Caractérisation de la pierre de Courville (Lutétien, Marne, France). Relations entre diagenèse et propriétés pétrophysiques

Abstract

The Courville limestone, from the vicinity of Rheims (Fig. 1), forms part of a famous dimension stone formation, the “Calcaire Grossier”, widely used in prestigious gothic monuments (such as the cathedrals of Paris, Laon, and Soissons - Blanc, 1998; Blanc and Gely, 1997). In the Champagne-Ardenne region, this formation was intensively quarried from antiquity until the First World War, but now only the St. Julien quarry at Courville remains (Tourtebatte, 1995). The “Calcaire Grossier” shows a large vertical and horizontal facies variability throughout the Paris basin (Gély, 1996). Near Rheims, it is reduced to a thin sequence with two bioclastic limestone units: a lower unit containing quartz, Ditrupa strangulata and Echinolampas calvimontanus (the A6 sequence in Gely, 1996), and an upper unit containing Orbitolites complanatus, Alveolinidae and Miliolidae (Fig. 2). Description of the Courville stratigraphic succession Two cream-coloured beds are currently used in monuments: the “Liais”, also called the “1/2 roche”, at the base, and the “Roche” in the upper part of the quarry (Fig. 3). These two limestones are essentially bioclastic with a micritic matrix support; numerous foraminifera (Orbitolites complanatus, Miliolidae, Alveolinidae, Rotalidae) and calcareous algae (Dasycladacea) are observed, and it would appear that the Courville “Liais” is not very different from its Parisian equivalent (Blanc et al., 1990). In the middle of the “Liais”, a thin, blue layer (due to differential oxidation-reduction of iron oxides) can be seen; quarrymen and stonecutters claim that this specific blue layer, which in places is more than 10 cm thick, is harder than is typical for the “Liais”, but we have found no normalized measurements to quantify this observation. The micritic matrix within the two hard beds has been partly or totally recrystallized into microspar or sparite crystals, and some intragranular pores are filled with large moldic spar cements. Some soft interbeds exist between the two hard beds, but as they are easily cracked by frost, they are no longer used as building stone. The boundaries between the “Liais” and the overlying bed (called “Bousin”) and between the interbeds are graded, in places over as much as 10 cm of sediment. The soft beds contain squashed and less diversified bioclasts (essentially Orbitolites complanatus and Miliolidae), and a greater quantity of small detrital quartz grains (about 10% of total volume). The matrix is clearly micritic with very little recrystallization (into microspar). The “Bousin” bed is overlain by two other soft beds known as the “Moellons” and “Oven” beds, the name of the latter deriving from its use during mining techniques (spear quarrying of the “Oven” bed leads to collapse of the overlying “Roche” bed). As for the “Roche” bed at the top of the quarry, its microfacies (biomicrosparite with various foraminifera) is similar to that of the “Liais”, with some silicification (related to chert from the upper part of this lithological series). The micritic matrix is largely recrystallized into microspar or spar calcite, and gastropod bioclasts seem to be more common than in the “Liais” bed. Petrophysical characterization of Courville Stone Although only the two hardest limestone beds (the “Liais” and “Roche” beds) are currently used as building stone, others such as the “Moellon” bed were sometimes used in the past in local buildings. The Courville building stone is now used mainly for the restoration of Rheims cathedral and other prestigious monuments, but is also sold to the United States as a renowned and ancient building stone. The hard limestones are very similar from a sedimentological standpoint (biomicrosparite - packstone), but their petrophysical properties are different (Table 1): a porosity range from 13.5% (“Roche” bed) to 21.7% (“Liais” bed) allows us to quantify the classification used by quarrymen for these beds. Similarly, the difference between the hard beds and soft beds (“Liais” and “Roche”) and the soft beds (“Moellon”, “Bousin” and “Oven”), is very marked from a petrophysical point of view, but less variable from a sedimentological point of view; for example, the porosity of the “Liais” (21.7% ± 0.9) changes within 10 cm to 35% (top of bed) and then jumps to 50.2% ± 2.6 in the overlying “Bousin” bed. Although the sedimentological microfacies of the various beds in the Courville quarry appear to be generally similar, the matrix shows recrystallization in the harder beds, whereas it is clearly micritic in the poor-quality beds. Relation between diagenetic state and porosity Since comparison of the different matrix recrystallization states using light microscopy is unsatisfactory because of it’s subjectivity, we also used an image-analysis technique (with Microvision instrument™ image suite) called “ternary signature” (Fronteau et al., 1999b; Fronteau, 2000). The special feature of this technique is the ability to consider and quantify three phases in each image, and since the sum total is equal to 100, it can be represented on a ternary diagram. Sample signatures are obtained by repeated measurements on the same thin-section (about 25 to 40 image analyses for each sample). Four kinds of sample were considered: “Liais” samples from the cream-coloured area and from the blue seam, a soft and frost-susceptible “Bousin” sample from the “Bousin” bed, and lastly a sample from the “Roche” bed - the hardest variety quarried in the Courville area. With this image-analysis technique, thresholds are fixed; Phase A (mainly black) matches pixels between 0 and 95 (dark matrix, iron oxides and macroporosity: image capture in polarized light), Phase B matches pixels from 96 to 165 (micritic matrix, some bioclasts or glauconitic elements), and Phase C (mainly light) matches pixels between 166 and 255 (microspar, spar, quartz). Signatures of all the Courville limestone samples plot close to one edge of the ternary diagram (Fig. 5), which indicates a Phase A close to zero. We found that meso- or macroporosity only represents 0.5 to 2.0% of the total volume (intragranular or moldic porosity, Choquette and Pray, 1970), and that the porosity of Courville building stone is mostly due to infra- or micropores (pore-throat size <5 m). The main part of the Courville Stone’s pore systems is represented by intramatrix pore types that cannot be observed using photonic microscopy. Relations between sedimento-diagenetic states and porosity variations can, however, be established from Hg-injection porosimetry or normalized measurements (hydrostatic weighing, NF B 10.503) and image analysis (quantification of micritic degree, Fig. 7). The hardest limestones, which provide good building stone, have a significant percentage of Phase C (light-grey values, Figs. 5 and 6), which indicates a significant matrix recrystallization rating (from micrite to microspar or sparite), with a mean value of 35% for the cream-coloured “Liais” and 62% for the “Roche” and the blue-coloured “Liais”. In contrast, the soft and porous beds (i.e. the “Bousin”) have 77% to 93% Phase B indicating a large proportion of non-recrystallized primary matrix. The blue-coloured “Liais” seam shows the same Phase B percentage as the “Roche” bed, and image analysis confirms the quarrymen’s observations: recrystallization is best developed in the middle of the bed where the stone has the same behaviour as that from the “Roche” bed, the hardest of the Courville sequence. A direct correlation thus clearly appears to exist between total porosity and the initial state of the matrix (Fig. 7). For the micritic limestones, quantification by image analysis may be directly linked to total porosity (even if, in practice, microporosity cannot be observed using a light microscope). Beds with dominant micritic features (such as the “Bousin” and “Four” beds) are porous and easily cracked by frost, unlike beds with dominant microsparitic features (such as the “Liais” and “Roche” beds), which are more resistant to frost. Sedimento-diagenetic microfacies characterization allows us to predict the weathering behaviour of these limestones (Fronteau, 2000). Furthermore, diagenetic transformations and micritic matrix features may determine water and air permeability (Cérépi et al., 2000), which either allows or prevents salt penetration (gypsum, thenardite, etc.) and affects limestone resistance to weathering in urban areas (Phillipon et al., 1992).

Key words: Limestone, Lutetian, Dimension stone, Microfacies, Porosity, Diagenesis, Marne France