Geomorphology and Processes in the Upper Congo Canyon and its Tributary Canyon

Abstract

Introduction The Congo Canyon, situated at the mouth of the Congo River of the west coast of Africa, is one of the largest submarine canyons in the world (Figs. 1A, 1B; Shepard, 1973). This canyon is unique among the valleys of the ocean floor in that it extends 20 miles into the Congo River mouth estuary (Fig. 2A). The Congo River, with the second largest continental drainage in the world (Fig. 1B), delivers sediments to the canyon which in turn feeds one of the largest deep-sea fans in the ocean, the Congo Fan (Zaire Fan, e.g. Savoye et al., 2009; Marsset et. al., 2009; Babonneau et. al., 2002 and others). The Congo River drainage basin with various tributaries on land has a 3.5x 106 km2 extent (Leturmy et al., 2003, Fig. 1A). The watersheds of the Congo River drainage basin are related to various geological features (Leturmy et al., 2003). To the west of the drainage basin, Precambrian igneous and metamorphic rock outcrops are exposed within a 500-800m high topographic (coastal) bulge (Fig. 1A). The uplift of this coastal bulge is inferred to be of upper Miocene or later origin (Lavier et al., 2001; Leturmy et al., 2003). To the north of the drainage basin, the Central African rift appears as a 900 m maximum elevation with Lower Cretaceous deposits on its southern flank. To the east of the basin, the East African Rift forms a morphological barrier since at least Middle Miocene, presently as high as 3400m (Fig. 1A). To the south of the Congo River drainage basin, is the relatively flat Kalahari plateau with a mean elevation of about 1000m. The Congo River drainage basin, thus surrounded by watersheds of generally low elevations, straddles the Equator and is presently covered by tropical forests. Most of the river drainage basin has low topographic gradients (Fig. 1B). The low gradients of the basin (Fig. 1B) and thick vegetation result in low rates of mechanical erosion along the river course. In addition, there are also numerous lakes in the drainage basin that trap significant amounts of the river sediments. Especially, much of the coarse-grained sediment bed load is deposited within the interior basin above (updip of) Stanley-Pool Lake (Heezen et al, 1964, Marsset et al., 2009, Fig. 1B). Thus, although the Congo River is the second largest in water discharge in the world, its suspended sediment load is relatively low, ranked 17th in the world. Downstream from Stanley- Pool, the Congo River during its course through the coastal bulge of Precambrian rocks drops through a series of cataracts (waterfalls) on its way to its estuary (Fig. 2). The sediments of the Congo River, especially its bed load below Matadi (Fig. 2) consists largely of material the river erodes while flowing through Precambrian rocks. Much of sediment load that the Congo River ultimately transports to the sea is deposited at the head of the Congo Canyon and in its estuary. The sediments at the head of the canyon become remobilized and form powerful sediment-gravity flows and turbidity currents that have been instrumental in breaking telegraphic cables laid across the canyon floor (Heezen et al., 1964; Shepard and Emery, 1973; Khripounoff et al., 2003). Since the Congo Canyon and the Congo River are presently connected, persistent and strong turbidity currents breaking the cables, and coinciding with major river floods and storms have been documented (Heezen et al.,1964; Khripounoff et al., 2003). In addition, longshore drift from south to north is persistent and strong in the area of the Congo River confluence and the Congo Canyon, and resulted in the formation of bars and spits near the mouth of the Congo River estuary (Savoye et al., 2009). Some of these bars and sediments in the estuary are eroded and flushed from time to time by the river during floods and transported to the canyon that contribute to the formation of sediment-gravity flows. The persistent northward longshore drift created the Ponte Padrao spit across the estuary (Fig. 2) and appears to be also instrumental in creating and maintaining some tributary canyons that connect to the main canyon from south. In addition to the canyon head sediment-instabilities and longshore drift, canyon flank instabilities form a third process that may contribute sediments to the Congo Canyon sediment transport.