This volume, Sedimentary Successions of the Arctic Region and their Hydrocarbon Prospectivity , developed around maps of the sedimentary successions of the Arctic Region, and contains a brief, but comprehensive compilation of geological and geophysical data characterizing all significant sedimentary successions in the Arctic, which cover 57% of the polar area north of 64° N. Its two main goals are to provide, based on present-day knowledge and data, a characterization of all Arctic sedimentary successions (or sedimentary accumulations) and to supply a snapshot of hydrocarbon-related exploration in the Arctic at the end of the first quarter of this century. To achieve these goals, we represent sedimentary successions as consisting of one or several ‘tectono-sedimentary elements’ (TSEs) based on the main tectonic regimes that formed accommodation space for accumulation of sediments. A TSE characterization template has been developed as an efficient method of organizing and presenting the most important information about the stratigraphy, structure and petroleum geology of a TSE, including the most significant exploration facts. This organizational architecture is the backbone of the volume and is a key feature that distinguishes it from other studies of Arctic sedimentary basins. The online volume includes six large-size foldout maps portraying the mapped TSEs in the Circum-Arctic context, including tectonic grain of the consolidated basement, anomalous gravity and magnetic fields, location of the Arctic sampling sites and seismic profiles.

The Chukchi Borderland (ChB) is a prominent bathymetric structure located between the deep-water Chukchi Abyssal Plan and the Canada Basin in the Arctic Ocean. This region represents a block of extended continental crust that was tectonically connected to both Siberia and North America before the formation of the Canada Basin. The interior of the ChB is dissected by normal faults into high-standing blocks and troughs that define the first-order structural elements of the Chukchi Plateau, the Northwind Basin and the Northwind Ridge. The post-Hauterivian (Brookian) strata thin from 16 km in the North Chukchi Basin (south of the ChB) to 4–5 km in the Northwind Basin. The basin fill records a history of alternating periods of tectonic extension and quiescence, as reflected in distinct depositional cycles. In this chapter, we describe the ChB as a composite tectono-sedimentary element (CTSE) using original and published 2D multi-channel seismic reflection profiles, tied to the well-calibrated stratigraphy of the Chukchi Shelf and integrated with potential data. We also provide a brief summary of potential hydrocarbon plays based on analogies with the Arctic Alaska Basin.

The modern Arctic has been formed through a series of continent–continent collisions, accretion of terranes and phases of crustal extension. The Neoproterozoic Timanian, Paleozoic Caledonian and Uralian, and late Mesozoic Verkhoyansk–Kolyma, Chukotkan and Brookian orogenies formed several large fold-and-thrust belts (FTBs). The FTBs are exposed across vast areas of continents and continue offshore to form a complex tectonic basement for thick sedimentary basins, playing an important role in the history of accumulation and deformation of younger unmetamorphosed sedimentary successions that are the subject of this volume. Recognition of the importance of FTBs in the Arctic geological history and their role as a controlling factor of development of Arctic sedimentary basins resulted in this chapter, in which we review the current state of knowledge about Arctic FTBs and highlight questions that remain to be addressed. Enclosure D, a map showing boundaries of the FTB and their internal first-order structural fabric, is a part of the overview.

The Albertine Rift of East Africa is a prime example of active early continental rifting in the world today. Geological field surveying of onshore modern-day sedimentary depositional environments here interprets older outcrop and constructs a chronostratigraphic framework based on global glacial–interglacial climatic cyclicity for the past 1.1 million years.

The North Chukchi–Podvodnikov (NChP) and Zhokhov–Wrangel (ZhW) composite tectono-sedimentary elements (CTSEs) occupy the northern parts of the East Siberian Sea and adjacent parts of the deep-water Podvodnikov and Toll basins of the Arctic Ocean. The NChP CTSE formed as a rift basin and includes one of the largest depocentres in the Arctic, the North Chukchi Basin. It contains a 23 km-thick succession of presumably Cretaceous and Cenozoic deposits, which may be underlain by either exhumed mantle or by incipient oceanic crust. Long-offset multichannel seismic profiles and seismic refraction data allow many details of the basin's geology to be imaged. The ZhW CTSE is located in the front of Late Mesozoic New Siberian–Chukchi Fold-and-Thrust Belt. In the Cretaceous and Cenozoic, it was dissected both by late contractional deformation and by succeeding extension, which was probably related to the boundary between the Eurasian and North American lithospheric plates. In this chapter we summarize the geology of the NChP and ZhW CTSEs, and propose a stratigraphic model based on seismic data calibrated with the drilled stratigraphy of the US Chukchi Sea. We also briefly speculate on possible hydrocarbon plays and systems based on an analogy with Arctic Alaska.

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.

Appendices A1–A3 and Acknowledgements for GSL Memoir 61, Sedimentary Dynamics in the Albertine Rift Valley, Equatorial East Africa .

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 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 intrabasinal fault intersections. The majority of Lake Edward and Lake Albert active springs, or palaeosprings, are also associated with precipitation of localized tufa–travertine limestones. The cooler tufas may contain calcitized plant roots, leaf imprints and freshwater gastropods. Active scavenging of uranium (U) can be demonstrated in the algae and cyanobacteria that inhabit active spring mouths, and corresponding tufa–travertines are depleted in radioactive U, potassium (K) and thorium (Th) elements. The source for concentrated bicarbonate ions ( HCO 3 − ) in groundwater at depth – needed to precipitate limestones at the surface – remains problematic. However, rare earth element (REE) plus yttrium (Y) (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.

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.

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 intrabasinal 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 recognizable marginal lacustrine sediments, such as sand berms and runnel lagoons.