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

Abstract

Peritidal carbonates of the Lower Jurassic (Liassic) Gibraltar Limestone Formation, which form the main mass of the Rock of Gibraltar, are replaced by fine and medium crystalline dolomites. Replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an ≈460‐m‐thick platform succession. The fine crystalline dolomite has δ18Ο values either similar to, or slightly higher than, those expected from Early Jurassic marine dolomite, and δ13C values together with 87Sr/86Sr ratios that overlap with sea‐water values for that time, indicating that the dolomitizing fluid was Early Jurassic sea water. Absence of massive evaporitic minerals and/or evaporite solution‐collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation. The occurrence of peritidal facies, a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions (salinity 72–199‰). The medium crystalline dolomite has some δ18Ο and δ13C values and 87Sr/86Sr ratios similar to those of Early Jurassic marine dolomites, which indicates that ambient sea water was again a likely dolomitizing fluid. However, the spread of δ18Ο, δ13C and 87Sr/86Sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow (≈500 m) burial or that dolomitization was multistage. These data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation. Taking earlier models into consideration, it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities (normal, penesaline to hypersaline), provided that 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.

Origin of dolomite in the Middle Ordovician peritidal platform carbonates in the northern Ordos Basin, western China

“…The Ma 5 5 submember deposits were largely replaced by type 1 dolomite on the shallow water carbonate platform, whereas the thinly bedded dolomite sequences (Fig. 4) and the lack of massive gypsum/anhydrite suggest that the type 1 dolomite formed from penesaline seawater in a near-surface environment, with magnesium ions provided by a combination of high-and low-frequency sea-level changes (Qing et al 2001). The development of dolomite in a zone of mixed meteoric and marine water has been questioned and considered to be unlikely by many researchers (Hardie 1987;Melim et al 2004).…”

“…Various models have been proposed to explain the origin of dolomite in carbonate platforms (Warren 2000;Machel 2004;Swart 2015), including regional subsurface flow models (sometimes referred to as burial-flow models) at elevated temperatures (Jones and Rostron 2000) and ''early'' synsedimentary models, such as those involving seepage reflux (Adams and Rhodes 1960). These models have been used to explain the massive dolomite in syndepositional evaporites (Jones and Rostron 2000;Qing et al 2001) or in seawater with elevated salinity (Rott and Qing 2013;Read et al 2012;Rivers et al 2012). However, the development of small-scale, discrete dolomites in platform carbonates in association with elevated salinity seawater and little gypsum is less well documented.…”

Lower Jurassic Foraminifera and Calcified Microflora from Gibraltar, Western Mediterranean

“…The rocks are arranged in high-frequency, metre-scale, shallowing-upward cycles with emergent inter-to supra-tidal caps and represent repeated phases of tidal¯at progradation (Bosence et al 2000). Dolomitization within the lower three members of the formation has partially destroyed both the depositional fabric and evidence for microbiota within the rock (Qing et al 2001). However, although microfossil absence from the older three members may in part be due to dolomitization, as microfossils have not been found in the limestones of the Keightley Member their occurrence as recorded here is considered to broadly re¯ect their original stratigraphic distribution.…”

The impact of eustasy, tectonics, and paleoclimate on the dolomitization of the syn-rift Neogene carbonate platforms in the Red Sea coastal area, Egypt

“…It very possibly documents lowstand basin restriction (Qing et al, 2001;Hass and Demeny, 2002;Teedumae et al, 2004). The peritidal shallowing-upward sequences with rhythmic sedimentation have been mostly interpreted as a consequence of autocyclic and progradation mechanisms.…”

“…The penecontemporaneous origin of the Um Diheisi dolomite is documented by the presence of fine crystalline texture (Qing et al, 2001). This suggests that the Um Diheisi dolomite crystallized rapidly, being consistent with dolomitization by hypersaline sea water in a peritidal setting.…”