction[
Fig[ 00[ Map shows the distribution of mass transport features "slumps and debris ~ows# as a function of sedimentary unit[
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Fig[ 01[ Model for the evolution of the deposition in the Congo Fan since the Late Paleogene[ The shaded areas show the depocentres for "a# unit CF2 "Late Paleogene:Early Neogene#\ "b# unit CF1 "Late Neogene#\ "c# lower unit CF0 "Early Quaternary#\ and "d# upper unit CF0 "Late Quaternary#[ The black arrows represent the distributions by the Kouilou:Niari River and the Congo River\ and the open arrow marks the material contributed by the Benguela Current "BC#[
pick!up of the Congo River drainage away from the East African Rift "Frostick + Reid\ 0878^ Frostick\ 0886#[ As a result\ more sediment was carried into the Atlantic[ The main material was transported via the Congo Canyon and also deviated to the northwest where a second depo! centre was created "Fig[ 01b#[ Little material found its way to the area south of the canyon "Fig[ 01b#[ The depocentre of lower unit CF0 again shrank as can be seen in Figs 01c and 02a[ Obviously\ less and less material was distributed by the KNR during Early Quat! ernary while the amount of sediment carried by the Congo River increased[ The Congo Fan\ and thus the canyon as well\ was built out farther to the west\ and a large amount of the contributed sediments was trans! ported into the middle fan where our lines do not cover it "Fig[ 02a#[ A signi_cant component of the material was de~ected to the north by the Benguela Coastal Current "Fig[ 02a#[ The onset of the Benguela Current in Late Miocene "Dean et al[\ 0873# led to a further modi_cation of the sedimentary environment[ The establishment of estuarine!in~uenced upwelling "Berger et al[\ 0887# led to an increased productivity and\ moreover\ reworking of shelf sediments as observed in ODP Leg 064 Site 0965 "Berger et al[\ 0887#[ The depocentre observable in our
data thus moved westward with insigni_cant quantities being laid down south of the canyon "Fig[ 01c#[ During the Late Quaternary\ the size of the depocentre at the northern rim of the Congo Fan shrank further "Fig[ 01d#[ The KNR has contributed even less material while the main sediments contributed by the Congo River have been guided to the middle and lower fan by the Congo Canyon[ A small component has again been de~ected to the north by the Benguela Coastal Current and deposited on the northern rim of the fan "Figs 01d and 02b#[ This could explain the observed distribution of upper unit CF0[ Since the amount of sediment seized by the Benguela Coastal Current is less than during the Early Quaternary "lower unit CF0# the size of the depocentre is further reduced while its location has moved upslope[
5[ Conclusions Neogene sedimentation on the Congo Fan has been interpreted using a regional set of high resolution seismic lines[ The stratigraphy developed by Uenzelmann!Neben et al[ "0886# is extended to four seismic lines located south of the Congo Canyon[ Information supplied by Berger et
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enable a depositional pattern as diverse as that observed[ Fault zones initiated during the rifting of the Atlantic have been observed all along the West African coast\ and it appears plausible for a river to take up its path along a fault[ Re~ection amplitude anomalies have been observed in the area south of the Congo Canyon[ Those anomalies are connected to the presence of both thermogenic and biogenic gas in the sediments[ Thermogenic gas originates from middle Cretaceous black shales\ the biogenic gas has been created in younger sediments deposited by the Benguela Current[ A model for the evolution of Neogene and Quaternary sedimentation proposes that the pattern changed from a single sediment source\ the KNR\ in Late Pale! ogene:Early Neogene time to two sediment sources rep! resented by the KNR and the Congo River\ in Middle:Late Miocene[ Since the Early Quaternary\ the Congo River has been the prevailing sediment source with the Congo Canyon guiding the material farther to the west onto the lower fan[ Acknowledgements I am grateful for the support and assistance of the o.cers and crew of RV Sonne during the expedition[ Furthermore\ the manuscript has greatly bene_ted from the comments of three unknown reviewers[ This is Alfred! Wegener!Institut contribution No 0330 and contribution No of the Joint Research Project 150 The Late Quat! ernary of the South Atlantic[ The work was funded by the German Federal Ministry of Research and Technology "BMBF# under contract No 92R316[ The author is responsible for the contents of this paper[
References Fig[ 02[ Isopach maps of lower unit CF0 "a# and upper unit CF0 "b#[ Note the di}erence in isopach interval[
Ala\ M[ A[\ + Selley\ R[ C[ "0886#[ The West African Coastal Basins[ In Selley "Ed[#\ African Basins[ Sedimentary Basins of the World\ "pp[ 062Ð075#[ Berger\ W[\ Wefer\ G[\ Richter\ C[ et al[ "0887#[ Le` 064 Preliminary Report[ Ocean Drilling Program\ Preliminary Report\ 064\ College Station\ TX "Ocean Drilling Program#[ Bolli\ H[ M[\ + Ryan\ W[ B[ F[ et al[ "0867#[ Initial Reptors of the Deep Sea Drillin` Project\ 39\ Washington] U[S[ Govt Printing O.ce[ Brice\ S[ E[\ Cochran\ M[ D[\ Pardo\ G[\ + Edwards\ A[ D[ "0871#[ Tectonics and Sedimentation of the South Atlantic Rift Sequence] Cabinda\ Angola[ In Watkins and Drake "Eds[#\ Studies in Con! tinental Mar`in Geolo`y\ "pp[ 4Ð07#[ AAPG Mem[\ 23[ Clark\ J[ D[\ Kenyon\ N[ H[\ + Pickering\ K[ T[ "0881#[ Quantitative analysis of the Geometry of Submarine Channels] Implications for the Classi_cation of Submarine fans[ Geolo`y\ 19\ 522Ð525[ Dean\ W[ E[\ Hay\ W[ W[\ + Sibuet\ J[!C[ "0873#[ Geologic Evolution\ Sedimentation\ and Paleoenvironment of the Angola Basin and Adjacent Walvis Ridge] Synthesis of Results of Deep Sea Drilling Project Leg 64[ In W[ W[ Hay\ J[!C[ Sibuet et al[ Init[ Rept[ DSDP\ 64\ "pp[ 498Ð433#[ Washington] U[S[ Govt Printing O.ce[ Droz\ L[\ Rigaut\ F[\ Cochonat\ P[\ + Tofani\ R[ "0885#[ Morphology
al[ "0887# has allowed a modi_cation of the previous de_nition of unit CF0 and re~ector CF!A[ Furthermore\ the identi_cation of an internal re~ector CF!A!int within unit CF0 as representing marine isotope stage 5[5 is now possible[ Additionally\ marine isotope stages 7 and 01 "re~ector CF!A# are identi_ed[ All units are much thinner south of the Congo Canyon than in the north[ This is probably due to both a longer lasting salt diapirism in the south and a preferred depo! sition north of the canyon resulting in stronger subsid! ence[ A hypothesis regarding the tectonic control of the Congo Canyon has been presented[ A tectonic control of the canyon\ i[e[\ a fault zone beneath the canyon would
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