Geology Reference
In-Depth Information
Important studies of pedogenic carbonates deal with
late Pleistocene and Holocene calcretes on the Baha-
mas, Florida Keys, Yucatan, Barbados, Persian Gulf,
Western Australia, and the Mediterranean (see Basics
and references with Code K030).
of well-horizonted soil profiles. Characteristic features
are distinct profiles made up of a succession of differ-
ent horizons and specific microfacies attesting to the
presence of a vegetated soil cover. 'Rhizogenic cal-
cretes' produced by calcification associated with plant
roots occur in the fluvial sequences of floodplains
(Wright et al. 1995), palustrine and lake margin areas
with relatively high water-tables, on subaerially exposed
marine carbonates (Wright 1994), in carbonate dunes
(Hay and Reeder 1978) or in beachrock deposits.
(2) Non-pedogenic calcretes, derived from laterally
migrating groundwater and composed of well-indu-
rated, variably extensive sheets of mottled carbonate
(Goudie 1983). In contrast to the rather thin pedogenic
calcretes (a few meters), these calcretes may have a
thickness of more than 10 meters as shown by Austra-
lian examples. Compared with pedogenic calcretes,
non-pedogenic calcretes are characterized by the ab-
sence of rhizoconcretions, black pebbles and peloids,
and by more silica than carbonate.
Calcareous paleosols typically contain a higher pro-
portion of mineral insoluble residue than the enclosing
limestone units. Two models have been developed for
explaining the mechanisms that lead to the concentra-
tion of insoluble residue: (1) Selective dissolution and
removal of calcium carbonate from carbonate host rock,
and (2) accretion of wind-driven dust at the limestone
exposure surface without major dissolution of the lime-
stones (Multer and Hoffmeister 1968). Most authors
favor the first explanation. Calcretes are defined as 'sec-
ondary accumulations of calcium carbonate (Low- and
High-Mg calcite) in nearsurface settings, which result
from the cementation and/or replacement of host ma-
terial by the precipitation of calcium carbonate from
soil water or ground water' (Wright 1990). In addition
to the calcium carbonate derived from the host rock,
eolian input is still an important factor (Goudie 1973).
Currently, two basically different models are being
discussed with respect to the formation of calcretes.
The commonly held model, that calcretes are pedogene
structures resulting from leaching of carbonate from
the surface horizon, is questioned by the second model,
claiming that calcretes are complex sedimentary for-
mations that have undergone phases of deposition, pe-
dogenesis, and transformation of sediments during early
diagenesis heavily related to microbially influenced pro-
cesses. According to the first model, calcrete is formed
in the interior of a soil; according to the second model
calcretes are accumulations that build up by aggrada-
tion from bottom to top (Verrecchia 1996).
Critical factors in the formation of caliche
are (1) cli-
mate (a significant net annual moisture deficit in the
soil; control on dust accretion caused by dust storms),
(2) topography, (3) vegetation, (4) presence of carbon-
ate and oxalic phases, (5) carbonate content, texture,
porosity and permeability of the hosting substrate
(Thériault and Desrochers 1993), (6) action of micro-
organisms (Loisy et al. 1999) as well as (7) exposure
time (which may range between a few tens of thou-
sands, as shown by Holocene calcretes, and millions
of years as suggested by Carboniferous calcretes).
Vegetation is the main factor in
rhizogenic calcretes
(Klappa 1979; Wright et al. 1995) exhibiting textures
which can be interpreted as calcification in, on or around
roots: Intracellular calcification creates
Microcodium
-
type microstructures. Extracellular calcification around
roots produces
laminar rhizolites
(Wright et al. 1988)
consisting of vertical rhizoconcretions or as calcare-
ous cementation around root mats (Pl. 20/6). Incom-
plete calcification of root mats may be responsible for
the formation of rhizogenic calcretes characterized by
peloidal and coated grain fabrics (Calvet and Julia
1983). Exposure time, substrate composition and veg-
etation control the
maturity levels
of calcrete profiles.
Relatively mature levels can be recognized from the
sequence of horizons forming the calcrete profiles and
by the number of calcrete microfabrics (which increase
with increasing maturity).
A number of
models
for pedogenic calcrete profiles
has been developed, focused on the progressive increase
in the volume of secondary calcium carbonate within
Classification:
Calcretes are classified according
(a) to their micromorphology and hydrological setting
or (b) dolomite content (Netterberg 1978). Using mi-
cromorphological criteria, massive 'alpha calcretes' (ex-
hibiting a dense pseudonodular microfabric with com-
plex microcracks, microsparite and coarse-crystalline
calcite, floating sediment grains (Pl. 128/6), and calcretes
exhibiting microfabrics of biogenic (particularly fun-
gal) origin, called 'beta calcretes', can be distinguished
(Wright 1990). The latter group is characterized by mi-
crobial coatings, calcified tubules,
Microcodium
, alveo-
lar septal fabrics, and needle fibre calcite (Pl. 128).
Two calcrete types
are
differentiated according to
their hydrologic setting:
(1) Pedogenic calcretes derived from vertically per-
colating soil water and typically occurring in the form
