Geology Reference
In-Depth Information
Lucid synopses of the most common diagenetic envi-
ronments were published by Matthews (1974), Long-
man (1980) and Harris et al. (1985). Box 7.1 lists some
key papers.
low subsurface, characterized by the mixing of fresh-
water and marine water. In the vadose zone water is
concentrated by capillarity at grain contacts; in the
phreatic meteoric zones water fills all pores. Both zones
comprise subenvironments distinguished by differences
in water movement (active circulation or stagnant con-
ditions), solution and precipitation processes and re-
sulting cements and porosity types (Longman 1980).
7.2.1 Meteoric, Marine and Burial Diagenesis
The location of the major diagenetic environments is
shown in Fig. 7.2, the processes that control these en-
vironments in Fig. 7.3. Diagenetic zones pass vertically
and laterally into others. Many carbonate rocks have
changed from one diagenetic zone to another during
their history because of sea-level fluctuations, tectonic
movements, and increasing or decreasing burial.
Meteoric freshwater lenses:
In coastal and island
settings meteoric freshwater lenses extend down hun-
dreds of meters (Fig. 7.3). Freshwater phreatic lenses
are caused by underground streams entering the near-
coast subsurface area and floating on saltwater of ma-
rine origin beneath or bounded at the base by imper-
meable layers (Matthews 1968). Recent island fresh-
water lenses show different zones of solution, stagna-
tion, and active flow (Longman 1980; Budd 1988;
Vacher 1988). Examples are known from Florida, the
Bahamas, Barbados and Bermuda (Land 1970; Matthews
1968; Halley and Harris 1979). The diagenetic pattern
of freshwater lenses (Saller 1984; Budd 1991) is ex-
plained by the hydrological Ghyben-Herzberg model,
which refers to freshwater lenses floating on seawater
on a homogeneous porous island or cay aquifer. The
model suggests that the freshwater lens extends below
mean sea level approximately 40 times the height of
the water table above mean sea-level. Small drops in
sea-level, therefore, may cause relatively deeply ex-
tending lenses recorded by specific cement associations.
7.2.1.1
Meteoric (Freshwater) Diagenesis
Meteoric is a Greek word meaning freshwater de-
rived from the Earths atmosphere. Freshwater diagen-
esis takes place in continental areas, along shelf mar-
gins, upon platforms with islands, and on atolls or iso-
lated platforms where sediments rise above sea level
(Fig. 7.3). Much of our knowledge of meteoric carbon-
ate diagenesis is based on studies of exposed Pleistocene
limestones (e.g. Bermudas, Barbados, Jamaica, Red
Sea; Land 1986; Vollbrecht 1990). Meteoric diagen-
esis starts with the loss of magnesium from High-Mg
calcite followed by the gradual disappearance of ara-
gonite and the replacement of aragonite by calcite.
Freshwater diagenesis
occurs in the meteoric vadose
zone and the meteoric phreatic zone as well as the shal-
7.2.1.2 Mixing Zone and Marine Vadose
Environment
The mixing of waters with dissimilar chemistries has
the potential for geochemical activity. Specific diage-
netic conditions characterized by the mixing of mete-
oric and marine waters may exist in shallow subsur-
face near-coastal settings (mixing zone) and at the
coastal interface of land and sea (marine beaches,
shoreface), characterized by rapidly changing condi-
tions (marine vadose environment). The mixing of wa-
ters in the
mixing zone
may be partly responsible for
the precipitation of aragonite and dolomite cementa-
tion in vugs (Kimbell and Humphrey 1994), dolomiti-
zation (Badiozamani 1973; Cander 1994; see Sect.
7.8.2.2) and chertification. Mixtures of salt water and
fresh water contribute significantly to the dissolution
of carbonate rocks on coasts (Sanford and Konikow
1989). The diagenetic criteria of the
marine vadose zone
are similar to those of the meteoric vadose zone (pen-
dant and meniscus cements), but the cementation is
Fig. 7.3.
Relationships of major diagenetic environments in
the shallow subsurface of an ideal permeable carbonate sand
island
. Differences in pore space fluids and in the possibility
for circulation of pore waters cause different dissolution and
precipitation processes, which are recorded by specific types
and assemblages of carbonate cements. The subaerial sedi-
ments of the meteoric vadose zone are affected by rain water.
Sediments in the meteoric phreatic zone are subject to active
water circulation (e.g. inner ramps, outer platforms, wind-
ward parts of reefs) or stagnant conditions (e.g. lagoons,
deeper ramps). Modified from Longman (1980).
