Page 331 - Carbonate Facies in Geologic History

318 Shoaling upward Shelf Cycles and Shelf Dolomitization

epigenetic dolomite formed along veins and fractures, preferential stylolitic solu-
tion of calcite, and dedolomitization by waters enriched in CaS0 4 all are late
diagenetic processes modifying the results of earlier and more stratigraphically
controlled dolomitization.

Conclusions

Stratigraphic evidence favors dolomitization associated with platforms and pos-
itive areas. These could have been preferred locations for at least two processes
logically causing the replacement by migrating fluids: (1) density reflux of high
MgjCa brines from sabkha evaporation which could have occurred penecontem-
poraneously, and (2) movement down a hydrologic gradient of a phreatic water
lens mixing with connate water or purely marine water along old coast lines. This
is a post-depositional process. The common cyclicity of upward shoaling or fill-in
cycles of shelf strata offers evidence for repeated fluctuations of relative sea level
to aid in either or both these processes. Study of distribution of dolomitized banks
and reefs in basins shows that in some areas fluid migration from shelves was
important and that dolomitization in other basins may have been associated with
sea-level lowering. In several examples of regional dolomitization it appears to be
postdepositional rather than penecontemporaneous and to have operated
through fluid migration during tectonic adjustment.