(Warkworth soil, Fig. 16.7, right) and the fresh, unweathered, parent rocks associated with each soil. Jongmans found
that the different saprolites reflected a difference in the parent
materials and thus how they weathered. The red saprolite is
formed on a coarse sandstone enriched in mafic minerals
(e.g. pyroxenes) and volcanic rock fragments, together,
critically, with calcite (Pakiri Formation containing volcanogenic Parnell Grit). This parent material becomes porous as the calcite dissolves amidst the coarse Fe-rich mineral
grains, favouring the synthesis of red haematite in a
‘box-like’ microfabric, provided the soil is well drained (and
therefore likely to be in an upper landscape position). In
contrast, the yellow saprolite is formed on fine-grained
mudstone with little or no mafic material and no calcite
(Pakiri Formation without Parnell Grit). It therefore remains
massive and non-porous during weathering, lacking the right
conditions to form red Fe oxides (probably it contains yellow goethite together with lepidocrocite in orange mottles).
The red weathering has a patchy, erratic distribution in
the landscape as the bedded volcanogenic sandstone and
siltstone parent materials are often highly folded and convoluted, and discontinuous because of erosion. The
red-weathered saprolite is thus considered to be a relict
feature because it has been overprinted by topdown pedogenesis in upper profiles, or because the contemporary soils
are saturated (gleyed), or both.
b. Red weathering in other parts of New Zealand
Although the red weathering puzzle in Northland has been
solved, it is not clear when or how the red weathering seen in
other parts of New Zealand, commonly associated with Ultic
Soils, was developed. Pockets of deep, red-weathered materials
occur on greywacke surfaces throughout New Zealand as well
as on granodiorites in northern Stewart Island and throughout
eastern Otago and Canterbury. Red weathered soils are the
norm in northeastern Marlborough and western Wellington.
Kaolinitic red-weathered material in New Zealand appears to
be commonly found in areas of relatively low tectonic uplift
rates where the stability may have helped preserve the highly
weathered material. An alternative possibility is that the
red-coloured kaolinitic material is aeolian dust derived from
Australia that has been deposited over long time periods on
relatively stable surfaces in New Zealand (Chap. 15).
Another hypothesis is that the reddening occurred in a
notably warmer interval in the distant past, either in the
Quaternary (since 2.6 million years ago), in the Pliocene (5.3–
2.6 million years ago), or Miocene (23–5.3 million years ago),
or even in the Cretaceous (145–66 million years ago) (see grey
box). It has been observed that the sedimentary deposits
forming the earliest part of the Cenozoic (last 66 million years,
such as the Broken River Formation) are quartz- and kaolinclay rich. Such deposits came from a strongly, and presumably
deeply, weathered greywacke regolith which some have
attributed to weathering on a “Cretaceous peneplain” surface
(about 80 million years ago) prior to the greywacke basement
subsiding beneath the ocean.
Could this be the ultimate Ultic Soil?
Today, Central Otago has a low level of tectonic
activity similar to that of the Northland region. We
might expect to find Ultic Soils in Otago but, generally, their formation has been prevented by erosion
related to periods of severe cold as glaciers advanced
and retreated in the Southern Alps further west. Nevertheless, very old Semiarid Soils do occur (Chap. 15).
However, near Alexandra, in Central Otago, there is
perhaps the oldest soil in New Zealand (Fig. 16.8). It is
a paleosol, which is a fossil soil formed in an earlier time
presumably under warmer, wetter, conditions than those
of the present. The paleosol has been exposed in an old
gold mining area, near the shore of Butchers Dam.
The white and brilliant red colours of the paleosol
contrast with the subdued grey-brown colours of the
overlying Late Quaternary (*20,000 years old) fluvioglacial gravels and pebbles underlying the
present-day surface soil. The top horizon of the paleosol is the white clay that is thought to correspond to
the ancient land surface. Both the white and red
horizons have 55% clay with clay minerals dominated
by kaolinite. The irregular, sharp nature of the
boundary suggests a pedologic origin. The red is likely
to be the Fe-oxide mineral haematite. Not shown in the
figure is underlying white silt with 22% clay, again
dominated by kaolinite, with the ghost rock structure
of highly weathered schist.
The Butchers Dam soil is an important historical
marker. It supports the occurrence of a lengthy period
of stability on the erosion surface where soil weathering rates have exceeded those of erosion.
16.4 Key Soil Properties
16.4.1 Soil Composition
Ultic Soils comprise the clayey weathering products of
mainly siliceous sedimentary rock of varying compositions
and textures. The types of clays formed are wide-ranging but
typically are dominated by kaolinite and halloysite, along
with abundant smectite including montmorillonite in wetter
soils, and minor micaceous clays including illite and
258
16 Ultic Soils
that the different saprolites reflected a difference in the parent
materials and thus how they weathered. The red saprolite is
formed on a coarse sandstone enriched in mafic minerals
(e.g. pyroxenes) and volcanic rock fragments, together,
critically, with calcite (Pakiri Formation containing volcanogenic Parnell Grit). This parent material becomes porous as the calcite dissolves amidst the coarse Fe-rich mineral
grains, favouring the synthesis of red haematite in a
‘box-like’ microfabric, provided the soil is well drained (and
therefore likely to be in an upper landscape position). In
contrast, the yellow saprolite is formed on fine-grained
mudstone with little or no mafic material and no calcite
(Pakiri Formation without Parnell Grit). It therefore remains
massive and non-porous during weathering, lacking the right
conditions to form red Fe oxides (probably it contains yellow goethite together with lepidocrocite in orange mottles).
The red weathering has a patchy, erratic distribution in
the landscape as the bedded volcanogenic sandstone and
siltstone parent materials are often highly folded and convoluted, and discontinuous because of erosion. The
red-weathered saprolite is thus considered to be a relict
feature because it has been overprinted by topdown pedogenesis in upper profiles, or because the contemporary soils
are saturated (gleyed), or both.
b. Red weathering in other parts of New Zealand
Although the red weathering puzzle in Northland has been
solved, it is not clear when or how the red weathering seen in
other parts of New Zealand, commonly associated with Ultic
Soils, was developed. Pockets of deep, red-weathered materials
occur on greywacke surfaces throughout New Zealand as well
as on granodiorites in northern Stewart Island and throughout
eastern Otago and Canterbury. Red weathered soils are the
norm in northeastern Marlborough and western Wellington.
Kaolinitic red-weathered material in New Zealand appears to
be commonly found in areas of relatively low tectonic uplift
rates where the stability may have helped preserve the highly
weathered material. An alternative possibility is that the
red-coloured kaolinitic material is aeolian dust derived from
Australia that has been deposited over long time periods on
relatively stable surfaces in New Zealand (Chap. 15).
Another hypothesis is that the reddening occurred in a
notably warmer interval in the distant past, either in the
Quaternary (since 2.6 million years ago), in the Pliocene (5.3–
2.6 million years ago), or Miocene (23–5.3 million years ago),
or even in the Cretaceous (145–66 million years ago) (see grey
box). It has been observed that the sedimentary deposits
forming the earliest part of the Cenozoic (last 66 million years,
such as the Broken River Formation) are quartz- and kaolinclay rich. Such deposits came from a strongly, and presumably
deeply, weathered greywacke regolith which some have
attributed to weathering on a “Cretaceous peneplain” surface
(about 80 million years ago) prior to the greywacke basement
subsiding beneath the ocean.
Could this be the ultimate Ultic Soil?
Today, Central Otago has a low level of tectonic
activity similar to that of the Northland region. We
might expect to find Ultic Soils in Otago but, generally, their formation has been prevented by erosion
related to periods of severe cold as glaciers advanced
and retreated in the Southern Alps further west. Nevertheless, very old Semiarid Soils do occur (Chap. 15).
However, near Alexandra, in Central Otago, there is
perhaps the oldest soil in New Zealand (Fig. 16.8). It is
a paleosol, which is a fossil soil formed in an earlier time
presumably under warmer, wetter, conditions than those
of the present. The paleosol has been exposed in an old
gold mining area, near the shore of Butchers Dam.
The white and brilliant red colours of the paleosol
contrast with the subdued grey-brown colours of the
overlying Late Quaternary (*20,000 years old) fluvioglacial gravels and pebbles underlying the
present-day surface soil. The top horizon of the paleosol is the white clay that is thought to correspond to
the ancient land surface. Both the white and red
horizons have 55% clay with clay minerals dominated
by kaolinite. The irregular, sharp nature of the
boundary suggests a pedologic origin. The red is likely
to be the Fe-oxide mineral haematite. Not shown in the
figure is underlying white silt with 22% clay, again
dominated by kaolinite, with the ghost rock structure
of highly weathered schist.
The Butchers Dam soil is an important historical
marker. It supports the occurrence of a lengthy period
of stability on the erosion surface where soil weathering rates have exceeded those of erosion.
16.4 Key Soil Properties
16.4.1 Soil Composition
Ultic Soils comprise the clayey weathering products of
mainly siliceous sedimentary rock of varying compositions
and textures. The types of clays formed are wide-ranging but
typically are dominated by kaolinite and halloysite, along
with abundant smectite including montmorillonite in wetter
soils, and minor micaceous clays including illite and
258
16 Ultic Soils