Page 330 - Carbonate Facies in Geologic History

The Stratigraphy of Dolomite 317

5. In several instances where carbonate buildups on shelf margins are well-dolomitized,
there is little or no dolomitization of forereef talus blocks derived from the bordering reef or
forereef. This clearly indicates dolomitization at a somewhat later period than reef growth.
Examples are: the Canning basin foreslope exotic boulders of Devonian age, northwest
Australia; the Miette-Ancient Wall debris flows (Late Devonian) in the Canadian Rockies
(Chapter IV); the Triassic Cipit blocks in the Dolomites and the lower talus blocks of the
Permian Reef Complex (Chapter VIII). Adams and Rhodes (1960) proposed a theory of
penecontemporaneous reflux dolomitization for Guadalupian strata; it is also possible that
dolomitization of the shelves surrounding the Delaware basin occurred toward the end of
Permian time when the overlying Salado evaporites were extensively deposited beyond the
confines of the basin. Dolomitization also affected the coarse and once permeable talus in the
middle foreslope of the shelf margin rather than the micritic (less permeable?) sponge-algal
facies of the upper slope.
A related example is found in the Ordovician exotic boulders in the Marathon fold belt of
West Texas (Wilson, 1954; McBride, 1969; Young, 1970). The blocks derived from the Lower
Ordovician Ellenburger group are practically all limestone whereas the environmentally
equivalent shelf facies is highly dolomitized. The best interpretation is that Ellenburger
dolomitization occurred after the boulders were emplaced (i.e., in late Canadian and Middle
Ordovician time during the formation of the widespread North American pre-Simpson un-
conformity).
6. A consideration of dolomitization during geologic time also permits the inference that
"somewhat later" diagenetic processes are at work to cause it. It is well known (Chilingar
1956) that Paleozoic and Precambrian carbonates are more dolomitized than those of the
Mesozoic and later time and that the geologic record contains much more dolomite than can
be accounted for when compared with what is forming today. Inasmuch as there is not much
evidence that Paleozoic tidal flats were more extensive over the world than in the Mesozoic,
one may assume that Paleozoic limestones have simply had more time for dolomitizing
waters to pass through them-that much dolomitization is not penecontemporaneous but
due to continuous processes operating over vast periods of time. These lines of argument
obviously favor the hypothesis of ground-water or connate-water movement as the mechanism
rather than refluxing or upward pumping of penecontemporaneously formed evaporitic
brines.
Extensive shelves around the Alberta basin of Canada provided sites of dolomi-
tizing fluids which migrated basinward: The Late Devonian banks of the Alberta
basin are extensively dolomitized except where more or less isolated by shales and
farthest removed from the platforms. Note the presence of limestone in Golden
Spike, Redwater, and Swan Hills banks on Fig. IV-20. In the strata exposed in the
mountains of Alberta there is not much evidence of penecontemporaneously
deposited shelf evaporite whose formation may have produced the dolomitizing
fluids. Slight evaporitic conditions occurred later at a time of regional shallowing
and silt deposition (Alexo Formation). There is, however, abundant evaporite in
the Alberta shelf south of the basin both contemporaneous with the Leduc reefs
(Duperow Formation) and also later in the Devonian (Winterburn Group and
Stettler beds).
There exist exceptions to all the above generalizations of stratigraphic rela-
tionships of dolomite and limestone. The chief reason for this may be the varia-
tions in timing between dolomitization and calcite cementation which inhibited
permeability and early diagenesis. Often grainstones are preserved as limestone
because early cementation rendered them less permeable than lime muds. Fur-
thermore, subsequent diagenetic history may to an important extent overprint
early, more regional patterns of dolomitization. Local leaching of calcite from
partly dolomitized zones along faults, later episodes of secondary dolomitization,