Page 187 - Petrology of Sedimentary Rocks

Methods for Producin Dolomite. The two most commonly accepted ways of
formin~dolomr -- te are with the hypersaline sabkha model, saline waters supplied
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either by supratidal flooding or by capillary upsucking; (2) by a reflux of heavy brines
through porous sediments beneath the saline basins. Both these models rely upon using
a brine that is always hypersaline. Hypersaline dolomite certainly exists, but we
propose that it is much easier and of much greater importance to produce dolomite by
dilution with fresh water. This can happen in several ways: (I) the schizohaline
environment, wherein hypersaline brines are periodically mixed with fresh waters in a
near-surface environment; (2) mixture in the shallow subsurface of evaporitic, highly
saline brines with fresh water; (3) mixture of normal sea water or its connate equivalent
with meteoric water, as in a salt-water/freshwater-lens contact zone; and (4) phreatic
meteoric water collecting Mg purged from magnesian-calcite during diagenesis.

Schizohaline environment. Many shallow hypersaline environments are subjected
to episodic flushing by fresh water. A sabkha can be flooded with monsoon rains; or a
shallow hypersaline bay may be flushed by storms or hurricanes. In either case the
salinity drops suddenly and drastically to nearly fresh conditions, only to be built slowly
back to hypersaline conditions, as the normal evaporitic regime regains mastery. Such
environments, characterized by wild swings between hypersaline and nearly fresh
condition, with essentially no times of normal marine salinity, are designated as
“schizohaline” environments.

Why is a schizohaline environment such an ideal place to form dolomite? Let us
assume a typical sabkha with perhaps five times normal salinity and a Mg/Ca ratio of
double normal, say 7: I. Adding fresh water drops the salinity drastically, but the
MgICa ratio remains almost as high as it was initially because of the low concentration
of total salts, including Ca and Mg, carried by the diluting water. On the diagram this
illustrated by a line dropping almost vertically and the compositon of the water plunges
deeply into the dolomite field.

For example, diluting one part normal sea water with 9 parts of average river
water with about 100 ppm total dissolved solids (typically including 20 ppm Ca and 7
ppm Mg) decreases the salinity of the mixture to about one-tenth-normal sea water, but
changes the Mg/Ca ratio only from 3: I to 2.2: I. Diluting a typical hypersaline
sabkha brine with 9 parts of river water would drop the Mg/Ca ratio only from 7:1 to
6:l. Water of such composition in the subsurface then should continue to precipitate
dolomite until the Mg/Ca ratio approaches l:2.

Repetition of this process every several decades might allow significant quantities
of dolomite to form. In such a dynamic environment of repeated flushing, effective
transport of Mg ions through the system is insured. The large freshwater head
developed during flooding may mix with and transport Mg-rich, diluted saline waters
into the deep subsurface as a thick lens, and provide the “pump” necessary for massive
diagenesis.

Subsidence or uplift--Even without the effect of flooding, gradual subsidence of
hypersaline sabkhaoeon sediments as deposition continues may allow penetration
of meteoric waters from a landward freshwater table into the hypersaline-saturated
sediments. A similar chemical revolution ensures. A fall in sea level will have the same
effect, or uplift and emergency of sabkha sediments will raise the hypersaline system
permanently into the vadose freshwater zone.

Mixing of fresh water with normal sea water--Many examples are known where a
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freshwater lens of huge size overlies sea water, with a zone of mixing in between

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