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Block tilting of the North Provence early Cretaceous carbonate margin: stratigraphic, sedimentologic and tectonic data

Géosystèmes carbonatés, Université de Provence, 13331 Marseille cedex 3. jean-pierre.masse{at}univ-provence.fr

In the western part of the Castellane tectonic arc, the so-called " Provence platform area ", corresponding to the foreland of the Alpine nappes (figs. 1–2), is marked by Tithonian-Berriasian shallow water carbonates capped by hemipelagic sediments deposited from the Valanginian up to the Aptian-Albian. A detailed biostratigraphic study of the Berriasian succession, based on calcareous algae and foraminifera, allows us to distinguish a Lower to Middle Berriasian, with Clypeina sulcata, Clypeina isabellae and Holosporella sarda, from an Upper Berriasian with Pfenderina neocomiensis, Danubiella cernavodensis, Falsolikanella campanensis and Macroporella praturloni (fig. 3). We performed a field survey of 30 sites located from Quinson to the west, and Escragnolles to the east (figs. 4–5) including the study of measured stratigraphic sections and the collection of samples for biostratigraphic interpretations. These stratigraphic investigations show that below the Valanginian beds, the Berriasian platfom carbonate succession, is locally incomplete, i.e. Upper Berriasian beds are frequently absent. During the Early and Middle Berriasian, depositional environments are marked by a strong bathymetric instability, with frequent subaerial exposure events, and a significant marine restriction; by contrast, during the Late Berriasian, the overall biological diversity increases and water agitation as well, which means a significant marine opening towards the basin. The Upper Berriasian hiatus is consequently regarded as the result of a Berriasian/Valanginian and/or a lowermost Valanginian erosion (fig. 6). The spatial distribution of complete or truncated Berriasian successions identifies east-west bands, in each band truncated series are located northward and complete series are located southward. Bands are limited by thrust or strip faults interpreted as palaeofaults reactivated during the Alpine orogeny (fig. 7). These fault-bounded blocks, 3 to 10 km in width, known as the Aiguine, La Palud-sur-Verdon, Carajuan-Audibergue and Peyroulles-La Foux blocks, are southerly rotated by 1 to 2^o. We regard this structural architecture as the result of basinward tilting of blocks. Due to their rotation, the uplifted parts were eroded whereas the depressed parts were protected against erosion (fig. 8). Such a dynamic behavior reflects a distensive tectonic regime, which has been active at least during the Valanginian, that is after the drowning of the North-Provence carbonate platform. These structural events are considered as the regional expression of the Neocimmerian tectonic phase coupled with an enhancement of the Atlantic rifting. The orientation of the major Alpine structural elements (folds and faults) of the Castellane arc, is mostly inherited from these early Cretaceous tectonic events.

View larger version (45K): [in this window] [in a new window]   FIG. 1. – Geographical and geological framework of the study region in SE France. FIG. 1. – Cadre géographique et géologique général de la région d’étude dans le SE de la France.

View larger version (63K): [in this window] [in a new window]   FIG. 2. – Geological and structural framework of the Bas-Verdon. FIG. 2. – Cadre géologique et structural du Bas-Verdon.

View larger version (39K): [in this window] [in a new window]   FIG. 3. – Répartition stratigraphique des algues calcaires et des foraminifères utilisés pour la datation des séries carbonatées de plate-forme du Berriasien et du Valanginien basal. FIG. 3. – Stratigraphic distribution of calcareous algae and foraminifera used for dating the Berriasian – basal Valanginian platform carbonates.

View larger version (27K): [in this window] [in a new window]   FIG. 4. – Stratigraphic study : geographic location of study sites. Localities with a detailed stratigraphic section are underlined and numbered from I to VI. FIG. 4. – Localisation géographique des sites d’étude stratigraphique. Les localités ayant fait l’objet de coupes stratigraphiques sont soulignées et assorties d’une numérotation de I à VI.

View larger version (40K): [in this window] [in a new window]   FIG. 5. – Stratigraphic correlations. FIG. 5. – Corrélations stratigraphiques.

View larger version (78K): [in this window] [in a new window]   FIG. 6. – Panoramic view of the La Palud-sur-Verdon plateau, viewed from the southern part of the Verdon canyon, showing the erosional wedge of the Berriasian platform carbonates overlain by lower Valanginian hemipelagic sediments. In the background the Barbin overthrust is indicated. FIG. 6. – Panorama du plateau de La Palud-sur-Verdon, vu du sud des gorges du Verdon, montrant le biseau d’érosion des calcaires de plate-forme du Berriasien, sous la couverture sédimentaire hémipélagique du Valanginien inférieur. En arrière plan est indiquée la base du chevauchement de Barbin.

View larger version (45K): [in this window] [in a new window]   FIG. 7. – Alpine structures and Eocretaceous palaeostructures of the North-Provence margin. a– Alpine structures, 1– Neogene and Quaternary of Riez-Va-lensole, 2– Alpine nappes, 3– moderately displaced Provence zone, 4– autochthonous zone, 5– submeridian grabens (Early Oligocene), 6– normal faults, 7– thrusts. b– Palaeostructures from the Berriasian-Valanginian transition, 1– Aiguine block bound by the Aiguine fault and La Palud-sur-Verdon fault , 2–La Palud-sur-Verdon block bound northward by the Trigance fault, 3– Carajuan-Audibergue block bound northward by the La Ferriere fault, 4– Peyrol-les-La Foux block bound northward by the Saint-Auban fault. FIG. 7. – Structures alpines et paléostructures éocrétacées de la marge nord-provençale. a– structures alpines, 1– Neogène et Quaternaire de Riez-Valen-sole, 2– nappes alpines, 3– couverture provençale décollée, 4– autochtone, 5– grabens subméridiens (début de l’Oligocène), 6– failles normales, 7– chevauchements. b– Paléostructures de la transition Berriasien-Valanginien, 1– bloc d’Aiguine limité par les failles d’Aiguine et de La Palud-sur-Verdon, 2– bloc de La Palud-sur-Verdon limité au nord par la faille de Trigance, 3– bloc de Carajuan-Audibergue limité au nord par la faille de la Ferrière, 4–bloc de Peyroulles-La Foux limité au nord par la faille de Saint-Auban.

View larger version (45K): [in this window] [in a new window]   FIG. 8. – Architecture of the North Provence margin at the end of Valanginian. Cartographic sketch and NE-SW profile from the Bas-Verdon area showing the tilted block arrangement, erosional wedges of the Berriasian platform succession, and their sealing by the Valanginian hemipelagic cover. FIG. 8. – Architecture de la marge nord-provençale à la fin du Valanginien. Schéma cartographique et profil NE-SW de la région du Bas Verdon montrant la configuration en blocs basculés, les biseaux d’érosion de la série de plate-forme berriasienne, et leur scellement par la série hemipélagique valangi-nienne.

Key Words: Provence • Early Cretaceous • Stratigraphy • Sedimentology • Contemporaneous tectonics