The relationship between submarine canyon fill and sea-level change: an example from Middle Miocene offshore Gabon, West Africa

SEDIMENTARY GEOLOGY ELSEVIER

Sedimentary Geology 90 (1994) 61-75

The relationship between submarine canyon fill and sea-level change: an example from Middle Miocene offshore Gabon, West Africa Erik Skovbjerg Rasmussen Geological Survey of Denmark, Thoravej 8, DK-2400 Copenhagen NE, Denmark (Received May 4, 1993; revised version accepted September 16, 1993)

Abstract

A series of Middle Miocene submarine canyons has been mapped in offshore Gabon, West Africa using a combination of seismic facies analysis and well information. Three distinct seismic facies are recognized in the canyon fill; the evolution of these facies is inferred to be related to sea-level change. During sea-level lowstand, deposition was characterized by lateral accretion on the southern canyon wall whilst progressive erosion occurred on the northern wall. This depositional pattern was controlled by a local coast-parallel current, that flowed northwestwards along West Africa during the Middle Miocene. As relative sea level rose, the erosion on the northern wall ceased and the canyons were filled by sediments showing an aggradational stratal pattern. Finally, during the early part of the transgression, the canyons were filled with horizontal or subhorizontal sedimentary strata. The sedimentary fill is thought to consist of alternating sands and mud within its lower part, but as relative sea level rose, clayey sediments gradually dominated the canyon fill.

1. Introduction In recent years an increasing number of papers has been published describing the morphology and depositional conditions of submarine fan systems, e.g. Wilde et al. (1978), Piper and N o r m a r k (1983), Mitchum (1985), McHargue and Webb (1986), Nelson and Maldonado (1988), Alonso et al. (1991), and Posamentier and Erskine (1991). Exploration for hydrocarbons in offshore G a b o n within the last decade has led to the acquisition of high-quality multichannel seismics. The present study concentrates on an area of 5000 km 2

(Fig. 1) in the southern part of the Gabonese waters. Within the area of investigation, a series of subparallel canyons has been mapped, running perpendicular to the slope (Fig. 2). The canyons are denoted A to H, from south to north. The canyons are highly erosive in their eastern, proximal reaches (on the slope), but basinwards they become narrower and develop complex channel levee deposits. The width and length of the canyons are 1-3 km and 45 km, respectively. The aim of this p a p e r is to describe the architecture of the canyon fill and the likely relation-

0037-0738/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0037-0738(93)E01 18-Y

62

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61- 75

ship of this fill to depositional processes. Although the Neogene succession offshore Gabon contains a series of stacked canyons, this study concentrates on the best expressed canyons of presumed Middle Miocene age.

2. Geological setting 2.1. General The basin west of Gabon was initiated as part of the crustal extension and rifting that occurred within the Gondwana craton in Late Jurassic to

Early Cretaceous times. The rift-phase initiated the separation of Africa and South America which continued by sea-floor spreading from AptianAlbian to the present. During the rifting episode in the Late Jurassic and most of the Early Cretaceous, lacustrine and fluviatile sediments were deposited in the basin. The so-called transitional phase, the opening of the South Atlantic, was initiated in the Aptian, and a basal interval of marine transgressive sands was deposited as a basal facies. The sands grade upwards into mud-rich facies with local intercalations of carbonates. The progressive reduction in clastic input reflects the termination of the tec-

I 11 ° East

10 ° 30

3 ° 30"

South

PortGentil~~'~'''~

,

1Okra

_m

I Fig. 1. Map of the pre-Quaternary deposits in the area investigated.

E.S. Rasmussen /Sedimentary Geology 90 (1994) 61-75

63

from Late Cretaceous to Oligocene times, but salt tectonism is still active today.

tonic movements and complete peneplanation of the source area. This resulted in deposition of several evaporitic depositional cycles during the Aptian reflecting successive transgressive and regressive pulses (Brink, 1974). During the drift phase, from the late Early Cretaceous (Albian) to the present, more than 3 km of deep-sea, shelf to near-shore sediments were deposited in response to thermal subsidence of the margin (Ussami et al., 1986). This is now highlighted by a general westward tilting of the area. The main phase of salt tectonism occurred

2.2. Tertiary At the beginning of the Palaeocene compressional tectonics prevailed resulting in folding of the Cretaceous strata. This compressive phase was caused by the anticlockwise rotation of the African plate culminated during the Eocene (Lutetian). During the Miocene the compression ceased, but later strike-slip movements have

~

11J East ~30'--

\ \ \ N

-~.=-

A~... m ..~ !

5a

"x "" ~ghe/fease

Geosection I Seismic section with refererme to figure number

C,nyon qlOkm ,

Fig. 2. Middle Miocene canyons A - H within the area investigated and seismic lines used in the study. The seismic sections shown in figures are indicated.

64

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61-75

A SW

;t-'~"~! ":" ~"

A NE

unconformity

~

E

F

Lll~m

Fig. 3. Cross-section showing the Tertiary strata. The Middle Miocene unconformityis marked with an arrow and the canyons E and F are also indicated.

caused offset of the Tertiary strata (Jansen et al., 1984). During earliest Tertiary times mud-dominated sediments with some carbonate intercalations were laid down in the area. This was followed by deposition of alternating muds and sands; the deposits become more sandy upwards, reflecting an overall regressive pattern. However, a major hiatus occurred during Late E o c e n e and Oligocene times, and resulted in fluvial erosion of the shelf sediments (Jansen et al., 1984). Another hiatus has been reported from the Miocene (Giroir et al., 1989). The formation of these unconformities is believed to be related to both tectonism and eustatic sea-level changes. Fig. 3 shows a geological cross-section of the slope off Southern Gabon. One important factor in the deposition of marine sediments in this area during the Tertiary was the development of the Benguela Current in Miocene time (Siesser, 1980). From that time, sediments discharged from rivers along West Africa were transported in a northerly direction (Siesser, 1978). The onset of the Benguela Current was a result of the development of the circum-Antarctic Current and was an important

factor in the establishment of the humid climate around the Equator which still prevails today. 2.3. Quaternary

Quaternary deposits on the shelf are very thin, ca. 20 m (Jansen et al., 1984), and confined to depressions in the sea floor. The Quaternary sediments are thought to originate from the Zaire River (formerly the Congo River) farther south of Gabon.

3. M e t h o d

The study is based on multichannel seismic surveys shot between 1985 and 1989 with excellent resolution and depth penetration. The line spacing is quite variable from less than 1 km in the eastern part of the study area to more than 10 km in the western part of the area (Fig. 2). The large distance between seismic lines over much of the area makes it difficult to map the three-dimensional architecture of the canyon fill. Consequently, this paper presents the two-dimensional architecture of the canyons.

Fig. 4. Cross-sections of the canyons (A, B, and C). For legend see Fig. 5. Published by permission of Geco-Prakla Exploration Services.

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61-75

NW

C

L_~

.

- _ : ~

.

__

.

.

_ - - .

~

.

.

_ _ .

~

~

- -

~

- - _ - - - - -

.

:

-----

.

.

.

~

.

~

~

.

i

i

- ~

f

-

X

,

.

~

-

, , ~ ~ : ~

~

. . ~ ~ f ~ . . .j. . ~ ,

~-~'~..--~')>"~_~=-----

.

---

NW

--~....~__~

.

~ ~. ~~/t

~-

SE

.

~

--~--=_

_~------7~.z.~.--..._.-~-

~

.

8 3 5 m b e l o w msL

~ ~

65

-

~

...........

~

~

- ~.,..~.....--.......~

~

~

~

-

.

~-

. . . ~

ii

- - - .....................

~.

~

.~ ~

8 1 4 m b e l o w msl.

.

--------

.

E,o~,

SE

~-%~-.~-~#,'~.~Z~--__~-~_ ~_"~'~_ •~ ~ z ~ -......--.

_

_ i~.,~~f

- - ~ - - ~ ~

r,oo.

66

E.S. Rasmussen/'Sedimentary Geology 90 (1994) 61-7.5

Seismic facies have been identified using the principles of Sangree and Widmier (1977), Brown and Fisher (1980) and Mitchum (1985), and the interpretation is based on the concepts of Posamentier and Vail (1988) and Posamentier et al. (1988). The age of the sediments has been determined from wells that lie within the study area, but do not penetrate the canyons.

on transverse sections. The longitudinal development of the different facies is difficult to determine on the basis of the present data set. The canyon fill overlies a distinct unconformity of probably Middle Miocene age (Fig. 3). 4.1. Facies 1

Facies 1 is characterized by high- to mediumamplitude reflectors with relatively good continuity (Figs. 4A and 4B). The internal reflection pattern is oblique showing a progradational seismic pattern. The reflectors dip up to 25 ° towards the north. Internal convergence of reflectors occur frequently and the stratigraphically oldest re-

4. Seismic facies description Three different facies have been found within the canyons. The definition of the facies is based

NW

I~-.~

6 0 9 m below msl.

~

_~

~-~'.':~:.~t/L

/

I I

~

--

"/-~

1kin

[_~

~i~

.__._.. '~:-!:::'~'.~. ". '.z~:. ".. c..:-..' , - . ~ . . : : : : ~ - : ~:i::::;::::::~

•

•

;

•

,_L.'.'.

~:~)','.;,'P--

Facies 1 Facies 2 ~

Facies 3

Fig. 5. Two cross-sections showing the lateral variation of Facies 2 within canyon H. Published by permission of Geco-Prakla Exploration Services.

67

E .o

L~

LT~

g .o

68

E.S. Rasmussen / Sedimentary Geology 90 (1994) 61- 7_5

flectors onlap the southern side of the canyon wall. The base is formed by downlap on a gently dipping surface. The downlapping surface is well expressed by a high-amplitude continuous reflector. The upper boundary is formed either by bending of the reflectors (becoming concordant with the upper boundary) or terminations shown as toplap. 4.2. Facies 2a

Facies 2a is characterized by medium-amplitude reflectors showing a sigmoidal seismic pattern (Figs. 4A and 4B). The reflectors dip ca. 20° towards the north. A characteristic feature of Facies 2a is the onlap on the northern side of the canyon wall. Some internal convergence of reflectors may occur. The dips of the reflectors are lower than that of Facies 1. The base and top are concordant with the boundaries. 4.3. Facies 2b

Facies 2b is similar to Facies 2a, but the reflectors dip towards the south (Fig. 5B). Facies 2b has only been recognized within canyon H. 4.4. Facies 3

Facies 3 is characterized by medium-amplitude reflectors showing a parallel to subparallel reflection pattern. Onlap is seen on both sides of the canyon walls. The upper boundary is concordant. The overall facies geometry is lensoid. As mentioned earlier the longitudinal development of the canyon deposits is more difficult to document. Due to the narrow width of the canyons, an understanding of their longitudinal geometry would demand a very dense seismic grid. However, canyon A is almost straight and a single seismic line (Fig. 6) covers parts of this canyon. Facies 1 shows a parallel to subparallel reflection pattern within the upper part of the canyon. The amplitude is variable with good continuity. A few minor convex features have been recognized. Towards the basin, the facies shows a seismic reflection pattern characterized by several thin

parallel reflectors with very good continuity, Whether this reflection pattern shows progradation or draping over the canyon floor is not clear due to a series of small faults, but from observations from other seismic sections the latter interpretation is preferred. Farther basinwards, the facies seems to thin-out and the reflection pattern becomes more transparent. Facies 2 is characterized by a parallel to subparallel reflection pattern throughout the section, and thins-out in a more regular way than Facies 1. Facies 3 is not very well exposed on any of the available seismic data and consequently not described in the longitudinal section.

5. Sedimentological interpretation As no wells penetrate the canyons, the following interpretation is based solely on the seismic pattern. It has been proposed that the formation of submarine canyons on continental slopes is related to sea-level fall (Vail et al., 1977; Shanmugam and Moiola, 1988; Haq, 1991). During such periods the lowered base level, commonly below the shelf edge, causes widespread subaerial exposure, steeper gradients and therefore enhanced erosion of canyon-like features. The steep slopes and increased sediment influx generate frequent slumping and sediment gravity flows which may further enhance erosion in the canyons. Canyon formation may also have been enhanced by slope failure caused by the escape of gas-hydrates during lowstands (May et al., 1983; Haq, 1994) a n d / o r minor tectonism. After the incision of the canyons in the slope sedimentation was initiated within these canyons. The relative position of the three facies in the canyon fill is consistent. Facies 1 was laid down first, followed by Facies 2a or 2b and Facies 3 which represents the final canyon fill. However, some of the canyons are mainly filled with Facies 3 (Fig. 4B). Facies 1 always prograded in a northerly direction. During this phase, the canyon was still open and active, but migrated towards the north by progressive erosion on the northern wall and contemporaneous deposition on the southern

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61-75

wall. Some of the canyons show a weakly "climbing" character in a northerly direction, indicating a minor rise of sea level (Fig. 4A). The observed deposition by lateral accretion may reflect the influence of a northerly directed current along the slope. The ocean circulation pattern of the study area was closely linked to the opening of the Atlantic during the Late Cretaceous and Tertiary; since the Miocene, the northerly directed Benguela Current has dominated the SE Atlantic (Siesser, 1980). The study area is located north of the main area of influence of this current, but the present circulation pattern in the area is dominated by a northerly directed current along the coast of Gabon (e.g. Van Weering and van Iperen, 1984) and during the Quaternary, sediment transport from the Zaire River into the study area has been reported by Jansen et al. (1984). Consequently, a similar circulation pattern probably

NW

69

existed in the Middle Miocene. Deposition was concentrated within the canyons during this stage (deposition of Facies 1) and characterized primarily by lateral accretion; consequently it is inferred to have occurred during a lowstand period with no or only minor relative rise of sea level (Fig. 7b). The high continuity and relatively high amplitude of the reflectors may indicate deposition of a lithology with varying acoustic impedance, e.g. sands alternating with muds. The sediment is thought to have largely originated in the south and been transported northwards by the submarine current. Sediment gravity flows from the shelf and land area towards the east as well as erosional products from the canyon wall, were probably deposited in the basin as a series of interfingering lobes. Confirmation of this assumption requires a more closely spaced seismic grid.

Relative sealevel High ~

SE Formation of the canyon

Low

b

Lateral accretion

High Low

C

Aggradation of canyon fill

Low

Depostion of horizontal strata within the canyon

High

"L'CZ::::~

High

d

..:.::::iii!!::::"'...!~ ...~ ."

==.... ==================== ..:".:"/ .': ~';.~,~t,.-.":

~..-_.....~.:.:~.:?.~:~ ........

Fig. 7. Evolution of canyons in relationship to changes in relative sea level.

":'-.'S-_~

• ..

...o

Low " .'.~----_'.L/

7[)

E.S. Rasmussen/Sedimentao' Geology 90 (1994) 61-75

The sediments of Facies 2a were laid down largely after the erosional processes had terminated and sedimentary processes within the canyon were dominantly depositional. The sediment fill within the central part of the canyon began to aggrade indicating a continuous rise of sea level. Even though base level rose during deposition, the sediments were still mainly confined to the canyons as only thin deposits of the same age have been recognized between the canyons. This canyon fill was laid down during a relative rise of sea level (Fig. 7c). The low continuity of the reflectors in Facies 2a may suggest a more patchy deposition of sands within mainly clayey deposits. The depositional environment for Facies 2b is probably similar to that of Facies 2a and Facies 2b is a lateral variation of Facies 2a as explained below. Canyon-fill deposits, analogous to Facies 2 have been described in McHargue and Webb (1986). They distinguish three possible processes responsible for the deposition of inclined canyon fill: (1) slumping of canyon walls, (2) pointbar deposition, and (3) shelf sediment influx. As regards the first of these possible causes, no slumping features have been recognized on the seismic, although this observation is limited by the seismic resolution. However, during the deposition of Facies 2, it is believed that only minor erosion occurred on the northern wall as this facies is characterized by aggradation of sediments rather than lateral accretion. Consequently, slumping is thought to be only of minor importance. The process of pointbar deposition may be a common feature within the most meandering canyons, for example in canyon H (Fig. 5). Here deposition of both Facies 2a and 2b have been recognized reflecting deposition on both sides of the canyon walls depending on the direction of the meander loops. The third process, shelf sediment influx, in the particular case of sediment influx from the slope, may also seem to be an important process for the formation of Facies 2 where inclined reflectors within the canyons continue into intercanyon areas in the same manner as described by McHargue and Webb (1986). It is thus likely that the

Facies 2 canyon fills accumulated as the result of interaction of sediment transport from the shelf and the N W - S E running current at the slope. Facies 3 represents the final fill of the canyons. In most cases the fill onlaps both sides of the canyon walls, suggesting that the influence of the current was significantly reduced. It is proposed that Facies 3 was deposited in the early phase of a transgression when only relatively small amounts of mostly fine-grained sediments were transported to the basin during episodic events, such as major storms or earthquakes. Such conditions generally prevail during early highstand. During the late highstand, which is a major progradational phase on the shelf (Posamentier and Vail, 1988), it is likely that sediment was also transported to the slope and basin plain. Such sediments may then have been deposited as hemipelagic deposits both in canyons and intercanyon areas producing a continuous sediment drape upon older strata. Continuous sedimentation in the area has only been recognized in a few cases, indicating that deposition of Facies 3 was probably related to the early part of the transgressional phase (Fig. 7d). Some of the canyons are dominated by fill of Facies 3, e.g. canyon B (Fig. 4B), and do not show the characteristic three-phase evolution. These canyons may have been formed late, probably during deposition of Facies 2. The development of these late canyons may have been due to avulsion of canyons due to plugging of some of the earlier canyons. Consequently the mass flow/ turbiditic currents had to create new pathways as described by Walker (1978). As noted above, the lithology of the submarine canyon fill can only be inferred. However, some general types of turbiditic systems are inferred to be active during the described evolution (Mutti, 1985; Posamentier et al., 1991). Based on Mutti's terminology, turbiditic systems can be subdivided into three types. Type 1 is a depositional system in which the bulk of the sandstone occurs in nonchannelized and elongated lobes in the outer region. Type 2 is a depositional system in which sandstone facies are predominantly deposited in the lower part of channels and areas beyond the channel mouth, whereas type 3 represents a de-

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61-75

ized by a lack of deposition within canyons and is a consequence of lateral accretion. Therefore, it can not be compared to the established model mentioned above. Facies 2 and 3 may have affinities with the turbidite systems of types 2 and 3

positional system characterized by small sandstone-filled channels enclosed in predominantly muddy successions. Facies 1 is exceptional in the way that deposition occurred during a period normally character-

_1!

•

sy

~

71

Shelf

Slope

Slope

f oor

Basin flooJ

•

~..~ ~-.~.~;!

D

C ::~;,t..~."

Shelf

Shelf

Slope

Basin floor

,~:,,;-;;'.~;~;~.,,~;~.,~;:,,-.-.,;.:,;.-:~..,.~,.'... ',:.;~?~¢ Basin ,,., ,~.~,~,,:, ~...,#~;~t" :~.:,~J,:, floor

LEGEND J J Area with non-deposition

~

Area with deposition

~..=

Sediment influx

Fig. 8. Schematic illustration of the evolution of the depositional environment (based partly on Walker, 1978).

72

E.S. R a s m u s s e n / S e d i m e n t a r y '

NW --

-

_

-

-

~

-~

-

- - - -

Z--

~

• " Basinal deposits synchronous with Facies 2 . . . . . . . . . . •

,

--~

._.

~ ~ _

----____.__...__ ~ ..~ .

~

~

~

~

~-"~

-

~ ~ - ~

--~1300m

~~

~--~

below msl.j._-~ SE

~

__ . . . . . .

------=

~"

------~----- ~,,~....._

~--/ i / / Basinal deposits synchronous with Facies 1

Z5

G e o l o g y 90 (1994) b l -

~ ~

.---....~~

~

S<~

--~..._"~ ~

i

£

-

-

:~::

~..~,,':'.-.~;,:r.;'..:...,,,:'.,-.~".;.,-'~'..

, "~'~';" -~ ~ ~~. . . . ~...>._-..,~.. ' ~ . "~: ~. ~::.~:t'..'":".~.'.'.:':'7: ' . ~: ": " .: ". -~' ,. ' .- ". ,C"::. ! ' ~ r

- ~ ~ . .

".:'"

~..-z-..-~.------~__.___

~ . . , ~ - L . _ ~ _

~ "

_---- - ---- 2.

lOOms 1kin Fig. 9. Cross-section showing the deeper part of the canyons B, C, and D where deposition occurred as complex levee deposits. Basinal deposits synchronous with Facies 1 and Facies 2 are shown. Published by permission of Geco-Prakla Exploration Services.

which are related to relative rise of sea level according to Posamentier et al. (1991).

6. D i s c u s s i o n

The evolution of the canyon fills on the G a b o n continental slope points towards deposition within a single sea-level cycle. During this cycle, cutting

and deposition were confined to the period with lowstand of sea level and perhaps the very early part of a transgression. When comparing the evolution of the canyon fill with the development of lowstand deposits described by Posamentier and Vail (1988) and Posamentier et al. (1991), many similarities are evident (see below). During the period of relative sea-level fall, incision and erosion occurred resulting in canyon

73

E.S. Rasmussen /Sedimentary Geology 90 (1994) 61-75

formation on the slope and probably deposition of fans on the lower slope and basin floor (Fig. 8A). The canyon formation represents the lowstand fan situation. At the time of deposition of Facies 1 (Fig. 8B), deposition occurred only by lateral accretion within the canyons, a consequence of limited accommodation space caused by the local current. This is the period during which the canyon both acted as a funnel for sediments laid down in the deep-sea environment (Fig. 9) and contemporaneously in the canyon due to lateral migration. This phase may correspond to the transition between the lowstand fan and the early lowstand wedge (Posamentier et al., 1991) during which the rate of eustatic sea-level fall decreases resulting in relative stillstand of sea level or a slow relative sea-level rise. Deposition on the basin floor at this time may have been characterized by the initiation of channel levee deposits (Posamentier et al., 1991). During deposition of Facies 2, an aggradational depositional pattern is recognized suggesting an increase in accommodation space. At this time sediments were laid down within the canyon without significant erosion of the walls (Fig. 8C). This phase may correspond to the late lowstand wedge (Posamentier et al., 1991) when deposition is important within the canyons and the channel levee systems are abandoned. During the deposition of the lowstand wedge, the rate of creation of new space increases as the rate of relative sea level gradually increases. Facies 3 is characterized by a horizontal stratal pattern and represents the final fill within the canyon. In this case, the availability of sediments was the limiting factor rather than the accommo-

Stratal pattern of canyon fill -.~.................... • ..-..-..

dation space. This phase may reflect the complete drowning of the shelf and consequent reduction in sediment supply to the deeper marine environment from the adjacent shelf (Fig. 8D). The order of the sea-level cycle inferred to have been responsible for the deposition of the canyon fills off the coast of Gabon cannot be established because of the poor biostratigraphical control in the study area. Fig. 10 shows the inferred relationship of the canyon fill to eustatic and relative sea-level changes. Such an interpretation assumes that eustatic sea-level change was responsible for the evolution of the canyon fill. However, the evolution of submarine canyons can also be explained by a simple tectonic model. Minor uplift may have resulted in emergence of the shelf, resulting in incision on the shelf and canyon formation on the slope due to lowering of the base level. Following uplift, resumed subsidence of the area would have resulted in a relative sea-level rise producing the stratal patterns that have been observed in this study. A study of the structural evolution of the shelf does indicate uplift during the Tertiary (author's unpublished data), but there is no specific tectonic event that can be correlated with the canyons described here. An additional argument against the tectonic model may be the time interval over which the unconformity was formed. A distinct unconformity with a series of canyons as observed in this study suggests processes that acted over a short time period. It is commonly proposed that only eustatic sea-level changes, and in particular those caused by glacio-eustasy, can be responsible for such relatively rapid changes in base level. Furthermore, as the Middle Miocene

Relative . sea level , , a

""-":~

. ;.~ !~:. :? ; ::-~!,-~ .., .... •

.~

...;..

~,N'4";~-~" b c,¢.,/

i

Eustatic sea level

-.:.-... " ~ "

High

LOW .,,%'--.., High /.,. -,,-. Low

Fig. 10. Schematicillustrationshowingthe relationshipbetweenstratal pattern, relativesea level, and eustaticsea level.

74

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61-75

succession consists of repeated periods of canyon formation, the eustatic sea-level model is preferred here (author's unpublished data).

7. Conclusion In the Middle Miocene a series of submarine canyons was formed on the continental slope of the coast of south Gabon. The formation of these canyons is inferred to be related to eustatic sealevel fall. During the subsequent sea-level rise, the canyons were filled with sediment. This sediment fill displays three distinct seismic facies which can be related to the progressive increase in accommodation space and lower-energy conditions during sea-level rise. The early canyon fill is probably composed of alternating sands and muds, but as the sea level rose, clayey sediments became dominant.

8. Acknowledgements The author want to thank Ron Steel, John Korstg~ird, Claus Andersen, Stefan Hultberg, and Jon Ineson for comments and fruitful discussions. Norsk Hydro a./s. and The Danish Research Academy are thanked for financial support. Geco-Prakla Exploration Service and Norsk Hydro a./s. are thanked for permission to publish the seismic on which this study is based. The figures were drafted by Alice Rosenstand.

9. References Alonso, B., Canals, M., Got, H. and Maldonado, A., 1991. Sea valleys and related depositional systems in the Gulf of Lion and Ebro continental margins. Bull. Am. Assoc. Pet. Geol., 75: 1195-1214. Brink, A.H., 1974. Petroleum geology of Gabon Basin. Bull. Am. Assoc. Pet. Geol., 58: 216-235. Brown, L.F., Jr. and Fisher, W.L., 1980. Seismic stratigraphy interpretation and petroleum exploration. Am. Assoc. Pet. Geol., Continuing Education Course Note Ser., 16, 181 pp. Giroir, G., Merino, E. and Nahon, D., 1989. Diagenesis of Cretaceous sandstone Reservoir of the South Gabon Rift Basin, West Africa: mineralogy, mass transfer, and thermal evolution. J. Sediment. Petrol., 59: 482-493. Haq, B.U., 1991. Sequence stratigraphy, sea-level change, and

significance for the deep sea. Int. Assoc. Sedimentoi., Spec. Publ., 12: 3-39. Haq, B.U., 1994. Deep-sea response to eustatic change and significance of gas hydrates for continental margin stratigraphy. In: H.W. Posamentier, C.P. Summerhayes, B.U. Haq and G.P. Allen (Editors), Sequence Stratigraphy and Facies Associations. Int. Assoc. Sedimentol., Spec. Publ., in press. Haq, B.U., Hardenbol, J. and Vail, P.R., 1987. Chronology of fluctuating sea level since the Triassic. Science, 235:1 t561167. Jansen, J.H.F., Giresse, P. and Moguedet, G., 1984. Structural and sedimentary geology of the Congo and Southern Gabon continental shelf; a seismic and acoustic reflection survey. Neth. J. Sea Res., 17 (2-4): 364-384. May, J.A., Warme, J.E. and Slater, R.A., 1983. Role of submarine canyons on shelfbreak erosion and sedimentation: modern and ancient examples. Soc. Econ. Paleontol. Mineral., Spec. Publ., 33: 315-332. McHargue, T.R. and Webb, J.E., 1986. Internal geometry, seismic facies, and petroleum potential of canyons and inner fan channels of the Indus submarine fan. Bull. Am. Assoc. Pet. Geol., 70: 161-180. Mitchum, R.M., Jr., 1985. Seismic stratigraphic expression of submarine fans. in: O.R. Berg and D.G. Woolverton (Editors), Seismic Stratigraphy, II. An Integrated Approach. Am. Assoc. Pet. Geol., Mem., 39: 117-136. Muni, E., 1985. Turbidite systems and their relations to depositional sequences: In: G.G. Zuffa (Editor), Provenance of Arenites. NATO-ASI Series, Reidel, Dordrecht, pp. 65-93. Nelson, C.H. and Maldonado, A., 1988. Factors controlling depositional patterns of Ebro turbidite systems. Mediterranean Sea. Bull. Am. Assoc. Pet. Geol., 72: 698-716. Piper, D.J.W. and Normark, W.R., 1983. Turbidite depositional patterns and flow characteristics, Navy Submarine Fan, California Borderland. Sedimentology, 30: 681-694. Posamentier, H.W. and Erskine, R.D., 1991. Seismic expression and recognition criteria of ancient submarine fans. In: P. Weimer and M.H. Link (Editors), Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems. Springer-Verlag, New York, N.Y. Posamentier, H.W. and Vail, P.R., 1988. Eustatic controls on clastic deposition, 11. Sequence and systems tract models. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 125-154. Posamentier, H.W., Jervey, M.T. and Vail, P.R., 1988. Eustatic controls on clastic deposition. I. Conceptual framework. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. van Wagoner (Editors), Sea Level Change--An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 109-124. Posamentier, H.W., Erskine, R.D. and Mitchum, R.M., Jr., 1991. Models for submarine-fan deposition within a sequence-stratigraphic framework. In: M.H. Weimer and M.H. Link (Editors), Seismic Facies and Sedimentary Processes of Submarine Fans and Turbidite Systems. Springer-Verlag, New York, N.Y.

E.S. Rasmussen/Sedimentary Geology 90 (1994) 61-75 Sangree, J.B. and Widmier, J.M., 1977. Seismic stratigraphy and global changes of sea level, Part 9. Seismic interpretation of elastic depositional facies. In: C.E. Payton (Editor), Seismic Stratigraphy--Application to Hydrocarbon Exploration. Am. Assoc. Pet. Geol., Mem., 26: 83-97. Shanmugam, G. and Moiola, R.J., 1988. Submarine fans: characteristics, models, classification, and reservoir potential. Earth-Sci. Rev., 24: 383-428. Siesser, W.G., 1978. Leg 40 results in relation to continental shelf and onshore geology. Init. Rep. DSDP, XL: 965-979. Siesser, W.G., 1980. Late Miocene origin of the Benguela upwelling system off northern Namibia. Science, 208: 283285. Ussami, N., Karner, G.D. and Bott, M.H.P., 1986. Crustal detachment during South Atlantic rifting and formation of Tucano-Gabon Basin system. Nature, 322: 629-632. Vail, P.R., Mitchum, R.M., Jr., Todd, R.G., Widmier, J.M.,

75

Thompson, S., III, Sangree, J.B., Bubb, J.N. and Hatlelid, W.G., 1977. Seismic stratigraphy and global changes of sea level. In: C.E. Payton (Editor), Seismic Stratigraphy--Applications to Hydrocarbon Exploration. Am. Assoc. Pet. Geol., Mem., 26: 49-212. Van Weering, T.D.E. and van Iperen, J., 1984. Fine-grained sediments of the Zaire deep-sea fan, southern Atlantic Ocean. In: D.A.V. Stow and D.J.W. Piper (Editors), Finegrained Sediments: Deep-Water Processes and Facies. Geol. Soc. London, Spec. Publ., 15: 95-113. Walker, R.G., 1978. Deep-water sandstone facies and ancient submarine fans: models for exploration for stratigraphic traps. Bull. Am. Assoc. Pet. Geol., 62: 932-966. Wilde, P., Normark, W.R. and Chase, T.E., 1978. Channel sands and petroleum potential of Monterey deep-sea fan, California. Bull. Am. Assoc. Pet. Geol., 62: 967-983.