Late Dolomitization in Basin Limestones of the Southern Apennines Fold and Thrust Belt (Italy)

Iannace, Alessandro ¹; Gasparrini, Marta ²; Gabellone, Tatyana ¹; Mazzoli, Stefano ¹
(1)Dipartimento di Scienze della Terra, Università di Napoli "Federico II", Napoli, Italy. (2) Département Sédimentologie et Stratigraphie, IFP-Energie Nouvelles, Rueil Malmaison, France.

The Triassic pelagic carbonates of the Lagonegro Units of southern Apennines fold and thrust belt are frequently dolomitized, both on outcrops and in the subsurface. Field survey and analyses of cuttings from oil wells has shown that the dolomites belong to two different generations. In the Lucania area they have fabrics and petrographic features typical of zebra or saddle dolomites, whereas to the north fine crystalline replacive dolomites prevail. The latter dolomites are genetically linked to the widespread, climatically controlled early diagenetic replacement which characterized the Norian shallow carbonate seas of western Tethys.

A comprehensive regional study is in progress on the saddle type dolomites to better define the relationships of fluid flows with the structural history of the tectonic belt. Field data indicates that the zebra fabrics were controlled by the regular bedding of the micritic limestones, resulting in replacement and void-filling events in a dilatational stress-field favoured by an overpressured fluid regime.

δ¹³C and ⁸⁷Sr/⁸⁶Sr values overlap those of the Upper Triassic seawater, whereas the δ¹⁸O values are depleted compared to the coeval seawater. Fluid salinities were in the range 2-6 wt % NaCl eq, whereas, homogenisation temperatures corrected for the maximum burial hydrostatic pressure indicate dolomite formation below 115°C. Thus, even by taking the maximum possible trapping conditions the fluids had temperatures below those of peak burial (130-160°C). They better fit to a second phase of deformation, occurring after the exhumation had begun and before the succession passed the isotherm of 100-110°C (apatite fission track data from literature) between 5 and 4 Ma under 3-4 km of burial.

It is suggested that dolomitization was due to formations waters squeezed out from surrounding lithologies and partially diluted by meteoric waters driven downward from the already exposed part of the accretionary prism. Further geochemical and fluid inclusion analyses are in progress in order to constrain the fluid flow paths and evolution during the stacking of the different tectonic units. It can be argued that this fluid-flux in the belt possibly also affected the Miocene foreland, represented by the Apulian Platform carbonates, now buried below the nappe stack, which host the major oil fields of continental Europe.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.