IAS 2010
Stable isotope stratigraphy and diagenesis of the CretaceousApulia platform (southernItaly)
Carl Jacquemyn
Rudy Swennen
Paola Ronchi
Several quarries in the Murge area were studied to comprehend the karstification processes of the Apulia carbonate platform. The selected outcrops are of different age and depositional setting. Two quarries consist of sub- to peritidal cycles that are Aptian in age. Two other quarries consist of Cenomanian rudist banks topped by a paleosol and overlain by Senonian peritidal beds. Stable isotopes were used to correlate between the different quarries, detect the karstification influence zone within the host rock and understand the specific diagenetical products that are present (speleothems, hardgrounds, …).
Micrite samples were analyzed to assess the original stable isotope signature during sedimentation and the overprinting by recrystallization related to meteoric water infiltration. Stable isotope results plot in a narrow δ18O-range (-4,5‰ to -2,5‰) but in a wide δ13C-range (-5,5‰ to +3,5‰), reflecting meteoric diagenetic overprinting. Results from different outcrops plot in different clusters. The Cenomanian rudist banksfrom different outcrops plot in exactly the same range. Results of the subtidal versus the peritidal Aptian cycles are separated by a δ13C-shift of -1,5‰. In the Cenomanian outcrops several hardgrounds occur. Stable carbon-isotope profiles across hardgrounds show a local positive excursion of +1,5‰ due to early marine cementation, and can be correlated accurately between the outcrops. A paleosol separates the Cenomanian from the Senonian deposits. A distinct negative δ13C-excursion occurs at this level and relates to soil-forming diagenesis. The overlying Senonian deposits are 2‰ δ13C more depleted than the Cenomanian rudist banks. Due to the impermeable nature of the paleosol, the Cenomanian deposits were protected from meteoric fluids that could only penetrate where the paleosol was breached by fractures. Several series of samples were taken at the same stratigraphic level, but at increasing distance from karst pockets. The carbon stable isotope results of these samples revealed an influence zone smaller than 1m around the karst pockets. Inside this zone a decrease in δ13C of 1,4‰ occurs that fadesaway exponentially from the karst cavity.
Diagenetic features such as cements and speleothems were analyzed to reveal different phases of cementation and the influence of flow rate within dissolutional cavities (karst pockets, vugs, …). The δ18O results of the cements cover the same range as the micrite samples. The δ13C range stretches from -11,5‰ to +3,0‰. The sampled speleothems have different morphologies (stalactite, euhedral crystals,crust, …), but their isotopic signature is identical. Therefore they likely precipitated from thesame fluid but in different regimes (phreatic, vadose,…). The results of the cements generallyplot in 2 groups. One group occursclose to the original limestone micrite composition. Thesecalcite cements relate mainly to precipitates that formed early inside fenestrae and molds, after the dissolution of(aragonitic) bioclasts. However, this phase does not occur in all molds. Thesecond group of cements plot close to the speleothem cluster, but is less δ13C depleted. The latter precipitated fromthe same fluids as the speleothems but underwent more rock-water interaction. Because thesecements precipitated inside small vugs and molds, flow rate likely was lower, when compared to flowrates inside large karst cavities and more interaction with the host rockwas possible.