The late Ypresian (early Eocene) Jdeir Formation was deposited in the Mesozoic‐Cenozoic Gabes‐Tripoli Basin, offshore Libya. The basin developed on the northern passive margin of the African Plate and was relatively unstable being affected by syn‐sedimentary tectonic movements. Deposition was coeval with a relative rise of sea‐level and the subsequent highstand. A lower, thinly‐developed nummulitic bank facies with restricted distribution records the transgressive event and is succeeded by more micritic sediments that record the time of maximum flooding. The succeeding sea‐level highstand is represented by a thick, and widely developed, progradational‐aggradational nummulitic sequence that displays lateral changes across WE‐ESE trending facies belts. Three major lithofacies are recognized in the Jdeir Formation: Nummulites packstone‐grainstone, Alveolina‐Orbitoliteswackestone‐packtone, andFragmental‐Discocyclina‐Assilina wackestone‐packstone, depositedin bank, back‐bank, and fore‐bank environments, respectively. The formation passes to the NNE into the pelagic lithofacies of the Hallab Formation; landward, to the south, it passes into shoreline evaporitic facies of the Taljah Formation. The lithofacies were structurally controlled by contemporaneous and/or syndepositional tectonic movements, with nummulitic facies tending to develop on uplifted areas. Petrographic and petrophysical studies indicate that porosity in the Jdeir Formation is controlled by depositional environment, tectonic setting and diagenesis. The combined effects of salt tectonics, a major unconformity at the top of the formation and meteoric diagenesis have produced excellent‐quality reservoir facies at the Bouri oilfield and in other areas. Porosity is highest in the nummulitic bank facies and lowest in the Alveolina‐Orbitolites micrite facies. Good to excellent reservoir quality occurs in the upper part of the nummulitic packstone‐grainstone facies, especially where these sediments overlie structurally high areas. High rates of dissolution found at the crests of domes and anticlines suggest that early diagenetic processes and features are, in part, structurally controlled. Future exploration success will depend on investigation of similar structures within the Gabes‐Tripoli Basin. Both porosity initiation and preservation are related to early depositional and diagenetic processes. The wide time‐gap between hydrocarbon generation and reservoir formation points to the role of the seal in porosity preservation and rules out the assumption that early emplacement of oil had preserved the porosity.

The Quaternary deposits of the Gefara Plain are comprised of three major facies – alluvial fan facies of sands and gravels which blankets the base of the Jebel Nafusa escarpment; floodplain facies of fluvial sands and silts with aeolian dunes which underlies most of the Gefara Plain and, aeolian coastal dune facies, predominantly calcarenitic, which is confined to a narrow coastal zone. Well exposed sections in the eastern part of the plain allow sub-division of the floodplain deposits into a number of mappable units defined and bounded by surfaces of stratigraphic discontinuity. The lithologies of these units are described and compared from selected type sections. The relationships between the units, their geometry, and the nature of the surfaces of stratigraphic discontinuity are discussed. It is proposed that, the lithostratigraphic sequence displayed by the Quaternary floodplain deposits of the eastern Gefara region provides a firm basis for correlation, not only with floodplain deposits in other parts of the plain but also with the other major facies developed in the area. In addition, the recognition of significant stratigraphic breaks in the sedimentary sequence combined with facies analysis lays a firm foundation for an interpretation of the geological history of the region.

Vertical, tubular borings, either simple or subdivided into cells by horizontal, spirally ornamented partitions of cemented fine silt, are common in the uppermost part of Unit Q3 of the Quaternary floodplain deposits in the eastern part of the Jeffara Plain. The structures display many features similar to those found in nests constructed by tube-dwelling, solitary wasps and bees. The size of the cells, the spiral construction of the partitions and the general architecture of the cells is closely comparable to the nest structure of the large carpenter bee Xylocopa virginica krombeini Hurd., and it is suggested that the Quarternary nests were constructed by a bee of Xylocopid type.

In recent years basic regional geological mapping of northern Libya by the Industrial Research Centre, Tripoli, together with the UNESCO Libyan Valleys Survey of eastern Tripolitania and the work of the author and Dr S. M. Ghellali in the Jifarah Plain and Jabal Nafusah, have greatly added to our knowledge of Quaternary deposits in the region. This work is described and evaluated against the background of earlier studies. The succession of major depositional, soil forming and erosional events which characterise the fluvio-aeolian deposits of the Jifarah Formation can be recognised in, and thus used to subdivide, laterally equivalent aeolianite, fangravel and wadi gravel deposits in the Jifarah Plain. Comparisons of this succession with others from throughout the region indicate that the events are of regional significance and that the Jifarah sequence provides a sound framework for lithostratigraphic correlation and a basis for facies analysis and chronostratigraphic studies.This review is largely limited to the northern margins of Libya north of the 30th parallel (Fig. 1). It is convenient to subdivide the zone into several regions. These are:(a) The Jifarah Plain and foothills of the Jabal Nafusah.(b) The Hammada Al Hamra Plateau and major valleys of eastern Tripolitania.(c) The coastal region from Misratah to Ajdabiyah.(d) Cyrenaica from Soluq to Al Bardia.

Sedimentary rocks of the Llangranog area are subdivided into the Tresaith and Llangranog Formations of Ashgill age and the Gaerglwyd and Allt Goch Formations of Llandovery age, formations which form the basis for mapping inland areas. Folds and cleavage follow a northeast to west-southwest arcuate trend. In a narrow zone near the coast, cleavage dips southeast and folds verge to the northwest; inland, the opposite prevails. Faults exhibit two main trends, northeast and north-northeast. Some cross-cutting occurs but, more commonly, trends combine to give arcuate anastomosing patterns on which southeastward directed reverse movements across east-northeast portions is translated into high angle sinistral oblique slip on sections trending north-northeast.