Geoscience Reference
In-Depth Information
(a)
(b)
(c)
(d)
Figure 8.13
Photomicrographs of pedogenic and groundwater silcretes: (a) grain supported to floating fabric glaebular pedogenic
silcrete from Stuart Creek, South Australia, consisting of quartz grains surrounded by a microquartz and opal matrix (plain polarised
light; scale bar 2 mm); (b) grain supported to floating fabric pedogenic silcrete from Stuart Creek, South Australia, consisting of
quartz grains surrounded by a microquartz and opal matrix and exhibiting anatase-rich geopetal laminations (plain polarised light;
scale bar 2 mm); (c) partially silicified nonpedogenic calcrete from a depth of 4.0 m beneath the floor of Kang Pan, near Kang,
Botswana, showing patchy replacement of carbonate cement (left of view) by chalcedonic and cryptocrystalline silica (right of
view), with a sharp boundary between carbonate and silica cements (cross-polarised light; scale bar 0.5 mm); (d) complex floating
fabric pan/lacustrine silcrete from Sua Pan, Botswana, consisting of quartz grains cemented by chalcedonic and cryptocrystalline
silica, which is partially replaced by micritic calcite. The sample also contains voids which are lined by opal and chalcedony and
infilled with late-stage calcite (cross-polarised light; scale bar 0.5 mm). Micrographs (a) and (b) both courtesy of John Webb.
over time (Dove and Rimstidt, 1994). Amorphous opal-
A transforms to near-amorphous opal-CT, better-ordered
opal-CT, cryptocrystalline quartz or chalcedony and fi-
nally microcrystalline quartz (see Nash and Hopkinson,
2004). The extent of paragenesis is determined by pro-
cesses including silica complexation, adsorption by clay
minerals and the neoformation of clays and other silicates
(Williams, Parks and Crerar, 1985), and may be accom-
panied by textural changes.
A range of geopetal structures have been described.
ure 8.13(a) and (b)), cusp-like structures containing lay-
ers of silica and TiO
2
, or silica and iron or manganese
oxides (Frankel and Kent, 1938; Terry and Evans, 1994;
Ballesteros, Talegon and Hernandez, 1997; Thiry
et al.
,
2006). Laminated and unlaminated conical or cap-like
structures have been identified on top of sediment clasts
(Callen, 1983; Van Der Graaff, 1983; Thiry and Milnes,
1991). Glaebules (concretionary or nodular structures;
see Brewer, 1964) are documented within F- and M-
fabric silcretes of presumed pedogenic origin (Summer-
