(BEING CONTINUED FROM 29/05/16)
THE NEOGENE FOSSILIFEROUS KARSTIC FILLINGS OF BAIXAS
(PYRENEES-ORIENTALES, FRANCE) AS EVIDENCE OF THE OLIGO-MIOCENE
TENSION IN NORTHERN MEDITERRANEAN AREA
Jean-Pierre Faillat, Jean-Pierre Aguilar,Marc Calvet et Jacques Michaux (France)
Fossiliferous karstic fillings raise various questions: their relation with karstification, quality of paleontologic dating, and geologic interest. A new approach to these issues results from the existence in the Eastern Corbieres, near Baixas (Fig. 1), of some fifty fossiliferous deposits distributed between the lower Miocene and the present. These beds, contained in carbonate rocks and forming the Baixas Plateau, are divided into three main categories: wall superficial deposit, pocket and fissure fillings whose direction can be demonstrated (Fig.2). Only the latter, called fossiliferous filling fissures (abbreviation FRF in French) are dealt with in the present paper.
The most characteristic FRF, 15 in number, belong to the Burdigalian, Langhian and Serravallian stages (Fig. 3). The filling of some FRF ipolyphased. The fossils consist of remains of small mammals, particularly rodents. The variability of the populations they release is comparable to that of populations extracted from other deposits of similar origin, to that of populations collectefrom lacustrine or paludous origin deposits and to that observed in present species. FRF faunas, homogeneous from the chronological point of view, indicate a fast settling of the sediment.
The fissures, 5 to 100m long, show subvertical walls, that are little corroded and have very well preserved a parallelism. These observations indicate that the walls have not been exposed to solving agents for a long time, considering what is known about how fast they act in a karstic environment. The span of time between the fissure opening and its filling has been nearly instantaneous at geologic scale. The rates of the detrital sedimentation in a karstic environment, known as being very high, indicate that the filling itself is just as fast. In conclusion, the FRF’s date the geologic events responsible for their formation.
The directional organization of the FRF’s has been compared with the results of a photo-geological study of morpho-structural alignments of the Baixas sector (1432 alignments read at 1/10,000, over 6 km2). These was quite a good coincidence between the main modes of the different distributions of alignments and directions of FRF. The directional sector most represented is N 10 to N 80 with a maximum between N 20 and N 50. The Miocene FRF’s correspond to it (Fig.4).
The directions of numerous Eastern Pyrenees karstic cavities show a NE-SW mode whereas some others follow a N-S and E-W mode. Such a varied distribution might prove that karstic action alone cannot account for the opening of fissures that originated FRF. Such action would have implied all the directions of the fracture network as well as vertical bedding joints. In this hypothesis, the FRF’s narrow sector localization would mean the-improbable-persistence of karstic erosion conditions which are well defined and did not vary over approximately 8 million years.
The alternative hypothesis is the FRF tectonic opening of the Baixas Plateau. It is confirmed by microtectonic observations-at a pluri-centimetric scale: 1. the presence of fissures, satellite of the FRF n° 202, presenting perfectly parallel walls, a broken outline and sharp angles, 2. the existence of some horizontal stylolithic planes affecting the surrounding rock (Fig. 5). These facts show the occurence of a distension causing the opening of the fissures, filled almost immediately after. Near the FRF, tension gashes en echelon, developing vertically, are set in action as normal fault. The calcite crystal axes of the tension gashes as well as their elongation reveal a NW-SE extension.
Fig.1. Location map
1. Paleozoic; 2. Ante-albian Mesozoic; 3. Albian marls;4. Plio-Quaternary; 5. thrust; 6. fault.
Fig.2. Schematic representation of the different types of fossil mammal bearing sites according to
their morphology and their degree of karstification. A: typical sub-paralleled wall fissure
with fossiliferous filling (FRF); B: karstified FRF with more or less concretionary walls;
C: karstic pocket; D: soil.
The exceptional conservation of the fissures pleads in favour of the successive FRF opening, followed by a filling. The fissure opening, by fits and starts, was due to the impact of the tectonic action which, at regional level, caused the great NE-SW faults of the Languedoc-Rousillon to work again. This origin for the Miocene FRF of the Baixas Plateau fits well into the regional geodynamic history marked by the formation of the Western Mediterranean. But the Baixas Plateau FRF show that the tension observed in the region during Aquitanian and Burdigalian goes on up to the middle Miocene and confirms recent work. The chronological extension of the Miocene FRF and the presence of sites of similar age which belong to the two other categories of karstic fillings show that the topographic surface has changed little during and since their formation. Finally, the FRF of the Baixas Plateau prove that in the case of carbonate rocks massifs a particular category of beds may serve as basis to disclose the geological history of the region.
Fig.3. Time distribution of the fossil mammal bearing sites. Hachured, the FRF.
Fig.4. Directional distribution of the FRF (line length is proportional to age uncertainty) and frequency histogram of photogeologic alignments at the 1/10,000 scale (area: 6km2; total number, 1,432; cumulative length: 934 km)
Fig.5. Scheme of FRF 202 (orientation N 27° ) and of satellite fissures
according to photography of a NNW-SSE vertical sawing plane.
1. Post-albian Breccia of Baixas; 2. sediment of the first filling phase (undetermined age);
3. sediment of the second filling phase (Burdigalian); 4. concretions of diverse age;
5. corrosion or sampled zone; 6. cracks and horizontal stylolithic planes.
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