Page 323 - Carbonate Facies in Geologic History

310 Shoaling upward Shelf Cycles and Shelf Dolomitization

Dolomitization of Carbonate Banks and Interior Shelf Cycles

Origin of Dolomite

Dolomite rock is very common in the geologic record but American geologists
have argued about its genesis, at least since 1911-1917, dates which marked the
publication of Steidtmann's and Van Tuyl's papers on its origin.
Stratigraphic and petrographic evidence show that most dolomite (dolostone) in
the geologic record results from a replacement process in which about half of the
Ca in a CaC0 3 molecule is substituted by Mg, forming CaMg (C0 3h. The end
result is a rock composed of a mosaic of rhombs varying from 5 to 250 microns in
which original fabric is partly or completely obliterated. The mosaic may be
loose-fitting or dense. Vuggy moulds of originally dense and resistant calcite
particles may be present.
The kinetics of the process are slow, offering one explanation for the lack of
dolomite on the sea floor and for its rarity in Holocene sediments as compared
with its abundance in the geologic record. The conditions necessary for dolomiti-
zation are (1) a sufficiently porous and permeable calcareous sediment to act as
host for the Mg replacement; (2) a fluid of the correct chemical composition to
react, capable of dissolving CaC0 3 and releasing Mg; and (3) a long-enduring
supply of Mg and (4) a hydrodynamic head to force great volumes of water
through the sediment. This is important because all natural waters have a rela-
tively low content of dissolved salts.
Several processes probably produce the dolomite observed in the geologic
record. Two genetic types are readily distinguished in the field: (1) strati-
graphically controlled, early diagenetic dolomite, and (2) coarse dolomite
precipitated hydrothermally along faults and veins. A variety of dolomite-forming
situations are known (Table X-4) and distinction between them may not be easy.
Combinations of processes operating over long periods of time undoubtedly
account for many complex relationships and inhibit clear-cut and consistent
generalizations supported by field and petrographic observations.
Important advances in our understanding of dolomite genesis have been made
from recent studies of Holocene dolomite (Table X-4). Three of the processes
observed in the Holocene and Pleistocene are clearly associated with emergent
platforms and their attendant shelf cycles and probably constitute the most im-
portant mechanisms for forming stratigraphically controlled dolomite. They are
discussed briefly below, followed by discussion of the stratigraphic evidence bear-
ing on environmental conditions and timing of the dolomitization process.
Adams and Rhodes (1960), Deffeyes et al. (1965) and Illing et al. (1965) pro-
vided a dolomitization theory based on the development of Mg-rich brines
through evaporation. The first two groups of authors proposed that dense saline
brines whose Mg/Ca ratios had been raised by loss of Ca through evaporative
precipitation of gypsum and anhydrite in tidal flats, ponds, and supratidal areas
(sabkhas) migrated regularly down through lime sediment and dolomitized it
(evaporative reflux). Subsequent study of evaporative flats in other carbonate
areas has indicated that Mg enrichment of brines might also occur when inflow-
ing sea water repeatedly dissolves Mg calcite in earlier-formed carbonate sedi-