Abstract The Early Cambrian palaeogeographical enigma arises when tectonic reconstructions are made using palaeoclimatic v. palaeomagnetic data that result in possibly contradictory tropical, mid-latitude, and south polar locations for major continents. For example, NW Africa and Cadomia may have lain in a tropical zone (0° to ±30° latitude) based on the presence of archaeocyath reefs, minor evaporites, and carbonate platforms at c. 520 Ma ± 5 Ma or, alternatively, NW Africa and Cadomia may have lain in a south polar zone (90° to 60° south latitude) based on palaeomagnetic constraints. Greater Avalonia may have evolved independently from NW Africa if a dropstone constraint implying polar latitudes at c. 530 Ma and a palaeomagnetic constraint implying c. 50° latitude at c. 505 Ma are accommodated. We show here how counterclockwise rotation of Gondwana during the Cambrian about an interior axis may solve the enigma. Gondwanan apparent polar wander becomes consistent with tropical conditions inferred for NW Africa when adjusted to accommodate constraints placing the south pole near Peru for c. 540–520 Ma. Concurrent counterclockwise rotation of Baltica and Gondwana during the Middle Cambrian may have facilitated separation of Greater Avalonia from Baltica across dextral shear zones.

Abstract In Europe, North Africa and Asia Minor, the remains of Pennsylvanian sedimentary basins bearing continental deposits either intimately mixed with shallow-marine strata or deposited in exclusively continental settings are preserved. Long-lasting research on these basins allowed the definition of regional stages and substages based on marine fauna and terrestrial flora, later extended by terrestrial and freshwater faunal biostratigraphies. Glacioeustatically driven marine bands provide laterally widespread correlation markers; however, where such bands are missing only biostratigraphic control exists. Resolution of biostratigraphic zonations combined with gaps in sedimentary successions and variable quality of the fossil record throughout the basin fills do not allow in all cases a precise correlation between the Pennsylvanian basins in Europe and, in turn, the timing of tectonic, climatic and biotic events, and thus an absolute complete understanding of the response of terrestrial and freshwater biota to climate changes across eastern tropical Pangaea. A helpful tool is new radioisotopic ages of intercalated volcaniclastics that reveal the partial diachroneity of some widely used biostratigraphies. We attempt to present the current state of the art to stimulate further research to mitigate gaps in our knowledge.

Abstract The Ordovician of North and West Africa comprises three main transgressive–regressive sequences understood as ‘second-order’ cycles of 10–15 myr duration. Tide- to wave-dominated shallow-marine clastic successions, preserving incidental bryozoan carbonates to the north, include fluvial deposits over the most proximal southern stretches of the platform. The boundary with Cambrian strata remains unclear but the latter are progressively less represented to the south in the undifferentiated ‘Cambro-Ordovician’. To the north, graptolites, brachiopods and trilobites combined with palynomorphs provide a robust biostratigraphic frame. Maximum flooding intervals occurred in the early to middle Tremadocian, middle Darriwilian and middle to late Katian. Two events interfered with an overall long-term transgressive trend. The ‘intra-Arenig’ (late Floian?) tectonic event highlighted palaeohighs coinciding with Paleoproterozoic basements. Gondwanan drainage basins were reorganized, which had an impact on sediment sourcing and distribution of detrital material (e.g. zircons) feeding the pre-Variscan Europe. The second event is the end-Ordovician glaciation. The domain supported the greatest part of the Hirnantian glaciers and may also have preserved pre-Hirnantian glacial archives. It is not until the very latest Ordovician that offshore conditions developed far inland; it is however suspected that this inundation benefited from a transient postglacial isostatic flexure.

Abstract The importance of the full understanding of the Holocene geomagnetic field spans from human history and archaeology, to palaeoclimatic changes and engineering, to geomagnetic field modelling and unravelling of the geomagnetic field characteristics and anomalies. Unfortunately, the dearth of data from large under-covered areas such as oceans, the African and South American continents, and the southern hemisphere (only 4–6% of the global datasets) dramatically limits our knowledge of the temporal and spatial evolution of the geomagnetic field and its application. Here, a review of all data from the African continent is presented in order to encourage and motivate a new generation of palaeomagnetic and archaeomagnetic studies. New data will sharpen the palaeomagnetism as a dating tool, improve our knowledge of local/global geomagnetic features, and will help to finally answer some of the fundamental questions in palaeomagnetism, like the temporal and spatial distribution of the palaeointensity peaks described, and the origin and evolution of the South Atlantic Anomaly.

Abstract: Compound-specific isotope analysis (CSIA) of individual organic compounds is a powerful but underutilized tool in petroleum exploration. When integrated with other organic geochemical methodologies it can provide evidence of fluid histories including source, maturity, charge history and reservoir processes that can support field development planning and exploration efforts. The purpose of this chapter is to provide a review of the methodologies used for generating carbon and hydrogen isotope data for mid- and high-molecular-weight n -alkanes. We discuss the factors that control stable carbon and hydrogen isotope compositions of n -alkanes and related compounds in sedimentary and petroleum systems and review current and future applications of this methodology for petroleum exploration. We discuss basin-specific case studies that demonstrate the usefulness of CSIA either when addressing particular aspects of petroleum exploration (e.g. charge evaluation, source rock–oil correlation, and investigation of maturity and in-reservoir processes) or when this technique is used to corroborate interpretations from integrated petroleum systems analysis, providing unique insights which may not be revealed when using other methods. CSIA of n -alkanes and related n -alkyl structures can provide independent data to strengthen petroleum systems concepts from generation and expulsion of fluids from source rock, to charge history, connectivity, and in-reservoir processes.

Abstract: Late Ordovician ( c. 445 Ma) glacial sandstones form important gas reservoirs in the Illizi Basin, SW Algeria. These reservoirs have a high degree of depositional and diagenetic complexity, such that understanding and predicting reservoir quality (RQ) presents a major challenge to their economic development. Porosity is typically 1–10%, but reaches up to 15% and permeability is typically <10 −15 m 2 (<1 mD), but locally reaches >10 −13 m 2 (>100 mD). The key questions addressed herein concern the development and distribution of this RQ variability, specifically why has good RQ been locally preserved? Primary depositional fabric exerts a strong control on RQ. Muddy sandstones are either highly compacted or pervasively cemented by quartz and microporous illite, and have very poor RQ. Only fine- to medium-grained, moderately well sorted, clean sandstones can contain good RQ, but texturally and mineralogically similar sandstones span a wide range of porosity and permeability. This range is primarily driven by the degree of quartz cementation, with incomplete cementation resulting in the best RQ. Quartz overgrowths in incompletely cemented clean sandstones are patchy and non-luminescent in scanning electron microscopy with cathodoluminescence (SEM-CL), possibly indicating slow growth rates. There is tentative evidence to link incomplete quartz cementation with oil charging of the reservoir. An alternative or additional explanation of RQ preservation may be related to limited silica supply in the centres of the thickest, stacked, clean sandstones, where the better RQ tends to reside. The results of this study imply that sustained high-energy depositional processes, coupled with an early oil charge, are prerequisites for retaining the best RQ. This has important implications for the exploration and development of Late Ordovician glacial sandstones in the Illizi Basin, and potentially similar plays elsewhere.

Abstract: A reactive transport model (RTM) is used to simulate the diagenetic evolution of a siliciclastic reservoir with a known burial and thermal history. The diagenetic phenomena occurring in two temperature regimes are simulated: kaolinite/chlorite formation at low/medium temperatures by means of isothermal zero-dimensional (0D) flush models, and smectite illitization and quartz precipitation at high temperatures by means of 0D and 1D non-isothermal models. Zero-dimensional models show that at 30°C kaolinite forms only in freshwater from K-feldspar and quartz. In river or seawater, muscovite is stable instead of kaolinite. Calcite formation depends on pH and total inorganic carbon. At 50°C, seawater promotes Mg-chlorite formation from mica alterations. At 70–110°C, evaporated seawater favours smectite–illite transformation and quartz precipitation. The non-isothermal 1D models are used to simulate the diagenesis of a compacting clay expelling fluid into an underlying sandstone. An sensitivity analysis of the clays’ thermodynamic and kinetic parameters is carried out to assess the possibility of modelling the transformation of smectite into illite in the clay, with the concomitant formation of a quartz cement in the adjacent sandstone. The results of the numerical simulations point out that the extent of the smectite–illite conversion and quartz precipitation is dependent primarily on the availability of potassium: temperature, however, does not seem to play a major role.