Page 326 - Carbonate Facies in Geologic History

Petrographic Evidence of Time of Dolomitization 313

lying marine or saline connate water would be a dolomitizing zone; this front
could pass through a considerable thickness of sediment as sea level drops. The
same conditions would occur during marine transgression.
Some additional Mg may become dissolved in ground water from Mg calcite
in freshly deposited marine sediment on shelves which periodically emerged and
were subjected to tropical rainfall. Wide, flat, low platforms such as Florida and
Campeche might have undergone many episodes of drowning, deposition of a
relatively thin sequence of marine sediment, emergence, and erosion without
having left a trace in the geologic record. The thickness of preserved sediment even
on great platforms of major subsidence is very limited when modern rates of
accumulation of shallow-water calcium carbonate are considered. Goodell and
Garman (1969) pointed out that, based on rates of modern Bahaman-type sedi-
mentation, the Bahama Banks should have been the site of eight times as much
shallow-water carbonate accumulation in the last 120 million years as they pos-
sess. This argues for persistently intermittent sedimentation, exposure, and per-
haps cannibalization of carbonate sediments as they are deposited. During a
general period of carbonate bank sedimentation there could have been many
minor transgressions and regressions.
The theory of dolomitization through mixing of meteoric marine water does
not necessitate a strong evaporative climate to concentrate Mg in interstitial
brines by precipitation of CaS04' It rather utilizes a climate of tropical rainfall to
provide a fluctuating lens of fresh water overlying saline brines as seen today in
Yucatan and Florida.
Both evaporative reflux and the above model of a migrating front of brackish
water (Dorag dolomitization of Badiozamani, 1973) require the flowage of ground
water through supratidal coastal sediment and the dolomitizing of permeable
sediment deposited in more normal marine conditions. Both models require a
relatively positive area (mainland or island) to generate the particular type of
water required and to furnish a head for the great volume of migrating fluid. In
both situations extensive dolomitization through thick sections may be aided by
the tendency toward prograding carbonate sedimentation-the formation of
shoaling upward or fill-in sedimentation. Seaward building of land surfaces over
poorly consolidated marine sediment also brings progradation of near-surface
diagenetic environments to alter the underlying carbonate. sediment.

Petrographic Evidence of Time of Dolomitization

A common problem in dolomitization is not only the origin of the fluid and its
movement but its timing as well; some petrographic observations have important
bearing on this. In some instances they indicate that dolomitization was not
necessarily penecontemporaneous with sedimentation although its map patterns
show it to be persistently associated with former shelves and positive areas, which
could have supplied the proper water. Zenger (1972a, b) has pointed out that
many stratigraphically controlled dolomites contain sedimentary and organic
structures indicative of the supratidal-intertidal environments in which Holocene
dolomite is now forming and in which the model of penecontemporaneous sab-