Mise en évidence micropaléontologique d'un flysch ypréso-lutétien, ex-crétacé, à matériel triasique resédimenté, dans la coupe de Bidart-Nord (Pyrénées Atlantiques, sud-ouest de la France)

Abstract

The Bidart-North section (North-Pyrenean Zone, Pays Basque and Bigorre Cretaceous flysch Subzone) is situated along the Atlantic coastline (Caseville beach), to the South of Biarritz (Pyrénées Atlantiques dept.), between the Bidart picnic area and the Handia-llbarritz cliffs. The topic of this paper is to propose a review of the Danian-Lutetian interval along this famous section, particularly concerning the discovery of Ypresian-Lutetian microfaunal assemblages within the Caseville Flysch deposits (previously Late Cretaceous in age), including a "Triassic" olistostrome at the base and overlying the Paleocene pink limestones. The lateral development of similar olistostromes to the East and to the West is also discussed for the Lasseube area (Bearn) and the Guipuzcoa coastline (Spain). Review of the Bidart-North section 1 - The "Iridium anomaly level" of the Cretaceous-Tertiary (K/T) boundary. It overlies the latest Maastrichtian Bidart Marls corresponding to the Mayaroensis Zone and dated of 67.7 ± 0.4 Ma by means of the grade-dating method applied to the globotruncanid assemblages. It consists of black clays (7 cm) containing rare reworked globotruncanids associated with benthonic foraminifera (and siliciclastics in the neighbouring Loya section). Thus the Iridium level can be regarded as a low stand deposit (LST) and the K/T boundary as a depositional sequence boundary. Within other sections of the Basque Country, such as the famous Zumaya section (Spain), these clays contain Zone P0 assemblages indicating the earliest part of the Danian. 2 - The "Paleocene pink limestone Formation" (50 m). It includes at less, six depositional sequences, SDT1 to SDT6, of increasing thickness, composed (when they are the most complete) of low stand (LST) brecciated limestones and high stand (HST) hemipelagic mudstones and red marls: a) SDT1 (1.50 m} begins with the Iridium level and red marly limestones (Zone P1A, Lower Danian) with several reworked Cretaceous globotruncanid and heterohelicid species (LST). Above, it includes HST mudstones exclusively containing small "globigerinids" ; b) SDT2 (2.50 mj consists of a monogenic breccia (LST) and white limestones (HST) from Zone PIB (Upper Danian); c) SDT3 (5 m), tectonically truncated, is only represented by slumped breccia; d) SDT4 (20 m) contains, within its LST detrital marly limestones, a well-preserved and rich assemblage of Cretaceous planktonic taxa associated with rare "globigerinids" from Zone P2 (Latest Danian) which shows up the weakness of the corresponding stratigraphie gap proposed by Haslett (1994); e) SDT5 (20 m) is characterized by its thick HST white limestone/red marl alternation assigned to the Upper Selandian (Zone P3B); f) SDT6 is limited to a few metres of LST marls and marly limestones from Zone P4 (Thanetian), particularly rich in reworked globotruncanids and heterohelicids. 3 - The "Chaotic complex", reworking Triassic material (20 m). Overlying the Paleocene limestones, by means of a submarine erosional surface, this red complex corresponds to two decimetric thick flows of megabreccia separated by a few metres of Ypresian flysch (Zone P6, Lower Ypresian). It contains heterogenic and heterometric carbonate blocks coming from the Mesozoic basement (Triassic, Lias, Dogger-Malm, Upper Cretaceous), which could be metamorphic ("dipyre"), within a red/green/black evaporitic matrix. This complex has been previously assigned to a diapiric structure, to a tectonic Triassic slice along a major fault or to a reworked intra-Lutetian Triassic olistostrome. Considering the absence of an important stratigraphic hiatus between the Paleocene limestones and the overlying Ypresian-Lutetian flysch, we agree with the last interpretation and regard the chaotic complex as a LST deposit marking the base of this flysch. 4 - The Ypresian - Lutetian Flysch of the Caseville beach (100 m at least). It is strongly folded and disappears to the North under the Quaternary of "Pavillon Royal". It consists of a fining-up succession of flint-bearing calciturbidites previously assigned tu the Upper Cretaceous (Bayonne sheet, 1:50,000 scale) by comparison with the "Fucoïd flysch" of Hendaye and Saint-Jean de Luz and considered as a "large olistolith" reworked within the chaotic complex. The previous authors seem to have underestimate the reworking of micro faunas: the benthonic assemblage observed within the clastic interval of turbidites is Vraconnian (= Latest Albian) in age when the hemipelagites from the top of the turbidites contain abundant Upper Cretaceous planktonic foraminiferal assemblages (reworked) associated with typically Ypresian - Lutetian (Zones P9 and P10) "globigerinids". Immediately to the North, the series is represented by more recent formations such as the nummulite-bearing limestones ("Biarritzian") of Peyre-que-Bleve and Peyreblanque and the Handia marly limestones (zones P11 to P14, Middle Lutetian to ? Bartonian). This last unit includes at its base a new chaotic complex (Ilbarritz complex) with Triassic and Liassic blocks, similar to the first one but more recent in age. Possible lateral equivalents of the chaotic complex reworking Triassic material and of the overlying Ypresian-Lutetian Bidart flysch. 1) Atlantic Spanish coastline (Cuipuzcoa Province): The oldest flysch outcrops ("Cuipuzcoa flysch") of Fuentarrabia (Cabo Higuer, SE) which are known above the Paleocene limestones are Ypresian in age (Zone P6). The overlying "Jaizkibel sandstones" are also dated as Ypresian (Zone P7) at their base. These datings already suggest correlations between the Ypresian-Lutetian flysch of Bidart-North and the Guipuzcoa-Jaizkibel turbiditic complex. 2) Bearn: Close to Pau, at the North-Pyrenean/Sub-Pyrenean transition zone, the Paleocene pink limestones (or "Lasseube limestones") are covered by calcarenitic turbiditic then marly Upper Paleocene series including, for the earlier authors, a long "Triassic" slice (evaporites, ophite) ponctually associated with globotruncanid-bearing marls (Upper Cretaceous). In fact this structure could be regarded as a gravitational chaotic complex, comparable to that at Bidart, reworked within the Zone P4- Zone P6A interval (Thanetian-Ypresian transition) and containing, in various places, reworked planktonic foraminiferal assemblages. In conclusion, the discovery of micropaleontologic data proving the Ypresian-Lutetian age of the "ex-Cretaceous" flysch of Bidart-North shows the importance of the foraminiferal redepositions within the tectogenic North-Pyrenean and Sub-Pyrenean series and, particularly, within the Early Tertiary turbiditic deposits. It ulso shows the interest in systematically looking for these reworked benthonic and planktonic foraminifera within the "Triassic" red and green clays in order to find evidences of resedimentation within more recent olistostromes.