Seismic Geomorphology of a Late Cretaceous Turbidite Channel system in 4 Deepwater Kribi / Campo sub-basin , offshore Cameroon 5

In this study, a seismic reflection dataset and well-log data were integrated to investigate the geometry and internal configuration of a turbidite channel system within the Late Cretaceous interval of the deep-water Kribi-Campo sub-basin, offshore Cameroon. This interval is characterized by a well-developed submarine channel system consisting of an early and a late-stage channel. Morphologically, the submarine channel system has a northeast-southwest trend and is U-shaped in cross-section with a length of 56 km within the study area. The early-stage channel has a relatively straight morphology and varies in width and depth from 3 to 5 km and 89 to 197 m, respectively. However, the late stage of the channel is characterized by a narrower (1 to 3 km) and shallower (41 to 103 m) incision, with sinuous morphology carved into the early channel infill. The changing interaction of differential tectonic subsidence, relative sea level, source sediment supply and slope gradient change are considered to be the major control on the geometry and internal characteristics of the submarine channel system. Sag subsidence during the Campanian led to basin deepening and the widespread development of basinal sediments as submarine fans and promotion of submarine channel system development. The filling of the channel system occurred during a long-term Maastrichtian relative sea level rise, punctuated by falls in relative sea level. Sand appears to have been fed to the channel system by the palaeo-Sanaga and palaeo-Nyong Rivers, with sand rich aprons developed were these rivers debouched into the study area. The early stage of the submarine channel is dominated by coarse-grained sediments in the southwest and fine-grained sediments in the northeast, while the late-stage channel is mainly filled with fine-grained sediments. The presence of coarse-grained sediments occur within the submarine channel axis downstream represents a potential for hydrocarbon reservoirs with enhanced petrophysical qualities due to a low depositional gradient. The geomorphological analysis of this ancient submarine channel system along the western African margin, as presented in this study, has broad implications in the understanding of the distribution of deep-water sediments with potential for hydrocarbon exploration in the region.


Introduction
The study area is located along the continental slope of the Kribi-Campo sub-basin 136 approximately 40 km off the coast of Cameroon (Fig. 1). It is situated in water depths ranging 137 from 600 to 2000 m (Fig. 1). The Douala/Kribi-Campo Basin is divided into two sub-basins, 138 namely the Douala sub-basin in the northern part and the Kribi-Campo sub-basin in the south 139 (Fig. 1). The southern sub-basin, which was investigated in this study, covers an area of 6150 140 km 2 . The Kribi-Campo sub-basin covers an area of 45 km 2 onshore and stretches NNE-SSW 141 on continental margin, between 2°10' and 3°20'N, and 9° and 10°30'E ( Fig. 1). 142 The evolution of the Kribi-Campo sub-basin is closely related to the opening of the South  (Turner, 1999). This Oligocene unconformity was underpinned by igneous intrusions during 184 lithospheric thinning and volcanism that was associated with the formation of Cameroon 185 volcanic arc in the Neogene (Seranne et al., 1992).

186
The post-rift sequence in the study area is characterized by sand-rich turbidity channel 187 belts and basin floor to toe-of-slope fans (Wornardt, 1999  The dataset analyzed in this study consists of a high-resolution 3D seismic reflection 205 survey and borehole data from the Kribi-Campo Sub-basin offshore Cameroon (Fig.1).  The study integrates well log data (gamma rays, resistivity, density, neutron, and sonic 224 logging) from two offshore wells, W1 and W2. W1 reached a total depth of 4747 m and W2 a 225 total depth of 4090 m below the seafloor corresponding to a stratigraphic interval ranging 226 between Albian to Recent. The wells cover the interval of interest and biostratigraphic data 227 were not available. Formations tops of the two wells and the checkshots data for the well W1 228 were used to correlate the seismic and borehole data. Well data from the W1 wellbore were 229 used to constrain the lithology and ages of the different horizons and deposits interpreted, as  The approach used here consists of the seismic interpretation of ten horizons (KC-1 to 234 KC-9, and the seafloor) (Fig. 2). These seismic horizons were tied to the well W1 using the 235 checkshot data and the interval of interest was divided into two main seismic units based on the 236 recognition of reflection termination patterns such as onlap, erosional truncations, seismic 237 facies/configuration, and vertical stacking patterns (Mitchum et al., 1977). In the present study,    Logbaba Formation (Fig. 4).

290
The KC-3 is located at approximately 4900 ms TWT and is characterized by a high-291 amplitude peak reflection with good continuity (Fig.4). This horizon is characterized the Kribi shows values ranging from -3700 ms upstream to -5100 ms downstream (Fig. 6a). These two 298 extreme values correspond respectively to a high and low topographic area on either side of the 299 study area. It is separated by a steep slope on the side of the continent that becomes increasingly 300 soft towards the seabed. The horizon KC-4 is located at approximately 4600 ms TWT and is a 301 peak reflection with high amplitude. The horizon is characterized by downlaps onto an erosional 302 surface and marks the change from a low frequency sequence below to a higher frequency 303 sequence above (Fig. 5). The isochore map shows values ranging from -3200 ms to -4800 ms, 304 respectively at two downstream and upstream ends of the sub-basin (Fig. 6b). The contours 305 lines of isochore map have a preferred NE-SW direction and have a folded shape in the central 306 part (Fig. 6b).

307
The surface KC-4 is incised by a NE-SW trending channel and covers the Kribi High in 308 the east (Figs. 4a and 4b). On the basin floor at the more distal end of the depositional system 309 (i.e., around P1), the surface marks the base of the relatively low-amplitude Tertiary package 310 (Fig. 4a). This contrasts with the underlying Cretaceous sequences which in cross-section are 311 more channelized and display higher amplitudes (Fig. 5). The thickest area reaches 1560 m (Vp 312 = 2400 m/s) in the east, and the average thickness of the unit is 1080 m (Fig. 6c). The interval of interest in this study (Campanian-Maastrichtian succession) has been 314 divided into two seismic sub-units: seismic unit 1 (SU1) and seismic unit (SU2) based on the 315 differences in the internal seismic reflection configurations (Fig. 5). SU1 consists of sub parallel 316 and aggradational reflections (Fig. 5). SU 1 is generally characterized by low amplitudes 317 reflectors with limited occurrence of high amplitude reflectors, with maximum thickness in the 318 east (Fig. 5). The high amplitude seismic facies display an aggradational pattern with parallel 319 and continuous reflectors displaying fan-shaped geometry (Table 1, Fig.5). SU2 forms the 320 uppermost unit in the Late Cretaceous, and consists of low to high amplitude, sub-parallel and 321 continuous reflectors. A large incision occurs within this unit, which is interpreted as a 322 submarine channel, characterized by high-amplitude reflections at its base (Table 1, Fig. 5). Five seismic facies (SF1 to SF5) were identified in the study interval and can be 326 interpreted to represent five specific depositional settings (Table 1). and has a sinuous morphology in plan view (Fig. 9). This facies is comparable to "mud-filled  (Table 1) Table 1). In map view, it occurs the SE part in the study area (Figs. 9c and 9d). This facies 351 correspond to the sand body which can be interpreted to fan deposits, and it is like those  The submarine channel observed in unit SU2 is U-shaped in cross-section (Fig. 7). It has  Table   365 1). The late stage of the channel is dominated by the SF2 facies. In addition, SF1 and SF2 are 366 inside the channel system and the seismic facies SF3 and SF4 are located outside of the system 367 ( Figs. 7b and 7c). SF3, mainly occurs in the unit containing the submarine channel. SF4 is seen 368 outside of the early channel belt and occurs only locally ( Fig. 7; Table 1). These reflections 369 typically dip away from the channel axis and decrease in amplitude away from the channel axis 370 (Fig. 7b).

371
To analyze the evolutionary history and infilling of the submarine channel system, unit 372 SU2 was divided into four intervals below the top of the channel that corresponds to the KC 04 373 horizon (Figs. 7a and 8). The well-log in the vicinity of the submarine channel system indicates 374 that the thickness between the top and base of the channel is approximately 130 m (Fig. 8a).

375
The gamma-ray motif shows a medium serrated peak and, in some places, a low gamma-ray 376 peak. The well-log petrofacies of this submarine channel consists of the clay interbedded with 377 layers of sands (Fig. 8a). The early-stage channel is visible on all the maps and is characterized 378 by relatively linear morphology (Fig. 9). The channel is 56 km long and 3-5 km wide (Fig. 9), 379 with an incision depth of 89-197 m (Fig. 10). In contrast, at the late stage, the channel could  (Fig. 9d). This high amplitude RMS channel fill observed in the horizon slice corresponds in 387 cross section to seismic facies SF1 and the low amplitude RMS fill corresponds to seismic 388 facies SF2. The late-stage channel is narrower and has a sinuous morphology and is located 389 within the early-stage channel, which is wider and has a straight shape (Fig. 9). The dimension 390 of the late-stage channel is 1-3 km wide, the length is about 56 km (Fig. 9), and the depth vary 391 from 41 to 103 m (Fig. 10).  393 There is a significant morphological variation along the submarine channel system 394 ( Figs. 9 and 10). In the northeastern portion, near the sediment source area, the channel 395 morphology varies considerably when compared to the southwestern portion which is 396 characterized by significantly greater width and smaller depth (Fig. 10).

397
The depth profile of the early channel thalweg shows an exponential trend and is divided 398 into three intervals (1, 2, and 3) that correspond to three segments (x, y, and z) based on the 399 channel gradient variations ( Fig. 11a; Table 2). The gradient of the early-stage channel is 2.64° 400 in the first segment (Fig. 11a). Between 12 km and 33 km, in segment y, the channel gradient 401 decreases to 2.02°. In the rest of the channel, segment z, the channel slope decreases between 402 33 and 44 km, and reaches its lowest value of 0.40° (Fig. 11a). varies between 3300 m to 1115 m (Fig. 11b). 412 The depth profile of the early-stage channel thalweg also shows remarkable variation 413 along the channel path (Fig. 11a), correlating with the variation in the depths of the early 414 channel (Fig. 11c). A plot of channel thalweg versus along channel distance also revealed three 415 intervals (Figs. 11a and 11c; Table 2). The first interval (0 -12 km) begins with the lowest value 416 of channel depth to the northeast of the seismic survey (Fig. 11c), followed by an increase to 417 170 m at 9 km in the early channel (Figs. 11c). The depth of the early-stage channel in this 418 interval ranges from 89 m to 171 m. In interval 2, between 12 and 33 km, the channel depth 419 begins with an increase from 109 m at 12 km to 179 m at 15 km (Fig. 11c). Then, the channel 420 depth decreases to its minimum value of 87 m at 24 km, before fluctuating by increasing 421 between 153 m and 183 m for the rest of the interval (Fig. 11c). The third interval (33 to 44 422 km) has the highest value of early channel depth, 197 m at 35 km (Fig. 11c). In this interval, 423 the depth of the early-stage channel begins with an increase followed by a decreasing trend after 424 reaching its maximum depth. The depth fluctuates between 112 m and 197 m.

425
The width/depth ratio of the early-stage channel varies from 18 to 54 ( Fig. 11d; Table   426 2) in the three intervals along the channel. The first interval begins with a decrease in the ratio 427 and fluctuates along the rest of the interval between 27 to 42 for the early-stage channel. The 428 ratio fluctuates within interval 2 (13 and 33 km), reaching its maximum value from 51 to 24 429 km before decreasing to its minimum value from 17 to 25 km in the early-stage channel.

430
Between 33 and 44 km, the width/depth ratio in interval 3 shows an increasing trend to the 431 northeast of the study area, where it reaches 43 (Fig. 11d).        505 Turbidite channel systems are one of the most common types of hydrocarbon reservoirs 506 found along the West Africa margin and elsewhere (Weimer et al., 2000). Therefore, the 507 discovery of these late Cretaceous submarine channels system, have implications for 508 hydrocarbon prospectivity in the deep-water Kribi-Campo sub-basin.

509
The early-stage channel consists of coarse grain sediments alternating with fine grain 510 sediments rather than being isolated on a basal erosional surface, suggesting multiple barriers were deposited in the high slope gradients (segment x) (Figs. 9d and 12). As a result, the channel 520 system with gentle gradients and coarse-grained sediments offers the highest potential for

522
Another potential application of this study lies in the well-log motif of the submarine 523 channel system where various stages of channel evolution have distinct logs responses (Fig. 8).

524
The basal coarse-grained lags of the early-stage channel in well W1 show a large kick in GR 525 and display a serrate GR log motif with some blocky/bell-shaped intervals (Well W1 in Fig. 8).

526
The late-stage channel fills are mainly characterized by a serrate GR motif with some low-527 amplitude bell-shaped GR intervals (Fig. 8a). The log responses observed in this study is similar 528 to those reported from other slope channel systems (e.g., Fig. 11    complex deposits in study interval. The location of the seismic section is shown in Fig. 1. c)