Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 8.9
Scanning electron microscope images of calcrete cements from valley calcretes exposed in the Okwa and Hanehai
valleys in the central Kalahari, Botswana: (a)
α
-fabric micritic cement with quartz grains and shell fragments; (b) quartz grain with
microspar grain-coating cement; (c) hollow fungal filaments encrusted with micrite and small bladed and acicular calcite crystals;
(d) needle-fibre and calcified fungal filaments (plus possible fruiting spores) lining a void with needle-fibres crossing the void.
and fungal hyphae (Klappa, 1978, 1979a, 1979b; Verrec-
chia and Verrecchia, 1994). Various concentric carbon-
ate forms may also be present, including pellets, ooids
and pisoliths (Bachman and Machette, 1977; Hay and
Wiggins, 1980). Rhombic calcite crystals within calcrete
pisoliths may accrete by the displacement of host mate-
rial (Folk, 1971; Chafetz and Butler, 1980). Other nodular
structures are usually micritic, as are the laminae which
characterise many calcretes.
Two end members of calcrete microtexture can be iden-
tified (Wright, 1990). Alpha fabrics, which correspond
to the k-fabrics of Gile, Peterson and Grossman (1966),
consist of skeletal grains within a micritic to sparitic crys-
talline groundmass. They are often densely cemented and
may contain mineral grains or clasts 'floating' within the
carbonate matrix. Considerable care is needed when us-
ing crystal size within alpha-fabrics as a palaeoclimatic
indicator or as a means to distinguish groundwater from
have occurred (Wright, 2007). This can lead to the de-
velopment of microspar and even sparry cements over
time and can only be identified with certainty through the
use of cathodoluminescence petrography (Budd, Pack and
Fogel, 2002). Beta-fabrics, in contrast, exhibit abundant
biogenic features, most notably the calcified remains of
fungi and roots. Fungi can produce carbonate textures
including alveolar septal fabrics and needle-fibre calcite
(Verrecchia and Verrecchia, 1994; Borsato
et al.
, 2000)
(Figure 8.9(d)). Calcification associated with plant roots
also generates a range of diagnostic textures (Klappa,
1980; Warren, 1983; Semeniuk and Searle, 1985; Alonso-
Zarza, 1999). Some calcrete laminae have been recog-
nised to be of organic origin (Klappa, 1979a, 1979b;
Wright, 1989; Verrecchia
et al.
, 1991) and one crystal
texture consisting of millimetre-long calcite prisms has
been attributed to a hypothetical colonial bacterium,
Mi-
crocodium
(e.g. Esteban, 1974; Klappa, 1978), or to the
