J. Garciaveigas et al., THE MESSINIAN SALT OF THE MEDITERRANEAN - GEOCHEMICAL STUDY OF THE SALT FROM THE CENTRAL SICILY BASIN AND COMPARISON WITH THE LORCA BASIN (SPAIN), Bulletin de la Societe geologique de France, 166(6), 1995, pp. 699-710
A geochemical study has been performed on Messinian halite deposits fi
om the Central Sicily Basin and the results compared with those publi
shed for the Lorca Basin (Spain), in order to provide a depositional m
odel for these marine salt formations and to improve our understanding
of the Messinian evaporitic event. Halite samples from boreholes and
mine galleries from the Salt Member of the Gessoso Solfifera Formation
of the Caltanissetta Basin (Sicily) were studied petrographically and
geochemically. The bromine content of halite increases from the base
of the Salt Member to the horizons containing kainite (layer B) up to
150 ppm. Upwards, the bromine content decreases and at the top of the
member it drops down below 13 ppm. thus reflecting a marked dilution o
f the mother brine, which resulted in the precipitation of almost brom
ine-free salt. This dilution has been attributed to the inflow of cont
inental waters in the literature. Fluid inclusion compositions at the
top of the unit demonstrate the SO4-rich character of the brine, which
is only slightly depleted in SO4 with respect to normal evaporated se
awater and shows a significantly Mg and K content, indicating the mari
ne origin of the brine which controlled the final precipitation. This
is in agreement with the petrographically well-established primary ori
gin of kainite. In the case of the Saline Unit from the Lorca Basin (S
E Spain), bromine profiles are essentially similar to those described
above, whereas fluid inclusion compositions at the top of the unit rev
eal the Mg, K and SO4-poor character of the brine and reflect an inflo
w of continental waters into the basin which were responsible for fina
l dilution and bromine-free salt precipitation. Thus, in the Lorca bas
in, which occupied a marginal position in the Mediterranean Basin, dil
ution and salt reprecipitation at the top of the salt unit occurred wh
en the basin was cut off from the sea and became completely isolated a
nd desiccated. In the Caltanissetta basin, which occupied a relatively
more central position, similar saline sediments were formed al the to
p of the Salt Member as a result of fresh marine waters inputs. Accord
ingly, in the Sicilian basin, the existing unconformity at the top of
the Lower Evaporite Unit does not imply subaerial exposure or complete
desiccation of the marine basin. Local tectonism probably controlled
the different hydrochemical evolutions of these basins.