Page 179 - Petrology of Sedimentary Rocks

2. Sabkha, or evaporitic salt flat. Salinity 5-10 times that of normal sea
water, MgICa from 5: I to 100: I. Fibrous to micri tic aragonite or Mg-
calcite; aphanocrystalline dolomite if Mg/Ca exceeds 5 or lO:l; also
evapori te minerals.

3. Subtidal cement near the sea water/sediment interface. Salinity and
Mg/Ca ratio same as normal sea water. Physically stable substrate the main
key, to allow slow crystallization of cements. Includes reef cement, shallow
submerged shelf (Persian Gulf), and Ocean-floor cements. Mainly micritic
and fibrous Mg-calcite and aragonite; some dolomite and rare calcite (by
very slow crystallization).
B. Meteoric waters
I. Surface fresh waters (lake, creeks). Salinity about I% that of sea
water; Mg/Ca ordinarily I :I0 to l:2. Calcite the chief form; in waters of
low ion strength (stream) calcite crystallizes as rhombs. In unusual lakes
with high Mg/Ca ratio, Mg-calcite, aragonite and even dolomite form, even
in waters of low salinity.

2. Shallow soils (air/sediment contact). Calcite in caliche can form
micrite or rhombs. Dolomite reported where high-Mg waters present.

3. Vadose zone in calcarenites. Fresh water, very low Mg/Ca ratio.
Sparry calcite, of ten minute rhombohedra.

4. Phreatic fresh-water zone. Salinity very low, Mg/Ca also generally
low ( I : IO to I :3). Sparry calcite, of ten in large, poi kilotopic crystals,
extending to edge of pore without intervening crust. Most precipitation
probably near water table (subsurface air/water contact). Upon weathering
the tendency is to flush sediments with waters with a very low Mg/Ca ratio.
Removal of Mg from the rocks allows recrystallization of micrite to
microspar or pseudospar, and replacement of dolomite by sparry calcite
(dedolomi tization).

C. Subsurface waters
I. Deep subsurface waters, largely of meteoric derivation. Sal ini ty
rather low, Mg/Ca typically I:6 to l:2. Sparry calcite, generally
anhedral. Some subsurface waters have Mg/Ca near I: I, and here dolomite
can form, limpid if crystallized from dilute solutions.

2. Deep subsurface waters, mainly of marine derivation. Salinity ranges
up to several times sea water; Mg/Ca usually low, however, I:4 to I :2.
Mg is selectively removed from subsurface connate brines because it is
snatched out by clay minerals and by formation of dolomite. Removal of Mg
from sea water means that Mg-poisoning no longer hampers calcite growth,
and sparry calcite mosaic can form.

3. It is suspected that the greatest precipitation of subsurface carbonate
should take place in the meteoric salt/water contact zone, e.g., near the
base of the fresh water lens.

Dolomite formed rapidly at high salinities in’sabkha environments tends to be in
very tiny crystals and may be poorly ordered. Dolomite formed by fairly concentrated

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