Dolomitization by penesaline sea water in Early Jurassic peritidal platform carbonates, Gibraltar, western Mediterranean

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Fo rmation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedde d or laminated dolomites in the lower 100 m of an approximate to 460-m-thic k platform succession. The fine crystalline dolomite has delta O-18 values either similar to, or slightly higher than, those expected from Early Juras sic marine dolomite, and delta C-13 values together with Sr-87/Sr-86 ratios that overlap with sea-water values for that time, indicating that the dolo mitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution-collapse breccias in these carbonate roc ks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restri cted microbiota and rare gypsum pseudomorphs are also consistent with penes aline conditions (salinity 72-199 parts per thousand,). The medium crystall ine dolomite has some delta O-18 and delta C-13 values and Sr-87/Sr-86 rati os similar to those of Early Jurassic marine dolomites, which indicates tha t ambient sea water was again a likely dolomitizing fluid. However, the spr ead of delta O-18, delta C-13 and Sr-87/Sr-86 values indicates that dolomit ization occurred at slightly increased temperatures as a result of shallow (approximate to 500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive d olomitization in thick peritidal carbonates in the absence of evaporite pre cipitation. Taking earlier models into consideration, it appears that repla cement dolomites can be produced by sea. water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided tha t there is a driving mechanism for fluid migration. The Gibraltar dolomites confirm other reports of significant Early Jurassic dolomitization in the western Tethys carbonate platforms.