surface horizon which is often much thicker than in other
soils, typically *40 cm or so in depth.
7.3 Soil-Landscape Relationships
7.3.1 Introduction
The geographic distribution of the Melanic Soils is controlled by underlying rock types and the distribution of
surficial materials. Where calcium or magnesium-rich rocks
are exposed at the ground surface, Melanic Soils form. Two
areas of New Zealand show a concentration of Melanic
Soils: the Waipara region of North Canterbury and the
hinterland of Oamaru in North Otago and its northern
extension into South Canterbury. Melanic Soils also occur
on the volcanic terrain around Banks Peninsula and Dunedin, on tuffaceous greywacke in Southland, and on other
widely dispersed small outcrops of limestone/calcareous
mudstone or ultramafic rocks throughout the rest of New
Zealand.
However, not all limestone, calcareous, or basaltic outcrops in a landscape support Melanic Soils. Other materials
may cover the limestone or basalt and prevent Melanic Soil
formation. The cover beds include loess, tephra, and colluvial debris derived from various materials occurring upslope.
For example, in the Waikato region, there are widespread
limestone deposits, such as near the world-famous Waitomo
Caves, but the soils are predominantly Allophanic Soils
formed mainly in the overlying tephra deposits. Areas of
limestone and mafic gabbro rocks also occur in the Fiordland
region, and the Mt. Arthur region of Nelson, where we might
expect to find Melanic Soils. However, the formation of
Melanic Soil has been prevented at many sites by erosion or
by extreme acid leaching under acid-litter-forming native
forest under high rainfall.
7.3.2 Melanic Soils Associated with Limestone
Terrain: Waipara and Oamaru
In the Waipara region of North Canterbury, the major geological features comprise limestone ridges (supporting
Rendzic Melanic Soils), associated with calcareous sandstone on hills and rolling land with Orthic Melanic Soils.
Clayey alluvial silts, clays, or gravels occur at downland
margins on foot slopes, and on fans (with Orthic Melanic
Soils and Vertic Melanic soils). It has been suggested that
the Argillic Orthic Melanic Soils (Tokarahi soils) described
in both North Canterbury and North Otago were developed
on loess that was originally calcareous.
In the Oamaru hinterland, substantial areas of limestone,
marl, and calcareous mudstones occur but Melanic Soils
have been suppressed by quartzofeldspathic loess cover.
Where loess is absent or thin, Orthic Melanic Soils prevail.
Vertic Melanic Soils in Waipara occur on clayey basin-infill
alluvium that incorporates calcareous gravels. A comparable
geological setting results in a similar suite of soils in
Oamaru, but there, the Vertic Melanic Soils occur on
weathered basaltic, and calcareous-basaltic, tuffs.
7.3.3 Melanic Soils in Hilly Volcanic Terrain:
Otago and Banks Peninsula
The soils of Banks Peninsula and Otago Peninsula, and their
pattern in the landscape, have similarities. The hills are
dominantly formed from a basaltic suite of mafic volcanic
rocks (basalt and andesite in Banks Peninsula, and basalt,
phonolite, and trachyte in Otago Peninsula). The volcanos
were active 11–6 million years ago. The soil pattern on these
volcanic hills is controlled by
• deposition of a loess blanket that has smothered Melanic
Soil genesis on lower ridges and flanks of the hills,
leading to the formation of Pallic Soils,
• exposure of weathered volcanic mafic rocks with the
formation of Melanic Soils,
• high leaching and depression of pH on higher altitude
sites, with consequent increase of acidity and loss of Si in
soil solution, and thus prevention of smectite clay formation, leading to the development of Brown Soils
instead.
The Melanic Soils are thus confined to the intermediate
altitude zone. The zone has irregular boundaries because it
has been controlled by deposition of loess subject to topographic effects on wind transport and deposition, as well as
by rainfall and associated leaching.
7.3.4 The Waiareka Loess Gap
The highly productive market gardens and cropping land on
the easy-rolling landscape immediately to the south of
Oamaru (Fig. 7.1) owe their productivity to the Waiareka
series of Vertic Melanic Soils. The Waiareka clay soils are
formed in weathered mafic volcanic rocks. There are also
minor areas of the Oamaru series (Rendzic Melanic Soils) on
limestone ridges. Surrounding the Melanic Soils are soils
formed in a blanket of loess (mainly Fragic and Laminar
Pallic Soils). The loess blanket is near continuous in the
surrounding landscape apart from the area of Waiareka soils
where there appears to be a ‘hole’ in the loess blanket. The
hole has been informally called the Waiareka loess gap, but
what is the origin of this apparent gap? One favoured
7.2 Soil Profile Genesis
105
soils, typically *40 cm or so in depth.
7.3 Soil-Landscape Relationships
7.3.1 Introduction
The geographic distribution of the Melanic Soils is controlled by underlying rock types and the distribution of
surficial materials. Where calcium or magnesium-rich rocks
are exposed at the ground surface, Melanic Soils form. Two
areas of New Zealand show a concentration of Melanic
Soils: the Waipara region of North Canterbury and the
hinterland of Oamaru in North Otago and its northern
extension into South Canterbury. Melanic Soils also occur
on the volcanic terrain around Banks Peninsula and Dunedin, on tuffaceous greywacke in Southland, and on other
widely dispersed small outcrops of limestone/calcareous
mudstone or ultramafic rocks throughout the rest of New
Zealand.
However, not all limestone, calcareous, or basaltic outcrops in a landscape support Melanic Soils. Other materials
may cover the limestone or basalt and prevent Melanic Soil
formation. The cover beds include loess, tephra, and colluvial debris derived from various materials occurring upslope.
For example, in the Waikato region, there are widespread
limestone deposits, such as near the world-famous Waitomo
Caves, but the soils are predominantly Allophanic Soils
formed mainly in the overlying tephra deposits. Areas of
limestone and mafic gabbro rocks also occur in the Fiordland
region, and the Mt. Arthur region of Nelson, where we might
expect to find Melanic Soils. However, the formation of
Melanic Soil has been prevented at many sites by erosion or
by extreme acid leaching under acid-litter-forming native
forest under high rainfall.
7.3.2 Melanic Soils Associated with Limestone
Terrain: Waipara and Oamaru
In the Waipara region of North Canterbury, the major geological features comprise limestone ridges (supporting
Rendzic Melanic Soils), associated with calcareous sandstone on hills and rolling land with Orthic Melanic Soils.
Clayey alluvial silts, clays, or gravels occur at downland
margins on foot slopes, and on fans (with Orthic Melanic
Soils and Vertic Melanic soils). It has been suggested that
the Argillic Orthic Melanic Soils (Tokarahi soils) described
in both North Canterbury and North Otago were developed
on loess that was originally calcareous.
In the Oamaru hinterland, substantial areas of limestone,
marl, and calcareous mudstones occur but Melanic Soils
have been suppressed by quartzofeldspathic loess cover.
Where loess is absent or thin, Orthic Melanic Soils prevail.
Vertic Melanic Soils in Waipara occur on clayey basin-infill
alluvium that incorporates calcareous gravels. A comparable
geological setting results in a similar suite of soils in
Oamaru, but there, the Vertic Melanic Soils occur on
weathered basaltic, and calcareous-basaltic, tuffs.
7.3.3 Melanic Soils in Hilly Volcanic Terrain:
Otago and Banks Peninsula
The soils of Banks Peninsula and Otago Peninsula, and their
pattern in the landscape, have similarities. The hills are
dominantly formed from a basaltic suite of mafic volcanic
rocks (basalt and andesite in Banks Peninsula, and basalt,
phonolite, and trachyte in Otago Peninsula). The volcanos
were active 11–6 million years ago. The soil pattern on these
volcanic hills is controlled by
• deposition of a loess blanket that has smothered Melanic
Soil genesis on lower ridges and flanks of the hills,
leading to the formation of Pallic Soils,
• exposure of weathered volcanic mafic rocks with the
formation of Melanic Soils,
• high leaching and depression of pH on higher altitude
sites, with consequent increase of acidity and loss of Si in
soil solution, and thus prevention of smectite clay formation, leading to the development of Brown Soils
instead.
The Melanic Soils are thus confined to the intermediate
altitude zone. The zone has irregular boundaries because it
has been controlled by deposition of loess subject to topographic effects on wind transport and deposition, as well as
by rainfall and associated leaching.
7.3.4 The Waiareka Loess Gap
The highly productive market gardens and cropping land on
the easy-rolling landscape immediately to the south of
Oamaru (Fig. 7.1) owe their productivity to the Waiareka
series of Vertic Melanic Soils. The Waiareka clay soils are
formed in weathered mafic volcanic rocks. There are also
minor areas of the Oamaru series (Rendzic Melanic Soils) on
limestone ridges. Surrounding the Melanic Soils are soils
formed in a blanket of loess (mainly Fragic and Laminar
Pallic Soils). The loess blanket is near continuous in the
surrounding landscape apart from the area of Waiareka soils
where there appears to be a ‘hole’ in the loess blanket. The
hole has been informally called the Waiareka loess gap, but
what is the origin of this apparent gap? One favoured
7.2 Soil Profile Genesis
105