Page 325 - Carbonate Facies in Geologic History

312 Shoaling upward Shelf Cycles and Shelf Dolomitization

ment and mixes with earlier-entrapped brines and salts saturating the bottom
sediment (Bathurst, 1971). Under such conditions there would be a continual
addition of the more soluble Mg salts to the brines and a continual loss of the Ca
by organic or chemical precipitation of CaC0 3 , steadily increasing the Mg/Ca
ratio. The eventual seepage of such brine through sediment underlying supratidal
areas or below the bottoms of shallow ephemeral lakes and ponds would induce
dolomitization.
The necessary movement of large amounts of fluid for dolomitization in tidal
flats poses a problem. The hydrodynamic head set up by the formation of dense
evaporative surface brines over permeable sediment saturated with normal ma-
rine water has been considered sufficient to move the required volumes of water.
But such water must move downward through relatively impermeable sabkha
sediment in a topographically flat area where no relief is present to help form a
hydrodynamic head. To date, field evidence has not demonstrated a great amount
of downward movement of such water although the natural laboratory provides
sediments deposited and altered by dolomitization only in the last few thousand
years. Field evidence on Persian Gulf sabkhas does seem to indicate a mechanism
for creating thin widespread stratigraphically controlled dolomite beds associated
with intertidal sediments.
A good Holocene example of dolomitization, principally by refluxing evapo-
rative brines through intertidal to subtidal sediments from supratidal areas, has
yet to be described. However, the reflux mechanism can be inferred in several
natural evaporating lakes and bays where gypsum is being abundantly deposited
and where the more soluble and abundant NaCl and Mg salts must be escaping
(Deffeyes et al. 1965).
A theory based on reverse water movement, upward pumping by evaporation
of sea water on sabkhas has been proposed by Hsu and Siegenthaler (1969) to
explain Holocene dolomite occurrences on evaporative flats in the Persian Gulf.
Gypsum and dolomite replacements down to 1.5 m below sabkha surfaces have
been observed in the Persian Gulf. Dolomite in Caribbean tropical regions also
occurs on supratidal areas but is confined to thin hard surface or near-surface
crusts, only a few cm thick. The dolomitization has taken place in the last 3000
years. Evaporative pumping offers a reasonable explanation for replacement do-
lomite associated solely with intertidal-supratidal sediments.
Another process of dolomitization related to persistently emergent areas has
been proposed (Hanshaw et al. 1971; Badiozamani, 1973; Land, 1973a,b). It has
been pointed out by Runnels (1969) and Matthews (1971) that fresh phreatic water
with only a minor amount of Mg, when combined with marine water, forms a
fluid which may be undersaturated with respect to calcite. The solubility of calcite
is greatest with intermediate ionic strength.
Saturation with respect to dolomite increases continuously with increasing sea
water added to phreatic water. Badiozamani (1973) calculated that in brackish
water "in the range of 5-30% sea water, the solution is undersaturated with
respect to calcite and many times supersaturated with respect to dolomite." The
model proposed necessitates a continuous supply of Mg derived from sea water
and mixing with meteoric water during constant fluctuations of sea level. During
emergence, the interface where the phreatic lens of fresh water impinges on under-