Page 329 - Carbonate Facies in Geologic History

316 Shoaling upward Shelf Cycles and Shelf Dolomitization

dolomitized. This is commonly, but not always, so. Differences in original texture
and in degree of cementation at the time of dolomitization in some instances
caused differences in permeability which in turn controlled susceptibility to re-
placement. In the late Devonian Duperow Formation of the Williston basin, beds
immediately above the anhydrite in the cycle are dolomitized for a meter or less.
This perhaps indicates that the dolomitizing fluid from the evaporite itself was
forced up into the overlying bed when the sulfate layer compacted. Examples
cited in this chapter of a close vertical and lateral association of dolomite and
anhydrite cycles include the Duperow formation, the Arab zones of the Persian
Gulf, and the San Andres Middle Permian of West Texas, the Triassic Keuper and
Hauptdolomite of central Europe.
A significant large-scale relationship between dolomite and evaporite deposi-
tion is seen in the regional distribution of the Middle Pennsylvanian in the west-
ern United States. The Desmoinesian is dolomitic only where it surrounds the
three evaporitic basins of this age (northern Denver-Julesburg, central Colorado,
and Paradox basins). Pennsylvanian strata surrounding other positive areas in the
southern part of the North American craton are nondolomitic.
Some early diagenetic dolomitization is later than the depositional cycles in
whose strata it is found: Conceivably, a period of tectonic subsidence of a basin
resulting in relative uplift of its edges shortly after an episode of deposition would
induce movement of fluids through the shelves and into the basin center. Such a
history would permit dolomitization by the mixture of meteoric, connate, and
marine water.
Examples of "somewhat later" dolomitization vary from broad regional pat-
terns around shallow basins to areas of individual carbonate banks.
1. Wilson (1967b) demonstrated that the Devonian Duperow dolomitization pattern
followed that ofthe Devono-Mississippian uplift and peripheral erosion around the Williston
basin rather than precisely the depositional facies and thickness patterns of Duperow sedi-
mentation.
2. Whereas dolomite is roughly peripheral to the Williston basin in the middle Madison
Group and confined to the shelf facies belts, it is also highly developed in north-central and
western Montana and cross-cuts isopach lines and facies belts in that area (Sun River Dolo-
mite) (Fig. II-8). Here it must be post-depositional.
3. In the Dolomites of northern Italy, a few of the great Middle Triassic banks with
central lagoonal and intertidal facies are not very dolomitized. The exceptions are those
overlain by thicker sequences of argillaceous Raibl or buried by volcanic sediments instead of
Raibl beds in the normal evaporitic facies. The inference is that the time of dolomitization
followed the construction of the great Ladinian banks and· was related to the subsequent
evaporitic shelf deposits of the overlying Upper Triassic Raibl and Hauptdolomit (Leonardi,
1967).
4. Where basinal carbonate buildups are buried in later evaporites they are commonly
dolomitized but the reason for this is not entirely clear. Examples include the pinnacle reefs of
Michigan basin and the Zama-Rainbow area of northern Alberta, Canada (Chapter IV).
Detailed subsurface correlations, using petrophysical logs, indicates that the evaporitic cycles
between the buildups were formed during periods of somewhat lowered sea level and at times
when at least the tops of the buildups were exposed subaerially. It remains an open question
as to whether later fluid migration laterally from the overlying and surrounding evaporites, or
ground-water movement at times when the pinnacles were exposed was responsible for
dolomitization. Jodry (1969) argued that connate water derived from compaction of sur-
rounding evaporites and limestones flowed upward through the Michigan basin pinnacle
reefs and dolomitized them.