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.