The East Siberian Sea–Chukchi Sea Prograded Margin (ESCPM) and Makarov Oceanic Basin (MOB) include interconnected sedimentary accumulations with gradual facies transitions that occupy a significant part of the Amerasia Basin and the adjacent Siberian Arctic continental margin. The ESCPM contains a succession of Cenozoic clinothem featuring shelf-margin progradation caused by a rapid influx of siliciclastic material from NE Asia into the adjacent Amerasia Basin and represents a single tectono-sedimentary element (TSE) as per the volume's terminology. The MOB is a most distant and isolated from continental depositional systems part of the Arctic Ocean. It consists of two first-order sedimentary accumulations with distinct depositional styles and provenances. The lower, Passive margin TSE is composed of a fragment of the presumably Paleozoic-Mesozoic Barents and North Kara passive continental margin. The upper, Synoceanic TSE was formed following the separation of continental block of the Lomonosov Ridge from the Eurasian continental margin at ∼ 56 Ma and during the opening of the Eurasian Basin. It includes mostly Eocene to Holocene hemipelagic and pelagic deposits and ice-rafted sediments. Each of these accumulations is characterised as a TSE, and the MOB itself is considered as a composite TSE (CTSE). Assessments of petroleum potential in both elements rely on regional geological constraints, sedimentary architecture, and modelling. Direct hydrocarbon indicators have been detected in seismic profiles in both ESCPM and MOB. In the latter, petroleum generation likely began in the Jurassic or Late Cretaceous, peaking in the Paleocene and possibly extending into the Miocene. Reservoir rocks are inferred in Cretaceous and Cenozoic strata. In ESCPM TSE, Paleocene-middle Eocene sedimentary successions are considered to include potential source rocks, associated with main flooding surfaces and clinothem bottomset deposits.

The North Kara Shelf and adjoining Severnaya Zemlya Archipelago represent a region of a complex tectonic interplay between two large Arctic fold-and-thrust belts: the Early Mesozoic Pay-Khoy–Novaya Zemlya–South Taimyr belt in the SW and the Late Paleozoic Taimyr–Severnaya Zemlya belt in the SSE and east; and an inferred early Carboniferous deformation zone in the north. Contractional deformations of different ages formed a series of basement highs and inverted sediment-cored swells that separate deeply subsided sedimentary basins. In the framework of this volume's terminology, this region consists of two composite tectono-sedimentary elements (CTSEs): the Vize–Ushakov CTSE and the North Kara CTSE. The principal difference between these elements is in the age of the underlying basement rocks, which, in turn, determines the stratigraphic span of the overlying sedimentary successions. In this chapter we characterize the geology of this remote Arctic region based on the available seismic data supported by published results and our own studies.

A persistent problem that has hampered a general understanding of Albertine Rift early continental rift basins has been the lack of a high-resolution stratigraphic framework within which to correlate sediments across their onshore outcrop. Crucial to resolving this problem is resolving how the three-dimensional stacking architecture of sediments is controlled by a combination of evolving rift basin structure and the glacial climatic cyclicity experienced at the Equatorial Tropics of East Africa during deposition. Rift basin structural styles and subsidence rates are modelled and their merits discussed in light of the field evidence gathered from around the Albertine Rift. After establishing the mechanism of structural development in the basins, the impact of glacially-driven climatic oscillations on sedimentary lithofacies is also incorporated to produce a combined glacial climatic cyclicity model (GCCM) for Albertine Rift basin evolution. This superimposes lake level transgressive – regressive cyclicity upon a tectonic regime of fault propagation and depocentre retreat away from rift margins. The model predicts the resulting stratigraphic architecture that should develop in outcrop areas and is supported by field evidence of not only lithofacies characteristic of arid (glacial) and wet (interglacial) climatic conditions but also cycles of 3-D terrace development, channel incision and sediment backfilling.

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 syn-rift 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’ that might be expected to characterise early continental rift basins are outlined.

Heat and groundwater flow through a rift basin are an integral part of its geodynamics, but predicting what is happening below the surface is often difficult due to a lack of direct information. Field observations on the occurrence, subaerial distribution, temperature and geochemistry of freshwater springs may help to form an idea of groundwater flow through the basin and what it is, or has been, flowing through. Hot springs in the Albertine Rift are common and their occurrence is directly linked to deep-seated main rift bounding faults, or major intra-basinal fault intersections. The majority of Lake Edward and Lake Albert active, or paleo-springs, are also associated with precipitation of localised tufa-travertine limestones. The cooler tufas may contain calcitised plant roots, leaf imprints and freshwater gastropods. Active scavenging of uranium can be demonstrated in the algae and cyanobacteria that inhabit active spring mouths and corresponding tufa-travertines are depleted in radioactive K, U and Th elements. The source for concentrated HCO3- in groundwater at depth, needed to precipitate limestones at the surface, remains problematic. However, REE+Y geochemistry of the tufa-travertines suggest end-member sources of either carbonatites or marine limestones, indicating the possibility of a pre-Neogene rift sequence beneath the Albertine Rift.

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 U and 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.

Whilst Albertine Rift marginal lacustrine and deltaic depositional environments each produce characteristic lithofacies that 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 ox-bow lakes are accompanied by development of characteristic ichno- and rhizo- fabrics 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 sand bodies, containing termite nests, correlating with petroleum reservoir intervals in the subsurface.

One of the most obvious geomorphological features that can form at a coastline is a delta. In the Albertine Rift, at the long-axial shorelines of lakes away from rift border faults at the basin flanks, and where there is a gentle, offshore topographic dip to the rift valley floor, then a long-axial delta system (LADS) can form. Low-level aerial photography demonstrates that these have a relatively high sediment discharge over a remarkably short period of time, prograding tens of metres in only a few years. This produces an elongate delta finger lobe, with associated characteristic geomorphological features, such as dextral and sinistral spits, barrier and mouth bars. Yet, no delta finger lobes are more than a few kilometres long, indicating that incremental movement along rift bounding and intra-basinal faults change the dip of the rift valley floor and cause forced avulsion in rivers and rapid abandonment of a LADS. Together, these factors produce easily identifiable intervals within Pleistocene - Holocene rift-fill stratigraphy that display coarsening-up parasequence sets within an overall 3-5 m deltaic package. Typically, these will be stacked in the stratigraphy between recogniseable marginal lacustrine sediments, such as sand berms and runnel lagoons.

Marginal lacustrine depositional environments in the Albertine Rift are highly sensitive to short-term, climatically controlled lake level fluctuations. As such, they act as one of the prime recorders of paleo-shoreline transgression or regression in rift-fill stratigraphy. In both Lakes Edward and Albert, modern day seasonal winds blow parallel with the long-axis of the rift basin, developing wave dominated shorelines at the end of a lake, but sheltered rift flanks display coastal features associated with longshore drift. The different types of shoreline around Albertine Rift lakes today produce characteristic geomorphological features and associated lithofacies, with key sedimentary structures and fauna/flora that can include hippopotamus, elephant, crocodile and fish remains. These can be identified in Pleistocene – Holocene onshore sedimentary successions around the Lake Edward and Lake Albert rift basins, providing a record of glacial – interglacial climatic cyclicity in the Equatorial Tropics of East Africa. Northern hemisphere interglacials result in a wet phase and lake level rise in the Albertine Rift, with marginal lacustrine shorelines back-stepping landward. Whereas, high latitude glacials cause corresponding periods of aridification and rift lake level fall, with paleo-shorelines prograding out towards the centre of the basin.

Maturation and accumulation of economic quantities of hydrocarbons are a common sedimentary component of continental rift basins around the world, both ancient and modern. Thus, groundwater is not the only fluid that may migrate along major rift faults away from basinal depocentres. De-risking petroleum exploration in rift basins requires demonstrating the presence of working petroleum play systems and the presence of leaking hydrocarbons to the surface is a primary way of demonstrating they are operating. Oil seeps have been known in Lake Albert since 1925 and the Kingfisher, Mputa-Waraga and Jobi-Rii petroleum play systems are due to go into full oil production. Lake Edward, however, has remained a virgin frontier exploration area. Tufa-travertine limestones along the eastern rift bounding fault of the Rwenzori Mountains have yielded organic residues which GCMS analyses indicate have a mature, oil-like distribution. Biomarkers suggest a marine shale source rock, the origin of which is at odds with both oils from Lake Albert and generation within the present EARS. A Lake Edward Slick Survey also sampled oil on the water surface that has the same biomarker signatures as the tufa-travertine-derived residues. Together they suggest maturation from a deeper pre-Neogene (?Mesozoic–Paleogene) rift sequence.