Laurence Droz

Morphology and recent evolution of the Zaire turbidite system (Gulf of Guinea)

The Zaire (Congo) turbidite system developed in an east-west direction off the Congo, Zaire, and Angola continental margins in the southeastern Atlantic Ocean. It includes an incised canyon that deeply penetrates into the Zaire River estuary, and a deep-sea fan that extends over ≈900 km toward the Angola abyssal plain. Cable breaks at the canyon and channel axis and occurrence of Holocene muddy turbidites on the fan attest to the present-day activity of the Zaire system. This activity is linked to the connection between the canyon and the river, which allows a permanent feeding of the deep basin. Being the only large mud-rich fan known to have been active during the Holocene highstand of sea level, the Zaire fan is unique among other fans and constitutes an original model, intermediate between the large inactive muddy fans and the smaller active sandy fans. The morphology as well as the architecture of the Zaire fan are not strongly different from inactive mud-rich systems. The fan is characterized by nine tightly meandering channels at the axial part of stacked-up, lens-shaped channel/levee bodies with typical high-amplitude channel facies and stratified levee facies on seismic profiles. These channel/levee bodies are suggested to belong to two superimposed turbidite systems. The Zaire system includes the youngest channel and the southern fossil channels, while the ancient Kouilou/Niari system includes the northern channels. Morphological parameters and sinuosity of the youngest channel are comparable to that measured for the presently inactive Amazon fan. This observation suggests that the present-day morphology is mainly a remnant of the lowstand period, the highstand period being, in the case of active systems, a period of reshaping by erosion. The highstand activity of the Zaire fan is well expressed on the EM12 acoustic imagery and 3.5 kHz profiles. This activity is restricted to the present-day axial part of the fan, on both sides of the youngest channel, and appears to be both depositional on the levees and erosional inside the channel. Depositional activity on the levee-overbanks includes a restricted late stage of fine-grained sediment deposition and an early stage of coarser-grained sediment deposition. The two depositional stages are separated by a subtle unconformity and are associated with strong backscatter variations, from very low close to the channel (late fine-grained turbidites of Holocene age) to very high (early coarser-grained turbidites) laterally away. Areal distribution of these recent deposits suggests a 60-km-long zone of sediment by-passing or reduced deposition at the canyon mouth during the Holocene.

Architecture of an active mud-rich turbidite system: The Zaire Fan (Congo-Angola margin southeast Atlantic): Results from ZaïAngo 1 and 2 cruises

Multichannel seismic data newly acquired during two ZaAngo surveys now provide an almost complete view of the Quaternary architecture of the Zaire Fan. Extending laterally from the southern Gabon margin to the Angola margin and longitudinally more than 800 km, the overall fan consists of three main individual fans that were deposited successively as overlapping depocenters. The individual fans are composed of channel/levee systems exhibiting similar seismic facies, external configurations, and organization to those described in other large mud-rich systems (e.g., the Amazon Fan). In particular, high-amplitude reflection units with a high oil-reservoir potential are recognized almost systematically as a basal sole for channel/levee systems. They possibly include true high-amplitude reflection packets related to avulsion processes below the avulsion points and coarse-grained basal levees related to the initial stages of levee aggradation subsequent to the avulsion. Correlations with Ocean Drilling Program Leg 175 Site 1077 indicate that the studied part of the Zaire Fan began to build in the late Pleistocene (780 ka). During the upper Quaternary, a great number of channel/levee systems (more than 80) were developed, possibly explained either by its permanent activity even during high sea level conditions or by the low Zaire River inputs. The frequent occurrence of channel entrenchment of either old or recent channels is another characteristic specific to the fan. Overdeepening of channels is probably partly caused by regressive erosion inside the parent channel in response to an avulsion and also in part because of other causes that are not fully understood.

Rhone Deep-Sea Fan: Morphostructure and Growth Pattern

A detailed bathymetric survey of the Rhone deep-sea fan and its feeder canyon using Sea-Beam, reveals morphologic features such as very tight meanders of the canyon and channel courses, cutoff meanders, and downslope narrowing of the inner channel floor. Striking similarities exist between these deep-sea features and some continental landforms, especially in alluvial plain areas or desert environments. Sea-Beam also reveals evidence of huge slump scars affecting the slope and fan. The superficial structure of the Rhone Fan results from the stacking of numerous lenticular acoustic units displaying specific seismic characters in which we recognized channel and levee facies. Except in the upper fan area, these units have not been constant; they have generally migrated, owing to shifting of the channel throughout fan evolution. Construction of the fan probably began as early as the early Pliocene and continued to the close of the Wurmian (late Wisconsinian). In our opinion, the fans growth pattern could be associated with climatic fluctuations. The principal sedimentary mechanism responsible for the growth of the fan appears to be turbidity currents, but mass gravity flows have also been an important factor in building the fan by occasionally blocking the main channel and forcing it to migrate.

Plio-pleistocene evolution of the Var deep-sea fan off the French Riviera

Abstract The Var deep-sea fan was deposited during the Pliocene and Quaternary seaward progradation of the Var delta within a steep-sided, flat-floored basin formed during the Messinian salinity crisis. The prominent palaeo-Var canyon was cut on the slope at the same time. Following the Early Pliocene transgression, Var sediments were initially trapped in the coastal ria formed by sea-level incursion of the head of the Messinian valley. By Middle Pliocene time Gilbert-type deltas had prograded to the break of slope (modern coastline) allowing coarse sediment to reach the basin. Quaternary coastal uplift and sediment supply from the glaciated Alps resulted in a supply of coarse sediment to the fan by the braided Var River throughout the Quaternary, even during sea-level highstands such as the present. In the earliest Pliocene, hemipelagic clays accumulated while coarse sediment was trapped in the Var ria. Through the rest of the Pliocene, a leveed fan valley extended southward from the Var canyon, which periodically switched locations through avulsion to the east. At about the beginning of the Quaternary, this eastward spillover channel became the main distributary and re-occupation of the older south-trending channels was blocked by rapid levee progradation and aggradation of mud waves by overbanking of turbidity flows on the Var sedimentary ridge. During the Quaternary, channel shifting to the north occurred. The Quaternary Var fan has an unusually steep concave longitudinal profile compared with most passive margin fans. The Laurentian fan has a similar profile to the Var; both have a high levee and a gravel and pebble-floored fan valley that is essentially a bypass zone, compared with the aggradational channel-levee systems (with lower levees) in most passive margin fans. This is a consequence of the constant supply of coarse sand, gravel and pebbles to the fan, even during highstands of sea level. The erosive capability of this coarse sediment in ignitive turbidity currents resulted in flushing out of the upper and middle fan valley system. Thick muddy turbidity currents, probably resulting from slumping caused by canyon incision, produced the prominent mud waves on the levee. The concave longitudinal profile led to rapid sediment deposition at critical points in the valley system as a result of turbidity flow expansion; this in turn promoted the development of a left hook in the valley system.

The Nile deep sea fan: preliminary results from a swath bathymetry survey

We present and discuss a set of data, mainly swath bathymetry, backscatter images and a few seismic data, that have been recently recorded over large areas of the Nile deep-sea fan between water depths of 1200 and 3000 m. These data demonstrate the presence of at least three distinct morphostructural provinces where interacting sedimentary, tectonic, and salt tectonic processes control the present day morphology and the recent evolution of the major Mediterranean deep-sea fan. The role of the ongoing collision presently occurring between the continental Eratosthenes Seamount and the Cyprus Arc may have been, and still is, a determining parameter for the development of the Nile deep-sea fan.

The Ebro and the Rhone deep-sea fans: First comparative study☆

Abstract Bathymetric and continuous-seismic profiles enable us to define the main morphological and structural outlines of the Ebro and Rhone deep-sea fans. Despite great differences of morphology and volume, these two fans are characterized by a similar structure with a 500 milliseconds thick series of superposed lenticular acoustic units lying above more homogeneous sediments. These units are tentatively attributed to channel-infilling, associated with overbank deposits shifting through time. Nevertheless, some of them may represent large sedimentary slides, especially off the Ebro river.

L'éventail profond de la marge celtique: stratigraphie et évolution sédimentaire

Seismic data collected on the Celtic Fan during Sedifan cruises (European Program Enam II), stratigraphically correlated to DSDP site 400 (leg 48) show that fan deposition was initiated during the Early Miocene. It progressively extended towards the middle of the Bay of Biscay during the Plio-Quaternary. The fan includes 3 main seismic units with several channel/levee bodies disturbed by mass-movement deposits. Depocenters shifted several times, and the present-day morphology ultimately results from a last shift of detritic inputs to the Whittard System.

Small submarine fans on the eastern margin of Corsica: Sedimentary significance and tectonic implications

Abstract The small turbidite systems and their network of distributary channels lying on the base of slope and basin off the eastern margin of Corsica exhibit physiographic and acoustic characteristics similar to those encountered in typical deep-sea fans. In most cases, the fans appear to be connected to sinuous canyons that incise into the shelf break and display varying shapes along the continental slope. They begin as large highly meandering erosional troughs. Then they become progressively depositional and bordered by lateral prominent levees and show evidence of branching. In their most distal part the height of levees decrease and lobes appear unchannelled. Seismically, the lobes are characterized in their middle and lower parts by superposition of channel-levee units, showing high amplitude reflectors with poor penetration in channels, and bedded facies with better penetration on levees. Only the most distal domains located beyond the channelized areas of the lobes are formed entirely by high amplitude reflectors with poor penetration that are interpreted as coarse deposits of sand or silt. The majority of these sand-rich fans, and especially those located in the northern sector of the margin, are related to the insular drainage system of Corsica. The sandrich fans located in the southern sector may have been structuraly controlled. By the lack of any obvious actual connexion with insular drainage, they suggest earlier association with high continental relief, subsequently removed by erosion or tectonic collapse. The southernmost canyon of this area (Caprera canyon) records structural trends of the Sardinian crystalline basement, and is also probably related to tectonic movements linked with the opening of the Tyrrhenian Sea.

A high sinuosity, laterally migrating submarine fan channel-levee-overbank: results from DSDP Leg 96 on the Mississippi Fan, Gulf of Mexico

Abstract DSDP Leg 96 drilled four sites in a channel-levee-overbank system on the Mississippi Fan, Gulf of Mexico, approximately 300 km from the present Mississippi River Delta in water depths of about 2500 m (Sites 617, 620, 621 and 622). Apart from the uppermost 20–25 cm of Holocene marly foraminiferal ooze in most of the drilled sites, the entire cored intervals are in the Pleistocene Ericson Y Zone. Eight sedimentary facies are recognized: (1) biogenic oozes and muddy oozes; (2) calcareous muds; (3) clays and muds; (4) silty muds and muddy silts; (5) silt-laminated muds; (6) silts and sands; (7) muddy gravels and pebbly muds, and (8) gravels. Sediment accumulation rates on this part of the fan during the Wisconsin glaciation were as much as 11 m/1000 yrs, although most of the sediments probably accumulated from discrete, geologically instantaneous events. Site 621 and Site 622 are located within a prominent channel, Site 617 on an adjacent levee, and Site 620 in overbank deposits approximately 18 km northeast from the channel sites. In this part of the fan, there is one prominent high sinuosity channel, asymmetric in cross section and flanked by levees with probable ridge-and-swale topography. Near these drill sites, the channel width is 3–4 km and its bathymetric relief ranges from 25–45 m. Downfan, the dimensions of the channel decrease. Site 617 (to 191.2 m sub-bottom) and Site 620 (to 422.7 m sub-bottom) mainly comprise fine-grained, thin-bedded turbidites, with Site 617 tending to be slightly coarser grained and showing considerably more evidence of wet-sediment deformation. Site 621 (to 214.8 m sub-bottom), in the channel axis near the deepest part of a meander, contains mainly muds with a downhole increase in the silt content above 195 m sub-bottom, where pebbly muds overlie clean gravel that was obviously washed during core-retrieval and probably was a sandy gravel or gravelly sand. Site 622 (to 208 m sub-bottom) shows similar lithologies to Site 621 although the sediments generally contain more silt, and towards the base of the hole become thoroughly laminated silts and sandy silts: pebbles within muds and silts occur at 199 m sub-bottom. Based on overall grain size trends over tens of metres, the channel sites show ill-defined fining-upward sequences, whereas the levee and overbank sites show coarsening-upward sequences, although the upper part of Site 617 is a fining-upward sequence. Biogenic components of sediments at the channel sites are dominated by shallow-water benthic foraminifera derived from the continental shelf, with the coarser grained clastic intervals containing reworked late Cretaceous planktonic foraminifera and radiolaria from the Upper Mississippi River Valley. The levee and overbank sites have a larger percentage of Quaternary radiolaria, pelagic algal cysts, and more planktonic foraminifera than the channel sites. Seismic reflection profiles across this most recent fan channel show high-amplitude reflectors in the lower part of the channel fill, thought to correspond to the coarsest grained channel lag deposits. Isopach maps show that the lag deposits are up to 6.5 km wide, slightly more than 200 m thick, and that the northernmost meander belt has migrated about 2 km laterally, 1.2 km downfan, and has climbed 175 m stratigraphically (Kastens and Shor, 1985; Sterling et al. , 1985). Evolution of the meander belt shows features common to point-bar migration in high sinuosity fluviatile systems. While the location of Sites 617, 620, 621 and 622 have been drilled within a middle fan environment, the width/depth ratios and the fact that this channel is a single conduit in this part of the fan, perhaps suggest a more appropriate comparison with many inner or upper fan environments that have been described in the literature.

Seismic evidence of widespread mass transport deposits in the Rhône deep-sea fan: Their role in the fan construction

Abstract In the Rhone deep-sea fan mass transport deposits are represented either by transparent or by chaotic echo-facies. They cover a very large area of this fan both superficially and interbedded in the sedimentary column. These deposits played an important role in the fan construction by blocking the paths of detrital feeding and leading to the creation of new ones.