Page 327 - Carbonate Facies in Geologic History

314 Shoaling upward Shelf Cycles and Shelf Dolomitization

kha dolomitization may be reasonably applied. But he has also thoroughly dem-
onstrated that there is fully as much dolomite, whose petrographic character
indicates deposition in completely marine condition instead of in supratidal eva-
poritic areas.
It is interesting (and somewhat puzzling) that petrographic observations show
much regional dolomitization, even in originally open marine sediments, to have
been relatively early in the diagenetic history of the sediment. Some control on
amounts and location of dolomite is commonly evidenced by differences in per-
meability of the original fabric. In many rocks showing partial replacement,
micritic matrix, micritic peloids, aragonitic bioclasts, and dense calcitic shells are
dolomitized, in that order. Presumably this sequence is based on ease of solubility
and the accessibility to fluid of the replaced fabric. In sediments demonstrating
such progressive dolomitization the process must have occurred before complete
calcitic cementation. Its end product is commonly porous, fossil-moldic, sucrose-
textured dolomite with all the calcite removed (Murray, 1960).
In other layers and in the same sections, however, may exist grainy beds of
finely crystalline, dense dolomite, interbedded with partially dolomitized lime-
stone and with beds of pure sucrose dolomite. In many such beds all types of
fragments and matrix are replaced, preserving the most delicate internal shell
structures as well as the micrite with tiny dolomite crystals. Such beds must have
been more or less thoroughly cemented before dolomitization since fabric per-
meability exercised no control on the degree of dolomitization. Chert nodules,
which preserve delicate fossils incorporated in sucrose porous dolomite which has
obliterated all unsilicified bioclasts, show that much dolomitization was late
enough to have been preceded by the first stages of silicification.
In summary, petrographic evidence often indicates a complex history of multi-
stage dolomitization in which the process may vary in time relative to other
diagenetic events such as anhydrite replacement, silicification, and calcitic cementa-
tion. Thus, in many instances petrographic evidence on timing of dolomitization
is equivocal and contradictory, indicating replacement probably occurring over
long time periods or during periods of rapidly changing condition.

The Stratigraphy of Dolomite

One way of ascertaining when dolomitization occurred, and what was its origin, is
by its lateral and vertical stratigraphic patterns. Stratigraphic studies show gener-
ally that shelves or positive areas are preferentially dolomitized. Many, but not
all, had evaporites on their landward or interior sides. Sediments in all three types
of shoaling cycles may be dolomitized. Observations also indicate that in some
examples, despite its early occurrence, the process was not necessarily penecon-
temporaneous with sedimentation.
Shelves are preferentially dolomitized compared to the basins which they bor-
der: Examples are manifold in the geologic record: The Middle Permian of West
Texas (Galley, 1958, p. 431, Fig. 31), the Madison Group of the Williston basin
(Sloss et aI., 1960, p. 28), the Red River Ordovician of the Williston basin (Fig. X-
21), the Silurian of the shelves around the Michigan and Appalachian basins