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The geology of the Boulonnais area of northern France

This seminar focuses on upper Jurassic and Cretaceous rocks exposed in cliffs between Boulogne and Cap Blanc-Nez. These are formed by a shallow-marine to intertidal sequence of lower Kimmeridgian to Tithonian age, Albian claystones, Albo-Aptian greensands and chalks of Cenomanian to Coniacian age (Purbeckian and Wealden facies are present in the area, but are not well exposed). As is distinct from the English Weald structure (a continuation of the Boulonnais-Artois anticline), Palaeozoic strata of the core of the London-Brabant massif outcrop in the Boulonnais and it is possible to examine inland sequences of Frasnian age and Visean carbonates of the massif paléozoïque de Ferques. An itinerary is included below.

The coast between Boulogne Harbour and Wimereux: exposes a section through the lower Kimmeridgian to upper Portlandian and includes two shallow-marine to intertidal sandbodies (the Grès de la Crèche and Grès des Oies). Both coarsen-upward and exhibit abundant trace fossils, including Thalassinoides, Rhyzocorallium, Skolithos and Diplocraterion. Sedimentary structures are better preserved in the higher sandbody and include swaley cross beds, as well as low-angle beach laminations. Intervening shales incorporate phosphatic nodule-beds (one of which is equivalent to the rotunda horizon of England) and evidence variation in oxygenation levels. Fauna are abundant and include pavlovid ammonites and many different genera of bivalves. Zoophycos burrows are also present, but may be difficult to observe. An additional section through the higher sandbody at Pointe des Oies demonstrates the lateral variation which can characterize shoreface sandbodies over relatively short distances and includes spectacular soft-sediment deformed horizons.

The massif paléozoïque de Ferques: the unconformity between the Bathonian Calcaires de Rinxent and Visean carbonates is exposed in Le Courgain mineral railway cutting, as are oolite grainstone facies of the Visean Formation de Haut Blanc and mounded, dolomitic carbonates of the Tournasian Dolomie de Hure. Couderousse mineral railway cutting exposes a section through a Waulsortian-type mound of the Dolomie des Noces and its enclosing Schistes de Beaulieu of Frasnian age.

The foreshore and cliffs at Cran de Quette on the western headland of Cap Gris-Nez: expose a section through the Argiles de Châtillon and Grès de la Crèche. The shales include coquina shell beds and sandier intervals with starved, wave-ripple lamination, whilst the overlying sandbody shows considerable sedimentological variation compared to exposures north of Boulogne harbour. The lower part accumulated as a sandwave complex in deeper parts of a shoreface system and is succeeded by a bioturbated unit in which Diplocraterion marks the transition to upper shoreface/intertidal conditions. The top of the unit exposes low-angle beach laminations with superimposed linguoid dunes and includes limonite-cemented horizons which possibly formed as a result of penecontemporaneous exposure.

The foreshore of the northern headland of Cap Gris-Nez: exposes the west-northwest-trending Nord-Artois Shear Zone which is developed in several repeat sections through the Argiles de Châtillon, Grès de la Crèche and Argiles de la Crèche. Sedimentological details are not obvious but this locality provides a rare opportunity to examine structure associated with a major strike-slip fault zone.

A coastal section through Cretaceous strata between le Petit Blanc-Nez and Cap Blanc-Nez: exposes greensands of the Albo-Aptian formations de Wissant and des Gardes, highly fossiliferous claystones of the Albian Formation de St Pô and chalks of Cenomanian to Turonian age. The latter include fractured chalks of the Formation de Cran, which form an intermittent springline perched above the marly Formation du Petit Blanc-Nez. The top of these permeable chalks corresponds to the base of the Formation d'Escalles which, whilst being intensely fractured, does not flow. Hardgrounds and other sedimentological features are well displayed at several levels, as are faults and joint systems, some of which are supposed to evidence at least two phases of movement.

  Further information may be obtained from:

Dr Lou Macchi,

Reservoir Associates International, Laburnum House, Shocklach, Cheshire SY14 7BT, U.K.

Tel: +44 1829 250562, fax: +44 1829 250444