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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