Le Paléocène et I'llerdien (p.p.) du Bassin sud-aquitain : lithostratigraphie, biostratigraphie et analyse séquentielle

Abstract

The lithosfratigraphy and biostratigraphy of the South Aquitaine Basin have been revised using outcrop and deep well data (Figs. 1 and 2). The new stratigraphic interpretations, supported by sequence stratigraphy, have enabled a basin-scale correlation with four formations being assigned to the Paleocene and Ilerdian. The Lasseube Formation (Cuvillier et al., 1963; Figs. 3 to 10), which groups all the pelagic limestone between the top of the Cretaceous and the argillaceous-arenaceous formations of the Thanetian, is represented by two facies: the " globigerinid chalky limestone " or " lithographic limestone " (Gubler and Pomeyrol, 1946; Cuvillier et al., 1963) representing a calm pelagic environment, and a turbiditic limestone known as " lithothamnion chalky limestone " or " conglomeratic limestone " (Gubler and Pomeyrol, 1946; Cuvillier et al., 1963; Piaziat, 1975 a). The former facies predominates in wells, whereas the latter facies is restricted to outcrop. The Pont-Labau Formation (Hay and Mohler, 1967) includes an argillaceous detrital unit with carbonate intercalations. A phase of intense erosion, clearly marked on the well-log correiations, separates it from the Lasseube Formation (Figs. 4 to 7). The top of the Pont-Labau Formation is marked by the transgressive Middle Ypresian Gan Marls, preceded by a period of emergence. We propose a subdivision of the Pont-Labau Formation into seven members with, from base to top: The Guillempau Sands (former lower Sands, Anonymous, 1962): sandstone, in places conglomeratic, micaceous and lignitic, coarse-grained sandstone and arenaceous clay, characteristic of a proximal turbiditic environment; The Latapy Marls (Pelagic foraminiferal Marl, Spicule Marl of Flicoteaux, 1967, 1972): light-grey pelagic marl containing some detrital fragments at the base; The Néez Beds (Operculina and discocyclina detrital Limestone of Flicoteaux, 1967, 1972; Upper Limestone of Boltenhagen, 1966): interbedded detrital limestone (brecciated or arenaceous with fragments of organisms such as Operculina heberti and Orbitoclypeus seunesi), clay and argillaceous limestone, capped with a bed of micritic limestone; The Libaros Beds (Transition zone and sandstone with calcareous-dolomitic cement of Flicoteaux, 1967): regressive argillaceous-arenaceous unit overlain by the carbonate Houns Beds or directly by the Pillardou Beds; The Pillardou Beds (Upper Sands of Cuvillier et al., 1963): interbedded sands/sandstone with a glauconitic calcareous cement and arenaceous marl with radiolaria and pelagic foraminifera; The Cassoua Marl: bathyal silty marl overlying Lasseube Formation limestone or to the west of Dax, directly overlying the Maastrichtian (Figs. 3 and 8). The Artigueloutan Marls, a homogenous globigerina silty marl marking the top of the Pont-Labau Formation in the North Pyrenean Trough. This Lower Ypresian unit will be described in a subsequent publication. The Arcet Formation (Jacquot and Raulin, 1888), which consists of dolomitic limestone, sub-reef limestone and gravelly limestone representing the different carbonate-platform facies, and is subdivided into two members: The Buane Beds (Boulanger, 1968): calcareous-dolomitic unit in the east and north of the basin (Figs. 6 to 10), limestone with Orbitociypeus seunesi and Operculina heberti in the west; in places, this member shows two Microcodium-bearing intervals in the Audignon Anticline; The Houns Beds (Deloffre, 1988): slightly argillaceous limestone, locally arenaceous with Orbitoclypeus seunesi and Operculina heberti, from which a gravelly limestone with Glomalveolina primaeva is developed in the Audignon Anticline. At Dax, the depositional environment was deeper with globigerina limestone at the base and limestone with Orbitoclypeus at the top (Fig 8,). In the Tarbes area, this member consists of two parts separated by an Microcodiumbearing interval, seen in the Jacque 1 well (Fig 7,): the lower Houns Beds with Glomalveolina primaeva (Figs. 4 and 5) and the upper Houns Beds belonging to the G. levis Zone (Fig 5). The Meilhan Formation (Sztràkos, 1996) is represented by nummulitic and alveolinid limestone, separated from the underlying Houns Beds by an erosional gap recognized throughout the basin. Fluvial sandstone with variegated or lignitic clay intercalations (Louer Sands of Platel, 1990; Fig. 9) is interbedded within the limestone around the Louer and Landes-de-Siougos anticlines, making it possible to distinguish a lower and an upper Meilhan Limestone (Figs. 3, 5 to 10). Dating shows that the age of the Lasseube Formation limestone varies according to the region: it is assigned to the Parvularugoglobigerina eugubina -Planorotalites pseudomenardii Zones and the NP1-NP7 Zones at Bidart (Martini, 1971; Tsizaraina, 1984; Haslett 1994; Fondecave-Wallez et al., 1994, 1995; Fig. 3), to the base of the Morozovella pseudobulloides - Planorotalites pusillus pusilius Zones, and the base ofthe NP1-NP4 Zones in the Béarn region (Hay and Mohler, 1967; Plaziat et al., 1975; Seyve, 1984 a, b; Fig. 4). The Pont-Labau Formation begins in the Planorotalites pusillus pusillus Zone and ends in the Morozovella subbotinae Zone, i.e. between NP4 (partim), NP1O and probably NP11 Zones (Hay and Mohler 1967; Martini, 1971; Romein, 1979; Seyve, 1984 a; Fig. 4). Currently available data suggest that the Arcet Formation begins in the Morozovella pseudobulloides Zone and ends in the Planorotalites pseudomenardii Zone (Besème, 1962; Blanc, 1973; Boulanger, 1968; Platel, 1990). The Meiihan Formation belongs to the Alveolina cucumiformis and Nummulites fraasi Zones and the NP 9 Zone (Schaub, 1981; Sztràkos, 1996). Figure 11 summarizes the stratigraphic position of and correlations between the Paleocene and Ilerdian formations and rnernbers in the Adour Basin. Based on new dating and sequence-stratigraphy analysis, the correlations have been extended to the whole of the Adour Basin. The sedimentary evolution of each sequence is described within the setting of the South Aquitaine Basin. Stacking the depositional sequences reveals major cycles separated by major discontinuities, recognized both at outcrop and in wells (Figs. 11 and 18). The Danian and basal Thanetian form a sedimentary unit with five depositional sequences (P1 to P5), in part eroded before the major Early Thanetian transgression comprising two depositional sequences (P6 and P7). The next two sequences (P8 and P9-E1) form another, strongly retrograding cycle throughout the South Aquitaine Basin, although this is commonly eroded before the Ilerdian cycle with sequences P10-E2 and P11-E3. Sequence P11-E3 overlies an erosion surface that affects all or part of the earlier cycles in the Adour Basin, whereas in the northern Corbières, along the Détroit de Carcassonne extension, the transgressive peaks of sequences P8 ta P11-E3 interrupt the thick continental sedimentary succession. The correlations and the sequence-stratigraphy interpretation have revealed, in the South Aquitaine Basin, two paleogeographic domains with their mutual boundary beneath the Lannemezan bank. To the west, in the Adour Basin, the marine domain extended progressively northwards during the Thanetian and the Ilerdian (Figs. 12 to 17). Bathyal sedimentation took place in the south in the North Pyrenean Trough whereas platform sedimentation occurred in the north. The boundary between these two domains remained generally stable during the Paleocene and the Ilerdian, trending roughly along the axis of the Arzacq Syncline. Although the depositional environments were relatively deep, lacunas are common in the succession with each sequence being preceded by localized or regional erosion. Thus, certain anticlinal structures show syndepositional play strongly influencing the shorelines by creating eroded zones during the Thanetian and the Ilerdian. To the east, from Comminges to BasLanguedoc, the marine domain fades out in a basin being filled with continental material undergoing general and continuous subsidence. Superimposed on the differences in geodynamic evolution between the two parts of the South Aquitaine Basin, is a succession of sedimentary cycles, observed in the Paris Basin and in the South Pyrenean Basin and with transgressive peaks common to all three basins. This may have a eustatic signification that needs to be confirmed outside the western European plate.