Chapter 7. Albertine Rift basin deltas

Whilst Albertine Rift marginal lacustrine and deltaic depositional environments each produce characteristic lithofacies which together can form a complex stratal architecture, ≥80% of the onshore area of these early continental rift basins is actually dominated by fluvial systems and their associated rift valley terrestrial environments. Albertine Rift rivers today can be classified as flank fan drainage, flank drainage rivers or long-axial systems. Dependent upon slope gradient and sinuosity, and reflecting the bed or suspended load transported, alluvial channels can cycle through four main stages before entering a delta distributary system at the lake shoreline. Higher energy flows closer to rift margins are dominated by gravel bed loads, transitioning to mixed and then suspended loads as gradients decrease out across the rift valley floor. Typical fluvial geomorphological features such as riffle–pool sequences, channel and side bars, meanders, point bars and oxbow lakes are accompanied by development of characteristic ichnofabrics and rhizofabrics in riparian sands and interfluve silts and clays. The four different fluvial stages can be recognized in onshore Pleistocene–Holocene sedimentary successions of both Lake Edward and Lake Albert, with well-sorted fluvial sandbodies, containing termite nests, correlating with petroleum reservoir intervals in the subsurface.

The Albertine Rift is dominated by Lakes Albert and Edward, which together represent two of the great rift lakes of East Africa. Rift basin lakes form one of the most obvious geomorphological features in an early continental rift basin, as crustal extension proceeds, causing the rift valley floor to subside and diverting the regional drainage pattern into the depocentre. Modern-day open lacustrine sedimentation in Albertine Rift lakes is dominated by deposition of organic-rich clays and diatomites, with fan deltas and turbidites delivering clastics from the flanks and long axes of the rift basins, respectively. Oxidation colours in the clays can be used to indicate bottom-water anoxia and seasonal mixing. Field evidence from Plio-Pleistocene rift-fill sediments demonstrates that diatomite horizons mark lacustrine transgressions, with deeper clay intervals recording elevated uranium (U) and thorium (Th) concentrations. Alkaline lake marly limestones, with algal mats, were discovered in two areas of onshore Lake Edward and preserve exceptionally high U and Th signatures. Similar values were identified in lacustrine clays of northern Lake Albert and suggest the development of restricted alkaline lake conditions within the Albertine Rift during its geological evolution were more common than previously acknowledged.

A comparison of sedimentary aliphatic hydrocarbon distribution between the southern Okinawa Trough and a nearby river with high sediment discharge

Tidally-influenced fluvial channel systems from the Miocene Malay Basin, Malaysia: Evidence from core facies and seismic geomorphological analyses

Nature of particulate organic matter in the River Indus, Pakistan

In the past two decades, the dynamic sedimentation process of the Yangtze Estuary has been seriously disturbed by coupled human interventions from the river basin to the estuary, especially the impoundment of the Three Gorges Dam in 2003 and the large-scale Deep-water Navigational Channel (DNC) regulation project in 1998–2010. This study investigated the changes in sedimentary dynamic and geomorphological processes in the turbidity maximum zone (TMZ) by analyzing the historical and present data for current, salinity, suspended sediment, and bathymetry. The results show that the decreased riverine sediment input caused a lagging decrease in suspended sediment concentration in the TMZ during the flood seasons. The DNC caused changes in the flow structure, sediment transport, and geometry of the TMZ in the North Passage (NP) and the South Passage (SP). In the NP, decreased ebb transport in the upper reaches led to landward migration of the TMZ during low discharges, while increased ebb transport in the middle and lower reaches caused the seaward migration of the TMZ during high discharges. As the associated topography of the TMZ, the mouth bar in the NP was mostly removed by channel dredging. However, rapid deposition at the location of the previous mouth bar indicates the formation of an incipient bar. In the SP, increased ebb transport after the DNC-induced disappearance of the TMZ and the mouth bar in the upper reaches and the seaward migration of the TMZ in the middle and lower reaches. Therefore, we found that the construction of dams and large-scale estuarine projects changed the sediment dynamics and geomorphological processes of the TMZ and even affected the long-term evolution of the estuary. Construction regulation projects in the TMZ, intended to narrow the cross-section and enhance seaward sediment transport, may produce the opposite effect. Before and after engineering projects, their impacts on estuarine processes need to be carefully estimated.

The tectonically active early continental Albertine Rift basins are dynamic entities, constantly evolving through time. Periods of crustal extension cause rift valley floor subsidence to create accommodation space and accumulate sediment in basinal depositional centres (‘depocentres’). Periods when stress fields change across a rift can move the maximum rupture along main rift-bounding faults to shift depocentre locations over time, often producing a series of synrift phases in basin development. In the Albertine Rift, the Lake Edward basin displays superb structural geomorphology that, in this study, could be mapped across the whole width of the rift valley. Often cited in the literature as a classic asymmetric half-graben, it is now clear that this basin is undergoing a change in rift phase, developing into an extremely asymmetric graben with a faulted eastern flexural arch. Tectonics does not operate alone, but forms a coupled dynamo with climate, that together drive and control the sedimentary fill of an early continental rift basin. The theoretical effects of this are modelled for Lake Edward to indicate basinal deposition when tectonics and climate are in, and out, of phase. Finally, the ‘sedimentary geodynamic elements’ which might be expected to characterize early continental rift basins are outlined.

This paper presents qualitative estimates of sediment discharge from opposite valley flanks in the S–N-oriented Val Lumnezia, eastern Swiss Alps, and relates inferred differences in sediment flux to the litho-tectonic architecture of bedrock. The valley flank on the western side hosts the deep-seated Lumnezia landslide where an area of ca. 30 km2 has experienced slip rates of several centimetres per year, potentially resulting in high sediment discharge to the trunk stream (i.e. the Glogn River). High slip rates have resulted in topographic changes that are detectable on aerial photographs and measurable with geodetic tools. In contrast, a network of tributary channels dissects the valley flank on the eastern side. There, an area of approximately 18 km2 corresponding to < 30% of the surface has experienced a change in the landscape mainly by rock avalanche and rock fall, and the magnitudes of changes are below the calibration limit of digital photogrammetry. We thus infer lower magnitudes of sediment discharge on the eastern tributaries than on the western valley side, where landsliding has been the predominant erosional process. These differences are interpreted to be controlled by the dip-slope situation of bedrock on the western side that favours down-slope slip of material. Morphometric investigations reveal that the western valley side is characterized by a low topographic roughness because this valley flank has not been dissected by a channel network. It appears that high sediment discharge of the Lumnezia landslide has inhibited the establishment of a stable channel network and has largely controlled the overall evolution of the landscape. This contrasts to the general notion that channelized processes exert the first-order control on landscape evolution and formation of relief and needs to be considered in future studies about landscape architecture, drainage network and sediment discharge.

Prolonged impact of earthquake-induced landslides on sediment yield in a mountain watershed: The Tanzawa region, Japan

The Tambores alluvial fan is located within the hyper-arid Atacama Desert of northern Chile. We examine evidence of the range of flow processes operative in this environment from a combination of Pleistocene-Holocene fan deposits and a recent (2001) flood event (16 m 3 s −1 ) in the fan feeder channel and upper alluvial-fan area. The field evidence suggests that peak flows recorded in the older deposits generated extensive sheetflood events dominated by antidune deposition in the upper fan area. These extreme, supercritical flows were generated by floods with sustained high sediment and water discharges and high stream power. Easily erodable alluvial source materials ensured high sediment discharge could be maintained within flood events. High stream power was ensured as a function of the tectonically exacerbated gradients within the source area. The 2001 event indicates the rapid rheological changes that can occur within an individual flood event, ranging from hyperconcentrated streamflow to mudflow. The flow deposits vary little in maximum clast size either between the varying flood events in the upper fan area, or down the fan gradient. This is due to a limited calibre of sediment being produced from the source area. The study highlights: (1) the range of flow rheologies that can be generated from a hyper-arid catchment both within and between flood events of varying magnitude and the associated difficulties in generating a reliable stratigraphy from the resultant deposits; (2) the high stream power and sediment discharge associated with major flood events and thus the nature of flood hazard in the catchment and on the fan; and (3) the limitations of sedimentological information such as maximum clast size as an indicator of peak flow characteristics in ancient deposits.

The architecture of the Western Andes is remarkably constant between southern Peru and northern Chile. An exception, however, is present near Arica at 18°S, where the Andes change their strike direction by ca. 50° and the Coastal Cordillera is absent over an along‐strike width of 50 km. Although this feature has been mentioned in several previous studies, no effort has been made yet to describe and explain this peculiar morphology of the Western Central Andean forearc. Here, we propose a large‐scale model to explain the Myr‐long low uplift rate of the Arica Bend concerning seismic coupling and continental wedge‐top basin evolution. New geomorphic and sedimentologic data are integrated with seismicity and structural data from the literature to interpret the post‐Oligocene pattern of uplift, erosion and sediment transport to the trench. Results show that the Arica Bend has been marked by exceptionally low coastal uplift rates over post‐Oligocene timescales. In addition, this uplift anomaly at the Arica Bend correlates with relatively high sediment discharge to the corresponding trench segment since late Oligocene time. We interpret that before 25 Ma, the forming seaward concavity of the subduction zone induced trench‐parallel extension at the curvature apex of the overriding forearc. The subsequent low uplift rate would have then triggered a feedback mechanism, where the interplay between relatively low interplate friction, low coastal uplift and relatively high sediment discharge favoured Myr‐long relative subsidence at the Arica Bend, in contrast to Myr‐long uplift of the Coastal Cordillera north and south of it.

Various styles of deposition of Pleistocene glacigenic deposits in Essex, England

Large-scale channel fill structures in Late Ordovician glacial deposits in Mauritania, western Sahara

Approximately 600 km2of Deep‐Tow side‐looking sonar data were collected at the eastern intersection of the Mid‐Atlantic Ridge with the Kane Transform. The merged digital image mosaic provides a synoptic view of the oceanic crust exposed across major escarpments at this ridge‐transform intersection (RTI). We characterized this large acoustic data set using textural attributes extracted from the backscatter image by means of a gray level co occurrence matrix method and Fourier fractal analysis. False‐color texture attribute maps created from these methods aid in the interpretation and discrimination of surficial deposits and bedrock units on the seafloor. Principal components analysis performed on the textural attributes reduced the dimensionality of the textural feature vector and optimized imago texture discrimination. The textural attributes were calibrated to ground truth geologic data and extrapolated to regions with no ground data. The resultant texture and classification maps are consistent with the results of submersible studies but reveal more details of the seafloor geology than the conventional visual interpretation of the backscatter image and ground geologic observations. The inferred distribution of gabbros, basalts, surficial sediments, and rubble deposits shown in the classification map provides new constraints on seafloor geology at the eastern Kane RTI. Over 60% of the southern transform valley wall is covered by pelagic sediments and talus or rubble deposits, with intermittent exposures of coarse‐grained gabbroic rocks and basaltic volcanic rocks. The western rift valley wall is classified as mainly massive outcrops of gabbroic rocks. The contact between the gabbroic outcrops on the western rift valley wall and the pillow basalt terrane on the rift valley floor can be traced for several kilometers along the base of the median valley wall. This new perspective supports the interpretation of the “inside‐corner massif” as an “oceanic core complex” which is being degraded by mass wasting along tectonically active escarpments of the bounding median valley and transform valley walls.

Seven dives in the submersible ALVIN and four deep-towed (ANGUS) camera lowerings have been made at the eastern ridge-transform intersection of the Oceanographer Transform with the axis of the Mid-Atlantic Ridge. These data constrain our understanding of the processes that create and shape the distinctive morphology that is characteristic of slowly-slipping ridge-transform-ridge plate boundaries. Although the geological relationships observed in the rift valley floor in the study area are similar to those reported for the FAMOUS area, we observe a distinct change in the character of the rift valley floor with increasing proximity to the transform. Over a distance of approximately ten kilometers the volcanic constructional terrain becomes increasingly more disrupted by faulting and degraded by mass wasting. Moreover, proximal to the transform boundary, faults with orientations oblique to the trend of the rift valley are recognized. The morphology of the eastern rift valley wall is characterized by inward-facing scarps that are ridge-axis parallel, but the western rift valley wall, adjacent to the active transform zone, is characterized by a complex fault pattern defined by faults exhibiting a wide range of orientations. However, even for transform parallel faults no evidence for strike-slip displacement is observed throughout the study area and evidence for normal (dip-slip) displacement is ubiquitous. Basalts, semi-consolidated sediments (chalks, debris slide deposits) and serpentinized ultramafic rocks are recovered from localities within or proximal to the rift valley. The axis of accretion-principal transform displacement zone intersection is not clearly established, but appears to be located along the E-W trending, southern flank of the deep nodal basin that defines the intersection of the transform valley with the rift floor.