Geology Reference
In-Depth Information
Fig. 2.3.
Non-marine carbonate depositional environments.
used in a broader sense, embracing various descriptive
terms associated with horizons of subaerial exposure
(e.g.
caliche
). Important processes are chemical and
physical weathering of host carbonate rock, soil devel-
opment and accumulation of calcium carbonate within
the soil by evaporation of saturated porewaters driven
to the surface by capillary forces. These processes may
result in a physicochemically and microbially controlled
formation of hard crusts on the surface or in the upper
horizon of soils (
duricrusts
). Depending on climatic
conditions and the host substrate, the duricrusts con-
sist of carbonate (forming
calcretes
or
dolocretes
) or
other mineral phases (e.g. silica forming
silcretes
,
gypsum ->
gypscretes
, hematite and goethite ->
ferri-
cretes
).
The following discussion is limited to near-surface
accumulations of calcium carbonate (Low-Mg calcite)
in terrestrial, alternating wet settings, generally called
calcrete
or
caliche
.
These distinctive carbonates result
from multiple processes induced by life, climate and
exchange between mineral phases:
'Calcrete' was proposed for calcareous-cemented
deposits in Africa, the roots of the word caliche are
Latin, meaning lime and limestones (Reeves 1976).
Calcrete and caliche are used synonymously or sepa-
rately (calcrete in order to describe highly indurated
parts of caliche profiles).
Other names are calcareous duricrust, cornstone,
kalkar (Agarwal et al. 1992) or 'Krustenkalk'. These
calcium-rich soil accumulations cover an estimated
20 million km
2
or about 13% of the total land surface
(Yaalon 1988). They are widespread in temperate (e.g.
Mediterranean), semiarid and arid climates, particularly
in tropical and subtropical latitudes (e.g. Caribbean)
with sparse rainfall and a mean annual temperature of
about 18 °C. 'Calcicretes (syn. calcretes)' and 'dolo-
cretes' can be differentiated.
Box 2.2.
Carbonates originating in non-marine environments.
Nonmarine carbonates formed in terrestrial subaerially exposed settings
Pedogenic carbonates, paleosols, caliche/calcrete:
Formed by the accumulation of calcium carbonate within
unconsolidated carbonate-rich soils.
Palustrine carbonates:
Carbonates formed in lacustrine and transitional non-marine/marine environments originating
by short-term oscillations of the water levels and characterized by a mixture of subaerial and freshwater facies
criteria.
Cave carbonates, karst:
Formed by the precipitation of calcium carbonate within caves resulting in 'speleothems' (e.g.
flowstones, dripstones).
Eolian carbonates:
Sedimentation of fine-grained, wind-born sand-sized carbonate material (-> eolianites).
Glacial carbonates:
Dissolution and recrystallization of glacially transported carbonate debris, occurs in glacial-marine
deposits.
Nonmarine carbonates formed in terrestrial aquatic settings
Freshwater carbonates: Travertine, calcareous tufa, calcareous sinter:
Formation of carbonate deposits at warm, hot
ore cold spring orifices in subaquatious settings (e.g. pools), by combined biotic and abiotic processes.
Lacustrine carbonates:
Deposition and precipitation of calcium carbonate in lakes of different salinity (freshwater
lakes, salt lakes, playa lakes) and different settings.
Fluvial carbonates:
Carbonate deposits formed in rivers, creeks and waterfalls, originating from the combined activity
of biotic and abiotic processes
.
